Speciation of trivalent actinides and lanthanides in digestive media

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. Nevertheless, little is known about the speciation of not only trivalent actinides (An(III)) but also trivalent lanthanides (Ln(III)), non-radioactive chemical analogs of An(III), in human 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, part of an international unified bioaccessibility method (UBM), developed by the Bioaccessibility Research Group of Europe (BARGE) (Wragg et al., 2009). To verify the model, natural human saliva samples were also investigated (Barkleit et al., 2017).
The speciation of trivalent curium (Cm(III)) and europium (Eu(III)) in the gastrointestinal tract and in human natural saliva has been studied by means of time-resolved laser-induced fluorescence spectroscopy (TRLFS). The standard model body fluids and the natural saliva samples were spiked in vitro with Cm(III) or Eu(III) with a trace metal concentration.
The dominant chemical species in the body fluids were determined by linear combination fitting (LCF) analysis based on the reference spectra for individual components in the body fluids. The results indicates the formation of inorganic- (60-90%) and organic species (10-40%) of Cm(III)/Eu(III) in the salivary media. Ternary M(III) complexes containing phosphate and carbonate anions with the additional counter-cation calcium is found to be the main inorganic species, while the complexes with the digestive enzyme α-amylase and the protein mucin represent the major part of the organic species.
When the M(III) reached the stomach, the metal complexes are dissociated due to the high acidic conditions in the stomach. That is, Cm(III) and Eu(III) are mainly present as aquo complexes, while a small part (about 20%) is coordinated by the protein pepsin. When entering the intestine the M(III) strongly interact with the protective protein mucin (about 65%) and inorganic ligands (mainly carbonate and phosphate).
These speciation results in different body fluids of the gastrointestinal tract pointed out that An(III) and Ln(III) are coordinated by both inorganic and organic molecules in the human digestive system. 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 the whole body fluids.

Wragg, J., Cave, M., Taylor, H., Basta, N., Brandon, E., Casteel, S., Gron, C., Oomen, A., van de Wiele, T., 2009. British Geological Survey Open Report OR/07/027, Keyworth, Nottingham, 90 pp.
Barkleit, A., Wilke, C., Heller, A., Stumpf, T., Ikeda-Ohno, A., 2017. Trivalent f-elements in human saliva: a comprehensive speciation study by time-resolved laser-induced fluorescence spectroscopy and thermodynamic calculations. Dalton Trans. 46, 1593-1605