Incorporation of phosphatidylcholine into spherical and discoidal lipoproteins during incubation of egg phosphatidylcholine vesicles with isolated high density lipoproteins or with plasma.

Incubation of phosphatidylcholine vesicles with isolated high density lipoproteins (HDL) or with whole plasma results in transfer of phospholipid into HDL. To investigate the mechanisms of this process, small unilamellar vesicles of egg phosphatidylcholine were incubated with human HDL3 (d 1.125 to 1.210 g/ml) or with plasma and were subsequently analyzed by density gradient ultracentrifugation and negative stain electron microscopy. Incubation of vesicles with isolated HDL3 resulted in formation of both discoidal lipoproteins (1.04 to 1.06 g/ml) and phospholipid-enriched spherical lipoproteins (1.10 to 1.14 g/ml). The lipid bilayer discoidal lipoproteins had dimensions approximately 22 X 5.4 nm and contained phospholipid, apoA-I, and apoA-II. The phospholipid-enriched sperical HDL were increased in diameter and decreased in density compared to HDL3. With maximum phospholipid uptake these lipoproteins had a similar density, size, and lipid composition to HDL2a, a phospholipid-rich subfraction of HDL isolated from human plasma between 1.100 to 1.125 g/ml. Scanning microcalorimetry of the phospholipid-enriched HDL showed, compared to HDL3, a decrease in the temperature and enthalpy of lipoprotein denaturation. This altered denaturation pattern was similar to that of plasma HDL2a. Following incubation of vesicles in plasma, there was an increase in phospholipid-rich lipoproteins within the HDL2 density range (1.070 to 1.125 g/ml), and disappearance of lipoproteins from the HDL3 density range (1.125 to 1.210 g/ml). The phospholipid-rich HDL2 contained a major peak (d 1.095 to 1.125 g/ml), which consisted of only spherical particles and a smaller peak (d 1.070 to 1.095 g/ml), which included a population of lipid bilayer discs. Thus, during incubation of vesicles with isolated HDL3, there is insertion of phospholipid into HDL3, producing spherical lipoproteins which have some properties in common with HDL2a. Discoidal lipoproteins are also formed, probably as a result of interaction of vesicles with small amounts of apoA-I and apoA-II released from HDL3. The uptake of phosphatidylcholine by HDL in whole plasma occurs by similar mechanisms.