Catalytic activity and stereoselectivity of purified forms of rabbit liver microsomal cytochrome P-450 in the oxygenation of the (-) and (+) enantiomers of trans-7,8-dihydroxy-7,8-dihydrobenzo[α]pyrene

Purified forms of rabbit liver microsomal cytochrome P-450 (phenobarbital-inducible P-450(LM) ₂ and β-naphthoflavone-inducible P-450(LM) ₄) were examined for catalytic activity in the conversion of the (-) and (+) enantiomers of trans-7,8-dihydroxy-7,8-dihydrobenzo-[α]pyrene to stereoisomeric, highly reactive and mutagenic 7,8-dihydroxy-9,10-oxy 7,8,9,10-tetrahydrobenzo[α]pyrenes. In the reconstituted enzyme system, P-450(LM) ₄ from both phenobarbital- and β-naphthoflavone-induced animals has much higher catalytic activity than P-450(LM) ₂ with either enantiomer of the trans-7,8-diol. Both forms of the cytochrome exhibit greater activity toward the (-) than the (+) isomer of the substrate, but this is more striking with P-450(LM) ₄. The relative amounts of diol-epoxides formed from either enantiomer of the substrate differ markedly with the form of the cytochrome used. P-450(LM) ₂ gives somewhat more diol-epoxide I than diol-epoxide II with both substrates. In contrast, P-450(LM) ₄ gives almost exclusively diol-epoxide I from the (-)trans-7,8-diol and more diol-epoxide II than diol-epoxide I from the (+)trans-7,8-diol. Thus, P-450(LM) ₄ is highly stereospecific in oxygenating at the 9,10 double bond, regardless of the absolute configuration of the hydroxyl groups at the 7 and 8 positions of the substrate.