Influence of irradiation conditions on the measurement of DNA double-strand breaks by pulsed-field gel electrophoresis.

The induction of gamma-ray-induced DNA double-strand breaks (DSBs) determined by pulsed-field gel electrophoresis was compared in the DNA of intact Chinese hamster ovary (CHO) cells imbedded in agarose plugs, and in the isolated DNA of agarose-imbedded CHO cells that had been lysed before irradiation, to determine whether these irradiation conditions would influence their measurement. DSB-induction in irradiated cells or isolated DNA was measured as the loss of DNA from the plug compared with the unirradiated control. Chromatin protein was completely digested by the lysis buffer and as such did not affect DNA migration upon electrophoresis, whereas concentrations of EDTA as low as 10(-5) M affected the induction of DSBs in the isolated DNA. The gel plugs required several hours of washing with PBS to remove the contaminating EDTA from the lysis buffer. Once the residual EDTA was removed, DNA DSB induction as a function of dose was 70 times greater in isolated DNA than in the DNA of intact cells.