Isolation and characterization of α-glucosidase inhibitory constituents from Rhodiola crenulata

•Four α-glucosidase inhibitors were isolated from the roots of Rhodiola crenulata.•We analyzed the inhibition mode of purified α-glucosidase inhibitors.•B2-3′-O-gallate, ECG and HETB were found to be potent α-glucosidase inhibitors.

α-Glucosidase inhibitors can reduce postprandial plasma glucose levels and suppress postprandial hyperglycemia. Using a fractionation technique and an inhibitory activity assay, four α-glucosidase inhibitors were isolated from the roots of Rhodiola crenulata. Their chemical structures were identified as epicatechin (EC), epicatechin-(4β,8)-epicatechin gallate (B2-3′-O-gallate), epicatechin gallate (ECG) and 2-(4-hydroxyphenyl)ethyl 3,4,5-trihydroxybenzoate (HETB) using mass spectrometry and NMR spectroscopy analysis. The half maximal inhibitory concentration (IC50) values of B2-3′-O-gallate, ECG, HETB and EC were 0.31 ± 0.01, 0.71 ± 0.01, 4.77 ± 0.22 and 29.85 ± 2.20 μM, respectively. Compared to a known α-glucosidase inhibitor (quercetin, IC50 = 5.30 ± 0.11 μM), B2-3′-O-gallate, ECG and HETB showed a strong α-glucosidase-inhibitory effect. The inhibition kinetics study indicates that B2-3′-O-gallate and ECG are mixed-competitive inhibitors, while HETB is a competitive inhibitor. The inhibition constants (Ki) of B2-3′-O-gallate, ECG and HETB were 0.30 ± 0.03, 0.21 ± 0.04 and 3.10 ± 0.09 μM, respectively. These data show that B2-3′-O-gallate, ECG and HETB are potent α-glucosidase inhibitors.