Decay Of Iodine-125 Research Articles

Hoogsteen or not Hoogsteen? Iodine-125 Radioprobing of the P53-Induced DNA Deformations

Radioprobing is suitable for tracing the DNA and RNA trajectories in nucleoprotein complexes in solution. The method is based on analysis of the single-strand breaks produced by decay of iodine-125 incorporated in the C5 position of cytosine.

64 Hoogsteen or not Hoogsteen? Iodine-125 radioprobing of the p53-induced DNA deformations

Radioprobing is suitable for tracing the DNA and RNA trajectories in nucleoprotein complexes in solution. The method is based on the analysis of the single-strand breaks produced by decay of iodine-125 incorporated in the C5 position of cytosine (Karamychev et al., 1999, 2012). Here, we used radioprobing to study the conformation of DNA in complex with the DNA binding domain (DBD) of the tumor-suppressor protein p53. Two recently crystallized DNA-p53 DBD complexes have different conformations of the CATG motifs: one with the Hoogsteen A:T pairs (Kitayner et al., 2010) and the other with the Watson–Crick pairs (Chen et al., 2010). The two complexes differ in the sequence of the central YYY|RRR junction: the first one has the C|G step and the second has the T|A step. Thus, it is interesting to apply the radioprobing method to the two DNA sequences used in crystallography to see if the local changes (T|A to C|G) in the center of the p53 response element would produce significant distortions in the CATG motifs. To this aim, the iodine-containing cytosine was incorporated in the duplexes containing p53-binding sites, in one of the two CATG motifs and the frequencies of DNA breaks were analyzed. Frequencies of breaks are negatively correlated with the iodine–sugar distances, thus, one can evaluate the changes in these distances upon DNA binding to a protein. The radioprobing distances obtained for both DNA sequences proved to be consistent with the Watson–Crick structure observed by Chen et al. (2010). We did not find any evidence of the Hoogsteen A:T base pair formation in the DNA-p53 DBD complexes in solution using our radioprobing method. The most significant changes in the break frequency distributions were detected in the central segment of the p53 binding site, YYY|RRR, which are consistent with an increase in DNA twisting in this region and local DNA bending and sliding (Nagaich et al., 1999). We interpret these p53-induced DNA deformations in the context of p53 binding to nucleosomal DNA (Sahu et al., 2010).

Radiation Dose-Rate and DNA Damage

In their article, Olipitz et al. (2012) examined signs of DNA damage after chronic exposure of C57Bl6 mice to low-level ionizing radiation (3 mGy/day). For 5 weeks, mice were irradiated continuously with 35.5 keV X-rays produced by the decay of iodine-125, yielding an accumulated dose of 105 mGy. The observed effects were compared with those from acute irradiation by X-ray machine at 1,700 mGy/day up to the same accumulated dose.

Study of charge transport mechanisms in 125I-induced DNA damage at various temperatures

Purpose: Iodine-125 decay induces localized DNA damage by three major mechanisms: (1) Direct damage by the emitted Auger electrons, (2) indirect damage by diffusible free radicals, and (3) charge neutralization of the residual, highly positively charged, tellurium daughter atom by stripping electrons from neighboring residues. The charge neutralization mechanism of 125I-induced DNA damage is poorly understood. Charge transport along a DNA molecules can occur by either a hopping mechanism initiated by charge injection into DNA and propagated by charge migration through DNA bases along the DNA length, or by a tunneling mechanism in which charge transfers directly from a donor to an acceptor residue. In the first case additional damage in DNA nucleotides can be inflicted by the traveling charge; therefore, it is important to learn if charge hopping plays a role in 125I-decay-induced DNA damage. In our previous work, we determined that at 193K the charge hopping mechanism was not an appreciable component of the mechanism of 125I-induced DNA damage. However, the question whether this is also the case at higher temperatures remained open.Methods: In the current study we used a well-known chemical barrier for charge hopping, 8-oxo-7, 8,-dihydroguanine (8-oxo-G), to assess the role of this mechanism in 125I-decay-induced DNA damage at the following temperatures: 198, 253, 277 and 298 K.Results: We found that varying the temperature had little effect on the distribution of 125I-induced DNA breaks, as well as on the breaks found at the 8-oxo-G probe both with and without piperidine treatment.Conclusions: We thus conclude that charge transport by the hopping mechanism is not a major factor in 125I-decay-induced DNA damage at biologically relevant temperatures.

