Chemical and Biological Consequences of the Radioactive Decay of Iodine-125 in Plasmid DNA

Doubly labeled $[{\rm U}\text{-}{}^{14}{\rm C},\ 5\text{-}{}^{125}{\rm I}]\text{iododeoxycytidine}$ (IdC) triphosphate was synthesized and incorporated enzymatically into defined positions of the plasmid pBR322. After storage under various conditions, the stable end products were analyzed using radio-GC, radio-HPLC, and electron microscopy. In addition, solutions of ^{14}{\rm C}\text{-}{\rm IdC}\text{-labeled}$ DNA containing Na125 I as an internal radiation source were studied to investigate the influence of internal radiolysis. Transmutation of the covalently bound 125 I leads to complete destruction of the labeled nucleotide, giving rise to 14 CO2 and ^{14}{\rm CO}$ as major products. Fragmentation of the pyrimidine base is independent of solvent and DNA configuration. Internal radiolysis caused by Na125 I leads to only minor damage. Electron microscopy studies reveal that decay-induced double strand breaks (dsb) occur both at the site of decay and in areas as far as hundreds of base pairs apart from that site. Number and distribution of the breaks is strongly dependent on solvent and DNA configuration. A direct correlation exists between the extent of fragmentation of the nucleotide and the mean number of dsb.