HLTH 340 Final Exam Study Notes PDF

HLTH 340 – Final Exam Study Notes SECTION A Principles of Toxicology Definitions (xenobiotic, toxicokinetic toxicant etc.) o Environmental toxicology: study of harmful effects of many different chemical, biological, and physical agents on living organisms in the ecosy...

HLTH 340 – Final Exam Study Notes SECTION A Principles of Toxicology Definitions (xenobiotic, toxicokinetic toxicant etc.) o Environmental toxicology: study of harmful effects of many different chemical, biological, and physical agents on living organisms in the ecosystem, including humans. o Environmental toxicology: describes chemical transport, fate, persistence and bioaccumulation of substances and their effects at the population/community level. o Toxicology: The study of poisons and its harmful effects. o Toxicant: Hazardous substances including chemicals that can pose harmful effects on living beings. o Dosage: Amount of substance taken into body, used to determine toxicological effects. Usually standardized by body weight and duration. Basics of Toxicological Analysis o Exposure: how people come into contact with xenobiotic. o Toxicokinetic: describes what happens to xenobiotic in body. o Toxicodynamic: describes what the xenobiotic does to the body. Toxicokinetic vs toxicodynamic processes o 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. o 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 o Majority of chemical of exposure to humans occur from naturally occurring compounds through diet of food plants. o Paracelsus (1493-1541) ▪ “The dose makes the posion” o Mathieu Orfila (1787-1853) ▪ “Father of toxicology” ▪ First great exponent of forensic medicine Factors to adverse effects from toxicants  o Intrinsic toxicity o Dose o Exposure conditions o Individual susceptibility Intrinsic toxicity o Characterized by key physical-chemical properties of toxicant: ▪ Molecular structure and functional groups; ▪ Stability/Reactivity; ▪ Solubility/Insolubility ▪ Chemical species/form; ▪ Volatility; o Toxicants may or may not have a threshold. o Several measures used to describe toxicity of toxicants. Toxic Effect Classification o Categorized by site of toxicity and mechanism of action. Exposure routes o Critical to determine dosage! o Routes have different rates of absorption, distribution, and metabolism. o NOTE: Exposure routes are different from exposure pathways. Exposure Responses o 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 o Haber’s Law: As exposure duration increases, the smaller amount of toxicant can cause adverse effects. What differentiates chronic toxicity from acute? o When a toxicant accumulates. o The rate of absorption exceeds rate of elimination. o Each dose of toxicant causes irreversible toxicity. o There is an insufficient time for recovery between doses. Internal/external dose o 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.  o Internal Dose ▪ The amount of toxicant available to cause harm. ▪ Measured via human biomonitoring Exposure Conditions Factors o Magnitude o Route (e.g., ingestion, inhalation, dermal) o Duration (e.g., acute, sub-chornic, chronic) o Frequency (e.g., # of times exposed, time between exposure) Susceptibility o Toxicants effect everyone differently. o Susceptibility and sensitivity (NOT Vulnerability) are treated as synonyms. o Factors: ▪ Sex ▪ Age and life-stage ▪ Nutrition and medical history ▪ environmental exposures ▪ Genetic background/ethnicity ▪ Species differences Thalidomide o Used as sedative and anti-nauseant during pregnancy in 1956 o Human teratogen causing missing/malformed limbs o These negative effects led to more stricter drug regulations and control over drug use and development. Risk Assessment Definitions o 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. o COPC: Contaminant of Potential Concern Assessment framework o Hazard Identification: Determines what health problems are caused by exposure to the hazard. o Exposure Assessment: Evaluates how individuals or populations come into contact with a hazard (e.g., through air, water, soil, food). o Hazard Characterization: Describes the nature and magnitude of adverse effects that could be caused by the hazard. o Risk Assessment: Combines information from hazard identification and exposure assessment to estimate the likelihood of health risks. o Risk Management: Identifies and implements measures to control the risk. o Risk Communication: Involves communicating risk findings and mitigation strategies to stakeholders and the public. What determines potential risk?  o Chemical concentration o The route of exposure o Inherent toxicity What