In vitro α-glucosidase inhibitory activity of isolated fractions from water extract of Qingzhuan dark tea

Background: Natural products have being used as potential inhibitors against carbohydrate-hydrolyzing enzymes to treat diabetes mellitus. Chinese dark tea has various interesting bioactivities. In this study, the active compounds from Qingzhuan dark tea were separated and their anti-diabetic activity was examined using an in vitro enzymatic model. Methods: The chloroform, ethyl acetate, n-butanol, sediment and residual aqua fractions of a Chinese dark tea (Qingzhuan tea) were prepared by successively isolating the water extract with different solvents and their in vitro inhibitory activities against a-glucosidase were assessed. The fraction with the highest inhibitory activity was further characterized to obtain the main active components of Qingzhuan tea. Results: The ethyl acetate fraction had the greatest inhibitory effect on a-glucosidase, followed by n-butanol, sediment and residual aqua fractions (with the IC50 values of 0.26 mg/mL, 2.94 mg/mL, 3.02 mg/mL, and 5. 24 mg/mL, respectively), mainly due to the high content of polyphenols. Among the eight subfractions (QEF1-8) isolated from the ethyl acetate fraction, QEF8 fraction showed the highest a-glucosidase inhibitory potential in a competitive inhibitory manner (the K-i value of 77.10 mu g/mL). HPLC-MS analysis revealed that (-)-epigallocatechin gallate (EGCG) and (-)-epicatechin gallate (ECG) were the predominant active components in QEF8. Conclusion: These results indicated that Qingzhuan tea extracts exerted potent inhibitory effects against a-glucosidase, EGCG and ECG were likely responsible for the inhibitory activity in Qingzhuan tea. Qingzhuan tea may be recommended as an oral antidiabetic diet.