Γ-ray Damage Research Articles

γ-Ray radiolysis of acetohydroxamic acid in HNO3 and its radiolytic product

Acetohydroxamic acid (AHA) is a novel salt-free reagent used for the separation of Pu and Np from U in the advanced Purex process. This paper reports the γ-ray damage of AHA in HNO3 and its radiolytic product. For 0.2 mol L−1 AHA in 0.2–2.0 mol L−1 HNO3 irradiated at a dose of 5–25 kGy, the radiolytic rate of AHA is 6.63–77.5%, and it increases with the HNO3 concentration and absorbed dose. The main radiolytic gases are N2O and H2, with volume fractions of (0.500–16.2) × 10−2 and (1.30–11.8) × 10−3, respectively, and they increase with the absorbed dose; the H2 volume fraction decreases with increasing HNO3 concentration. The main liquid radiolytic products are CH3COOH and HNO2, and their concentrations are (3.40–19.7) × 10−2 and (0.200–4.80) × 10−3 mol L−1, respectively, which increase with the HNO3 concentration. Since a significant concentration of HNO2 is present in the irradiated AHA-HNO3 solution, a holding reductant must be used to destroy HNO2 and stabilize Pu(III) and Np(V) when AHA is applied for the separation of Pu and Np from U.

γ-ray damage of N,N-diethyl-hydroxylamine in water and its radiolytic products at lower dose

N,N-Diethyl-hydroxylamine (DEHA) is a novel salt-free reductant used in the reprocessing of spent fuel, this paper reports on γ-rays damage of 0.1–0.5 mol L−1 DEHA irradiated 5–25 kGy and its radiolytic products. DEHA is relative stable against radiation, its radiolysis rate increases with the dose, but decreases with increasing initial DEHA concentration. The main radiolytic products are hydrogen, ethene, methane, acetaldehyde and ammonium ion, their quantities increase with initial DEHA concentration. Hydrogen and liquid radiolytic products quantities increase with the dose, the dependence of hydrocarbon quantity on dose relates to original DEHA concentration and dose.

900-MHz Microwave Radiation Enhances γ-Ray Adverse Effects on SHG44 Cells

Mobile phones are widely used globally. However, the biological effects due to exposure to electromagnetic fields (EMF) produced by mobile phones are largely unknown. Environmental and occupational exposure of humans to γ-rays is a biologically relevant phenomenon. Consequently studies were undertaken to examine the interactions between γ-rays and EMF on human health. In this study, exposure to 900-MHz EMF expanded γ-ray damage to SHG44 cells. Preexposure EMF enhanced the decrease in cell proliferation induced by γ-ray irradiation and the rate of apoptosis. The combination of EMF and γ-ray exposure resulted in a synergistic effect by triggering stress response, which increased reactive oxygen species, but the expression of hsp70 at both mRNA and protein levels remained unaltered. Data indicate that the adverse effects of γ-rays on cellular functions are strengthened by EMF.

Interstrand Cross-Links: A New Type of γ-Ray Damage in Bromodeoxyuridine-Substituted DNA

Interstrand cross-links (ICL) represent one of the most toxic types of DNA damage for dividing cells. They are induced both by natural products (e.g., psoralens + UVA) and by several chemical agents, some of which are used in chemotherapy (e.g., carboplatin and mitomycin C). Although repair mechanisms exist for interstrand cross-links, these lesions can induce mutations, chromosomal rearrangements, and cell death. Here, we report, for the first time, the formation of ICL by gamma-rays in brominated DNA. It is well established that the radiosensitization properties of bromodeoxyuridine (BrdUrd) result primarily from the electrophilic nature of the bromine, making it a good leaving group and leading to the irreversible formation of a uridinyl radical (dUrd(*)) or uridinyl anion (dUrd-) upon addition of an electron. We observe that the radiolytic loss of the bromine atom is greatly suppressed in double-stranded compared to single-stranded DNA. We have used a model DNA containing a bulge, formed by five mismatched bases, and have observed a linear dose-response for the formation of strand breaks on the single-stranded regions of both the brominated strand and the opposite nonbrominated strand. Surprisingly, we have observed the formation of interstrand cross-links exclusively in the mismatched region. Thus, we propose that the radiosensitization effects of bromodeoxyuridine in vivo will almost certainly be limited to single strand regions such as found in transcription bubbles, replication forks, mismatched DNA, and possibly the loop region of telomeres. Our results suggest that interstrand cross-links may contribute to the radiosensitization effects of BrdUrd. These findings may have profound implications for the clinical use of bromodeoxyuridine as a radiosensitizer, as well as for the development of targeted radiosensitizers.

