Comparative evaluation of rosmarinic acid, methyl rosmarinate and pedalitin isolated from Rabdosia serra (MAXIM.) HARA as inhibitors of tyrosinase and α-glucosidase

Rabdosia serra has been used in traditional Chinese medicine for centuries. In order to illustrate the pharmaceutical activity of R. serra as hypoglycaemic and skin-whitening agents, rosmarinic acid (confirmed as the major compound in R. serra), methyl rosmarinate and pedalitin isolated from R. serra were evaluated for their inhibitory effects and mechanisms on tyrosinase and α-glucosidase. The inhibitory effects on both tyrosinase and α-glucosidase were in decreasing order, pedalitin>methyl rosmarinate>rosmarinic acid. The IC50 values for the tyrosinase and α-glucosidase activity inhibited by pedalitin were 0.28 and 0.29mM, respectively. Both rosmarinic acid and methyl rosmarinate were considered as noncompetitive inhibitors of tyrosinase, while pedalitin was suggested to be a mixed-type inhibitor of tyrosinase. In the assay of α-glucosidase inhibition, rosmarinic acid was found to be a competitive inhibitor, whereas both methyl rosmarinate and pedalitin were considered as mixed-type inhibitors.