The self-association of the apo-Gln-I and apo-Gln-II polypeptides of human high density serum lipoproteins.

The major polypeptide components of human high density lipoprotein, apo-Gln-I and apo-Gln-II, self-associate at pH 8.3 and ionic strength 0.045. The dimeric (MW = 56,800) association constant for apo-Gln-I is 1.3 X 10(4)liters/mol. apo-Gln-II is tetrameric at all experimentally accessible concentrations. Self-association of apo-Gln-I is accompanied by minor conformational alterations distinct from that induced by saturating levels of bound amphiphilic ligands. These results are discussed with respect to the design of reconstitution experiments between the apoproteins and lipids.     

Plasma phospholipid transfer protein-mediated reactions are impaired by hypochlorite-modification of high density lipoprotein

The two main functions of phospholipid transfer protein (PLTP) are the transfer of phospholipids between plasma lipoproteins and the conversion of high density lipoprotein (HDL), where prebeta-HDL particles are generated. HDL is considered an anti-atherogenic lipoprotein due to its function in the reverse cholesterol transport, where prebeta-HDL accepts cellular membrane cholesterol from peripheral tissues. However, the anti-atherogenic properties of native HDL may be abolished by oxidation/modification. Hypochlorous acid/hypochlorite (HOCl/OCl-)-a potent oxidant generated in vivo only by the myeloperoxidase-H2O2-chloride system of activated phagocytes-alters the physiological properties of HDL by generating a pro-atherogenic lipoprotein particle. Therefore, we have studied the effect of HOCl on the function of HDL subclass 3 (HDL3) and triglyceride-enriched HDL3 (TG-HDL3) in PLTP-mediated processes in vitro. Modification of HDL3 and TG-HDL3 with increasing HOCl concentrations (oxidant:lipoprotein molar ratio between 25:1 and 200:1) decreased the capacity of the corresponding lipoprotein particles to accept phospholipids. Although binding of PLTP to unmodified and HOCl-modified lipoprotein particles was similar, the degree of PLTP-mediated HDL conversion was decreased upon HOCl oxidation. PLTP released apolipoprotein A-I (apoA-I) from HOCl-modified HDL3, but the particles formed displayed no prebeta-mobility. Based on these findings, we conclude that the substrate properties of HOCl-modified HDL3 and TG-HDL3 in PLTP-mediated processes are impaired, which indicates that the anti-atherogenic properties of HDL are impaired.     

Distinct HDL subclasses present similar intrinsic susceptibility to oxidation by HOCl

The heme protein myeloperoxidase (MPO) functions as a catalyst for lipoprotein oxidation. Hypochlorous acid (HOCl), a potent two-electron oxidant formed by the MPO-H₂O₂-chloride system of activated phagocytes, modifies antiatherogenic high-density lipoprotein (HDL). The structural heterogeneity and oxidative susceptibility of HDL particle subfractions were probed with HOCl. All distinct five HDL subfraction were modified by HOCl as demonstrated by the consumption of tryptophan residues and free amino groups, cross-linking of apolipoprotein AI, formation of HOCl-modified epitopes, increased electrophoretic mobility and altered content of unsaturated fatty acids in HDL subclasses. Small, dense HDL3 were less susceptible to oxidative modification than large, light HDL2 on a total mass basis at a fixed HOCl:HDL mass ratio of 1:32, but in contrast not on a particle number basis at a fixed HOCl:HDL molar ratio of 97:1. We conclude that structural and physicochemical differences between HDL subclasses do not influence their intrinsic susceptibility to oxidative attack by HOCl.     

Copper binding before polypeptide folding speeds up formation of active (holo) Pseudomonas aeruginosa azurin.

