HLTH 340 Final Exam Study Notes PDF

**HLTH 340 -- Final Exam Study Notes** **SECTION A** **Principles of Toxicology** - Definitions (xenobiotic, toxicokinetic toxicant etc.) - **[Environmental toxicology:]** study of harmful effects of many different chemical, biological, and physical agents on living organ...

**HLTH 340 -- Final Exam Study Notes** **SECTION A** **Principles of Toxicology** - Definitions (xenobiotic, toxicokinetic toxicant etc.) - **[Environmental toxicology:]** study of harmful effects of many different chemical, biological, and physical agents on living organisms in the ecosystem, including humans. - **[Environmental toxicology:]** describes chemical transport, fate, persistence and bioaccumulation of substances and their effects at the population/community level. - **[Toxicology:]** The study of poisons and its harmful effects. - **[Toxicant:]** Hazardous substances including chemicals that can pose harmful effects on living beings. - **[Dosage:]** Amount of substance taken into body, used to determine toxicological effects. Usually standardized by body weight and duration. - Basics of Toxicological Analysis - [Exposure:] how people come into contact with xenobiotic. - [Toxicokinetic:] describes what happens to xenobiotic in body. - [Toxicodynamic:] describes what the xenobiotic does to the body. - Toxicokinetic vs toxicodynamic processes - [Toxicokinetic Processes:] - **Absorption:** how toxicants enter the body through external membrane barriers. - **Distribution**: how toxicants move through circulatory fluids to organs and tissues. - **Metabolism**: how our body processes toxicants to various metabolites. - **Excretion:** How the body removes the toxicant/metabolites. - [Toxicodynamic Processes:] - The biological effects caused by the toxicant to our body. - E.g., Oxidative Stress; Electrophilic Attack; DNA Damage; Endocrine Disruption;... - History of Toxicology - Majority of chemical of exposure to humans occur from naturally occurring compounds through diet of food plants. - **[Paracelsus (1493-1541)]** - "The dose makes the posion" - **[Mathieu Orfila (1787-1853)]** - "Father of toxicology" - First great exponent of forensic medicine - Factors to adverse effects from toxicants - Intrinsic toxicity - Dose - Exposure conditions - Individual susceptibility - Intrinsic toxicity - Characterized by key physical-chemical properties of toxicant: - Molecular structure and functional groups; - Stability/Reactivity; - Solubility/Insolubility - Chemical species/form; - Volatility; - Toxicants [may] or [may not] have a threshold. - Several measures used to describe toxicity of toxicants. - Toxic Effect Classification - Categorized by [site of toxicity] and [mechanism of action]. - Exposure routes - Critical to determine dosage! - Routes have different rates of absorption, distribution, and metabolism. - [NOTE]: Exposure routes are different from exposure pathways. - Exposure Responses - Classified through [duration] and [frequency] of exposure: - Acute - Exposure \< 24 hours - 1 dose - Subacute - Exposure up to a month - Subchronc - Exposure between 1-3 monthsz - Chronic - Exposure \> 3 months - **[Haber's Law:]** As exposure duration increases, the smaller amount of toxicant can cause adverse effects. - What differentiates chronic toxicity from acute? - When a toxicant accumulates. - The rate of absorption exceeds rate of elimination. - Each dose of toxicant causes irreversible toxicity. - There is an insufficient time for recovery between doses. - Internal/external dose - **[External Dose]** - [The amount of toxicant that has entered the body.] - Requires bioavailability adjustments - Compared against exposure limits for risk assessment - Provides details on driver of exposure. - **[Internal Dose]** - The amount of toxicant available to cause harm. - Measured via human biomonitoring - Exposure Conditions Factors - Magnitude - Route (e.g., ingestion, inhalation, dermal) - Duration (e.g., acute, sub-chornic, chronic) - Frequency (e.g., \# of times exposed, time between exposure) - Susceptibility - Toxicants effect everyone differently. - Susceptibility and sensitivity (NOT Vulnerability) are treated as synonyms. - Factors: - Sex - Age and life-stage - Nutrition and medical history - environmental exposures - Genetic background/ethnicity - Species differences - Thalidomide - Used as sedative and anti-nauseant during pregnancy in 1956 - Human teratogen causing missing/malformed limbs - These negative effects led to more stricter drug regulations and control over drug use and development. **Risk Assessment** - Definitions - **[Risk Assessment:]** The process of establishing information regarding acceptable levels of a risk and/or levels of risk for an individual, group, society, or the environment. - **[COPC:]** Contaminant of Potential Concern - Assessment framework - **[Hazard Identification:]** Determines what health problems are caused by exposure to the hazard. - **[Exposure Assessment:]** Evaluates how individuals or populations come into contact with a hazard (e.g., through air, water, soil, food). - **[Hazard Characterization:]** Describes the nature and magnitude of adverse effects that could be caused by the hazard. - **[Risk Assessment:]** Combines information from hazard identification and exposure assessment to estimate the likelihood of health risks. - **[Risk Management:]** Identifies and implements measures to control the risk. - **[Risk Communication:]** Involves communicating risk findings and mitigation strategies to stakeholders and the public. - What determines [potential risk]? - Chemical concentration - The route of exposure - Inherent toxicity - What components must be present for an adverse risk to exist? - Receptor - Exposure - Hazard - [NOTE]: If 1 of the components is missing, there can be no risk. - Risk Assessment Paradigm - **[Problem Formulation:]** Gather and interpret information to understand problem to define scope of assessment and identify key elements (e.g. chemicals, routes of exposure, etc\...) - **[Exposure Assessment:]** Determine nature and magnitude of exposure to individuals/populations, if any, based on receptor, site characteristics, and contaminants. - **[Hazard Assessment:]** Evaluates toxicological properties of hazard, like [dose-response relationships] and [exposure limits] to understand how the hazard can affect health. - **[Risk Characterization:]** Using data from exposure/hazard assessment to estimate potential health risk. - **[Risk Management:]** Identify and implement actions to reduce/eliminate health risk. - **[Scientific Communication:]** Informing the public/stakeholders about risks and mitigations measures taken against hazard. - **[Collection and Validation of Data:]** Data collected and validated throughout the process to ensure accuracy/reliability of risk assessment. - Hazard vs. Risk - Certain things can be hazardous (venomous snakes), but there's a low probability of being exposed to the hazard (getting bit and injected by the snake), so the risk is low. - Exposure pathways - Classifications: - Completed: Health risk since public can be exposed to COPC. - Potentially: Potential health risk. - Eliminated: No health risk. - Refers to the source to receptor pathway of a potential contaminant. - Exposure pathways vs. exposure routes - The exposure pathways refer to the path the COPC takes to reach the receptor, while the exposure route specifically describes how the COPC enters the body. - Dose-Response (and associated values) - "The dose makes the posion" - Generally, the higher the dose, the more severe the response. - Relationship is synthesized from observation studies. - Help describe correlation between exposure and induced effects. - **[NOAEL = No Observed Adverse Effect Level]** - Used in determining exposure limits for chemicals that have a threshold. - Usually exaggerate score by factor of 10 or even 1000. - Uncertainty factors - UF1 \~ **[Inter-species differences]** - UF2 \~ **[Inter-individual differences]** - Receptor Characterization - A lot of factors must can cause significant variations in receptors like: - Lifestyle and habits - Physical characteristics - Sensitive receptor groups - Etc.... - Exposure activity factors - E.g., toddlers/gardeners have higher and more direct contact with soil and dust so are exposed to toxicants found in there, which should be considered when setting exposure limits. - Bodyweight - Dose per kilogram can have huge amount of varying effects of toxicants in population, making it important to consider. - Contaminant Characterization - Concentration in environment - Speciation (metals) - Congeners (organics) - Bioavailability - Bioavailability Adjustments - Most relevant in oral/dermal exposures - Expressed as fraction: \~1-100% / \~0.01-1.00 - Data provided through silico models and vitro models. - Risk assessment and exposure calculations and associated values (HQ, ILCR, ADI/TDI etc) - [Exposure Limits: ] 1. Identify critical effect(s) 2. Calculate threshold for critical effects(s). E.g., NOAEL 3. Apply Uncertainty factors. - Equation: [\$exposure\\ limit = \\frac{\\text{NOAEL}}{UF\_{1}\*UF\_{2}}\$]{.math.inline} - **[NOTES:]** - Data is usually derived from regulatory databases. - Exposure limits appear differently for threshold vs. non-threshold. - May be designated for specific routes e.g., oral slope factor - Exposure limits often vary - [LD50:] - Refers to a dose that results in a 50% mortality rate. - The lower value the more deadlier the substance! - [Contaminated Soil:] - Exposure (µg/kg/day) = IR~soil~ \* C~soil~ \* AF~GIT~ \* (ED/365) / BW - IR~soil~ = Amount of soil ingested per day (g/day) - C~soil~ = Chemical concentration is soil (ug/g) - AF~GIT~ = Oral bioavailability of