Evaluation of Phytoconstituents, Antioxidants Potential, Cytotoxic, Antimicrobial Activities and Mineral Composition of Vigna subterranea (L) Verdic. Extracts

Abstract

Vigna subterranea is a leguminous drought tolerant crop of African origin which has food, nutrition and medicinal value. Nuts were investigated for their phytochemicals, cytotoxicity, antioxidants, antimicrobial activity and mineral composition.Nut landraces were selected from Western part of Kenya. Extraction of the six nut cultivars was done using single extraction method of 1:1 ratio of methanol: dichloromethane solvent. Extracts were analyzed for their phytochemical composition using gas chromatography-mass spectroscopy, cytotoxic activity by MTT assay against two human cancer cells (DU 145 & Hep 2) and a non-cancerous cell (Vero), antioxidant potential against DPPH, antibacterial activity to evaluate against Staphylococcus aureus, Escherichia coli, Pseudomonas auriginosa using the Kirby-Baurer diffusion test and antifungal activity to assay against the Candida albicans and mineral composition analysis for macro and micro elements. Data for cytotoxicity and antioxidant activities were expressed as mean of three-independent experiments. The analysis was performed using one way ANOVA and two-tailed students t-test from SPSS program. With the aid of NIST library the peaks from GC-MS analyses were compared and identified. Terpenoids, saponins, flavonoids and alkaloids were identified. The dark brown variety extract exhibited cytotoxicity against Hep 2 with IC50 value of 15.5 μg/ml but showed less (41.1 μg/ml) for DU 145 cell line. The extracts exhibited cytotoxicity against Vero cells on dilution by giving IC50 values with dark brown varieties at 19.2 μg/ml. Furthermore, the extracts of Vigna subterranea varieties of dark brown and cream spotted had the highest cytotoxicity against Hep 2 cancer cell line with IC50 of 15.5 and 17.9 μg/ml respectively. Additionally, manganese, magnesium and potassium, sodium, iron and calcium were identified in the extracts. Extracts inhibitory potentials towards the isolated strains were varied with E. coli showing zone of inhibition at 27±0.7 mm, S. aureus; 25.3±0.2 mm, and P. aeruginosa at 25.1±0.2 mm compared to 37.0±0.5, 41.3±0.9 and 42.3± 0.9 respectively for ceftriaxone. The black variety extracts had the greatest (91.1 %) inhibitory effect on mycelial growth of C. albicans at 4 μg/ml as compared to xv Clotrimazole (95.3%) at the same concentration with insignificant values from negative control. The antioxidant potential on comparison of ascorbic acid and cultivars of Vigna subterrenea against DPPH scavenging assay method gave IC50 values at 10.1 and 25± 1.4 μg/ml respectively. GC-MS analysis of extracts led to identification of several phytocomponents viz. farnesyl alcohol, 2-chloroethyl linoleate, 7-octadecenoic acid methyl ester, methyl behenate, trans-2- methyl-4-n-butylthiane- s,s –dioxide, methyl arachate, (-Z)-octadecenyl ester, 9,17-decadecadienal (Z) and palmitic acid. These results indicate the possible potential medicinal use of nuts because of the biological activities it exhibits. Use of GC-MS analysis is the first step towards understanding the nature of phytocomponents in this nut. The isolation of individual bioactive components and modifications are necessary for future studies and to help find new and better drugs.