Stereochemical aspects in the 4-vinylcyclohexene biotransformation with rat liver microsomes and purified P450s. Monoepoxides and diols

The stereochemical course of the biotransformation of 4-vinylcyclohexene (V CH, 1) by liver microsomes from male and female control and induced rats an d purified rat F450 2B1 and 2E1 has been determined. The epoxidation of 1, catalyzed by male microsomes, occurs on both the endo- and exocyclic double bond to give four isomeric epoxides, cis-4-vinylcyclohexene 1,2-epoxide (2 ), trans-4-vinylcyclohexene 1,2-epoxide (3), (4R*,7S*)-4-vinylcyclohexene 7 ,8-epoxide (4), and (4R*,7R*)-4-vinylcyclohexene 7,8-epoxide (5). On the ot her hand, microsomes from female rats catalyzed primarily the endocyclic ep oxidation. The stereoselectivity of this process was strongly dependent on gender and P450 induction. Only the phenobarbital and pyrazole, at lower le vels, were able to enhance the epoxidation of 1 and mostly on the endocycli c double bond. Also, P450 2E1 and 2B1 in a reconstituted system were able t o perform the epoxidation of 1 primarily on its endocyclic double bond. The metabolites, cis- and trans-4-vinylcyclohexene 1,2-epoxide (2 and 3, respe ctively) and the isomeric 4-vinylcyclohexene 7,8-epoxides (4 and 5), were r apidly biotransformed into the corresponding vicinal diols by mEH-catalyzed hydrolysis. The reaction of the endocyclic epoxides occurred with good sub strate diastereo- and enantioselectivity favoring the hydrolysis of epoxide s (1S,2R,4S)-3 and (1R,2S,4S)-2 to give, before 50% conversion, selectively (1R,2R,4S)-diol (6). At variance, the hydrolysis of the exocyclic epoxides was characterized by a high level of substrate enantioselection associated with a very low, if any, level of substrate diastereoselection, the two ep oxides, (4R,7S)-4 and (4R,7R)-5, being hydrolyzed practically with the same rate. On the basis of the major resistance to mEH hydrolysis, the endocycl ic epoxides, (1R,2S,4R)-3 and (1S,2R,4R)-2, are expected to be further oxid ized, in a stereochemical manner, to the specific mutagenic diepoxides whic h are thought to play a crucial role in VCH ovotoxicity. Thus, VCH ovotoxic ity may be markedly affected by the reactivity of the diepoxidic stereoisom ers formed and detoxicated.