The structure of serum lipoproteins as analysed by X-ray small-angle scattering

Peptides at the Interface: Self-Assembly of Amphiphilic Designer Peptides and Their Membrane Interaction Propensity

Self-assembling amphiphilic designer peptides have been successfully applied as nanomaterials in biomedical applications. Understanding molecular interactions at the peptide–membrane interface is crucial, since interactions at this site often determine (in)compatibility. The present study aims to elucidate how model membrane systems of different complexity (in particular single-component phospholipid bilayers and lipoproteins) respond to the presence of amphiphilic designer peptides. We focused on two short anionic peptides, V₄WD₂ and A₆YD, which are structurally similar but showed a different self-assembly behavior. A₆YD self-assembled into high aspect ratio nanofibers at low peptide concentrations, as evidenced by synchrotron small-angle X-ray scattering and electron microscopy. These supramolecular assemblies coexisted with membranes without remarkable interference. In contrast, V₄WD₂ formed only loosely associated assemblies over a large concentration regime, and the peptide promoted concentration-dependent disorder on the membrane arrangement. Perturbation effects were observed on both membrane systems although most likely induced by different modes of action. These results suggest that membrane activity critically depends on the peptide’s inherent ability to form highly cohesive supramolecular structures.

Computational studies of plasma lipoprotein lipids

Plasma lipoproteins are macromolecular assemblies of proteins and lipids found in the blood. The lipid components of lipoproteins are amphipathic lipids such as phospholipids (PLs), and unesterified cholesterols (UCs) and hydrophobic lipids such as cholesteryl esters (CEs) and triglycerides (TGs). Since lipoproteins are soft matter supramolecular assemblies easily deformable by thermal fluctuations and they also exist in varying densities and protein/lipid components, a detailed understanding of their structure/function is experimentally difficult. Molecular dynamics (MD) simulation has emerged as a particularly promising way to explore the structure and dynamics of lipoproteins. The purpose of this review is to survey the current status of computational studies of the lipid components of the lipoproteins. Computational studies aim to explore three levels of complexity for the 3-dimensional structural dynamics of lipoproteins at various metabolic stages: (i) lipoprotein particles consist of protein with minimal lipid; (ii) lipoprotein particles consist of PL-rich discoidal bilayer-like lipid particles; (iii) mature circulating lipoprotein particles consist of CE-rich or TG-rich spheroidal lipid-droplet-like particles. Due to energy barriers involved in conversion between these species, other biomolecules also participate in lipoprotein biological assembly. For example: (i) lipid-poor apolipoprotein A-I (apoA-I) interacts with ATP-binding cassette transporter A1 (ABCA1) to produce nascent discoidal high density lipoprotein (dHDL) particles; (ii) lecithin-cholesterol acyltransferase (LCAT) mediates the conversion of UC to CE in dHDL, driving spheroidal HDL (sHDL) formation; (iii) transfer proteins, cholesterol ester transfer protein (CETP) and phospholipid transfer protein (PLTP), transfer both CE and TG and PL, respectively, between lipoprotein particles. Computational studies have the potential to explore different lipoprotein particles at each metabolic stage in atomistic detail. This review discusses the current status of computational methods including all-atom MD (AAMD), coarse-grain MD (CGMD), and MD-simulated annealing (MDSA) and their applications in lipoprotein structural dynamics and biological assemblies. Results from MD simulations are discussed and compared across studies in order to identify key findings, controversies, issues and future directions. This article is part of a Special Issue entitled: Biosimulations edited by Ilpo Vattulainen and Tomasz Róg.

Combined light and electron microscopy using diaminobenzidine photooxidation to monitor trafficking of lipids derived from lipoprotein particles

