We previously reported (E)-β-phenyl-α,β-unsaturated carbonyl scaffold ((E)-PUSC) played an important role in showing high
tyrosinase inhibitory activity and that derivatives with a 4-substituted
resorcinol moiety as the β-phenyl group of the scaffold resulted in the greatest
tyrosinase inhibitory activity. To examine whether the 4-substituted
resorcinol moiety could impart
tyrosinase inhibitory activity in the absence of the α,β-unsaturated carbonyl moiety of the (E)-PUSC scaffold, 10 urolithin derivatives were synthesized. To obtain more candidate samples, the
lactone ring in synthesized urolithins was reduced to produce nine reduced urolithins. Compounds 1c (IC50 = 18.09 ± 0.25 μM), 1h (IC50 = 4.14 ± 0.10 μM), and 2a (IC50 = 15.69 ± 0.40 μM) had greater mushroom
tyrosinase-inhibitory activities than
kojic acid (KA) (IC50 = 48.62 ± 3.38 μM). The SAR results suggest that the 4-substituted
resorcinol motif makes an important contribution to
tyrosinase inhibition. To investigate whether these compounds bind to human
tyrosinase, a human
tyrosinase homology model was developed. Docking simulations with mushroom and human tyrosinases showed that 1c, 1h, and 2a bind to the active site of both tyrosinases with higher binding affinities than KA. Pharmacophore analyses showed that two
hydroxyl groups of the 4-substituted
resorcinol entity act as hydrogen bond donors in both mushroom and human tyrosinases. Kinetic analyses indicated that these compounds were all competitive inhibitors. Compound 2a inhibited cellular
tyrosinase activity and melanogenesis in α-
MSH plus
IBMX-stimulated B16F10
melanoma cells more strongly than KA. These results suggest that 2a is a promising candidate for the treatment of skin pigment disorders, and show the 4-substituted
resorcinol entity importantly contributes to
tyrosinase inhibition.