Toxicity of Radioprotective Agents to Yeast

These studies consider the toxic effects to baker's yeast of four sulfhydryl compounds frequently used as radioprotectants: cystamine, cysteamine, cysteine, and dithioerythritol (DTE). All but cysteine markedly reduce colony formation at concentrations above 10^-4 M. The rate of action on colony formation varied. Cystamine required at least an hour of exposure to produce an effect, and then it developed slowly, while the others manifested their action rapidly. All the agents inhibit exogenous respiration with dextrose to some degree, while their action on endogenous respiration was either small and variable or a stimulation. All cause a small but significant increase in the number of cells staining with methylene blue dye, but this is quite insufficient to account for the inhibition of colony formation by death of the cells. None of the effects measured bear a consistent relation to the radioprotective ability of these compounds in single cells or in higher organisms. Therefore, it is unlikely that identical or closely correlated mechanisms underlie the toxic effects and the ability to protect against x-ray damage. Nevertheless, the toxic concentrations in the case of all but cysteine lie close to or are lower than those needed for radioprotection. This raises questions as to the extent to which the "protected" cell population resembles the "unprotected" cells in expression of radiation effects.