contaminant (unitless) - ED = Exposure Duration (Number of days per/year) - BW = Receptor body weight (kg) - [Hazard Quotient (HQ)] - [\$HQ = \\frac{\\text{External\\ Dose}}{\\text{Tolerable\\ Daily\\ Intake}}\$]{.math.inline} - Tolerable Daily intake (TDI) can be replaced to any exposure limit: - Reference Dose (RfD) - Acceptable Daily Intake (ADI) - Upper Limit (UL) - Risk defined as HQ: - \> 1.0 (If included all exposure pathways) - \> 0.2 (If included only 1 exposure pathway) - [Incremental Lifetime Cancer Risk (ILCR)] - [\$ILCR = exposure\\left( \\frac{\\text{μg}}{kg\*d} \\right)\*Cancer\\ Slope\\ Factor\\left( \\frac{\\text{μg}}{kg\*d} \\right)\^{- 1}\$]{.math.inline} - The higher the CSF, the more potent the carcinogen - ILCR compared to benchmark to determine risk vs. negligible risk - Risk if ILCR: - \> 1 x 10^-6^ - \> 1 x 10^-5^ - Values determined by US FDA to be negligible risk. - Risk Management & Communication: - **Key Elements**: Awareness of potential problems, public engagement, development of scientific knowledge, political will, and societal values. - **Strategies:** - **[Precautionary Principle:]** Act when potential harm is significant, even if not fully understood. - **[ALARA (As Low As Reasonably Achievable):]** Minimize risks to the lowest feasible level. - **[Comparative Risk Analysis:]** Compare risks (e.g., pesticide A vs. pesticide B) to choose the least harmful option. - **Public Perception:** Public tends to fear unfamiliar or uncontrollable risks (e.g., chemicals) more than familiar, voluntary ones (e.g., smoking). - Challenges in Risk Assessment: - **Data Gaps:** Lack of comprehensive toxicity data for many chemicals. - **Extrapolation Issues:** Using animal studies to predict human risks can be unreliable due to differences between species. - **Real-World Exposure:** Humans are exposed to complex mixtures of pollutants, not single chemicals, complicating risk assessment. - Exposure Pathways and Routes: - **Pathways:** Source to receptor via environmental media (air, water, soil, etc.). Completed pathways indicate potential health risk. - **Routes:** How the contaminant enters the body (inhalation, ingestion, dermal contact). - Bioavailability vs. Bioaccessibility: - **Bioavailability:** Contaminant is absorbed into the body and can cause harm. - **Bioaccessibility:** Contaminant is available for uptake but hasn\'t crossed a biological barrier yet. - Focus on bio accessible contaminants for risk assessment, as they indicate potential exposure - Hazard and Risk: - **Hazard:** Potential to cause harm (e.g., toxic chemical). - **Risk:** Likelihood of exposure leading to harm. A substance may be hazardous but not pose significant risk without exposure - Risk-Specific Dose (RsD): - For carcinogens, RsD represents the daily intake level that corresponds to an acceptable risk level (e.g., 1-in-a-million chance of cancer)) **SECTION B** - Bradford-Hill Criteria (Brunekreef 2008 reading) 1. Strength of the association 2. Consistency 3. Specificity of the association 4. Temporality 5. Biological gradient 6. Plausibility 7. Coherence 8. Experimental Evidence 9. Analogy - Human Microbiome Project (HMP) Goals 10. Determine if people share a core human microbiome 11. Understand if changes in microbiome relates with changes in health 12. Develop tech and bioinformatic tool to support these goals 13. Address ethical, legal, and social implications raised by HMP - Microbiome - We are widely colonized by microbiomes. - Partly transferred from mother. - Several transitions in first year of life and remains constant till 65. - Influenced by genetics, environment, diet, and other factors. - They can cause catastrophic damage if they move where they don't belong - GI flora most diverse - Bacteroides -- people who eat plenty of protein and animal fats - Prevotella -- people who eat more carbs, especially fibre - Ruminococcus - GI microbial activity Million Dollar Question - Are **uncertainty factors** currently used in risk assessment sufficiently protective for populations made more susceptible by the activity of their microbiome? - Influence of microbiome on health - **[Metabolic Functions:]** Microbiomes play a crucial role in digesting food that the body can\'t process alone, producing essential nutrients like vitamins. - **[Immune System Regulation:]** Microbiomes help regulate both innate and acquired immunity, protecting against infections and reducing chronic inflammation. - **[Disease Association:]** Shifts in microbial balance have been linked to various diseases, including allergies, autoimmune disorders, obesity, and even mental health conditions like autism spectrum disorders (ASDs) and irritable bowel syndrome (IBS). - **[Drug Metabolism:]** Microbiomes metabolize drugs and other compounds, affecting how medications work in the body and influencing responses to toxicants. - Sites of absorption - The GI tract - Lungs - Skin - Categories of drug administration - Enteral - Through alimentary canal (*i.e.*, sublingual, oral, and rectal). - Parenteral - Through all other routes (i.e., intravenous, intraperitoneal, intramuscular, subcutaneous, etc.). - Cell Membranes - Membrane lipid bilayer is formed through hydrophobic interactions. - Fluidity depends on structure and fatty acid saturation. - Saturated fatty acids clump together closely, so hard for enzymes to break it down. - Unsaturated fatty acids are more fluid like, so healthier. - Passive Transport - **[Fick's Law:]** Chemicals will move from high concentration to low concentration. - **[Paracellular diffusion:]** small hydrophilic molecules (\~500-600 Da) crosses through aqueous pores in membrane. - **[Transcellular diffusion:]** large hydrophobic molecules diffuse across lipid domains of membrane. - Small lipophiles permeate via passive diffusion. - Hydrophiles cannot permeate. - Paracellular is quicker. - Transcellular passive diffusion - Requires no system or energy source - Random migration by individual solute molecules - Cannot concentrate substances (*i.e.*, no pumping action) - Bidirectional -- flow in or out of tissue - Direction governed by concentration gradient - Absorption rate determined by: - Surface area of barrier - Concentration gradient - *Permeability* of the substance through the membrane - Physiochemical properties that affect absorption - **[Molecular weight:]** Small molecules with \< 500 Daltons pass easier. - **[Hydrophobicity (lipophilicity)]** molecules pass easier - **[Ionization:]** Molecules with +/- charge pass worse. - **[Polarity (H-bonding):]** molecule with uneven charge pass worse. - Cell junction functions - Mortar between cells - Permeability seal -- selective barrier to diffusion - Cell-to-cell communication - K~ow~ (partition coefficient) - Relative solubility in lipid (lipophilicity) vs. water (hydrophilicity) - *K~ow~* = Concentration~(octanol)~ ÷ Concentration~(water)~ - A white background with black dots Description automatically generated - *K~ow~* express in log~10~ units - *K~ow~* \> 10 (or log *K~ow~* \> 1): Lipophilic - *K~ow~* = \~1-9 (or log *K~ow~* = \~0-1): Amphiphilic - *K~ow~* = \~0-1 (or log *K~ow~* \< 0): Hydrophilic - Acidity - Non-ionized species of the acid/bases are preferentially absorbed transcellularly. - All modes of transport across membranes - Passive Diffusion -- Lipophiles - Facilitated Diffusion -- Hydrophiles - Active Transport -- Hydrophiles - Lipinski's "Rule of Five" - Poor transcellular passive absorption when 2+ are true: - 5+ H-bond donors in molecular structure. - 10+ H-bond acceptors in molecular structure - MW \> 500 - Log Kow \> 5 - Carrier-mediated absorption - Large glycoprotein molecules - Help specific hydrophilic solutes to cross membrane barriers - Active transport characterizations - Movement of chemicals against a gradient - Saturation at high chemical concentration - Selectivity for certain structural features - Competitive inhibition of similar structured chemical - Requirement of energy to facilitate transport - Ion transporter specificity: - Effective ionic radius - Positive or negative charge - Class of metal ion - Ion transporter factors: - Affinity for tranposrt protein - Saturation - Competition - regulation - Relevant transport proteins - NA^+^/K^+^ pump - Uses a ATP to pump 3 Na+ molecules out of the cell and 2 K+ molecules into the cell. - Divalent cation transporters - Pumps divalent cations from lumen into bloodstream transcellularly. - Passive diffusion if high concertation of divalent cation. - Active transport if low concertation with ATP. - TRPV6 (ECaC2) - Pumps calcium from intestine into bloodstream transcellularly actively. - Upregulated by active vitamin D - Upregulated by estrogen - Lead uses same transporter to enter bloodstream - High calcium intake can competitively inhibit lead absorption - Endocytosis - Active transport where cell engulfes molecule(s) using energy - For large polar molecules - Different varities: - **[Phagocytosis]:** Engulfing of solid particles (e.g., large molecules). - **[Pinocytosis:]** Engulfing of extracellular fluids. - **[Receptor-mediated endocytosis:]** Absorbs specific molecules (e.g., metabolites, hormones) via receptor-specific vesicles. - Example: manganese (Mn2+) - Enters brain actively via endocytosis. - Exocytosis - Expels substances from cell - Brunekreef 2008 - **[Environmental Epidemiology:]** Focuses on how physical, chemical, and non-infectious biological factors in the environment impact health and disease within populations. It contrasts with occupational epidemiology and nutritional epidemiology, which deal with workplace and food-related exposures, respectively. - **[The London Smog of 1952:]** A landmark event in environmental health where over 4000 deaths were attributed to air