DNA strand breakage and repair in human kidney cells after exposure to incorporated iodine-125 and cobalt-60 gamma-rays.

Biological effects of 125I incorporated into DNA exceed those to be expected from the absorbed radiation dose by a factor 3--30. The reason for this discrepancy suggests special mechanisms introduced by 125I transmutation, decays by K-capture leading to the emission of an average of 6 low energy electrons including Auger electrons and to a highly positively charged daughter nuclide. The effect of 125I decay on DNA strand breakage and subsequent repair was studied. Human kidney cells T in the deep frozen state (-196 degrees C) were exposed to incorporated 125I and 60Co gamma-rays. The number of DNA single strand breaks (ssb) was determined by DNA centrifugation in alkaline sucrose gradients. DNA repair was studied by incubating the cells after thawing at 37 degrees C. For 125I decay in frozen cells which were kept with or without dimethyl-sulfoxide, 4 and 6 ssb were measured per decay. The gamma-rays produced 1ssb per 26 eV absorbed energy. Most of this damage was repaired 30 to 40 min after onset of incubation. No repair of the damage caused by 125I was observed. The high efficiency of 125I decays for the production of unreparable ssb provides evidence for the high radiotoxicity of this isotope. The observed lack of repair may in part be due to the high numbers of at least 2,000 125I decays per cell nucleus necessary for the assay system. Damage from many 125I decays may interfere with enzymatic repair processes.