A technique to estimate the apparent surface pressure of emulsion particles using apolipoproteins as probes.

The apparent pressures in the surface monolayer of emulsion particles can be estimated by comparing the absorption of an apolipoprotein to planar lipid monolayers and to emulsions. Lipids are spread at an air-water interface in a Pockels/Langmuir surface balance and the adsorption of [14C]-labeled apolipoproteins placed in the subphase is studied as a function of surface pressure using the surface radioactivity method. An apoprotein surface concentration/initial lipid surface pressure curve (gamma/pi i) is constructed. The maximum apolipoprotein surface concentration gamma e of emulsions is derived from standard emulsion/apolipoprotein binding isotherms. The apparent emulsion surface pressure is then estimated by comparing gamma e to the gamma/pi i curve. Apolipoprotein A-I has been used as an example of a probe to estimate the effective surface pressure in approximately 1000 A diameter egg yolk phosphatidylcholine/cholesterol/triolein emulsion particles. When the cholesterol content of emulsions is low, the surface pressure of the emulsion is about 17 dyne cm-1. At high cholesterol concentrations (0.49 cholesterol/phospholipid mole ratio) the surface pressure is increased to 25 dyne cm-1. The addition of the maximum amounts of apoA-I to these particles raises the effective surface pressure of the emulsion to about 30 dyne cm-1 and stabilizes the particles.