Single- and Double-Strand Break Formation in DNA Irradiated in Aqueous Solution: Dependence on Dose and OH Radical Scavenger Concentration

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Abstract
The yields of single- and double-strand breaks (SSB and DSB) in calf thymus DNA, after 60Co .gamma. irradiation in dilute aqueous solution, have been determined via molecular weight measurements using a low-angle laser light scattering technique. The irradiations were administered to to N2O-containing solutions of DNBA in the absence and presence of oxygen and with different concentrations of the OH radical scavengers phenol, tertiary butanol, and methanol. OH radicals were found to produce SSB linearly with dose with a G value of 55 nmol J-1 and 54 nmol J-1 in deoxygenated and oxygenated solutions, respectively. DSB were formed according to a linear-quadratic dose relationship and the G value of linearly formed DSB were GDSB.alpha.(r.t.) = 3.5 nmol J-1 in deoxygenated and 3.2 nmol J-1 in oxygenated solution. The ratio of GSSB/GDSB.alpha.(r.t.) = .gamma. of 19 .+-. 6 was independent of the scavenger concentration in the case of tertiary butanol and methanol-containing solutions. GDSB.alpha.(r.t.) is interpreted to result from a radical site transferred from a sugar moiety of the cleaved strand to the complementary intact strand. This process of radical transfer and subsequent cleavage of the second strand occurs with a probability of about 6 .+-. 2% in the presence of oxygen at all scanger concentrations studied. These data on scavenging capacity on GDSB.alpha.(r.t.) suggest that the double-strand breakage produced via radical transfer remains higher than that resulting from direct effect, up to scavenging capacities of about 109 s-1.

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