Dithiocarbamate Toxicity toward Thymocytes Involves Their Copper-Catalyzed Conversion to Thiuram Disulfides, Which Oxidize Glutathione in a Redox Cycle without the Release of Reactive Oxygen Species

We have reported previously that diethyldithio-carbamate (DDC) and pyrrolidine dithiocarbamate (PDTC) induce apoptosis in rat thymocytes. Apoptosis was shown to be dependent upon the transport of external Cu ions into the cells and was accompanied by the oxidation of intracellular glutathione, indicating the inducement of pro-oxidative conditions (C. S. I. Nobel, M. Kimland, B. Lind, S. Orrenius, and A. F. G. Slater,J. Biol. Chem.270, 26202–26208, 1995). In the present investigation we have examined the chemical reactions underlying these effects. Evidence is presented to suggest that dithiocarbamates undergo oxidation by CuIIions, resulting in formation of the corresponding thiuram disulfides, which are then reduced by glutathione, thereby generating the parent dithiocarbamate and oxidized glutathione (glutathione disulfide). Although DDC and PDTC were found to partially stabilize CuIions, limited redox cycling of the metal ion was evident. Redox cycling did not, however, result in the release of reactive oxygen species, which are believed to be scavengedin situby the dithiocarbamate. DDC and PDTC were, in fact, shown to prevent copper-dependent hydroxyl radical formation and DNA fragmentation in model reaction systems. The thiuram disulfide disulfiram (DSF) was found to induce glutathione oxidation, DNA fragmentation, and cell killing more potently than its parent dithiocarbamate, DDC. Of particular importance was the finding that, compared with DDC, the actions of DSF were less prone to inhibition by the removal of external copper ions with a chelating agent. This observation is consistent with our proposed mechanism of dithiocarbamate toxicity, which involves their copper-catalyzed conversion to cytotoxic thiuram disulfides.