Photochemical reduction of ferric iron by chelators results in DNA strand breaks

The induction of single-strand breaks (SSB) in phi X174 RFI DNA by Fe(III) chelates under cool-white fluorescent (CWF) light or in darkness was investigated. Under CWF light, ferric iron (25 microM) without the addition of a low-molecular-weight (LMW) chelator or with the addition of a LMW chelator, such as citrate, nitrilotriacetate (NTA), or EDTA, generated SSB in 11, 30, 13, or 0% of the treated, closed-circular DNA, respectively. Addition of H2O2 increased the formation of DNA SSB to 93, 75, 62, or 25, respectively. In the dark, DNA SSB were not detected in the absence of H2O2. The addition of H2O2 resulted in 85% (none), 8% (citrate), 18% (EDTA), or 42% (NTA) DNA with SSB. The formation of DNA SSB was completely inhibited by a strong Fe(II)-specific chelator, ferrozine. Various .OH scavengers (e.g., mannitol, 5,5-dimethyl-1-pyrroline-N-oxide, dimethyl sulfoxide) completely inhibited DNA SSB formation in the presence of a LMW chelator, but only partially inhibited in the absence. These results suggest that Fe(II) and .OH or similarly reactive species may be involved in the induction of DNA SSB. The evolution of CO2 from chelators, such as EDTA or citrate, required CWF light and Fe(III), suggesting the participation of chelators in the reaction with Fe(III). Under CWF light, catalase inhibited and superoxide dismutase (SOD) stimulated formation of DNA SSB, indicating H2O2 was generated from O2-. and was required for DNA damage. In the dark, SOD inhibited formation of DNA SSB, suggesting O2-. was required for reaction(s) other than formation of H2O2. These results suggest that in the light the photochemical reduction of Fe(III) by the chelators results in formation of DNA SSB without the addition of H2O2. In the dark, the addition of H2O2 was required for the formation of DNA SSB, suggesting a different mechanism for DNA strand break formation.