Macromolecular Repair and Free Radical Scavenging in the Protection of Bacteria against X-Rays

Abstract

The sensitivity of E. coli B/r to X-irradiation under various conditions was determined from the survival of colony-forming ability when plated on nutrient agar. The kinetics of radiation protection by mercaptoethanol in the presence of various amounts of oxygen revealed its action to be twofold. Firstly, at low oxygen concentrations, mercaptoethanol reduced the sensitivity in competition with oxygen. In this 40 [mu][image] oxygen competed about equally with 10 m[image] mercaptoethanol. At 100 m[image], it reduced by 35% the level of the plateau of radiosensitivity at high oxygen concentrations, an effect that was not in competition with oxygen. The latter property is shared by ethanol, methanol, and nitrite and is presumably due to scavenging of free radical intermediates. Nitric oxide at 300 [mu][image]. sensitized anoxic cells, but protected by up to 50% at higher concentra tion. Intercomparison of the effectiveness of these substances in protecting cells, with published values for their reaction rates with the aqueous free radicals, leads to the conclusion that, in normally hydrated E. coli, free radical intermediates are responsible for about half the total injuries and under aerobic conditions they have properties comparable to those of OH radicals. In contrast, substances such as nitrate, carbon dioxide, and oxygen known to react with reducing species, H", or the aqueous electron did not protect the cells. The intercomparison of the actions of these several substances provides a method for determining the fraction of X-ray injury to cells due to free radical intermediates and an indication of their properties.