Characterization of the structure of polydisperse human low-density lipoprotein by neutron scattering

Modeling Small-Angle X-ray Scattering Data for Low-Density Lipoproteins: Insights into the Fatty Core Packing and Phase Transition

Atherosclerosis and its clinical consequences are the leading cause of death in the western hemisphere. While many studies throughout the last decades have aimed at understanding the disease, the clinical markers in use today still fail to accurately predict the risks. The role of the current main clinical indicator, low density lipoprotein (LDL), in depositing fat to the vessel wall is believed to be the onset of the process. However, many subfractions of the LDL, which differ both in structure and composition, are present in the blood and among different individuals. Understanding the relationship between LDL structure and composition is key to unravel the specific role of various LDL components in the development and/or prevention of atherosclerosis. Here, we describe a model for analyzing small-angle X-ray scattering data for rapid and robust structure determination for the LDL. The model not only gives the overall structure but also the particular internal layering of the fats inside the LDL core. Thus, the melting of the LDL can be followed in situ as a function of temperature for samples extracted from healthy human patients and purified using a double protocol based on ultracentrifugation and size-exclusion chromatography. The model provides information on: (i) the particle-specific melting temperature of the core lipids, (ii) the structural organization of the core fats inside the LDL, (iii) the overall shape of the particle, and (iv) the flexibility and overall conformation of the outer protein/hydrophilic layer at a given temperature as governed by the organization of the core. The advantage of this method over other techniques such as cryo-TEM is the possibility of in situ experiments under near-physiological conditions which can be performed relatively fast (minutes at home source, seconds at synchrotron). This approach now allows the monitoring of structural changes in the LDL upon different stresses from the environment, such as changes in temperature, oxidation, or external agents used or currently in development against atherosclerotic plaque build-up and which are targeting the LDL.

Enrichment of serum low-molecular-weight proteins using C18 absorbent under urea/dithiothreitol denatured environment

Serum low-molecular-weight proteins (LMWPs, molecular weight<30kDa) are closely related to the body physiological and pathological situations, whereas many difficulties are encountered when enriching and fractionating them. Using C(18) absorbent (100 A) enrichment and fractionation under urea/dithiothreitol (DTT) denatured environment followed by 60% acetonitrile (ACN) elution, serum LMWPs could be enriched more than 100-fold and were evaluated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), two-dimensional gel electrophoresis (2-DE), and isotope-coded affinity tag (ICAT) labeling quantification. Proteins existing in human serum at low nanograms/milliliter (ng/ml) levels, such as myeloid-related proteins (MRPs), could be identified directly from 2-DE coupled with matrix-assisted laser desorption/ionization tandem time-of-flight mass spectrometry (MALDI-TOF/TOF MS) and LTQ-Orbitrap MS. Sixteen proteins were confidentially identified and quantified using ICAT labeling and liquid chromatography-tandem mass spectrometry (LC-MS/MS). By virtue of its easy operation and high reproducibility to process large quantity complex serum samples, this method has potential uses in enriching LMWPs either in serum or in cell and tissue samples.

The assembly of apoB-containing lipoproteins: a structural biology point of view

Atherosclerosis is a widespread disease caused by the deposition of lipids on arterial walls. Such lipid plaques in coronary arteries can be fatal. Although many factors related to diet, life-style, etc. contribute to the worsening of the ailment, the primary cause, the lipids in the circulatory system, come from a series of low-density lipoproteins. These lipoproteins are necessary for the transport of lipids to and from different organs. It would be valuable to medicine and the field of drug design if a more detailed understanding of the organization of lipid and protein in these molecules were available. Unfortunately because of heterogeneity in their size and lipid composition, all classes of the low-density serum lipoproteins appear to be not amenable to the most widely used method for obtaining detailed atomic data - X-ray crystallography. However there appears to be a recently identified homolog that is relatively homogeneous, and crystal structures have been obtained. Used as a molecular model, the homolog serves as a source of conformational information that might help to unravel the processes involved in the lipid loading of the low-density lipoproteins. The review attempts to give a brief summary of the structural biology of the serum low-density lipoproteins relative to the molecular model of lipovitellin.

