Effect of cholesterol on the formation of micellar complexes between bovine A-I apolipoprotein and L-alpha-dimyristoylphosphatidylcholine

Bovine apolipoprotein (apo)A-I displays more enhanced antioxidant and anti-atherosclerotic activity in lipid-free and lipid-bound states than human and porcine apoA-I

Apolipoprotein A-I (apoA-I) is a major component of high-density lipoprotein (HDL), which displays anti-atherosclerotic activity in plasma. In the current study, we compared the functional and structural characteristics of human, bovine and porcine apoA-I as regards their antioxidant ability and protein stability. In the lipid-free state, the immunoreactivity of bovine and porcine apoA-I differed from that of human apoA-I and bovine and porcine apoA-I exhibited greater resistance to denaturation induced by urea treatment. Bovine apoA-I showed the weakest binding ability of dimyristoyl phosphatidylcholine; however, bovine apoA-I formed slightly larger reconstituted HDL (rHDL) particles with palmitoyl oleoyl phosphatidylcholine, with a higher number of apoA-I-containing particles. Bovine and porcine apoA-I comprised of pentameric structures, whereas human apoA-I in the rHDL state consisted of trimeric structures. Although apoA-I from all three species showed a similar content of α-helicity in the lipid-free state (approximately 53%), bovine apoA-I showed a lower α-helicity content (approximately 66%) compared with human apoA-I (approximately 74%) in the rHDL state. Bovine apoA-I was more resistant to denaturation and glycation upon treatment with urea and fructose, respectively. Furthermore, bovine apoA-I showed a greater inhibition of cupric ion-mediated low-density lipoprotein (LDL) oxidation and uptake of acetylated LDL by macrophages compared with human or porcine apoA-I in the lipid-free and lipid-bound states. In conclusion, bovine apoA-I has unique functional properties in the lipid-free and lipid-bound states, and displays significantly enhanced anti-atherosclerotic activity.

Mechanisms responsible for the compositional heterogeneity of nascent high density lipoprotein

Apolipoprotein (apo) A-I-containing nascent HDL particles produced by the ATP binding cassette transporter A1 have different sizes and compositions. To understand the molecular basis for this heterogeneity, the HDL particles produced by apoA-I-mediated solubilization of phospholipid (PL)/free (unesterified) cholesterol (FC) bilayer membranes in cell and cell-free systems are compared. Incubation of apoA-I with ATP binding cassette transporter A1-expressing baby hamster kidney cells leads to formation of two populations of FC-containing discoidal nascent HDL particles. The larger 11-nm diameter particles are highly FC-enriched (FC/PL = 1.2/1 mol/mol) relative to the smaller 8 nm particles and the cell plasma membrane (FC/PL = 0.4/1). ApoA-I-mediated spontaneous solubilization of either multilamellar or unilamellar vesicles made of a membrane-PL mixture and FC yields discoidal HDL particles with diameters in the range 9–17 nm and, as found with the cell system, the larger particles are relatively enriched in FC despite the fact that all particles are created by solubilization of a common FC/PL membrane domain. The size-dependent distribution of FC among HDL particles is due to varying amounts of PL being sequestered in a boundary layer by interaction with apoA-I at the disc edge. The presence of a relatively large boundary layer in smaller discoidal HDL promotes preferential distribution of phosphatidylserine to such particles. However, phosphatidylcholine and sphingomyelin which are the primary PL constituents of nascent HDL do not exhibit selective incorporation into HDL discs of different sizes. This understanding of the mechanisms responsible for the heterogeneity in lipid composition of nascent HDL particles may provide a basis for selecting subspecies with preferred cardio-protective properties.

Nanodiscs as a new tool to examine lipid-protein interactions

Nanodiscs are self-assembled discoidal fragments of lipid bilayers 8-16 nm in diameter, stabilized in solution by two amphipathic helical scaffold proteins. As stable and highly soluble membrane mimetics with controlled lipid composition and ability to add affinity tags to the scaffold protein, nanodiscs represent an attractive model system for solubilization, isolation, purification, and biophysical and biochemical studies of membrane proteins. In this chapter we overview various approaches to structural and functional studies of different classes of integral membrane proteins such as ion channels, transporters, GPCR and other receptors, membrane enzymes, and blood coagulation cascade proteins which have been incorporated into nanodiscs. We outline the advantages provided by homogeneity, ability to control oligomerization state of the target protein and lipid composition of the bilayer. Special attention is paid to the opportunities afforded by nanodisc system for the detailed studies of the role of different lipid properties and protein-lipid interactions in the functional behavior of membrane proteins.

