The effect of 1,10-phenanthroline (OP) on repair of bleomycin (Bleo)-induced double-strand breaks in Ehrlich ascites tumor cells was studied using nondenaturing filter elution. 1,10-Phenanthroline is a metal chelator which is believed to inhibit strand breakage by Bleo through competition for intracellular iron. Cells were treated with 25 μM Bleo for 1 h, washed free of unincorporated drug, and then reincubated in the absence or presence of OP. In the absence of OP, relative elution (with respect to cells irradiated with 50 Gy and used as an internal standard) decreased in a first-order process with a half-time for repair of 2.4 h. 1,10-Phenanthroline at$10\ {\rm nmol}/10^{5}$ cells (50 μM) accelerated the net decrease in relative elution, producing a biphasic response with half-times of 5.3 h and less than 30 min for the two components. Thus functionally active Bleo remained in cells after they were washed free of unincorporated drug. 1,10-Phenanthroline at a concentration of$3.1\ {\rm nmol}/10^{5}$ cells did not result in a similar net acceleration of repair of double-strand breakage, though it increased the rate of repair of single-strand breakage as measured by alkaline elution. The differences in repair observed in response to different OP concentrations are discussed in terms of models for double-strand breakage by Bleo. After 4-5 h repair, relative elution from Bleo-treated cells remained at about 40% of that achieved at the end of 1-h Bleo treatment in either the presence or absence of the$10\ {\rm nmol}\ {\rm OP}/10^{5}$ cells, demonstrating that some double-strand breaks were resistant to repair. In contrast, the level of residual relative elution in cells irradiated with 50 Gy following repair, as a percentage of that induced initally, was significantly lower (7-10%). The biphasic decrease in relative elution proceeded with half-times similar to those for repair of Bleo-induced breaks in the presence of OP, suggesting that similar processes are involved in repair of damage by both modalities.

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