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.

Alterations in the structure of apolipoprotein B-100 determine the behaviour of LDL towards thromboplastin

Apolipoprotein B-100 acts as an inhibitor of thromboplastin activity independently of the tissue factor pathway inhibitor (TFPI) associated with plasma lipoproteins. Analysis of the primary structure of Apo B-100 showed a higher than expected occurrence of lysine groups in the receptor-binding region. In order to demonstrate the participation of lysine groups of Apo B-100 in the inhibition of thromboplastin, thromboplastin and Apo B-100 were incubated together in the presence of poly-L-lysine, poly-L-arginine, lysine and arginine monomers. The inhibition of thromboplastin by Apo B-100 was completely suppressed in the presence of poly-L-lysine. Poly-L-arginine was found to be less effective and neither lysine or arginine monomers had any significant effect on the inhibitory effect of Apo B-100. Alterations in the structure of Apo B-100 reconstituted in lipid vesicles resembling LDL, brought about by lipid peroxidation and lipid loading were examined by means of Fourier transform infra-red spectroscopy. It was found that, upon oxidation without the addition of cupric ions, the apolipoprotein attains a more exposed conformation with an increase in alpha-helical structure. This increase occurred at the expense of beta-structure. On lipid loading, an increase in beta-structure at the expense of the alpha-helix, was demonstrated. It is therefore proposed that the variable action of LDL towards thromboplastin derives from alterations in the secondary structure of the Apo B-100, particularly the receptor-binding region.

Crystallization and preliminary X-ray analysis of a low density lipoprotein from human plasma

Single crystals of human plasma low density lipoprotein (LDL), the major transport vehicle for cholesterol in blood, have been produced with a view to analysis of the three-dimensional structure by x-ray crystallography. Crystals with dimensions of approximately 200 x 100 x 50 microm have been reproducibly obtained from highly homogeneous LDL particle subspecies, isolated in the density ranges d = 1.0271-1. 0297 g/ml and d = 1.0297-1.0327 g/ml. Electron microscopic imaging of ultrathin-sectioned preparations of the crystals confirmed the existence of a regular, quasihexagonal arrangement of spherical particles of approximately 18 nm in diameter, thereby resembling the dimensions characteristic of LDL after dehydration and fixation. X-ray diffraction with synchrotron radiation under cryogenic conditions revealed the presence of well resolved diffraction spots, to a resolution of about 29 A. The diffraction patterns are indexed in terms of a triclinic lattice with unit cell dimensions of a = 16. 1 nm, b = 39.0 nm, c = 43.9 nm; alpha = 96.2 degrees, beta = 92.1 degrees, gamma = 102 degrees, and with space group P1.

Time-course studies by synchrotron X-ray solution scattering of the structure of human low-density lipoprotein during Cu(2+)-induced oxidation in relation to changes in lipid composition