Cofactors often stabilize the native state of the proteins; however, their effects on folding dynamics remain poorly understood. To uncover the role of one cofactor, we have examined the folding kinetics of Pseudomonas aeruginosa azurin, a small blue-copper protein with a copper cofactor uniquely coordinated to five protein residues. Copper removal produces apo-azurin which adopts a folded structure identical to that of the holo-form. The folding and unfolding kinetics for apo-azurin follow two-state behavior. The extrapolated folding time in water, tau approximately 7 ms, is in good agreement with the topology-based prediction. Copper uptake by folded apo-azurin, to govern active (holo) protein, is slow (tau approximately 14 min, 50:1 copper-to-protein ratio). In contrast, the formation of active (holo) azurin is much faster when copper is allowed to interact with the unfolded polypeptide. Refolding in the presence of 10:1, 50:1, and 100:1 copper:protein ratios yields identical time-trajectories: active azurin forms in two kinetic phases with folding times, extrapolated to water, of tau = 10 +/- 2 ms (major phase) and tau = 190 +/- 30 ms (minor phase), respectively. Correlating copper-binding studies, with a small peptide derived from the metal-binding region of azurin, support that initial cofactor binding is fast (tau approximately 3.7 ms) and thus not rate-limiting. Taken together, introducing copper prior to protein folding does not speed up the polypeptide-folding rate; nevertheless, it results in much faster (> 4000-fold) formation of active (i.e., holo) azurin. Living systems depend on efficient formation of functional biomolecules; attachment of cofactors prior to polypeptide folding appears to be one method to achieve this.     

Role of high-density lipoprotein in transport of circulating bilirubin in rats

The analbuminemic rat strain established by Nagase et al. (Nagase, S., Shimamune, K., and Shumiya, S. (1979) Science 205, 590-591) exhibits hereditary deficiency in albumin biosynthesis. Serum bilirubin concentration is rather lower in homozygous (aa) rats (0.009 +/- 0.002 mg/dl) as compared with heterozygous (Aa) rats (0.047 +/- 0.009 mg/dl) or wild-type Sprague-Dawley (AA) rats (0.034 +/- 0.006 mg/dl) as evidenced by high pressure liquid chromatography analysis of bilirubin. After intravenous administration of various amounts of [heme-3H]hemoglobin in rats, [3H]bilirubin derived from [3H]heme of hemoglobin in vivo is more efficiently excreted into bile in aa rats than in Aa or AA rats. [3H]Bilirubin is exclusively bound with high-density lipoprotein (HDL) in aa rats, and a significant amount of [3H]bilirubin is shown to bind with HDL in Aa or AA rats in vivo. Scatchard plots revealed that [3H]bilirubin is bound with HDL in three binding modes depending on the molar ratio of [3H]bilirubin to HDL: Kd = 0.8 X 10(-7) M (molar ratio, 0.02-0.06), Kd = 1.6 X 10(-6) M (molar ratio, 0.06-0.41), and Kd = 1.2 X 10(-4) M (molar ratio, 0.79-9.02). Even under extreme conditions of excess hemoglobin administration, the molar ratio remains under 0.041; and thus, expected the Kd value would remain around 0.8 X 10(-7) M. Binding of [3H]bilirubin to rat serum albumin revealed two distinct binding modes depending on the molar ratio of [3H]bilirubin to rat serum albumin: Kd = 3.6 X 10(-7) M (molar ratio, 0.03-0.21), and Kd = 5.0 X 10(-6) M (molar ratio, 0.21-2.46). Under physiological conditions in Aa or AA rats, the former mode would be more reliable than the latter. Thus, HDL could bind with approximately 4.5 times higher affinity than rat serum albumin in Aa or AA rats under physiological conditions in vivo.     

