Mechanisms of DNA strand breakage by ionizing radiation

The effects of added scavengers on cellular radiosensitivity reveal that the hydroxyl radical is the major species responsible for the indirect effect. The reaction is mechanistically complex since many products have been identified. The interaction between DNA and the hydroxyl radical is also kinetically complicated, as might be expected for a diffusion controlled reaction of a macromolecule. This non-homogeneous kinetic behavior results in a variation of the second order rate constant with hydroxyl radical lifetime. The authors have accumulated sufficient data to quantify this variation over a wide range of radical lifetimes. Although there is some kinetic evidence for the existence of damage clustering, more direct experimental evidence from product analysis has proven elusive. The authors reasoned that linked DNA molecules may offer an alternative experimental approach to evaluate the LMDS model. Under conditions where hydroxyl radical diffusion distances are relatively large (i.e. low scavenger concentration), two linked plasmids should behave towards ionizing radiation as if they were unlinked. Therefore, they prepared a concatenated DNA substrate consisting of two singly linked supercoiled double stranded rings of unequal size (2,817 and 1,065 bp), and compared product distributions after irradiation under various conditions