Inhibitory Actions of Lagerstroemia speciosa (L.) Pers. Aqueous and Ethanolic Leaf Extracts against Carbohydrate-digesting Enzymes

Context: The leaves of Lagerstroemia speciosa (L.) Pers. (banaba) have been consumed traditionally in several Southeast Asian countries for the treatment of diabetes mellitus. The plasma glucose lowering actions of this medicinal plant were proposed to be linked with several mechanisms of action. However, its inhibitory actions against α-amylase and α-glucosidase, which play a major role in the regulation of postprandial plasma glucose, have not been clearly established. Aims: This study thus aimed to investigate the effects of Lagerstroemia speciosa (L.) Pers. aqueous and ethanolic leaf extracts (LSA and LSE extracts, respectively) on in vitro carbohydrate-digesting enzyme activities and enzyme kinetics. Settings and Design: In vitro carbohydrate-digesting enzyme activity assay. Materials and Methods: The activities of α-amylase and α-glucosidase were indicated by the amounts of maltose and p-nitrophenol generated in the reactions, respectively. Statistical analysis used: The data were analysed by using one-way ANOVA, followed by Bonferroni post-hoc test. Results: The LSA extract significantly inhibited both α-amylase and α-glucosidase enzymes with the IC₅₀s of 1.21±0.16 and 49.71±0.86 μg/mL, respectively. Meanwhile, the LSE extract selectively inhibited α-amylase activity (IC₅₀ = 22.21±4.00 μg/mL) with no apparent inhibition against α-glucosidase activity. Both LSA and LSE extracts inhibited α-amylase enzyme in a mixed inhibition manner whilst the LSA extract also acted as a mixed inhibitor against α-glucosidase enzyme. The extracts possessed higher binding affinities toward the enzymes, indicated by the lower Ki values, when compared to acarbose (positive control). Conclusion: These results suggest the potential use of the extracts for a control of postprandial plasma glucose.