Interaction of apolipoprotein A-I with lecithin-cholesterol vesicles in the presence of phospholipase C

Here we study the anti-nucleating mechanism of apolipoprotein A-I (apo A-I) on model biliary vesicles in the presence of phospholipase C (PLC) utilizing dynamic light scattering (DLS), steady-state fluorescence spectroscopy, cryogenic transmission electron microscopy (cryo-TEM), and UV/Vis spectroscopy. PLC induces aggregation of cholesterol-free lecithin vesicles from an initial, average size of 100 nm to a maximal size of 600 nm. The presence of apo A-I likely inhibits vesicle aggregation by shielding the PLC-generated hydrophobic moieties, which results in vesicles of an average size of 200 nm. A similar phenomenon is observed in cholesterol-enriched lecithin vesicles. Whereas PLC alone produces aggregates of 300 nm, no aggregation is observed when apo A-I is present along with PLC. However, the ability of apo A-I to inhibit aggregation is temporary, and after 8 h, a broad particle size distribution with sizes as high as 800 nm is observed. Apo A-I possibly induces the formation of small apo A-I/lecithin/cholesterol complexes of about 5-20 nm similar to the discoidal pre-HDL complexes found in blood when it can no longer effectively shield all the DAG molecules. Concomitant with formation of complexes, DAG molecules coalesce into large oil droplets, which account for the large particles observed by light scattering. Thus, apo A-I acts as an anti-nucleating agent by two mechanisms, anti-aggregation and microstructural transition. The mode of protection is dependent on the cholesterol content and the relative amounts of DAG and apo A-I present. This study supports the possibility of apo A-I solubilizing lipids in bile in a similar fashion as it does in blood and also delineates the mechanism of formation of the complexes.

Lipid absorption and transport in ruminants

The objective of this paper is to review new insights on the biological mechanisms of absorption and transport of lipid in ruminants, especially the modern concepts and analytical methods used in studies on structural properties and intravascular and tissue metabolism of lipoproteins and their factors of variation. The intestinal absorption of lipids (including long-chain fatty acids) is detailed, and variations in the qualitative and the quantitative aspects of absorption with diet composition, especially for high fat diets, are presented. Also, structural properties and distribution characteristics of lipoprotein classes in different lymphatic and blood vessels are compared across several animal species. Physicochemical and hydrodynamic properties of the lipoprotein particles and their apolipoprotein moieties are given for the main classes of lipoproteins. Finally, lipoprotein metabolism is discussed in relation to development and physiological, nutritional, and hormonal status. Intravascular metabolism of lipoproteins, including the role of lipolytic enzymes and lipid transfer proteins, is presented. Characteristics of the intestinal and hepatic synthesis of lipoproteins and apolipoprotein fractions are compared, especially through experiments stimulating the hepatic secretion of triglyceride-rich lipoproteins. Different methods of measurement of lipoprotein tissue uptake or secretion in ruminants are discussed.

Apolipoprotein A-I stabilizes phospholipid lamellae and thus prolongs nucleation time in model bile systems: an ultrastructural study

To explore the mechanisms whereby apolipoprotein A-I inhibits the nucleation of cholesterol crystals, we performed an ultrastructural study using supersaturated model bile systems. Vesicles, micelles and phospholipid lamellae were consistently separated by gel permeation chromatography either in the absence or presence of apolipoprotein A-I. Furthermore, apolipoprotein A-I coeluted with phospholipid lamellae. A sequential study using transmission electron microscopy revealed that phospholipid lamellae without apolipoprotein A-I showed a rapid transformation, with formation of multilamellae and fusion followed by microcrystal nucleation. In contrast, lamellae with apolipoprotein A-I showed little transformation. In conclusion, apolipoprotein A-I stabilizes the phospholipid lamellae, thereby inhibiting the nucleation of cholesterol crystals in supersaturated model bile systems.

