Reconstitution of lipoproteins. I. Lipid–protein interaction of high density apoproteins, purified apoA-I and apoA-II with dimyristoyl-lecithin and dimyristoyl-lecithin:cholesterol vesicles studied by isomeric spin-labelled lecithins

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Abstract
Reconstitution of high density lipoprotein [HDL] like particles from lecithin, cholesterol and HDL apoproteins was studied. Sonicated mixtures of sn-3-dimyristoyl-lecithin [DML] and cholesterol (molar ratio 10:1) were incubated with total human plasma HDL apoproteins and purified apoprotein A-I and apoprotein A-II in the presence of isomeric (5''-, 12''- and 16''-(N-oxyl-4",4"-dimethyloxazolidine)stearoyl) spin-labeled lecithin probes. The supernatant obtained after centrifugation of the incubation mixture at 12,000 .times. g for 5 min contained reconstituted lipoproteins with an average protein:lipid weight ratio of 0.52-0.53. To determine the position of apoprotein A-I and apoprotein A-II with respect to the lipid bilayer, the thermotropic behavior of reconstituted lipoproteins was monitored by ESR spectroscopy using isomeric spin-labeled lecithin probes. Comparison of experimental spectral parameters (transition temperature, temperature range and cooperative unit) obtained from reconstituted lipoproteins with liposomes of DML and DML:cholesterol (10:1) showed a relative increase of transition temperature and broadening of the transition range with 5- and 12-(N-oxyl-4'',4''-dimethyl-oxazolidine)stearic acid containing lecithin probes when reconstituted lipoproteins contained either apoprotein A-I or apoprotein A-II. With 16-(N-oxyl-4'',4''-dimethyloxazolidine)stearic acid containing lecithin probe, the transition temperatures and the temperature range relative to those values measured with 5''- and 12''-(N-oxyl-4",4"-dimethyloxazolidine) lecithin probes were higher and narrower, respectively, with a tendency towards values obtained for liposomes not containing apoproteins. The cooperative unit in DML:apoprotein recombinants monitored with 5''- and 12''-(N-oxyl-4",4"-dimethyloxazolidine) lecithin probes was smaller than or approximately equal to those measured with 16''-(N-oxyl-4",4"-dimethyloxazolidine) lecithin probes when these systems contained either apoprotein A-I or apoprotein A-II. Apoproteins A-I and A-II in lipoprotein recombinants of the composition used in the study are apparently partially embedded within the hydrocarbon portion of the lipid bilayer of reconstituted lipoproteins, but do not span the entire lipid bilayer. Relevance of these results to the mechanism of lecithin:cholesterol acyltransferase activation is discussed.