Diaminobenzidine (DAB) photooxidation is a method for conversion of fluorescent signals into electron-dense precipitates that are visible in the electron microscope. Recently, we have applied this method to analyze organelles involved in holo-high density lipoprotein (HDL) particle uptake at the ultrastructural level. In the present work we extended the spectrum of molecules visualized via photooxidation to monitor the uptake of HDL-derived lipids in HepG2 cells. By the combined light-electron microscopic method and with the aid of the DAB photooxidation technique, it became possible for the first time to visualize different intracellular pathways of lipoprotein particle-derived lipids and analyze the compartments involved at the ultrastructural level. HDL-Alexa 568 was used to visualize holo-HDL particle uptake. Reconstituted HDL particles containing the fluorescent cholesterol analogues Bodipy-cholesterol, Bodipy-cholesteryl oleate, or cholesteryl Bodipy-ester were used to visualize uptake of the HDL-associated sterol. In Bodipy-cholesteryl oleate and cholesteryl Bodipy-ester, the cholesterol moiety or the fatty acid moiety is fluorescently labeled, respectively; in contrast, Bodipy-cholesterol is an analogue of free cholesterol. The cellular compartments involved in their intracellular routes after uptake were analyzed in the fluorescence and electron microscope after DAB photooxidation. Bodipy-cholesterol was found to be localized in tubular endosomes and multivesicular bodies (MVBs), in the trans-Golgi network, and in stacked Golgi cisternae. In contrast, HepG2 cells incubated with HDL containing Bodipy-cholesteryl oleate or cholesteryl Bodipyester gave an uptake pattern comparable to that of holo-HDL particles, with MVBs being involved. Bodipy-cholesteryl oleate was also found in lysosomes. These results indicate that HDL-derived cholesterol and cholesteryl ester are transported by different intracellular pathways in HepG2 cells. Thus, the DAB photooxidation method enables the analysis of intracellular transport of lipoprotein particle-derived lipids at the light and at the ultrastructural level.

Atomistic simulations of phosphatidylcholines and cholesteryl esters in high-density lipoprotein-sized lipid droplet and trilayer: clues to cholesteryl ester transport and storage

Cholesteryl esters (CEs) are the water-insoluble transport and storage form of cholesterol. For both transport and storage, phospholipids and proteins embrace the CEs to form an amphipathic monolayer that surrounds the CEs. CEs are transported extracellularly in lipoproteins and are stored intracellularly as cytoplasmic lipid droplets. To clarify the molecular phenomena related to the above structures, we conducted atomistic molecular-dynamics simulations for a spherical, approximately high density lipoprotein sized lipid droplet comprised of palmitoyl-oleoyl-phosphatidylcholine (POPC) and cholesteryl oleate (CO) molecules. An additional simulation was conducted for a lamellar lipid trilayer consisting of the same lipid constituents. The density profiles showed that COs were located in the core of the spherical droplet. In trilayer simulations, CO molecules were also in the core and formed two denser strata. This is remarkable because the intra- and intermolecular behaviors of the COs were similar to previous findings from bulk COs in the fluid phase. In accordance with previous experimental studies, the solubility of COs in the POPC monolayers was found to be low. The orientation distribution of the sterol moiety with respect to the normal of the system was found to be broad, with mainly isotropic or slightly parallel orientations observed deep in the core of the lipid droplet or the trilayer, respectively. In both systems, the orientation of the sterol moiety changed to perpendicular with respect to the normal close to the phopsholipid monolayers. Of interest, within the POPC monolayers, the intramolecular conformation of the COs varied from the previously proposed horseshoe-like conformation to a more extended one. From a metabolic point of view, the observed solubilization of CEs into the phospholipid monolayers, and the conformation of CEs in the phospholipid monolayers are likely to be important regulatory factors of CE transport and hydrolysis.

Modular Structure of Solubilized Human Apolipoprotein B-100

Being intimately involved in cholesterol transport and lipid metabolism human low density lipoprotein (LDL) plays a prominent role in atherogenesis and cardiovascular diseases. The receptor-mediated cellular uptake of LDL is triggered by apolipoprotein B-100 (apoB-100), which represents the single protein moiety of LDL. Due to the size and hydrophobic nature of apoB-100, its structure is not well characterized. Here we present a low resolution structure of solubilized apoB-100. We have used small angle neutron scattering in combination with advanced shape reconstruction algorithms to generate a three-dimensional model of lipid-free apoB-100. Our model clearly reveals that apoB-100 is composed of distinct domains connected by flexible regions. The apoB-100 molecule adopts a curved shape with a central cavity. In comparison to LDL-associated apoB-100, the lipid-free protein is expanded, whereas according to spectroscopic data the secondary structure is widely preserved. Finally, the low resolution model was used as a template to reconstruct a hypothetical domain organization of apoB-100 on LDL, including information derived from a secondary structure prediction.

