Chemical modification and complex formation studies with jack bean proteinase inhibitor

Pretreatment of the purified jack bean inhibitor with enterokinase activated human pancreatic preparation for 1 hr decreased its inhibitory capacity against crystalline bovine alpha-chymotrypsin by 30% but did not affect its trypsin inhibitory activity. Preincubation of the inhibitor with bovine chymotrypsin for 60 min resulted in partial loss of the inhibitory potency. Complex formation studies by gel chromatography on Sephadex G-100 indicated that the trypsin-inhibitor and chymotrypsin-inhibitor complexes dissociated to release inactivated inhibitor and active proteinases. Gel chromatography of the inhibitor in presence of 1.5 M ammonium sulphate indicated that the inhibitor showed a tendency to aggregate without loss of biological activity. However, in 4.2 M salt medium after 3 hr, antichymotryptic activity was lost completely without any effect on antitryptic activity. Treatment with methylamine, a nucleophile, caused a greater loss of antichymotryptic activity. Trinitrobenzene sulphonate and ethylacetamidate, the amino group modifiers, affected only the antichymotryptic activity. Treatment with ninhydrin, a specific arginine modifier, at pH 9.0 abolished the antitryptic activity whereas only 50% of the antichymotryptic activity was lost. Diethylpyrocarbonate, a histidine reagent, also decreased only the antitryptic activity. Modification of tryptophan and cysteine residues of the inhibitor had no effect on its inhibitory potency. Treatment with mercaptoethanol and sodium borohydride caused nearly 50% loss of antitryptic and antichymotryptic activities. Chloramine-T, a reagent that modifies methionine residues, inactivated the inhibitor.