Stereoselective formation and hydration of benzo[ c]phenanthrene 3,4-and 5,6-epoxide enantiomers by rat liver microsomal enzymes

Abstract

The K-region 5,6-epoxides, formed in the metabolism of benzo[ c]phenanthrene (BcPh) in the presence of an epoxide hydrolase inhibitor 3,3,3-trichloropropylene 1,2-oxide (TCPO) by liver microsomes from untreated, phenobarbital-treated, 3-methylcholanthrene-treated, and polychlorinated biphenyls (Aroclor 1254)-treated rats of the Sprague-Dawley and the Long-Evans strains, were found by chiral stationary phase high-performance liquid chromatography analyses to be enriched (58–72%) in the 5 S,6 R enantiomer. In the absence of TCPO, the metabolically formed BcPh trans-5,6-dihydrodiol was enriched (78–86%) in the 5 S,6 R enantiomer. The major enantiomer of the BcPh 3,4-epoxide metabolite was found to be enriched in the 3 S,4 R enantiomer which undergoes racemization under the experimental conditions. The major enantiomer of the 5,6-dihydrodiol metabolite was elucidated by the exciton chirality circular dichroism (CD) method to have a 5 S,6 R absolute stereochemistry. Absolute configurations of enantiomeric BcPh 5,6-epoxides were determined by CD spectral analyses of the enantiomeric methoxylation products derived from each of the two BcPh 5,6-epoxide enantiomers. Optically pure BcPh 5 S,6 R-epoxide was enzymatically hydrated exclusively at the C 6 position to form an optically pure BcPh 5 S,6 S-dihydrodiol. However, optically pure BcPh 5 R,6 S-epoxide was hydrated at both C 5 and C 6 positions to form a BcPh trans-5,6-dihydrodiol with a (5 S,6 S):(5 R,6 R) enantiomer ratio of 32:68.