Evidence for a hypoxic fixation reaction leading to the induction of ssb and dsb in irradiated DNA

Abstract

Purpose: To measure hypoxic chemical fixation processes of radiation damage in both isolated plasmid DNA and in GSHdepleted E. coli cells. Materials and methods: Plasmid pBR322 DNA was irradiated with a single 5 ns pulse of 400 keV electrons under hypoxic conditions. At pre-set times, immediately before or after the electron pulse, the chamber containing the DNA was exposed to a high-pressure shot of hydrogen sulphide (H2S) gas. Results: DNA irradiated before contact with the H2S pulse was more sensitive to the production of both single strand breaks (ssb) and double strand breaks (dsb) than DNA irradiated after the addition of H2S. The post-irradiation protection of DNA by H2S was time-dependent, having first-order rate constants of 21 s-1 for ssb and 10 s-1 for dsb. Conclusions: This is the first direct kinetic evidence for the involvement of a hypoxic fixation reaction in the production of DNA damage by ionizing radiation. It indicates that long-lived radical damage is induced in DNA which, even at times of 20-50 ms after irradiation, can be chemically repaired, or rescued, by the addition of a thiol agent. This reaction may partially explain the predicted decrease in oxygen enhancement ratio (OER) with linear energy transfer (LET) on the basis of the increased clustering of radicals produced on the DNA by tracks of ionizing radiation. As radical multiplicity increases with LET there is a greater chance that some of the radicals will become fixed in the absence of oxygen leading to an increased probability of damage under hypoxia and a reduction in the OER.