No abstract

Uranium is a naturally occurring heavy metal. Its extensive use in the nuclear cycle and for military applications has focused attention on its potential health effects. Acute exposures to uranium are toxic to the kidneys where they mainly cause damage to proximal tubular epithelium. The purpose of this study was to investigate the biological consequences of acute in vitro uranyl exposure and the influence of uranyl speciation on its cytotoxicity. NRK-52E cells, representative of rat kidney proximal epithelium, were exposed to uranyl-carbonate and -citrate complexes, which are the major complexes transiting through renal tubules after acute in vivo contamination. Before NRK-52E cell exposure, these complexes were diluted in classical or modified cell culture media, which can possibly modify uranyl speciation. In these conditions, uranium cytotoxicity appears after 16 h of exposure. The CI50 cytotoxicity index, the uranium concentration leading to 50% dead cells after 24 h of exposure, is 500 microM (+/-100 microM) and strongly depends on uranyl counterion and cell culture medium composition. Computer modeling of uranyl speciation is reported, enabling one to draw a parallel between uranyl speciation and its cytotoxicity.

Ralstonia metallidurans CH34, a soil bacterium resistant to a variety of metals, is known to reduce selenite to intracellular granules of elemental selenium (Se 0 ). We have studied the kinetics of selenite (Se IV ) and selenate (Se VI ) accumulation and used X-ray absorption spectroscopy to identify the accumulated form of selenate, as well as possible chemical intermediates during the transformation of these two oxyanions. When introduced during the lag phase, the presence of selenite increased the duration of this phase, as previously observed. Selenite introduction was followed by a period of slow uptake, during which the bacteria contained Se 0 and alkyl selenide in equivalent proportions. This suggests that two reactions with similar kinetics take place: an assimilatory pathway leading to alkyl selenide and a slow detoxification pathway leading to Se 0 . Subsequently, selenite uptake strongly increased (up to 340 mg Se per g of proteins) and Se 0 was the predominant transformation product, suggesting an activation of selenite transport and reduction systems after several hours of contact. Exposure to selenate did not induce an increase in the lag phase duration, and the bacteria accumulated approximately 25-fold less Se than when exposed to selenite. Se IV was detected as a transient species in the first 12 h after selenate introduction, Se 0 also occurred as a minor species, and the major accumulated form was alkyl selenide. Thus, in the present experimental conditions, selenate mostly follows an assimilatory pathway and the reduction pathway is not activated upon selenate exposure. These results show that R. metallidurans CH34 may be suitable for the remediation of selenite-, but not selenate-, contaminated environments.Microorganisms play a major role in the biogeochemical cycle of selenium in the environment (12). Certain strains that are resistant to selenium oxyanions, and reduce selenite (Se IV ) and/or selenate (Se VI ) to the less available elemental selenium (Se 0 ) (7), could be potentially used for the bioremediation of contaminated soils, sediments, industrial effluents, and agricultural drainage waters.Ralstonia metallidurans CH34 is a soil bacterium characteristic of metal-contaminated biotopes. It is resistant to a variety of heavy metals and metalloids including Cr VI , Co II , Ni II , Cu II , Zn II , As V , Cd II , Hg II , and Pb II . The genes for metal resistance are located in two large plasmids (pMOL28 and pMOL30). Their function and regulation are well understood for some of these elements (18). This bacterial strain is also resistant to selenite, and detoxification is realized by the incorporation of this oxyanion and its subsequent reduction to red Se 0 , as shown by X-ray absorption spectroscopy (24). This study also revealed that the Se 0 granules were localized mainly in the cytoplasm. In contrast to previously cited metals and metalloids, the genes involved in selenite resistance have not yet been identified, and the exact mechanism of selenite bioreduction is still unknown. R....