Partial characterization of a subtilisin inhibitor from black bean seeds

The subtilisin inhibitor (SI), isolated from black beans (Phaseolus vagaris) has a molecular weight of 8918 and it contains four half cystine residues. The N-terminal residue is arginine and the C-terminal is lysine. The subtilisin inhibiting site is composed of an Ala-Leu or Ile bond. This site interacts in addition to subtilisin (S) with elastase from human leukocytes (HLE), both of which compete for the inhibitor. Also, HLE-cleaved SI (SIHLE) is inactive against HLE as well as against S. It was shown that the inhibition of S by SI resembles a temporary inhibition, involving a very slow release of enzyme activity. The trypsin reactive site is a Lys-Val bond, and its interaction with bovine trypsin is very weak. No complexes of SI or trypsin-cleaved SI (SIT) with trypsin could be detected on cellulose-acetate membrane electrophoresis, while SIT forms a clearly visible complex with S.