The principal classes of xenobiotics of medical relevance are drugs, chemical carcinogens, naturally occurring compounds in plant foods, and various compounds that have found their way into our environment by one route or another, such as polychlorinated biphenyls (PCBs), insecticides and other pesticides. Metabolism of xenobiotics include two phases:
- Phase 1, e.g, hydroxylation. the major reaction involved is hydroxylation, catalyzed by members of a class of enzymes referred to as monooxygenases or cytochrome P450s isoforms .
The other reaction include reduction and hydrolysis, which perform by same enzymes species.
- Phase 2, conjugation (e.g. glucronic acid). the hydroxylated or other compounds produced in phase 1 are converted by specific enzymes to various polar metabolites by conjugation with glucuronic acid, sulfate, acetate, glutathione, or certain amino acids, or by methylation. The overall purpose of the two phases of metabolism of xenobiotics is to increase their water solubility (polarity) and thus excretion from the body.
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