Lateral interactions of pig apolipoprotein A-1 with egg yolk phosphatidylcholine and with cholesterol in mixed monolayers at the triolein-saline interface

Interfacial tensions of egg yolk phosphatidylcholine (PC) and cholesterol monolayers adsorbed at the triolein-saline interface were measured in the presence and absence of pig apolipoprotein A-1 (apoA-1) in the saline phase. In the absence of apoA-1, the adsorptions of PC and cholesterol at the interface from the triolein phase are cooperative, showing large lateral attractive interactions between the PC molecules and the cholesterol molecules in the monolayer. In the presence of apoA-1, the PC adsorption is anti-cooperative, indicating strong lateral attractive interactions between the PC and the apoA-1 molecules, i.e., apparently, repulsive lateral interactions between the PC molecules. On the other hand, lateral interactions of very low magnitude are observed between the cholesterol and apoA-1 molecules in the monolayer. Values of the lateral interaction energy are evaluated from the adsorption data by the Defay-Prigogine-Flory theory of monolayers. The large difference in lateral interaction energy with apoA-1 between PC and cholesterol in a mixed monolayer is discussed in connection with current problems in lipoprotein catabolism: reverse cholesterol transport, alterations in affinity of lipid particles to apoA-1, and formation of high-density lipoproteins and abnormal lipoproteins.

Plasma lipid transport in the preruminant calf, Bos spp: primary structure of bovine apolipoprotein A-I

The preruminant calf (Bos spp.) is a model of considerable interest with regard to hepatic and intestinal lipoprotein metabolism (Bauchart et al., J. Lipid Res. (1989) 30, 1499-1514 and Laplaud et al., J. Lipid Res. (1990) 31, 1781-1792). As a preliminary step towards future experiments dealing with HDL metabolism in the calf, we have purified apoA-I from this animal and determined its complete amino acid sequence. Thus, approx. 10% of calf apoA-I was shown to contain a propeptide, with the sequence Arg-His-Phe-Trp-Gln-Gln. Enzymatic cleavage of apoA-I resulted in 10 proteolytic peptides. The complete apoA-I sequence was obtained after alignment of peptides on the basis of their homologies with those from rabbit apoA-I. Thus calf apoA-I consists of 241 amino acid residues, and exhibits high sequence homology with all mammalian apoA-I's studied to date. The bovine protein contained 10 hydrophobic amphipathic helical regions, occurring between residues 43-64, 65-86, 87-97, 98-119, 120-141, 142-163, 164-184, 185-206, 207-217 and 218-241. A computer-constructed phylogenetic tree showed that bovine apoA-I was more closely related to its dog counterpart, including the presence of a single methionine, than to the corresponding macaque and human proteins. Comparative predictions of the respective antigenic structures of human and bovine apoA-I's using the Hopp-Woods algorithm indicated similar positions for all 13 detectable antigenic sites, among which 7 were of identical, or closely related, amino acid composition. This finding was confirmed by demonstration of partial immunological identity between the two proteins upon immunodiffusion analysis, a result obtained using a monospecific rabbit antiserum against bovine apoA-I. Finally, comparison of sequence homology between bovine apoA-I and the lecithin:cholesterol acyl transferase (LCAT) activating region of human apoC-I suggests that several LCAT activating domains may be present in calf apoA-I.

Characterization and amino-terminal sequence of apolipoprotein AI from plasma high density lipoproteins in the preruminant calf, Bos spp

The major apolipoprotein of calf plasma high-density lipoproteins, apo-AI, has been isolated and characterized. Apolipoprotein AI (apo-AI) was separated from the protein moiety of high-density lipoproteins (d 1.090-1.180 g/ml) by preparative electrophoresis in SDS-polyacrylamide gels followed by electrophoretic elution. Purified calf apo-AI had an Mr of approx. 27,000-28,000 in SDS-polyacrylamide gels, resembling human apo-AI. The amino acid composition of calf apo-AI displayed an overall similarity to that of its human and other mammalian counterparts (baboon, dog, badger, rabbit, rat and mouse), but differed in having higher proportions of glutamic acid, alanine and isoleucine. Amino-terminal amino acid sequence analysis up to the 47th residue showed close homology between calf apo-AI and those of the mammals with which it was compared. However, residues 2, 7, 20 and 22 in calf AI (i.e. aspartic acid, serine, glutamic acid and isoleucine, respectively) were substituted by glutamic acid, proline or glutamine, aspartic acid, and valine or leucine respectively, in the other mammals.

