Factors Influencing the Radioprotective Action of Cysteine: Effects in Escherichia coli Due to Drug Concentration, Temperature, Time, and pH

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Abstract
Protection by L-cysteine against the lethal action of 250-kv X-rays was studied in washed, buffered, dilute suspensions (4 x 104 cells/ml) of E. coli, strains B and B/r, initially in equilibrium with air. Cysteine protection was the end result of events in 2 sequential periods, a temperature-dependent reaction period, during which a state of protection developed, followed by a temperature-independent irradiation period, during which some of the toxic effects of the absorbed X-rays were prevented or corrected. The degree of protection that developed during the reaction period depended on the interrelated effects of temperature (0.1[degree] to 37[degree]C), pH (5.1 or 7.8), drug concentration (0.0001 to 1.0 [image]) , and time (3.5 to 60 minutes). pH changed the shape of the curve relating protection to log drug concentration from linear (pH 5.1) to S-shaped (pH 7.8). The 2 curves crossed in the region of half-maximum protection (3 x 10-3 to 10-2 [image]), so that at low concentrations more protection was obtained at pH 5.1 but at high concentrations the reverse was true. The great variety of results that could be obtained by varying the experimental conditions indicated the importance of physiological and biochemical factors in determining protection. In addition, they illustrated how different results in different laboratories can be largely a matter of experimental design. With respect to the anoxia theory of cysteine action, the results showed that anoxia was the major factor under certain conditions. Under other conditions, however, the autoxidation of cysteine in the medium could not induce anoxia; yet significant protection occurred. It was concluded that some kind of non-anoxic mechanism was at work. This conclusion was supported by finding that the maximum value of the protection ratio ranged from 4 to 6, above the value of 3 predicted by the anoxia theory.

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