Mechanism of DNA damage by thiocyanate radicals

Abstract

Purpose : It was previously shown that γ-irradiation of aqueous solutions of plasmid DNA in the presence of millimolar concentrations of thiocyanate ions leads to the formation in very high yields of sites recognized by the 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 thiocyanate ions was irradiated with 137 Cs γ-rays. After irradiation, aliquots 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, increasing ionic strength, increasing nitrite or iodide concentration, and decreasing oxygen concentration. Conclusions: The observations suggest that the monomeric thiocyanate radical SCN •is an intermediate in the reaction, and that the yields of FPG-sensitive sites are determined by competition between the disproportionation of the dimeric radical anion (SCN) 2 •- and the fate of a one-electron oxidized guanine species in DNA. The latter can react with oxygen to produce an FPGsensitive site or can be reduced without producing an FPGsensitive site. The results help to clarify the mechanisms responsible for DNA damage by the direct eÚect of ionizing radiation.