Tyrosinase Inhibitory Constituents from the Roots of Morus Nigra: a Structure-activity Relationship Study

The phytochemical profiles of Morus nigra roots and twigs were compared by HPLC with those of the old and young twigs of Morus alba which are known to contain oxyresveratrol and mulberroside A as major components. It was found that M. nigra root extract contains some unknown natural products with potential tyrosinase inhibitory activity. The extract (95% ethanol) of the roots of M. nigra was further investigated in this study. One new compound, 5'-geranyl-5,7,2',4'-tetrahydroxyflavone, and twenty-eight known phenolic compounds were isolated. Their structures were identified by mass spectrometry and NMR spectroscopy. Nine compounds, 5'-geranyl-5,7,2',4'-tetrahydroxyflavone, steppogenin-7-O-beta-D-glucoside, 2,4,2',4'-tetrahydroxychalcone, moracin N, kuwanon H, mulberrofuran G, morachalcone A, oxyresveratrol-3'-O-beta-D-glucopyranoside and oxyresveratrol-2-O-beta-D-glucopyranoside, showed better tyrosinase inhibitory activities than kojic acid. It was noteworthy that the IC(50) values of 2,4,2',4'-tetrahydroxychalcone and morachalcone A were 757-fold and 328-fold lower than that of kojic acid, respectively, suggesting a great potential for their development as effective natural tyrosinase inhibitors.