Binding and metabolism of benzo[a]pyrene and 7,12-dimethylbenz[a]anthracene by seven purified forms of cytochrome P-450

Abstract

Seven different forms of cytochrome P-450 have been purified from rat liver microsomes. The major 3-methylcholanthrene (MC) inducible cytochrome P-450 (form c) exhibits the greatest activity toward both benzo[a]pyrene (BP) (58 min-1) and 7,12-dimethylbenz[a]anthracene (DMBA) (29 min-1) and forms substantially high spin, high affinity complexes (Kd = 10 nM) with both hydrocarbons. Cytochrome P-450d, a minor MC-inducible form, has far lower activity for metabolism of both polycyclic aromatic hydrocarbons (PAH), yet also forms high affinity complexes (Kd approximately 100 nM) with both PAH, retaining the full high spin state of the free cytochrome. Although two phenobarbital (PB)-induced forms (P-450's b and e) differ by only 13 amino acids, they exhibit significant differences in metabolism of PAH and in complex formation. Whereas P-450b is only active in metabolism of DMBA (9.8 min-1 versus 1.9 min-1 for BP), P-450e has low activity for both substrates (3.3 and 1.2 min-1). Nevertheless, P-450e forms a high affinity complex (Kd approximately 100 nM) with both PAH that enhances the proportion of the high spin state (from 30% to 70%). Failure to displace n-octylamine (NOA) suggests binding that is removed from the heme. P-450b remains low spin in the presence of PAH and NOA is again not displaced. In addition, the two forms can be distinguished by their regioselectivities for both PAH. P-450's a, h, and pregnenolone-16 alpha-carbonitrile (PCN) exhibit little activity toward BP or DMBA, but P-450 PCN does form a low spin complex with BP (not DMBA). Regioselectivity in metabolism of DMBA by PB-induced microsomes does not agree with that of the major constituent forms. Only the minor, less active purified forms (e and a) mediate substantial 12-hydroxylation and 3,4-epoxidation of DMBA. Thus, additional factors in microsomal reactions must contribute to these differences.