DNA-DAMAGE PRODUCED BY EXPOSURE OF SUPERCOILED PLASMID DNA TO HIGH-LET AND LOW-LET IONIZING-RADIATION - EFFECTS OF HYDROXYL RADICAL QUENCHERS

Production of DNA damage by exposure to ionizing radiation was measure d in two in vitro systems. A supercoiled plasmid of 7.3 kbp was isolat ed and exposed in an aqueous environment to Co-60 gamma rays and JANUS 0.85 MeV fission-spectrum neutrons. Dose responses for the production of single-strand breaks (ssbs) and double-strand breaks (dsbs) were c omputed from the conversion of the supercoil to its relaxed and linear forms. The relative effectiveness (neutrons:gamma-rays) for destructi on of genetic transforming activity of M13 viral DNA was 0.23, close t o that for ssb production, in contrast with the situation for biologic al effects such as lethality, mutagenesis and cellular transformation measured in mammalian cells, where RBEs are > 1. The role of hydroxyl (OH) radicals in DNA damage induction by neutrons was investigated by exposure of plasmid in the presence of known quenchers of this species . Of four quenchers tested, all were able to reduce the yields of both ssbs and dsbs. These findings are consistent with a model for ssb and dsb induction by high Linear energy transfer radiation that involves OH radical mediation.