New flavonoid glycosides from two Astragalus species (Fabaceae) and validation of their antihyperglycaemic activity using molecular modelling and in vitro studies

Phytochemical investigation of the methanol extracts of the aerial parts of Astragalus turkestanus and A. xanthomeloides using various chromatographic techniques resulted in the isolation and structural elucidation of two new flavonoid glycosides, namely, 7-methoxy kaempferol-3-O-alpha-L-arabinosyl-(1 -> 6)-beta-D-galactopyranoside (3) from the former and kaempferol-3-O-alpha-L-rhamnopyranosy1-7-0-a-L-rhamnopyranosyl-(1 -> 6)-beta-D-galactopyranoside (6) from the latter. In addition five other known compounds were isolated for the first time from these species. In silico molecular modeling study was carried out on a-glucosidase (PDB ID 3TOP; 2.88 A degrees) to assess the alpha-glucosidase inhibitory activity of the isolated compounds. Both compounds 3 and 6 displayed the highest fitting score with Delta G equals to -82.10 and -67.66 Kcal/mol in the pH-based ionization method and -68.55 and -67.83 Kcal/mol in the rule-based ionization method, respectively. alpha-Glucosidase inhibitory activity was further asserted in vitro using an ELISA assay that is measured colorimetrically at 405 rim. Compound 3 showed a significant alpha-glucosidase inhibitory activity displaying IC50 of 50.31 mu g/mL approaching that of acarbose (IC50 = 30.57 mu g/mL). All the tested samples showed no significant cytotoxic activity against Caco-2 (human epithelial colorectal adenocarcinoma) cells using the MIT assay (IC50 > 400 mu g/mL). Thus; both new compounds could offer promising natural antihyperglycaemic entities without any marked toxicity, which could be useful in the pharmaceutical industry.