Purification to homogeneity and characterization of a form of cytochrome P-450 with high specificity for benzo[alpha]pyrene from beta-naphthoflavone-pretreated rat liver microsomes.

A highly purified cytochrome P-450 has been isolated from beta-naphthoflavone-pretreated rat liver microsomes with a 9% recovery. The cytochrome has an absorbance maximum in the carbon monoxide-reduced difference spectrum at 446 nm and is designated cytochrome P-446. Cytochrome P-446, as isolated, appears to have low spin ferric iron and has a specific content of 18.1 nmol/mg of protein. Cytochrome P-446 migrates as a single band of molecular weight 53,400 in sodium dodecyl sulfate polyacrylamide gels. This value is in good agreement with the results of sedimentation equilibrium determinations (55,000). The amino acid composition of cytochrome P-446 indicates that the protein contains 35% hydrophobic residues. The amino acid composition, in general, resembles that of most other cytochromes P-450. Cytochrome P-446 has a turnover number for benzo[alpha]pyrene of 53 nmol/min/nmol of cytochrome P-446. In addition, cytochrome P-446 actively hydroxylates benzphetamine, ethylmorphine, p-nitroanisole, p-nitrophenetole, and 7-ethoxycoumarin at varying rates.