Mechanism of DNA damage by thiocyanate radicals

Purpose: It was previously shown that gamma-irradiation of aqueous solution s of plasmid DNA in the presence of millimolar concentrations of thiocyanat e ions leads to the formation in very high yields of sites recognized by th e base excision repair endonuclease formamido-pyrimidine-DNA N-glycosylase (FPG). The authors wished to characterize the mechanism responsible for the production of these FPG-sensitive sites. Materials and methods: An aqueous solution of plasmid DNA containing thiocy anate ions was irradiated with Cs-137 gamma-rays After irradiation, aliquot s were treated with FPG. Break yields were determined using neutral agarose gel electrophoresis. Results: The yield of FPG-sensitive sites decreased with decreasing enzyme activity, increasing thiocyanate concentration, increasing dose-rate, incre asing ionic strength; increasing nitrite or iodide concentration, and decre asing oxygen concentration. Conclusion: The observations suggest that the monomeric thiocyanate radical SCN. is an intermediate in the reaction, and that the yields of FPG-sensit ive sites are determined by competition between the disproportionation of t he dimeric radical anion (SCN)(2)(.-) and the fate of a one-electron oxidiz ed guanine species in DNA. The latter can react with oxygen to produce an F PG-sensitive site or can be reduced without producing an FPG-sensitive site . The results help to clarify the mechanisms responsible for DNA damage by the direct effect of ionizing radiation.