Metabolism of benzo[a]pyrene: conversion of (+/-)-trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene to highly mutagenic 7,8-diol-9,10-epoxides.

Abstract

Metabolites of (.+-.)-trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene formed by rat liver microsomes and a highly purified monoxygenase system were analyzed by high pressure liquid chromatography. Four stereoisomeric tetraols of 7,8,9,10-tetrahydrobenzo[a]pyrene, known solvolysis products of the 2 highly mutagenic stereoisomers of the 9,10-epoxide of the 7,8-dihydrodiol, were identified as products. The ratio of the 2 highly unstable diol epoxides formed (7.beta.,8.alpha.-dihydroxy-9.beta.,10.beta.-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene, diol epoxide 1; 7.beta.,8.alpha.-dihydroxy-9.alpha.,10.alpha.-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene, diol epoxide 2) ranged from about 1.7-0.4. The diol epoxides are sufficiently reactive to alkylate phosphate buffer (pH 7.4) at 37.degree.. Microsomes, particularly those from control animals, formed a substantial amount of an additional metabolite that appears to be phenolic. In analogy to benzo[a]pyrene, the metabolism of the 7,8-dihydrodiol shows similar induction after pretreatment of rats with phenobarbital or 3-methylcholanthrene. Neither diol epoxide appears to be a substrate for epoxide hydrase based on the ratios of tetraols formed in the presence or absence of epoxide hydrase. In view of the known carcinogenicity of benzo[a]pyrene-7,8-oxide and 7,8-dihydrodiol and the marked mutagenicity of the stereoisomeric diol epoxides, both of these diol epoxides qualify for consideration as ultimate carcinogen(s) of benzo[a]pyrene.