A molecular genetic dissection of the evolutionarily conserved N terminus of yeast Rad52.

Rad52 is a DNA-binding protein that stimulates the annealing of complementary single-stranded DNA. Only the N terminus of Rad52 is evolutionarily conserved; it contains the core activity of the protein, including its DNA-binding activity. To identify amino acid residues that are important for Rad52 function(s), we systematically replaced 76 of 165 amino acid residues in the N terminus with alanine. These substitutions were examined for their effects on the repair of gamma-ray-induced DNA damage and on both interchromosomal and direct repeat heteroallelic recombination. This analysis identified five regions that are required for efficient gamma-ray damage repair or mitotic recombination. Two regions, I and II, also contain the classic mutations, rad52-2 and rad52-1, respectively. Interestingly, four of the five regions contain mutations that impair the ability to repair gamma-ray-induced DNA damage yet still allow mitotic recombinants to be produced at rates that are similar to or higher than those obtained with wild-type strains. In addition, a new class of separation-of-function mutation that is only partially deficient in the repair of gamma-ray damage, but exhibits decreased mitotic recombination similar to rad52 null strains, was identified. These results suggest that Rad52 protein acts differently on lesions that occur spontaneously during the cell cycle than on those induced by gamma-irradiation.

ELECTRON SPIN RESONANCE STUDY ON RADIOLYTIC DAMAGE IN BaCl2 CRYSTALLITE

The x-ray and γ-ray radiolytic damage in luminous host material BaCl2 and its recovery have been studied by the electron spin resonance method. It is found that the x-ray radiolytic damage in the sample can be recovered completely under sunlight for only two minutes, but the complete recovery of γ-ray damage is not easy.

Cytosar-U (ARA-C) induced changes in mouse jejunal crypt epithelial kinetics and radiosensitivity to gamma rays and fast neutrons

Pretreatment with the S phase specific cytotoxic agent Cytosine Arabinoside (Ara-C) protects the intestinal stem cells from gamma radiation injury by nearly tenfold. Studies were undertaken to test whether an altered cell age distribution could account for the reported duration and degree of Ara-C induced protection and to measure the degree of protection from the high energy neutrons of the Fermilab Cancer Treatment Facility. Twelve hours after treatmentwith Ara-C, B6CF 1/ANL mice wereexposed to increasing single doses of either 137Cs γ-rays or neutrons from the Fermilab accelerator, or a split dose of neutrons with intervals of 1, 2, and 3 hours. The number of regenerating microcolonies per jejunal circumference in Ara-C treated and irradiated animals was compared to irradiated controls. Another group of mice was given Ara-C but in the 12-hour interval between Ara-C and irradiation, colcemid was given every 3 hours to continuously block and kill cells in mitosis. The results suggest that Ara-C given 12 hours prior to neutron irradiation protects intestinal stem cells to nearly the same degree as it does from 137Cs γ-ray damage. Furthermore, the control split-dose recovery ratio to neutron irradiation at 1, 2, or 3 hours was 1.8 and was unchanged 12 hours after Ara-C. Colcemid reduced the crypt cell population to less than half the normal 250 cells per crypt; however, the cell survival curve was unaltered from the survival curve 12 hours after Ara-C. These results suggest that Ara-C recruits intestinal clonogenic stem cells, but inhibits their normal passage through DNA synthesis. These cells, responsible for intestinal mucosal regeneration, appear to be held in a radioresistant portion of the cell cycle for a period of about 10–16 hours after Ara-C.

Characterization of Rapid Recovery from γ-Ray Damage in Plateau-Phase Human Diploid Fibroblasts

Rapid recovery was investigated in density-inhibited, stationary-phase human diploid cells subcultured to low density at various times after a single dose of radiation. The influence of total dose, postirradiation temperature, and cell-to-cell contact on recovery was examined. The cultures were exposed to /sup 60/Co ..gamma.. rays at a dose rate of 50 rad per second. When the irradiation and postirradiation temperatures were maintained at 25 or 37/sup 0/C, recovery, as manifested by an enhancement in survival, was similar during the first 30 min, whereas it was reduced at later times in the 25/sup 0/C compared with the 37/sup 0/C cultures. This result suggests the existence of different rapidly and slowly acting components to recovery. No recovery was noted at any time with incubation at 4/sup 0/C. The recovery observed in density-inhibited cultures was consistently greater than that in less dense cultures, suggesting an effect of cell-to-cell contact and cell cycle distribution. Following single doses of 200 to 900 rad, progressive enhancement in relative recovery occurred with increasing doses over a period of 2 to 90 min, illustrating the importance of the relationship of the dose to the rapid recovery process.

