The alkaline elution of bromodeoxyuridine-containing (BrdUrd) DNA and chlorodeoxyuridine-containing (CldUrd) DNA was studied in two CHO lines, the parental AA8 and a mutant line, EM9, which has a defect in repairing strand breaks and a 12-fold elevated baseline frequency of SCE. BrdUrd-DNA was found to have alkali-labile sites as well as direct breaks, neither of which were increased significantly by prior treatment of AA8 cells with an inhibitor (benzamide) or poly(adenosine diphosphoribose) polymerase. CldUrd-DNA, which gives higher frequencies of SCEs than BrdUrd-DNA, had more strand breaks than BrdUrd-DNA in AA8 cells after treatment with benzamide, while without benzamide there was no difference. The accumulation of breaks in CldUrd-DNA by benzamide was shown to occur rapidly, to reach a maximum by 90 min, and to be readilyt reversible after benzamide removal. Under all conditions, EM9 cells had more strand breaks than AA8. These observed differences in strand breaks were not due to differenced in incorporation efficiencies. For the different halogenated pyrimidines and cell types, there was a good correlation between the number of strand breaks and reduction of plating efficiencies.
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