Non-Repairable Strand Breaks Induced by ¹²⁵ I Incorporated into Mammalian DNA

When (125)I is incorporated into Chinese hamster DNA (via (125)I-labeled iododeoxyuridine) and the cells are stored at 77 degrees K, the resulting decays of the isotope cause 4 to 5 breaks/single-strand per disintegration. On the average, about 50% of these breaks are repaired. In contrast, under the same conditions of storage and in the same range of total strand breaks/cell, 70-100% of the breaks induced by x-radiation are repaired. Thus, the extreme toxicity of (125)I when incorporated into DNA is correlated with the unrepaired breaks caused by decay of this isotope. These results suggest that unrepaired DNA strand breaks may be important in cell killing after treatments which damage DNA.