Tb3+ binding to human erythrocyte spectrin resulting in conformation change and aggregation

Spectroscopic studies of interactions involving horseradish peroxidase and Tb3+

The spectroscopic properties of interactions involving horseradish peroxidase (HRP) and Tb3+ in the simulated physiological solution was investigated with some electrochemical and spectroscopic methods, such as cyclic voltammetry (CV), circular dichroism (CD), X-ray photoelectron spectroscopy (XPS) and synchronous fluorescence (SF). It was found that Tb3+ can coordinate with oxygen atoms in carbonyl groups in the peptide chain of HRP, form the complex of Tb3+ and HRP (Tb-HRP), and then lead to the conformation change of HRP. The increase in the random coil content of HRP can disturb the microstructure of the heme active center of HRP, in which the planarity of the porphyrin cycle in the heme group is increased and then the exposure extent of the electrochemical active center is decreased. Thus Tb3+ can inhibit the electrochemical reaction of HRP and its electrocatalytic activity for the reduction of H2O2 at the Au/Cys/GC electrode. The changes in the microstructure of HRP obstructed the electron transfer of Fe(III) in the porphyrin cycle of the heme group, thus HRP catalytic activity is inhibited. The inhibition effect of Tb3+ on HRP catalytic activity is increased with the increasing of Tb3+ concentration. This study would provide some references for better understanding the rare earth elements and heavy metals on peroxidase toxicity in living organisms.

Nature of cerium(III)- and lanthanum(III)-induced aggregation of human erythrocyte membrane proteins

To clarify the nature of the aggregation of membrane proteins (MP) induced by lanthanide cations (Lns), the interaction of cerium(III) (Ce3+) and lanthanum(III)(La3+) with erythrocyte membrane proteins was studied by means of SDS-PAGE, light scattering measurement, fluorescence, CD and FTIR spectra. The results showed that Ce3+ and La3+ induce protein aggregation not only by Lns non-covalent binding and cross-linking, but also by oxidative cross-linking through disulfide bond formation. As demonstrated by intrinsic fluorescence, CD and FTIR spectra studies, the aggregation was accompanied by the conformation changes with tryptophane residues exposing to more hydrophobic environment and the decreasing alpha-helix and beta-sheet contents. By stopped-flow studies, protein aggregation was shown to be a slow change, which is initiated by rapid Lns binding and then followed by subsequent conformational changes.

Gadolinium induces domain and pore formation of human erythrocyte membrane: an atomic force microscopic study

Lanthanide cations bind to human erythrocyte membranes and enhance cell permeability. It was postulated that this effect is due to their likeness with calcium ions, which have been used to induce perforation of cells. However, the nature and mechanism of the perforation are still not clear. In the present work, the change in surface topography of erythrocyte membranes exposed to various gadolinium species was imaged with an atomic force microscope (AFM) in order to get direct evidence of perforation. The images of the whole cell and regions in nanometer scale showed that the normal surface is featured by closely packed nanometer size particles. The AFM images showed that Gd(3+) binding to erythrocytes led to domain structure at low concentration and pore formation at higher concentration. The domain structures that appeared after incubation with 1.0x10(-6)-1.0x10(-5) mol/l Gd(3+) solution for 30 min are featured by the particles aggregated to form ranges and the separations among them enlarged to gorges. With a higher concentration, 2.5x10(-5) mol/l Gd(3+), the further aggregation developed into crater-shaped 'pores'. By washing with EDTA the 'pores' can be resealed but the domain structure remained. The anionic complex of Gd(3+), [Gd(Cit)(2)](3-) of this concentration, can only induce the domain structure formation. The domain and 'pore' structures mediated by Gd(3+) concentrations might be responsible for both enhanced permeability and perforation. The mechanism of Gd-induced domain formation and perforation is discussed on the basis of aggregation of membrane proteins and the coexistence of different phases of membrane lipids resulting from Gd(3+) binding.

The reaction of lanthanide ions with n-doxyl stearic acids and its utilization for the ESR study on the permeability of lipid-bilayer of erythrocyte membrane to gadolinium ions

The reaction of lanthanide ions with n-doxyl stearic acid (nDS) spin labels (n = 5,7,12,16) was investigated by the electron spin resonance technique in aqueous solution. Among the lanthanides, the Gd3+, Tb3+, Tm3+ and Ce3+ ions strongly quenched the ESR signal of spin labels, but the effects of La3+, Eu3+ and Lu3+ are very weak. The quenching effects are featured by: (1) the dependence on the concentration of lanthanide ions; (2) no obvious changes of the ESR line shape in the presence of lanthanide ions; (3) the quenching constant decreases in the order: Gd3+ > Tb3+ > Tm3+ > Ce3+; (4) the quenching effects of lanthanide ions are found to strikingly correlate with their magnetic properties. These findings indicate that the interaction of lanthanide ions with nitroxide oxygen leading to the reduction of ESR signal amplitude is dominated by their magnetic characteristics rather than the coordination effect. By labeling erythrocyte membrane with nDS, n = 5,7,12,16 at different depths, we studied the diffusion of Gd3+ into the lipid-bilayer of erythrocyte membrane by monitoring the reduction processes of the ESR signals of nitroxide spin labels located at different depths of membrane lipid-bilayer after addition of Gd3+. These results revealed that the Gd3+ ions can penetrate into the lipid-bilayer, though the entry rate is slow. It was shown that the Gd3+ ions bind to the membrane and enhance the permeability of extracellular ascorbate into erythrocyte membrane. The transport mechanism of Gd3+ ions through the lipid-bilayer might be involved in the Gd3+ cation-induced pore formation in the surface of membrane.