Lipid composition influences the shape of human low density lipoprotein in vitreous ice

Earlier cryo-electron microscopic studies have indicated that the normal low density lipoprotein (N-LDL) has a discoid shape when its core is in the liquid-crystalline state. In the present study, we investigated whether the shape of LDL depends on the physical state and/or the lipid composition of the lipoprotein core. Using a custom-built freezing device, we vitrified NLDL samples from either above or below the phase-transition temperature of the core (42 and 24 degrees C, respectively). Cryo-electron microscopy revealed no differences between these samples and indicated a discoid shape of the N-LDL particle. In contrast, TG-enriched LDL (T-LDL) did not have discoid features and appeared to be quasi-spherical in preparations that were vitrified from either 42 or 24 degrees C. These results suggest that the shape of NLDL is discoid, regardless of the physical state of its core, whereas T-LDL is more spherical. Aspects that may influence the shape of LDL are discussed.

Directed self-assembly of monodisperse phospholipid bilayer Nanodiscs with controlled size

Using a recently described self-assembly process (Bayburt, T. H.; Grinkova, Y. V.; Sligar, S. G. Nano Letters 2002, 2, 853-856), we prepared soluble monodisperse discoidal lipid/protein particles with controlled size and composition, termed Nanodiscs, in which the fragment of dipalmitoylphosphatidylcholine (DPPC) bilayer is surrounded by a helical protein belt. We have customized the size of these particles by changing the length of the amphipathic helical part of this belt, termed membrane scaffold protein (MSP). Herein we describe the design of extended and truncated MSPs, the optimization of self-assembly for each of these proteins, and the structure and composition of the resulting Nanodiscs. We show that the length of the protein helix surrounding the lipid part of a Nanodisc determines the particle diameter, as measured by HPLC and small-angle X-ray scattering (SAXS). Using different scaffold proteins, we obtained Nanodiscs with the average size from 9.5 to 12.8 nm with a very narrow size distribution (+/-3%). Functionalization of the N-terminus of the scaffold protein does not perturb their ability to form homogeneous discoidal structures. Detailed analysis of the solution scattering confirms the presence of a lipid bilayer of 5.5 nm thickness in Nanodiscs of different sizes. The results of this study provide an important structural characterization of self-assembled phospholipid bilayers and establish a framework for the design of soluble amphiphilic nanoparticles of controlled size.

Structure of Triglyceride-Rich Human Low-Density Lipoproteins According to Cryoelectron Microscopy

Low-density lipoprotein (LDL) particles from normolipidemic individuals contain a cholesteryl ester-rich core that undergoes a thermal transition from a liquid crystalline to an isotropic liquid phase between 20 and 35 degrees C. LDL from hypertriglyceridemic patients or prepared in vitro by the exchange of very low-density lipoprotein for LDL cholesteryl esters is triglyceride-rich, does not have a thermal transition above 0 degrees C, and exhibits impaired binding to the LDL receptor on normal human skin fibroblasts. Cryoelectron microscopy of LDL quick-frozen from 10 (core-frozen) and 40 degrees C (core-melted) revealed ellipsoidal particles with internal striations and round particles devoid of striations, respectively. Cryoelectron microscopy of triglyceride-rich LDL prepared in vitro revealed particles similar to the core-melted normolipidemic LDL, i.e., round particles without striations. These data suggest that the LDL core in the liquid crystalline phase is characterized by the appearance of striations, whereas LDL with a core that is an isotropic liquid lacks striations. It is suggested that freezing the LDL core into a liquid crystalline phase imposes structural constraints that force LDL from a sphere without partitions to an ellipsoid with partitions. We further suggest that the striation-defined lamellae are a structural feature of a liquid crystalline neutral lipid core that is a determinant of normal binding to the LDL receptor and that conversion of the neutral lipid core of LDL to the isotropic liquid phase via an increase in the temperature or via the addition of triglyceride partially ablates the receptor binding determinants on the LDL surface. This effect is likely achieved through changes in the conformation of apo-B-100. These data suggest that the physical state of the LDL core determines particle shape, surface structure, and metabolic fate.

