The sulfhydryl proteinase ficin has been isolated from commercial preparations by salt fractionation and chromatography on carboxymethylcellulose. During the isolation, proteolytic activity was reversibly inhibited by sodium tetrathionate. Several active components were found, but only the major one was characterized. In its inactive form, the purified enzyme could be stored as a salt precipitate at 4° for at least 15 months without any loss of activity. The major component was homogeneous by several criteria. Its amino acid composition, molecular weight, amino-terminal residue, and optical rotatory dispersion were determined. Its enzymatic specificity was studied using the oxidized B chain of insulin as a substrate. It was found that a wide variety of peptide bonds were hydrolyzed, but peptide bonds following an aromatic residue appeared to be hydrolyzed more efficiently than the others.
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