Enhancement of Cysteamine Cytotoxicity by Hyperthermia and Its Modification by Catalase and Superoxide Dismutase in Chinese Hamster Ovary Cells¹

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° during treatment; however, at 37° and 44° 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°-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°, 0.01 at 37°, and 0.00008 when the 44°-heat treatment was also used. Cysteamine toxicity was not modified by the addition of superoxide dismutase (10 µg/ml) but was completely blocked by the addition of catalase (50 µg/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.