Metabolic pathways of carcinogenic chromium

The products of hexavalent chromium [Cr(VI)] reduction by glutathione (GSH) alone or in the presence of equimolar quantities of aspartate (Asp) and/or glutamate (Glu) and a chromium-containing material extracted from bovine liver were studied by ultraviolet-visible spectrum (UV-vis) studies, electrospray mass spectrometry (ES-MS), electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR). Reduction of chromate by GSH was followed by UV-vis and NMR, revealing the formation of a paramagnetic complex in which GSH acts as a ligand. ES-MS and EPR measurements provided unequivocal evidence of a dimeric Cr(V)2GSH2 species in which the two metal ions are bridged by the -Glu carboxylate. The analysis of the 1H and 13C shifts experienced by GSH protons and the values of paramagnetic contributions to proton spin-lattice relaxation rates provided a set of constraints for structural determination. The same experiments were repeated in the presence of an equimolar concentration of Asp, revealing the formation of a dimeric Cr(V) paramagnetic complex in which the two metals are now bridged by Asp. Nuclear magnetic resonance dispersion profiles show that water is not displaced by Asp and that the correlation time of this complex is slowed by the increased complexity. When Glu is also included in the solution in equimolar concentration to GSH and Asp, data are consistent with the formation of many mono- and dinuclear species, with the three ligands competing with each other. Finally, the spectroscopic investigation of the chromium-containing material extracted from bovine liver revealed the presence of a complicate mixture of Cr(IV) or Cr(V) complexes, among which some Cr(V)-GSH species are present alone or with other ligands in the metal coordination sphere.