NADPH-supported and arachidonic acid-supported metabolism of the enantiomers of trans-7,8-dihydrobenzo[a]pyrene-7,8-diol by human liver microsomal samples.

Using a new sensitive reverse-phase HPLC assay relying on UV detection at 344 nm, the capacity of 18 human liver microsomal samples to support NADPH-dependent, cytochrome P450-mediated oxidation and arachidonic acid-dependent oxidation of the enantiomers of trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene (B[a]P-7,8-DHD) was determined. The (-)-7R,8R-enantiomer, the preferred substrate of cytochrome P450, formed 94% diolepoxide 2 (anti-isomer; 7R,8S-dihydroxy-9S,10R-epoxy-7,8,9,10-tetrahydrobenzo[a]-pyrene) measured as derived alcohols, and the (+)-7S,8S-enantiomer formed 67% diolepoxide 1 (syn-isomer; 7S,8R-dihydroxy-9S,10R-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene). Arachidonic acid-supported oxidations gave approximately 70% diolepoxide 2 from each enantiomer. The involvement of different sets of cytochrome P450 isozymes was supported by incubations in the presence of alpha-naphthoflavone (alpha-NF) (50 microM) and correlation studies. In the absence of alpha-NF, a positive correlation was found between the metabolism of the (-)-enantiomer but not the (+)-isomer of B[a]P-7,8,-DHD and the relative content of P450IA2. In the presence of alpha-NF, the P450IIIA3/4 content correlated positively with the metabolism of both the (+)-enantiomer and the (-)-enantiomer. Gestodene (100 microM) inhibited the alpha-NF-stimulated metabolism, confirming the involvement of cytochrome P450IIIA3/4. No difference was found between the extent of arachidonic acid-supported, peroxyl radical-mediated metabolism of the (+)- and (-)-enantiomers of B[a]P-7,8-DHD. The metabolism was almost completely abolished by 2 microM butylatedhydroxyanisole and 100 microM nordihydroguaiaretic acid, confirming the free radical nature of the reaction.