Interactions of peroxyquinols with cytochromes P450 2B1, 3A1, and 3A5: influence of the apoprotein on heterolytic versus homolytic O-O bond cleavage.

In studies of the mechanisms involved in oxygen activation by cytochromes P450 (P450s), organic hydroperoxides are frequently used to model the putative peroxyferrous complex formed immediately prior to oxygen-oxygen bond cleavage during the reaction cycle of this monooxygenase. Heterolysis of the O-O bond by ferric P450 generates the catalytically competent oxo-ferryl intermediate analogous to peroxidase Compound I, and homolysis produces the Compound II analog capable only of one-electron oxidations. As P450s have been shown to catalyze both modes of O-O bond scission, the present investigation was focused on the influence of the apoprotein on the relative contributions of these competing processes. Liver microsomes from rats treated with the P450-inducing agents phenobarbital, beta-naphthoflavone, and dexamethasone, as well as purified P450s 2B1, 3A1, and 3A5 were incubated with 2,6-di-tert-butyl-4-hydroperoxy-4-methyl-2, 5-cyclohexadienone (BHTOOH). Ratios of heterolysis to homolysis were determined by analyzing the products derived from this hydroperoxide. The data demonstrate that BHTOOH is cleaved with a ratio of approximately 1.0 with all of the liver microsomal or purified P450s investigated, except with liver microsomes from dexamethasone-treated rats or with P450s 3A1 and 3A5. In these cases, heterolysis predominated over homolysis by factors of 2.5 to 4.0. On the other hand, microsomes rich in P450 2B1 catalyzed predominantly heterolysis with analogs of BHTOOH containing smaller alkyl substituents. The data are consistent with a requirement for solvent access to the peroxyferrous complex and general acid catalysis of heterolytic O-O bond cleavage by water.