Incubation of isolated Chinese hamster ovary cell nuclei, equilibrated in an atmosphere containing 2% O2, with glutathione, cysteine, or cysteamine resulted in a decrease in the number of X-ray-induced DNA double-strand breaks (DSBs), determined by pH 9.0 filter elution. In the absence of exogenous thiol, no sensitization was observed with the addition of N-ethylmaleimide, indicating that endogenous thiols were not present at significant levels. Protection by 0.3 mM glutathione was not enhanced by the addition of exogenous glutathione S-transferases or by glutathione peroxidase. The data were analyzed according to a simple competition model with various hypotheses. Cysteamine was more than an order of magnitude more effective than the other thiols tested, on a molar basis, in preventing DSB formation. Depending on the hypothesis used to evaluate the data, glutathione was either much less effective, on a molar basis, in preventing the bulk of the DSBs or was capable of preventing only ∼55% of the damage, regardless of concentration. These data suggest that natural thiols other than glutathione may contribute to cellular radioprotection even if their concentration is much lower than that of glutathione. The data also suggest that despite the relative inefficiency of glutathione as a radioprotector, some areas of oxygenated tissues-where the oxygen tension falls below 2%-may be protected by glutathione concentrations in the physiological range of 3-20 mM.

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