Studies on Soybean Trypsin Inhibitors: II. Accidentally Modified Kunitz Soybean Trypsin Inhibitor*

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Abstract
A soybean trypsin inhibitor (Kunitz) whose several amino acid residues such as methionine, tryptophan and tyrosine were modified was obtained during the purification of the trypsin inhibitor from soybean flakes, and the properties and modified amino acid residues of the inhibitor were investigated. 1. This inhibitor was separated into monomer, dimer and polymer by gel filtration, and the monomer and the dimer had 83 and 40%, respectively, of the inhibitory-activity of the native inhibitor. 2. The modified residues were methionines at residues 100 and 130, tryptophan at 133 and tyrosines at 17 and/or 18. 3. Two methionines were oxidized to methionine sulfoxide, a part of tyrosines-17 and/or -18 was chlorinated to monochlorotyrosine and tryptophan-133 was modified into unknown forms. 4. It was deduced that the modification of these amino acid residues in the protein had been caused by chlorine during dialysis of the protein against tap water.

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