Chapter 47: Xenobiotic Metabolism & Detoxification

Xenobiotic Metabolism & Detoxification from Harper’s Illustrated Biochemistry presents a comprehensive overview of the metabolism of xenobiotics, defined as chemical compounds foreign to the body, including therapeutic drugs, food additives, and environmental pollutants. Understanding these pathways is critical for grasping the principles of pharmacology and toxicology, as the body’s ability to process these substances determines their therapeutic efficacy and potential toxicity. The text details the biphasic nature of xenobiotic metabolism, primarily occurring in the liver. Phase 1 metabolism focuses on functionalization, mainly through hydroxylation reactions catalyzed by the cytochrome P450 superfamily (CYP) of heme-containing monooxygenases located in the smooth endoplasmic reticulum. These reactions utilize NADPH and molecular oxygen to introduce polar groups into lipophilic substrates, rendering them more reactive. Significant attention is given to the genetics and regulation of CYP isoforms, explaining how enzyme induction leads to drug-drug interactions (such as phenobarbital inducing CYP2C9) and how genetic polymorphisms contribute to individual variations in drug response. Phase 2 metabolism involves conjugation reactions where the functionalized metabolites are combined with endogenous hydrophilic substrates—such as glucuronic acid, sulfate, or glutathione—to form highly water-soluble polar compounds that can be readily excreted in urine or bile. The chapter elaborates on specific mechanisms like glucuronidation via UDP-glucuronic acid, sulfation using adenosine 3-phosphate-5-phosphosulfate (PAPS), and the vital role of glutathione S-transferases in neutralizing electrophilic compounds. Beyond detoxification, the text explores the broader metabolic roles of glutathione, including its function as an intracellular antioxidant and its participation in amino acid transport across membranes via the gamma-glutamyl cycle. Finally, the summary addresses the deleterious effects of metabolism, describing how certain inert precursors (prodrugs or procarcinogens) can be biologically activated into toxic forms that covalently bind to DNA or proteins, induce cytotoxicity, trigger immune responses by acting as haptens, or initiate carcinogenesis, despite protective mechanisms like epoxide hydrolase.