Antioxidant Activity and Starch-digesting Enzyme Inhibition of Selected Thai Herb Extracts

Abstract

The objective of this study was to screen the selected Thai herb extracts for antioxidant activity and potential inhibition of starch-digestive enzymes. Ethanolic extracts of eight Thai herbs, leaves of Coccinia grandis, leaves and fruits of Aegle marmelos Corr., fruits of Momordica charantia, leaves and sheaths of Moringa oleifera Lam., leaves of Lagerstroemia speciosa L., and bark of Cinnamomum verum J.Presl, were selected for evaluation in terms of their polyphenol compounds, antioxidant activity, and in vitro potential inhibition against a-amylase and a-glucosidase. The contents of the total phenolic compounds, flavonols, and flavonoids ranged from 20.31 mg gallic acid (GA)/g extract to 391.80 mg GA/g extract, from 5.22 mg quercetin (QE)/g extract to 52.39 mg QE/g extract, and from 5.15 mg QE/g extract to 108.74 mg QE/g extract, respectively. A high correlation between the total phenolic content and the total flavonoid content was observed. The antioxidant activities were expressed as the concentration of extract required to exhibit 50% inhibition of DPPH and ABTS radical scavenging activities, ranging from 0.04 mg/mL to 3.48 mg/mL and from 0.03 mg/mL to 1.74 mg/mL, respectively. The leaves of L.speciosa with cold extraction had the highest total phenolic content, total flavonoid content, and antioxidant activity (both DPPH and ABTS methods). The antioxidant activity correlated well with the flavonoid and the phenolic contents. The potential inhibition (IC50) against human saliva a-amylase varied from 0.57 mg/mL to >5 mg/mL, porcine pancreas a-amylase varied from 0.19 mg/mL to >5 mg/mL, and yeast a-glucosidase varied from 0.04 mg/mL to >5 mg/mL. The leaves of L.speciosa with cold extraction had the highest potential of yeast a-glucosidase inhibition. As for a-amylase inhibition, C.verum bark extracted using hot method showed the best activity. The inhibitory activity of these herb extracts against the starch-digesting enzymes displayed significant correlation with phenolic content, flavonoid content, and antioxidant activity. In conclusion, the results indicated that L.speciosa leaves and C.verum bark extracts could be important Thai herbs with high potential antioxidant and starch-digestive inhibition activities.