Isolation and Characterization of the Subunits of a Heat-labile α-Amylase Inhibitor from Phaseolus vulgaris White Kidney Bean

抄録

The heat-labile one of the two α-amylase inhibitors of the white kidney bean (Phaseolus vulgaris) was found to be composed of three kinds of subunits, and they were isolated and characterized. The α-subunit was free from tryptophan and cysteine and the β-subunit contained no methionine or cysteine. There was no marked resemblance in tryptic peptide maps between the α- and β-subunit polypeptides. The α-subunit contained 30% by weight of carbohydrate, mainly made up of high mannose-oligosaccharides, and the sugar moiety of the β-subunit amounted 7% and appeared to be predominantly composed of xylomannose-type oligosaccharides. The largest subunit, γ, was very similar in molecular features to a postulated αβ-dimer and its N-terminal sequence coincided with that of the α-subunit. The molecular weights of the polypeptides of α, β-, and γ-subunits were shown to be 7, 800, 14, 000, and 22, 000, respectively, by SDS-PAGE. It seemed likely that the α- and β-subunits are common to both of the inhibitors and that the heat-lability of this inhibitor arises from the γ-subunit.