Uranium bioavailability and toxicity are closely linked to the metal’s speciation in solution. However in biological fluids or in media classically used for cell culture - and subsequently for in vitro cell exposure -, uranium is rarely present as free-ion since these media contain non-negligible concentrations of potential ligands such as phosphate, bicarbonate and/or calcium. The chemical form of uranium that is internalized in cells and interferes with biological processes is of major concern. Uranium toxicity and accumulation were evaluated in vitro on NRK-52E cells, model for rat renal proximal tubule. Uranium intracellular accumulation begins after 12 h exposure to 600 µM U; toxicity appears as soon as cells accumulated 25 to 30 mg U/g protein. Modification of uranium speciation in the exposure medium induces great changes in toxicity and cell accumulation. Comparison of toxicity and accumulation results to theoretical uranium speciation, calculated with the J-Chess computer program, shows that free-ion concentration can not explain the total uranium intracellular accumulation. Low molecular weight U complexes, such as UO2(CO3)34- but also UO2PO4- could be implicated in U cellular accumulation and toxicity.
These postulates will be verified by direct U speciation in cell exposure medium by XAFS analysis.