Partial Amino Acid Sequence of an Alkaline Protease Inhibitor, API-2 (b and c)

Abstract

API-2b and API-2c (alkaline protease inhibitors) were directly subjected to Edman degradation. It was elucidated that API-2C was lacking in Ala-Pro-Ser-Leu-Gly-Ala located in the NH₂-terminal region of API-2b, so the two inhibitors were heterogeneous in the NH₂-terminal region. Cleavage of the three methionyl bonds in API-2c with cyanogen bromide resulted in three fragments, designated as Peptide 1, Peptide 2 and Peptide 3. It was found that Peptide 1 was located at the NH₂-terminal region, Peptide 2 was in the center and Peptide 3 made-up the COOH-terminal region of the API-2 molecule. Partial sequence data showed that most threonine, alanine, glycine and leucine residues were distributed in the NH₂-terminal half of the molecule, and the rest of the molecule contained most of the aromatic residues.

The amino acid sequence around the reactive site of API-2 (b and c) was very similar to that of S-SI (Streptomyces subtilisin inhibitor) with the exception of the isoleucine residue in place of valine residue in S-SI.