Phosphatidylserine Monolayers Research Articles

Physical studies of phospholipids: XI. Ca 2+ binding to monolayers of phosphatidylserine and phosphatidylinositol

Measurements have been made of the binding of radioactive Ca 2+ to unimolecular films of purified phosphatidylserine and phosphatidylinositol at various film pressures. Three theoretical treatments based on the mass equation have been applied to the experimental data to evaluate the binding of Ca 2+ in terms of K A , the apparent association constant, and n, the fraction of phospholipids molecules acting as binding sites for a Ca 2+. At low film pressures (or low charge densities) the binding of Ca 2+ follows the mass equation. Deviations from the mass equation are observed with increasing film pressure (charge density) and are attributed to electrostatic interaction between adjacent binding sites. The ratio of Ca 2+ bound/lipid reaches a maximum at about 30 dynes/cm when the spacing between phospholipid molecules is most favourable for two-point electrostatic attachment of Ca 2+. Ca 2+ bound to phosphatidylserine and phosphatidylinositol form a Stern layer and do not penetrate into the plane of the Gouy potential.

Biophysical properties of phospholipids. I. Interaction of phosphatidylserine monolayers with metal ions

Biophysical properties of phospholipids. I. Interaction of phosphatidylserine monolayers with metal ions

Membrane model: Association of inorganic cations with phospholipid monolayers

The present work is relevant to the problem of identifying the membrane molecule or molecules which may provide negatively charged groups for ion exchange. The monolayer approach has been used and the following experiments done: (i) association of cations with polar groups of purified phospholipids has been measured in terms of surface pressure-area diagrams, (ii) adsorption of Ca2+ to the monolayer and its dependence on pH has been measured to determine the relation between ionization of the monolayer polar groups and ion-exchange properties, and (iii) the displacement of Ca2+ from phosphatidylserine films by Na+, K+, or H+ at different degrees of monolayer packing has been measured to determine whether or not one ion is more effective than another in displacing Ca2+. The following findings suggest that phosphatidylserine and phosphatidylethanolamine may provide negatively charged groups for ion exchange at cell surfaces: (a) Ca2+ in the hypophase reduces the surface pressure (condenses the monolayer) as compared with Na+ or K+. This reduction in surface pressure is not changed by addition to the hypophase of Na2+ or K+ up to 10 mM but is completely prevented by either Na+ or K+ at 100 mM. (b) Uptake of Ca2+ by phosphatidylsernie and phosphatidylethanolamine monolayers measured by isotope technique shows that there is 1 Ca2+ adsorbed per phosphatidylserine and 0.35 Ca2+ per phosphatidylethanolamine molecule at pH7. (c) Displacement of Ca2+ adsorbed to phosphatidylserine films by Na+, K+, or H+. This experiment shows that monovalent cations are in competition with Ca2+ for the negatively charged sites of phosphatidylserine monolayers. (d) More Ca2+ is displaced from phosphatidylserine films by either Na+ or K+ at low surface pressure than at high surface pressure. This is interpreted in terms of peri-polar groups volume available for ion intrusion. (e) Relation between ionization of the polar groups of phosphatidylserine and phosphatidylethanolamine and ion-exchange properties measured by studying the adsorption of Ca2+ to the monolayers and its groups of phosphatidylserine and phosphatidylethanolamine, and the carboxyl group of phosphatidylserine seem to be ionized.