Isolation and characterization of antioxidant constituents of the fruit of Telfairia occidentalis Hook F (Cucurbitaceae)

Purpose: To evaluate the antioxidant property of the fruit of Telfairia occidentalis and isolate the components responsible for the antioxidant activity. Methods: The fruit pericarp was macerated with methanol and the extract obtained successively partitioned with n-hexane, dichloromethane and ethyl acetate. The in vitro antioxidant activity of the extract and fractions was evaluated using 2, 2-diphenyl-1-picryl hydrazyl (DPPH) radical scavenging, reducing power, nitric oxide scavenging, total antioxidant and hydrogen peroxide scavenging assays. The n-hexane fraction, which had the highest DPPH scavenging and total antioxidant activities, was subjected to column and thin layer chromatography to isolate the components. The isolated compounds were identified by ultraviolet-visible (UV), nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR) spectroscopy and mass spectrometry. Results: Among the fractions tested, n-hexane had the best total antioxidant activity of 99.44 % at 20 mg/ml (p < 0.05) compared to ascorbic acid at 99.71 % of 20 mg/ml. This fraction also had the highest DPPH radical scavenging activity of all the fractions (p < 0.05) at all test concentrations. For nitric oxide scavenging activity, the whole extract, and the chloroform and aqueous fractions exhibited activity ranging from 92.29 to 97.66 % compared to 98.93 % for ascorbic acid. The hydrogen peroxide scavenging activity of the extract and fractions ranged from 92.60 to 96.23 % compared with of the standard, ascorbic acid (101.68 %). The major components of the n-hexane fractions were alpha- amyrin and beta-amyrin. Conclusion: The fruit pericarp of Telfairia occidentalis possesses good DPPH radical scavenging activity. This is the first time the antioxidant activity of the fruit (pericarp) and the presence of alpha- and beta- amyrins in Telfairia occidentalis have been reported.