Structural studies of wheat monomeric and dimeric protein inhibitors of alpha-amylase

Two wheat monomeric protein inhibitors of alpha-amylase with mol.wt. 12000, designated inhibitors 0.28 and 0.39 according to their gel-electrophoretic mobilities, showed almost identical circular-dichroism spectra in both the far and near u.v. at different pH values as well as in the presence or absence of dissociating and reducing agents. Both inhibitors (0.28 and 0.39) were readily inactivated by reduction of the five disulphide bridges present in each inhibitor molecule. These properties are very similar to those exhibited by the wheat dimeric protein inhibitor of alpha-amylase with mol.wt. 24000, designated inhibitor 0.19 according to its gel-electrophoretic mobility. The N-terminal sequence of the 0.19 inhibitor was determined without separating its subunits and compared with that of the 0.28 inhibitor reported by Redman [(1976) Biochem. J. 155, 193--195]. Petide 'maps' from tryptic digests of reduced and carboxymethylated inhibitors 0.19 and 0.28 were compared. One molecule of reducing sugar is covalently bound per inhibitor-0.19 protomer and inhibitor-0.28 molecule. The results obtained strongly support previous findings indicating the structural equivalence of inhibitor 0.28 with each inhibitor-0.19 protomer and the common phylogenetic origin of these protein alpha-amylase inhibitors from wheat kernel.