components must be present for an adverse risk to exist? o Receptor o Exposure o Hazard o NOTE: If 1 of the components is missing, there can be no risk. Risk Assessment Paradigm o 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...) o Exposure Assessment: Determine nature and magnitude of exposure to individuals/populations, if any, based on receptor, site characteristics, and contaminants. o Hazard Assessment: Evaluates toxicological properties of hazard, like dose- response relationships and exposure limits to understand how the hazard can affect health. o Risk Characterization: Using data from exposure/hazard assessment to estimate potential health risk. o Risk Management: Identify and implement actions to reduce/eliminate health risk. o Scientific Communication: Informing the public/stakeholders about risks and mitigations measures taken against hazard. o Collection and Validation of Data: Data collected and validated throughout the process to ensure accuracy/reliability of risk assessment. Hazard vs. Risk o 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 o Classifications: ▪ Completed: Health risk since public can be exposed to COPC. ▪ Potentially: Potential health risk. ▪ Eliminated: No health risk. o Refers to the source to receptor pathway of a potential contaminant. Exposure pathways vs. exposure routes o 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) o “The dose makes the posion” ▪ Generally, the higher the dose, the more severe the response. o Relationship is synthesized from observation studies.  o Help describe correlation between exposure and induced effects. o 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. o Uncertainty factors ▪ UF1 ~ Inter-species differences ▪ UF2 ~ Inter-individual differences Receptor Characterization o A lot of factors must can cause significant variations in receptors like: ▪ Lifestyle and habits ▪ Physical characteristics ▪ Sensitive receptor groups ▪ Etc.… o 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. o Bodyweight ▪ Dose per kilogram can have huge amount of varying effects of toxicants in population, making it important to consider. Contaminant Characterization o Concentration in environment o Speciation (metals) o Congeners (organics) o Bioavailability Bioavailability Adjustments o Most relevant in oral/dermal exposures o Expressed as fraction: ~1-100% / ~0.01-1.00 o Data provided through silico models and vitro models. Risk assessment and exposure calculations and associated values (HQ, ILCR, ADI/TDI etc) o Exposure Limits: 1. Identify critical effect(s) 2. Calculate threshold for critical effects(s). E.g., NOAEL 3. Apply Uncertainty factors. 𝑁𝑂𝐴𝐸𝐿 ▪ Equation: 𝑒𝑥𝑝𝑜𝑠𝑢𝑟𝑒 𝑙𝑖𝑚𝑖𝑡 = 𝑈𝐹1 ∗𝑈𝐹2 ▪ 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 o LD50: ▪ Refers to a dose that results in a 50% mortality rate. ▪ The lower value the more deadlier the substance!  o Contaminated Soil: ▪ Exposure (µg/kg/day) = IRsoil * Csoil * AFGIT * (ED/365) / BW IRsoil = Amount of soil ingested per day (g/day) Csoil = Chemical concentration is soil (ug/g) AFGIT = Oral bioavailability of contaminant (unitless) ED = Exposure Duration (Number of days per/year) BW = Receptor body weight (kg) o Hazard Quotient (HQ) 𝐸𝑥𝑡𝑒𝑟𝑛𝑎𝑙 𝐷𝑜𝑠𝑒 ▪ 𝐻𝑄 = 𝑇𝑜𝑙𝑒𝑟𝑎𝑏𝑙𝑒 𝐷𝑎𝑖𝑙𝑦 𝐼𝑛𝑡𝑎𝑘𝑒 Tolerable Daily intake (TDI) can be replaced to any exposure limit: o Reference Dose (RfD) o Acceptable Daily Intake (ADI) o Upper Limit (UL) Risk defined as HQ: o > 1.0 (If included all exposure pathways) o > 0.2 (If included only 1 exposure pathway) o Incremental Lifetime Cancer Risk (ILCR) 𝜇𝑔 𝜇𝑔 −1 ▪ 𝐼𝐿𝐶𝑅 = 𝑒𝑥𝑝𝑜𝑠𝑢𝑟𝑒 ( ) ∗ 𝐶𝑎𝑛𝑐𝑒𝑟 𝑆𝑙𝑜𝑝𝑒 𝐹𝑎𝑐𝑡𝑜𝑟 ( ) 𝑘𝑔∗𝑑 𝑘𝑔∗𝑑 The higher the CSF, the more potent the carcinogen ILCR compared to benchmark to determine risk vs. negligible risk Risk if ILCR: o > 1 x 10-6 o > 1 x 10-5 ▪ Values determined by US FDA to be negligible risk. Risk Management & Communication: o Key Elements: Awareness of potential problems, public engagement, development of scientific knowledge, political will, and societal values. o 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. o 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: o Data Gaps: Lack of comprehensive toxicity data for many chemicals. o Extrapolation Issues: Using animal studies to predict human risks can be unreliable due to differences between species. o Real-World Exposure: Humans are exposed to complex mixtures of pollutants, not single chemicals, complicating risk assessment.  