Iodine-125 decay in a synthetic oligodeoxynucleotide. I. Fragment size distribution and evaluation of breakage probability.

Lobachevsky, P. N. and Martin, R. F. Iodine-125 Decay in a Synthetic Oligodeoxynucleotide. I. Fragment Size Distribution and Evaluation of Breakage Probability. Incorporation of (125)I-dC into a defined location of a double-stranded oligodeoxynucleotide was used to investigate DNA breaks arising from decay of the Auger electron-emitting isotope. Samples of the oligodeoxynucleotide were also labeled with (32)P at either the 5' or 3' end of either the (125)I-dC-containing (so-called top) or opposite (bottom) strand and incubated in 20 mM phosphate buffer or the same buffer plus 2 M dimethylsulfoxide at 4 degrees C during 18-20 days. The (32)P-end-labeled fragments produced by (125)I decays were separated on denaturing polyacrylamide gels, and the (32)P activity in each fragment was determined by scintillation counting after elution of fragments from the gel. The relative fragment size distributions were then normalized on a per decay basis and converted to a distribution of single-strand break probabilities as a function of distance from the (125)I-dC. The results of three to five experiments for each of eight possible combinations of labels and incubation conditions are presented as a table showing the relative numbers of (32)P counts in different fragments as well as graphs of normalized fragment size distributions and probabilities of breakage. The average numbers of single-strand breaks per (125)I decay are 3. 3 and 3.7 in the top strand and 1.3 and 1.5 in the bottom strand with and without dimethylsulfoxide, respectively. Every (125)I decay event produces a break in the top strand, and breakage of the bottom strand occurs in 75-80% of the events. Thus a double-strand break is produced by (125)I decay with a probability of approximately 0.8.

Double-Strand Break Yield Following 125I Decay Effects of DNA Conformation

The decay of iodine-125 (125I) is accompanied by the emission of low-energy electrons that dissipate most of their energy in approximately 10 nm from the decay site. In mammalian cells, the .OH generated by these electrons are also confined to a small volume. Iodine-125 is thus an excellent probe for assessing the radiobiologic effects produced by .OH in close proximity to the site of a decaying atom. We have compared in pUC19 plasmids (naked DNA) and in Chinese hamster V79 lung fibroblasts (chromatin) the modulation by the .OH scavenger dimethyl sulfoxide (DMSO) of 125I-induced DNA double-strand breaks (DSB). The data indicate that DMSO cannot protect plasmid DNA against DSB damage from 125I decaying within a few angstroms from DNA. However, DMSO attenuated DSB production in V79 cells following the decay of DNA-incorporated 125I, thus suggesting that chromatin structure fosters some DSB formation by indirect mechanism(s). DSB production depends on the environment and/or conformation of DNA. Consequently, current biophysical modeling of DNA damage that is based on naked and non-compacted DNA is inadequate for explaining radiobiologic effects at the cellular level.

Strand Breaks after the Decay of Iodine-125 in Proximity to Plasmid pBR322 DNA

To elucidate the kinetics of DNA strand breaks caused by low-energy Auger electron emitters in proximity to DNA molecules, we synthesized (125)I-labeled 2-iodoacridine (2-(125)IA), which intercalates with DNA, and 4-iodoacridine (4-(125)IA), which does not. Supercoiled DNA from pBR322 plasmid, labeled with 3H, was purified and incubated with 2-(125)IA or 4-(125)IA in aqueous solution. Reaction mixtures were stored at 4 degrees C to accumulate radiation dose from the decay of (125)I, and DNA was resolved by gel electrophoresis into supercoiled (DNA-I), nicked-circular (DNA-II) and linear (DNA-III) forms, representing undamaged DNA, single-strand breaks (SSBs) and double-strand breaks (DSBs), respectively. Gamma irradiation from an external (137)Cs source led to an exponential decrease in DNA-I with a D0 value of 10.8 +/- 0.3 Gy. Under identical conditions, the D0 values for 2-(125)IA and 4-(125)IA were 22.4 +/- 0.6 x 10(11) disintegrations and 4.7 +/- 0.4 x 10(11) disintegrations, respectively. External gamma irradiation and 4-(125)IA produced SSB/DSB ratios of 26.5 +/- 2.1 and 15.9 +/- 2, respectively, while that for 2-(125)IA was 0.6. The average number of DSBs from each decay of (125)I was 0.67 for 2-(125)IA and 0.27 for 4-(125)IA. The results indicate that the decay of (125)I bound to a DNA-intercalating compound produces DSBs 2.5-fold more efficiently than (125)I bound to a nonintercalating compound and support the theoretical expectations that predict a DSB yield that is highly dependent on the proximity of the Auger electron emitter to DNA.

