[3H]cholesterol transfer from microemulsion particles of different sizes to human fibroblasts.

A new technique for preparing microemulsion particles of well-defined sizes and compositions is presented. Utilization of these microemulsions is advocated as lipoprotein models in studies of lipid transport and metabolism, rather than the currently used phospholipid-cholesterol vesicles. The emulsion particles consisted of egg phosphatidylcholine and cholesterol as surface lipids and cholesteryl oleate as core lipid. They were prepared by a combined injection and sonication technique and size-separated by a two-step procedure of gel filtration chromatography and density gradient centrifugation. By varying the ratios of core and surface material, particles covering a size range of 20-200 nm in diameter could be produced. The adequacy of these microemulsions as lipoprotein models was tested by studying the transfer of [3H]cholesterol and [14C]cholesterol oleate from the particles to cultured human fibroblasts. Up to a particle size of 100 nm, there was a slight increase of [3H]cholesterol transfer. The transfer of [14C]cholesteryl oleate was very slow, yet measurable. Studies of the exchangeability of cholesterol between the microemulsion core and surface phases indicated that all cholesterol can be transferred from microemulsions to cultured cells as a single pool.