Structure-activity relationships in the free-radical metabolism of xenobiotics.

Many xenobiotics, including naturally occurring compounds, drugs, and environmental agents, are metabolized both in vivo and in vitro to free-radical intermediates. The one-electron reduction of nitroaromatic compounds, quinones, and a wide variety of other chemicals is catalyzed enzymatically by a number of reductases and dehydrogenases. Structure-activity studies have shown that the cytotoxicities of nitroaromatic compounds and quinones are related to their one-electron reduction potentials (E1(7)). Other factors such as oil:water partition coefficients may also be important. Xenobiotics may also be oxidized to free radicals by peroxidases. Hammett's rules apply to the one-electron oxidation of simple meta- or para-substituted phenols and amines by horseradish peroxidase, compound I.