Interactions of La2+ with phosphatidylserine vesicles: binding, phase transition, leakage, 31P-NMR and fusion.
The interaction of La2+ with phosphatidylserine vesicles is studied by differential scanning calorimetry, 140La binding, 31P-NMR chemical shifts and relaxation rates, carboxyfluorescein and [14C]sucrose release, X-ray diffraction and freeze-fracture electron microscopy. In the presence of La3+ concentrations above 1 mM and an incubation temperature of 38 degrees C, i.e., at the phase transition temperature of the complex La/phosphatidylserine, the binding ratio of La/lipid exceeds a 1/3 ratio, reaching saturation at a 1/2 ratio. Analysis, employing a modified Gouy-Chapman equation, indicates a significant increase in the intrinsic binding constant of La/phosphatidylserine when the La3+ concentrations exceeds the threshold concentration for leakage. The analysis illustrates that at the molecular level the binding of La3+ can be comparable to or even weaker than that of Ca2+, but that even when present at smaller concentrations La3+ competes with and partially displaces Ca2+ from membranes or other negatively charged surfaces. The results suggest that the sequence La3+ greater than Ca2+ greater than Mg2+ reflects both the binding strength of these cations to phosphatidylserine as well as their ability to induce leakage, enhancement of 31P spin-lattice relaxation rates, fusion and other structural changes. The leakage, fusion, and other structural changes are more pronounced at the phase transition temperature of the La/lipid complex.
D004578:Electron Spin Resonance Spectroscopy; D005614:Freeze Fracturing; D007811:Lanthanum / Q000378:metabolism*; D008563:Membrane Lipids; D008567:Membranes, Artificial; D008722:Methods; D008854:Microscopy, Electron; D008968:Molecular Conformation; D010718:Phosphatidylserines / Q000378:metabolism*; D011868:Radioisotopes; D013395:Sucrose / Q000378:metabolism; D014961:X-Ray Diffraction
D008563:Membrane Lipids; D008567:Membranes, Artificial; D010718:Phosphatidylserines; D011868:Radioisotopes; D013395:Sucrose; D007811:Lanthanum