ABSTRACT
Biotransformation refers to the specific phase of “metabolism” during which one chemical is transformed to another by a biotic system, most often an enzyme. Living systems have several mechanisms for metabolizing and detoxifying xenobiotics. The microsomal monooxygenases are versatile enzyme systems that oxidize endogenous steroids, fatty acids, prostaglandins, and xenobiotics. Conjugating groups are usually transferred by endogenous enzyme systems, and either the conjugating agent or the xenobiotic is frequently activated to a high energy intermediate. Several different families of sulfotransferases have been identified in the soluble fraction of many tissues. Substrates that undergo conjugation by these enzymes include phenols, alcohols, steroids, arylamines, and bile salts. The epoxide rearranges to the chloroethanol which is conjugated to glutathione. Acylation in xenobiotic biotransformation involves either activation of acetate that is transferred to an amine, or direct activation of a xenobiotic to an acid so an endogenous amino acid can be conjugated to it. Besides the O-methyltransferases, N-methyltransferases and thiol S-methyltransferases are important.
