Trypsin and chymotrypsin inhibitor from chick peas. Selective chemical modifications of the inhibitor and isolation of two isoinhibitors.

The trypsin and chymotrypsin inhibitor from chick peas (CI) is stable in HCl 0.001 M -- 0.01 M and in KOH 0.01 M -- 0.05 M even after 24 h. Increased KOH concentrations decrease considerably the inhibitory activity already after 1 h. Maleyation and succinylation of the inhibitor resulted in almost full loss of its trypsin-inhibitory activity but had no effect on the chymotrypsin-inhibitory activity. A series of modifications directed towards tyrosyl residues showed that iodination influenced only the chymotrypsin-inhibitory activity; however, nitration and arsanilation affected not only the chymotrypsin-inhibitory activity but also the trypsin-inhibitory activity. Treatment of the inhibitor with CNBr and chloramine T resulted only in a decrease in the chymotrypsin-inhibitory activity indicating that the only methionine is involved in the chymotrypsin-inhibitory activity. When CI-fragment A, previously treated with trypsin at pH 3.75, was further treated with carboxypeptidase B, a release of three lysyl residues per mole protein was found. CI was separated by equilibrium chromatography on SP-Sephadex column into two isoinhibitors, CII and CIII, respectively. Both inhibited trypsin and chymotrypsin with the same specific activity as CI. They differed from each other only in a glutamyl, aspartyl, glycyl and alanyl residue.