Structural characterisation of nucleoside loaded low density lipoprotein as a main criterion for the applicability as drug delivery system

The potential role of human low density lipoprotein (LDL) particles as delivery system for lipophilic, cytotoxic drugs critically depends on their structural integrity. In the present study, LDL particles were loaded with antineoplastic prodrugs, i.e. monooleoyl (MOT)- and dioleoyl (DOT)- thymidine esters by different techniques. Using the reconstitution method MOT shows the highest incorporation efficiency with over 80% of the initial drug associated with LDL. In contrast, for the more lipophilic DOT the incorporation efficiency for reconstitution, dry film as well as dimethylsulfoxide method was extremely low. Structural changes upon drug loading were monitored by differential scanning calorimetry (DSC) and small angle X-ray scattering (SAXS). The results show that the influence of MOT and DOT is predominantly confined to the surface monolayer of LDL seen as a destabilisation of the protein moiety and a small increase in particle diameter. The core lipid region of the LDL-drug complexes remains essentially unaffected, as verified by undisturbed core lipid arrangement and core lipid melting behaviour.

NMR studies of lipoprotein structure

Early NMR structural studies of serum lipoproteins were based on (1)H, (13)C, (31)P, and (2)H studies of lipid components. From the early studies information on composition, lipid chain dynamics and order parameters, and monolayer organization resulted. More recently, selective or complete isotopic labeling techniques, combined with multidimensional NMR spectroscopy, have resulted in structural information of apoprotein fragments. Finally, use of heteronuclear three- and four-dimensional experiments have yielded solution structures and protein-lipid interactions of intact apolipoproteins C-I, C-II, and A-I.

Three-dimensional structure of low density lipoproteins by electron cryomicroscopy

Human low density lipoproteins (LDL) are the major cholesterol carriers in the blood. Elevated concentration of LDL is a major risk factor for atherosclerotic disease. Purified LDL particles appear heterogeneous in images obtained with a 400-kV electron cryomicroscope. Using multivariate statistical and cluster analyses, an ensemble of randomly oriented particle images has been subdivided into homogeneous subpopulations, and the largest subset was used for three-dimensional reconstruction. In contrast to the general belief that below the lipid phase-transition temperature (30 degrees C) LDL are quasi-spherical microemulsion particles with a radially layered core-shell organization, our three-dimensional map shows that LDL have a well-defined and stable organization. Particles consist of a higher-density outer shell and lower-density inner lamellae-like layers that divide the core into compartments. The outer shell consists of apolipoprotein B-100, phospholipids, and some free cholesterol.

Modification of the lipid-protein interaction in human low-density lipoprotein destabilizes ApoB-100 and decreases oxidizability

The interactions of the lipid and protein moiety of human low-density lipoprotein (LDL) and their influence on the oxidation behavior of LDL were modified using an amphipathic peptide, melittin, as a probe. The interaction of melittin with the LDL phospholipid surface resulted in a destabilization of apolipoprotein B-100 (apoB-100) as monitored by differential scanning calorimetry, while the characteristics of lipid core melting remained nearly unchanged. Binding of melittin caused a restriction of lipid chain mobility near the glycerol backbone, but not in the middle or near the methyl terminus of the fatty acyl chains as observed by electron paramagnetic resonance. Also, upon melittin addition, the level of copper binding to apoB-100 and the oxidizability of LDL by Cu2+ ions were greatly reduced, as indicated by abolished tryptophan fluorescence quenching upon Cu2+ binding and, during oxidation, prolongation of the lag phase of oxidation, attenuated consumption of alpha-tocopherol, and a lowered maximal rate of conjugated diene formation. This reduction of oxidizability could not be reversed by increasing the Cu2+ concentration. It is deduced that interaction of Cu2+ and alpha-tocopherol is required for reductive activation of the metal. It can be abolished by interfering with the interactions between apoB-100 and the lipid moiety of LDL which modifies the conformation of LDL and, as a consequence, hinders copper binding to apoB-100.

Microphase separation in low density lipoproteins. Evidence for a fluid triglyceride core below the lipid melting transition

