Quantitative assessment of the contribution of clustered damage to DNA double-strand breaks induced by Co-60 gamma rays and fission neutrons

The induction of DNA strand breaks by fission neutrons was studied in aqueo us plasmid (pBR322) DNA under aerobic conditions for a wide range of hydrox yl radical ((OH)-O-.) scavenger concentrations and was compared to the indu ction of strand breaks by Co-60 gamma rays. Strand breaks were measured usi ng agarose gel electrophoresis coupled with sensitive P-32-based phosphor i maging. Yields are reported for DNA single-strand breaks (SSBs) and double- strand breaks formed linearly with dose (alpha DSBs), The fraction of alpha DSBs that were dependent on the multiply damaged site (MDS) or clustered d amage mechanism was also calculated using a model. G values for SSBs and al pha DSBs declined with increasing (OH)-O-. scavenging capacity. However, wi th increasing (OH)-O-. scavenging capacities, the decrease in yields of str and breaks for fission neutrons was not as pronounced as for gamma rays. Th e percentage of alpha DSBs for gamma rays was dependent on (OH)-O-. scaveng ing capacity, appearing negligible at low scavenging capacities but increas ing at higher scavenging capacities. In contrast, fission neutrons induced high percentages of alpha DSBs that were approximately independent of (OH)- O-. scavenging capacity. The levels of alpha DSBs formed by the MDS mechani sm after exposure to fission neutrons are consistent with the expected dist inctive features of high-LET energy deposition events and track structure, The results also confirm observations made by others that even for low-LET radiation, the MDS mechanism contributes significantly to DNA damage at cel l-like scavenging conditions. (C) 1999 by Radiation Research Society.