Exposure Pathways and Routes: o Pathways: Source to receptor via environmental media (air, water, soil, etc.). Completed pathways indicate potential health risk. o Routes: How the contaminant enters the body (inhalation, ingestion, dermal contact). Bioavailability vs. Bioaccessibility: o Bioavailability: Contaminant is absorbed into the body and can cause harm. o Bioaccessibility: Contaminant is available for uptake but hasn't crossed a biological barrier yet. o Focus on bio accessible contaminants for risk assessment, as they indicate potential exposure Hazard and Risk: o Hazard: Potential to cause harm (e.g., toxic chemical). o Risk: Likelihood of exposure leading to harm. A substance may be hazardous but not pose significant risk without exposure Risk-Specific Dose (RsD): o 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 1. Determine if people share a core human microbiome 2. Understand if changes in microbiome relates with changes in health 3. Develop tech and bioinformatic tool to support these goals 4. Address ethical, legal, and social implications raised by HMP Microbiome o We are widely colonized by microbiomes. o Partly transferred from mother. o Several transitions in first year of life and remains constant till 65. o Influenced by genetics, environment, diet, and other factors. o They can cause catastrophic damage if they move where they don’t belong o 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 o 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 o Metabolic Functions: Microbiomes play a crucial role in digesting food that the body can't process alone, producing essential nutrients like vitamins. o Immune System Regulation: Microbiomes help regulate both innate and acquired immunity, protecting against infections and reducing chronic inflammation. o 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). o Drug Metabolism: Microbiomes metabolize drugs and other compounds, affecting how medications work in the body and influencing responses to toxicants. Sites of absorption o The GI tract o Lungs o Skin Categories of drug administration o Enteral ▪ Through alimentary canal (i.e., sublingual, oral, and rectal). o Parenteral ▪ Through all other routes (i.e., intravenous, intraperitoneal, intramuscular, subcutaneous, etc.). Cell Membranes o Membrane lipid bilayer is formed through hydrophobic interactions. o 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 o Fick’s Law: Chemicals will move from high concentration to low concentration. o Paracellular diffusion: small hydrophilic molecules (~500-600 Da) crosses through aqueous pores in membrane. o Transcellular diffusion: large hydrophobic molecules diffuse across lipid domains of membrane. ▪ Small lipophiles permeate via passive diffusion. ▪ Hydrophiles cannot permeate. o Paracellular is quicker.  Transcellular passive diffusion o 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 o Absorption rate determined by: ▪ Surface area of barrier ▪ Concentration gradient ▪ Permeability of the substance through the membrane Physiochemical properties that affect absorption o Molecular weight: Small molecules with < 500 Daltons pass easier. o Hydrophobicity (lipophilicity) molecules pass easier o Ionization: Molecules with +/- charge pass worse. o Polarity (H-bonding): molecule with uneven charge pass worse. Cell junction functions o Mortar between cells o Permeability seal – selective barrier to diffusion o Cell-to-cell communication Kow (partition coefficient) o Relative solubility in lipid (lipophilicity) vs. water (hydrophilicity) o Kow = Concentration(octanol) ÷ Concentration(water) o o Kow express in log10 units o Kow > 10 (or log Kow > 1): Lipophilic o Kow = ~1-9 (or log Kow = ~0-1): Amphiphilic o Kow = ~0-1 (or log Kow < 0): Hydrophilic Acidity o Non-ionized species of the acid/bases are preferentially absorbed transcellularly. All modes of transport across membranes o Passive Diffusion – Lipophiles o Facilitated Diffusion – Hydrophiles o Active Transport – Hydrophiles Lipinski’s “Rule of Five” o 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 o Large glycoprotein molecules ▪ Help specific hydrophilic solutes to cross membrane barriers Active transport characterizations o Movement of chemicals against a gradient o Saturation at high chemical concentration o Selectivity for certain structural features o Competitive inhibition of similar structured chemical o Requirement of energy to facilitate transport Ion transporter specificity: o Effective ionic radius o Positive or negative charge o Class of metal ion Ion transporter factors: o Affinity for tranposrt protein o Saturation o Competition o regulation Relevant transport proteins o NA+/K+ pump ▪ Uses a ATP to pump 3 Na+ molecules out of the cell and 2 K+ molecules into the cell. o 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. o 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 o Endocytosis ▪ Active transport where cell engulfes molecule(s) using energy For large polar molecules Different varities: o Phagocytosis: Engulfing of solid particles (e.g., large molecules). o Pinocytosis: Engulfing of extracellular fluids. o Receptor-mediated endocytosis: Absorbs specific molecules (e.g., metabolites, hormones) via receptor- specific vesicles.  