The structural organization of the neutral lipid core in human low density lipoproteins (LDL) was investigated in physicochemically defined, distinct human LDL subspecies in the density range of 1. 0244-1.0435 g/ml by evaluation of the core lipid transition temperature, chemical composition, and the behavior of spin-labeled core lipids. Calorimetric studies were performed on more than 60 LDL preparations, and the transition temperature, which varied between 19 and 32 degreesC, was correlated to the chemical composition and revealed a discontinuity at a critical cholesteryl ester to triglyceride ratio of approximately 7:1. For electron spin resonance studies, several LDL preparations were probed with spin-labeled cholesteryl esters and triglycerides, respectively. In LDL with a high triglyceride content, both labels exhibited similar mobility behavior. In contrast, in LDL with only small concentrations of triglycerides, the behavior of labeled cholesteryl esters and labeled triglycerides differed distinctly. The cholesteryl esters were strongly immobilized below the transition temperature, whereas the triglycerides remained fluid throughout the measured temperatures. These results suggest that the critical cholesteryl ester to triglyceride mass ratio of 7:1 corresponds to two concentric compartments with a radial ratio of 2:1, where the liquid triglycerides occupy the core, and the cholesteryl esters form the frozen shell. At higher triglyceride contents, the triglyceride molecules insert into the cholesteryl ester shell and depress the peak transition temperature of the LDL core, whereas at lower triglyceride contents, excess cholesteryl esters are dissolved in the core.

Thermal stability of apolipoprotein B100 in low-density lipoprotein is disrupted at early stages of oxidation while neutral lipid core organization is conserved

The time course of the unfolding characteristics of the protein moiety and of the thermotropic behavior of the core-located apolar lipids of highly homogeneous low-density lipoprotein (LDL) subspecies (d 1.030-1.040 g/mL) have been evaluated during transition metal- and azo radical-induced oxidation using differential scanning calorimetry. Apolipoprotein B100 (apo-B100) structure was highly sensitive to oxidative modification; indeed, a significant loss of thermal stability was observed at initial stages irrespective of whether oxidation was mediated by site-specific binding of copper ions or by free radicals generated during decomposition of azo compounds. Subsequently, thermal protein integrity was destroyed, as a result of potentially irreversible protein unfolding, cross-linking reactions, and aggregation. Our results suggest that even minimal oxidative modification of apo-B100 has a major impact on the stability of this large monomeric protein. By contrast, the core lipids, which consist primarily of cholesteryl esters and triglycerides and play a determinant role in the thermal transition occurring near physiological temperature, preserved features of an ordered arrangement even during propagation of lipid peroxidation.

Molecular acoustic as a new tool for the study of biophysical properties of lipoproteins

The method of measurement of velocity and absorption of ultrasound at a fixed frequency (7.2 MHz) and measurement of density were used to study the physical properties of high- (HDL3) and low- (LDL) density lipoproteins. We found substantial changes in velocity number [u] and absorption number [alpha lambda] on temperature, which reflect structural changes in the hydrophobic core of LDL at the thermotropic-phase transition. The absorption number revealed broad changes in temperature for both classes of lipoproteins (LP). The density of LP also depends on temperature but in considerably less degree than the acoustic parameters. The values of acoustic parameters were determined, showing that LDL and HDL3 greatly differ with respect to adiabatic compressibility.

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.

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.

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

Low-density lipoproteins (LDL) in plasma are constructed from a single molecule of apolipoprotein B-100 (M(r) 512000) in association with lipid (approximate M(r) 2-3 x 10(6)). The gross structure was studied using an updated pulsed-neutron camera LOQ with an area detector to establish the basis for the interpretation of structural changes seen during dynamic studies of LDL oxidation. Neutron-scattering data for LDL in 100% 2H2O buffers emphasize their external appearance. Guinier analysis on a continuous-flux neutron camera D17 revealed pronounced concentration-dependences in the radius of gyration, RG, and the intensity of forward scattering, I(0) (equivalent to the M(r) of LDL) between 0.5 and 11 mg of LDL protein/ml. LDL preparations from different donors gave different RG values. When extrapolated to zero concentration, RG values ranged between 8.3 and 10.6 nm and were linearly correlated with M(r), which is consistent with a spherical structure. The distance-distribution function P(r) in real space showed a single maximum at 9.1-10.9 nm, which is just under half the observed maximum dimension of 23.1 +/- 1.2 nm expected for a spherical structure. The neutron radial-density function p(r) exhibited a plateau of high and featureless density at the centre of LDL. LDL can be modelled by a polydisperse assembly of spheres with two internal densities and a mean radius close to 10.0 nm in a normal distribution of radii with a standard deviation of 2.0 nm. The data are consistent with recent electron-microscopy and ultracentrifugation data.(ABSTRACT TRUNCATED AT 250 WORDS)

