The point of view of the final metabolite c Not discussed in the text

reactions of reduction are also known. In the simplest and broadest perspective, metabolic redox reactions involve the transfer of one oxygen atom (monooxygenations), two oxygen atoms (dioxygenations), one or two electrons, or a hydride anion (Table 1).3'4 This vast topic of metabolic redox reactions receives comprehensive coverage in Chapter 5.05.

There is, however, an additional class of functionalizations, namely, the reactions of hydrolysis.4,5 Here, the degree of oxidation of the substrate remain unchanged, hence the term 'nonredox' reactions, which is sometimes found in the literature. A few authors like to consider hydrolyses as conjugation reactions, an unfortunate and confusing viewpoint given that are they fulfil none of the criteria by which conjugations are recognized (see later).

Reactions of conjugations are by far the most varied when considering the chemical entities being transferred to the substrate, or better here, coupled to the substrate. For example, the endogenous conjugating moiety (the 'endocon,' as it has been termed) can be a small and hydrophobic fragment (the methyl group), a large and hydrophobic molecule (cholesterol or a diglyceride), or a large and hydrophilic one (glucuronic acid or glutathione). Table 1 is offered here to help the reader acquire a synthetic and schematic understanding of the reactions of drug metabolism.

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