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J. Cosmet. Sci., 64, 99–110 (March/April 2013) 99 Inhibitory mechanism of red globe amaranth on tyrosinase YAN MU, LIN LI, YONG ZHOU, HAI-LIU WEI, and SONG-QING HU, Guangdong Province Key Laboratory for Green Processing of Natural Products and Product Safety, College of Light Industry and Food Sciences, South China University of Technology, Guangdong 510640, People’s Republic of China (Y.M., L.L., H.-L.W., S.-Q.H.) and School of Software Technology, Dalian University of Technology, Liaoning 116024, People’s Republic of China (Y.Z.). Accepted for publication June 19, 2012. Synopsis Tyrosinase inhibitors from natural plants are currently attracting great interest. In this study, vanillic acid (VA) from red globe amaranth fl ower was identifi ed as an effective tyrosinase inhibitor. The 50% inhibitory concentration values of VA were 0.53 and 0.63 mg/ml for the monophenolase and diphenolase activities of tyrosinase, respectively. VA did not function as a simple copper chelator, and it did not induce detectable changes in the enzyme conformation. An investigation into the interaction between VA and tyrosinase by docking method revealed that VA was bound to residues at the entrance to the dicopper center. This suggests that VA could strongly inhibit tyrosinase activity by hampering the binding of substrates to tyrosinase. Because of the stability of the complex, VA hindered binding of monophenol substrates better than that of diphenol substrates, which resulted in different inhibitory effi cacies. A study of the mechanism of tyrosinase inhibition provided new evidence to elucidate the molecular mechanism of depigmentation by red globe amaranth plant. INTRODUCTION Melanin plays an important role in skin pigmentation, and it is synthesized from tyro- sine by tyrosinase (EC 1.14.18.1). Melanin is widely distributed in nature it is found in many organisms, including microorganisms, plants and animals and catalyzes two key reactions in the melanin biosynthesis pathway: the hydroxylation of monophenol to o-diphenol (monophenolase activity) followed by the oxidation of o-diphenol to the corresponding o-quinone (diphenolase activity), which can polymerize spontaneously to form melanins (1–3). It is well documented that tyrosinase is an essential enzyme, and it is thought to be the rate-limiting enzyme in melanin synthesis. (4). Although the production of melanin in human skin is a major defense mechanism against solar irradiation, the abnormal production of the pigment can lead to melasma, freckles, Address all correspondence to Song-Qing Hu at fesqhu@scut.edu.cn.