ETHANOL AS A MODIFIER OF RADIATION SENSITIVITY OF LIVING CELLS AGAINST UV-C RADIATION.

The protection of Escherichia coli bacteria and the yeast Saccharomyces cerevisiae against UV-C radiation by ethanol was studied. It was found that the fraction of surviving cells increases with increasing ethanol concentration. The specific protection depends on the dose rate, concentration range of ethanol, and it is higher for yeast compared to the bacteria.

RADIOPROTECTIVE EFFECT OF HYDROXYL RADICAL SCAVENGERS ON PROKARYOTIC AND EUKARYOTIC CELLS UNDER VARIOUS GAMMA IRRADIATION CONDITIONS.

The influence of various hydroxyl radical scavengers such as methanol, ethanol and dimethyl sulfoxide on radiation sensitivity of prokaryotic cells (bacteria Escherichia coli) and eukaryotic cells (yeast Saccharomyces cerevisiae and V79 cells-Chinese hamster pulmonary fibroblasts) irradiated by 60Co gamma radiation was investigated. The dependence of radiation sensitivity on dose rate in range from 1.8 to 100 Gy h-1 was evaluated. Survival of cells irradiated by increasing dose rates was followed using clonogenic assay. Specific protective effect was found to be a nonmonotonous function of dose rate with typical maximum at the dose rate range from 50 to 55 Gy h-1 in all studied cell types.

Effects of irradiation conditions on the radiation sensitivity of microorganisms in the presence of OH-radical scavengers.

PURPOSE: The purpose of the paper was to investigate the protective effect of some scavengers of OH radicals (hydroxyl radicals) on the radiation sensitivity of bacteria (in some cases also yeast) under normoxic (in air) or hypoxic (suboxic) conditions and to compare the obtained results with those published earlier for the yeast, all in a wide interval of irradiation conditions. Another aim was to investigate a possible impact of the reaction order of the reaction between the scavengers and the OH radicals on the protection of the cells. MATERIALS AND METHODS: In order to study the protective effect of OH scavengers we used various concentrations of methanol and potassium formate (in some cases also ethanol) in isotonic salt solutions. These solutions containing living bacteria (Escherichia coli) or yeast (Saccharomyces cerevisiae) were irradiated with 60Co radiation using various doses and dose rates. Irradiation was performed in air, in some cases under the hypoxic conditions. The number of surviving cells was determined prior to and after irradiation in suspension with and without scavengers. The surviving fractions after irradiation with and without scavenger were evaluated. RESULTS: The surviving fraction of bacterial cells increases linearly with increasing concentration of both scavengers. The fraction of surviving cells does not increase with increasing concentration of the scavengers under suboxic conditions. The protective effect Ϭ increases linearly with increasing scavenging efficiency and this dependence is much sharper under normoxic conditions than under suboxic ones. The specific protection k is much higher for the methanol than for the potassium formate. CONCLUSIONS: The basic characteristics of the impact of scavengers of OH radicals on radiation sensitivity of both bacteria and yeast are the same in a wide interval of doses and dose rates. The specific protection effect is much higher under the normoxic conditions. This protective effect is inversely proportional to the rate constant of the reaction between the scavenger and the OH radicals. It seems to be obvious that the presence of oxygen during irradiation is a necessary condition for the protective action of the scavengers which may be partially controlled by some transport processes and may be connected with the radiation sensitivity of the cells. On the other hand, the change of the reaction order of the reaction of the scavenger with the OH radicals has turned out to be unimportant.

At the crossroad of photochemistry and radiation chemistry: formation of hydroxyl radicals in diluted aqueous solutions exposed to ultraviolet radiation.

Formation yields of ˙OH radicals were precisely determined in aqueous solutions of coumarin-3-carboxylic acid and ferrous sulfate (i.e., Fricke dosimeter) exposed to 253.7 nm radiation delivered from a continuous source. Quantum yield of ˙OH radicals was determined as ∼0.08, i.e., roughly one out of twelve photons, efficiently absorbed in UV-illuminated solutions, produced one ˙OH radical. Energetically, a water molecule should undergo a correlated action of at least two 4.9 eV photons delivering enough energy for direct H-OH dissociation (5.0-5.4 eV). We suggest a mechanism based on an interaction of two water molecules, both in long-living triplet states. An intermolecular transfer of excitation energy provided a sufficient amount of energy for the dissociation of one water molecule into ˙OH and H˙ radicals. In an aqueous solution of phospholipids, quantum yields of hydroperoxides formed under these irradiation conditions decreased with total effectively absorbed energy (i.e. a dose), similar to the radiation chemical yields obtained during an exposure to ionizing radiation, such as gamma rays from radionuclide sources. Under 253.7 nm irradiation, one ˙OH radical causes a peroxidation of 34 phospholipid molecules. This implicates chain mechanism of the reaction.

Delayed effects of accelerated heavy ions on the induction of HPRT mutations in V79 hamster cells.

