On the Nature of the Oxygen Effect on X-ray-induced DNA Single-strand Breaks in Mammalian Cells

Abstract

The initial yield of DNA single-strand breaks in X-irradiated Chinese hamster ovary cells was about 4-fold higher in aerated samples than in those under extreme hypoxia (2), with DNA breakage yields of 3·6 × 10⁻⁹ and 0·85 × 10⁻⁹ dalton⁻¹ krad⁻¹ for air and N₂ irradiations, respectively. In cells in which metabolism had been inhibited by heating before irradiation, a maximal DNA break yield of about 8 × 10⁻⁹ dalton⁻¹ krad⁻¹ was found for samples irradiated in air or under moderate hypoxia (200 p.p.m. O₂), while extreme hypoxia (2) gave a yield of 4·3 × 10⁻⁹ breaks dalton⁻¹ krad⁻¹. When cysteamine was added to heated cells irradiated under extreme hypoxia, the DNA break yield was close to that obtained for viable cells under extreme hypoxia. Similarly, when the reducing agent, Na₂S₂O₄, was added before irradiation to remove O₂ chemically, the breakage yield for heated cells was close to that obtained for viable cells under extreme hypoxia. A 10-fold difference in the radiation effect was observed between maximal sensitivity (8 × 10⁻⁹ dalton⁻¹ krad⁻¹) seen in heated cells under air or 200 p.p.m. O₂ and the near to minimal sensitivity (0·85 × 10⁻⁹ dalton⁻¹ krad⁻¹) seen in viable cells under 2. A reasonable interpretation of these findings is that at radiation-induced free radical sites, fast chemical reactions occur which result in the fixation or the restoration of the damage. Thus, an oxygen-enhancement effect exists in mammalian cells for the X-ray-induction of DNA single-strand breaks; this effect appears to depend mainly upon radiation chemical events.