Partially purified white bean amylase inhibitor reduces starch digestion in vitro and inactivates intraduodenal amylase in humans

Abstract

Whether commercial, bean-derived α-amylase inhibitor preparations failed to decrease starch digestion in humans because of insufficient antiamylase activity, destruction by gastrointestinal secretions, or decreased activity in the presence of starch is unknown. We used a simple partial purification procedure to markedly concentrate the inhibitor (sixfold to eightfold by total protein content, and 30–40-fold by dry weight). Compared with a commercial preparation and crude bean extract, this partially purified inhibitor inactivated intraduodenal, intraileal, and salivary amylase in vitro faster and more completely (p < 0.001); its specific activity was not affected by exposure to gastric juice and was only minimally reduced by duodenal juice. Whereas the rate of amylase inhibition by inhibitor was markedly slowed in the presence of nondietary liquid starch, dietary solid starch had only a minimal effect. Consequently, the partially purified inhibitor had no effect on liquid starch digestion, but decreased in vitro digestion of dietary starch in a dose-dependent manner (p < 0.001). Perfusion of the partially purified inhibitor (2.0, 3.5, or 5.0 mg/ml at 5 ml/min) into the duodenum of humans rapidly inhibited >94%, >99%, or >99.9% of intraluminal amylase activity. We conclude that commercial amylase inhibitors failed to decrease starch digestion in vivo mainly because they have insufficient antiamylase activity. However, a partially purified inhibitor with increased specific activity is stable in human gastrointestinal secretions, slows dietary starch digestion in vitro, rapidly inactivates amylase in the human intestinal lumen, and, at acceptable oral doses, may decrease intraluminal digestion of starch in humans. Such an inhibitor therefore deserves study.