Inhibition of α-amylase activity by insoluble and soluble dietary fibers from kiwifruit (Actinidia deliciosa)

The mechanisms underlying the physiological benefits of dietary fibers are multifaceted, and the inhibitory effect of the binding of these fibers to digestive enzymes such as α-amylase may be one of the factors contributing to their nutritional functionality. In this study, soluble and insoluble dietary fibers (SDF and IDF) were isolated from mature kiwifruit and investigated for their inhibitory action against α-amylase using an in vitro digestion method. Both IDF and SDF showed a dose-dependent decrease in the initial starch hydrolysis velocity. When used at the same concentration, IDF showed a higher α-amylase inhibition effect. The analysis of the Michaelis–Menten kinetics of the hydrolysis reaction of α-amylase in maize starch with IDF indicated a mixed type of inhibition and the competitive inhibitor constant (Kic) for the enzyme–inhibitor (EI) complex was 4 mg/mL. The inhibition between SDF and α-amylase was observed to be non-competitive, and the dissociation constant for the EI complex was 12.5 mg/mL. The results suggested that IDF and SDF may, in general, have different mechanisms of action for their amylase inhibition activity and that both IDF and SDF obtained from kiwifruit can attenuate starch hydrolysis in vitro, thereby having potential applications in the food and nutraceutical industries.