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 melanosomes, such thiol donor(s) must cross the membrane barrier from the cytosol to the melanosome interior. Cysteine and/or glutathione (GSH) were proposed as suitable thiol donors, although uptake of these compounds into melanosomes was not previously characterized. In this study, we show that cysteine is transported, in a temperature- and concentration-dependent manner, across membranes of melanosomes derived from murine melanocytes. Additional proof that cysteine uptake results from a carrier-mediated process and is not due to simple diffusion or to a membrane channel, was obtained in countertransport experiments, in which melanosomes preloaded with cysteine methyl ester took up significantly more [35S]cysteine than did unloaded controls. In contrast, we were unable to detect any significant uptake of [35S]GSH over a wide concentration range, in the presence or in the absence of reducing agent. This study is the first demonstration of melanosomal membrane transport of cysteine, and it strongly suggests that free cysteine is the thiol source utilized for pheomelanin synthesis in mammalian melanocytes.