Characterization and functional properties of the α-amylase inhibitor (α-AI) from kidney bean ( Phaseolus vulgaris) seeds

Abstract

Alpha-amylase inhibitor (α-AI) from kidney bean ( Phaseolus vulgaris L. cv Tendergreen) seeds has been purified to homogeneity by heat treatment in acidic medium, ammonium sulphate fractionation, chromatofocusing and gel filtration. Two isoforms, α-AI1 and α-AI1′, of 43 kDa have been isolated which differ from each other by their isoelectric points and neutral sugar contents. The major isoform α-AI1 inhibited human and porcine pancreatic α-amylases (PPA) but was devoid of activity on α-amylases of bacterial or fungal origins. As shown on the Lineweaver–Burk plots, the nature of the inhibition is explained by a mixed non-competitive inhibition mechanism. α-AI1 formed a 1:2 stoichiometric complex with PPA which showed an optimum pH of 4.5 at 30°C. Owing to the low optimum pH found for α-AI activity, inhibitor-containing diets such as beans or transgenic plants expressing α-AI should be devoid of any harmful effect on human health.