S-PI was crystallized from culture filtrate of Streptomyces naniwaensis by salting out with ammonium sulfate, adsorption on active carbon, extraction with methanol, gel filtration on Sephadex LH-20, column chromatography on silica gel and rechromatography on Sephadex LH-20. The molecular weight of S-PI was estimated 644 and its molecular formula was assumed C31H57N5O9 from the results of mass spectral analysis and elementaly analysis. S-PI gave positive Rydon-Smith reaction but was negative to ninhydrin reaction. One mg of crystalline pepsin was completely inhibited by 18 μg of S-PI. S-PI also inhibited various acid proteases but the rate of inhibition was different on each acid protease.
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Satoi et al. (1971) studied this question.