Radiotoxicity of Intranuclear ¹²⁵I Atoms Not Bound to DNA

Summary

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.