Evidence for distinct binding sites in the cumene hydroperoxide-dependent metabolism of benzo[a]pyrene catalyzed by cytochrome P-450

A few constitutive cytochrome P-450 isozymes in male rat liver microsomes catalyzed the metabolism of benzo[ a ]pyrene (BP) in cumene hydroperoxide (CHP)-dependent reactions, which produced predominantly 3-hydroxyBP and BP quinones. This process varied with the concentration of CHP. At 0.05 mM CHP, 3-hydroxyBP was the major metabolite. An increase in CHP concentration reduced 3-hydroxyBP formation but increased the level of BP quinones. This change in metabolic profile was reversed by preincubation with pyrene. Pyrene selectively inhibited quinone formation and enhanced 3-hydroxyBP formation. Naphthalene, phenanthrene and benz[ a ] anthracene nonspecifically inhibited total metabolism. BP binding to microsomal protein correlated with quinone formation, suggesting a common precursor reactive intermediate. BP metabolism by female rat liver microsomes also depended on CHP concentration but was much less effective than that in the male. With females, quinones were the major metabolites at all CHP concentrations, and their formation was again modulated by pyrene. These data indicate that two distinct binding sites are responsible for the formation of 3-hydroxyBP and BP quinones.