Cr(III) exerts stronger structural effects than Cr(VI) on the human erythrocyte membrane and molecular models

Chromium exists in many oxidation states, of which only the hexavalent Cr(VI) and the trivalent Cr(III) ions are stable under environmental conditions. It is generally reported that Cr(VI) is highly toxic while Cr(III) is relatively innocuous, although others have reported just the opposite. On the other hand, despite the many studies on chromium toxicity, and particularly after the knowledge that Cr(VI) anions readily enter the erythrocytes where they are reduced to Cr(III), there are practically no reports on the structural effects induced by chromium compounds on the erythrocyte membrane. With the aim to better understand the molecular mechanisms of the interaction of Cr(III) and Cr(VI) with cell membranes, CrCl3, and K2CrO4 were incubated with intact erythrocytes, isolated unsealed human erythrocyte membranes (IUM), and molecular models of the erythrocyte membrane. These consisted in bilayers built-up of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylcholine (DMPE), phospholipid classes present in the outer and inner monolayers of the erythrocyte membrane, respectively. The capacity of Cr(III) and Cr(VI) to perturb the bilayer structures of DMPC and DMPE was evaluated by X-ray diffraction, DMPC large unilamellar vesicles (LUV) and IUM were studied by fluorescence spectroscopy, and intact human erythrocytes were observed with scanning electron microscopy (SEM). In all these systems, it was found that Cr(III) induced considerably higher structural perturbations than Cr(VI).