Purification, characterization and sequence determination of a double-headed trypsin inhibitor peptide from Trichosanthes kirilowii (a Chinese medical herb).

A double-headed trypsin inhibitor peptide was isolated and purified from the root of Trichosanthes kirilowii Maxim (Cucurbitaceae), a Chinese medical herb, by 2.5% trichloroacetic acid and heat treatment followed by affinity chromatography with immobilized trypsin and ion-exchange chromatography. This inhibitor, consisting of 41 amino-acid residues with three pairs of disulfide bonds was sequenced. Two active domains were found to be located at two disulfide loops composed of eight (Pos. 17-24) and nine (Pos. 29-37) amino-acid residues, respectively. It inhibits two molecules of trypsin simultaneously and might be regarded as the smallest double-headed trypsin inhibitor (Mr = 4575) so far known. The chemical modification of the inhibitor with cyclohexandione and citraconic anhydride showed that Arg20-Gly21 and Lys30-Leu31 corresponded to the two reactive sites, respectively. The discovery of the Trichosanthes inhibitor is of importance not only for the study on the structure-function relationship of proteinase inhibitor peptides but also for the search for low molecular mass inhibitors of clinical value among Chinese medical herbs.