Anti-tyrosinase Activity Of Constituents From The Leaves Of Morus Alba L.

Since melanin formation is the most important determinant of the color of mammalianskin, the inhibition of melanin formation may result in a reduction in skin darkness.Melanin is biosynthesized from tyrosine by enzymatic oxidation as well as anautooxidation process. Tyrosinase, the rate-limiting enzyme, catalyzes tyrosine to3,4-dihydroxyphenylalanine (L-DOPA) and further oxidizes it to dopaquinone, which isused for the ultimate formation of melanin. Therefore tyrosinase inhibitors may havepotential for treating abnormal pigmentation disorders and for use as skin-whitening agentsin the cosmetic industry. Several tyrosinase inhibitors, including arbutin and kojic acidhave been widely used for the purpose of skin whitening, especially in Northeast Asia.Some plant extracts such as Glycyrrhizae radix and Morus radix have also been used. Butthere is always a need for new skin-whitening agents.The leaves of Morus alba L. (Moraceae) have been used as a blood pressure depressantin China, South Korea and Japan from old times. Many flavones and their derivatives wereisolated as active principles from the root of this plant, but the constituents of the leaveshave not yet been thoroughly investigated. We have investigated the anti-tyrosinase activityof the leaves of this plant and found that the EtOAc extract of the leaves showedanti-tyrosinase activity. In order to investigate the relationship of the constituents and theanti-tyrosinase activity of the leaves, we have studied the constituents of the EtOAc extractof the leaves of M. alba.Seven known compounds were isolated from the leaves of M. alba, namelyβ-sitosterol(1), octanoic acid (2), daucosterol (3), kaempferol-3-O-β-D-glucopyranoside (4),quercetin-3-O-β-D-glucopyranoside (5), quercitrin (6), morin-3-O-β-D-glucopyranoside(7), were evaluated for their anti-tyrosinase activity. Compounds 4 and 5 were hydrolyzedwith HCl to give kaempferol (8) and quercetin (9) as their aglycones, and these were alsotested for their anti-tyrosinase activity. Kaempferol (8) and quercetin (9) showedanti-tyrosinase activity with 50% inhibitory concentrations (IC₅₀) values of 0.32 and 0.92mM, which showed much stronger activity than arbutin. Furthermore, compounds 4-7showed DPPH free radical scavenging effect with IC₅₀ values of 5.7, 4.6, 4.9, 4.2μM andABTS cation radical scavenging effects with IC₅₀ values of 15.4, 6.5, 9.2μM and 3.6μM, respectively.