pollution. This was an early example of environmental epidemiology being used to assess the health effects of environmental factors, leading to public health policies like the Clean Air Act. - **[Definition of \"Environment\" in Epidemiology]**: Refers to external factors like air, water, food, and soil, excluding social or occupational factors but including passive smoking and home exposures. - **[Risk Assessment Components:]** - [Hazard Identification]: Determining if a substance or environmental factor can cause harm. - [Exposure-Response Assessment]: Establishing the relationship between exposure level and health effects. - [Exposure Assessment:] Measuring or estimating the level of exposure. - [Risk Characterization]: Quantifying the proportion of exposed populations that might experience adverse effects. - **[Example of Hazard Identification:]** Studies linking PVC flooring to increased asthma risk in children, showcasing the hazards of chemical exposures from household materials. - **[Exposure Assessment Methods:]** - [Direct Measurement:] Using instruments to measure exposure (e.g., personal air monitors). - [Modeling:] Predicting exposure levels based on proximity to pollution sources. - **[Exposure-Response Relationship]**: This relationship describes how changes in exposure levels correspond to changes in health outcomes. For example, passive smoking (ETS) was linked to lung cancer risk in non-smoking individuals exposed to smoking spouses. - Biomarker - A chemical/metabolite/product that is measured in the human body. - Measurable indicator of some biological state or condition. - Used to allow us to make bioavailability adjustments from environmental monitoring data, so we don't overestimate potential health risks and related outcomes. - Obtaining the **biologically effective dose** - Human biomonitoring (not specific studies) - Directly measure exposure of toxicants in people by measuring *biomarker* in human specimens (e.g. urine or blood). - Provides internal dose of chemical via all routes of exposure. - [Purpose:] - Estimate dose absorbed in body. - Provide a measure of health risk. - Find new chemicals in environment and human tissues. - Monitor changes in exposures. - Determine distribution of exposures in population. - Identify vulnerable groups in population. - [Sexton et al. six major uses:] - Identifying priority exposures - Recognizing time trends in exposures - Identifying at-risk populations - Establishing reference ranges for comparisons - Providing integrated dose measurements - Evaluating exposure prevention efforts - [Limitations (biomarkers):] - (Usually) unable to define sources, pathways, or duration of exposure. - ONLY demonstrates presence of chemical. - Unable to determine toxic external dose. - Lack of useful **health-based guidance values** for most chemicals. - Types of Biomarkers - Biomarkers of Exposure - How much of a biomarker is found in a compartment of an organism. - Biomarkers of Effect - Magnitude of biochemical, physiologic, behavioral, or other alterations in organism which signify association of possible health impairment or disease. - Biomarkers of Susceptibility - Indicators either genetic or physiological of an organism to be able to respond to a exposure of a chemical substance. - Significance of route of exposure - Concentration and properties of toxicant vary with route of exposure. - One route may absorb a high percent while another route absorbs a low amount of toxicant. - A toxicant may be non-toxic from one route, but highly toxic via another. - Highlights the importance of route of exposure. - Absorption by GI tract - Many environmental toxicants precent within food. - Incidental ingestion is the most common route of exposure. - Absorption occurs across entire GI tract. - Significant absorption occursf when toxicant is in gut lumen as molecular solution. - Absorption depends on pH of gut lumen, pKa, and lipid solubility of compound. - Organic acid/bases are absorbed by simple diffusion in GI tract where it exists in most lipid soluble form (non-ionized). - Toxicants can compete with actual required nutrients. - **[First-pass effect:]** - Toxicants are bio transformed by GI tract or liver before entering blood stream. - Nutritional status is important of a person since changes can affect the GI tract, body composition and fluids, etc.. - Lead -- Pathology and trends in policy and historical - ***Flint water crisis*** - Flint, Michigan, in April 2014 - Michigan changed water source. - Water contaminated by lead because city failed to apply corrosion inhibitors, so aging pipes leached lead into water supply. - [Sources of Lead:] - Leaded paints - Auto exhaust in soils - Pica in young children - Lead dust contamination - Lead shot - Lead drinking water pipes in older homes - **[BLL (Blood Lead Level):]** current research correlates to internal dose rather than external - Uncertainty with BLL and health risks - Clearly document adverse effects at BLL of 10 μg/dL - Sufficient evidence of adverse health effects at BLL of 5 μg/dL to as low as 1-2 μg/dL. - Harmful to all ages, infants/children are the most susceptible. - [Children may face:] - Neurodevelopmental effects - Reduction of IQ - Attention-related behaviours - Hyperactivity and behavioral disorders - BLL increase of 1 μg/dL correlates to approx. IQ point deficit. - Lead poisoning prevented to vulnerable people (infants/pregnant women/elderly) by: - Calcium supplements - Diet with dairy products, adequate vitamin D - Estrogen supplements for post menopause women - Adult lead poisoning symptoms: - Reduced CNS sensitivity - Chronic anemia, hypertension, kidney problems - **[Law of Unintended Consequence]** - Lead pipe replacements may pose a higher risk than leaving as is. - Disturbing old pipes may cause the release of lead. - Toxicants and Lungs - Lungs poor barrier to xenobiotics from entering bloodstream. - Very large surface area with thin membrane highly perfused with blood. - Epithelium in lungs limited, only allowing slow absorption of highly water-soluble compounds. - Compounds that pose risks to lung absorptions are: - Gases and vapours - Aerosols and particles - Mucosa in nose can trap gas molecule if they are very water soluble or reach with cell surface components. - May concentrate xenobiotics in nose tissue. - Molecules cross from alveolar space to blood through concentration balancing on both sides. - Controlled by blood-to-gas partition coefficient (unique to each gas) - **[Particulates]**: particles, dust, must or fume that have been suspended in air. - **[Protection against particulates:]** - Airway geometry - Humidity - Mucociliary clearance - Alveolar macrophages - Particle size and effect: - "Coarse" - \>= 5 µm - Deposited in nasopharyngeal region - "Fine" - Approx. 2.5 µm - Deposited in tracheobronchial region of lungs - "ultra-fine" / nanoparticles - \< 100 nm in size - Penetrate alveolar sacs of lungs - Deposited in alveolar region of the lungs - Lead understood - Pose greatest risk - **[Particulates removal:]** - Mucociliary escalator - Phagocytosis - Lymphatic excretion - **[NOTES:]** - Overall all are inefficient - Some remain in alveoli indefinitely, and stimulate local network of collagen fibers to form alveolar plaque or nodule. - Toxicants and Skin - Largest organ and provides good barrier to surrounding environment (relatively impermeable). - [Absorption through skin depends on:] - Concentration - Duration of contact - Solubility - Physical condition of skin - Part of the body exposed - [Three major layers of skin:] - **[Epidermis:]** outer layer, whose function is to protect, absorb, feel, secrete, excrete, and regulate. - **[Dermis:]** for tensile strength of skin. It functions to regulate temperature and supply epidermis with nutrient-saturated blood. - **[Subcutaneous Tissue:]** layer of fat and loose, connective tissue with large blood vessels and nerves. Also provides insulation and cushioning. - Chemicals are absorbed mostly through epidermis while some can also enter through sweat glands or hair follicles. - Chemicals pass seven layers of epidermis before reaching dermis where they can enter bloodstream or lymph. - **[Stratum corneum]** is the outermost and thickest layer making it the primary barrier to absorption of xenobiotics. **SECTION C** - Additional ADME terms - [Disposition] = **[distribution]** + **[metabolism]** - [Clearance] = **[metabolism]** + **[excretion]** - Factors affecting distribution - [Broadly:] - Physicochemical properties of the chemical - Blood flow - The rate of diffusion out of the capillary bed into the cells of a particular organ or tissue - Affinity of a xenobiotic for various tissues - [Specifically:] - Circulatory system anatomy (**[first-pass effect]**) - Toxicant partitioning between **[blood components]** - Blood flow patterns (**[perfusion kinetics]**) - Blood-tissue partitioning (**[tissue bioavailability]**) - Internal membrane barriers - Tissue sequestering and mobilization. - Metabolic biotransformation reactions - Ionic trapping of toxicants (or metabolites) in tissue - Routes for xenobiotic to move to target organ - [Portal circulation] - **[Portal vein]** carries blood from Gi track to liver - [Systemic circulation] - **[Pulmonary vein]** carries blood from lung to rest of body via heart. - [Lymphatic circulation] - **[Alimentary duct]** carries lymph from GI track to the system circulation - First-pass effect - Xenobiotics from gut enter liver via **[hepatic portal vein.]** - Xenobiotics from systemic circulation enter liver via **[hepatic artery.]** - Liver filters lipophiles (fraction) in **[hepatocytes.]