RECRUITMENT OF CELL PHOSPHOLIPIDS AND CHOLESTEROL BY APOLIPOPROTEINS A-II AND A-I - FORMATION OF NASCENT APOLIPOPROTEIN-SPECIFIC HDL THAT DIFFER IN SIZE, PHOSPHOLIPID-COMPOSITION, AND REACTIVITY WITH LCAT

Studies were carried out to determine whether apolipoprotein (apo) A-I I, like apoA-I, can recruit phospholipid and cholesterol from cell mem branes, thereby forming nascent apoA-II-specific HDL. ApoA-II and apoA -I were purified from plasma and each was incubated with CHO cells at a concentration of 10 mu g/ml. Lipid-containing complexes were isolate d from the medium in both cases; the composition of the apoA-II- and a poA-I-specific complexes were similar where percent protein, phospholi pid, and cholesterol were 35 +/- 3, 38 +/- 2, and 25 +/- 1 for apoA-II , respectively, and 40 +/- 2, 35 +/- 1, and 24 +/- 2 for apoA-I, respe ctively. On a per mole of apolipoprotein basis, apoA-I recruited signi ficantly more phospholipid and cholesterol than dimeric apoA-II sugges ting that apoA-I with its greater number of alpha helices binds more l ipid. By electron microscopy, nascent apoA-II- and apoA-I-specific par ticles were predominantly discoidal in morphology. ApoA-II complexes w ere unique in their nondenaturing polyacrylamide gradient gel size dis tribution as six distinct populations of particles with diameters of 8 .1, 9.3, 10.4, 11.8, 13.1, and 14.6 nm were routinely noted, compared with apoA-I which formed only three major populations with diameters o f 7.3, 9.2, and 11.0 nm. Nascent apoA-I complexes incubated with purif ied lecithin:cholesterol acyltransferase (LCAT) were transformed into predominantly 8.4 nm particles. The latter is similar in size to plasm a HDL(3a), LpA-I particles, suggesting that extracellularly assembled apoA-I-lipid complexes can directly give rise to a major plasma LpA-I subpopulation upon interaction with LCAT. Unlike apoA-I, apoA-II-lipid complexes could not serve as substrates for LCAT and did not undergo transformation. This study also demonstrates, for the first time, that apoA-II and apoA-I show a preference in phospholipid recruitment from membranes. Although phosphatidylcholine is the major phospholipid rem oved by both apolipoproteins, apoA-II preferentially recruits phosphat idylethanolamine (PE) as its second most abundant phospholipid while a poA-I recruits sphingomyelin. As PE is usually associated with the inn er leaflet of the membrane, it is likely that dimeric apoA-II, compare d with apoA-I, can penetrate farther into the membrane and extract PE. This ability of apoA-II to insert more deeply into the lipid milieu m ay explain the known ability of apoA-II to resist dissociation from th e mature HDL particle.