Reduction of Epoxy Derivatives of Benzo(a)pyrene by Microsomal Cytochrome P-450¹

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 4,5-oxide reductase was increased about seven times by pretreatment 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 dinucleotide phosphate-supported reduction of benzo(a)pyrene 4,5-oxide. Moreover, the xanthine oxidase-supported reduction of benzo(a)pyrene 4,5-oxide was increased up to 20-fold by riboflavin. Both stimulations by riboflavin were completely blocked by carbon monoxide.

Benzo(a)pyrene 7,8-oxide was reduced by rat liver microsomes as rapidly as was benzo(a)pyrene 4,5-oxide, but phenanthrene 9,10-oxide and trans-stilbene oxide were reduced at slower rates. No detectable reductase activities were seen when benzo(a)pyrene 7β,8α-diol-9α,10α-epoxide, benzo(a)pyrene 7β,8α-diol-9β,10β-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 × g supernatant fraction to the incubations containing microsomes. These results suggested that epoxide metabolites formed by microsomal cytochrome P-450 were reduced to the parent hydrocarbon at significant rates in physiological environments.