Current topics in radiation research quarterly最新文献

Synchronized suspension cultures of Chinese hamster ovary cells (CHO) were labeled with various doses of 3H-thymidine or 125I-iododeoxyuridine to evaluate the cytocidal effects of intranuclear radionuclide decay. Damage produced by radionuclide decay outside the cell nucleus was studied on cells exposed to 125I labeled, monovalent concanavalin A. After labeling, the cells were resynchronized in G1-phase and incubated for 36 h at 4 degrees C to permit dose accumulation. Cell lethality was evaluated by the standard colony assay. Based on radionuclide incorporation data, cellular dimensions, and subcellular radionuclide distributions, the cumulative dose to whole cells, cell nuclei, and cellular cytoplasm was calculated from the known decay properties of 3H and 125I. As expected, DNA associated 125I (LD50: 60 decays/cell; 45 rad) was much more toxic to CHO cells than 3H (LD50: 1350 decays/cell; 380 rad) 380 rad) or external X-irradiation (LD50: 330 rad). In contrast, membrane associated 125I was surprisingly non-toxic (LD50: 19 600 decays/cell). At 19 600 decays/cell the dose to the cell membrane was approximately 52 krad and the overlap dose into the cytoplasm was about 2470 rad. Even at these high dose levels, membrane damage or cytoplasmic damage apparently did not contribute significantly to radiation induced cell death. With 19 600 decays on the plasma membrane the CHO nuclei received an overlap dose of about 410 rad. As can be seen from the LD50 data for 3H and X-rays, a nuclear dose of 410 rad should be sufficient to account for 50% cell death. These findings indicate that, although intranuclear decay by electron capture is extremely destructive, identical decay events in the plasma membrane cause only minimal cell damage. This parallels our earlier studies on 67Ga labeled leukemia cells which showed that electron capture decay in the cytoplasm is also highly ineffective in killing mammalian cells. It therefore appears that radiation-induced cell lethality in dividing mammalian cells results primarily from nuclear damage. Cytoplasmic or membrane contributions to radiation-induced cell death, if any, must be minimal. By implication, these findings refute the enzyme release hypothesis and similar theories designed to explain mitotic death in terms of cytoplasmic or membrane damage rather than nuclear damage.

The shape of the survival curve for cells inactivated by tritium decay in DNA is modified by the presence of halogenated pyrimidines in the DNA in a manner analogous to their effect on X-ray induced reproductive death. The large shoulder found for tritium suicide is removed completely by coincorporation of 10(-6) M IUdR. The oxygen enhancement ratio for 125I and tritium disintegrations in unsynchronized Chinese hamster cells was determined for cells permitted to accumulate damage from these events at 4 degrees C. The oxygen enhancement ratio for 125I induced damage is 1.4. This is much smaller than the OER found for tritium decay which is similar or more than that found for X-ray exposure under the same conditions. These results suggest that the nature of the lesions produced by 125I decay in DNA are analogous to those produced by high LET radiation while those lesions produced by tritium are similar to lesions produced by roentgen rays. In synchronous V79 cells the effects of 125I induced damage in different regions of the mammalian cell DNA was examined taking advantage of the fact that DNA replication in hamster nuclei follows a time-dependent three dimensional pattern. The experiments indicate that 125I decays accumulated in the G2-period of the cell cycle have different efficiences for the induction of reproductive death depending on the region of the DNA which is labeled. The efficiency for the induction of reproductive death appears to be a maximum in DNA that replicates in V79 cells near the end of the DNA replication cycle. Electron capture events are dramatically efficient in the production of lethal chromosome aberrations. In CHO cells synchronized in the G1-stage of the cell cycle stored in the frozen state the efficiency for the induction of dicentric and ring chromosomes is 0.03. The dose response curve for the induction of these aberrations is linear in contrast to the curvilinear response found for roentgen ray exposure under the same conditions. Data on this kind suggest that there may exist "critical" regions within mammalian cell nuclei where chromatin fibers from two different chromosomes are in close proximity to each other and both are damaged non-repairably by a single electron capture event.

