Structural analysis of the cysteine-containing peptides from the major 3-methylcholanthrene-induced isozyme of cytochrome P-450 (P-450c) in rat liver microsomes.

Cytochrome P-450c, the major 3-methylcholanthrene-inducible isozyme of cytochrome P-450 in rat liver microsomes, was subjected to proteolytic digestion after S-carboxymethylation of the protein, and the peptides were resolved by high-pressure liquid chromatography. Since it is now recognized that cytochromes P-450 contain a thiolate as the axial fifth ligand of the heme, seven peptides containing eight cysteines were subjected to microsequence analysis. One cysteine-containing peptide (Tsa-56) was shown to possess 46-69% homology with a common peptide found in five other cytochromes P-450 but is not anticipated to be the heme-binding segment on the basis of X-ray crystallographic results obtained with Pseudomonas putida cytochrome P-450cam. Analysis of the other cysteine-containing peptides in cytochrome P-450c revealed two peptides (Tsa-54 and T-46) of only limited homology with the highly conserved region of cytochromes P-450cam, P-450LM2, P-450b, and P-450e that are all presumed to contain the heme-binding cysteine. Another peptide that contained two cysteines and a stretch of hydrophobic residues (Tsa-47) had limited sequence homology with a similar peptide found in several other cytochromes P-450. This domain is located a short distance from the proposed heme-binding cysteine in other cytochromes P-450. Sequence analysis of a cysteine-containing peptide (T-30) from another 3-methylcholanthrene-inducible rat liver cytochrome P-450 (cytochrome P-450d) revealed 91% homology with peptide T-46 from cytochrome P-450c, but this peptide shows no significant homology with any of the cysteine-containing peptides from other cytochromes P-450.(ABSTRACT TRUNCATED AT 250 WORDS)