Current topics in radiation research quarterly最新文献

In view of environmental and biological implications, the decay-induced incorporation of tritium from T2 into nucleosides in aqueous solutions has been studied by chromatographic methods. It could be shown that at T2 concentrations of about 1 to 2 mCi/ml, labeling of thymidine or deoxyuridine mainly occurs via the 3HeT+ decay ion rather than via beta--radiolysis. In a saturated solution of thymidine, the number of thymidine molecules labeled per tritium decay (L-value) is only 8 X 10(-4), concomitantly 0.6 HTO molecules are also formed, together with CH3T (L = 0.2) and an unidentified organic product (L = 10(-3). Compared to identical concentrations of HT and HTO in aqueous deoxyuridine solutions, the labeling efficiency of T2 is an order of magnitude higher.

Effects of 125I decay in DNA were investigated by measurements of strand breaks and lethal efficiencies of the decays. In bacteriophages T1 and T4, local decay effects were compared with effects of the emitted electrons by induction of both single (ssb) and double strand breaks (dsb) in the intact phage head and in extended free state DNA. Most dsbs were found to result from local decay effects whereas most real ssbs are caused by the electrons. A simple one-to-one relationship seems to exist in the phages between the decays of 125I, numbers of dsbs and lethal effects. In E. coli rec+ and recA repair of dsbs was studied in addition to lethal decay efficiencies. In rec+ more than 70% of the dsbs were repaired within 1 h at 37 degrees C. No repair was observed in recA. The probability of lethality per 125I decay per completed genome was found to be 0.37 for rec+ and 0.93 for recA cells. The number of lethal events per unrepaired dsb was found to be practically equal to unity. Unrepaired dsbs thus seem to be the primary mechanism of lethality caused by 125I decay, and all unrepaired dsbs seem to be lethal.

Two strains of murine leukaemic lymphoblasts: L5178Y-R and L5178Y-S differing in the X-ray sensitivity by a factor of ca. 2 and showing X-ray-UV-light cross-sensitivity were exposed to: (a) 104 muCi/ml of HTO for time intervals from 25 to 600 h, (b) 10 muCi/ml of L-[4,5(n)-3H]lysine for 25 to 600 h, and (c) 0.05 muCi/ml of [methyl-3H] thymidine for 4 to 600 h. After completion of the exposure, survival was determined by cloning or by backward extrapolation of growth curves. In the case of HTO and 3H-thymidine exposure time the survival relationship passed through broad minima (75--200 and 50--400 h for (a) and (c), respectively). After longer exposures, higher survivals were observed reaching values close to 1 in the extreme cases. The results of shorter exposures indicate that L5178Y-R and L5178Y-S cells considerably differ in their susceptibilities to the tritiated compounds and that this difference increases with the shift of 3H localization towards DNA.

Iodine-125, which is used for the clinical diagnosis of the thyroid gland, has a high probability of decay by K-capture followed by the emission of low-energy electrons having ranges of up to 23 micrometer. Thus one has high local concentrations of energy deposition comparable in size to that of the smaller follicles. In this paper the local distribution of energy deposition inside and outside the follicles of the human thyroid gland is calculated for follicle sizes of 20--400 micrometer. Also the local dose rate at the position of the nuclei of the follicle cells is determined as a function of the follicle size. Possible dosimetric approaches to the problem of radiobiological effectiveness are discussed, at first in general and then for the example of the inactivation of follicle cells due to the incorporation of 125I.

For the estimate of the radiation exposure of man and for the calculation of the risk of artificial tritium from nuclear power plants, organic tissue-bound tritium is of decisive importance. In model experiments, a tritium incorporation of 61 to 71% was found from tritiated water (HTO) into organic matter of planctonic algae under under reproducible conditions and this was related to the theoretical value. In further experiments the tritium release from these high tritiated algae was of interest. Kept in darkness in tritium-free, non-sterile river water, so that autolytic processes and bacterial decomposition could occur, the concentration of HTO was measured over a period of three weeks. A relatively long half-life of tissue-bound tritium was found under various temperature conditions. Therefore it must be considered that a significant retention of tritium in biological matter has to be taken into account in a natural ecosystem. In streams into which the cooling water of a nuclear reactor is released all conditions are found already for a long turnover and cycling of artificial tritium in living organisms as well as the conditions for a favourable transport of tritium by food chains to man.

