Reduction of epoxy derivatives of benzo(a)pyrene by microsomal cytochrome P-450.

Benzo(a)pyrene 4,5-oxide was rapidly reduced to benzo(a)pyrene by rat liver microsomes in the presence of reduced nicotinamide adenine dinucleotide phosphate when incubated anaerobically. The activity of benzo(a)pyrene 4,5oxide reductase was increased about seven times by pretreat ment of rats with 3-methylcholanthrene. The addition of riboflavin to the incubation mixture resulted in the increase (up to 10-fold) in the reduced nicotinamide adenine dinucbeotide phosphate-supported reduction of benzo(a)pyrene 4,5-oxide. Moreover, the xanthine oxidase supported reduction of benzo(a)pyrene 4,5-oxide was in creased up to 20-fold by riboflavin. Both stimulations by ribo flavin were completely blocked by carbon monoxide. Benzo(a)pyrene 7,8-oxide was reduced by rat liver micro somes as rapidly as was benzo(a)pyrene 4,5-oxide, but phen anthrene 9,10-oxide and trans-stibbene oxide were reduced at slower rates. No detectable reductase activities were seen when benzo(a)pyrene 7fl,8a-diol-9a,1 Oa-epoxide, benzo(a) pyrene 7f3,8a-diol-9f1,10f1-epoxide, styrene oxide, and several other epoxide compounds were used as the substrates. The activity of microsomal benzo(a)pyrene 4,5-oxide reductase was markedly depressed when incubated under an atmosphere of air. The inhibition by oxygen of the benzo(a)pyrene 4,5-oxide reductase was partially reversed by the addition of a liver 105,000 x g supernatant fraction to the incubations containing microsomes. These results suggested that epoxide metabobites formed by microsomal cytochrome P-450 were reduced to the parent hydrocarbon at significant rates in physiological envi ronments.