New olean-15-ene type gymnemic acids from Gymnema sylvestre (Retz.) R.Br. and their antihyperglycemic activity through α-glucosidase inhibition

Abstract

A mixture of gymnemic acids was precipitated from the water extract of leaves of Gymnema sylvestre (Retz.) R.Br. ex Sm. (Asclepiadaceae) by acidification with 2 N H2SO4. The chromatographic separation of the mixture afforded five new gymnemic acids (1-5). The compounds were characterized as Δ15 oleanane glycosides on the basis of extensive spectral data analysis. The compounds (1-5) showed dose dependent inhibition of α-glucosidase, which was found to be comparable to acarbose (IC50 95 μg/ml). Maximum inhibition was achieved with compound 4 (IC50 57 μg/ml) followed by 3 (IC50 62 μg/ml), 1 (IC50 80 μg/ml), 2 (IC50 120 μg/ml) and 5 (IC50 128 μg/ml). The results revealed that the overall pattern of hydroxyl and acyl substitutions of compounds affected their inhibitory activity. In oral sucrose tolerance test, pre-treatment with crude gymnemic acid mixture and isolated compounds 1 and 4 at a dose of 10 mg/Kg b.w. significantly blunted the effect of sucrose challenge in mice. Based on these results, the antihyperglycemic effect of G. sylvestre can be, at least partly, attributed to the inhibition of α-glucosidase by its gymnemic acids. The current study provides relatively more direct evidence explaining the effectiveness of G. sylvestre against hyperglycemia.