Enhancement of cysteamine cytotoxicity by hyperthermia and its modification by catalase and superoxide dismutase in Chinese hamster ovary cells.

Chinese hamster ovary cells were exposed to the sulfhydryl compound cysteamine at concentrations ranging from 0 to 8 mM for 120 min. No toxicity was found in cells maintained at 5 degrees during treatment; however, at 37 degrees and 44 degrees a paradoxical toxicity was observed, i.e., substantial toxicity was observed at cysteamine concentrations of 0.2 to 1 mM but decreased at higher drug concentrations. When drug-treated cells were exposed to a 30-min 44 degrees -heat treatment (surviving fraction, 0.15 in the absence of drug) toxicity was markedly enhanced. At 0.4 mM cysteamine, the surviving fraction was approximately 0.6 at 5 degrees, 0.01 at 37 degrees, and 0.00008 when the 44 degrees -heat treatment was also used. Cysteamine toxicity was not modified by the addition of superoxide dismutase (10 micrograms/ml) but was completely blocked by the addition of catalase (50 micrograms/ml) over the drug concentration range of 0.2 to 2.0 mM. Cysteamine autoxidation as measured by O2 uptake at 0.4 mM proceeds through hydrogen peroxide (H2O2) production as evidenced by the regeneration of O2 upon the addition of catalase. In contrast, at 4.0 mM cysteamine, O2 regeneration was not pronounced. The data suggest that the production of H2O2 is the first reaction step in the mechanism of cysteamine toxicity. The subsequent production of highly reactive oxygen species like hydroxyl radicals (.OH) from H2O2 in the presence of reduced metal (Fenton chemistry) probably leads to the observed cellular toxicity.