Dual Effect of Oxygen on the Induction and Repair of Single-strand Breaks in the DNA of X-irradiated Mammalian Cells

Abstract

The induction and rejoining of single-strand DNA breaks was studied in Chinese hamster cells exposed to radiation under oxic or anoxic conditions. The number of breaks was determined by the alkaline sucrose gradient technique either immediately after irradiation or after a period of oxic or extremely hypoxic incubation. A linear relationship between the dose and the inverse of the molecular weight of irradiated DNA was noted when oxygen was present or absent during radiation exposure. A dose-modifying effect of oxygen by a factor 2·3 was calculated. When quinacrine was used to inhibit rejoining of breaks during irradiation, the dose-modifying factor increased to 2·7. The energy required to induce one single-strand break was estimated to be 29 eV in oxygen, and 77 eV in anoxia. When the cells were incubated after irradiation under oxic conditions for 30 min, about 80 per cent of the oxically-induced breaks, and about 50 per cent of the hypoxically-induced breaks were repaired. The rejoining process was greatly inhibited when post-irradiation incubation was made under anoxic conditions. In this case, not more than 10–20 per cent of the breaks were repaired independently whether exposure to radiation was made in the presence or absence of oxygen.