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

Abstract

Doubly labeled [U-14C, 5-125I]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 14C-IdC-labeled DNA containing Na125I as an internal radiation source were studied to investigate the influence of internal radiolysis. Transmutation of the covalently bound 125I leads to complete destruction of the labeled nucleotide, giving rise to 14CO2 and 14CO as major products. Fragmentation of the pyrimidine base is independent of solvent and DNA configuration. Internal radiolysis caused by Na125I 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.

Chemical Effects of the Decay of 125l in [14C , 125l] lodobenzene in Different Solvents

“…This proves that the uracil ring efficiently breaks down following the Auger effect. A similar pattern of gaseous fragmentation products from doublylabeled DNA molecules was reported by LLNZ and STÖCK-LIN [14]. These gases were not detected when 12S I was not incorporated into, but added to the DNA in solution.…”

“…As mentioned before, this was achieved by monitoring the 14 C-labeled products formed in solutions of CH 3 ,2S I as internal source of 12S I and [U-14 C] C 6 H 5 I in the same solvents as used in double labeling experiments. The results can be summarized as follows: (1) In the benzene solution 95% of 14 C was recovered as the parent [U-14 C] C 6 H S I; (2) In the hexane matrix, the recovery of the initial 14 C activity was 77% (6% or 11% as 14 C 6 H 6 and 72% or 65% 14 C 6 H s Iat 77K and 295 K, respectively); (3) In the methanol samples, 88% of the 14 C activity was recovered, of which 16% i4 C 6 H 6 and 71% 14 C 6 H S I at 77K, at 295 Κ 19% I4 C 6 H 6 and 70% 14 C 6 H 4 I; (4) In none of the systems 14 C-products other than benzene and iodobenzene were present.…”

“…Many authors tend to believe that a highly localized energy deposition by low energy electrons around the decay site is primarily responsible for the biological damage (e. g. the strand breaks) [10,12]. However, the results of radiochemical studies seem to imply that while the action of electrons affects the sugar-phosphate moiety, the base to which 125 1 is attached is most probably damaged due to a charge neutralization [14].…”

“…Radiochemical studies on iodouracil [13] and isolated DNA [ 14] appeared to be substantial for the comprehension of the mechanism of damage produced by the incorporated 125 1. Using the double labeling technique, DEUTZMANN and STÖCKLIN [13] demonstrated that in aqueous solutions of 12S I-iodouracil the major products were CO and C0 2 .…”

Mechanisms Underlying Production of Double-Strand Breaks in Plasmid DNA after Decay of125I-Hoechst

“…The DSB yields in naked DNA produced by decaying 125 I have been determined repeatedly. In these investigations (18,20,30,31,33,(50)(51)(52)(53)(54)(55), the 125 I atom has been positioned in proximity to the DNA molecule either after DNA incorporation of an iodinated base analog (e.g. 125 IdUrd for thymidine or 125 IdC for deoxycytidine) or after a DNA intercalator or groove binder has been radioiodinated and added to the DNA samples.…”