Identification of chemical constituents from the bark of Larix kaempferi and their tyrosinase inhibitory effect

Most of the wood bark produced by the forestry production is discarded in spite of containing many kinds of the phytochemical ingredients. The aim of the present study was to identify secondary metabolites from the bark of Larix kaempferi generated as waste material and evaluate their potential as cosmetic agents. Eighteen compounds, including a novel phenanthrene, 4,6,7-trihydroxyphenanthrene-2-O-β-D-glucopyranoside (16), were isolated from the bark of L. kaempferi and identified by matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF-MS) and nuclear magnetic resonance (NMR). In addition, the tyrosinase inhibitory activity of these compounds was evaluated. Procyanidin B7 (18) exhibited the most potent inhibition with IC50 values of 31.0 μM and 61.8 μM when using L-tyrosine and L-dopa as the substrate, respectively, which were similar to those of the positive control, kojic acid. Interestingly, quercetin-3-O-α-L-rhamnopyranoside (10) was shown to possess the tyrosinase inhibition although the other series of 3-glycoylated flavonols were not active, suggesting that the rhamnosyl group at C-3 and the hydroxyl group at C-3ʹ played an indispensable role in the anti-tyrosinase activity. These findings indicate that a number of constituents from L. kaempferi bark may have potential as additives in cosmetics.