Characterization of phospholipids in pre-alpha HDL: selective phospholipid efflux with apolipoprotein A-I.

Previously, we have shown that lipid-free apoA-I, when incubated with fibroblasts, will produce lipoproteins of pre-alpha mobility (Asztalos, B. F., et al. 1997. Arterioscler. Thromb. Vasc. Biol. 17: 1630-1636). In order to understand the nature of these pre-alpha particles, we further characterized their lipid content. The pre-alpha particles are high density lipoproteins, having a median density of 1.08 g/ml. They have a surface charge of -18.45 mV. The phospholipid composition of these particles showed that they have 4% each of phosphatidyl ethanolamine and inositol; 69% phosphatidyl choline and 18% sphingomyelin. This phospholipid composition is different from those of plasma HDL (81% phosphatidyl choline, 13% sphingomyelin), plasma membrane on the fibroblasts, and whole fibroblast phospholipid. To demonstrate that the pre-alpha mobility resides in the lipids, lipids from pre-alpha lipoproteins were reconstituted with lipid-free apoA-I. The resultant particles retained their pre-alpha mobility. We conclude that apoA-I may react with specific regions of plasma membrane to acquire this unusual lipid composition and that pre-alpha mobility is caused in part by the unusual phospholipid composition.