Chemische und biologische Folgen des radioaktiven Zerfalls von Iod-125 in Plasmid DNA

The consequences of the decay of iodine-125 incorporated into DNA were studied on a molecular basis. Major emphasis was placed on the following questions : (i) the fate of the labelled nucleotide after the decay of the constituent 125, atom, (ii) the spatial relationship between strand breaks and the position of the labelled nucleotide within the DNA strand and (iii) the specific products formed from strand breaks. Doubly ($^{14}$C and $^{125}$I) labelled 5-iodo-2'-deoxycytidine 5'-triphosphate (IdCTP) was synthesized and incorporated enzymatically into the SalI-cutting site of the plasmid pBR 322. Part of the radioiodinated DNA was treated with T4-DNA ligase in order to restore the circular structure of the native plasmid molecule. After 4 months of storage under various conditions the stable end products were analyzed by radio GC, radio HPLC and electron microscopy. As the decay not only leads to transmutation effects but also to radiolysis effects caused by the emitted electron, for comparison the experiments were not only carried out with doublylabelled DNA but also with solutions of $^{14}$C-labelled DNA containing Na$^{125}$I as internal radiation source. The results clearly indicate that radiolysis alone causes only minor damage. Transmutation of the covalently bound iodine, on the other hand, leads to complete destruction of the labelled nucleotide, giving rise to $^{14}$CO$_{2}$ and $^{14}$CO as main products. [...