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Tyrosinase inhibition by extracts and constituents of stembark of Sideroxylon inerme L
The stembark of Sideroxylon inerme is traditionally used as a paste in KwaZulu-Natal province, South Africa for lightening skin. . Skin lightening can be achieved by the use of inhibitors of tyrosinase, a key enzyme in the formation of the pigment melanin. A methanolic extract of the stembark gave inhibition of monophenolase tyrosinase in a colorimetric assay with IC50 value of 82.1±2.7µg/mL . TLC examination of this extract using silica gel/ethyl acetate:formic acid: acetic acid: water 13.5:10:1: 1.5 showed two inhibitory zones when sprayed for monophenolase inhibitory activity . Column chromatography (Sephadex LH-20 with water:ethanol gradient from 0% to 100% ethanol) with TLC monitoring of eluants led to the isolation of epicatechin gallate and procyanidin B1, their structures being determined by spectroscopic methods.
When tested in the colorimetric assay these compounds were found not to be active against diphenolase (IC50 >200µg/mL) but displayed activity against monophenolase with IC50 values of 30±1.9µg/mL (65.5µM) for epigallocatechin gallate and 200±2.2µg/mL (346.0µM) for procyanidin B. Using the DPPH colorimetric test for radical-scavenging activity  both compounds showed a strong effect with IC50 values of 1.33µg/mL (2.9µM) for epigallocatechin gallate and 1.68µg/mL (4.9µM) for procyanidin B.
The traditional use of the bark of S. inerme is therefore supported by these findings and the major activity appears to be due to epigallocatechin gallate.
This work was supported by the NRF/Royal Society
References: 1. Van Wyk E., Gericke N (2000) People's plants: A guide to useful plants of southern Africa. Briza. Pretoria. 2. Nerya, O. et al. (2003)J. Agric. Food Chem. 51:1201–1207. 3. Wangthong, S. et al. (2007) Biochem. Chromatog. 21:94–100. 4. Toit, R. et al. (2001) Toxicology 166:63–69.