The addition of l-cysteine-HCl to growing cultures of E. coli 15T- log. phase cells, resulted in a decrease of their sensitivity to X-ray radiation, even when cysteine was removed from the cell-suspensions prior to irradiation. Cysteine proved to be effective as a radiosensitivity-modifying agent at concentrations of 1-10 mM. Simultaneously cysteine increased the cellular AS-SH level of the bacteria. This rise was found to depend on the doses of cysteine and the length of the incubation period. Extracellular cysteine at concentrations of 0·05 mM and upwards caused a detectable SH surplus intracellularly, which increased up to 5 mM. Higher concentrations of cysteine failed to induce a further rise of the bacterial SH content. At a given concentration the AS-SH level increasing effect of cysteine set in 30 sec after adding it to the cultures, the maximum level being reached around the 15th min. No direct correlation was found to exist between the changes in the radiosensitivity of the cells and their enhanced endogenous SH level.
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