Studies on Trypsin Inhibitors in Sweet Potato

II. Modification of Amino Acid Residues in Inhibitor III

抄録

To clarify the character of the functional groups of a trypsin inhibitor from a sweet potato, chemical modification of the inhibitor and kinetic studies of the modified inhibitor have been carried out. As a result of these studies, it was found that antitryptic and antiplasmic activities were only slightly affected by modification of two amino groups, one sulfhydryl and two tryptophanyl residues in the inhibitor. It was shown, however, that the inhibitor partially lost antitryptic activity on nitration of two tyrosyl residues, whereas modification of one tyrosyl residue enhanced the inhibitory activity, and that the activity significantly decreased on modification of one arginyl, one histidyl and some carboxyl groups. This suggests that the functional groups of the inhibitor which are involved in the interaction with trypsin [EC 3. 4. 21. 4] are at least one arginyl, one histidyl and several carboxyl groups in addition to one tyrosyl residue. The K_i for the complex between trypsin and the inhibitor in which one histidyl or some carboxyl groups were modified was found to be greater than that of the native inhibitor, while K_ifor the complex between the enzyme and the inhibitor with one tyrosine modified or one tryptophan modified decreased. It was found that the effect of chemical modification on the antiplasmic activity of the inhibitor was less than on the antitryptic activity, although inhibitor in which two histidyl residues were modified showed antiplasmic activity reduced almost to zero. These data suggest the existence of many bonds between the two moieties involved in the complex.