The heterogeneous distribution and accumulation of radionuclides in discrete areas of cellular and subcellular dimensions is called microdistribution. The biological effect of microdistributed radionuclides with low-range emissions is determined by the degree of irradiation of the radiosensitive microareas of the body. The critical microareas of the body are nuclei of such cells which (1) are radiosensitive, (2) are essential to maintaining life, (3) are irreplaceable, (4) have a long life span and/or renew themselves. In this sense, the stem cell nuclei are considered critical microareas of the body. Stem cells constitute only a small fraction of the total body's cellularity. In case of concentration of radionuclides in stem cell nuclei, such as from incorporated labeled DNA precursors, there is a total congruence of the radionuclide microdistribution with the radiosensitive microarea, and the biological effect is expected to be enhanced over that from a homogeneous distribution of the same amount of radionuclides. This situation is discussed for 3H, 14C and 125I incorporate into mice as tracers of DNA precursors. The average labeling intensity of the bone marrow cell nucleus was taken to represent the average labeling intensity of the stem cell nucleus. The dose to the stem cell nucleus, then, is derived from the number and energy of decays originating in the nuclear mass of 270 X 10(-12) g. The transmutation effect from isotopic decay in DNA is considered in order to arrive at dose equivalents. On the basis of known data on labeling efficiency of bone marrow and on stem cell proliferation kinetics in the mouse, the infinite accumulation of decays in and the total expected dose to the stem cell nucleus was calculated for intravenous injection or ingestion of 1 muCi 3H-TdR per g body weight. The distribution factor and an annual limit on intake for the mouse model was suggested. Corresponding data are presented for 14C-TdR and 125I-UdR. A special situation is given for the case of hot particles where there is a random relationship between microdistributed radionuclides and critical microareas of the body. In this instance, theory predicts a decreased biological effect in comparison to the situation where the same amount of radionuclides is homogeneously distributed. There is experimental evidence that supports the theoretical predictions particularly for the case of 236Pu dioxide in the human lung.

Two hypotheses have been advanced to explain the extreme biological toxicity of DNA associated 125I: (a) high local concentrations of radiation energy from low energy Auger electron; (b) charge-induced molecular fragmentations in the DNA. To distinguish between these two hypotheses an attempt was made to evaluate the molecular events associated with electron capture decay, to calculate the microdistribution of radiation energy after 125I decay, and to relate the microdosimetry data to the biological toxicity of 125I and 3H. The cellular damage produced by the two radionuclides was evaluated on synchronized Chinese hamster ovary cells (CHO) labeled with various doses of 3H-thymidine or 125I-iododeoxyuridine. As expected, 125I (LD50: 45 rad; D0: 74 rad) proved much more toxic to CHO cells than either 3H (LD50: 380 rad; D0: 250 rad) or external X-irradiation (LD50: 330 rad; D0: 230 rad). To evaluate the molecular mechanism of 125I toxicity, iododeoxyuridine labeled with both 125I and 14C was synthesized and the effect of 125I decay on the molecular structure of iododeoxyuridine was studied by monitoring the fate of 14C activity after 125I decay. The results of this experiment indicate that 125I decay does not cause molecular fragmentation in iododeoxyuridine, only deiodination. Moreover, microdosimetry calculations show that at least in small target spheres more radiation energy is deposited on the average by decaying 125I than by a high LET alpha-particle traversing a sphere of equal diameter. These findings greatly strengthen the hypothesis that the high LET-type damage produced by Auger emitters results from high local concentrations of radiation energy rather than from charge-induced fragmentation of the DNA.

Rats were given 239Pu citrate i.v. and the mitochondria of hepatocytes were studied. A reduction of oxygen consumption per mg protein with time was seen. We have investigated the nucleotides involved in nuclear phosphorylation. After mitochondrial extraction according to Schneider and Hogeboom, [1950], a new method was used for estimating CMP, NAD, 5' AMP, NADP, ADP and GTP. Flavine mononucleotide was not determined. The plutonium content was followed from day 0 to day 65 post injection. Mitochondrial nucleotides were studied on days 5 and 11. On day 5, all nucleotide levels were reduced by 20 to 70% except AMP which increased and GTP which remained constant. On day 11, all nucleotides had decreased by 70 to 100% except NAD which increased by 20 to 30%. The results suggest that plutonium citrate given intravenously has a time dependent effect on the energy metabolism of the liver.

The incorporation and distribution of tritiated thymidine (3H-TdR) and tritiated water (HTO) have been measured in newborn rats exposed to various levels of tritium by continuous infusion into pregnant rats from day 9 until term. In the animals exposed to HTO, the tritium activity was homogeneously distributed, while 3H-TdR led to accumulation of DNA-bound and homogeneously distributed tritium. The incorporated activity and the specific activity of DNA from ovaries which showed a reduction of total oocyte number by approximately 50% were used to estimate the dose absorbed by the ovarian cell nuclei in both systems. From the absorbed dose, a factor of 3.7 was calculated for the "internal relative biological effectiveness" of DNA-bound tritium as compared to homogeneously distributed 3H under the restrictive assumption that the static description of the system at birth reflects the situation during the time of dynamic development of the ovaries when the toxic effect occurs. The influence of these dynamic factors of changing nuclear size and tritium incorporation during the sensitive period is weighed against the possibility that the continuous 3H-TdR infusion during pregnancy might represent a model in which DNA-bound tritium shows a higher effectiveness than homogeneously distributed tritium.

