Purification, Characterization and Sequence Determination of a Double-headed Trypsin Inhibitor Peptide fromTrichosanthes kirilowii(a Chinese Medical Herb)

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Abstract
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 3 pairs of disulfide bonds, was sequenced. Two active domains were located at 2 disulfide loops composed of 8 (Pos. 17-24) and 9 (Pos. 29-37) amino-acid residues, respectively. It inhibits 2 molecules of trypsin simultaneously and might be regarded as the smallest double-headed trypsin inhibitor (MW = 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 2 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.

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