Fluorescence study of the motional states of core and surface lipids in native and reconstituted low density lipoproteins

Evaluation of Methods for the Measurement of Low-Density Lipoprotein Cholesterol

A high concentration of low-density lipoprotein cholesterol (LDL-C) in plasma is one of the strongest risk factors for atherosclerotic cardiovascular disease and mortality. The most common approach to determining LDL-C in the clinical laboratory is the Friedewald calculation. There is an increased interest to improve the accuracy of LDL-C estimated by this equation. The expert panel convened by National Cholesterol Education Program has recommended the development of accurate direct methods to measure LDL-C. Several homogeneous and fully automated methods have been introduced in recent years that show improved precision and accuracy over earlier methods, especially the Friedewald calculation. Each of the atherogenic particles in plasma—very-low, intermediate-, and low-density lipoprotein—as well as lipoprotein (a), contain one molecule of apolipoprotein B (apoB) and thus, plasma total concentration of apoB reflects the number of atherogenic particles. Several studies suggested that the measurement of apoB could improve the prediction of risk of coronary artery disease. Thus, in addition to the newly developed direct assays, alternative calculation procedures have been proposed that also take into consideration total serum apoB concentration for the estimation of LDL-C and the presence of small, dense LDL particles. The new generation of homogenous methods for the measurement of LDL-C and the use of serum apoB concentration for the estimation of LDL-C can contribute to the accurate LDL-C determination.

Lipid–protein interactions in human plasma LDL evidenced by magnetic resonance

Low density lipoprotein (LDL) particles exhibit extremely complex three-dimensional structural organization which is still not understood at the molecular level. The aim of this study was to provide the experimental evidence of a direct non-covalent interaction of the protein part with the lipid matrix. The approach was based on the combination of (1)H NMR (600 MHz) spectroscopy with thiol-specific spin labeling of the protein (apoB). It is shown that the spectral peaks assigned to the methyl head groups of phosphatidylcholine and sphingomyelin in the (1)H spectra of LDL exhibit line broadening when otherwise free thiol groups of apoB are covalently modified by methanethiosulfonate spin label. The effect is similar in the presence of water soluble paramagnetic compound. These results indicate that fragments of apoB, which are part of the receptor binding region, are directly in contact with the solvated phospholipid head groups of the lipid matrix.

Differences in the binding capacity of human apolipoprotein E3 and E4 to size-fractionated lipid emulsions

We describe sensitive new approaches for detecting and quantitating protein-lipid interactions using analytical ultracentrifugation and continuous size-distribution analysis [Schuck (2000) Biophys. J.78, 1606-1619]. The new methods were developed to investigate the binding of human apolipoprotein E (apoE) isoforms to size-fractionated lipid emulsions, and demonstrate that apoE3 binds preferentially to small lipid emulsions, whereas apoE4 exhibits a preference for large lipid particles. Although the apparent binding affinity for large emulsions is similar (Kd approximately 0.5 micro m), the maximum binding capacity for apoE4 is significantly higher than for apoE3 (3.0 and 1.8 amino acids per phospholipid, respectively). This indicates that apoE4 has a smaller binding footprint at saturation. We propose that apoE isoforms differentiate between lipid surfaces on the basis of size, and that these differences in lipid binding are due to a greater propensity of apoE4 to adopt a more compact closed conformation. Implications for the role of apoE4 in blood lipid transport and disease are discussed.

Modulation of apolipoprotein E-mediated plasma clearance and cell uptake of emulsion particles by cholesteryl ester

Cholesteryl ester, along with triglyceride (TG), is the major core component of plasma lipoproteins. We investigated the effect of core composition on the physical state and metabolic behavior of lipid emulsions, as model particles of lipoproteins. Fluorescence studies using 1,6-diphenylhexatriene analogs showed that although cholesteryl oleate (CO) significantly decreased core mobility, the surface rigidity of phosphatidylcholine (PC) monolayers was independent of core composition. When intravenously injected into rats, the increased amount of core CO tended to retard TG emulsion removal from plasma, and the initial clearance rate was correlated with the amount of apolipoprotein E (apoE) bound from plasma. In addition, PC liposomes with a similar emulsion particle size showed negligible binding of apoE and were cleared at a slower rate compared to all emulsions. Furthermore, the effect of CO on the binding behavior of apoE to the emulsion surface and the emulsion uptake by hepatocytes was assessed in vitro. Replacing core TG with CO was found to decrease the apoE binding capacity to emulsions markedly without changing the binding affinity and thereby to reduce the cell uptake of emulsion particles by HepG2 cells. These results indicate that the physical state of core lipids, which can be modulated by CO content, plays a role in emulsion metabolism through the alteration in apoE binding.

Structure of low density lipoprotein (LDL) particles: basis for understanding molecular changes in modified LDL

Low density lipoprotein (LDL) particles are the major cholesterol carriers in circulation and their physiological function is to carry cholesterol to the cells. In the process of atherogenesis these particles are modified and they accumulate in the arterial wall. Although the composition and overall structure of the LDL particles is well known, the fundamental molecular interactions and their impact on the structure of LDL particles are not well understood. Here, the existing pieces of structural information on LDL particles are combined with computer models of the individual molecular components to give a detailed structural model and visualization of the particles. Strong evidence is presented in favor of interactions between LDL lipid constituents that lead to specific domain formation in the particles. A new three-layer model, which divides the LDL particle into outer surface, interfacial layer, and core, and which is capable of explaining some seemingly contradictory interpretations of molecular interactions in LDL particles, is also presented. A new molecular interaction model for the beta-sheet structure and phosphatidylcholine headgroups is introduced and an overall view of the tertiary structure of apolipoprotein B-100 in the LDL particles is presented. This structural information is also utilized to understand and explain the molecular characteristics and interactions of modified, atherogenic LDL particles.

