Complex Formation of Microbial Alkaline Protease Inhibitor (S-SI) with Subtilisin BPN′ and Its Properties

Microbial alkaline protease inhibitor, S–SI, was investigated on the interaction with subtilisin BPN′ Inhibitory equivalent of S–SI to subtilisin BPN′ was determined that one molecule of S–SI (MW = 23,000) inhibited two molecules of subtilisin BPN′ (MW = 27,700). The S–SI-subtilisin BPN′ complex was isolated by gel filtration on Sephadex G–100 and rhombic crystals were obtained. DIP- and ZAGPCK-subtilisin BPN′ did not form such complex with S–SI. Homogeneity of the complex was determined by disc electrophoresis. The isoelectric point of the complex was pH 5.5. Assay of S–SI dissociated and amino acid analysis of the complex indicated that one subunit (a half molecule) of S–SI was combined with one molecule of subtilisin BPN′ From molecular weight determination, it was clarified that the complex was composed of one molecule (consist of 2 subunits) of S–SI and two molecules of subtilisin BPN′.