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'.
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Sato et al. (1974) studied this question.