In Vitro and In Vivo Digestibility of Putative Nutraceutical Common-Bean-Derived Alpha-Amylase Inhibitors

The inhibition of the alpha-amylase digestive enzyme impedes starch digestion by blocking access to the active site of the enzyme, thereby playing a role in the prevention of obesity and type 2 diabetes. Plant-derived alpha-amylase inhibitors (αAIs) are promising nonpharmacological alternatives for the prevention of these diseases. Alpha-amylase inhibitor-1 (αAI-1) present in common bean (Phaseolus vulgaris) is derived from a precursor protein. In this study, the effect of digestion on the digestibility, immune reactivity, and bioactivity of αAI-1 was assessed from four varieties of Hungarian common bean (Phaseolus vulgaris), with special regard to the precursor protein. For this purpose, αAI-1 was tested in both matrix (native flour and cooked flour) and purified forms under in vitro and acute rat in vivo digestion experiments. The effect of digestion on αAI-1s was monitored by lab-on-a-chip (LOC) electrophoresis, SDS-PAGE/immunoblot, and inhibitory activity analyses by native PAGE. After both in vitro and in vivo digestion, we established that αAI-1 was not degraded even after 60 min gastric digestion and showed immune-reactive properties as well. Although the activity of the purified αAI-1 was lost, that of αAI-1 in the flour matrix (noncooked and cooked) was retained in the stomach. Presumably, in the beans, αAI-1 polypeptides became active due to the pepsin digestion of the precursor protein. The latter samples were also tested in vivo in the small intestine and their resistance and immune reactivity were observed, but αAI-1 did not show activity, as αAI-1 polypeptides were probably complexed by pancreatic amylases. From these results, we can assume that the αAI-1-rich bean protein preparation can affect the carbohydrate metabolism; thus, it could be a promising ingredient for weight loss purposes.