** - Remaining lipophiles enter systemic blood via **[hepatic vein]**. - **[Hepatocytes]** secrete lipophiles to **[bile]**. - **[Biliary excretion]** carries lipophiles to gut. - Blood Fractions - **[Whole blood]** - Plasma + erythrocytes - **[Plasma]** - Aqueous liquid carrying RBC with dissolved proteins, clotting factors, trace elements. - **[Serum]** - Plasma -- fibrinogen + trace elements + proteins (not involved in clotting) - Blood partitioning - **[Free plasma phase]** -- xenobiotic molecules dissolved as free solute in water (Hydrophiles). - **[Protein bound phase]** -- xenobiotics reversibly bound to large plasma proteins (lipophiles). - Plasma proteins - [Albumin (most common)] -- neutral lipophilic and mildly acidic - [Lipoproteins] -- strongly lipohilic molecules - [Special carrier proteins] -- e.g., transferrin for iron and other metal ions - Erythrocytes -- selectively bind to certain metal ions (e.g., Fe, Zn, Pb) - ALAD - ALAD1: - Most common form - More Pb in their plasma and bone - Increased ALA accumulation - Increased chelation efficacy - More sensitive to CNS - ALAD2: - Relatively common among Caucasians (20%); very rare for people of colour - Asparagine Lysine substitution at residue 59 - More Pb in whole blood, RBC - Less ALA accumulation - Less chelation efficacy - More sensitive to kidney - Tissue distribution - Toxicants often produce effects at **[target tissue]**. - **[Critical effect]** is the effect that occurs with lowest administered or observed dose. - **[Bioavailability]** is internal dose that can pose harm to target tissue. - **[Oral bioavailability:]** % of toxicant absorbed from GI tract. - **[Effective Blood Concentration:]** fraction of toxicant freely dissolved in blood plasma. - **[Tissue/blood partition:]** degree of toxicant permeation from blood to specific tissue. - Perfusion-limited and partition-limited tissue distribution - **[Tissue perfusion rate:]** rate of blood flow to organs - Highly perfused tissues most vulnerable (e.g., liver, kidney, etc.) - Poorly perfused tissues least vulnerable (e.g., skin, fat, etc.) - **[Partition-limited tissue distribution:]** xenobiotic partition between high- and low-fat tissue. - Mainly determined by **K~ow~** - **[Carrier-mediated transport:]** - Ionic/polar xenobiotics can use selective membrane channels or pumps. - Distribution also depends on liver metabolism of xenobiotics. - Lipophilic metabolites bind to plasma protein. - Persist in body. - Hydrophilic metabolites - Rapidly excreted by kidney - Know what PBPK models are (but nothing too specific) - stands for **[Physiologically Based Pharmacokinetic Model.]** - Simulates toxicokinetics of xenobiotic. - Models represent the body with compartments such as lungs, liver, fat, blood, and other tissues. - Different parameters, such as **[blood flow rates]** and **[concentrations]** are used to calculate how substances move between compartments. - These models help in predicting toxicity, pharmacokinetics, and dosage of xenobiotics (foreign substances). - Important for risk assessment and regulatory decision-making in toxicology and pharmacology. - Internal membrane barriers - A additional protective layer for vulnerable tissue to restrict uptake of some xenobiotics from the blood to tissue. - **[Anatomical barriers:]** - Endothelial cells of blood capillaries have tight junctions to prevent paracellular uptake. - **[Physiological barriers:]** - Capillary endothelium cells with [selective carrier-mediated uptake channels] for beneficial nutrients and regulator factors. - These barriers have efflux pumps to remove any xenobiotic that has entered. - **[Non-static:]** - Permeability changes of internal membranes due to many factors. - Injury, infection, stress may alter barrier function. - May not be fully mature in early life. - May become less effective with old age. - Blood-brain barrier - Brain restricted by **[2 barriers]**: - Blood-brain barrier (BBB) - Blood-cerebral spinal fluid barrier (BCSFB) - Non-bullet-proof barriers, but keep most toxicants from entering brain. - BBB formed by endothelial cells of blood capillaries in brain. - **[Endothelial cells form tight junctions]** with adjacent cells by: - Tight seal between cells - Prevent diffusion of polar compounds paracellularly. - Diffusion of lipophilic compounds prevented by **[efflux transporters]**. - Glial cells provide another layer of protection. - BBB **[not fully developed at birth]**, so chemicals are more toxic for newborns. - Junctions are immature "leaky" - Placenta - Another strictly regulated internal membrane only allowing exchange of vital molecules. - Most vital nutrients transported by **[active transport]**. - Most xenobiotics enter through simple diffusion - To prevent xenobiotics to reach fetus, many layers. - **[BCRP (Breast Cancer Resistance Protei]n**) key role in projection: - [Highest expression] in placenta vs any other organ - Limits absorption across many barriers in body. - Enhances excretion of xenobiotics in liver/kidney - Excretion of vitamins into breast milk - MPTP - Back-street chemist made MPTP accidentally instead of MPP (synthetic heroin), which when ingested show symptoms of **[Parkinson's disease]**. - Caused strong degeneration of dopaminergic neurons containing neuromelanin. - Highly lipophilic so crossed BBB quickly. - Damaged/destroyed dopamine producing neurons in **[substantia nigra]**. - MPP+ inhibits mitochondrial complex 1 - Cell death - Accumulation of free radicals. - CYP2D6 - A gene when expressed can **[metabolize centrally]** acting drugs, neurotoxins, neurochemicals. - Reduce/eliminate severity of drugs. - **[Increased expression]** for alcohol consumers and smokers. - MPP+, Paraquat, Rotenone, and Lead - **MPP+**: Bioactivated metabolite of MPTP. Neurotoxic via **Complex I inhibition**, leading to **ROS** and **Parkinsonism**. Crosses **BBB** via passive diffusion. - **Paraquat**: A toxic herbicide that accumulates in the lungs, causing **lung fibrosis** and **pulmonary edema**. Acts as a **pro-oxidant**. - **Rotenone**: A botanical pesticide that is neurotoxic by inhibiting **Complex I**. Associated with **Parkinsonism**. Passes the **BBB** by passive diffusion. - **Lead**: Alters blood-bone partitioning due to **ALAD polymorphism**, leading to neurotoxicity. - **BBB Uptake**: Lead, MPP+ (as MPTP), and Rotenone cross the BBB, while Paraquat's mechanism is unclear. - **Consequences**: - **Lead**: IQ decrease. - **MPP+**: Chemical Parkinsonism due to brain accumulation. - **Paraquat**: **Pulmonary edema** and **fibrosis**. - **Rotenone**: Chemical Parkinsonism - Tissue Sequestration - **Tissue sequestration** occurs when a toxicant is stored in a specific tissue for an extended period. - The sequestered tissue may not be harmed directly by the toxicant. - Sequestration reduces toxicant concentrations in the blood and target tissues, potentially delaying toxic effects. - **Beneficial** by protecting against acute toxicity or overdose, allowing more time for excretion. - **Harmful**, by prolonging toxicant presence in body, so gradual accumulation. - Common storage tissues include **fat**, **bone**, **liver**, and **kidneys**. - For example, **lead** is stored in bones, while **paraquat** accumulates in the lungs. - Toxicant storage - [Binding to plasma proteins] - **[Albumin]** -- most abundant and can bind a large number of compounds. - α~1~-acid glycoprotein - transferrin, ceruloplasmin - α- and β-lipoproteins - [Liver and kidney] - High capacity - **[Ligandin:]** Cytoplasmic protein in liver with high affinity to many organic acids. - **[Metallothionein:]** Found in kidney/liver with high affinity for cadmium and zinc. In liver can hold Pb and concentrate 50-fold more than plasma. - Regulation of transition metals - Upregulated in response to sensitive metals. - [Fat] - Many highly lipophilic toxicants are distributed and concentrated in fat. - Storage in fat helps lower concentrate of xenobiotics in target tissues. - Helps reduce toxicity of compounds in obese people compared to lean. - [Bone ] - Fluoride, lead, and strontium may incorporate in **[bone matrix]**. - 90% lead in body eventually in skeleton - Mechanism by substituting bone components. - E.g., Pb^2+^ and Sr^2+^ may substitute for Ca^2+^ in the **[hydroxyapatite]** lattice matrix. - Not permanent, and can be released by osteoclastic activity - Molecular Sequestration - **[Transferrin]** - blood protein that bind and transports ferric iron (Fe^3+^) and similar transition metals (*e.g.