Fundamental research on the harmful effects of ionizing radiation on living cells continues to be of great interest. Recently, priority has been given to the study of high-charge and high-energy (HZE) ions that comprise a substantial part of the galactic cosmic ray (GCR) spectra that would be encountered during long-term space flights. Moreover, predictions of the delayed genetic effects of high linear energy transfer (LET) exposure is becoming more important as heavy ion therapy use is increasing. This work focuses mainly on the basic research on the delayed effects of HZE ions on V79 Chinese hamster cells, with emphasis on the induction of HPRT mutations after prolonged expression times (ET). The research was conducted under various irradiation conditions with accelerated ions 18O (E=35.2MeV/n), 20Ne (E=47.7MeV/n and 51.8MeV/n), and 11B (E=32.4MeV/n), with LET in the range from 49 to 149 keV/µm and with 60Co γ-rays. The HPRT mutant fractions (MF) were detected in irradiated cells in regular intervals during every cell culture recultivation (every 3days) up to approximately 40days (70-80 generations) after irradiation. The MF maximum was reached at different ET depending on ionizing radiation characteristics. The position of the maximum was shifting towards longer ET with increasing LET. We speculate that the delayed mutations are created de novo and that they are the manifestation of genomic instability. Although the exact mechanisms involved in genomic instability initiation are yet to be identified, we hypothesize that differences in induction of delayed mutations by radiations with various LET values are related to variations in energy deposition along the particle track. A dose dependence of mutation yield is discussed as well.

Degradation of phospholipids under different types of irradiation and varying oxygen saturation.

The effects of different types of radiation on the formation of peroxide forms of 2-dioleoyl-sn-glycero-3-phosphocholine were studied under various conditions. For the irradiation, an aqueous solution of small unilamellar vesicles was prepared. Variations in parameters such as the dose rate and molecular oxygen saturation levels were evaluated. Our study suggests that the mechanism of the peroxides formation process remains unchanged under irradiation by accelerated electrons, gamma and accelerated protons. The values of radiation chemical yields of the peroxidic form depend on the type of radiation, dose rate, and the saturation of molecular oxygen. The level of oxygen saturation strongly affects the values of radiation chemical yields as well, as the dissolved oxygen is an important agent participating in peroxidation and it is a source of free radicals during the radiolysis. The values of radiation chemical yields strongly suggest that the mechanism of radiation-induced peroxidation of phosphatidylcholines does not proceed via chain reaction.

Breaking DNA strands by extreme-ultraviolet laser pulses in vacuum.

Ionizing radiation induces a variety of DNA damages including single-strand breaks (SSBs), double-strand breaks (DSBs), abasic sites, modified sugars, and bases. Most theoretical and experimental studies have been focused on DNA strand scissions, in particular production of DNA double-strand breaks. DSBs have been proven to be a key damage at a molecular level responsible for the formation of chromosomal aberrations, leading often to cell death. We have studied the nature of DNA damage induced directly by the pulsed 46.9-nm (26.5 eV) radiation provided by an extreme ultraviolet (XUV) capillary-discharge Ne-like Ar laser (CDL). Doses up to 45 kGy were delivered with a repetition rate of 3 Hz. We studied the dependence of the yield of SSBs and DSBs of a simple model of DNA molecule (pBR322) on the CDL pulse fluence. Agarose gel electrophoresis method was used for determination of both SSB and DSB yields. The action cross sections of the single- and double-strand breaks of pBR322 plasmid DNA in solid state were determined. We observed an increase in the efficiency of strand-break induction in the supercoiled DNA as a function of laser pulse fluence. Results are compared to those acquired at synchrotron radiation facilities and other sources of extreme-ultraviolet and soft x-ray radiation.

Influence of various scavengers of •OH radicals on the radiation sensitivity of yeast and bacteria.

PURPOSE: To quantitatively investigate the influence of various •OH (hydroxyl radical) scavengers on the radiation sensitivity of yeast and bacteria, particularly to define the relationship between the protective effect of a scavenger and its •OH scavenging efficiency. MATERIALS AND METHODS: In order to study the protective effect of •OH scavengers we used various concentrations of four scavengers (methanol, potassium formate, ethanol and ascorbic acid) in isotonic salt solutions. These solutions containing live yeast (Saccharomyces cerevisiae) or bacteria (Escherichia coli) were irradiated with (60)Co isotope γ -radiation using two different doses and dose rates. The number of surviving cells was determined prior to and after irradiation both in suspension with and without scavengers. The surviving fractions after irradiation with and without the scavenger were evaluated. RESULTS AND CONCLUSIONS: The main results of the paper were: The surviving fraction increased approximately linearly within the measured interval with increasing concentration of the scavenger. The same dependences were found for the protecting effect depending on the scavenging efficiency. The slopes of these dependences (k) were found to be characteristic for each scavenger. The k value determined the degree in which the scavenging of •OH radicals participated in the protection of living cells. The protective effects of scavengers at the same scavenging efficiency were different and unique for each scavenger. No simple relation was found between the efficiency of scavenger k and the rate constant kOH of the reactions between scavengers and •OH radicals. Our results suggest that the studied scavengers effectively protected yeast and bacteria against ionizing radiation. Although the scavenging of •OH radicals seems to be important for protection of living cells, it is clearly not the only process on which the protection is based.

Radiation formation of colloidal silver particles in aqueous systems.

This paper reports on the formation of silver nanoparticles initiated by gamma and UV radiation in various aqueous solutions. Inorganic precursors were used for radiation and/or photochemical reduction of Ag(+) ions to a metallic form. The influence of various parameters on the nucleation and formation of colloid particles was studied. Attention was also focused on the composition of the irradiated solution. Aliphatic alcohols were used as scavengers of OH radicals and other oxidizing species. The influence of the stabilizers on the formation and stability of the nanoparticles was studied.