Variation of single-strand break yield with scavenger concentration for the SV40 minichromosome irradiated in aqueous solution.

We have measured the yield of single-strand breaks (SSBs) induced in aerobic aqueous solution by 137Cs gamma irradiation for the SV40 minichromosome as measured by agarose gel electrophoresis. Under these conditions, DNA SSBs are caused mainly by the hydroxyl radical. To characterize the reactivity of the SV40 minichromosome with the hydroxyl radical and to compare its behavior with that of naked DNA, we examined the variation of the G value for SSB formation, G(SSB), with the concentration of added hydroxyl radical scavengers. We find that simple competition kinetics is not applicable, but that a nonhomogeneous kinetics model is in much better agreement. Estimates of the efficiency of SSB induction per OH radical interaction with the SV40 minichromosome (0.04-0.05) indicate that this substrate is about five times more radioresistant than naked DNA at scavenging capacities < 10(8) s-1. At a DNA concentration of 50 micrograms ml-1, G(SSB) for the direct effect in the minichromosome is about 1 x 10(-5) mumol J-1 (2 x 10(-10) SSB Gy-1 Da-1), essentially equal to that for naked DNA.