Cholesterol efflux via HDL resecretion occurs when cholesterol transport out of the lysosome is impaired

Recently, we showed that holo HDL particle uptake and resecretion occur in physiologically relevant cell lines and that HDL uptake is mediated by scavenger receptor class B type I (SR-BI). Furthermore, we established that HDL resecretion is accompanied by [(3)H]cholesterol efflux. This study shows that HDL uptake and resecretion occur even when LDL uptake and cholesterol trafficking are disturbed. First, we used a set of inhibitors that block cholesterol transport out of the lysosome: chloroquine, imipramine, U18666A, and monensin. In all cases, HDL retroendocytosis occurred and HDL resecretion mediated [(3)H]cholesterol efflux, although to a lesser extent. Second, cell lines carrying somatic mutations in intracellular cholesterol transport were used: CHO 2-2 and CHO 3-6 cells accumulated LDL-derived lipid in the lysosome but showed all components of HDL retroendocytosis. SR-BI overexpression increased HDL uptake and resecretion and [(3)H]cholesterol efflux in these mutant cells. Finally, we used Niemann-Pick type C (NPC) patient fibroblast cells, which carry a defect in cholesterol transfer out of the lysosome. NPC fibroblast cells accumulate cholesterol in the lysosome as a result of a mutation in the NPC1 gene. Despite disturbed intracellular cholesterol transfer, NPC fibroblast cells exhibited HDL retroendocytosis and [(3)H]cholesterol efflux via HDL resecretion, although to a lesser extent. Thus, [(3)H]cholesterol efflux via HDL resecretion is independent of the cholesterol uptake pathway via the LDL receptor and may be an alternative way to remove excess cholesterol.     

Follicular fluid lipoproteins in the mare: Evaluation of HDL transfer from plasma to follicular fluid

Using a density gradient ultracentrifugal procedure, we have separated equine plasma and follicular fluid high-density lipoproteins (HDL). The density distribution of the follicular fluid HDL was clearly displaced towards the highest densities in comparison with that of plasma HDL. Similarly, an analysis of size distributions showed a decrease in follicular fluid HDL diameters (4.2 to 9.2 nm) compared to plasma HDL (5.5 to 9.5 nm). HDL were isolated into three subfractions on the basis of the disposition of the Sudan Black stained bands in the centrifuge tubes. Concentrations of each subfraction were clearly lower in the follicular fluid, and the relative percentages with regard to the plasma equivalents were inversely proportional to the molecular weights (23.8% for HDL-1, 49.9% for HDL-2 and 63.7% for HDL-3). The cholesterol/phospholipid molar ratio and the esterified/free cholesterol molar ratio were clearly increased in the follicular HDL-2 and HDL-3 subfractions. The apolipoprotein distribution in follicular fluid HDL was very close to that in plasma HDL. LCAT activity measured in human as well as equine samples was weaker in follicular fluid compared to plasma in both species (4.0 nmol of free cholesterol esterified per h per ml vs. 24 nmol per h per ml). Theoretical concentrations of follicular fluid HDL were calculated assuming that the HDL particles would be merely a filtration product undergoing no detectable metabolic modifications. Biochemical measurements showed that the lightest particules (HDL-1) were less numerous than suggested by the theoretical calculation. Thus, although follicular fluid HDL appear to be a filtration product of plasma HDL, they undergo metabolic transformations that we suggest may be linked to hormonal synthesis and reverse cholesterol transport.     

Affinity of lipid transfer protein for lipid and lipoprotein particles as influenced by lecithin-cholesterol acyltransferase

