DNA strand breakage by bivalent metal ions and ionizing radiation

Purpose: To investigate mechanisms of DNA breakage via the interaction of bivalent metal ion, thiol reducing agent and ionizing radiation, in *OH scavenging abilities comparable to those in cells. Materials and methods: We measured the effects of 10 min exposure to 200 μM Fe2+ vs. Fe3+ on the induction of single (SSB) and double (DSB) strand breaks in unirradiated and oxically irradiated SV40 DNA, in aqueous solution containing 75 or 750 mM glycerol and or 5 mM glutathione (GSH). Results: Fe2+ or GSH alone produced little DNA damage. However, their combination produced a dramatic increase in the production of both SSB and DSB. Experiments with ferric ion suggest that it produces DNA damage only after partial reduction to ferrous by GSH. Induction efficiencies for SSB in the presence of Fe2+ GSH showed additivity of the effects of radiation alone with those from Fe2+ GSH. However, the corresponding induction efficiencies for DSB demonstrated a 2.5-fold enhancement. Conclusions: Our results are consistent with a model in which reduced bivalent metal ions plus thiols, in the presence of O2, produce DSB in DNA primarily via local clusters of hydroxyl radicals arising from site specific Fenton reactions. The synergism observed between DSB production by Fe GSH and by ionizing radiation, also believed to occur via local clusters of hydroxyl radicals, is consistent with this model. Our results suggest that both normally present intracellular iron and ionizing radiation may be important sources of oxidative stress in cells.