The complete amino acid sequence of rabbit phenobarbital-induced liver microsomal cytochrome P-450

The complete amino acid sequence of the major phenobarbital-induced cytochrome P-450 (P-450LM2) from rabbit liver microsomes has been determined. The protein contains 489 amino acid residues in a single polypeptide chain and has Mr = 55,464. The sequence was compared with the amino acid sequence of P-450CAM and the nucleotide sequence of cDNA obtained from phenobarbital-induced rat liver cytochrome P-450 mRNA. These comparisons suggest that, despite functional similarities, the structural homology between microbial and microsomal cytochromes P-450 is limited to a single 8-residue region, and, in contrast, the structure of inducible microsomal cytochrome P-450 isozymes is highly conserved among mammalian species. Furthermore, we propose that the thiolate heme ligand of cytochrome P-450 is contributed by a cysteinyl residue near the COOH terminus, position 434 in the rabbit P-450LM2 sequence, based on the homology in this region with P-450CAM. The NH2 terminus of the protein from residues 1-310 is characterized by 8 hydrophobic segments 18-23 residues long, each of which is terminated by a cluster of charged amino acid residues. Residues 320-443 comprise a hydrophilic region which contains the putative heme binding cysteinyl residue as well as segments of homology with a constitutive rabbit cytochrome P-450 isozyme. The sequence data suggest that cytochrome P-450LM2 contains multiple transmembranous segments as well as a hydrophilic cytoplasmic domain. The hydrophilic domain contains regions of homology with several other cytochromes P-450, and thus appears to have an essential role in the biological function of the protein.