*, Manganese) - **[Ceruloplasmin]** - blood protein that binds and transports copper (Cu^2+^) and ferrous iron (Fe^2+^) - **[Ferritin]** - tissue protein (esp. liver) that binds ferrous iron (Fe^2+^) and sequesters it as ferric iron (Fe^3+^) - Deficiencies in these carriers can cause **[oxidative stress]**, and **[long term tissue damage]** in organs. - Organochlorines - **Organochlorine compounds (OC)** are **highly lipophilic** (fat-loving) and commonly found in animal-derived foods. - OCs are absorbed through the gut by **passive diffusion** and stored in **adipose tissue (fat)**. - OCs meet the **PBT criteria**: - **Persistent**: They are not easily broken down by metabolism. - **Bioaccumulative**: They accumulate in fat tissue and cannot be readily excreted. - **Toxic**: They are harmful when tissue concentrations exceed a certain threshold. - **Depot Mobilization**: - During **pregnancy** and **breastfeeding**, OCs stored in fat can be mobilized into the bloodstream. - These compounds can be redistributed to active tissues, especially the **fetus** and **breast milk**. - **Breast milk** may act as an excretion route, leading to high levels of contaminants being passed to the **infant**, who can absorb the OCs from the milk. - Bone and Lead sequestration - **Lead (Pb²⁺) enters the body** through calcium transporters (**TRPV6** and **DMT1**) in the gut. - **Brain** is the primary target for lead and crosses the **blood-brain barrier (BBB)** via calcium transporters. - **Lead sequestration in bones**: - **90% of Pb²⁺** is stored in bone, particularly in growing bones of **children**. - Pb²⁺ deposits are visible in X-rays, providing only **partial protection** from lead toxicity. - **Adults** accumulate Pb²⁺ in bones over time. - **Bone mobilization of lead**: - Pb²⁺ in bones can mobilize during **pregnancy** and **aging**, as **Ca²⁺** is released from bones. - **Men** experience less bone demineralization with age, while **women** face more extreme demineralization during **menopause** due to reduced estrogen. - Serum estrogen **[increase]** with age for men, so less extreme bone demineralization. - **Pregnancy and breastfeeding** increase the risk of Pb²⁺ mobilization, potentially harming both mother and child. - Calcium supplements help prevent mobilization especially in post-menopausal or pregnant women. **HLTH 340 -- Week 7 Study Notes** **Overview of Mercury in the Environment** - **Sources:** - **Natural:** Forest fires, volcanic eruptions. - **Anthropogenic:** Coal combustion, metal smelting, industrial processes (e.g., chlor-alkali plants). - **Transport:** - Elemental mercury (Hg⁰) is stable and can travel long distances in the atmosphere. - Deposited mercury in soil and water may be converted into more toxic organic forms. - Forests are sponges to mercury **Speciation of Mercury** - **Key Forms:** - **Elemental Mercury (Hg⁰):** Easily vaporized, long-range atmospheric transport. - **Inorganic Mercury (Hg²⁺):** Can be transformed by microorganisms into methylmercury. - **Methylmercury (MeHg):** Highly toxic, bioaccumulates and biomagnifies in aquatic food webs. - **Other Organic Mercury Compounds:** - **Methylmercury (MeHg):** Relatively hydrophilic, readily crosses biological membranes due to molecular mimicry. - **Ethylmercury (EtHg):** Also hydrophilic. - **Dimethylmercury:** Very lipophilic, extremely toxic. - **Thiomersal:** Very hydrophilic preservative form. **Note:** Methylmercury's toxicity partly arises from its ability to mimic methionine, allowing it to cross membranes like the blood-brain barrier (BBB) via the LAT1 transporter. **Mercury Cycling & Biomagnification** - **Conversion to Methylmercury:** - Aquatic bacteria convert inorganic mercury (Hg²⁺) into methylmercury (MeHg). - **Bioaccumulation & Biomagnification:** - Small aquatic organisms absorb MeHg. - Predatory fish (e.g., tuna, swordfish) accumulate higher levels. - Humans at the top of the food chain receive the highest doses through fish consumption. **Human Exposure and Health Effects** - **Main Exposure Route:** Dietary intake, primarily fish. - **Health Effects:** - Neurological damage (e.g., cognitive deficits, motor impairment). - Immune - reproductive system effects. - **At-Risk Populations:** - Indigenous communities relying on fish as staple food sources. - Populations in regions with high mercury deposition. - Sensitive sub-populations: women of childbearing age, pregnant individuals, young children. - **Cardiovascular Trends from Hg:** - S-PUFA is negatively associated with myocardial infarction. - Hair-HG is positively associated with myocardial infraction. **Mercury Guidelines & Advisories** - **Tolerable Daily Intakes (TDIs):** - Total Hg (General Population): \~0.71 µg/kg/day (BCS 2007). - Methylmercury (General Population): \~0.47 µg/kg/day (BCS 2007) and \~0.23 µg/kg/day (WHO 2003). - Methylmercury (Sensitive Populations): \~0.20 µg/kg/day (BCS 2007). - **Canadian Fish Consumption Guidelines:** - Retail Fish Limit: 0.5 ppm total mercury (except shark, swordfish, fresh/frozen tuna). - Advisories for Exempted Fish: - General adults: \~1 meal/week. - Women of childbearing age & young children: \~1 meal/month. - A non-enforceable guideline (0.2 ppm) often used in risk assessments for frequent consumers. - **Sport Fishing Advice in Ontario** - Size and number of fish you can safely eat. - How to choose fish with lowest levels of contaminants. - Fish to not eat. - How to prepare fish to reduce contaminants. - Contaminants in different Ontario fish. - [Note:] Muscular fish store less MeHg due to lack of fat. **Historical Case Study: Minamata Disease** - **Location:** Minamata Bay, Kyushu, Japan. - **Cause:** Industrial discharge of methylmercury-laden wastewater (notably from Chisso Corporation) into coastal waters. - **Impact:** - 900 deaths - 2200 affected with severe neurological symptoms (ataxia, sensory disturbances, speech impairment), with many fatalities. - [Congenital Minamata disease]: Neurological damage in infants exposed in utero. - **Recognition & Response:** - Identified as organic mercury poisoning in the late 1950s. - Widespread contamination led to government advisories and eventual global awareness. - **International Policy:** - Led to the Minamata Convention on Mercury, a global treaty to reduce mercury emissions, ban certain uses, and prevent future incidents. **Canadian Example -- Grassy Narrows & White Dog Reserves:** - Mercury pollution from a paper mill into the English-Wabigoon River system. - Long-term community health, economic, and psychosocial impacts. **Mercury in Breast Milk & The Role of Transporters** - **Xenobiotic Distribution in Breast Milk:** - Contaminants, including methylmercury, can partition into breast milk. - Breast milk is a critical exposure route for nursing infants. - **BCRP (Breast Cancer Resistance Protein):** - An efflux transporter that can influence the secretion of certain xenobiotics (including potentially mercury compounds) into breast milk. - Understanding BCRP function is crucial for predicting infant exposure and assessing risks associated with breastfeeding. **Global Perspective & Regulation** - **Global Problem:** - Majority of mercury deposited in some countries (e.g., Canada) originates from foreign sources (notably industrial regions such as China). - **Minamata Convention on Mercury:** - Established in 2013, entered into force in 2017. - Mandates reducing mercury use and emissions. - Bans new mercury mines and phases out certain mercury-containing products by 2020. **Molecular Sequestration: Selenium (Se) and Mercury (Hg)** - **Role of Selenium:** - Selenium (Se) has an extremely high affinity for mercury, surpassing sulfur by a factor of one million. - Selenoproteins can bind and sequester both inorganic and organic mercury. - Thought to serve as a "sink," potentially protective in target tissues (e.g., brain). - Controversy exists over whether selenium binding in brain tissue is truly protective or if it can also impair selenoenzyme function. **Key Point:** Selenium sequestration of mercury may mitigate toxicity but also raises concerns that mercury could effectively "capture" selenium, reducing its availability for essential selenoenzymes. **Case Study: MeHg Neurotoxicity and Selenium** - **Low vs. High MeHg Exposure:** - High-dose methylmercury (MeHg) exposure is clearly neurotoxic in children and adults. - Low-dose health implications are less certain due to conflicting epidemiological findings. - **Island Comparisons:** - **Seychelles:** Diet includes fish with relatively high MeHg but also rich in long-chain polyunsaturated fatty acids (LC-PUFA) and selenium. Limited adverse effects detected. - **Faroes:** Diet includes pilot whale meat/fat with moderate to high MeHg but lower LC-PUFA and selenium levels. More pronounced adverse outcomes observed. **Key Insight:** The presence of selenium (and beneficial nutrients like LC-PUFAs) may reduce MeHg's harmful effects, emphasizing the importance of nutrient-mercury balance. **Hg-Se Interaction Paradigms** - **Hypothesis 1 (Conventional):** Selenium acts as a mercury antagonist, neutralizing its toxicity. - **Hypothesis 2 (Proposed):** Mercury is a selenium antagonist. Mercury sequesters selenium, preventing it from forming selenoenzymes, and this sequestration itself causes harm. **Conclusion:** Both paradigms highlight the importance of the Hg:Se ratio in determining toxicity or protection. **Selenium Health Benefit Value (SeHBV)** - **Concept:** Assesses both mercury and selenium concentrations in fish to evaluate net risk or net benefit. - **Calculation:** - Convert Se and Hg concentrations in fish from µg/kg to µmol/kg (using molecular weights: Se = 78.96 g/mol, Hg = 200.59 g/mol). - Calculate SeHBV: - or alternate updated forms provided by Ralston. - **Interpretation:** - **SeHBV ≥ 0:** Selenium is in surplus relative to mercury (net benefit). - **SeHBV \< 0:** Mercury dominates relative to selenium (net risk). **Example (Northern Pike):**\ \[Se\] \~ 6.33 µmol/kg, \[Hg\] \~ 1.25 µmol/kg → SeHBV \> 0 indicates good selenium health benefit. **Reality Check on SeHBV** - SeHBV focuses on Hg and Se, not other nutrients (e.g., LC n-3 PUFA) or other contaminants (e.g., PCBs). - North Americans rarely lack selenium, but often require more LC n-3 PUFAs. - No regulatory agency currently uses SeHBV for risk assessments. - Real long-term solution: reduce global mercury emissions (e.g., Minamata Convention). **Comparing Mercury (Hg) and Lead (Pb) Exposure and Risks** **Key Differences for Fetal Toxicity:** **Mercury** **Lead** ----------------------- ---------------------------------------------------------------- -------------------------------------------- Sources Primarily fish (MeHg), some via amalgam; Air is transport only Many sources, including dust and soil Nutrient Interactions Selenium can reduce MeHg toxicity at targets Ca, Fe, Zn may reduce Pb absorption Storage Sites Hg mainly in muscle, some in hair Pb mainly in bone, some in teeth Biomarkers Hg in hair, blood, nails Pb in blood, bone, teeth Fetal Exposure Fetal blood Hg \> maternal blood Hg Fetal blood Pb = maternal blood Pb Retention Hg has relatively low retention Pb accumulates in bone, long-term storage Maternal Status Current intake \> body stores dominates Body burden \> current intake important Breastfeeding Breast milk Hg \< maternal blood Hg Breast milk Pb \ Gut \> Blood \> Placenta \> Other tissues. ### **Enzyme Induction:** - **Definition:** Increase in enzyme levels due to increased gene transcription in response to exposure to certain substances. - **Mechanism:** - **Xenobiotic Inducers:** Compounds that enhance the expression of CYP450 enzymes. - **Result:** Increased metabolic clearance of xenobiotics, potentially altering the efficacy or toxicity of drugs. **Examples:** - **Polychlorinated Biphenyls (PCBs):** - **Effect:** Increase CYP450 expression, affecting detoxification and bioactivation pathways. - **Implication:** Can lead to increased formation of reactive epoxides. **Study Question:** - **Q:** *How does enzyme induction by xenobiotics affect drug metabolism?