In vitro Free Radical Scavenging Activities of the Saponin-rich Fractions from Vernonia amygdalina Del. (Compositae)

Abstract

Vernonia amygdalina leaves is consumed fresh or dried after removal of bitter taste by the peoples of the South-Eastern Nigeria, especially the Igbos. It is also widely used in cooking and serves as a vegetable and ethno-medicinal herb against many ailments. Not much is known of its in vitro antioxidant potentials, especially the major saponins and those of its fractions. In this study, the 80 % methanol extract of the leaves was fractionated by analytical TLC-guided column chromatography, preparative TLC and further purified with HPLC using gradient concentrations of solvent mixtures of chloroform, n-butanol, methanol and distilled water yielding VASC, VASB, VASM70, VASM65 and VASM50. Quantitative phytochemical analysis gave their % total saponin content as 55.8, 64.6, 21.4, 78.8 and 33.8 respectively. The 5 saponin-rich fractions (VASC, VASB, VASM70, VASM65 and VASM50) showed no significant signs of toxicity in mice at oral doses of 1500 mg/kg although VASM50 at 1500 mg/kg caused increased urination and watery fecal deposits which disappeared after 18hrs. The saponin-rich fractions of Vernonia amygdalina were evaluated for in vitro free radical scavenging properties using 2, 2-diphenyl-1-picrylhydrazyl (DPPH.), ferric reducing antioxidant power (FRAP), hydrogen peroxide scavenging assay, hydroxyl radical scavenging assay, ABTS radical cation scavenging activity assay, anti-lipid peroxidation assay, B-carotene bleaching assay and superoxide anion using ascorbic acid, butylated hydroxytoluene (BHT), catechin and gallic acid as reference standards. The results showed that the saponin-rich fractions from Vernonia amygdalina leaves had a potent DPPH radical scavenging and FRAP activities comparable to those of reference standards used. The IC50 values were also comparable to those of standards especially that of VASM65. Vernonia species are known to contain abundant saponins and flavonoids which are polar compounds and readily soluble in methanol. Most saponins and their derivatives of plant origin are known to possess great antioxidant potentials. This may explain the above observed antioxidant activities and thus, the use of the leaves in different traditional curative therapies in Eastern Nigeria, and as vegetable.