Whereas the radiotoxicity of tritium has been extensively studied, comparatively little information exists on its long-term effects as a potential environmental pollutant, particularly at small dosage. This investigation was primarily aimed at assessing comparatively a possible carcinogenic potency of tritiated water versus radioactive precursors of DNA, RNA and proteins, namely tritiated thymidine, uridine and leucine in C57 Black mice. Tritium is largely released in the environment in the form of tritiated water. There are many uncertainties, however, as to how tritium is incorporated from tritiated water into cell constituents quantitively and qualitatively. In 1965, we reported on the carcinogenic effect of tritium in the form of tritiated thymidine on newborn C57 BL mice in the dose range of 0.3--1.5 muCi/g [Mewissen. 1965]. Hence the selection of tritiated water, and of tritiated precursors, in an attempt to evaluate their respective role in the tritium transfer process and to correlate their possible late effects with their specific patterns or sites of incorporation. This study deals with tritium incorporation from tritiated water and various precursors at the 1 or 10 muCi level. RSA values, i.e., the ratio of organically bound tritium per hydrogen content of dry tissue over aqueous tritium per hydrogen content of water, were estimated for newborn, juvenile and adult mice, at various time intervals (1, 8, 15, 22 and 29 days) following single administration of tritiated water, tritiated thymidine, uridine or leucine. The data available at this time show that administration of tritiated water (or precursors) result in a complex time dependent and age dependent residual activity dynamics both in the organic component and in the aqueous fraction of tissue. A few preliminary conclusions can be made. Following a single acute or brief exposure to tritiated water, values of activity become exceedingly small after a relatively short time period. In a steady state equilibrium, resulting from chronic exposure to tritiated drinking water, RSA values tend to stabilize. However, wide variations between various organs are to be expected, as suggested by their respective RSA values following a single exposure. In view of these observations, it would seem that a realistic estimate of the internal dose to the radiosensitive nucleus must take into consideration the age dependent incorporation of tritium from tritiated water, as well as the variation between organs. The carcinogenic risk has often been estimated from a uniform dose dependency model. The influence of time and space microdistribution of dose within tissues and more particularly at specific sites (such as DNA, RNA or protein) has received, as yet, little attention, as well as the relative contributions of the time sequence of dose absorption during the usually long latency period. Such factors, among others, may be critical in carcinogenesis from internal irradiation...

The Auger cascade consists of low-energy electrons which are emitted instead of characteristic X-rays when a vacancy in an inner electronic shell is filled. Such atomic vacancies are produced by orbital electron capture, internal conversion, and the photoelectric absorption of photons. The relative abundance of Auger electrons and their decrease with increasing atomic charge (Z) is explained. For iodine (Z = 53), about 30% of the energy is carried by Auger electrons and thus is dissipated locally. As the concept of average dose cannot always be applied to electron capture decay in cellular components, microdosimetric parameters, applied separately to each of the distinct types of low energy radiations involved, seem more appropriate. The development and applications of the relevant microdosimetric concepts are reviewed briefly. The approach explains the high biological effectiveness of 125I in different and unrelated systems, such as DNA molecules and the thyroid gland. The effects of 125I on thyroids of humans and experimental animals were compared to those produced by 131I by the Glasgow and the Beilinson groups. The experience of these teams in applying therapeutic doses of 125I in order to control thyrotoxic patients is evaluated.

Preimplanted mouse embryos were cultured in an in vitro-system from the two cell stage to blastocysts. In the control cultures about 92% of the incubated embryos developed to blastocysts. When tritiated compounds like tritiated water or tritiated thymidine were present in the incubation medium the number of blastocysts decreased with increased medium tritiated thymidine was about one thousand times more effective than tritiated water. Tritiated thymidine caused a more pronounced division delay than tritiated water which had a strong effect on the blastulation process. Further studies showed that the incorporation of tritiated thymidine into the DNA lead to high concentrations of tritium in the cell nucleus. Dose calculations are performed for the cell nucleus in the case of incubation with tritiated thymidine as well as with tritiated water. The different action of the tritiated compounds can apparently be explained by the specific incorporation of thymidine into the DNA.

Mice have been exposed continuously, in utero, to tritiated water (via the maternal drinking water) or to tritiated thymidine (infused continuously into the mother). In both cases the patterns of labeling and subsequent loss of tritium over an extended period have been studied. The technique of infusion in unrestrained mice and its application in the production of fully tritium-labeled offspring is described in some detail. These fully labeled mice are being used to study a number of early and late effects, in particular, gonad cell effects and carcinogenesis, following this form of internal irradiation. Some preliminary results are presented. Similar results produced a homogeneous irradiation from tritiated water are also reported.