Cryoelectron microscopy of low density lipoprotein in vitreous ice

In this report, images of low density lipoprotein (LDL) in vitreous ice at approximately 30 A resolution are presented. These images show that LDL is a quasi-spherical particle, approximately 220-240 A in diameter, with a region of low density (lipid) surrounded by a ring (in projection) of high density believed to represent apolipoprotein B-100. This ring is seen to be composed of four or five (depending on view) large regions of high density material that may represent protein superdomains. Analysis of LDL images obtained at slightly higher magnification reveals that areas of somewhat lower density connect these regions, in some cases crossing the projectional interiors of the LDL particles. Preliminary image analysis of LDL covalently labeled at Cys3734 and Cys4190 with 1.4-nm Nanogold clusters demonstrates that this methodology will provide an important site-specific marker in studies designed to map the organization of apoB at the surface of LDL.

Re-evaluation of the structure of low density lipoproteins

Low density lipoprotein (LDL) is an established atherogenic factor. Much effort has therefore been devoted to elucidation of its structure, yielding the generally accepted model according to which the neutral lipids (cholesterol ester and triglycerides) form a lipid core emulsified by phospholipids, cholesterol and the amphipathic Apolipoprotein B. Yet, the detailed structure of LDL is not clear. The present work was carried out with the aim of re-evaluating the LDL structure using the minimal number of assumptions: in view of the previously noted surface deficit (lack of sufficient PL and cholesterol to cover the surface of the lipid core) we have assumed that polar head groups are not covered by apo B. Other than that, we have 'allowed' Apo B to penetrate into the PL monolayers and the lipidic core and to pertrude into the solution (be elevated above the PL head group level). We have also 'allowed' neutral lipid penetration into the monolayer and variation of the thickness of the phospholipid monolayers within reasonable boundaries. Based on the established values of relevant constants (molecular weights and volumes, densities and surface areas) we have computed the radius of the particle, the penetration of Apo B into lipidic milieus and the fraction of the surface area covered by Apo B as functions of the LDL composition, the monolayer thickness and the 'elevation' of Apo B above this monolayer. These computations show that at least 40% of the LDL surface must be covered by protein and that the protein penetrates, on the average, only about a half of the PL monolayer. Thus it is not very likely to penetrate into the lipid core. These general features are preserved in the smaller LDL particles of hypertriglyceridemic patients. Assuming that no PL head group is covered by Apo B, the previously described immobilization of 20% of the phospholipids is likely to result from the interaction of Apo B with neighboring PL. According to our computations this can be regarded consistent with the previously proposed arrangement of the apo B as a '3-4 domain structure' or a long string configuration but inconsistent with 'one domain' or 'twenty domain' structures.

Structure of Colorado potato beetle lipophorin: differential scanning calorimetric and small-angle X-ray scattering studies

The structure of lipophorin, isolated from hemolymph of the Colorado potato beetle, was investigated by differential scanning calorimetry (DSC) and small-angle X-ray scattering. The DSC heating curves of intact lipophorin showed endothermic peaks that were similar to peaks obtained with the hydrocarbon fraction isolated from this lipophorin. The observed peaks correlated with the transition of the hydrocarbons from an ordered into a more disordered state. Changes in structure of the lipophorin particles with increasing temperature were also observed by small-angle X-ray scattering studies. The structural organization of lipophorin was further elucidated by simulation analysis, using a three-layered symmetrical sphere as a model. These studies revealed that lipophorin from the Colorado potato beetle is a sphere with a maximum diameter of 175 A. The sphere is composed of three radially symmetrical layers of different electron densities. The outer layer (37.5-39.5 A in thickness) is composed of phospholipid, apolipophorin I, and part of apolipophorin II. The middle layer (5-10 A) contains diacylglycerol, the rest of apolipophorin II, and probably beta-carotene. The core of the particle (40-45 A) only contains hydrocarbons. This structure differs from another model, previously proposed for cockroach and locust lipophorins [Katagiri, C., Sato, M., & Tanaka N. (1987) J. Biol. Chem. 262, 15857-15861], in the small size of the middle layer. The volume of the middle layer correlated well with the low diacylglycerol content of this lipophorin.

