Biol. Pharm. Bull. 30(9) 1711—1715 (2007)

layer of the dermis and is the major cellular component responsible for skin color. Although normal melanin pigmentation in the skin is the major defense mechanism against ultraviolet light, the production of abnormal pigmentation such as melasma, freckles, senile lentigines, and other forms of melanin hyperpigmentation can be a serious problem aesthetically. The novo synthesis of melanin involves the conversion of the amino acid, L-tyrosine to L-DOPA (3,4-dihydroxyphenylalanine), and then the oxidation of L-DOPA by tyrosinase, a rate-limiting enzyme, catalyzing step in melanin biosynthesis that yields dopaquinone. This altered tyrosinase process often causes abnormal accumulation of melanin pigments. Therefore, it has been common practice to include tyrosinase inhibitors such as kojic acid in the depigmentation ingredients of cosmetic products. Various tyrosinase inhibitors that have been reported include hydroquinone, ascorbic acid derivatives, azelaic acid, corticosteroids, retinoids and arbutin. Recently, phenolic compounds were shown to have a tyrosinase inhibitory effect, in addition to their known anti-oxidative and free radical-scavenging efficacies. These compounds also are shown to exhibit anti-inflammatory, antiviral, and anti-allergic activities in various biological systems. Some stilbenes have polyphenolic compounds that are reported to possess various biological activities that are anti-carcinogenic, anti-inflammatory, and inhibit tyrosinase. Resveratrol, a hydroxystilbene present at high concentrations in grape seeds and red wine, was recently shown to have a wide range of biological and pharmacological activities including as a potent antioxidant. Oxyresveratrol, one of the ingredients of the Mori cortex, is reported to exert a potent inhibitory effect on tyrosinase’s DOPA oxidase activity. At present, the production of skin-whitening cosmetics with oxyresveratrol as its basis depends on Mori cortex extract, because the quantity of the ingredient is limited and an effective method of synthesis has yet to be established. Therefore, a search for alternative materials continues, as with polyphenolic compounds, which have high bioactivity and easy accessibility. Through our continuous effort, we have found several potent tyrosinase inhibitors from natural materials that have an even greater potency than the well-known kojic acid. Recently, we succeeded in the synthesis of a new compound (patent application number: 102006-0071433), “HNB”, as a polyphenolic compound (Fig. 1). The aim of this current study was to characterize the inhibition mode of HNB on mushroom-derived tyrosinase. We also investigated the inhibitory effects of HNB on melanin biosynthesis, cell viability, and its depigmenting effect on the murine B16F10 melanoma cell line (B16 cells).