Isolation and characterization of antioxidant compounds from Annona senegalensis leaf extracts
Annona senegalensis leaf extract is used in the management of oxidative related diseases. In this work, we investigated the plant’s crude extracts for its bioactive constituents with a view to identify new and potent natural antioxidant compounds from natural source. A DPPH assay directed fractionation of the ethyl acetate fraction was achieved by open column chromatography using silica gel or Sephadex LH-20 as a stationary phase. Two compounds were isolated and characterized using Nuclear Magnetic Resonance (NMR) and infrared spectroscopy. The compounds were identified as: 2-(3,4-dihydroxyphenyl)- 3,4-chroman-3,5,7-triol, catechin (1), and quercetin-O-glycoside (2). Quantitative antioxidant activities of the extracts and isolated compound (1) were evaluated using: Total Antioxidant Capacity (TAC), Ferric Reducing Antioxidant Power (FRAP), DPPH spectrophotometric, Nitric Oxide scavenging (NO) and Ferrous Ion Chelation assays. In addition, Total Phenol Content (TPC) of each of the samples was determined. In DPPH assay, compound 1 exhibited highest activity, IC50 = 0.03 ± 0.01 mg/mL. The ethyl acetate and butanol fractions were the most active fractions. All the samples tested in this assay are less potent than the ascorbic acid used as a standard, IC50 = 0.01 ± 0.00 mg/mL. In NO assay, ethyl acetate fraction was the most potent among the solvent fractions but less potent than the ascorbic acid (standard). Compound 1 demonstrated comparable activity to the standard with an IC50 = 0.05 ± 0.00 mg/ mL. In ferrous ion chelation assay, ethyl acetate fraction exhibited the highest chelating activity followed by catechin (1) with IC50 of 0.24 ± 0.01 and 1.29 ± 0.03 mg/mL respectively. Both samples are less potent (IC50 =0.05 ± 0.00) than ethylenediamine tetraacetic acid used as a standard. In TAC assay, ethyl acetate fraction exhibited the best activity followed by catechin which showed a better activity expressed as ascorbic acid equivalence per gram (66.57 ± 2.73 and 40.12 ± 3.17 mg AAE/g) respectively. The reverse was observed in FRAP assay, catechin exhibited the best activity with 30.02 ± 0.21 mg AAE/g followed by ethyl acetate fraction which showed a better activity (22.09 ± 2.69 mg/mL). The amount of Total Phenolic Content in the samples ranged from 13.50 to 51.91 gallic acid equivalents (GAE mg/g) with catechin having the highest phenolic content followed by the ethyl acetate fraction. This study concluded that A. senegalensis leaf extracts and its constituent (catechin) possessed moderate antioxidant activity providing rationale for the ethnomedicinal uses of this plant in the management of oxidative stress related diseases.
Keywords: Annona senegalensis, Annonaceae, antioxidant activity