Reduction in γ-ray damage in hydrogenated silicon

Abstract Deep level transient spectroscopy has been used to observe the reduced concentrations of vacancy-related defects in γ-irradiated n-type Si containing hydrogen atoms. Data are presented on the efficiency and depth of this damage reduction as a function of the duration and temperature of the exposure to the plasma used to introduce the atomic hydrogen. A 3-hour exposure in an H plasma at 300°C prior to irradiation reduced the concentrations of the O-V, V-V and P-V centres by half or more to a depth of ∼20 μm, compared to the control samples.

Reduction in γ-ray damage in hydrogenated germanium

Abstract Deep level transient spectroscopy has been used to observe the reduced concentrations of γ-ray induced defects in Ge containing atomic hydrogen. Data are presented on the efficiency and depth of this damage reduction as a function of the duration and temperature of the exposure of the plasma used to introduce the atomic hydrogen. A 3-hour exposure in an H plasma at 300°C before irradiation reduced the γ-induced defect concentration by half or more to a depth of ∼40 μm, compared with that of the control samples.

The release of a deoxyribonucleic acid fragment after x-irradiation of Micrococcus radiodurans.

SummaryThe nature of the 3H activity released from M. radiodurans after γ-ray doses of 500 krads which cause no decrease in cellular viability has been investigated by gel filtration and alkaline sucrose-gradient centrifugation. Immediately after irradiation, oligonucleotides ∼7·0 × 105 daltons are released which are subsequently rapidly degraded in broth to thymine by exonucleolytic enzymes. Such enzymes are also capable of degrading mammalian DNA. Treatment with iodoacetamide 2 × 10−5 M results in a sensitization expressed as a loss of the shoulder of the dose-response curve. The study of DNA degradation products in M. radiodurans treated with iodoacetamide suggests that sensitization is due to inhibition of an enzyme which excises DNA from a DNA-membrane complex. Iodoacetamide appears not to inhibit the removal of thymine from irradiated DNA. The results suggest that at least two enzymes are involved in the repair of γ-ray damage in M. radiourans and that repair of the DNA-membrane complex is mainly re...

Involvement of the excision repair system in recovery of bacteriophage from gamma-ray damage sustained under oxic and anoxic condtions.

SummaryBacteriophages T1 and λvir irradiated with gamma-rays under anoxic conditions show a higher survival when assayed on an Escherichia coli Hcr+ strain than when assayed on a Hcr− derivative (uvrA B or C). Hardly any difference in sensitivity is seen for phage irradiated in the presence of oxygen.Assayed on a strain deficient in DNA polymerase I, the survival of irradiated T1 is lower than on the wild-type parent, for irradiation under both oxic and anoxic conditions.It is concluded that the damage sustained under oxic conditions, which is repaired by the excision repair system, does not need the products coded by the uvr genes, whereas part of the damage inflicted under nitrogen does.This conclusion is supported by the observation that the sensitivities of bacteriophages T4D and T4Dv1 for γ-rays differ under nitrogen, but are identical under oxygen.

Electron spin resonance of irradiated ferroelectric tri-glycine sulfate

A preliminary analysis of the electron spin resonance spectra of ferroelectric tri-glycine sulfate single crystals, which were subjected to γ-ray damage indicated that: (a) the principal stable radical Produced by γ-ray damage is NH 3CH-; (b) the orientations of the radicals are fixed in the crystal lattice; (c) the approximate values of the H c, the three H N, and the N Fermi coupling constants are 21.0, 14.5 and 3.2 G respectively; (d) the NH 3 group is rotating in the ferroelectric phase.

Annealing of γ-Ray Damage in Germanium

Isothermal annealing of germanium single crystals irradiated by γ-rays from Co 60 was studied. The variation of conductance with time was measured. The conductance curves are composed of three stages. To explain this fact, the model proposed by Fletcher and Brown may be employed with a certain modification. From the analysis of these curves, the activation energy of the direct recombination of vacancy interstitial pairs, that for the diffusion of interstitial atoms and that for the diffusion of vacancies were determined to be 0.765 eV., 0.741 eV. and 1.250 eV., respectively.