The covalent structure of rabbit phenobarbital-induced cytochrome P-450. Partial amino acid sequence and order of cyanogen bromide peptides.

The phenobarbital-induced isozyme of cytochrome P-450 was isolated from rabbit liver microsomes and subjected to chemical and proteolytic degradation. The resulting peptides were isolated exclusively by reverse phase high pressure liquid chromatography. Automated sequence analysis was employed to determine the linear arrangement of 385 residues and the order of all CNBr peptides. The NH2 terminus of the cytochrome P-450 consists of five hydrophobic segments 18 to 23 residues long, separated by polar regions of 8 to 15 residues. The COOH terminus contains a hydrophilic region 120 residues long as well as several hydrophobic segments. A tridecapeptide, previously found to be analogous to a tryptic peptide from a constitutive P-450 isozyme (Ozols, J., Heinemann, F. S., and Johnson, E. F. (1981) J. Biol. Chem. 256, 11405-11408) was positioned within the hydrophilic region of the COOH terminus. The amino acid sequence is interpreted in terms of a model for the topology of the cytochrome in the membrane which can be used as a basis for further experiments. Comparison of this sequence with the cytochrome P-450CAM sequence (Haniu, M., Armes, L. G., Tanaka, M., Yasunobo, K. T., Shastry, B. S., Wagner, G. C., and Gunsalus, I. C. (1982) Biochem. Biophys. Res. Commun. 105, 889-894) revealed a single homologous region which contains a cysteinyl residue. The amino acid sequence of his segment is as follows: P-450 microsomal/P-450CAM:Ile/Leu-Cys-Leu-Gly-Gln/Glu-Ser/Gly-Leu/Ile-Ala-Arg. The amino acid sequence data were also compared with the amino acid sequence deduced from the nucleotide sequence of phenobarbital-inducible cytochrome P-450 mRNA from rat liver (Kurijama-Fuji, Y., Mizukami, Y., Kawajiri, K., Sogawa, K., Muramatsu, M. (1982) Proc. Natl. Acad. Sci. U. S. A. 79, 2793-2797). This comparison suggests that the structure of the phenobarbital-induced isozyme is highly conserved among mammalian species.