Purification, characterization, and complete amino acid sequence of a trypsin inhibitor from amaranth (Amaranthus hypochondriacus) seeds

Abstract

A protein proteinase inhibitor was purified from a seed extract of amaranth (Amaranthus hypochondriacus) by precipitation with (NH4)2SO4, gel-filtration chromatography, ion-exchange chromatography, and reverse-phase high-performance liquid chromatography. It is a 69-amino acid protein with a high content of valine, arginine, and glutamic acid, but lacking in methionine. The inhibitor has a relative molecular weight of 7400 and an isoelectric point of 7.5. It is a serine proteinase inhibitor that recognizes chymotrypsin, trypsin, and trypsin-like proteinase activities extracted from larvae of the insect Prostephanus truncatus. This inhibitor belongs to the potato-I inhibitor family, showing the closest homology (59.5%) with the Lycopersicum peruvianum trypsin inhibitor, and (51%) with the proteinase inhibitor 5 extracted from the seeds of Cucurbita maxima. The position of the lysineaspartic acid residues present in the active site of the amaranth inhibitor are found in almost the same relative position as in the inhibitor from C. maxima.