The effects of lecithin-cholesterol acyltransferase (LCAT) on the transfer of cholesterol esters mediated by lipid transfer protein (LTP) and its affinity for lipid and lipoprotein particles were investigated. When the single bilayer vesicle preparations (containing phosphatidylcholine, cholesterol, cholesteryl ester, and apolipoprotein- (apo) A-I at the molar ratio of 90:30:1.2:0.18) or high density lipoprotein 3 (HDL3) were used as the cholesteryl ester donor and low density lipoproteins (LDL) as the acceptor, the transfer activity of LTP was enhanced by the addition of low concentrations of LCAT. In contrast, no enhancement of cholesteryl ester transfer was observed upon addition of LCAT to either the discoidal bilayer particle preparations (containing phosphatidylcholine, cholesterol, cholesteryl ester, and apo-A-I at the molar ratio of 90:30:1.2:1.0) or high density lipoprotein 2 (HDL2). Although both apo-A-I and apo-A-II promoted the transfer of cholesteryl ester from vesicles to LDL, the additional enhancement of the transfer by LCAT was observed only with the vesicles containing apo-A-I. Gel permeation chromatography of LTP/vesicle and LTP/HDL3 mixtures in the presence and absence of LCAT showed that the affinity of LTP for both the vesicles and HDL3 increased upon addition of LCAT. In contrast, neither HDL2 nor discoidal bilayer particles showed any significant enhancement of LTP binding upon addition of LCAT. By using LCAT covalently bound to Sepharose 4B, a maximal interaction between LTP and bound LCAT was shown to occur at the ionic strength of 0.16. Deviation from this ionic strength reduced the extent of the interaction. At the ionic strength of 0.01 and 0.5, the elution volume of LTP was identical to that of bovine serum albumin.     

Apolipoprotein-specific populations in high density lipoproteins of human cord blood

High density lipoproteins (HDL) in human cord blood have previously been shown to exhibit particle size profiles distinctly different from those of adult HDL. The adult HDL profile is comprised of separate contributions from two major apolipoprotein-specific populations; one population contains both apolipoproteins AI and AII (HDL(AIwAII], while the other has apolipoprotein AI without AII (HDL(AIw/oAII]. The present studies establish that cord blood HDL are also comprised of HDL(AIwAII) and HDL(AIw/oAII) populations whose particle size profiles closely reflect cholesterol and HDL-cholesterol levels in cord blood. Compared with the adult, cord blood HDL(AIwAII) profiles generally show both a greater subspeciation within HDL2a and HDL3b/3c size intervals as well as relative reduction of material in the HDL3a interval. In the cord blood HDL(AIw/oAII) profile, HDL2b(AIw/oAII) particles also show subspeciation with a major component that is consistently larger than that normally observed in the adult (11.2 vs. 10.3 nm). As in the adult, the HDL3a(AIw/oAII) component is present but, unlike the adult, its relative amount is low; hence, its peak is usually not discernable in the cord blood total HDL profile. Our studies show that the larger-sized HDL2b(AIw/oAII) of cord blood are enriched in phospholipid which probably accounts for their increased size. The protein moiety of the larger-sized HDL2b(AIw/oAII) has a molecular weight equivalent to four apolipoprotein AI molecules per particle similar to the normal-sized adult subpopulation. Phospholipid enrichment of cord blood HDL(AIwAII) subpopulations within the HDL2a size interval was not observed. However, the protein moiety of cord blood HDL2a(AIwAII) is unusual in that it exhibits an apolipoprotein AI:AII molar ratio considerably lower (0.8:1 vs. 1.6:1) than that of adult. We suggest that the unique particle size distribution of cord blood total HDL is due in large part to: (a) a specific enrichment of phospholipid in HDL2b(AIw/oAII) species, producing particles larger than normal adult counterparts and (b) an elevated proportion of apoAII carried by the HDL(AIwAII) particles that may influence subspeciation in the HDL3a/b/c size interval.     

Discoidal complexes containing apolipoprotein E and their transformation by lecithin-cholesterol acyltransferase

