Potent microbial and tyrosinase inhibitors from stem bark of Bauhinia rufescens (Fabaceae)

Abstract

The stem bark extracts of Bauhinia rufescens Lam. (Fabaceae) yielded 6-methoxy-7-methyl-8-hydroxydibenz[b,f]oxepin, α-amyrin acetate, β-sitosterol 3-O-β-D-xylopyranoside, 4-(2′-Hydroxyphenethyl)-5- methoxy-2-methylphenol, menisdaurin and sequoyitol. Their structures were determined using spectroscopic methods and comparisons with the literature data. For the antimicrobial assay Gram-positive and Gram-negative bacterial and fungal strains were tested, while the tyrosinase inhibition assay utilized L-DOPA as a substrate for the tyrosinase enzyme. 6-Methoxy-7-methyl-8- hydroxydibenz[b,f]oxepin, α-amyrin acetate, β-sitosterol 3-O-β-D-xylopyranoside, menisdaurin and sequoyitol showed weak to moderate activities with minimum inhibition concentration (MIC) values in the range of 112.5-900 μg/mL against all bacterial strains, while the MIC values for the fungal strains were in the range of 28.1-450 μg/mL. In the tyrosinase inhibition assay, α-amyrin acetate was found to be moderately active against tyrosinase with an inhibition of 62% at 0.1 mg/mL. This activity was lower than that of the positive control, kojic acid (85%).