Structural characterization of complex tannins from Euryale ferox fruit peels and their inhibitory mechanisms against tyrosinase activity and melanogenesis

Tyrosinase is a rate-limiting enzyme for melanogenesis and abnormal melanin production can be controlled by utilizing tyrosinase inhibitory substances. To develop potent and safe inhibitors of tyrosinase, complex tannins a narrowly distributed plant polyphenols were prepared from the fruit peel of Euryale ferox (EPTs) and then structurally characterized, as well as investigated for their inhibitory effects and the involved mechanisms against tyrosinase activity and melanogenesis. The structures of EPTs were established to consist of 63.49% hydrolyzable tannins and 36.51% flavan-3-ol units. EPTs inhibited both the monophenolase and diphenolase activities of tyrosinase efficiently. This outstanding inhibition was presumably ascribed to the strong copper-ion chelating ability of EPTs and the microenvironment modification and secondary structure rearrangement of tyrosinase caused by the formation of EPTs-tyrosinase complexes. Treatment of EPTs to B16F10 cells also decreased the intracellular tyrosinase activity, induced apoptosis and G2/M cell cycle arrest, suppressed melanoma cell proliferation and downregulated the mRNA expression of tyrosinase, TRP-1 and MITF, consequently leading to a distinct reduction in melanin content. Furthermore, EPTs exhibited powerful antioxidant properties, which maybe contributed to impeding the initial steps of melanin formation. This study offered theoretical guidance for the potential applications of EPTs in cosmetic, functional food and medical industries.