Antimicrobial and antioxidant flavonoids from the leaves of Oncoba spinosa Forssk. (Salicaceae)

BACKGROUND Naturally occurring flavonoids have been reported to possess various pharmacological properties. The aim of this study was to evaluate the antimicrobial and antioxidant activities of the MeOH extract and flavonoids from the leaves of Oncoba spinosa, a plant used for the treatment of syphilis, wounds and sexual impotence. METHODS The plant extract was prepared by maceration in methanol and sequentially fractionated by column chromatography. The structures of isolated compounds were elucidated on the basis of spectral studies and comparison with published data. The MeOH extract and its isolated compounds were evaluated for their antibacterial and antifungal activities by broth microdilution method. The 1,1-diphenyl-2-picrylhydrazyl (DPPH) and trolox equivalent antioxidant capacity (TEAC) assays were used to detect the antioxidant activity. The samples were tested spectrophotometrically for their hemolytic properties against human red blood cells. RESULTS The fractionation of the MeOH extract afforded five known flavonoids including kaempferol (1), quercetin (2), apigenin-7-O-β-D-glucuronopyranoside (3), quercetin 3-O-β-D-galactopyranoside (4) and quercetin 3-O-α-L-rhamnopyranosyl (1 → 6) β-D-glucopyranoside (5). The MeOH extract displayed weak to moderate antimicrobial activities (MIC = 256-2048 μg/ml). Quercetin 3-O-α-L-rhamnopyranosyl (1 → 6) β-D-glucopyranoside (5) and quercetin (2) were respectively the most active compounds against bacteria (MIC = 8-64 μg/ml) and fungi (MIC = 64 - 128 μg/ml). These tested samples also showed high radical-scavenging activities (EC50 = 5.08 - 70.56 μg/ml) and gallic acid equivalent antioxidant capacities (TEAC = 53.76 - 89.86 μg/ml) when compared with vitamin C (EC50 = 4.72 μg/ml). The MeOH extract and compounds 2-5 were non-toxic to human red blood cells indicating their high selectivity to be used as antimicrobial and antioxidant drugs. CONCLUSION The MeOH extract of O. spinosa as well as compounds 2 - 5 could be a potential source of natural antimicrobial and antioxidant products.