Effect of Modification of Reactive Lysine on Antitryptic and Antichymotryptic Activity of Proteinase Inhibitor from Cow Colostrum

Abstract

40% of the primary structure of the cow colostrum proteinase inhibitor (CTI) is homologous with the structure of the trypsin kallikrein inhibitor (TKI) from bovine organs; the positions of the reactive lysine residues are also the same in both inhibitors. Both CTI and TKI were modified by carbamoylation and the fully labeled derivatives were isolated by ion-exchange chromatography. The effect of the modification on the antitryptic and antichymotryptic activity of both inhibitors was investigated. The antichymotryptic activity of both inhibitors is not decreased after the modification. The antitryptic activity of modified TKI is retained, yet the dissociation constant of the complex of the modified inhibitor with trypsin is considerably increased; nevertheless, modified TKI is a good trypsin inhibitor. The antitryptic activity of modified CTI is hardly detectable. We explain this difference in the behaviour of both inhibitors by a replacement of basic residues Arg-17 and Arg-39 in TKI by neutral amino acids Ala-20 and Gln-42 in CTI.