The primary objectives of this study were to determine whether analogs to native discoidal apolipoprotein (apo)E-containing high-density lipoproteins (HDL) could be prepared in vitro, and if so, whether their conversion by lecithin-cholesterol acyltransferase (LCAT; EC 2.3.1.43) produced particles with properties comparable to those of core-containing, spherical, apoE-containing HDL in human plasma. Complexes composed of apoE and POPC, without and with incorporated unesterified cholesterol, were prepared by the cholate-dialysis technique. Gradient gel electrophoresis showed that these preparations contain discrete species both within (14-40 nm) and outside (10.8-14 nm) the size range of discoidal apoE-containing HDL reported in LCAT deficiency. The isolated complexes were discoidal particles whose size directly correlated with their POPC:apoE molar ratio: increasing this ratio resulted in an increase in larger complexes and a reduction in smaller ones. At all POPC:apoE molar ratios, size profiles included a major peak corresponding to a discoidal complex 14.4 nm long. Preparations with POPC:apoE molar ratios greater than 150:1 contained two distinct groups of complexes, also in the size range of discoidal apoE-containing HDL from patients with LCAT deficiency. Incorporation of unesterified cholesterol into preparations (molar ratio of 0.5:1, unesterified cholesterol:POPC) resulted in component profiles exhibiting a major peak corresponding to a discoidal complex 10.9 nm long. An increase of unesterified cholesterol and POPC (at the 0.5:1 molar ratio) in the initial mixture, increased the proportion of larger complexes in the profile. Incubation of isolated POPC-apoE discoidal complexes (mean sizes, 14.4 and 23.9 nm) with purified LCAT and a source of unesterified cholesterol converted the complexes to spherical, cholesteryl ester-containing products with mean diameters of 11.1 nm and 14.0 nm, corresponding to apoE-containing HDL found in normal plasma. Conversion of smaller cholesterol-containing discoidal complexes (mean size, 10.9 nm) under identical conditions resulted in spherical products 11.3, 13.3, and 14.7 nm across. The mean sizes of these conversion products compared favorably with those (mean diameter, 12.3 nm) of apoE-containing HDL of human plasma. This conversion of cholesterol-containing complexes is accompanied by a shift of some apoE to the LDL particle size interval. Our study indicates that apoE-containing complexes formed by the cholate-dialysis method include species similar to discoidal apoE-containing HDL and that incubation with LCAT converts most of them to spherical core-containing particles in the size range of plasma apoE-containing HDL. Plasma HDL particles containing apoE may arise in part from direct conversion of discoidal apoE-containing HDL by LCAT.     

Transformation of large discoidal complexes of apolipoprotein A-I and phosphatidylcholine by lecithin-cholesterol acyltransferase

Using a cholate-dialysis recombination procedure, complexes of apolipoprotein A-I and synthetic phosphatidylcholine (1-palmitoyl-2-oleoylphosphatidylcholine (POPC) or dioleoylphosphatidylcholine (DOPC] were prepared in mixtures at a relatively high molar ratio of 150:1 phosphatidylcholine/apolipoprotein A-I. Particle size distribution analysis by gradient gel electrophoresis of the recombinant mixtures indicated the presence of a series of discrete complexes that included species migrating at RF values observed for discoidal particles in nascent high-density lipoproteins (HDL) in plasma of lecithin-cholesterol acyltransferase-deficient subjects. One of these complex species, designated complex class 6, formed with either phosphatidylcholine, was isolated by gel filtration and characterized at follows: discoidal shape (mean diameter 20.8 nm (POPC) and 19.0 nm (DOPC]; molar ratio, phosphatidylcholine/apolipoprotein A-I, 155:1 (POPC) and 130:1 (DOPC); and both containing 4 molecules of apolipoprotein A-I per particle. Incubation of class 6 complexes with lecithin-cholesterol acyltransferase (EC 2.3.1.43) and a source of unesterified cholesterol (low-density lipoprotein (LDL] was shown by electron microscopy to result in a progressive transformation of the discoidal particles (0 h) to deformable (2.5 h) and to spherical particles (24 h). The spherical particles (diameter 13.6 nm (POPC) and 12.5 nm (DOPC) exhibit sizes at the upper boundary of the interval defining the human plasma (HDL2b)gge (12.9-9.8 nm). The spherical particles contain a cholesteryl ester core that reaches a limiting molar ratio of approx. 50-55:1 cholesteryl ester/apolipoprotein A-I. The deformable particles assume a rectangular shape under negative staining and, relative to the 24-h spherical product, are enriched in phosphatidylcholine. Chemical crosslinking (by dimethyl suberimidate) of the isolated transformation products shows the 24-h spherical particle to contain predominantly 4 apolipoprotein A-I molecules; products produced after intermediate periods of time appear to contain species with 3 and 4 apolipoproteins per particle. Our in vitro studies indicate a potential pathway in the origins of large, apolipoprotein A-I-containing plasma HDL particles. The deformable species observed during transformation were similar in size and shape to particles observed in interstitial fluid.     

