Reactions of xenobiotic metabolism are traditionally categorized into phase I and phase II reactions. Such a nomenclature is based on the fact that phase I reactions generally precede phase II, but it conveys no biochemical information whatsoever, and it neglects the very many examples of phase II reactions occurring without prior phase I reactions and even preceding them. This had led a number of authors to use the less euphonic but more informative terms of functionalization and conjugation reactions.1'2

Functionalization (phase I) reactions involve the creation of a functional group in the substrate molecule or the transformation of an existing functional group. The majority of such metabolic reactions are oxidations, but numerous

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