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 with the endonuclease-III (endo-III) of Escherichia coli. MATERIALS AND METHODS Plasmid DNA in aerobic aqueous solution was irradiated with one of five radiation types: 137Cs gamma-rays (LET approximately 0.3keV microm(-1)), 244Cm alpha-particles (140-190 keV microm(-1)), 4He ions (97 keV microm(-1)), 56Fe ions (143 keV microm(-1)) or 197Au ions (1,440 keV microm(-1)). The irradiated samples were then incubated with endo-III. SSB and DSB yields 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 after gamma-irradiation. This may reflect inhibition of the activity of endo-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 lower increases in SSB and DSB yields would still be expected. CONCLUSION The results provide evidence for clustered DNA damage after high LET irradiation.