Increases in the particle size of high-density lipoproteins induced by purified lecithin: cholesterol acyltransferase: effect of low-density lipoproteins

Homogeneous subpopulations of human high-density lipoproteins subfraction-3 (HDL3) have been incubated at 37 degrees C with purified lecithin: cholesterol acyltransferase, human serum albumin and varying concentrations of human low-density lipoproteins (LDL). Changes in HDL particle size and composition during these incubations were monitored. Incubation of HDL3a (particle radius 4.3 nm) in the absence of LDL resulted in an esterification of more than 70% of the HDL free cholesterol after 24 h of incubation. This, however, was sufficient to increase the HDL cholesteryl ester by less than 10% and was not accompanied by any change in particle size. When this mixture was incubated in the presence of progressively increasing concentrations of LDL, which donated free cholesterol to the HDL, the molar rate of production of cholesteryl ester was much greater; at the highest LDL concentration HDL cholesteryl ester content was almost doubled after 24 h and there was an increase in the HDL particle size up to the HDL2 range. In the case of HDL3b (radius 3.9 nm), there were again only minimal changes in particle size in incubations not containing LDL. In the presence of the highest concentration of LDL tested, however, the particles were again enlarged into the HDL2 size range after 24 h incubation. These HDL2-like particles were markedly enriched with cholesteryl ester but depleted of phospholipid and free cholesterol when compared with native HDL2. Furthermore, the ratio of apolipoprotein A-I to apolipoprotein A-II resembled that in the parent-HDL3 and was very much lower than that in native HDL2. It has been concluded that purified lecithin: cholesterol acyltransferase is capable of increasing the size of HDL3 towards that of HDL2 but that other factors must operate in vivo to modulate the chemical composition of the enlarged particles.     

Slow oxidation of high density lipoproteins as studied by EPR spectroscopy

There is relatively little information on the role of high density lipoprotein (HDL) oxidation in atherogenesis although there are indications that oxidation might affect atheroprotective activities of HDL. Recently we reported the study on LDL oxidation initiated and sustained by traces of the transition metal ions under conditions, which favor slow oxidation. Here we report the results of the analogous study on the oxidation of the two HDL subclasses. The oxidation process was monitored by measuring the time dependence of oxygen consumption and concentration of the spin-trapped free radicals using EPR spectroscopy. In both HDL2 and HDL3 subclasses, the dependence of the oxidation process on the copper/lipoprotein molar ratio is different from that in LDL dispersions. Comparison of the kinetic profiles of HDL2 and HDL3 oxidation revealed that under all studied experimental conditions HDL2 was more susceptible to copper-induced oxidation than HDL3.     

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.     

Volumetric determination of apolipoprotein stoichiometry of circulating HDL subspecies

