The toxicity, protective action against radiation injury and inhibition of DNA synthesis, and rejoining process by cysteamine and cystamine have been studied in cultured mammalian cells with the following results. Cysteamine (a) Toxicity: Although cysteamine in high concentrations is nontoxic, it is toxic in relatively low concentrations (0.5 mM). In low concentrations, it induces single-strand breaks of DNA in the irradiated and nonirradiated cells, depresses DNA synthesis in nonirradiated cells, and depresses the rate of rejoining of radiation-induced single-strand breaks of DNA in irradiated cells. (b) Protection against radiation damage: Cysteamine (over 5 mM) protects DNA molecules from radiation-induced single-strand breaks and double-strand cuts. The degree of protection by cysteamine increases with increasing cysteamine concentrations. (c) Inhibition of DNA synthesis and rejoining process: Cysteamine in high concentrations (over 5 mM) inhibits normal DNA synthesis of the nonirradiated cells and slows the rejoining of radiation-induced single-strand breaks in irradiated cells. Cystamine. Cystamine shows no toxic effect and affords little protection for DNA molecules from radiation-induced single-strand breaks. However, cystamine inhibits DNA synthesis of nonirradiated cells and the rejoining process of irradiated cells.

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