The role of sulfhydryl compounds in mammalian melanogenesis: the effect of cysteine and glutathione upon tyrosinase and the intermediates of the pathway.

The effect of cysteine and glutathione on mammalian melanogenesis has been studied. It has been shown that their action is mediated by two different mechanisms. (a) The reaction of the thiol groups with dopaquinone after the tyrosinase-catalyzed oxidation of tyrosine and dopa. This mechanism leads to the formation of sulfhydryl-dopa conjugates and finally sulfur-containing pigments, phaeomelanins instead of eumelanins. This fact might produce an inhibition of melanogenesis due to the slower rate of chemical reactions involved in the polymerization of such thiol-conjugates when compared to that of indoles. (b) The direct interaction between the sulfhydryl compounds and the tyrosinase active site. This interaction may regulate the activity of the enzyme. It is shown that Harding-Passey mouse melanoma tyrosinase is more sensitive to sulfhydryl compounds than mushroom tyrosinase. Cysteine always produces an inhibition of the tyrosinase hydroxylase and dopa oxidase activities of melanoma tyrosinase, this inhibition becoming greater as the cysteine concentration increases. On the other hand, glutathione produces an activation of the tyrosine hydroxylase activity below 3 mM and an inhibition at higher concentrations. The limit between the enzymatic activation and inhibition appears at glutathione concentrations similar to the physiological levels of this compound found in melanocytes. Although the switch from eumelanogenesis to phaeomelanogenesis occurs at much lower concentrations of glutathione, taking into account these data it is discussed that this sulfhydryl compound may regulate not only the type but also the amount of melanin formed inside melanocytes.