Example: manganese (Mn2+) o Enters brain actively via endocytosis. o Exocytosis ▪ Expels substances from cell Brunekreef 2008 o 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. o 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. o 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. o 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. o Example of Hazard Identification: Studies linking PVC flooring to increased asthma risk in children, showcasing the hazards of chemical exposures from household materials. o 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. o 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 o A chemical/metabolite/product that is measured in the human body. o Measurable indicator of some biological state or condition. o 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) o Directly measure exposure of toxicants in people by measuring biomarker in human specimens (e.g. urine or blood). o Provides internal dose of chemical via all routes of exposure. o 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. o 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 o 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 o Biomarkers of Exposure ▪ How much of a biomarker is found in a compartment of an organism. o Biomarkers of Effect ▪ Magnitude of biochemical, physiologic, behavioral, or other alterations in organism which signify association of possible health impairment or disease. o 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 o Concentration and properties of toxicant vary with route of exposure. o One route may absorb a high percent while another route absorbs a low amount of toxicant. o A toxicant may be non-toxic from one route, but highly toxic via another. o Highlights the importance of route of exposure. Absorption by GI tract o Many environmental toxicants precent within food. o Incidental ingestion is the most common route of exposure. o Absorption occurs across entire GI tract.  o Significant absorption occursf when toxicant is in gut lumen as molecular solution. o 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). o Toxicants can compete with actual required nutrients. o First-pass effect: ▪ Toxicants are bio transformed by GI tract or liver before entering blood stream. o 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 o 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. o 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 o BLL (Blood Lead Level): current research correlates to internal dose rather than external o 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. o 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 o BLL increase of 1 μg/dL correlates to approx. IQ point deficit. o 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 o Adult lead poisoning symptoms: ▪ Reduced CNS sensitivity  ▪ Chronic anemia, hypertension, kidney problems o 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 o Lungs poor barrier to xenobiotics from entering bloodstream. ▪ Very large surface area with thin membrane highly perfused with blood. o Epithelium in lungs limited, only allowing slow absorption of highly water-soluble compounds. o Compounds that pose risks to lung absorptions are: ▪ Gases and vapours ▪ Aerosols and particles o 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. o Molecules cross from alveolar space to blood through concentration balancing on both sides. ▪ Controlled by blood-to-gas partition coefficient (unique to each gas) o Particulates: particles, dust, must or fume that have been suspended in air. o Protection against particulates: ▪ Airway geometry ▪ Humidity ▪ Mucociliary clearance ▪ Alveolar macrophages o 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 o 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 o Largest organ and provides good barrier to surrounding environment (relatively impermeable). o Absorption through skin depends on: ▪ Concentration ▪ Duration of contact ▪ Solubility ▪ Physical condition of skin ▪ Part of the body exposed o 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. o Chemicals are absorbed mostly through epidermis while some can also enter through sweat glands or hair follicles. o Chemicals pass seven layers of epidermis before reaching