Biol. Pharm. Bull. 30(5) 873—879 (2007)

freckles and pigmented acne scars, are of serious concern to women. Their treatment usually involves the use of medicines or medicinal cosmetics containing depigmenting agents or skin-whitening agents. For example, hydroquinone and 4methoxyphenol are used as topical therapies for hyperpigmentation disorders. Active ingredients available for medicated whitening cosmetics in Japan include vitamin C and its stabilized phosphate and glucoside derivatives, kojic acid, arbutin, linoleic acid, tranexamic acid, 3-O-ethyl ascorbic acid and ellagic acid. Among them, kojic acid, arbutin, linoleic acid and ellagic acid are plant products. Kojic acid is a naturally occurring fungal metabolite produced from Asperigillus orzae. Arbutin is a naturally occurring glucosylated hydroquinone found in leaves of Arctostaphylos uva-ursi. Linoleic acid is contained in oilseed of Carthamus tinctorius and Helianthus annuus. Ellagic acid is contained in the shell of Caesalpinia spinosa fruits. These compounds inhibit the formation of melanin pigment by inhibiting the activity of tyrosinase, which is a key enzyme in melanin biosynthesis. Other tyrosinase inhibitors have been isolated from Morus alba L. (Moraceae), Machilus thunbergii (Lauraceae), Trichosanthes kirilowii (Cucurbitaceae), Piper longum (Piperaceae) and Artocarpus heterophyllus (Moraceae). On the other hand, linoleic acid, which is formed as a secondary metabolite in various plants, inhibits the formation of melanin pigment without inhibiting tyrosinase activity. Among the plant extracts we have studied, that of Arnica montana L. (Asteraceae) was also found to inhibit melanin accumulation without inhibiting tyrosinase activity. In this study, we isolated and identified the active compound, and investigated its mechanism of action.