Identification of a methionine residue as the reactive site for chymotrypsin in the double-headed proteinase inhibitor from the canine submandibular gland (author's transl)]

The canine submandibular inhibitor is double-headed with two independent reactive sites. Whereas the trypsin-reactive center (-Ala-Cys-Pro-Arg26-Leu-His-) is located in domain I, the chymotrypsin-reactive site (-Met-Cys-Thr-Met78-Asp-Tyr-) is located in domain II. The presence of a methionine residue in this inhibition center is supported by the findings that nitration with tetranitromethane abolishes neither trypsin nor chymotrypsin inhibition, whereas after alkylation of the methione residues, only trypsin inhibition is retained. Remarkably, another inhibitor from microbial sources [10] which also contains a methionine residue in the presumed reactive site also inhibits subtilisin but not chymotrypsin (or trypsin).