Tyrosinase inhibitory constituents from the stems of Maackia fauriei.

Bioassay-guided investigation of the stems of Maackia fauriei led to the isolation of seven flavonoid constituents, formononetin (1), genistein (2), daidzein (3), texasin (4), tectorigenin (5), odoratin (6) and mirkoin (7). Their structures were elucidated on the basis of spectral studies as well as by comparison with literature data. Tyrosinase inhibition activities were carried out on the isolated compounds. Among these, mirkoin (7) was identified as a potent tyrosinase inhibitor. It inhibited mushroom tyrosinase with an IC(50) value of 0.005 mm, which is ten times more active than kojic acid (IC(50) = 0.045 mm). The inhibition kinetics, analysed by Lineweaver-Burk plots, indicated mirkoin (7) to be a competitive inhibitor of tyrosinase when L-tyrosinase was used as a substrate. The results suggest that hydroxyl groups at C-4' in the B ring of flavonoids play an important role in the tyrosinase inhibition activities. Interestingly, compounds 4-7 were isolated for the first time from this plant.