We have previously shown that cells with a high content of the gold-binding protein metallothionein (MT) are resistant to the growth inhibitory effects of gold(III) chloride and auranofin. To investigate whether MT confers resistance to sodium aurothiomalate two cell lines of cultured human epithelial (HE) cells were used; the parental cell line (HE) and a substrain (HE100) containing high levels of cytosolic MT. Sodium aurothiomalate and thiomalic acid without gold both caused a dose-dependent growth inhibition of both cell lines when used in the concentration range 25-300 mumol/l culture medium and for four days' exposure. MT, despite binding about one third of the cytosolic gold, did not protect against the antiproliferative effect of sodium aurothiomalate. The gold and the thiomalate moieties were distributed differently within the cells; 30% of the cellular gold and 80% of the thiomalate were recovered in the cytosol. Gold was mainly protein bound in both cell lines, as shown by G75 Sephadex gel filtration of the cytosols. In the HE100 cells about 30% of the gold eluted with MT. The thiomalate eluted mainly with substances with molecular weights less than 1000. Cellular synthesis of MT was not observed during sodium aurothiomalate treatment. The results indicate that the sodium aurothiomalate molecule dissociates and support the suggestion that the thiomalate moiety is, in part, responsible for the antiproliferative effect of the drug.
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