Soybean trypsin inhibitor. Cleavage and identification of a disulfide bridge not essential for activity.

Soybean trypsin inhibitor could be partially reduced by treatment with 0.25 M sodium borohydride at 0” without loss of inhibitory activity toward trypsin. From the number of --SH groups released and the carboxymethylcysteine content of the carboxymethylated derivative, it was evident that two -SH groups had been produced under these conditions. A comparison of the diagonal electrophoretic maps of peptic digests of unmodified inhibitor, partially reduced and carboxymethylated inhibitor, and fully reduced and carboxymethylated inhibitor indicated that only one of the two disulfide bonds of the partially reduced derivative had been modified. Peptides in which the cystine residues had been converted to either cysteic acid or carboxymethylcysteinesulfone were isolated from the carboxymethylated derivatives of partially and fully reduced inhibitor. A comparison of their composition with the known sequence of amino acids surrounding the cystine residues of the inhibitor permitted the unequivocal identification of the disulfide bond which had been reduced by sodium borohydride.