Abstract:
Tyrosinase inhibitors are important for their potential applications in improving food quality, prevention of pigmentation disorder, and other melanin-related human health issues. The study was designed to evaluate the phytochemical constituents, antioxidant and tyrosinase inhibitory potentials of extracts of B. verticillata leaves with a view to evaluating the possible inhibitory activities of the extracts. The plant was extracted with methanol, acetone and dichloromethane and subsequently screened for the presence of secondary metabolites. The antioxidant potentials were evaluated by determining the total phenolic and flavonoid contents, reducing property, 2, 2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity, 2,2ʹ-Azinobis(3-ethylbenzo-thiazoline-6-sulphonate) (ABTS) radical scavenging ability, iron chelating property, hydroxyl radical scavenging ability and the extent of inhibition of Fe2+-induced lipid peroxidation in albino rat liver homogenate. The effects of extracts on tyrosinase were compared with kojic acid as standard inhibitor, using L-dihydroxylphenylalanin (L-DOPA) as substrate. Phytochemical analysis revealed that the extracts contain tannins, alkaloids, steroids, cardiac glycosides, and terpenoids while saponins was only present in methanol extract. The methanolic exytract was found to exhibit superior activity with the highest total phenolic content of 54.56 ± 0.55 (mg gallic acid equivalent /g extract). There was no significant difference between the total flavonoid content of the extracts (p< 0.05) and the reducing antioxidant power of the extracts were in order of 42.78 ± 0.48, 35.15 ± 1.10 and 33.42 ± 0.60 mg/g extract. In addition, methanolic extract exhibited the highest inhibition of lipid peroxidation, hydroxyl and ABTS radical scavenging and iron chelating effects. However, the strongest potency against DPPH radical was exerted by extract from acetone with percent inhibition of 41.69 ± 0.50 at 100 μg/ml. The extracts inhibited tyrosinase activity, which was concentration dependent with IC50 of 186.76, 199.6 and 267.66 μg/ml for methanol,
acetone and dichloromethane extracts respectively while kojic acid exhibited IC50 of 179.34 μg/ml. The results showed that B. verticillata leaf extracts contain molecules that exhibit potent and appreciable antioxidant and antityrosinase activity. It implies that the plant contain biomolecules that can be used as antioxidant agents.
