Metabolism in vitro of para-substituted styrenes: Kinetic observations of substituent effects

Abstract

The metabolism of six para-substituted styrene derivatives ( p-XC 6 H 4CH CH 2; X = CN, Br, Cl, Ph, CH 3, CH 3O) was studied in vitro using rat liver microsomes. With p-methyl-, p-methoxy- and p-phenylstyrene it was shown that metabolic attack was completely confined to the vinyl group (as is the case with styrene itself); aromatic hydroxylation, benzylic hydroxylation, and O-demethylation could not be detected. All six styrenes gave a type 1 difference binding spectrum with microsomes in vitro, and the role of cytochrome P450 in the metabolism of p-phenylstyrene was further confirmed by its inhibition by CO and stimulation by phenobarbital pretreatment. The spectral dissociation constants ( K s ) for the styrenes showed no dependence on the nature of the substituents and were confined to a narrow range of values, each being close to the K m value observed for the metabolism of that substrate. While the latter varied slightly more than K s , there was again no regular dependence on the properties of the substituent. Similarly the V m values obtained for these substrates showed no dependence on the nature of the substituents, which is in contrast to previously observed substituent effects on aromatic hydroxylation by P450 enzymes. These results, as well as related examples from other studies, are discussed in terms of chemical selectivity and mechanistic versatility on the part of a single multi-potential oxygen atom-transfer reagent generated by P450 enzymes.