Identification of α-glucosidase inhibitors from a new fermented tea obtained by tea-rolling processing of loquat (Eriobotrya japonica) and green tea leaves

Background: A new fermented tea produced by tea-rolling processing of loquat (Eriobotrya japonica) leaf with green tea leaf (denoted as LG tea) showed a potent antihyperglycaemic effect in maltose-loaded rats. The aim of this study, therefore, was to identify α-glucosidase inhibitors in the antihyperglycaemic tea product. Results: LG tea had a threefold higher maltase-inhibitory activity (IC₅₀ 0.065 mgdried extractm L_-1) than either the constituent loquat leaf or green tea alone. In addition, LG tea favourably inhibited maltase action rather than sucrase action. As a result of bioguided high-performance liquid chromatography separations of LG tea, theasinensin A, theasinensin B, strictinin and 1,6-digalloylglucose were newly identified asmaltase inhibitors with IC50 values of 142, 225, 398 and 337 μmol L^-1 respectively, along with previously identified catechins and theaflavins. Conclusion: Judging from the magnitude of the α-glucosidase-inhibitory contribution of each isolated compound to the overall inhibition of LG tea, catechins were the main candidates responsible for α-glucosidase or maltase inhibition in LG tea, followed by theaflavins, theasinensins, strictinin and 1,6-digalloylglucose.