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Questions and Answers
What term is used to describe compounds that are resistant to microbial degradation?
Which of the following is NOT a reason why some xenobiotic compounds remain recalcitrant?
Which of the following is an example of a recalcitrant compound?
What is the main difference between biodegradation and mineralization?
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Which property of xenobiotic compounds increases their bioaccumulation potential?
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What is one of the consequences of xenobiotics accumulating in the environment?
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Why are many synthetic polymers considered recalcitrant?
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Which of the following is NOT a typical use for polychlorinated biphenyls (PCBs)?
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What is the primary purpose of the metabolism of xenobiotics in the body?
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Which phase of xenobiotic metabolism involves reactions like hydroxylation?
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What is a characteristic of cytochrome P450?
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Which reaction is NOT a type of conjugation reaction involved in xenobiotic metabolism?
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What happens when certain hydrocarbons are ingested?
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What does the term recalcitrant refer to in the context of xenobiotics?
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Which of the following best defines xenobiotics?
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What role does NADPH play in the metabolism of xenobiotics?
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What primary factor influences the rate of transformation of xenobiotics?
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Which statement best describes bioavailability of xenobiotics?
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What is one key characteristic of aerobic metabolism in the biodegradation of contaminants?
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Which of the following best describes the outcome of complete biodegradation of hydrocarbons?
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How are aromatic hydrocarbon rings primarily processed during biodegradation?
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What metabolic pathway is primarily involved in the degradation of linear hydrocarbons?
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What role do microorganisms play in the biodegradation process of xenobiotics?
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What happens to alicyclic hydrocarbons during biodegradation?
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Study Notes
Xenobiotics
- Xenobiotics are foreign compounds to the body. Examples include drugs, food additives, pollutants, and artificial chemicals.
- Xenobiotics are metabolized in two phases to increase water solubility for excretion.
- Phase 1: Hydroxylation by monooxygenases or cytochrome P450.
- Phase 2: Conjugation with glucuronic acid, sulfate, acetate, glutathione, or certain amino acids (or by methylation).
- The overall goal of xenobiotic metabolism is detoxification.
- Xenobiotics may be prodrugs or procarcinogens.
- Cytochrome P450 is a hemoprotein, widely distributed across species, primarily found in the liver and small intestines.
- Most cytochrome P450 enzymes are inducible.
- Certain cytochrome P450 isoforms are involved in the metabolism of polycyclic aromatic hydrocarbons (PAHs) and are called aromatic hydrocarbon hydroxylases (AHH).
- Xenobiotic metabolism can be affected by species, genetic factors, gender, and age.
- Enzyme induction can occur by intake of phenobarbital or certain hydrocarbons.
Recalcitrant Molecules
- Recalcitrant molecules are compounds that remain in the environment for a long time and resist degradation.
- Many xenobiotic compounds are not broken down by microbes because they are:
- Not recognized as substrates by degradative enzymes.
- Highly stable and chemically inert.
- Insoluble in water and adsorbed to soil.
- Highly toxic.
- Too large to enter microbial cells.
Types of Recalcitrant Xenobiotics
- Examples include:
- Polychlorinated biphenyls (PCBs)
- Synthetic polymers
- Alkylbenzyl sulphonates
- Oil mixtures
- Pesticides
Polychlorinated Biphenyls (PCBs)
- PCBs contain two benzene rings linked covalently with halogen substitutes.
- They are used as plasticizers, insulators, and coolants.
Synthetic Polymers
- Produced as plastics, such as polyethylene, polystyrene, polyvinyl chloride, and nylon.
- Recalcitrant due to insolubility and high molecular weight.
Hazards of Xenobiotics
- Toxicity: Many xenobiotics are toxic to bacteria, eukaryotes, and humans.
- Carcinogenicity: Recalcitrant compounds accumulate in the environment, causing pollution.
- Bioaccumulation and Biomagnification: Many xenobiotics accumulate and magnify through the food chain.
Transformation of Xenobiotics
- Biodegradation: Decomposition of organic compounds into inorganic elements by living organisms.
- Mineralization: Complete decomposition of organic compounds into inorganic elements.
- Biotransformation: Change in the structure of a chemical compound, altering its properties.
Rates of Transformation
- The rate of xenobiotic transformation depends on:
- Chemical structure and concentration of the xenobiotic.
- Type and number of microorganisms capable of transforming the xenobiotic.
- Physicochemical properties of the environment.
Bioavailability of Xenobiotics
- Bioavailability depends on:
- State of the xenobiotic (solid, liquid, or gas).
- Water solubility.
- Adsorption and adherence to soil or sediment.
- Only the water-dissolved fraction is available to microorganisms.
Biodegradation of Xenobiotics
- General features:
- A wide range of metabolic pathways are involved in degrading xenobiotics.
- Biodegradation converts complex molecules into simpler ones, ideally CO2 and H2O.
Aerobic Metabolism
- Microorganisms use O2 to degrade contaminants.
Anaerobic Metabolism
- Microorganisms use other chemicals (e.g., nitrates, iron, sulfate, carbon dioxide, uranium, technicium, perchlorate) to degrade contaminants.
Biodegradation of Petroleum Compounds
- Linear molecules are degraded by β-oxidation to acetyl CoA, further metabolized through the TCA cycle.
- Alicyclic hydrocarbons are converted to carboxylic acids, which are further biodegraded via β-oxidation.
- Aromatic rings are hydroxylated by Dioxygenases to form diols.
- The rings are cleaved forming catechols.
- Catechol is metabolized via ortho ring cleavage or meta ring cleavage.
- Intermediates of the tricarboxylic acid cycle (TCA cycle) are produced.
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Description
Explore the fascinating world of xenobiotics, foreign compounds to the body that include drugs, pollutants, and food additives. This quiz covers their metabolic processes in two phases, the role of cytochrome P450, and factors affecting metabolism. Test your knowledge on how these compounds are detoxified and their potential roles as prodrugs or procarcinogens.
