Rapid identification of the α-glucosidase inhibitory compounds from Thunberg's Geranium (Geranium thunbergii Sieb. et Zucc.)

In the current study, our investigation of inhibitory compounds from the ethyl acetate-soluble fraction of 95% ethanol extract of Thunberg's Geranium (Geranium thunbergii Sieb. et Zucc.; TG) was followed by identification of the inhibitory compounds by a combination of HPLC microfractionation and an enzyme assay in a 96-well plate. Structural analyses of the active compounds were carried out using LC-MS (n) . The main compound inhibiting yeast alpha-glucosidase in this plant was tentatively identified as geraniin by LC-MS (n) . Six compounds, p-hydroxybenzoic acid (1) (IC50 > 362.32 mu M), brevifolin carboxylic acid (2) (IC50 > 171.23 mu M), geraniin (3) (IC50 =4.09 mu M), ellagic acid (4) (IC50 = 63.91 mu M), kaempferol-3-O-arabinofuranosyl-7-O-rhamnopyranoside (5) (IC50 =27.70 mu M), and kaempferitrin (6) (IC50 > 86.51 mu M) were isolated from the active EtOAc-soluble fraction of TG on yeast alpha-glucosidase. However, the 6 isolated compounds were very weak inhibitory activity against mammalian alpha-glucosidase and alpha-amylase. This study suggests that the developed HPLC microfractionation with an enzyme assay system is one of the powerful tools for rapid screening and identification of enzyme inhibitors in complex extracts.