Chemical speciation of An(III) and Ln(III) in human saliva

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 saliva. Saliva is one of the most important body fluids to understand the behavior of these metals in the digestive system, as it is the very first contact medium in the human body in case of oral ingestion.
We report the first speciation study of curium(III) and europium(III) in human saliva by means of time-resolved laser-induced fluorescence spectroscopy (TRLFS). For TRLFS measurements, fresh saliva samples from humans have been spiked in vitro with Cm(III) or Eu(III). The dominant chemical spe-cies in the human saliva was identified by a comparison of the sample spectra with reference spectra obtained for synthetic saliva and individual components of the body fluid. Linear combination analysis on the sample spectra indicates the formation of about 60-90% inorganic- and 10-40% organic species of Cm(III)/Eu(III) in the salivary media. A ternary M(III) complex containing phosphate and carbonate anions with the additional counter-cation calcium is formed as the main inorganic species. Complexes with α-amylase and mucin (to a minor extent) represent the major part of the organic species. Thermodynamic calculation of the speciation, based on the recently determined stability constants for Cm(III) and Eu(III) complexes with α-amylase, also supports the experimentally determined speciation.