Modeling the interaction of peroxynitrite with low-density lipoproteins. II: reaction/diffusion model of peroxynitrite in low-density lipoprotein particles

Peroxynitrite is a possible initiator for the free radical chain reaction that results in peroxidation of low-density lipoproteins (LDL) which is the first step in atherogenisis. This paper reports on the use of a diffusion/reaction model to examine the processes involved in peroxynitrite attack on LDL particles. Results indicate that because of the short distance involved, diffusion is much more rapid than chemical decomposition. Because of this decoupling the free radicals generated by peroxynitrite decomposition may be found at any point in the LDL particle. At the concentrations expected in physiological systems only a small proportion of LDL particles may contain peroxynitrite molecules. However, these particles may still be profoundly effected because of the long reaction chain length expected after initiation.

Aggregation of dimyristoylphosphatidylglycerol liposomes by human plasma low density lipoprotein

Turbidity (absorbance at 470 nm) measurements revealed human serum low density lipoprotein (LDL) to cause, within a few minutes and at physiological pH and [NaCl], the aggregation of liquid crystalline large unilamellar liposomes (LUVs) of dimyristoylphosphatidylglycerol (DMPG). No evidence for concomitant lipid or aqueous contents mixing was obtained with fluorescent assays for these processes, in keeping with the lack of fusion of LUVs. Involvement of apoB is implicated by the finding that tryptic digestion of LDL abrogates its ability to cause aggregation. Aggregation is not caused by VLDL, HDL2, or HDL3. Interestingly, also oxidised LDL failed to aggregate DMPG vesicles. Aggregation of DMPG LUVs by LDL did depend on the ionic strength of the medium as well as on the phase state of the lipid. More specifically, below the main transition temperature Tm maximal aggregation was seen in the presence of 25-100 mM NaCl, whereas slightly higher (up to 150 mM) [NaCl] were required when T>Tm. Aggregation due to LDL was also observed for dimyristoylphosphatidylserine as well as for dipalmitoylphosphatidylglycerol LUVs, whereas liposomes composed of either unsaturated acidic phospholipids or different phosphatidylcholines were not aggregated. Involvement of electrostatic attraction between the acidic phosphate of DMPG and cationic residues in apoB is suggested by the finding that increasing the content of dimyristoylphosphatidylcholine (DMPC) in DMPG liposomes reduced their aggregation and at XDMPC=0.50 no response was evident. Notably, increasing the mole fraction of 1-palmitoyl-2-oleyl-PG (POPG) in DMPG LUVs progressively reduced their aggregation by LDL and at XPOPG=0.50 there was complete inhibition. The latter effect of POPG is likely to be due to augmented hydration of the unsaturated lipid constituting a barrier for the contact between apoB and the vesicle surface. In keeping with this view, the presence of the strongly hygroscopic polymer, poly(ethylene glycol) at 1% (by weight) enhanced the aggregation and could partly reverse the inhibition by POPG.

Evidence for distinct behaviour of phosphatidylcholine and sphingomyelin at the low density lipoprotein surface

This study demonstrates that the use of high field 1H NMR spectroscopy permits individual detection of phosphatidylcholine and sphingomyelin molecules at the surface of native low density lipoprotein (LDL) particles. Distinct behaviour was observed for the choline head group -N(CH3)3 resonances of these different phospholipids revealing preferential immobilisation for phosphatidylcholine. This suggests the existence of reversible and irreversible phosphatidylcholine-apolipoprotein B interactions and is consistent with microdomain formation at the surface monolayer of LDL. The novel resonance assignment and results show that 1H NMR can provide efficient and practical means for future studies on the structure and dynamics at the LDL surface.

Physical states of surface and core lipids in lipid emulsions and apolipoprotein binding to the emulsion surface

Plasma triglyceride-rich lipoproteins vary in lipid composition during their metabolism. We investigated the effects of the lipid composition of emulsion particles, specifically those of cholesterol enrichment and core replacement (replacing core triglyceride with cholesteryl oleate), on the physical states of surface and core lipids. Steady-state and time-resolved fluorescence anisotropies were measured in lipid emulsions using 1,6-diphenylhexatriene to probe the core and 1,6-diphenylhexatriene analogues for the outer and inner hydrophobic portions of surface phospholipids. In the absence of cholesterol, core replacement had little effect on the surface rigidity, despite the large difference in core mobility. However, core replacement caused a marked increase in surface rigidity in the presence of cholesterol. Quenching experiments using the fluorescent cholesterol analogue, dehydroergosterol, indicated that core replacement allowed surface dehydroergosterol to redistribute from the inner to the outer regions in the emulsion surface. These results indicated that core replacement modulates the surface properties of the emulsion particles through the redistribution of cholesterol in the surface layers. Furthermore, core replacement significantly decreased the binding of apolipoprotein E to the emulsion surface, whereas the binding of apolipoprotein CII responded to the cholesterol enrichment. This binding behavior of exchangeable apolipoproteins may closely correlate with the location of surface cholesterol and the mobility of core lipids.