Redox reactivity of guanyl radicals in plasmid DNA

Purpose: It has been previously argued that ii-irradiation of plasmid DNA i n the presence of thiocyanate ions produces products recognized by the E. c oli base excision-repair endonuclease formamidopyrimidine-DNA N-glycosylase (FPG), and there that derive from an intermediate guanyl radical species. The wish was to characterize the reactivity of this intermediate with reduc ing agents. Materials and methods: Aqueous solutions of plasmid DNA containing either b romide or thiocyanate (10(-3) to 10(-1) mol dm(-3)) and also one of six oth er additives (azide, ferrocyanide, iodide, nitrile, promethazine, tryptopha n, 10(-7) to 10(-3) mol dm(-3)) were subjected to Cs-137 gamma -irradiation (662 keV). After irradiation, the plasmid was incubated with FPG. Strand b reak yields before and after incubation were determined by agarose gel elec trophoresis under neutral conditions. Results: The very high yields of FPG-sensitive sites in the presence of SCN - or Br- decreased significantly with increasing concentrations of all of t he sis additives, with promethazine and tryptophan being the most efficient additives, and azide and iodide the least. Conclusions: From the results it is possible to estimate values of the rate constants for the reduction of the DNA guanyl radical (5 x 10(5), 2 x 10(5 ), 10(7) and 10(7) dm(3) mol(-1) s(-1) for ferrocyanide, nitrite, promethaz ine and tryptophan respectively).