Redox reactivity of guanyl radicals in plasmid DNA.
OBJECTIVE
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.


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).
Milligan
J R
JR
Department of Radiology, University of California at San Diego, La Jolla, 92093-0610, USA.
Aguilera
J A
JA
Nguyen
J V
JV
Ward
J F
JF
D001965:Bromides / Q000737:chemistry; D004249:DNA Damage; D004269:DNA, Bacterial / Q000737:chemistry* / Q000235:genetics / Q000528:radiation effects; D004278:DNA, Superhelical / Q000235:genetics / Q000528:radiation effects; D043382:DNA-Formamidopyrimidine Glycosylase; D004305:Dose-Response Relationship, Drug; D004307:Dose-Response Relationship, Radiation; D016166:Free Radical Scavengers / Q000737:chemistry; D005609:Free Radicals / Q000737:chemistry*; D005720:Gamma Rays; D006147:Guanine / Q000737:chemistry* / Q000528:radiation effects; D017665:Hydroxyl Radical / Q000737:chemistry; D007454:Iodides / Q000737:chemistry; D009699:N-Glycosyl Hydrolases / Q000378:metabolism; D010084:Oxidation-Reduction / Q000528:radiation effects; D010957:Plasmids / Q000235:genetics* / Q000528:radiation effects; D013861:Thiocyanates / Q000737:chemistry
D001965:Bromides; D004269:DNA, Bacterial; D004278:DNA, Superhelical; D016166:Free Radical Scavengers; D005609:Free Radicals; D007454:Iodides; D013861:Thiocyanates; D017665:Hydroxyl Radical; D006147:Guanine; D009699:N-Glycosyl Hydrolases; D043382:DNA-Formamidopyrimidine Glycosylase