Sequence-specific DNA breaks produced by triplex-directed decay of iodine-125.

Triplex forming oligonucleotides (TFO) labeled with Auger emitters could be ideal vehicles to deliver radioactive-decay energy to specific DNA sequences, causing DNA breaks and, subsequently, inactivation of these sequences. To demonstrate this approach we labeled with 125I (two 125I per molecule on average) a purine-rich 38-mer which forms a stable triplex with a polypurine x polypyrimidine stretch in the human HPRT gene. Decay of 125I in the bound TFO was shown to cause sequence-specific double strand breaks (DSB) in the target HPRT sequence cloned into plasmid DNA. No sequence-specific breaks were observed if 125I-labeled TFO were not bound to the plasmid DNA. After 60 days of decay accumulation (one 125I half-life) approximately a quarter of all plasmid molecules contained sequence-specific DSB, corresponding to 0.3 site-specific DSB per decay. Sequencing gel analysis shows that the DNA breaks are distributed within a few bases of the maxima at those bases opposite to the positions of 125I in the TFO.

Photon spectral characteristics of a new double-walled iodine-125 source.

The photon spectral characteristics of a recently designed Iodine-125 source have been measured. The source has a physical length of 5 mm and a diameter of 0.8 mm. A thin tungsten filament coated with radioactive Iodine-125 is used as a radiographic marker and is encapsulated in a double wall titanium shell of uniform thickness all around. The photon spectral characteristics, measured with an intrinsic germanium (Ge) detector coupled to a multichannel analyzer, reveal that the seed emits the 27.4-keV K alpha and 31.4-keV K beta x rays and 35.5-keV gamma photons from the decay of Iodine-125. Because of their low energy, the tungsten x rays are not observed in the spectrum. The anisotropy of the radiation fluence for each of the above-mentioned photon energies was measured in planes containing the seed short and long axes. The 4 pi-averaged anisotropy factor for the total radiation fluence, i.e., sum of the above three photon energies is 0.92. The photon intensity radiated along the seed long axis is approximately equal to the intensity in the seed transverse direction due to the absence of end welds. The new Iodine-125 source is characterized by good radiographic visualization, greater structural strength due to double wall encapsulation design, and emission of more isotropic Iodine-125 photon spectrum.

Infinite thickness in autoradiographs of iodine-125.

Infinite thickness for iodine-125 in autoradiographs was measured by autoradiography using four types of nuclear emulsion. Using Ilford K5, Kodak AR 10 and Hyperfilm-3H, a value of 20 microns was obtained. Using Hyperfilm Betamax, infinite thickness was greater than 60 microns. The results are discussed in relation to the types of radiation emitted as a result of iodine-125 decay and in relation to published image spread data for this radioisotope.

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

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.

Range of Radiochemical Damage to DNA with Decay of Iodine-125

Studies of the length of DNA fragments produced upon decay of iodine-125-labeled deoxycytidine that was located at a single position within a DNA fragment of defined sequence demonstrate that most radiochemical damage occurs within 15 to 20 angstroms of the site of iodine-125 decay. However, DNA strand breakage was detectable up to 70 angstroms from the site of iodine-125 decay.