Probucol reduces the rate of association of apolipoprotein C-III with dimyristoylphosphatidylcholine

The effect of low concentrations of probucol and cholesterol on the association of dimyristoylphosphatidylcholine with human plasma apolipoprotein C-III was studied. Liposomes of dimyristoylphosphatidylcholine with or without probucol or cholesterol were prepared by swelling the lipids in buffer at 37 degrees C. The association of apolipoprotein C-III with the liposomes was determined at 24 degrees C by measuring the rate of clearing of turbidity at 400 nm following addition of protein. At a weight ratio of probucol/dimyristoylphosphatidylcholine of 1:25 (5 mol% probucol), the rate of clearing of liposomes was decreased by 60%; 5 mol% cholesterol had no effect on the clearing rate. Liposomes were then added to the preformed apolipoprotein C-III/lipid micelles. In the absence of probucol, the added liposomes cleared rapidly regardless of the presence or absence of cholesterol. With 5 mol% probucol, almost no decrease in absorbance was noted on addition of liposomes to the micelles. These data show that probucol reduces the rate of association of an apolipoprotein with lipid and suggests that the interaction of probucol with lipid may modify the assembly and/or metabolism of lipoproteins.

Membrane cholesterol uptake by recombinant lipoproteins

Recombinant lipoproteins, prepared with apo A-I isolated from human high density lipoprotein (HDL) and various phospholipids (PLs), were compared with respect to their ability to remove cholesterol (Chol) from labelled erythrocyte ghost membranes. It was found that uptake of Chol was essentially complete following an 8 h incubation at 37 degrees C. Quantitation of the amount of cholesterol taken up showed that recombinants prepared from bovine brain sphingomyelin (BBSM) or dipalmitoyl phosphatidylcholine (DPPC) acquired the highest proportion of Chol (80-140 mol/mol protein), whereas shorter chain phospholipids like dimyristoyl phosphatidylcholine (DMPC) acquired little or no membrane Chol. Chemical analysis of the incubation products indicated that this latter result was due to loss of PL, presumably to the membrane, with consequent disruption of the recombinant particle. Results with DPPC:A-I recombinants of differing PL/protein ratios and sizes showed that Chol uptake was fairly constant at 0.70 mol Chol/mol PL. It is concluded that discoidal, phospholipid-rich recombinant lipoproteins can effectively take up substantial amounts of Chol from physiological membranes, provided that the PLs utilized form micellar complexes which are capable of retaining their structural integrity during the incubation with the membranes.

Estimation of liposome integrity by 1H-NMR spectroscopy

A fast and simple method of 1H-NMR spectroscopic control of liposome membrane integrity is suggested. The method is based on the redistribution of intensities between two singlet 1H-NMR signals--from intraliposomal marker compound (nitrilotriacetic acid sodium salt, 1H-NMR signal at 4 p.p.m.) and from its complex with Eu3+ added to the external medium (NMR signal of the complex at - 1 p.p.m.). The method permits registration of the kinetics of liposome destruction under the action of detergent or serum. It is shown that the presence of cholesterol in the membrane makes it more stable.

Incorporation of cholesterol into serum high density lipoprotein apoprotein and recombinants

The uptake of cholesterol (CHL) by serum high density lipoprotein (HDL) delipidated apoproteins and phospholipid-apoprotein recombinants has been studied with two methods; by incubation with Celite-dispersed cholesterol or with cholesterol crystals. The apoproteins bind very small amounts of cholesterol with a maximum of about 6 micrograms/mg apoprotein. Recombinants with dimyristoyl phosphatidylcholine (DMPC) or egg phosphatidylcholine (EPC) as phospholipid component gave similar values for cholesterol uptake. The initial rate for uptake from Celite-cholesterol by recombinants was high (0.1 mol cholesterol/mol phospholipid/h) and somewhat higher than that for phospholipid vesicles. The maximal uptake was by gel filtration shown to depend on the size of the complexes with values about 0.95 mol cholesterol per phospholipid for vesicular complexes, 0.75 for discoidal complexes and between 0.5 and 0.2 for small 'protein-rich' complexes. During the incubation of recombinants with cholesterol there was considerable decomposition of discoidal complexes and formation of larger ones. The results show that phospholipid-apoprotein complexes are efficient acceptors for cholesterol but also that about 25% of the phospholipid in the discoidal complexes is excluded from interaction with cholesterol by interaction with apoprotein.

