Screening for potential α-glucosidase and α-amylase inhibitory constituents from selected Vietnamese plants used to treat type 2 diabetes

Ethnopharmacological relevance: The 18 plant species investigated in this study have been used as herbal antidiabetic remedies in Vietnamese traditional medicines. This study aimed to evaluate their ability to inhibit alpha-glucosidase and alpha-amylase, two key enzymes involved in serum glucose regulation. Materials and methods: Chloroform, ethanol and water extracts of 18 plants were screened for alpha-glucosidase and alpha-amylase inhibitory activity. Analytical-scale HPLC was subsequently used to investigate the most active extracts, where samples with low level of tannins were identified and fractionated into 96-well microplates, followed by alpha-glucosidase and alpha-amylase assessment of each well. High-resolution alpha-glucosidase and alpha-amylase inhibition profiles constructed from these assays allowed identification of HPLC peaks correlated with alpha-glucosidase and alpha-amylase inhibitory activity. The active constituents were subsequently isolated using preparative-scale HPLC and their structure was elucidated by HRESIMS and NMR. Results: Ethanol extracts of Nepenthes mirabilis, Phyllanthus urinaria, and Kandelia candel significantly inhibited alpha-glucosidase with IC50 values of 32.7 +/- 6.3, 39.7 +/- 9.7, and 35.4 +/- 13.9 mu g/mL, respectively. Water extracts of N. mirabilis, Phyllanthus amarus, P. urinaria, Lagerstroemia speciosa, Syzygium cumini, Rhizophora mucronata, and IC candel showed IC50 values of 3.3 +/- 0.8, 34.9 +/- 1.5, 14.6 +/- 4.6, 5.4 +/- 0.5, 20.9 +/- 1.8, 3.3 +/- 0.6, and 4.0 +/- 0.8 mu g/mL, respectively. In the alpha-amylase inhibition assay, ethanol extracts of K. candel and Ficus racemosa showed IC50 of 7.6 +/- 0.9 and 46.7 +/- 23.6 mu g/mL, respectively. Showing low tannin constituents as seen from HPLC profiles, P. amarus and P. urinaria water extracts and F. racemosa ethanol extract were subjected to microfractionation. Only high-resolution alpha-glucosidase inhibition profiles of P. amarus and P. urinaria water extracts showed several active compounds, which were isolated and identified as corilagin (1), repandusinic acid A (2), and mallotinin (3). IC50 of these compounds were 1.70 +/- 0.03, 6.10 +/- 0.10, and 3.76 +/- 0.15 mu M, respectively. Kinetics analysis revealed that 1 displayed a mixed type mode of inhibition with K-i and K-i values of 2.37 +/- 0.90 and 2.61 +/- 0.61 mu M, respectively, whereas 2 and 3 competitively inhibited alpha-glucosidase with Ki values of 4.01 +/- 0.47 and 0.65 +/- 0.11 respectively. Conclusion: Corilagin (1), repandusinic acid A (2), and mallotinin (3) were potent alpha-glucosidase inhibitors contributing significantly to the inhibitory effect observed for the water extracts of P. amarus and P. urinaria. (C) 2016 Published by Elsevier Ireland Ltd.