Cysteine transport in melanosomes from murine melanocytes

The synthesis of pheomelanin requires the incorporation of thiol-containing compound(s) during the process of mammalian melanogenesis. Since melanins are produced only in specialized, membrane-bound organelles, known as melan osomes, such thiol donor(s) must cross the membrane barrier from the cytoso l to the melanosome interior, Cysteine and/or glutathione (GSH) were propos ed as suitable thiol donors, although uptake of these compounds into melano somes was not previously characterized, In this study, we show that cystein e is transported, in a temperature- and concentration-dependent manner, acr oss membranes of melanosomes derived from murine melanocytes, Additional pr oof that cysteine uptake results from a carrier-mediated process and is not due to simple diffusion or to a membrane channel, was obtained in countert ransport experiments, in which melanosomes preloaded with cysteine methyl e ater took up significantly more [S-35]cysteine than did unloaded controls, In contrast, we were unable to detect any significant uptake of [S-35]GSH o ver a wide concentration range, in the presence or in the absence of reduci ng agent, This study is the first demonstration of melanosomal membrane tra nsport of cysteine, and it strongly suggests that free cysteine is the thio l source utilized for pheomelanin synthesis in mammalian melanocytes.