A review of plasma apolipoprotein A-I interactions with phosphatidylcholines

This review starts with a brief introduction to the properties of plasma high-density lipoproteins and their major protein component, apolipoprotein A-I, followed, in the following sections, by an account of experimental work from our laboratory on the interactions of apolipoprotein A-I with synthetic and natural phosphatidylcholines. The spontaneous reactions of phosphatidylcholine vesicles with apolipoprotein A-I are described in terms of the methods of observation, the properties of the reaction products (vesicular or micellar complexes of protein and lipid), and the kinetic controlling factors in the formation of the micellar products. A general detergent reconstitution method for the preparation of micellar complexes is presented, and applications of these particles in studies of the apolipoprotein-lipid interface and of enzymatic reactions are discussed.

Action of lecithin:cholesterol acyltransferase on model lipoproteins. Preparation and characterization of model nascent high density lipoprotein

Apolipoprotein A-I, the major protein of human plasma high density lipoprotein, is the primary activator of plasma lecithin:cholesterol acyltransferase. In vitro, the association of apolipoprotein A-I with physiological phosphatidylcholines can be catalyzed by mixing the protein and lipid with sodium cholate, which is removed by chromatography. The apolipoprotein A-I/phospholipid complex has the physical properties of an HDL, and when cholesterol is present the complex is a highly reactive substrate in the lecithin:cholesterol acyltransferase-catalyzed reaction. The relative reactivity of this complex compared with a number of other lipid-protein complexes is presented and discussed.

The apparent preferential interaction of human plasma high density apolipoprotein A-I with gel-state phospholipids

The enthalpy, entropy and free energy of activation was measured for the transfer of the tryptophan residues of apolipoprotein A-I from a more hydrophobic environment of a lipoprotein particle containing dimyristoylphosphatidylcholine (with or without 12% cholesterol) to an aqueous solvent in the presence of varying concentrations of guanidinium chloride. The free energy of activation was approximately 25 kcal/mol at 50 degrees C for all the conditions studied. The enthalpy of activation was greatest under conditions where a large degree of unfolding occurs when the protein dissociated from lipid. However, under these conditions the unfavourable activation enthalpy was compensated for by a favourable activation entropy resulting in the insensitivity of the free energy of activation to the condition of measurement. Apolipoprotein A-I has an apparent affinity for gel-state lipid which results from the very slow rate of dissociation of the lipoprotein particle below 40 degrees C. It is unlikely that the association of apolipoprotein A-I with dimyristoylphosphatidylcholine is thermodynamically stable only in the temperature region of the phase transition but that the association exhibits a large kinetic stability, especially at lower temperatures or in the absence of guanidinium chloride.

Studies of the interaction between apolipoproteins A and C and triacylglycerol-rich particles

We studied the interaction of apolipoproteins A-I, C, and HDL2 with phospholipid-stabilized, triacylglycerol-rich particles to learn more about the molecular mechanisms that underlie the metabolism of chylomicrons. Apolipoproteins A-I, C-I, C-III1 and C-III2 all bound to and destabilized triacylglycerol-rich particles, apparently by removing phospholipids from the particle surface. None of the apolipoprotein C tested, at any concentration, was, however, able to equal the disruptive effect of apolipoprotein A-I. The destabilizing effects of apolipoproteins A-I and C were not additive. Apolipoprotein C-III1 seemed to lessen the disruptive effect of apolipoprotein A-I by binding competitively to triacylglycerol-rich particles. Unexpectedly, previous binding of apolipoprotein A-I to triacylglycerol-rich particles nearly tripled the ability of these particles to bind apolipoprotein C. Destabilization of triacylglycerol-rich particles by apolipoprotein A-I was prevented by HDL2. The protective effect of HDL2 seemed to depend partly on transfer of unesterified cholesterol from HDL2, since the amounts of unesterified cholesterol and apolipoprotein A-I bound to the particles surface showed a strong negative correlation.

