Isolation and characterization of antioxidant and antimicrobial compounds from Anacardium occidentale L. (Anacardiaceae) leaf extract

Abstract

The extracts obtained from Anacardium occidentale leaf were investigated for antioxidant and antimicrobial activities. Bioassay-guided fractionation using DPPH autobiographic analysis of the EtOAc soluble fraction of the crude extract resulted in the isolation of agathisflavone (1) and a mixture of quercetin 3-O-rutinoside (2) and quercetin 3-O-rhamnoside (3). Characterization of the isolated compounds was carried out using nuclear magnetic resonance (1D and 2D) and mass spectrometry. The antioxidant activities of the samples were determined using three complementary tests, namely; DPPH-radical scavenging, total antioxidant capacity (TAC) and ferric reducing antioxidant power (FRAP). The antimicrobial activity of the samples was evaluated using the disc diffusion and broth microdilution methods. The results indicated that the mixture (ratio 2:1) of compounds (2) and (3) was most active in its capacity to scavenge free radicals in the DPPH assay [IC50 = 0.96 ± 0.01 μg/mL] compared to the ethyl acetate (EtOAc) fraction, the ascorbic acid and quercetin standards. The mixture of compounds (2) and (3) exhibited the highest activities in the TAC assay (5332.00 ± 3.38 mgAAE/g, 8562.00±5.43mgQE/g) and FRAP assay (15,136.00 ± 7.14 mgAAE/g, 199,530.00 ± 94.12 mgQE/g) compared to the EtOAc fraction. The fractions of the extract showed comparatively better activities against gram-negative than the gram positive ones with the broadest spectrum of activities demonstrated for the ethyl acetate fraction. No inhibitions were obtained against the fungal strains investigated. The isolated compounds from the plant possess essentially inhibitory rather than cidal activities against these organisms with the Minimum Bactericidal Concentrations (MBCs) >2.0mg/mL in all cases. The mixture of compounds 2 and 3 showed higher activity than compound 1 with the former having MIC as low as 0.25mg/mL against Pseudomonas aeruginosa and Clostridium sporogens, 0.5mg/mL against Staphylococcus aureus and 1.0mg/mL against Escherichia coli and Klebsiella pneumonia strains investigated while the latter has an MIC of 1.0mg/mL against most of the organisms.This study concluded that the extracts and isolated compounds had strong antioxidant and moderate antibacterial activities and could be effective in the management of oxidative stress related diseases. These findings also justified the use of this plant's extracts in folk medicine.