Title : Antioxidant activity, total phenolic and total flavonoid contents of the methanol extracts of the leaves and stems of milkweed, Gomphocarpus physocarpus (E.Mey)
The antioxidant activity of the methanol extracts of the stems and leaves of Gomphocarpus physocarpus (E.Mey.) (Apocynaceae) were assessed to determine the medicinal potential of this South African milkweed. The possible phytochemical sources, of the antioxidant activity were also evaluated by measuring the total phenolic (FolinCiocalteu method) and Flavonoid contents (Aluminium chloride method). Histochemical tests were conducted to confirm the location of these compounds. The EC50 based on the DPPH radical scavenging activity was similar for the leaf (9.93±0.11 µg/ml) and stem (11.37±0.46 µg/ml) extracts which was significantly lower than the standard antioxidant, ascorbic acid (13.52±0.41 µg/ml). The total phenolic (8.7±0.41 mg GAE/g DW) and flavonoid contents (2.91±0.21 mg QE/g DW) of the stem extracts are possibly responsible for the strong antioxidant activity. Histochemical analyses and fluorescence microscopy revealed the location of phenol accumulation to be in the laticiferous and parenchymatic tissues of the stem cortex and pith. It is possible that the plant accumulates phenolic compounds as part of its chemical defense strategy against insect herbivory. This defense strategy may in fact be a major source of novel antioxidant compounds.