dermis where they can enter bloodstream or lymph. o Stratum corneum is the outermost and thickest layer making it the primary barrier to absorption of xenobiotics. SECTION C Additional ADME terms o Disposition = distribution + metabolism o Clearance = metabolism + excretion Factors affecting distribution o 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 o 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 o Portal circulation ▪ Portal vein carries blood from Gi track to liver o Systemic circulation ▪ Pulmonary vein carries blood from lung to rest of body via heart. o Lymphatic circulation ▪ Alimentary duct carries lymph from GI track to the system circulation First-pass effect o Xenobiotics from gut enter liver via hepatic portal vein. o Xenobiotics from systemic circulation enter liver via hepatic artery. o Liver filters lipophiles (fraction) in hepatocytes. o Remaining lipophiles enter systemic blood via hepatic vein. o Hepatocytes secrete lipophiles to bile. o Biliary excretion carries lipophiles to gut. Blood Fractions o Whole blood ▪ Plasma + erythrocytes o Plasma ▪ Aqueous liquid carrying RBC with dissolved proteins, clotting factors, trace elements. o Serum ▪ Plasma – fibrinogen + trace elements + proteins (not involved in clotting) Blood partitioning o Free plasma phase – xenobiotic molecules dissolved as free solute in water (Hydrophiles). o Protein bound phase – xenobiotics reversibly bound to large plasma proteins (lipophiles). o 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 o Erythrocytes – selectively bind to certain metal ions (e.g., Fe, Zn, Pb) ALAD o ALAD1: ▪ Most common form ▪ More Pb in their plasma and bone ▪ Increased ALA accumulation ▪ Increased chelation efficacy ▪ More sensitive to CNS o 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 o Toxicants often produce effects at target tissue. o Critical effect is the effect that occurs with lowest administered or observed dose. o Bioavailability is internal dose that can pose harm to target tissue. o Oral bioavailability: % of toxicant absorbed from GI tract. o Effective Blood Concentration: fraction of toxicant freely dissolved in blood plasma. o Tissue/blood partition: degree of toxicant permeation from blood to specific tissue. Perfusion-limited and partition-limited tissue distribution o 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.) o Partition-limited tissue distribution: xenobiotic partition between high- and low- fat tissue. ▪ Mainly determined by Kow o Carrier-mediated transport: ▪ Ionic/polar xenobiotics can use selective membrane channels or pumps. o 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) o stands for Physiologically Based Pharmacokinetic Model. o Simulates toxicokinetics of xenobiotic. o Models represent the body with compartments such as lungs, liver, fat, blood, and other tissues. o Different parameters, such as blood flow rates and concentrations are used to calculate how substances move between compartments. o These models help in predicting toxicity, pharmacokinetics, and dosage of xenobiotics (foreign substances). o Important for risk assessment and regulatory decision-making in toxicology and pharmacology. Internal membrane barriers o A additional protective layer for vulnerable tissue to restrict uptake of some xenobiotics from the blood to tissue. o Anatomical barriers:  ▪ Endothelial cells of blood capillaries have tight junctions to prevent paracellular uptake. o 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. o 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 o Brain restricted by 2 barriers: ▪ Blood-brain barrier (BBB) ▪ Blood-cerebral spinal fluid barrier (BCSFB) o Non-bullet-proof barriers, but keep most toxicants from entering brain. o BBB formed by endothelial cells of blood capillaries in brain. o Endothelial cells form tight junctions with adjacent cells by: ▪ Tight seal between cells ▪ Prevent diffusion of polar compounds paracellularly. o Diffusion of lipophilic compounds prevented by efflux transporters. o Glial cells provide another layer of protection. o BBB not fully developed at birth, so chemicals are more toxic for newborns. ▪ Junctions are immature “leaky” Placenta o Another strictly regulated internal membrane only allowing exchange of vital molecules. o Most vital nutrients transported by active transport. o Most xenobiotics enter through simple diffusion o To prevent xenobiotics to reach fetus, many layers. o BCRP (Breast Cancer Resistance Protein) 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 o Back-street chemist made MPTP accidentally instead of MPP (synthetic heroin), which when ingested show symptoms