The position of the reactive site peptide bond in eggplant trypsin inhibitor molecule

The purified trypsin inhibitor from eggplant (Solanum melongena, L.) has an Arg-X bond in the reactive site which is selectively cleaved by limited hydrolysis with a catalytic amount of bovine trypsin. So-called trypsin-modified inhibitor was separated from the native one by QAE-Sephadex A-25 chromatography. After the cleavage of disulfide bonds of isolated modified inhibitor by means of reduction and S-carboxymethylation, two fragments (F-I and F-II) were obtained by gel filtration on Sephadex G-25. F-I and F-II were composed of 44 and 14 amino acid residues, respectively. The sum of both coincided with that of the native inhibitor. The N-terminal of F-I was blocked and the C-terminal was arginine. In F-II, the N-terminal was identified as asparagine and the C-terminal as serine. No N-terminal amino acid could be detected in the native inhibitor, as described in our previous paper (7). Therefore, it is concluded that the reactive site of eggplant trypsin inhibitor is an arginylasparagine bond located between residues 44 and 45.