DNA strand break yields after post-high LET irradiation incubation with endonuclease-III and evidence for hydroxyl radical clustering

Purpose: To determine the increase in single- (SSB) and double-strand break (DSB) yields after post-high LET irradiation incubation of plasmid DNA wit h the endonuclease-III (endo-III) of Escherichia coli. Materials and methods: Plasmid DNA in aerobic aqueous solution was irradiat ed with one of five radiation types: Cs-137 gamma -rays (LET similar to0.3 keV mum(-1)), Cm-244 alpha -particles (140-190 keV mum(-1)), He-4 ions (97 keV mum(-1)), Fe-56 ions (143 keV mum(-1)) or Au-197 ions (1440 keV mum(-1) ). The irradiated samples were then incubated with endo-III. SSB and DSB yi elds were quantified by agarose gel electrophoresis. Results: Endo-III incubation produced an increase in the SSB and DSB yields . The increases were in general lower after the high LET irradiation than a fter gamma -irradiation. This may reflect inhibition of the activity of end o-III by the nearby DNA damage expected from high LET radiation. It can be shown that even if the activity of endo remains unchanged, significantly lo wer increases in SSB and DSB yields would still be expected. Conclusion: The results provide evidence for clustered DNA damage after hig h LET irradiation.