of Parkinson’s disease. o Caused strong degeneration of dopaminergic neurons containing neuromelanin. o Highly lipophilic so crossed BBB quickly. o Damaged/destroyed dopamine producing neurons in substantia nigra. o MPP+ inhibits mitochondrial complex 1 ▪ Cell death  ▪ Accumulation of free radicals. CYP2D6 o A gene when expressed can metabolize centrally acting drugs, neurotoxins, neurochemicals. o Reduce/eliminate severity of drugs. o Increased expression for alcohol consumers and smokers. MPP+, Paraquat, Rotenone, and Lead o MPP+: Bioactivated metabolite of MPTP. Neurotoxic via Complex I inhibition, leading to ROS and Parkinsonism. Crosses BBB via passive diffusion. o Paraquat: A toxic herbicide that accumulates in the lungs, causing lung fibrosis and pulmonary edema. Acts as a pro-oxidant. o Rotenone: A botanical pesticide that is neurotoxic by inhibiting Complex I. Associated with Parkinsonism. Passes the BBB by passive diffusion. o Lead: Alters blood-bone partitioning due to ALAD polymorphism, leading to neurotoxicity. o BBB Uptake: Lead, MPP+ (as MPTP), and Rotenone cross the BBB, while Paraquat’s mechanism is unclear. o Consequences: ▪ Lead: IQ decrease. ▪ MPP+: Chemical Parkinsonism due to brain accumulation. ▪ Paraquat: Pulmonary edema and fibrosis. ▪ Rotenone: Chemical Parkinsonism Tissue Sequestration o Tissue sequestration occurs when a toxicant is stored in a specific tissue for an extended period. o The sequestered tissue may not be harmed directly by the toxicant. o Sequestration reduces toxicant concentrations in the blood and target tissues, potentially delaying toxic effects. o Beneficial by protecting against acute toxicity or overdose, allowing more time for excretion. o Harmful, by prolonging toxicant presence in body, so gradual accumulation. o Common storage tissues include fat, bone, liver, and kidneys. o For example, lead is stored in bones, while paraquat accumulates in the lungs. Toxicant storage o Binding to plasma proteins ▪ Albumin – most abundant and can bind a large number of compounds. ▪ α1-acid glycoprotein ▪ transferrin, ceruloplasmin ▪ α- and β-lipoproteins o 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. o 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. o Bone ▪Fluoride, lead, and strontium may incorporate in bone matrix. ▪90% lead in body eventually in skeleton ▪Mechanism by substituting bone components. ▪ E.g., Pb2+ and Sr2+ may substitute for Ca2+ in the hydroxyapatite lattice matrix. ▪ Not permanent, and can be released by osteoclastic activity Molecular Sequestration o Transferrin - blood protein that bind and transports ferric iron (Fe3+) and similar transition metals (e.g., Manganese) o Ceruloplasmin - blood protein that binds and transports copper (Cu2+) and ferrous iron (Fe2+) o Ferritin - tissue protein (esp. liver) that binds ferrous iron (Fe 2+) and sequesters it as ferric iron (Fe3+) o Deficiencies in these carriers can cause oxidative stress, and long term tissue damage in organs. Organochlorines o Organochlorine compounds (OC) are highly lipophilic (fat-loving) and commonly found in animal-derived foods. o OCs are absorbed through the gut by passive diffusion and stored in adipose tissue (fat). o 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. o 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  o Lead (Pb²⁺) enters the body through calcium transporters (TRPV6 and DMT1) in the gut. o Brain is the primary target for lead and crosses the blood-brain barrier (BBB) via calcium transporters. o 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. o 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. o 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: o Natural: Forest fires, volcanic eruptions. o Anthropogenic: Coal combustion, metal smelting, industrial processes (e.g., chlor-alkali plants). Transport: o Elemental mercury (Hg⁰) is stable and can travel long distances in the atmosphere. o Deposited mercury in soil and water may be converted into more toxic organic forms. o Forests are sponges to mercury Speciation of Mercury  Key Forms: o Elemental Mercury (Hg⁰): Easily vaporized, long-range atmospheric transport. o Inorganic Mercury (Hg²⁺): Can be transformed by microorganisms into methylmercury. o Methylmercury (MeHg): Highly toxic, bioaccumulates and biomagnifies in aquatic food webs. Other Organic Mercury Compounds: o Methylmercury (MeHg): Relatively hydrophilic, readily crosses biological membranes due to molecular mimicry. o Ethylmercury (EtHg): Also hydrophilic. o Dimethylmercury: Very lipophilic, extremely toxic. o 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: o Aquatic bacteria