Characterization of metal complexes with metallothioneins by capillary zone electrophoresis (CZE) with ICP-MS and electrospray (ES)-MS detection

The complementarity of CZE-ICP-MS and CZE-ES-MS couplings was evaluated for the characterization of metal complexes with metallothionein (MT) isoforms. The preparations characterized included the rabbit liver MT and its further purified MT-1 and MT-2 isoforms. The CZE-ICP-MS electrophoregrams showed the presence of several well (often baseline) separated Gaussian-shape peaks. ES-MS was used for the identification of these signals; mass spectra taken at the peak apexes were complex and indicated the co-existence of several metal complexes (possibly with different isoforms) within each peak. Many of these complexes could be identified on the basis of the molecular mass. The study highlights the attractive potential of CZE-ICP-MS and CZE-ES-MS when used in parallel for the characterization of metal complexes with biomolecular ligands. The inherent discrepancy between the sensitivities of ICP-MS and ES-MS is partly alleviated by the fact that the metal (detected by ICP-MS) constitutes only a small fraction of the total molecule (detected by ES-MS), and by the smaller dilution factor with the makeup sheath flow in ES-MS than in ICP-MS.