During labeling of mammalian cells with 125I-iododeoxyuridine for studies on single-strand break induction and repair, care must be taken to keep the amount of 125I incorporated per cell to very low levels. Under some conditions enough 125I can be incorporated during the incubation period (generally about one generation time) to damage the repair systems of cells so extensively, even before they are frozen, that they cannot repair any of the breaks induced by the 125I during the time they are frozen to accumulate these breaks.

Freshwater and marine algae are particularly useful for studying the radioactive contamination of aquatic systems. Acetabularia, Chlamydomonas and Porphyra were used to investigate the uptake and eventual biological effects of tritium. When the Acetabularia are grown in the presence of tritiated water, a significant amount of 3H is incorporated in the total nucleic acids and protein fraction. Chloroplasts of Acetabularia were isolated from whole cells and their DNA purified by the agarose procedure, before radioactivity analysis: a significant amount of 3H was incorporated into the chloroplast genome. Chlamydomonas was grown on minimal medium containing increasing concentrations of tritiated water. The increase in cell number was checked by microscope counting. The generation time was 9.6 h and seemed not affected even by the highest 3H concentration. Parallel experiments have shown that an appreciable amount of 3H was incorporated into the total organic matter of the plants. In the case of Porphyra, it was found that a very low level of 3H was incorporated into the total DNA of the plant.

Murine leukaemic lymphoblasts L5178Y-S were exposed to: (a) 104 muCi of 3HOH for time intervals from 25 to 600 h, (b) 10 muCi of L-[4,5(n)-3H] lysine for 25 to 600 h, and (c) 0.05 muCi of [methyl-3H]thymidine for 4 to 600 h. Extended post-exposure observations of growth disturbances and viability changes indicate marked differences between heritable lesions induced by the three tritiated compounds. After exposures to tritiated lysine and tritiated water, the damage was predominantly of non-lethal character while in the populations previously exposed to tritiated thymidine most of the cells eliminated during the post-exposure growth were lethally damaged. In all cases examined growth retardation was observed followed by growth at the normal rate. An exception concerned a cell culture exposed for 600 h to tritiated thymidine for which the slowed-down growth was observed for ca seventy cell generations.

Metaphase chromosomal aberrations were produced by 125I-labeled iododeoxyuridine (125I-UdR) incorporated into Chinese hamster Don cells at the end of the S-period of the cell cycle. Chromosome damage and the number of autoradiographic silver grains were recorded for whole cells, for chromosome pairs No. 4 and No. 5, and for the X and the Y chromosomes. The X and the Y chromosomes, which label late in S, were at least twice as heavily labeled as chromosome pairs No. 4 and No. 5--two readily recognizable autosomes of similar size. The incidence of chromosome damage was at least six times that which would have been expected from equivalent doses of X-rays and the incidence of damage was directly related to the number of silver grains over each chromosome. We estimate that it takes four to ten disintegrations to produce a visible chromosome aberration. The finding that chromosome damage is localized at the site of the 125I decay is most readily explained by the high flux of low energy Auger electrons occurring at the site of the decay of the incorporated 125I atom.

Though environmental biological equilibrium seems to be safeguarded as long as the radiological protection of man is assumed, a number of problems raised by environmental protection are not fully solved. In particular, there occur both macroscopic processes of concentration at some levels of the biological chains, and microlocalization at the cellular, subcellular and molecular levels. The long-term consequences of the non-uniform distribution of the radionuclides should therefore be assessed even if the contamination levels of the physical environment are very low. These aspects are of particular significance with alpha-emitters (plutonium and transplutonium elements), beta-emitters (chiefly tritium) and activation products, some of which are radioisotopes of biologically significant elements. The incidence of these processes is discussed in the light of recent and future developments in nuclear energy. This approach might open new avenues for investigations in environmental radiological protection. The concept of a concentration factor between the environment and the total organism must be supplemented by the concept of a localization and concentration factor at the subcellular level. The significance of the physico-chemical forms of the radionuclides and the parameters likely to modify the characteristics of membranes should be emphasized. Finally, an indication is given on priority measures in the area of investigations of the long-term consequences of low doses on environmental organisms.