The Modification of Damage Caused by Primary Ionization of Biological Targets

Abstract

Various experimental data are discussed in the light of the hypothesis that radiation-formed H2O2 and free radicals from water are responsible for a large part of damage by ionizing radiation to cell nuclei. New data show that radiosensitivity of bacteria is increased by O2 concentrations which are low compared with those effecting comparable changes, through HO2 radical action, in the radiosensitivity of certain chemical systems. It is concluded that the "indirect action" hypothesis for living cells cannot be upheld, and in particular that the action of HO2 radicals cannot explain the "oxygen effect". Most radiation damage to cell nuclei occurs through direct ionization of biological target molecules; it is suggested that these, being left highly reactive, are involved within a very short time in a "metionic reaction", the nature of which determines whether restoration of the target can subsequently occur. Modifying agents such as O2, by taking part in this reaction, influence the probability of restoration and hence the extent of the damage which becomes apparent.