Steroid 21-hydroxylase (cytochrome P-450) from porcine adrenocortical microsomes: microsequence analysis of cysteine-containing peptides.

The steroid 21-hydroxylase cytochrome P-450 from porcine adrenocortical microsomes was purified to homogeneity. The protein exhibited two NH2-terminal sequences, one of which was identical with the first but lacking the NH2-terminal methionine. The sequence was extremely hydrophobic but had little homology to the 17 alpha-hydroxylase/C17,20-lyase isolated from neonatal porcine testes or to rat or rabbit liver microsomal cytochromes P-450. The cysteine-containing fragments of the S-carboxymethylated protein were purified by high-performance liquid chromatography and sequenced. Three of the cysteine-containing peptides exhibited significant sequence homology with peptides from the major phenobarbital-induced rat liver cytochrome P-450 (P-450b) and two with peptides from cytochrome P-450cam (camphor methylene hydroxylase from Pseudomonas putida). The presence of conserved regions in the primary sequences of these proteins appears likely to provide clues to the nature of their heme-binding domains.