Chemical Effects of Iodine-125 Decay in Aqueous Solution of 5-Iodouracil. Ring Fragmentation as a Consequence of the Auger Effect

The chemical effects of the radioactive decay of125 I in the synthetic DNA precursor iodouracil were investigated by means of doubly labeled $[2\text{-}{}^{14}{\rm C},5\text{-}{}^{125}{\rm I}]\text{-iodouracil}$ with a double-labeling degree of 80%. After storage of the aqueous solutions under various conditions, a product analysis was carried out by radio gas chromatography and high-pressure liquid chromatography. The major products formed were ${}^{14}{\rm CO}$ and14 CO2 with a yield up to 65%. For comparison, aqueous solutions of $[2\text{-}{}^{14}{\rm C}]\text{-iodouracil}$ were irradiated with60 Co γ rays. In these experiments the major product was always uracil. Results of the two series of experiments unambiguously demonstrate that125 I decay, followed by the Auger effect, causes considerably greater damage in the immediate surroundings than that brought about by external irradiation. The formation of CO and CO2 in high yields clearly demonstrates that fragmentation of the parent molecule occurs ev...

Biological damage in testis by Iodine-125 in partially blocked thyroid of rats

Degeneration of testis has been observed after administration of Iodine-125 in potassiumperchlorate treated rats. Histological damage is associated with loss of DNA, RNA, acid phosphatase, total adenosine triphosphatase (ATPase) and Na/K dependent ATPase. Iodine-125 induced atrophic testis shows higher content of sodium and lower levels of potassium as compared to control testis. Damage of testis by Iodine-125 has been compared with atrophied testis, following gamma irradiation earlier reported. Auger effect due to Iodine-125 decay and transmutation at the sites of nuclei and plasma membrane of germinal cells seems to be the possible explanation for testicular damage caused by Iodine-125.

Molecular Consequences of the Auger Effect: The Fate of Iododeoxyuridine Following Iodine-125 Decay

Two hypotheses have been advanced to explain the extreme biological toxicity of DNA-associated125 I: (a) high local concentrations of radiation energy from low-energy Auger electrons; (b) charge-induced molecular fragmentation in the DNA. To distinguish between these two hypotheses, an attempt was made to evaluate the molecular events associated with the Auger effect. Doubly labeled iododeoxyuridine was synthesized, with125 I in number 5 position and14 C in number 2 position of the pyrimidine ring. To achieve the high degree of radiochemical purity required for this experiment, the doubly labeled iododeoxyuridine was subjected to three consecutive column separations using Biogel P-2 (minus 400 mesh). The final, pure sample of ${}^{14}{\rm C}-{}^{125}{\rm I}\text{-iododeoxyuridine}$ was incubated at 4°C for 76 days and then subjected to further column analysis. The fate of iododeoxyuridine after125 I decay was ascertained by monitoring the distribution of14 C activity in the elution pr...

Induction of lethality and DNA breakage by the decay of iodine-125 in bacteriophage T4.

SummaryIodine-125 has been incorporated into the DNA of coliphage T4 in the form of 5-iododeoxyuridine (IUdR), an analogue of thymidine. 125I decays by electron capture with consequent vacancy cascades and multiple ionization (Auger effect). This decay mode is known to cause extensive fragmentation of labelled small molecules. 125I-labelled T4 phage have been stored in a glycerol-casamino acid storage medium at −196°C during decay and assayed for the induction of lethalities and double-strand breaks (DSB's) in DNA. The lethal efficiency per decay, α(lethal), is 0·59 ± 0·08, in reasonable agreement with the previously determined value for 125I decay in T1 phage, α(lethal) = 0·49 ± 0·03. The efficiency of induction of DSB's, as determined by sedimentation of extracted DNA in neutral sucrose gradients, is 1·01 ± 0·13. These results indicate that every decay causes a DSB, but that some DSB's are non-lethal. α(DSB) is independent of storage temperature during decay (+4°C versus −196°C) and is also independent ...

The occurrence of double-strand breaks in coliphage T1-DNA by iodine-125 decay.

SummaryDNA of Coliphage T1 was labelled with 125I-iododeoxyuridine. Inactivation of plaque forming ability and sedimentation behaviour of DNA was studied during the radioactive decay of 125I. The absence of a temperature effect as well as the observation that a radical scavenger does not influence inactivation supports the view that ionizing radiation is not responsible for loss of phage viability. However, experimental evidence is given for DNA double-strand breaks being the relevant lethal event caused by electron vacancy cascades (Auger effect). The ratio of single- to double-strand breaks was calculated to be 1·1 : 1. The relationship of 1 : 1 was found between the numbers of lethal events and of double-strand breaks.