Dopaquinone addition products in cultured human melanoma cells

The concentrations of dopa, cysteinyldopas, 5-S-glutathionyldopa, gamma-glutamyl-5-S-cysteinyldopa and 5-S-cysteinylglycinedopa, were analysed in homogenates of cultured human melanoma cells and in culture media. Cysteinyldopas were found to be the major catechol in the cells, with a molar concentration more than a hundred times that of dopa. 5-S-Glutathionyldopa was found in the same amount as dopa, while the quantity of 5-S-cysteinylglycinedopa was one order of magnitude less. gamma-Glutamyl-5-S-cysteinyldopa was not present in detectable amounts. In the medium the concentrations of dopa, 5-S-cysteinylglycinedopa and of 5-S-glutathionyldopa were about one half of those in the cells, while the concentration of cysteinyldopas was about 2%. The ratio between 2-S-cysteinyldopa and 5-S-cysteinyldopa when incubating dopa and cysteine with tyrosinase was identical with the ratio between the analogically synthetised isomers of glutathionyldopa. Consequently, from the calculation of these ratios in cells and media one cannot deduce whether cysteinyldopas arise from the direct addition of cysteine to dopaquinone, or from degradation of glutathionyldopa. Oxidation of 5-S-glutathionyldopa gives a red chromophore with maximum absorption at 480 nm which develops into a black pigment.