Comparative antioxidant activity, total phenol and total flavonoid contents of two Nigerian ocimum species

Abstract

Antioxidants are compounds which act as a major defense against oxidative stress caused by free radicals. In this study, a comparative evaluation of the antioxidant properties, phenolic and flavonoid contents of the methanolic extracts of Ocimum gratissimum Linn and Ocimum canum Sims was carried out. Crude extracts of both plants were obtained by maceration of powdered plant materials in methanol (80%) for 24hrs. The phenolic and flavonoid contents were determined using standard methods while the antioxidant capacities were evaluated using six different in vitro radical scavenging assays: total antioxidant potential, reductive potential, I, 1-diphenyl-2-picryl-hydrazyl (DPPH) free radical scavenging, lipid peroxidation inhibition, hydroxyl radical and nitric oxide scavenging activity. The total phenolic content in O. gratissimum and O. canum were found to be 32.66 ± 6.21 and 17.19 ± 2.54 mg GAE/g dw while total flavonoid content gave 1.94 ± 0.23 and 0.67 ± 0.01 mg QUE/g dw, respectively. Both extracts had effective reductive potential as well as exhibited strong total antioxidant capacity with increasing concentration of extract. Comparatively, O. gratissimum exhibited a significantly (p < 0.05) higher capacity to quench the DPPH free radical with IC₅₀ value of 26.01 ± 3.2 µg/ml than O. canum, which has an IC₅₀ value of 60.45 ± 5.22 µg/m. O. gratissimum also significantly inhibited membrane lipid peroxidation and hydroxyl radical formation with IC₅₀ of 99.37 ± 8.56 µg/ml and 465.33 ± 21.62 µg/ml, respectively while O. canum correspondingly gave IC₅₀ values of 447.5 ± 35.61 µg/ml and 868.16 ± 43.05 µg/ml. In the nitric oxide scavenging activity, however, O. canum showed a stronger inhibitory effect than O. gratissimum as indicated by their IC₅₀ values of 277.22 ± 15.09 µg/ml and 731 ± 56.99 µg/ml, respectively. These activities are however several folds lower than those of butylated hydroxyl toluene (BHT), ascorbic acid and quercetin used as antioxidant standards. The results demonstrate that O. gratissimum has greater antioxidant capacity than O. canum because of its relatively higher radical scavenging activity in all antiradical tests carried out except the nitric acid scavenging test and higher contents of flavonoid and phenolic compounds. O. gratissimum is therefore more beneficial therapeutically than O. canum since it provides better defense against free radical induced oxidative stress, and this attribute probably explains its relative preference in ethnomedicine for both culinary and medicinal purposes amongst the Ocimum species widely cultivated in South Western Nigeria.

Keywords: Antioxidant activity, phenolic content, DPPH, Ocimum gratissimum, Ocimum canum.