Bioactive compounds from Nauclea latifolia leaf extracts

The leaf extracts of Nauclea latifolia (Smith) were investigated for its bioactive constituents to validate it reported ethnomedicinal uses (infectious and oxidative stress related diseases). Isolation of antioxidant compounds from the ethyl acetate fraction was achieved by a DPPH directed fractionation, which resulted in trans-3,4-dihydroxycinnamic acid (caffeic acid) (1), quercetin (2), quercetin-3-O-β-glycopyranoside (3), 3-caffeoylquinic acid (chlorogenic acid) (4), and 3,5-O-dicaffeoylquinic acid (5). The extracts and isolated compounds were evaluated for antioxidant activities using five complementary assays: total antioxidant capacity (TAC), inhibition of nitric oxide (NO) radical activity, 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging activity, ferric reducing antioxidant potential (FRAP) and metal chelating ability (MCA) spectrophotometrically. The total phenolic content (TPC) was also determined. Antimicrobial activities of the samples were evaluated using agar cup plate diffusion and broth microdilution method. Compound (2) was more potent in the DPPH assay [IC50 = 0.59 ± 0.04 to 21.3 ± 1.01 µg/mL] than the ascorbic acid [8.82 ± 0.23 µg/mL] used as a standard. All the isolated compounds exhibited a higher degree of inhibition of NO radical than the standard. The IC50 ranged from 5.17 ± 2.59 to 55.2 ± 0.66 µg/mL compared with the standard, ascorbic acid [IC50 = 69.3 ± 1.78 µg/mL]. In the metal chelating ability (MCA) assay, compound (5) had better activity [IC50 = 105 ± 2.73 µg/mL] than the standard, EDTA [IC50 = 122 ± 1.17 µg/mL]. Compound (2) had the best TAC and FRAP results, 10.9 ± 2.14 mg AAE/g and 1.68 ± 0.09 mg AAE/g respectively. N. latifolia extracts exhibited less antibacterial activities than the isolated compounds. The MIC of quercetin (2) against S. aureus was 0.156 mg/mL while quercetin 3-O-β-glucopyranose exhibited wider spectrum of activities with MIC in the range of 1 mg/mL for most of the bacteria strains. Also, 3,5-O-dicaffeoylquinic acid had an MIC of 1.25 mg/mL against P. aeruginosa. Compounds 4 and 5 are reported from N. latifolia for the first time. This study concluded that some of the isolated compounds had strong antioxidant and moderate antimicrobial activities.