Effect of DOPA-loading on glutathione-dependent 5-S-cysteinyldopa genesis in melanoma cells in vitro

The effect of dopa, cysteine, and glutathione on 5-S-cysteinyldopa (5-S-CD) genesis in melanoma cells cultured in normal and tyrosine- and cysteine-free media has been studied. In normal media only melanotic melanoma cells have been found to secrete 5-S-CD into the medium. In the presence of dopa and cysteine, both, media incubated with and without cells have been found to produce 5-S-CD. In the presence of dopa and glutathione, however, cell-free media did not show the presence of 5-S-CD. In contrast melanoma cell-cultured media has been found to contain large quantities of this amino acid. The optimum condition for maximum production of 5-S-CD via glutathione-dependent pathway has been found to be at the dopa concentration of 10(-5) M when glutathione is present at the concentration of 10(-5) M in the culture medium. Thus dopa concentration with regards to glutathione is 1:1 on the molar basis which is twice the dopa concentration required in in vitro noncellular tyrosinase system. It is suggested that higher dopa requirement in our melanoma cell culture system reflects the co-existence of eu- and pheomelanin synthesis taking place according to their genetically predetermined proportions.