In metabolism of xenobiotics all of the following reactions occur in phase one, EXCEPT:
Correct Answer: Conjugation
Description: Xenobiotic is a compound that is foreign to the body. The principal classes of xenobiotics of medical relevance are drugs, chemical carcinogens, and various compounds that have found their way into our environment by one route or another, such as polychlorinated biphenyls (PCBs) and ceain insecticides. In phase 1, the major reaction involved is hydroxylation, catalyzed mainly by members of a class of enzymes referred to as monooxygenases or cytochrome P450s. Hydroxylation may terminate the action of a drug, though this is not always the case. In addition to hydroxylation, these enzymes catalyze a wide range of reactions, including those involving deamination, dehalogenation, desulfurization, epoxidation, pre oxygenation, and reduction. Reactions involving hydrolysis (eg, catalyzed by esterases) and ceain other non-P450-catalyzed reactions also occur in phase 1. In phase 2, the hydroxylated or other compounds produced in phase 1 are conveed by specific enzymes to various polar metabolites by conjugation with glucuronic acid, sulfate, acetate, glutathione, or ceain amino acids, or by methylation. Conjugation reaction occurs in phase 2 and not in phase 1. Ref: Murray R.K. (2011). Chapter 53. Metabolism of Xenobiotics. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e
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