Redox reactivity of guanyl radicals in plasmid DNA.

Purpose: It has been previously argued that gamma-irradiation of plasmid DNA in the presence of thiocyanate ions produces products recognized by the E. coli 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 reducing agents.

Materials And Methods: Aqueous solutions of plasmid DNA containing either bromide or thiocyanate (10(-3) to 10(-1) mol dm(-3)) and also one of six other additives (azide, ferrocyanide, iodide, nitrite, promethazine, tryptophan, 10(-7) to 10(-3) mol dm(-3)) were subjected to 137Cs gamma-irradiation (662 keV). After irradiation, the plasmid was incubated with FPG. Strand break yields before and after incubation were determined by agarose gel electrophoresis 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 the six 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) dm3 mol(-1) s(-1) for ferrocyanide, nitrite, promethazine and tryptophan respectively).