Speciation of trivalent actinides and lanthanides in body fluids


Speciation of trivalent actinides and lanthanides in body fluids

Barkleit, A.; Wilke, C.; Heller, A.; Ikeda-Ohno, A.; Stumpf, T.

In case of incorporation into the human body, radionuclides potentially represent serious health risks due to their chemo- and radiotoxicity. In order to assess their toxicological behavior, such as transport, metabolism, deposition, and elimination from the human organisms, the understanding of their in vivo chemical speciation on a molecular level is crucial. Due to their high specific radioactivity with very long half-lives, trivalent actinides (An(III)) are considered to be some of the problematic radionuclides particularly in the geological repository of radioactive wastes. The reliable safety and health assessment of the waste repositories requires the information about the behavior of An(III) in vivo. Nevertheless, little is known about the speciation of not only An(III) but also trivalent lanthanides (Ln(III)), non-radioactive chemical analogs of An(III), in body fluids.
In order to improve our understanding of the behavior of An(III) and Ln(III) in the human body, the present study focuses on the chemical speciation of An(III) and Ln(III) in the gastrointestinal tract. The human gastrointestinal system was simulated by using an in vitro digestion model, which was developed by Oomen et al. [1] and is the basis of an international unified bioaccessibility protocol [2]. To verify the model, natural human saliva samples were included in the speciation investigation. Because An(III) and Ln(III) are excreted mainly by the kidney [3, 4], their speciation in natural human urine was investigated to complete the metabolic pathway from oral ingestion through the digestive system till elimination.
The speciation of curium(III) (Cm(III)) and europium(III) (Eu(III)) in the gastrointestinal tract as well as in human natural saliva and urine has been studied by means of time-resolved laser-induced fluorescence spectroscopy (TRLFS). The standard model body fluids and the natural saliva and urine samples were spiked in vitro with Cm(III) and Eu(III) in trace metal concentrations.
The dominant chemical species in the human saliva was identified by a comparison of the natural human sample spectra with reference spectra obtained for synthetic saliva and individual components of the body fluid. Linear combination fitting analysis on the sample spectra indicates the formation of 60-90% inorganic- and 10-40% organic species of Cm(III)/Eu(III) in the salivary media. Ternary M(III) complexes containing phosphate and carbonate anions with the additional counter-cation calcium are formed as the main inorganic species. Complexes with the digestive enzyme α-amylase and the protein mucin (to a minor extent) represent the major part of the organic species.
When the M(III) reached the stomach, the metal complexes were dissociated due to the high acidic conditions. That is, Cm(III) and Eu(III) are mainly present as the aqua ion, and only a small part (about 20%) is coordinated by the protein pepsin. When entering the intestine the metal ions are strongly bound by the protective protein mucin and inorganic ligands (mainly carbonate and phosphate).
After transporting into the bloodstream and transformation into the urine via the kidney, the speciation of the metal ions strongly depends on the pH of the urine. When the pH is slightly acidic, the formation of Cm(III) and Eu(III) citrate complex dominates, whereas ternary complexes with phosphate and calcium as the main ligands and the additional participation of citrate and/or carbonate occur at around near-neutral pH [5].
These speciation studies in different body fluids pointed out that An(III) and Ln(III) are coordinated by both inorganic and organic molecules in the human body. Proteins (e.g. α-amylase, pepsin, mucin) would be the important organic binding partners. Furthermore, ternary inorganic complexes containing phosphate and carbonate anions with the additional counter-cation calcium are expected to be formed as the main inorganic species in almost all the body fluids.
References
[1] Oomen, A. G., Rompelberg, C. J. M., Bruil, M. A., Dobbe, C. J. G., Pereboom, D. P. K. H., Sips, A. J. A. M., Arch. Environ. Contam. Toxicol. 44, 281-287 (2003).
[2] Wragg, J., Cave, M., Taylor, H., Basta, N., Brandon, E., Casteel, S., Gron, C., Oomen, A., van de Wiele, T., British Geological Survey Open Report OR/07/027, Keyworth, Nottingham, 90 pp. (2009).
[3] Menetrier, F., Taylor, D. M., and Comte, A., Appl. Radiat. Isot. 66, 632–647 (2008).
[4] Taylor, D. M., Leggett, R. W., Radiat. Prot. Dosim. 105, 193–198 (2003).
[5] Heller, A., Barkleit, A., Bernhard, G., Chem. Res. Toxicol. 24, 193-203 (2011).

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    9th International Conference on Nuclear and Radiochemistry NRC9, 29.08.-02.09.2016, Helsinki, Finland

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