Evaluation of antioxidant properties and alpha-amylase inhibitory activities of some medicinal plants

Abstract

Antioxidant compounds are claimed to have an impact in disease prevention. The objective of this study was to evaluate the total phenolics content, antioxidant and alpha- amylase inhibitory activities of the leaf extracts of Averrhoa bilimbi (AB), Argyreia populifolia (AP), Costus speciosus (CS), Desmodium gangeticum (DG), Solanum nigrum (SN), Spondias dulcis (SD),Wattakaka volubilis (WV) and Ziziphus rugosa (ZR) which are consumed in traditional and Ayurvedic medicine in Sri Lanka. The dried plant materials were sequentially extracted with n-hexane, ethyl acetate and methanol. The total phenolics content (TPC) was quantified in the ethyl acetate extracts using the Folin-Ciocaltaeu method and values were expressed as miligrams gallic acid equivalents per gram (mg GAE/g). Oxygen Radical Absorbance Capacity (ORAC) and Ferric ion Reducing Antioxidant Power (FRAP) was evaluated using a trolox standard curve, and both values were expressed as μmol Trolox Equivalents per gram (μmol TE/g). Activity of α- amylase inhibition was assessed using 3, 5-dinitrosalicylic acid reagent and values were expressed as IC50 (ppm). TPC values were highest in SD (12.5 ±0.54 mg GAE/g) while SN had the lowest value (1.33 ±0.01 mg GAE/g). ORAC values varied between 404.8 ± 32.80 and 2613 ±191.3 μmol TE/g and decreased in the order of SD > DG > AP > WV > SN > ZR > AB > CS. FRAP values were highest in SD (521 ± 25.0 μmol TE/g) while the lowest was reported for SN (66.9 ± 1.45 μ mol TE/g). Statistical analysis was conducted using SPSS version 20 (USA). FRAP values were linearly correlated to TPC values with an R² of 0.936. However, there was no significant correlation between TPC and ORAC values (R²=0.392). SD had the highest values for TPC, ORAC and FRAP assays. The IC₅₀ for α-amylase inhibitory activities was above 1000 ppm for all extracts. It may be concluded that some of these herbs are potent antioxidants and contain a considerable amount of phenolic compounds. Hence, further evaluation of their therapeutic potential may be carried out through activity fractionation.