convert inorganic mercury (Hg²⁺) into methylmercury (MeHg). Bioaccumulation & Biomagnification: o Small aquatic organisms absorb MeHg. o Predatory fish (e.g., tuna, swordfish) accumulate higher levels. o 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: o Neurological damage (e.g., cognitive deficits, motor impairment). o Immune o reproductive system effects. At-Risk Populations: o Indigenous communities relying on fish as staple food sources. o Populations in regions with high mercury deposition. o Sensitive sub-populations: women of childbearing age, pregnant individuals, young children. Cardiovascular Trends from Hg: o S-PUFA is negatively associated with myocardial infarction. o Hair-HG is positively associated with myocardial infraction. Mercury Guidelines & Advisories Tolerable Daily Intakes (TDIs): o Total Hg (General Population): ~0.71 µg/kg/day (BCS 2007). o Methylmercury (General Population): ~0.47 µg/kg/day (BCS 2007) and ~0.23 µg/kg/day (WHO 2003). o Methylmercury (Sensitive Populations): ~0.20 µg/kg/day (BCS 2007). Canadian Fish Consumption Guidelines: o Retail Fish Limit: 0.5 ppm total mercury (except shark, swordfish, fresh/frozen tuna). o Advisories for Exempted Fish: ▪ General adults: ~1 meal/week. ▪ Women of childbearing age & young children: ~1 meal/month. o A non-enforceable guideline (0.2 ppm) often used in risk assessments for frequent consumers. Sport Fishing Advice in Ontario o Size and number of fish you can safely eat. o How to choose fish with lowest levels of contaminants. o Fish to not eat. o How to prepare fish to reduce contaminants. o Contaminants in different Ontario fish. o 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: o 900 deaths o 2200 affected with severe neurological symptoms (ataxia, sensory disturbances, speech impairment), with many fatalities. o Congenital Minamata disease: Neurological damage in infants exposed in utero. Recognition & Response: o Identified as organic mercury poisoning in the late 1950s. o Widespread contamination led to government advisories and eventual global awareness. International Policy: o 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: o Contaminants, including methylmercury, can partition into breast milk. o Breast milk is a critical exposure route for nursing infants. BCRP (Breast Cancer Resistance Protein): o An efflux transporter that can influence the secretion of certain xenobiotics (including potentially mercury compounds) into breast milk. o Understanding BCRP function is crucial for predicting infant exposure and assessing risks associated with breastfeeding. Global Perspective & Regulation Global Problem: o Majority of mercury deposited in some countries (e.g., Canada) originates from foreign sources (notably industrial regions such as China). Minamata Convention on Mercury: o Established in 2013, entered into force in 2017. o Mandates reducing mercury use and emissions. o Bans new mercury mines and phases out certain mercury-containing products by 2020. Molecular Sequestration: Selenium (Se) and Mercury (Hg) Role of Selenium: o Selenium (Se) has an extremely high affinity for mercury, surpassing sulfur by a factor of one million. o Selenoproteins can bind and sequester both inorganic and organic mercury. o Thought to serve as a “sink,” potentially protective in target tissues (e.g., brain). o 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: o High-dose methylmercury (MeHg) exposure is clearly neurotoxic in children and adults. o Low-dose health implications are less certain due to conflicting epidemiological findings. Island Comparisons: o 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. o 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: o 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). o Calculate SeHBV: SeHBV=[Se][Hg]−[Hg][Se]SeHBV=[Hg][Se]−[Se][Hg] or alternate updated forms provided by Ralston. Interpretation: o SeHBV ≥ 0: Selenium is in surplus relative to mercury (net benefit).  o 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 Primarily fish (MeHg), some via amalgam; Many sources, including dust Sources Air is transport only and soil Nutrient Selenium can reduce MeHg toxicity at Ca, Fe, Zn may reduce Pb Interactions targets absorption Pb mainly in bone, some in Storage Sites Hg mainly in muscle, some in hair teeth Biomarkers Hg in hair, blood, nails Pb in blood, bone, teeth Fetal blood Pb = maternal Fetal Exposure Fetal blood Hg > maternal blood Hg blood Pb Pb accumulates in bone, long- Retention Hg has relatively low retention term storage Body burden > current intake Maternal Status Current intake > body stores dominates important Breast milk Pb