Tyrosinase Inhibitory And Hepatoprotective Constituents From Selected Natural Plants

In this study, two tyrosinase inhibitors which isoliquiritigenin and honokiol were isolated from the root of Dalbergia odorifera and Magnoliae Cortex, and their structures were indentified with NMR ('H-NMR and ¹³C-NMR) technique as well as Mass. The kinetic study isolated compounds were tested with mushroom tyrosinase. Effect of isoliquiritigenin (1) isolated from the heartwood of Dalbergia odorifera T. Chen (Leguminosae) on mushroom tyrosinase activity was investigated in vitro using L-tyrosine and L-3, 4-dihydroxyphenylalanine (L-DOPA) as the substrates. When L-tyrosine was used as a substrate, both isoliquiritigenin (1) and kojic acid, a positive control, inhibited tyrosinase activity in a concentration-dependent manner. IC₅₀ values of isoliquiritigenin (1) and kojic acid were 61.4 and 52.2 μM, respectively. However, isoliquiritigenin (1) showed week inhibitory effect on the oxidation of L-DOPA by tyrosinase with inhibition ratio of 9.1 ± 7.1 % at 100 μM. It is also suggested that 3-unsubstituted and 4-hydroxyl phenyl group in isoliquiritigenin (1) plays an important role on the inhibition of tyrosinase activity when L-tyrosine was used as a substrate. Analysis of Lineweaver-Burk plot showed that isoliquiritigenin (1) acts as a competitive inhibitor in case of L-tyrosine as a substrate. However, effect of the neolignans,honokiol (2) and magnolol (3), isolated from Magnoliae Cortex on mushroom tyrosinase activity was investigated in vitro using L-tyrosine as a substrate. Honokiol (2) inhibited tyrosinase activity significantly in a concentration-dependent manner, on the other hand, magnolol (3) did not show tyrosinase inhibitory effect. Honokiol exhibited tyrosinase inhibitory effect with IC50 value of 67.9 jjM, and proved to act as a non-competitive inhibitor by the analysis of Lineweaver-Burk plot.Phytochemical investigation of the MeOH extract of the root barks ofCudrania tricuspidata Bureau (Moraceae) and Sanguisorbae Radix, asguided by hepatoprotective activity in vitro, furnished thirteen compounds cudratricusxanthone A (4), cudraxanthone L (5), cudratricusxanthone E (6), and macluraxanthone B (7), cudraflavone B (8), ziyuglycoside 1 (9), -(-)catechin (10), methyl (3-D-glucoside gal late (11), gallic acid (12), gallocatechin (13), methyl gallate (14), 4,5-Dimethoxy-3-hydroxybenzoic Acid Methyl Ester (15). Compounds 4-7 and 13 showed the significant hepatoprotective effect on tacrine-induced cytotoxicity in human liver-derived Hep G2 cells. Compounds 4, 5, and 7 also exhibited the significant hepatoprotective effect on nitrofurantoin-induced cytotoxicity in human liver-derived Hep G2 cells.