Oleic acid allows more apoprotein A-1 to bind with higher affinity to large emulsion particles saturated with cholesterol

Apoprotein (apo) A-1 binding to large triolein-rich emulsion particles saturated with cholesterol has been examined as a function of the oleic acid content. Six emulsion systems were formed containing 0.3-1.0% (by weight) oleic acid, 82.9-86.3% triolein, 10.6-7.2% egg yolk phosphatidylcholine, and 6.7-5.5% cholesterol. The average emulsion particle diameters calculated from these lipid compositions ranged between 84 and 116 nm. Negative stain electron microscopy of an emulsion containing 1% oleic acid showed a polydisperse population of only large spherical particles with a mean diameter of 116 +/- 54 nm. The calculated cholesterol concentrations of the particles surface and core for the six emulsions were 43.3 +/- 1.1 and 5.6 +/- 0.2 mol%, respectively, and were rather constant. Therefore, when the surface oleic acid concentrations increased from 2.6 to 10.1 mol%, the phospholipid concentration decreased from 55.1 to 45.9 mol%. In the core, oleic acid increased at the expense of triolein. In the range studied a nearly 4-fold increase in the surface oleic acid content produces a similar increase in the binding capacity (N) and reduces the dissociation constant (Kd). The changes in the Kd and N values were linearly dependent on the surface oleic acid concentration. These data show that oleic acid allows more apoA-1 to bind with higher affinity to large emulsion particles saturated with cholesterol.