Wheat gluten protein inhibits α-amylase activity more strongly than a soy protein isolate based on kinetic analysis.

The objective of this study was to investigate the porcine pancreatic α-amylase (PPA) inhibitory activity of botanical food proteins, represented by soy protein isolate (SPI) and wheat gluten protein (WGP). The half maximal inhibitory concentration (IC50) was determined, and the kinetics of inhibition were investigated using Dixon, Cornish-Bowden, and Lineweaver-Burk plots. The results showed WGP functioned as mixed-type inhibitors with both competitive and uncompetitive inhibitory characteristics, while SPI showed competitive inhibitory effects on α-amylase. The competitive inhibition constants (Kic) of WGP and SPI were 5.753 and 30.212 mg/mL, respectively, and the uncompetitive inhibition constants (Kiu) of WGP was 45.110 mg/mL, respectively. The IC50 values of WGP and SPI were 3.086 and 33.899 mg/mL, respectively. For WGP, the lower Kic vs. Kiu for mixed-type inhibitors suggested that they bound more tightly to free PPA than the PPA-starch complex. Compared with SPI, WGP displayed a stronger inhibitory effect on α-amylase. These results indicated that SPI and WGP may delay the digestion of starchy foods by inhibiting starch hydrolytic enzymes, which may be of relevance in vivo during gastrointestinal digestion. The findings are also of important practical value for the development of carbohydrate-restricted diet and protein-based functional foods.