Although HDL is inversely correlated with coronary heart disease, elevated HDL-cholesterol is not always protective. Additionally, HDL has biological functions that transcend any antiatherogenic role: shotgun proteomics show that HDL particles contain 84 proteins (latest count), many correlating with antioxidant and anti-inflammatory properties of HDL. ApoA-I has been suggested to serve as a platform for the assembly of these protein components on HDL with specific functions - the HDL proteome. However, the stoichiometry of apoA-I in HDL subspecies is poorly understood. Here we use a combination of immunoaffinity chromatography data and volumetric analysis to evaluate the size and stoichiometry of LpA-I and LpA-I,A-II particles. We conclude that there are three major LpA-I subspecies: two major particles, HDL[4] in the HDL₃ size range (d = 85.0 ± 1.2 Å) and HDL[7] in the HDL₂ size range (d = 108.5 ± 3.8 Å) with apoA-I stoichiometries of 3 and 4, respectively, and a small minor particle, HDL[1] (d = 73.8 ± 2.1Å) with an apoA-I stoichiometry of 2. Additionally, we conclude that the molar ratio of apolipoprotein to surface lipid is significantly higher in circulating HDL subspecies than in reconstituted spherical HDL particles, presumably reflecting a lack of phospholipid transfer protein in reconstitution protocols.     

Specific interaction of the intermediate filament protein vimentin and its isolated N-terminus with negatively charged phospholipids as determined by vesicle aggregation, fusion, and leakage measurements

The interaction of the intermediate filament protein vimentin and its non-alpha-helical N-terminus with phosphatidylserine and phosphatidylinositol small unilamellar vesicles was investigated by measuring vesicle aggregation, fusion, and leakage. While the N-terminus suppressed Ca2(+)-induced fusion of phosphatidylserine vesicles, it caused their rapid aggregation in the absence of Ca2+; at a molar ratio of lipid to polypeptide of 25:3, the polypeptide/lipid complexes precipitated from the reaction mixture. This aggregation was efficiently diminished by NaCl. The phosphatidylinositol vesicles, on the other hand, became leaky when interacting with the N-terminus of vimentin, even at a molar ratio of lipid to polypeptide of 500:1. The leakage of phosphatidylinositol vesicles was suppressed by the addition of Ca2+ or NaCl to the reaction mixture. Intact vimentin also caused leakage of phosphatidylinositol vesicles, at low and high salt concentration. The results indicate specific and differential interactions of the N-terminus of vimentin with various negatively charged lipid species, although there is an electrostatic component common to these interactions.     

Synthesis of reconstituted high density lipoprotein (rHDL) containing apoA-I and apoC-III: the functional role of apoC-III in rHDL

Apolipoprotein (apo) C-III is a marker protein of triacylglycerol (TG)-rich lipoproteins and high-density lipoproteins (HDL), and has been proposed as a risk factor of coronary heart disease. To compare the physiologic role of reconstituted HDL (rHDL) with or without apoC-III, we synthesized rHDL with molar ratios of apoA-I:apoC-III of 1:0, 1:0.5, 1:1, and 1:2. Increasing the apoC-III content in rHDL produced smaller rHDL particles with a lower number of apoA-I molecules. Furthermore, increasing the molar ratio of apoC-III in rHDL enhanced the surfactant-like properties and the ability to lyse dimyristoyl phosphatidylcholine. Furthermore, rHDL containing apoC-III was found to be more resistant to particle rearrangement in the presence of low-density lipoprotein (LDL) than rHDL that contained apoA-I alone. In addition, the lecithin:cholesterol acyltransferase (LCAT) activation ability was reduced as the apoC-III content of the rHDL increased; however, the CE transfer ability was not decreased by the increase of apoC-III. Finally, rHDL containing apoC-III aggravated the production of MDA in cell culture media, which led to increased cellular uptake of LDL. Thus, the addition of apoC-III to rHDL induced changes in the structural and functional properties of the rHDL, especially in particle size and rearrangement and LCAT activation. These alterations may lead to beneficial functions of HDL, which is involved in anti-atherogenic properties in the circulation.     

