α-Glucosidase inhibitory effect of anthocyanins from Cinnamomum camphora fruit: Inhibition kinetics and mechanistic insights through in vitro and in silico studies

α-Glucosidase inhibitors are widely used to suppress postprandial glycemia in the treatment of type 2 diabetes mellitus. The present study evaluated the in vitro α-glucosidase inhibitory activity of three major pigment constituents of Cinnamomum camphora fruit, namely cyanidin, cyanidin 3-rutinoside, and cyanidin-3-O-glucoside. We found that cyanidin exerted strong inhibitory activity on α-glucosidase, with IC50 of 5.293 × 10-3 mM, whereas cyanidin 3-rutinoside and cyanidin-3-O-glucoside did not show inhibitory activity on α-glucosidase. The inhibitory activity of cyanidin was stronger than that of acarbose (IC50 1.644 mM), the current most commonly used drug for postprandial glycemia. Kinetic analysis indicated that cyanidin inhibited α-glucosidase through competition, with a Ki value of 0.0183 mM. Fluorescence spectrum titration showed only one binding site between cyanidin and α-glucosidase, and the binding constant was calculated. Further, molecular docking was conducted to simulate the binding interactions between cyanidin and α-glucosidase. Cyanidin was found to interact with several residues close to the catalytic site of α-glucosidase through π-π stack interaction and hydrogen bonds. The calculated binding energy of the cyanidin and enzyme complex was -105.031 kJ/mol. Molecular simulation and calculation showed that the van der Waals force played an essential role in the binding of α-glucosidase and cyanidin.