Proteolytic cleavage of horse liver cytochrome b5. Primary structure of the heme-containing moiety.

The amino acid sequence of the NH2-terminal segment of horse cytochrome b5, containing the heme binding site, has been determined. A fragment, representing residues 7 through 90, was obtained by tryptic cleavage of native cytochrome b5. Chymotryptic cleavage of native cytochrome b5 yields a peptide containing residues 1 through 98. Contrary to native cytochrome b5, neither derivative showed binding to horse liver microsomal vesicles. The complete primary structure of the polar moiety has been deducted from automated and manual sequence analysis of peptides obtained from tryptic and chymotryptic digests of native cytochrome and apocytochrome preparations. Glutamyl residues at positions 41, 42, 47, and 48 appear to be replaced by aspartyl residues in some molecules. Such microheterogeneity is not observed at glutamyl residues at other positions. The native cytochrome b5 preparation contains a blocked NH2-terminal residue.