Displacement of the human apoprotein A-I by the human apoprotein A-II from complexes of (apoprotein A-I)-phosphatidylcholine-cholesterol

Reassembly experiments, involving isolated human apoproteins A-I and A-II and (dimyristoylglycerophosphocholine)-cholesterol vesicles were performed with apoprotein mixtures at apoprotein A-I/A-II molar ratios varying between 0 and 3. The apoproteins were incubated at 24 degrees C. 28 degrees C and 32 degrees C with either pure dimyristoyl-glycerophosphocholine vesicles or with dimyristoylglycerophosphocholine cholesterol vesicles containing 2, 5, 10, 15 mol/100 mol cholesterol. The kinetics of association were followed by measuring the increase of the fluorescence polarization ratio after labeling the lipids with diphenyl hexatriene. The complexes were separated from the free protein by gradient ultracentrifugation. Total protein was assayed and the apoproteins A-I and A-II were quantified separately by immunonephelometry. The content of apoprotein A-I was also monitored by measuring the intrinsic tryptophan fluorescence. The results suggest that apoprotein A-II has a greater affinity than apoprotein A-I for the phospholipid-cholesterol vesicles and that apoprotein A-II is able to quantitatively displace apoprotein A-I from the lipid-protein complexes. The content of apoprotein A-II in the complexes increases proportionally to the concentration of apoprotein A-II in the incubation mixture until saturation is reached. At saturation the dimyristoylglycerophosphocholine/apoprotein A-II ratio in the complex is dependent upon the cholesterol content of the original vesicles and increases from 60 to 275 mol/mol between 0 and 15 mol/100 mol cholesterol. From these experiments one can calculate that 1 mol human apoprotein A-I is displaced by 2 mol human apoprotein A-II.

Reassembly of human apoproteins A-I and A-II with unilamellar phosphatidylcholine-cholesterol liposomes. Association kinetics and characterization of the complexes

The kinetics of association between the human apoprotein A-I and apoprotein A-II and cholesterol dimyristoyl phosphatidylcholine (DMPC) vesicles are compared in this study and the lipid-apoprotein complexes are characterized. The association kinetics are followed by turbidity measurements monitoring the decrease of the vesicular size and by fluorescence polarization measurements monitoring the decrease in the mobility of the phospholipid acyl chains during complex formation. The influence of the incubation temperature and of the cholesterol/DMPC ratio has been studied by both techniques. Under all incubation conditions the apoprotein A-II associates more readily with cholesterol-DMPC vesicles than apoprotein A-I, as the kinetics are faster and the complex yield larger. With both apoproteins optimal complex formation takes place around the phospholipid transition temperature and around 10 mol% cholesterol. The apoprotein A-I/lipid association seems restricted to this narrow range for the temperature and the cholesterol/DMPC ratio, while the apoprotein A-II still associates with vesicles containing 20 mol% cholesterol and at temperatures up to 32 degrees C. The lipid-apoprotein complexes were isolated by gradient ultracentrifugation and by gel chromatography. According to these data the apoprotein A-II associates more readily than apoprotein A-I with cholesterol-DMPC vesicles to form protein-rich complexes, whilst the optimal apoprotein A-I-lipid association requires a more disordered lipid structure.

Interaction of human plasma high-density lipoprotein HDL2b with discoidal complexes of dimyristoylphosphatidylcholine and apolipoprotein A-I

The interaction of HDL2b, a major subclass (d = 1.063 - 1.100 g/ml) of human plasma high-density lipoproteins, with discoidal complexes composed of dimyristoylphosphatidylcholine (DMPC) and apolipoprotein A-I (weight ratio, DMPC/apolipoprotein A-I (2.1 - 2.5:1); dimensions, 10.0 x 4.4 nm) was investigated. Incubation at 37 degrees C for 4.5 h of HDL2b with discoidal complexes resulted in a transfer of DMPC from the discoidal complexes to the HDL2b, a release of lipid-free apolipoprotein A-I from the discoidal complexes during such transfer, and a dissociation of some apolipoprotein A-I from the HDL2b surface. The number of discoidal complexes degraded during interaction with HDL2b depended on the initial molar ratio of HDL2b to discoidal complexes. Approximately one molecule of HDL2b was required for the degradation of one discoidal complex particle, and the degradation process appeared limited by the capacity of the HDL2b for uptake of DMPC. Degradation of discoidal complexes was also observed when human plasma LDL (d = 1.006-1.063 g/ml) was substituted for HDL2b in the interaction mixture.

