Radiotoxicity of intranuclear 125I atoms not bound to DNA

The radiotoxicity of 125I covalently bound to DNA is unusually high. This has been attributed both to the Auger electrons which result from the electron capture process accompanying 125I decay and to local transmutation effects which cause extensive damage to nearby structures. We introduced 125I into cell nuclei in the form of iodoantipyrine, a molecule which diffuses freely through cells, and we have compared the survival of these cells to those exposed to radiation from extracellular 125I-labelled albumin or 55Fe-labelled transferrin. We found a value for D0 of 34 rad for 125I decays occurring within the cell nucleus compared to 362 rad for extracellular 125I and 277 rad for extracellular 55Fe. Since transmutation effects are very short range and 125I was distributed uniformly throughout the nucleus rather than bound to DNA, most of the radiotoxicity of intranuclear 125I-labelled iodoantipyrine must be due to Auger electrons.