* - **A:** Induction increases CYP450 enzyme levels, accelerating the metabolism of drugs, which can decrease their therapeutic efficacy or increase the production of toxic metabolites. ### **Genetic Polymorphism of CYP450 Isoforms:** - **Definition:** Genetic variations that lead to differences in enzyme activity among individuals and populations. - **Impact:** - **Metabolic Rate Variability:** Leads to **ultraslow**, **slow**, **average**, **fast**, or **ultrafast** metabolizers. - **Clinical Implications:** Affects drug dosing, efficacy, and risk of adverse drug reactions. **Classification of CYP450 Isoforms:** - **Class I Isoforms (Non-Polymorphic):** - **Includes:** CYP1A1, CYP1A2, CYP2E1, CYP3A4. - **Characteristics:** - Well conserved with little genetic variability. - Enzymatic activity is relatively constant across individuals. - **Metabolize:** Many environmental xenobiotics and pro-carcinogens. - **Regulation:** Induced by xenobiotics. - **Class II Isoforms (Polymorphic):** - **Includes:** CYP2B6, CYP2C9, CYP2C19, CYP2D6. - **Characteristics:** - Highly polymorphic with significant genetic variability. - **Metabolize:** Drugs but usually not pro-carcinogens. - **Implications:** Unpredictable drug responses due to variable metabolism rates. **Study Question:** - **Q:** *Why is understanding CYP450 polymorphisms important in medicine?* - **A:** It helps in tailoring drug dosages to individual patients, reducing the risk of adverse reactions and improving therapeutic outcomes. ### **Importance to Various Fields:** - **Toxicologists:** - Understand species differences in toxicity. - Extrapolate animal data to humans. - Develop antidotes and treatments. - Determine whether toxicity is due to the parent compound or metabolites. - **Public Health Officials and Physicians:** - Identify susceptible populations. - Improve drug prescribing practices. - Avoid adverse drug reactions. - Implement personalized medicine approaches. **Phase II Metabolism (Conjugation Reactions)** ----------------------------------------------- **Definitions:** - **Conjugation:** The process of adding endogenous polar molecules to xenobiotics or their metabolites to increase water solubility. ### **Key Concepts:** - **Purpose of Phase II Metabolism:** - **Detoxification:** Converts lipophilic compounds to hydrophilic metabolites for excretion. - **Excretion:** Facilitates elimination via kidneys (urine) or bile (feces). - **Prevent Reabsorption:** Conjugated metabolites are less likely to be reabsorbed in the GI tract. - **Requirements for Conjugation:** - **Functional Groups:** Xenobiotics must have or acquire polar groups (-OH, -NH₂, -SH, -COOH). - **Cofactors:** Provide the conjugating group and energy. - **Transferases:** Enzymes that catalyze the conjugation reactions. ### **Types of Conjugation Reactions:** 1. **Glucuronidation:** - **Enzyme:** Glucuronyl Transferase (GT). - **Cofactor:** Uridine diphosphate glucuronic acid (UDPGA). - **Substrates:** Alcohols, phenols, carboxylic acids. - **Location:** Liver, kidney, intestine, lung, skin, prostate, brain. - **Characteristics:** - Most common Phase II reaction. - Increases water solubility for excretion. - Activity affected by age, hormones, environmental factors. 2. **Sulfation:** - **Enzyme:** Sulfotransferases. - **Cofactor:** 3\'-Phosphoadenosine-5\'-phosphosulfate (PAPS). - **Substrates:** Phenols, alcohols, arylamines, N-hydroxyl compounds. - **Location:** Liver, kidney, intestine. - **Characteristics:** - Less common. - Can produce reactive metabolites (toxic). 3. **Methylation:** - **Enzyme:** Methyltransferases. - **Cofactor:** S-Adenosylmethionine (SAM). - **Substrates:** Phenols, catechols, amines, N-heterocycles, sulfhydryl compounds. - **Location:** Liver, kidney, lung, CNS. - **Characteristics:** - Metabolites are often less water-soluble but generally inactive. 4. **Acetylation:** - **Enzyme:** N-Acetyltransferases. - **Cofactor:** Acetyl-CoA. - **Substrates:** Primary aromatic amines, sulfonamides. - **Location:** Liver, lung, spleen, gastric mucosa, red blood cells, lymphocytes. - **Characteristics:** - Major route for drugs with amino groups. - Metabolites may be less water-soluble. 5. **Glutathione Conjugation:** - **Enzyme:** Glutathione S-Transferases (GSTs). - **Cofactor:** Glutathione (GSH). - **Substrates:** Electrophilic compounds (epoxides, alkyl halides). - **Location:** Liver, kidney. - **Characteristics:** - Protects against reactive oxygen species (ROS) and electrophiles. - GSH is a critical antioxidant in cells. **Study Question:** - **Q:** *What is the role of glutathione in detoxification?* - **A:** Glutathione acts as a nucleophile to conjugate electrophilic xenobiotics, neutralizing them and preventing damage to cellular macromolecules. **Electrophiles and Nucleophiles in Toxicology** ------------------------------------------------ **Definitions:** - **Electrophile:** An electron-deficient species that seeks electrons; often reactive and can form covalent bonds with nucleophiles. - **Nucleophile:** An electron-rich species that donates electrons; includes molecules with lone pairs or pi bonds. ### **Electrophilic Xenobiotics and Reactive Metabolites:** - **Electrophiles:** - **Examples:** Epoxides, aldehydes, quinones, quinone imines. - **Reactivity:** Attack electron-rich nucleophilic sites in macromolecules (DNA, proteins). - **Toxicity:** Form covalent adducts leading to mutations, cancer, or cell death. - **Types of Adducts:** - **Alkylation:** Addition of small alkyl groups. - **Bulky Adducts:** Addition of large groups altering structure/function. - **Cross-Linking:** Covalent links between molecules. ### **Direct-Acting vs. Indirect-Acting Electrophiles:** - **Direct-Acting Electrophiles:** - **Reactive Parent Compound:** Contains electrophilic groups (e.g., chemical mustards, isocyanates). - **Characteristics:** Short-lived, rapidly hydrolyzed; cause immediate toxicity. - **Indirect-Acting Electrophiles:** - **Bioactivation Required:** Parent compound metabolized to reactive electrophile (e.g., via CYP450). - **Examples:** Epoxides formed from PAHs. ### **Target and Non-Target Nucleophiles:** - **Target Nucleophiles:** - **Essential Biomolecules:** Proteins with -SH, -NH₂, -OH groups; DNA bases. - **Damage:** Covalent binding leads to dysfunction. - **Non-Target Nucleophiles:** - **Glutathione (GSH):** - **Function:** Scavenges electrophiles, protecting essential molecules. - **Limitation:** Finite supply; depletion leads to increased toxicity. **Study Question:** - **Q:** *Why is glutathione depletion dangerous in the context of electrophile exposure?* - **A:** Without sufficient GSH, electrophiles can attack vital cellular components, leading to toxicity and cell damage. **Case Studies and Examples** ----------------------------- ### **Sulfur Mustard (Mustard Gas):** - **Chemical Warfare Agent:** - **Type:** Direct-acting electrophile. - **Effects:** Blistering agent causing skin and lung damage; chronic exposure linked to lung cancer. - **Mechanism of Action:** - **Alkylation of Biomolecules:** Reacts with nucleophilic sites, causing cellular dysfunction. ### **Bhopal Disaster (1984):** - **Incident:** - **Chemical Released:** Methylisocyanate (MIC). - **Cause:** Water entered MIC storage tank, leading to exothermic reaction and gas release. - **Effects:** Immediate respiratory and ocular irritation; long-term health impacts. - **Methylisocyanate (MIC):** - **Reactivity:** Direct-acting electrophile. - **Toxicity:** Reacts with nucleophiles in the body, forming adducts and causing damage. ### **Fluoroacetate (Lethal Synthesis):** - **Source:** - **Natural Occurrence:** Found in certain plants (e.g., Gifblaar). - **Use:** Rodenticide (rat poison). - **Mechanism:** - **Mimics Acetate:** Converted to fluorocitrate in the body. - **Inhibition of Aconitase:** Blocks the citric acid cycle, halting energy production. - **Symptoms:** - Nausea, abdominal pain, ventricular fibrillation, seizures, coma, death. **Study Question:** - **Q:** *What is lethal synthesis in the context of fluoroacetate poisoning?