Mg2+, Ca2+, and Mn2+ bound on anionic phospholipids resist desalting dialysis: Evaluation of binding parameters using stern adsorption isotherms

Abstract Desalting dialysis experiments were undertaken to provide insight into the specific adsorption of Mn +2 , Ca +2 , and Mg 2+ and the extracted membrane lipids DGDG, PC, PA, PG, and PS. The analysis of remaining cation and phosphate in dialysis bags permits one to trace the adsorption isotherm. The remaining surface charge of the lipid vesicles following the adsorption of cation allowed the evaluation of the intrinsic fixation constant, K I , using the Stern isotherm function as applied by Nir et al. (22) to divalent cation fixation or to an ion exchange reaction at the surface of phosphatidylserine vesicles. Only the portion of the data corresponding to a cation concentration lower than the aggregation threshold value can be reasonably simulated by the partial isotherm function. A saturation in the fixation of the cation to anionic phospholipids occurred. It is mainly attributed to a modification in the conformation of phospholipids in order to protect neighboring sites of fixation following a divalent cation binding. Modifying the above function to include a factor n , a saturation index for the accessibility of sites on the surface of vesicle permits one to stimulate adequately the fixation data and to determine K I of specific fixation of divalent cation to some anionic phospholipid. The values of K I obtained for PS are 2, 7, and 3 M −1 for the fixation of Mg +2 , Mn +2 , and Ca +2 , respectively; for the formation of MgPA, MnPA, and CaPA complexes, the values of K I are 3, 1.5, and 6 M −1 , respectively; and finally in the case of PG, only the fixation of Ca +2 gives quantitative results, 4 M −1 .