Alpha-Glucosidase Inhibitory Effect of Crude Extracts from Selected Philippine Medicinal Plants

Abstract

Background and Objectives: Despite the growing number of herbal preparations being marketed as
supplements to treat diabetes mellitus, no sufficient data are currently available to guarantee the effectiveness of such preparations. The study, therefore, aimed (1) to provide additional data by investigating the efficacy of four medicinal plants in reducing the breakdown of carbohydrates to glucose via a-glucosidase inhibition, and (2) to determine the active principles responsible for such activity.

Methodology: Spectrophotometric method using 96-well plates was employed to determine inhibition of
yeast a-glucosidase by the different plant extracts. IC50, a measurement of inhibitory activity, was extrapolated from the concentration-% inhibition graphs generated. Phytochemical screening was done using the standard methods available in literature for the determination of reducing substances, tannins or polyphenols, flavonoids, glycosides, steroids and triterpenoids and alkaloids.

Results: Out of the 16 crude extracts prepared, the ethanolic extracts of Lagerstroemia speciosa and Syzygium samarangense showed concentration-dependent % inhibition, with IC50 of 116.64 ± 6.71 ug/mL and 111.76 ± 7.88 ug/mL, respectively. Phytochemical screening of L. speciosa ethanolic extract indicated presence of tannins, triterpenes or sterols, flavonoids and anthraquinones. Tannins, sterols or triterpenes and flavonoids were identified in the crude ethanolic extract of S. samarangense. Upon subjecting this to modified Kupchan fractionation, the ethyl acetate and aqueous fractions showed the highest activity against the enzyme with IC50 of 124.45 ± 3.23 ug/mL and 107.19 ± 2.78 ug/mL, respectively. Flavonoids were the only phytochemicals detected exclusively in these fractions of S. samarangense.

Conclusion and Recommendations: Among the four medicinal plants with reported anti-diabetic property, L. speciosa and S. samarangense were able to inhibit a-glucosidase. Triterpenes or sterols are considered the active constituents in L. speciosa, while flavonoids are those considered active in S. samarangense. Further characterization through isolation and elucidation of the a-glucosidase inhibitor/s present in the crude ethanolic extract is needed to identify the exact chemical composition of the enzyme inhibitor.