BIOACTIVE CONSTITUENTS AND TYROSINASE INHIBITORY ACTIVITIES OF SELECTED PLANTS

Abstract

The traditional use of plants against skin disease, and especially for cosmeticeutical purposes, is a common practice n the domestic medicine of many cultures. This study investigated the anti-tyrosinase and other related properties - phytochemical compositions, antioxidant and antibacterial properties of some selected plant parts used traditionally against skin disease, and especially for cosmeticeutical purposes, and the bioactive compound(s) in the most potent sample responsible for its skin lightening effect. Four plants part: Daturametel L. and Borreria verticillataL. leaves, the stem dye of Baphia nitida Lodd. and bark of Erythrophleum suaveolens (Guill & Perr.) were extracted with five solvent system; water, methanol, acetone, dichloromethane and hexane. Tyrosinase inhibitory screening of the extracts showed a dose-dependent inhibition, with the best two o-diphenolase activity on Mushroom tyrosinase observed for Erythrophleum suaveolens acetone extract (78.73%; IC50 of 39.2±0.20 µg/ml) and Borreria verticillata methanolic extract (80.82%; IC50 of 32.5±0.15 µg/ml) at 400 µg/ml which was comparable to that of kojic acid, the standard inhibitor. These best two extracts: Borreria verticillata methanolic extract and Erythrophleum suaveolens acetone extracts showed the presence of phytochemicals: glycosides, alkaloids, tannins, terpenoids, steroids, cardiac-glycosides and flavonoids. Also saponins was observed only in the former while the later showed little presence of phlabotannins and anthraquinones. Both extracts also exhibited a dose dependent activity in the antioxidant property assays considered and showed moderate antibacterial activity with the best inhibition observed for Borreria verticillata at concentration 0.1 mg/ml and zone of inhibition diameter of 18.5 mm for S. aureus and S. mercesens at 24 hours. Inhibitory kinetics by Erythrophleum suaveolens acetone extract was reversible and the extract was determined to be a mixed inhibitor while Borreria verticillata methanol extract diphenolase activity was also reversible and its inhibition non-competitive. Gas chromatography and Mass spectrometry results for the Erythrophleum suaveolens acetone extract revealed the presence of: Oleic acid (22.33%), n-Hexadecanoic acid (13.28%), 1,2,3-Benzenetriol (10.49%) and 1,3-Benzenediol (9.27%), while for Borreria verticillata methanolic extract the most abundant compounds were 9,12,15-Octadecatrienoic acid and n-Hexadecanoic acid. Fractionation of the best tyrosinase inhibiting extract (Erythrophleum suaveolens acetone extract) revealed the chloroform fraction was the most potent and its inhibition mechanism; reversible and non-competitive. The compounds identified in the fraction included fatty acids (Palmitic acid and Oleic acid), phyto sterols (stigma sterol and gamma-Sit sterol) and triterpenoids (Lupeol and Cycloeucalenol acetate). In silico study revealed interaction was done between constituents in the fraction and the tyrosinase architecture with the best docking energies observed for triterpenoids: Lupeol (-12.6 kcal.mol-1) and Cycloeucalenol acetate (-10.5 kcal.mol-1). Also the interaction of inhibitor with the enzyme showed that binding was done with another site different from the predicted active site of tyrosinase. Hence suggesting binding of inhibitor was done with an allosteric site which also confirms the non-competitive inhibition of the fraction. This study therefore confirms the antipigmenting properties of the studied plant parts used in folklore for cosmeceutical purposes and some of the bioactive constituents present in the most potent extracts’ contributing to its depigmenting property.