METABOLISM OF (-TRANS-BENZO[A]PYRENE-7,8-DIHYDRODIOL BY 3-METHYLCHOLANTHRENE-INDUCED RAT-LIVER HOMOGENATES())

Using a new sensitive reverse-phase HPLC assay with on-line radioactiv ity detector, metabolism of (+)-transbenzo[a]pyrene-7,8-dihydrodiol (B [a]P diol) to the ultimate carcinogen benzo[a]pyrene-7,8-diol-9,10-epo xide (B[a]PDE) was studied using 3-methylcholanthrene-induced rat live r homogenates. The results demonstrate that the stereoselectivity of B [a]PDE formation is a function of the concentration of the cellular co nstituents in the incubation media. At more dilute concentrations of t he homogenate, the ratio of anti- to syn-B[a]PDE was the highest and d ecreased as the homogenate protein was increased in the incubation med ium. However, there was a marked and parallel decrease of free B[a]PDE and DNA-bound radioactivity with increasing concentrations of cellula r constituents in the incubation medium. The decreased DNA-bound radio activity appears to be due to the preferential binding of B[a]PDE to g lutathione and to proteins as the homogenate concentration was increas ed in the incubation media. These results indicate that liver homogena tes, while apparently preserving the function of microsomes, present a dditional opportunities to study the interrelationship among cytochrom e P450 monooxygenase activity, water-soluble conjugates, and binding o f B[a]P diol metabolites to macromolecules in the study of benzo[a]pyr ene-induced carcinogenesis.