Purification and characterization of two Kunitz family subtilisin inhibitors from seeds of Canavalia lineata

Two subtilisin inhibitors (CLSI-II and -III) were purified from seeds of Canavalia lineata by extraction with water, ammonium sulfate precipitation, and chromatographies on DEAE-Toyopearl and hydroxyapatite. The two inhibitors have the same molecular weight of about 22,000, and quite similar amino acid compositions. They contain five half-cystine residues and tend to dimerize through an intermolecular disulfide bridge due to the presence of a single cysteine residue. CLSI-III only inhibited subtilisin-type serine proteases, while CLSI-II showed a wider inhibitory specificity. Though the two inhibitors have almost identical thermal labilities, CLSI-II is more stable as to extreme pH than CLSI-III. They are considered to be Kunitz type inhibitors on the basis of several properties.