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Purpose: It has been previously argued that the use of the one-electron oxidants (SCN)[sub 2][sup •-] and Br[sub 2][sup •-] with plasmid DNA leads to the formation of DNA guanyl radicals. These guanyl radical species are intermediates in the DNA damage produced by processes such as photo-ionization and ionizing irradiation. The present paper evaluates the use of thallium(II) ions (Tl[sup II]OH[sup +]) as the one-electron oxidant, and also determines rate constants for the reduction (repair) of guanyl radicals in plasmid DNA by a variety of reducing agents including the biologically important compounds ascorbate and glutathione. Materials and methods: Aqueous solutions of plasmid DNA containing 10[sup -3] mol dm[sup -3] thiocyanate or thallous ions and a reducing agent (azide, nitrite, ferrocyanide, hexachloroiridate(III), iodide, ascorbate, glutathione, glutathione disulphide, methionine, tyrosine, 5-hydroxyindole-3-acetic acid, 10[sup -7]-10[sup -4] mol dm[sup -3]) were irradiated with [sup 137]Cs γ-rays (662 keV). After irradiation, the plasmid was incubated with the E. coli base excision repair endonuclease formamidopyrimidine-DNA N-glycosylase (FPG). Strand break yields after incubation were quantified by means of agarose gel electrophoresis. Results: High yields of FPG-sensitive sites produced by the oxidants (SCN)2[sup •-] and Tl[sup II]OH[sup +] were strongly attenuated by the presence of the reducing agents. Conclusions: From the results, it is possible to arrive at estimates of the rate constants for the reduction of the DNA guanyl radical by the reducing agents. Values lie in the range 10[sup 4]-10[sup 7] dm[sup 3] mol[sup -1] s[sup -1]. Using the values for ascorbate and glutathione, it is possible to estimate an upper limit on the order of milliseconds for the lifetime of DNA guanyl radicals under cellular conditions. The implication is that there may well be a significant chemical repair of DNA base damage by the direct effect of ionizing radiation. [ABSTRACT FROM AUTHOR]

Purpose: To determine the increase in single- (SSB) and double-strand break (DSB) yields after post-high LET irradiation incubation of plasmid DNA with the endonuclease-III (endo-III) of Escherichia coli. Materials and methods: Plasmid DNA in aerobic aqueous solution was irradiated with one of five radiation types: [sup 137]Cs γ-rays (LET ~0.3keV μm[sup -1]), [sup 244]Cm α-particles (140-190 keV μm[sup -1]), [sup 4]He ions (97 keV μm[sup -1]), [sup 56]Fe ions (143keV μm[sup -1]) or [sup 197]Au ions (1440keV μm[sup -1]). The irradiated samples were then incubated with endo-III. SSB and DSB yields were quantified by agarose gel electrophoresis. Results: Endo-III incubation produced an increase in the SSB and DSB yields. The increases were in general lower after the high LET irradiation than after γ-irradiation. This may reflect inhibition of the activity of endo-III by the nearby DNA damage expected from high LET radiation. It can be shown that even if the activity of endo remains unchanged, significantly lower increases in SSB and DSB yields would still be expected. Conclusion: The results provide evidence for clustered DNA damage after high LET irradiation. [ABSTRACT FROM AUTHOR]

Purpose: To determine the increases in SSB and DSB yields after post gamma-irradiation incubation of plasmid DNA with the Escherichia coli base excision repair endonucleases formamidopyrimidineDNA N-glycosylase (FPG) and endonuclease III (endo III). Materials and methods: Aqueous solutions of plasmid DNA were irradiated with [sup 137]Cs gamma-rays in the presence of 10[sup -4]-10[sup -1] mol dm[sup -3] formate. After irradiation, aliquots were treated with FPG and/or endo III. SSB and DSB yields were then determined using gel electrophoresis. Results: Both SSB and DSB yields were found to increase after enzyme incubation, with the increase in the DSB yield being approximately equal to the square of the increase in the SSB yield. The correlation between the increases in the SSB and DSB yields was unaffected by the scavenger concentration during irradiation. Conclusion: Under the conditions used, the majority of DSB appear to be formed from two hydroxyl radical attacks. [ABSTRACT FROM AUTHOR]