Analysis of substrate specificity and endopeptidyl activities of the cathepsin B-like proteinase from Helicoverpa armigera.

The cathepsin B-like proteinase from Helicoverpa armigera (HCB) is involved in the degradation of yolk proteins during embryonic development. In order to gain insight into the substrate specificity of this proteinase, various proteins from animals and plants were tested as substrates. The specific cleavage sites of this enzyme on endopeptide bonds were assayed using bovine serum albumin (BSA) as a substrate. Results showed that BSA was degraded into several fragments, which suggests that HCB cleaves BSA at specific endopeptidyl sites. The amino acid sequences of the BSA derived peptides were determined, revealing cleavage of the bonds between residues Arg81-Glu82, Val423-Glu424 and Gly430-Lys431. This suggests that the minimum requirement for a scissile bond to be recognized by HCB is the presence of an ionic amino acid at the P1 ' position and the P1 position can vary. These observations suggest that HCB cleaves bonds at the N-terminal side of ionic amino acid residues giving HCB a wide range of substrates, though other factors dictating the substrate specificity of this enzyme remains to be clarified. Our results provide new evidence that HCB functions as an endopeptidase on some proteins.