Substrate-binding region of cytochrome P-450SCC (P-450 XIA1). Identification and primary structure of the cholesterol binding region in cytochrome P-450SCC.

Cytochrome P-450SCC (P-450 XIA1) from bovine adrenocortical mitochondria was investigated using a suicide substrate: [14C]methoxychlor. [14C]Methoxychlor irreversibly abolished the activity of the side-chain cleavage enzyme for cholesterol (P-450SCC) and the inactivation was prevented in the presence of cholesterol. The binding of [14C]methoxychlor and cytochrome P-450SCC occurred in a molar ratio of 1:1 and the cholesterol-induced difference spectrum of cytochrome P-450SCC was similar with the methoxychlor-induced difference spectrum. [14C]Methoxychlor-binding peptides were purified from tryptic-digested cytochrome P-450SCC modified with [14C]methoxychlor. Determination of the sequence of the amino-acid residues of a [14C]methoxychlor-binding peptide allowed identification of the peptide comprising the amino-terminal amino-acid residues 8 to 28.