Low-density lipoproteins (LDLs) in plasma are constructed from a single molecule of apolipoprotein B-100 (apoB) (M(r) 512,000) in association with lipid [approximate M(r) (2-3) x 10(6)]. LDL oxidation is an important process in the development of atherosclerosis, and can be imitated by the addition of Cu2+ ions. Synchrotron X-ray scattering of LDL yields curves without radiation damage effects at concentrations close to physiological. The radius of gyration RG for preparations of LDL from different donors ranged between 12.1 and 16.0 nm, with a mean of 13.9 nm. At 4 degrees C, the distance distribution curve P(r) indicated a maximum dimension of 25-27 nm for LDL, a peak at 19.5 nm which corresponds to a surface shell of protein and phospholipid head groups in LDL, and submaxima between 1.7 and 13.5 nm, which correspond to an ordered lipid core in LDL. LDL from different donors exhibited distinct P(r) curves. For oxidation studies of LDL by X-rays, data are best obtained at 4 degrees C at a concentration of > or = 2 mg of LDL protein/ml together with controls based on non-oxidized LDL. LDL oxidation (2 mg of apoB/ml) was studied at 37 degrees C in the presence of 6.4, 25.6 and 51.2 mu of Cu2+/g of apoB. Large changes in P(r) were reproducibly observed in the inter-particle distance range between 13 and 16 nm shortly after initiation of oxidation. This corresponds to the phospholipid hydrocarbon in LDL, which has either increased in electron density during oxidation or become increasingly disordered. After 25 h, the structural changes subsequently spread to regions of the P(r) curves assigned to surface apoB and the central core of cholesteryl esters and triacyl-glycerols. Lipid analyses were carried out under the same solution conditions. The alpha-tocopherol and beta-carotene antioxidant contents of LDL were consumed within 1-2 h. Analyses of the formation of thiobarbituric acid-reactive substances and lipid hydroperoxides indicated that arachidonic acid was preferentially oxidized before the maximal formation of lipid hydroperoxides at 8-12 h after initiation of oxidation. High-performance TLC showed that phosphatidylcholine was continuously converted into lysophosphatidylcholine during oxidation, which is consistent with the early changes in the X-ray P(r) curves. The neutral core lipids became modified only after 12-15 h of oxidation. The combination of X-ray scattering structural analyses with biochemical analyses shows that the oxidation of LDL first affects the outer shell of surface phospholipid, then it spreads towards damage of apoB and the internal neutral lipid core of LDL.

Time-course studies by neutron solution scattering and biochemical assays of the aggregation of human low-density lipoprotein during Cu(2+)-induced oxidation

The oxidative modification of low-density lipoproteins (LDL) is recognized to be a key event in the development of atherosclerotic plaques on artery walls. The characteristics of LDL oxidized by cells of the artery wall can be imitated by the addition of Cu2+ ions to initiate lipid peroxidation in LDL. Neutron scattering of LDL in 2H2O buffers enables the time course of changes in the gross structure of LDL during oxidation to be continuously monitored under conditions close to physiological. Oxidation of LDL [2 mg of apolipoprotein B (apoB) protein/ml] was studied in the presence of 6.4, 25.6 and 51.2 mumol of Cu2+/g of apoB by incubation at 37 degrees C for up to 70 h. Neutron Guinier analyses showed that the radius of gyration RG (indicative of size) and the forward-scattered intensity at zero angle I(0) (indicative of M(r)) continuously increased during oxidation, indicating that LDL had aggregated. Both the rate of aggregation and the change in RG and I(0) values after 10 and 50 h increased with Cu2+ concentration. Distance-distribution functions P(r) showed that, within 4 h, the maximum dimension of LDL increased from 23 to 55 nm. The P(r) curves of oxidatively modified LDL exhibited two peaks at 10-12 nm and 26 nm. The 10-12 nm peak corresponds to native LDL, and the 26 nm peak is assigned to the initial formation of LDL dimers and trimers and their progression to form higher oligomers. The growth of the 26 nm peak depended on Cu2+ concentration. Particle-size-distribution functions Dv(r) suggested that the polydisperse spherical structure of LDL ceased to exist after 30 h, at which point the LDL samples underwent a phase separation. Related, but not identical, changes in the I(Q) and P(r) curves were observed when native LDL was self-aggregated by brief vortexing. Parallel assessment of LDL protein modification by SDS/PAGE showed increased aggregation and degradation of apoB with increased Cu2+ concentrations, and that the main apoB protein band had diminished after 2-8 h, depending on the amount of Cu2+ added. The uptake and degradation of oxidized 125I-labelled LDL by mouse peritoneal macrophages occurred maximally within the first 10 h, and increased in proportion to the Cu2+ concentration. ApoB protein broke down within the first 10 h of oxidation, and this is the period when scavenger receptors on macrophages can recognize and internalize oxidized LDL. Within 10 h, the protein-lipid interactions responsible for the spherical LDL structure became destabilized by protein fragmentation.(ABSTRACT TRUNCATED AT 400 WORDS)