* - **A:** The metabolic conversion of a non-toxic compound into a toxic metabolite (fluorocitrate) that disrupts essential biochemical pathways. **Free Radicals and Oxidative Stress** -------------------------------------- **Definitions:** - **Free Radicals:** Molecules with unpaired electrons; highly reactive. - **Reactive Oxygen Species (ROS):** Chemically reactive molecules containing oxygen (e.g., superoxide anion, hydroxyl radical). ### **Sources of ROS:** - **Endogenous:** Cellular metabolism, mitochondrial respiration. - **Exogenous:** Environmental pollutants, radiation, toxins. ### **Targets of ROS:** - **Lipids:** Peroxidation of membrane lipids leading to cell damage. - **Proteins:** Oxidation alters function and structure. - **DNA:** Mutations and strand breaks. ### **Antioxidant Defense Mechanisms:** - **Enzymes:** - **Superoxide Dismutase (SOD):** Converts superoxide anion to hydrogen peroxide. - **Catalase (CAT):** Converts hydrogen peroxide to water and oxygen. - **Glutathione Peroxidase (GPx):** Reduces hydrogen peroxide and lipid peroxides using GSH. - **Non-Enzymatic Antioxidants:** - **Glutathione (GSH):** Thiol-containing tripeptide that scavenges free radicals. - **Vitamins:** Vitamin C and E. **Study Question:** - **Q:** *How does oxidative stress contribute to cellular damage?* - **A:** Excess ROS overwhelm antioxidant defenses, leading to oxidation of lipids, proteins, and DNA, resulting in cell dysfunction or death. **Re-evaluating Phase I and Phase II Classification** ----------------------------------------------------- ### **Issues with Traditional Classification:** - **Sequential Misconception:** - Not all compounds undergo Phase I before Phase II. - Some Phase II reactions can produce toxic metabolites. - **Mechanistic Incoherence:** - Grouping unrelated reactions (oxidations, reductions, hydrolyses) together. - Separating related reactions (hydrolysis and conjugation with nucleophiles). - **Exceptions:** - **Acetaminophen:** Can be directly conjugated without Phase I. - **Codeine:** Requires bioactivation to morphine via Phase I. ### **Phasing Out the Terms:** - **Proposal:** - Focus on the type of reaction rather than the phase. - Classify based on enzyme mechanisms and substrates. **Expert Opinion:** - **Quote by D. Josephy (2005):** - Criticizes the Phase I and II terms as inaccurate and misleading. - Suggests they are chemically incoherent and ignore enzyme understanding. **Key Takeaway:** - **Understanding Metabolism:** - Requires a mechanistic approach rather than rigid classification. - Emphasizes the complexity and variability of metabolic pathways. **Overall Key Takeaways** ------------------------- - **Metabolic Variability:** - Genetic polymorphisms and environmental factors significantly affect xenobiotic metabolism. - Personalized medicine is essential for effective and safe drug therapy. - **Importance of Detoxification Pathways:** - Phase II reactions generally detoxify compounds, but exceptions exist. - Awareness of metabolic pathways aids in predicting toxicity and drug interactions. - **Role of Electrophiles and Free Radicals:** - Reactive metabolites can cause significant cellular damage. - Antioxidant defenses are crucial in mitigating oxidative stress. - **Critical Evaluation of Metabolic Classifications:** - Recognize limitations of traditional Phase I and II classifications. - Adopt a more nuanced understanding of enzyme functions and reactions. **HLTH 340 -- Lecture \#10: Elimination of Xenobiotics** **Introduction** **Key Point:** Efficient elimination of toxic materials (xenobiotics) is critical for the survival of organisms. - **Xenobiotics:** Chemical substances that are foreign to the biological system. - As organism complexity increases, so does the complexity of xenobiotic elimination processes. - **Unicellular organisms:** Rely on passive diffusion to eliminate waste. - **Complex organisms:** Require advanced systems due to: - Increased size relative to surface area. - Compartmentalization. - Improved external membrane barriers. **Definitions** - **Excretion:** Removal of xenobiotics (and their metabolites) by excretory organs (e.g., urine, feces). - **Elimination:** The combined metabolic and excretory processes that clear xenobiotics from the body. - **Passive Excretion:** Movement of substances without energy input; effective at high plasma concentrations. - **Active Secretion:** Energy-dependent transport of substances out of the body via specific channels. - **First-Order Kinetics:** Rate of elimination depends on the plasma concentration of the xenobiotic. - **Zero-Order Kinetics:** Rate of elimination depends solely on the activity of the transport mechanism, not on plasma concentration. **Elimination Mechanisms** **Factors Influencing Elimination Rates** - **Physicochemical Properties:** - **Partition Coefficient (Kₚ):** Ratio of a substance\'s concentrations in a mixture of two immiscible phases at equilibrium. - **Dissociation Constant (pKa):** Measure of the strength of an acid in solution. - Polarity, molecular structure, shape, and weight. - **Exposure Level and Timing:** Amount and duration since exposure. - **Route of Exposure:** Inhalation, ingestion, dermal contact, etc. - **Distribution in Body Compartments:** Varies with perfusion rates and partitioning. - **Health Status:** Overall health and organ function (e.g., liver, kidneys). - **Biotransformation Rate:** Conversion of lipophilic xenobiotics to hydrophilic metabolites. - **Presence of Other Toxicants:** Interactions that may interfere with elimination. **Annotation:** Understanding these factors is crucial for predicting how a xenobiotic will behave in the body and its potential toxicity. **Main Routes of Excretion** 1. **Renal System (Urine)** - **Key Point:** Primary route for hydrophilic substances and their metabolites. - **Processes Involved:** - Filtration: Passive process in the glomeruli. - Excretion: Elimination via urine. - **Note:** Kidney damage can reduce excretion efficiency, leading to toxicity. 2. **Gastrointestinal (GI) Tract and Liver (Feces)** - **Key Point:** Major route for lipophilic substances after biotransformation. - **Processes Involved:** - **Metabolism:** Liver converts lipophilic xenobiotics to more hydrophilic forms. - **Excretion:** - **Biliary Excretion:** Liver secretes metabolites into bile, which enters the intestine. - **Enterohepatic Circulation:** Some substances may be reabsorbed, prolonging their presence in the body. - **Direct Secretion into GI Lumen:** Minor route involving transport across enterocytes. 3. **Respiratory System (Exhaled Air)** - **Key Point:** Efficient for eliminating gases and volatile substances. - **Processes Involved:** - Diffusion from blood into alveoli. - Exhalation of low molecular weight, volatile, lipophilic compounds. - **Unique Aspect:** Lipophilic substances are excreted more effectively than hydrophilic ones via the lungs. 4. **Other Routes** - **Breast Milk:** - High lipid content; acidic nature traps alkaline fat-soluble substances. - **Concern:** Lipophilic xenobiotics can pass to infants (e.g., DDT, PCBs). - **Sweat, Saliva, Tears, Semen:** - Minor routes with small amounts excreted. - Useful for biomonitoring and forensic analysis. - **Skin:** - Excretion of volatile chemicals through transdermal passage. **Study Question:** Why are lipophilic xenobiotics more effectively excreted via the lungs compared to other routes? **Chelation Therapy** **Definition:** A chemical process involving the administration of chelating agents to bind and remove heavy metals from the body. **Chelating Agents** - **Characteristics:** - Possess ligand binding atoms forming ring-like structures with metals. - **Common Agents:** - **EDTA (Ethylenediaminetetraacetic Acid):** Water-soluble, binds divalent metal cations. - **DMSA (Dimercaptosuccinic Acid):** Water-soluble, thiol-containing, binds metals and metalloids. - **BAL (Dimercaprol):** Lipid-soluble, binds metals like lead and arsenic. **Uses and Controversies** - **FDA-Approved for Lead Poisoning:** Effective in treating acute heavy metal exposures. - **Alternative Medicine Claims:** - Suggested benefits for atherosclerosis, arthritis, and other chronic conditions. - Lack of sufficient scientific evidence supporting these uses. **Benefits and Drawbacks** - **Benefits:** - Effective against acute exposures. - Form non-toxic complexes. - Remove metals from soft tissues. - Oral administration possible. - **Drawbacks:** - Redistribution of toxic metals. - Loss of essential metals. - Potential liver and kidney toxicity. - Pro-oxidant effects. - Side effects like headache and nausea. **Key Takeaway:** Chelation therapy is a valuable tool for heavy metal poisoning but must be used cautiously due to potential risks. **Elimination Kinetics** **First-Order Kinetics** - **Definition:** The rate of elimination is directly proportional to the plasma concentration of the xenobiotic. - **Characteristic:** Exhibits exponential decay; a constant fraction is eliminated per unit time. - **Graphical Representation:** Linear when plotted on a logarithmic scale. **Zero-Order Kinetics** - **Definition:** The rate of elimination is constant and independent of plasma concentration. - **Characteristic:** A constant amount is eliminated per unit time. - **Graphical Representation:** Linear decline on an arithmetic scale. **Biological Half-Life (T₁/₂):** The time required for the body burden of a substance to decrease by half. **Examples of Biological Half-Lives** **Substance** **Biological Half-Life** ------------------- -------------------------- Chloroform 1.5 hours Caffeine 2 to 10 hours Benzene 9 to 24 hours Lead in Blood 28 to 36 days Mercury 65 days PCBs 3 to 10 years Lead in Bone 10 years Cadmium in Bone 30 years Plutonium in Bone 100 years **Annotation:** Understanding half-lives aids in assessing how long a xenobiotic will persist in the body and potential long-term effects. **Ideal Characteristics of a Chelator** - High affinity for toxic metals. - Low toxicity to the body. - Ability to penetrate cell membranes. - Rapid elimination from the body. - High water solubility. - Forms non-toxic complexes. - Shares the same distribution pathway as the metal. **Combination Therapy Benefits:** - Uses multiple chelators to enhance metal removal. - Targets both intracellular and extracellular metals. - Reduces doses