Amino acid compositions and partial amino acid sequences of three highly purified forms of liver microsomal cytochrome P-450 from rats treated with polychlorinated biphenyls, phenobarbital, or 3-methylcholanthrene.

Three highly purified forms of rat liver microsomal cytochrome P-450 (P-450,, P-45Ob, P-450,) are shown to be distinct proteins based on comparisons of their amino acid compositions, automated sequence analyses of the first 19 amino acids, and carboxypeptidase analyses of the COOH-terminal sequences. These three forms of cytochrome P-450 were purified from liver microsomes of rats pretreated with a mixture of polychlorinated biphenyls (Aroclor 1254); P-450, and P-45Ob were also purified from phenobarbital-pretreated rats and P-450, and P-450, were purified from 3-methylcholanthrene-pretreated rats. All three hemoproteins have a similar content of hydrophobic amino acid residues (44%), but the three forms differ significantly in cysteine, tryptophan, histidine, arginine, and methionine content. The NH,and COOH-terminal residues for each form are: P-450., both methionine; P-450a, glutamic acid and serine; P-450,, isoleucine and leucine. The NHz-terminal sequences of the first 19 amino acids of each form of cytochrome P-460 showed very little homology; cytochromes P-450. and P-450, differed in 17 amino acids, P-450, and P-450a in 16 amino acids, and P-45Ob and P-450, in 14 amino acids. The COOHterminal sequences of the last seven amino acids revealed no homology among cytochromes P-450,, P-450,, and P-460,. Each form of cytochrome P-450 purified from rats pretreated with different inducers is identical in amino acid composition and NH2and COOH-terminal sequences. The different amino acid compositions and partial amino acid sequences, with our previously reported data on the lack of immunological relatedness of the hemoproteins, demonstrate that the three forms of rat liver cytochrome P-450 have different primary and tertiary structures. These data indicate that cytochromes P-450,, P-46Oa, and P-450, are separate gene products and not post-translational modifications of one primary gene product.