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Chem. Res. Toxicol. 2005, 18, 1150-1154
The Chemical Speciation of Uranium in Water Does Not Influence Its Absorption from the Gastrointestinal Tract of Rats S. Frelon,* P. Houpert, D. Lepetit, and F. Paquet IRSN, Direction de la Radioprotection de l’Homme, Service de Radiobiologie et d’Epide´ miologie, Laboratoire de Radiotoxicologie BP 166, F-26702 Pierrelatte Cedex, France Received December 10, 2004
Studies of the chemical speciation of uranium in water can enhance the knowledge of the mechanisms of its absorption from the gastrointestinal tract and its storage in the body. They can also help to improve the dosimetric models recommended by the International Commission on Radiological Protection (ICRP). The aim of this work was to assess the influence of uranium speciation on its absorption from the gastrointestinal tract by using both computer speciation modeling and direct measurement of the fractional absorption in vivo in rats after ingestion of five different samples of contaminated water. Preliminary ex vivo studies with human saliva and gastric juice showed that 90% of uranium was recovered with the natural components of the fluid studied. The computer studies of uranium speciation among the electrolytes of these fluids showed that under the set conditions, the chemical species changed in a broadly similar manner under the influence of fluid composition and pH. In vivo studies in rats validated these observations by indicating an average fractional absorption of about 0.4% for each of five different water samples. It is concluded that the chemical form of uranium in the water ingested did not influence its absorption into the body.
Introduction Biokinetic models are required to calculate radiation doses from internally deposited radionuclides. The postcontamination description of the behavior of an element in the body and the assessment of biological parameters, such as biodistribution, biokinetics, and excretion, are needed to establish these models, as realistically as possible on a physiological level. Of those parameters, transfer from the gastrointestinal tract (GIT), the entry route in the case of contamination through ingestion of an element, is often assessed by calculating the fractional absorption (f1), which determines the fraction of an element entering the blood circulation followed by the organs. In a 2005 report (1), the International Commission on Radiological Protection (ICRP) proposes f1 in relation to the solubility of the former ingested compound. The f1 is specific to both an ingestion mode and the mass ingested (2, 3) yet is also specific to the initial speciation because of the possible changes in properties of absorption and elimination of an element in relation to its chemical structure (4). For these reasons, scientists working in the field of radiological protection have highlighted the importance of postexposure radionuclide speciation in their distribution assessment and therefore dosimetry (5). In the case of uranium, an actinide that is both naturally present the whole length of the alimentary chain and implicated in broad-ranging coordination chemistry, there is very little data on its bioavailability from environment to mammals although the environ* To whom correspondence should be addressed. Tel: +33 4 75 50 75 50. Fax: +33 4 75 50 43 26. E-mail:
[email protected].
ment, and particularly water, represents a potential source of chronic contamination. A few studies report its transfer in environment, soils, and aquatic systems (6-9), but the assessment of uranium bioavailability for humans after oral intake is poorly documented at this time (7, 8). This work aimed to assess the influence of initial uranium speciation in water on uranium absorption from the GIT in the context of chronic contamination via drinking water. Ex vivo studies were performed to determine the distribution of uranium in fluids from the GIT and were followed by a computational uranium speciation approach (10) in the electrolytic fraction of these fluids. In both these approaches, the ICRP 30 model of the GIT was considered, i.e., separated compartments such as the mouth and stomach (11). Finally, an in vivo study was conducted to validate the results on an animal model by determining the uranium fractional absorption (f1) after ingestion of five different samples of contaminated water.
Experimental Procedures Chemicals. Uranium. Depleted uranium nitrate was purchased from Sigma-Aldrich (France). Uranium-233 nitrate was provided by CERCA (COGEMA, France). Composition of the Water Samples. Five different samples of water were used in this study. Three of them were mineral and purchased in public shops (I-III), one was distilled water (IV), and the other (V) was made up of water IV + salts but no carbonates. The ionic compositions of the five samples used are listed in Table 1. Methods and Approaches. Ex Vivo Study. Filtrations over cutoff membranes followed by uranium measurement in each fraction were performed to assess the distribution of
10.1021/tx049662i CCC: $30.25 © 2005 American Chemical Society Published on Web 06/24/2005
Chemical Speciation of Uranium in Water
Chem. Res. Toxicol., Vol. 18, No. 7, 2005 1151
Table 1. Ionic Composition (mg L-1) of the Water Samples Used for Experimentsa water (mg L-1) ions
I
II
III
IV
V
K+ ClNO3HCO3SO42Ca2+ Mg2+ Na+ pH at equilibrium
1 5 3.8 357 10 78 24 5 7.2
0 0 0 403 1479 555 110 14 7.0
5.7 8.4 6.3 65.3 6.9 9.9 6.1 9.4 7.0
0 0 0 0 0 0 0 0 6.5
0 0 0 0 1.9 555 110 14 7.1
a Waters I-III are mineral waters. Water V was prepared from water IV (distilled water, Sigma), by addition of different salts.
uranium within either saliva or a mixture of saliva and gastric juice. The method is based on the ability of such filters to separate biological molecules by molecular weight following centrifugation. Preparation of Labeled Solutions. Mineral water I was used to prepare the uranium solution to provide constant ionic composition throughout the experiment. The uranyl stock solution was prepared by dissolving depleted uranyl nitrate (240 mg L-1) in drinking water I and adding 233Uranyl nitrate (233U) (molar ratio of DU:233U, 5:1). The stock solution was stirred for 1 h to ensure that the uranium was totally dissolved. It was filtered through 0.22 µm filter units, and the uranium concentration was checked by a Kinetic Phosphorescence Analyzer (Chemchek, United States). Experiments with Biofluids. Both the saliva and the gastric juice used for these experiments were collected from healthy volunteers and pooled and, in the case of saliva, was used fresh, while the gastric juice was stored for 1 day at -80 °C. The volume ratios used in the experiments were calculated from published data given in Table 2. The first experiment with saliva consisted of adding uranyl stock solution to a volume of saliva (volume ratio of uranyl solution:saliva, 5:1) at 37 °C. The mixture was stirred and allowed to stand for 10 s before being centrifuged. For the second experiment, depleted uranyl nitrate in drinking water I was added, the mixture was stirred, and then, 233Uranyl nitrate (233U) (molar ratio of DU:233U, 5:1) was added. As in the first experiment, the mixture was stirred and allowed to stand for 10 s before being centrifuged. The experiment with gastric juice consisted of adding uranyl stock solution to a volume of saliva (volume ratio of uranyl solution:saliva, 5:1) at 37 °C. The mixture was stirred and allowed to stand for 10 s before adding a volume of gastric juice (volume ratio of uranyl solution:saliva:gastric juice, 5:1:1.45) at 37 °C. The mixture was stirred and then allowed to stand for 2 h at 37 °C before being centrifuged. Centrifugation Studies. An aliquot of 665 µL of the watersaliva or water-saliva-gastric juice mixture was centrifuged at 2000g for 15 min at 4 °C to separate the supernatant from
the residue fraction (Fresidue). Then, 500 µL of the supernatant was placed in an amicon filtration tube (Millipore Ltd., France) fitted with a 30 kDa cutoff membrane and centrifuged at 2000g for 15 min at 4 °C to separate the fractions of >30 kDa (F>30kDa) and
