α-Glucosidase and -amylase inhibitory activities of saponins from traditional Chinese medicines in the treatment of diabetes mellitus

Extracts of eleven traditional Chinese medicines (TCM) with a reputation of usefulness in treating diabetes mellitus were examined for α-glucosidase and α-amylase inhibitory activities in vitro. The extract with the highest activity was selected for further characterization. The extract of the root bark of Aralia taibaiensis (EAT) outperformed other extracts in the assays with IC50 values of 0.48 ± 0.01 mg/mL (BSG), 0.41 ± 0.02 mg/mL (SCG), 0.61 ± 0.03 mg/mL (BLA) and 0.77 ± 0.03 mg/mL (PPA), respectively. To identify which constituents were responsible for the activities, thirteen triterpenoid saponins were isolated from EAT and examined for their inhibitory effects against α-glucosidase and α-amylase. The results revealed that saponins 2, 3, 4 (IC50: 0.83 ± 0.05 μM for BSG and IC50: 0.72 ± 0.03 μM for SCG), 5, 6, 7, 9, 10, 11 and 12 (IC50: 1.07 ± 0.04 μM for BSG and IC50: 0.93 ± 0.05 μM for SCG) showed α-glucosidase inhibitory activities, while 2, 3, 4 (IC50: 0.93 ± 0.04 μM for PPA), 5, 6, 7, 9, 10, 11 and 12 (IC50: 1.02 ± 0.03 μM for PPA) possessed significant α-amylase inhibitory activities. In addition, the structure-activity relationship of the thirteen saponins was discussed based on their structure features and diabetes mellitus related activities. It is suggested that the glucuronic acid unit at C-3 of the aglycone is the imperative functional group of the antidiabetic activities, and two characteristic structural features are responsible for the remarkable α-glucosidase and α-amylase inhibitory activities.