Isolation and identification of α-glucosidase inhibitory constituents from the seeds of Vigna nakashimae: Enzyme kinetic study with active phytochemical

The α-glucosidase inhibition effects of the 80% ethanol extracts in the seeds of five Vigna species (V. nakashimae, V. nipponensis, V. umbellate, V. radiate, and V. angularis) were evaluated and their half-maximal inhibitory concentration (IC50) values showed considerable differences (p < 0.05) ranging from 7.3 to >900 μg/ml. V. nakashimae exhibited the most potent inhibition with IC50 value of 7.3 ± 1.1 μg/ml, followed by V. nipponensis (184.0 ± 9.5 μg/ml) and V. umbellata (520.0 ± 8.1 μg/ml). Bioactivity-guided fractionation of V. nakashimae seeds yielded three phenolics by silica gel chromatography and their structures were elucidated as gambiriin D (1), luteoliflavan-7-O-glucopyranoside (2), and catechin-7-O-glucopyranoside (3) using nuclear magnetic resonance (NMR) spectroscopy. In particular, gambiriin D (1) possessed strong inhibition activity with IC50 of 36.8 ± 2.3 μM through simple reversible slow-binding inhibition (kinetic parameters: k4 = 0.0048  μM−1s−1; Kiapp = 48 μM). Furthermore, this compound inhibited recombinant human aldose reductase with IC50 value of 12.0 ± 0.7 μM. Results suggest that V. nakashimae may be a potent α-glucosidase inhibition for health products.