Metabolism of benzo[a]pyrene VI. Stereoselective metabolism of benzo[a]pyrene and benzo[a]pyrene 7,8-dihydrodiol to diol epoxides

Abstract

(±)-7β,8α-Dihydroxy-9β,10β-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (diol epoxide- 1) and (±)-7β,8α-dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (diol epoxide- 2) are highly mutagenic diol epoxide diastereomers that are formed during metabolism of the carcinogen (±)- trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene. Remarkable stereoselectivity has been observed on metabolism of the optically pure (+)- and (−)-enantiomers of the dihydrodiol which are obtained by separation of the diastereomeric diesters with (−)-α-methoxy-α-trifluoromethylphenylacetic acid. The high stereoselectivity in the formation of diol epoxide- 1 relative to diol epoxide- 2 was observed with liver microsomes from 3-methylcholanthrene-treated rats and with a purified cytochrome P-448-containing monoxygenase system where the (−)-enantiomer produced a diol epoxide- 2 to diol epoxide- 1 ratio of 6 : 1 and the (+)-enantiomer produced a ratio of 1 : 22. Microsomes from control and phenobarbital-treated rats were less stereospecific in the metabolism of enantiomers of BP 7,8-dihydrodiol. The ratio of diol epoxide- 2 to diol epoxide- 1 formed from the (−)- and (+)-enantiomers with microsomes from control rats was 2 : 1 and 1 : 6, respectively. Both enantiomers of BP 7,8-dihydrodiol were also metabolized to a phenolic derivative, tentatively identified as 6,7,8-trihydroxy-7,8-dihydrobenzo[a]pyrene, which accounted for ∼30% of the total metabolites formed by microsomes from control and phenobarbital-pretreated rats whereas this metabolite represents ∼5% of the total metabolites with microsomes from 3-methylcholanthrene-treated rats. With benzo[a]pyrene as substrate, liver microsomes produced the 4,5-, 7,8- and 9,10-dihydrodiol with high optical purity (>85%), and diol epoxides were also formed. Most of the optical activity in the BP 7,8-dihydrodiol was due to metabolism by the monoxygenase system rather than by epoxide hydrase, since hydration of (±)-benzo[a]pyrene 7,8-oxide by liver microsomes produced dihydrodiol which was only 8% optically pure. Thus, the stereospecificity of both the monoxygenase system and, to a lesser extent, epoxide hydrase plays important roles in the metabolic activation of benzo[a]pyrene to carcinogens and mutagens.