needed, minimizing side effects. - Decreases loss of essential trace elements. **Study Question:** What are the advantages of using combination chelation therapy over a single chelating agent? **Key Takeaways** - **Elimination of Xenobiotics:** Crucial for reducing toxicity; involves both biotransformation and excretion. - **Routes of Excretion:** Urinary, fecal, and pulmonary are primary; others are secondary but can be significant in certain contexts. - **Factors Affecting Elimination:** Include chemical properties, health status, and presence of other substances. - **Chelation Therapy:** Effective for certain heavy metal poisonings; requires careful consideration due to potential risks. - **Kinetics of Elimination:** Understanding first-order and zero-order kinetics is essential for predicting how substances are cleared from the body. **Final Annotation:** Mastery of xenobiotic elimination processes is essential for predicting toxicological outcomes and designing effective treatments for poisoning. **HLTH 340 -- Lecture \#11: Elimination via the Urinary System** **Introduction** **Key Point:** The kidneys play a critical role in eliminating xenobiotics and maintaining homeostasis in the body. **The Urinary System Components** 1. **Kidneys** 2. **Ureter** 3. **Bladder** 4. **Urethra** **Focus:** The kidneys are the primary organs involved in the excretion of xenobiotics. **The Kidney** **Main Functions** - **Excretion of Metabolic Waste:** Removes waste products like urea, mineral ions, water, and xenobiotics via urine. - **Regulation of Blood Composition:** - **Water Balance:** Controls the amount of water excreted and reabsorbed. - **Ion Content:** Regulates electrolytes such as sodium and potassium. - **pH Balance:** Adjusts the secretion and reabsorption of hydrogen ions to maintain blood pH. - **Blood Pressure:** Influences blood pressure through water regulation. - **Red Blood Cell Production:** Secretes **erythropoietin** to stimulate red blood cell production when oxygen levels are low. **Annotation:** Understanding kidney functions is essential for grasping how xenobiotics are processed and eliminated. **The Nephron** **Definition:** The functional unit of the kidney responsible for filtering blood and forming urine. - **Approximately 1 million nephrons per kidney.** **Structure** 1. **Glomerulus:** Filters blood plasma. 2. **Proximal Convoluted Tubule (PCT):** Reabsorbs nutrients; secretes certain xenobiotics. 3. **Loop of Henle:** Concentrates urine. 4. **Distal Convoluted Tubule (DCT):** Further reabsorption and secretion. 5. **Collecting Duct (CD):** Final adjustments to urine composition. **Key Point:** The nephron is essential for filtering blood, reabsorbing necessary substances, and excreting waste products. **Mechanisms of Urinary Excretion** There are **four primary mechanisms** involved in the excretion of xenobiotics via the urinary system: **1. Filtration** - **Process:** Blood plasma is filtered through the glomerulus into the renal tubule. - **Factors Influencing Filtration:** - **Molecule Size:** Small molecules pass through; proteins and blood cells do not. - **Glomerular Filtration Rate (GFR):** Dependent on blood flow and pressure. - **Note:** Approximately 99% of the filtrate is reabsorbed; 1% is excreted as urine. **Study Question:** How does molecule size affect the filtration of xenobiotics in the glomerulus? **2. Passive Diffusion** - **Process:** Movement of lipid-soluble compounds from blood to renal tubules without energy expenditure. - **Factors Influencing Diffusion:** - **Ionization State:** Non-ionized compounds diffuse more readily. - **pH of Tubular Fluid:** Affects ionization and retention of compounds. - **Reabsorption Risk:** Non-ionized compounds may be reabsorbed back into the bloodstream. **3. Active Transport** - **Process:** Energy-dependent transport of chemicals from blood to tubular lumen via carrier proteins. - **Characteristics:** - **Specificity:** Transporters are specific for certain weak acids or bases. - **Saturation:** Can become saturated, limiting excretion rate. **4. Facilitated Diffusion** - **Process:** Similar to active transport but does not require energy. - **Role:** Assists in the movement of substances down their concentration gradient. **Factors Affecting Urinary Excretion** - **Plasma Protein Binding:** - Highly bound compounds have decreased filtration but can still undergo active transport. - **pH of Urine:** - **Weak Acids:** Excretion increased with alkaline urine. - **Weak Bases:** Excretion increased with acidic urine. **Key Takeaway:** Both the chemical nature of xenobiotics and physiological conditions influence their excretion. **Failures of Homeostasis** - **Kidney Disease:** Caused by diabetes, infections, or chemical poisoning; reduces excretion efficiency. - **Infections:** Bladder and kidney infections can impede function. - **Kidney Stones:** Crystallization of minerals can block urine flow. **Annotation:** Disruptions in kidney function can lead to increased toxicity due to xenobiotic accumulation. **Glomerular Filtration** - **Process:** Passive filtration of blood plasma through glomerular capillaries. - **Characteristics:** - Large pores allow molecules up to 60 kDa. - Filters about 20% of renal blood flow. - **Note:** Damage to glomeruli affects filtration efficiency. **Tubular Secretion and Reabsorption** **Tubular Secretion** - **Active Transport Mechanisms:** - **Organic Anion Transporters (OAT):** Transport organic acids. - **Organic Cation Transporters (OCT):** Transport organic bases. - **Multidrug Resistance Proteins (MDR/MRP):** Transport various xenobiotics. **Tubular Reabsorption** - **Passive Reabsorption:** - Dependent on ionization and lipid solubility. - **High Urinary pH:** Increases excretion of acids. - **Low Urinary pH:** Increases excretion of bases. - **Active Reabsorption:** - Involves transporters like OCTs and MRPs. **Study Question:** What role do OAT and OCT transporters play in xenobiotic elimination? **Xenobiotic Efflux Pumps** - **Definition:** Proteins that mediate active transport of hydrophobic xenobiotics out of cells. - **Types:** - **P-glycoprotein (P-gp/MDR1):** Transports a wide range of xenobiotics. - **Multidrug Resistance Proteins (MRP1-7):** Overlap in substrate specificity with P-gp. - **Function:** Protect tissues by limiting xenobiotic accumulation. **Key Point:** Efflux pumps are critical in preventing xenobiotic toxicity by facilitating their excretion. **Fecal Excretion** **Overview** - **Second Major Pathway:** Eliminates xenobiotics via the gastrointestinal tract. - **Processes Involved:** - Direct elimination of non-absorbed compounds. - Delivery to the GI tract via bile. - Secretion into the intestinal lumen by enterocytes. **Biliary Elimination** **Role of the Liver** - **First-Pass Effect:** Liver processes substances from the GI tract before they reach systemic circulation. - **Biotransformation:** Converts lipophilic xenobiotics to more hydrophilic metabolites. **Factors Affecting Biliary Excretion** - **Molecular Weight:** Larger compounds are excreted more readily. - **Conjugation:** Glutathione and glucuronide conjugates are preferentially excreted. - **Transporters:** - **MRP2:** Transports organic anions. - **BCRP:** Affinity for sulfated conjugates. - **MDR1:** Transports various substrates. - **MATE1:** Transports organic cations. - **BSEP:** Secretes bile salts. **Annotation:** Understanding biliary transporters is essential for predicting xenobiotic excretion via bile. **Enterohepatic Circulation** - **Definition:** Recycling of substances between the intestine and liver. - **Process:** - Xenobiotics excreted into bile enter the intestine. - May be reabsorbed into the bloodstream or excreted in feces. - **Implications:** - Prolongs the presence of xenobiotics in the body. - Deconjugation by intestinal microbes can enhance reabsorption. **Study Question:** How does enterohepatic circulation affect the elimination half-life of xenobiotics? **Hepatotoxicity: Cholestasis** - **Definition:** Reduction or stoppage of bile flow. - **Causes:** Liver disorders, bile duct obstruction, transporter impairment. - **Symptoms:** - Jaundice (yellowing of skin and eyes). - Dark urine, light-colored stools. - Accumulation of bile acids and xenobiotics in hepatocytes. - **Consequences:** Leads to liver toxicity due to buildup of substances normally excreted in bile. **Excretion of Lipophilic Xenobiotics** **Challenges** - **Poor Excretion:** Lipophilic substances are reabsorbed in kidneys and undergo enterohepatic circulation. - **Bioaccumulation:** Leads to increased body burden and potential toxicity. **Persistent Organic Pollutants (POPs)** - **Characteristics:** - Highly lipophilic organochlorine compounds. - Persistent in the environment and biota. - Examples include DDT and PCBs. - **Elimination:** Slow due to limited conjugation and excretion pathways. **Key Takeaway:** Lipophilic xenobiotics require biotransformation and active transport mechanisms for effective elimination. **Key Takeaways** - **Kidney Function:** Critical for excreting xenobiotics through filtration, secretion, and reabsorption mechanisms. - **Transporters:** Specific proteins facilitate the movement of xenobiotics across cell membranes. - **Biliary Excretion:** Liver processes and excretes xenobiotics into bile, affecting fecal elimination. - **Enterohepatic Circulation:** Can prolong the presence of xenobiotics in the body. - **Elimination Challenges:** Lipophilic substances pose difficulties due to reabsorption and bioaccumulation. **Final Annotation:** Mastery of urinary and biliary elimination processes is crucial for understanding xenobiotic detoxification and the factors influencing toxicity.