Protein inhibitors of trypsin from the seeds of Cucurbitaceae plants.

Seven trypsin inhibitors were isolated from the seeds of Cucurbitaceae plants: two from cucumber (Cucumis sativus) and red bryony (Bryonia diotica) and one from figleaf gourd (Cucurbita ficifolia), spaghetti squash (Cucurbita pepo var. (vegetable spaghetti) and water melon (Citrullus vulgaris). The inhibitors were purified by fractionation with ammonium sulphate, followed by ion-exchange chromatography and affinity chromatography using immobilized trypsin or anhydro-trypsin. The homogeneous inhibitors from cucumber and water melon are made up of 32 and 30 amino acid residues, respectively, whereas the remaining ones of 29 residues. All inhibitors contain three disulphide bridges and are free of threonine, phenylalanine and tryptophan. Inhibitors from spaghetti squash and CSTI IIb from cucumber are inactivated by acetylation of free amino groups whereas the remaining ones are inactivated by modification of arginine with 1,2-cyclohexanedione. Thus the P1 residues of the reactive sites of the inhibitors are lysine and arginine, respectively.