Influence of apolipoprotein A-I and apolipoprotein A-II availability on nascent HDL heterogeneity

It is important to understand HDL heterogeneity because various subspecies possess different functionalities. To understand the origins of HDL heterogeneity arising from the existence of particles containing only apoA-I (LpA-I) and particles containing both apoA-I and apoA-II (LpA-I+A-II), we compared the abilities of both proteins to promote ABCA1-mediated efflux of cholesterol from HepG2 cells and form nascent HDL particles. When added separately, exogenous apoA-I and apoA-II were equally effective in promoting cholesterol efflux, although the resultant LpA-I and LpA-II particles had different sizes. When apoA-I and apoA-II were mixed together at initial molar ratios ranging from 1:1 to 16:1 to generate nascent LpA-I+A-II HDL particles, the particle size distribution altered, and the two proteins were incorporated into the nascent HDL in proportion to their initial ratio. Both proteins formed nascent HDL particles with equal efficiency, and the relative amounts of apoA-I and apoA-II incorporation were driven by mass action. The ratio of lipid-free apoA-I and apoA-II available at the surface of ABCA1-expressing cells is a major factor in determining the contents of these proteins in nascent HDL. Manipulation of this ratio provides a means of altering the relative distribution of LpA-I and LpA-I+A-II HDL particles.     

Reactivity of HDL subfractions towards lecithin-cholesterol acyltransferase. Modulation by their content in free cholesterol

(1) Human HDL2 (d 1.070-1.125) and HDL3 (d 1.125-1.21) labelled with unesterified [14C]cholesterol, were incubated with a source of lecithin-cholesterol acyltransferase. For optimal activity, the reaction required the addition of albumin in excess, at least 3-times greater than the concentration of HDL-free cholesterol. Under such conditions, the reaction appeared saturable. HDL3 was found the most efficient substrate and the Vmax values expressed for 1.5 IU LCAT/ml and with an albumin/free cholesterol ratio of 3, were 8.3 nmol free cholesterol esterified/ml per h and 4.1 nmol/ml per h for HDL3 and HDL2, respectively. (2) HDL3 were modified in the presence of VLDL by inducing triacylglycerol lipolysis with a semipurified lipoprotein lipase from bovine milk. The newly formed HDL had gained free cholesterol and phospholipids, so that about 50% of these modified HDL, referred to as light-LIP-HDL3, were reisolated in the HDL2 density range. Light-LIP-HDL3 were enriched mostly in free cholesterol (+ 160%) and in phospholipid (+ 40%). Their reactivity towards LCAT was half-reduced compared to parent HDL3, which correlated well with a decrease in their phospholipid/free cholesterol molar ratio. Moreover, HDL3 artificially enriched in free cholesterol and exhibiting a comparable PL/FC behaved like lipolysis-modified HDL in their reactivity towards LCAT. (3) HDL3 were also modified by co-incubation with VLDL (post-VLDL-HDL3), or with VLDL and a source of lipid transfer protein (CET-HDL3). The latter treatment greatly affected the lipid composition of the core particle (-25% esterified cholesterol, +190% TG). In both cases, the moderate decreasing LCAT reactivity observed could be related to the phospholipid/free cholesterol ratio. Thus, like in artificial substrates, the lipid composition of the HDL surface may control the rate of LCAT-mediated cholesterol esterification.     

Subpopulations of apolipoprotein A-I in human high-density lipoproteins. Their metabolic properties and response to drug therapy

In this study immunological procedures were used to detect and quantify high-density lipoprotein (HDL) particles of differing apolipoprotein A composition. In the plasma of eight healthy female subjects, 45% of the total apolipoprotein A-I existed in particles (called '(AI)HDL') devoid of apolipoprotein A-II. The remainder circulated in association with apolipoprotein A-II at a molar ratio of approximately 1:1. Nicotinic acid selectively raised the plasma apolipoprotein A-I/A-II ratio by increasing the proportion of (AI)HDL particles. Probucol produced the opposite effect, lowering the plasma concentration of these particles. The kinetic properties of apolipoprotein A-I in total HDL and in the (AI)HDL particle were the same despite the fact that apolipoprotein A-I equilibration between these two species was incomplete. Therefore, there appear to be at least two apolipoprotein A-containing particle populations in HDL which are immunochemically and metabolically distinct.