Biochemical Nature of a Natural α-Amylase Inhibitor from Wild Amaranth (Amaranthus paniculatus) Seeds

Endogenous á-amylase inhibitors exist widely in animals, plants and microorganisms. These inhibitors show remarkable structure variety with different modes of inhibition and specificity against different á-amylases. To explore the alpha-amylase inhibitors in wild amaranth, a novel proteinaceous inhibitor of á-amylase, named WAI-1, was purified and its structure and function were investigated in this study. WAI-1 was one of the smallest proteinaceous inhibitors with a molecular weight of 986.5 Da. The structural analysis exposed that WAI-1 was a cyclic nonapeptide of nine amino acids, with pyroglutamate as the N-terminal. The hydrolysis in hydrochloric acid solution opened the loop of the side chain of WAI-1 at the N-terminal, but did not affect its inhibitory activity. However, the hydrolysis by trypsin disconnected arginine at the c-terminal, causing almost a full loss of its inhibitory activity. WAI-1 had good heat stability and specific inhibitory activity against á-amylases of the insects. The integrity of the molecular loop structure of WAI-1 was critical for its stability and inhibitory activity.