Interaction of human plasma high density lipoprotein HDL2 with synthetic saturated phosphatidylcholines

The interaction of human plasma high density lipoprotein HDL2 (d 1.063-1.125 g/ml) with sonicated dispersions of synthetic saturated phosphatidylcholines, dipalmitoyl- (diC16PC), dimyristoyl- (diC14PC), didodecanoyl- (diC12c), didecanoyl- (diC10PC), and dioctanoyl- (diC8PC) L-alpha phosphatidylcholine, was investigated. Incubation (4.5 hr, 37 C) of HDL2 with diC14PC, diC12PC, diC10PC and diC8PC followed by gradient gel electrophoresis of preparative ultracentrifugation resulted in a redistribution of apolipoprotein A-I (apoA-I). The extent of redistribution depended on the molar ratio of the phospholipid to HDL2 in the incubation mixture. Redistributed apoA-I occurred as lipid-free apoA-I and/or as complexes of apoA-I and/or as compelxes of apoA-I with phosphatidylcholine. Increasing the length of time of ultracentrifugation of the interaction mixtures did not increase the extent of redistribution. No redistribution of apoA-I was detected following incubation and gradient gel electrophoresis or preparative ultracentrifugation of mixtures of HDL2 with dispersions of diC16PC.

The involvement of the lipid phase transition in the plasma-induced dissolution of multilamellar phosphatidylcholine vesicles

Unsonicated liposomes prepared from dimyristoyl phosphatidylcholine were nearly completely dissolved during a 3 h incubation with rat plasma at or close to the phase-transition temperature of 24 degrees C. At 37 or 15 degrees C virtually no liposomal disintegration was observed even after 24 h of incubation. The liposomal solubilization, which was monitored by turbidity measurements or by determination of phospholipid sedimentability, was accompanied by the formation of a phospholipid-protein complex similar or identical to the one we previously reported to be formed from sonicated liposomes of egg phosphatidylcholine (Scherphof, G., Roerdink, F., Waite, M. and Parks, J. (1978) Biochim. Biophys. Acta 542, 296--307). Unsonicated multilamellar liposomes made of egg phosphatidylcholine were completely resistant to the dissolving potency of plasma when incubated at 37 degrees C. Liposomes from equimolar mixtures of dimyristoyl and dipalmitoyl phosphatidylcholine were only degraded by plasma in the temperature range between 30 and 35 degrees C at which temperature this cocrystallizing phospholipid mixture undergoes a phase transition. However, even at these temperatures the rate of dissolution of this mixture was significantly lower than of dimyristoyl phosphatidylcholine at 24 degrees C. In the dissolving process of this mixture a slight preference for the lower-melting component was observed. The ability of cholesterol to completely abolish the susceptibility of dimyristoyl phosphatidylcholine liposomes to plasma at a 1:2 molar ratio of cholesterol to phospholipid substantiates the essential role of the phase transition in the process of liposome solubilization. When liposomes of the monotectic mixtures dimyristoyl and distearoyl phosphatidylcholine or dilauroyl and distearoyl phosphatidylcholine were incubated with plasma at temperatures in between those at which the constituent lipids undergo a phase change in the mixture, the liposomes were slowly dissolved. Under those conditions a selective removal of the lipids in the liquid-crystalline phase was observed. It is concluded that for the plasma-induced dissolution of unsonicated liposomes, which is most probably achieved by interaction with (apo)lipoproteins, the presence of phase boundaries is required in much the same way as was first reported for phospholipases by Op den Kamp, J.A.F., de Gier, J. and Van Deenen, L.L.M. (1974) Biochim. Biophys. Acta 345, 253--256).