Tyrosinase inhibitory constituents from the bark ofPeltophorum dasyrachis(yellow batai)

Tyrosinase inhibitory activity-guided fractionation of the bark of Peltophorum dasyrachis (yellow batai) led to the isolation of the six active compounds which were characterised as six flavonoids: apigenin (1), (+)-2,3-trans-dihydrokaempferol (2), (+)-2,3-trans-dihydrokaempferol-3-O-α-L-rhamnoside (3), (+)-4′,7-dimethoxy-2,3-trans-dihydroquercetin (4), (+)-2,3-trans-dihydroquercetin (5) and (–)-2,3-trans-dihydroquercetin-3-O-α-L-rhamnoside (6). All compounds were isolated for the first time from the bark of P. dasyrachis. Moreover, all compounds were evaluated for tyrosinase activities towards L-DOPA as the substrate. We observed that compounds 2 and 5 showed potent inhibitory effects (IC50 values were 126 ± 3.2 and 210 ± 5.8 µM, respectively). In general, for flavonoids the 3′,4′-dihydroxy group's substituent is a more potent inhibitor than the 4′-hydroxy group substituent, i.e. quercetin > kaempferol. Interestingly, our result in the oxidation of L-DOPA showed that the 4′-hydroxy group substituent (compound 2) is a more potent inhibitor than the 3′,4′-dihydroxy group substituent (compound 5). This result showed a new relationship between tyrosinase inhibitory activities and flavonoids. The kinetic analyses by Lineweaver–Burk plots showed that both the compounds 2 and 5 behaved as competitive inhibitors of L-DOPA oxidation.