Dependence of radiation sensitivity on oxygen tension in Oedogonium—II. Effect of temperature and LET

A multiple-radical model for radiation action on DNA and the dependence of OER on LET

We have developed a multiple-radical model of the chemical modification reactions involving oxygen and thiols relevant to the interactions of ionizing radiations with DNA. The treatment is based on the Alper and Howard-Flanders equation but considers the case where more than one radical may be involved in the production of lesions in DNA. This model makes several predictions regarding the induction of double strand breaks in DNA by ionizing radiation and the role of sensitizers such as oxygen and protectors such as thiols which act at the chemical phase of radiation action via the involvement of free radicals. The model predicts a decreasing OER with increasing LET on the basis that as radical multiplicity increases so will the probability that, even under hypoxia, damage will be fixed and lead to lesion production. The model can be considered to provide an alternative hypothesis to those of "interacting radicals' or of "oxygen-in-the-track'.

Independence of the radiobiological oxygen constant, K, and the respiration rate of mammalian cells

The radiosensitivity, and the radiobiological oxygen-constant K for mouse Ehrlich ascites cells depend on the pre-irradiation growth conditions. Measurements of the oxygen-consumption rate of cells grown by different methods have been made to ascertain whether the differences in radiobiological properties could be associated with different rates of respiration. Inhibition of respiration by sodium amytal had no significant effect on either the radiosensitivity or the estimate of K.