Structure of human low-density lipoprotein subfractions, determined by X-ray small-angle scattering

The structure of low-density lipoprotein (LDL) particles from three different density ranges (LDL-1: d = 1.006-1.031 g/ml; LDL-3: d = 1.034-1.037 g/ml; LDL-6: d = 1.044-1.063 g/ml) was determined by X-ray small-angle scattering. By using a theoretical particle model, which accounted for the polydispersity of the samples, we were able to obtain fits of the scattering intensity that were inside the noise interval of the measured intensity. The assumption of deviations from radial symmetry is not supported by our data. This implies a spread-out conformation of the apolipoprotein B (apoB) molecule, which appears to be localized in the outer surface shell. A globular structure is not consistent with our data. Furthermore, different models exist concerning the structure of the cholesterol ester core below the phase transition temperature. The electron density data suggest an arrangement in which the steroid moieties are localized at average radii of 3.2 and 6.4 nm. Model calculations show that packing problems can only be avoided if approximately half of the acyl chains of each shell are pointing towards the center of the particle, the other half towards the surface. This arrangement of the acyl chains has never been proposed before. The LDL particles of different density classes differ mainly with respect to the size of the core but also with respect to the width of the surface shells. Model calculations show that the size of different LDL particles can be accurately predicted from the compositional data.

Structural changes in oxidised low-density lipoproteins and of the effect of the anti-atherosclerotic drug probucol observed by synchrotron X-ray and neutron solution scattering

The atherosclerotic properties of low-density lipoproteins (LDL) are thought to be strongly enhanced by oxidation. The lipid-lowering drug probucol reduces the susceptibility of LDL to oxidation. Synchrotron X-ray and high-flux neutron solution scattering curves were used to characterise the structural properties of human LDL, before and after modification by oxidation with Cu2+ and the addition of probucol, in order to evaluate these techniques. Analyses based on Guinier plots, simple two-shell spherical modelling, and the use of cubic splines and indirect transformation show that a 20-h incubation with Cu2+ ions (but not 6 h) causes some of the LDL to associate to form larger aggregated particles. Gel electrophoresis on Cu2+ -oxidised LDL shows a concomitant degradation of the apolipoprotein B-100 as well as the formation of high molecular mass forms. These experiments indicate that the apoprotein B-100 structure has been significantly disrupted by oxidation. The addition of probucol to LDL causes an increase in the polydispersity of LDL, as evidenced by small changes in the Guinier curves and some weakening of the minima in the X-ray scattering curves. No changes in the quasispherical shape of LDL are observed and gel electrophoresis indicates no changes. It is possible that probucol may exert its effect by increasing the range of sizes of LDL and that the lipid-lowering effect of probucol in vivo might be caused by the preferential catabolism of the higher molecular mass forms of LDL thus created.

Effect of fish oils on rat plasma lipoproteins

Plasma high (HDL) and low (LDL) density lipoproteins were isolated from rats fed a diet supplemented with either fish (menhaden) oil or hydrogenated coconut oil (control). Fluorescence polarization and electron spin resonance of labelled fatty acid probe molecules incorporated into the outer amphiphilic monolayer of HDL indicate molecular motion is restricted in the upper portion of the acyl chain following the fish oil diet, which is consistent with a 'hook' conformation predicted by preliminary molecular model calculations for n-3 fatty acids (the predominant component of fish oil). Negligible dependence on diet was observed in LDL. Thus, a HDL specific effect of dietary fish oil on molecular fluidity and order in the outer monolayer of rat lipoproteins is suggested.

Small-angle x-ray scattering of high- and low-affinity heparin

Solution characterization of heparin with high affinity (HA) and low affinity (LA) for antithrombin III was performed using the methods of small-angle x-ray scattering (SAXS), viscometry, and aqueous gel permeation chromatography (GPC). SAXS provided various topological parameters including the radius of gyration ([S2]1/2), radius of gyration of cross-section ([S2]q1/2), persistence length (a*), contour length (L), and mass parameters, e.g., overall molecular mass (Mr), and mass per unit length (Mq). The molecular weights of HA and LA pig mucosal heparins were found to be 14,900 and 11,500 and the respective radii of gyration were 40.1 and 33.6 A. The persistence lengths of HA and LA were 21.3 and 20.3 A, respectively. These parameters were compared to SAXS data of heparin [S. S. Stivala, M. Herbst, O. Kratky, and I. Pilz (1968) Arch. Biochem. Biophys. 127, 795-802] fractionated according to molecular weight only. It was found that the various experimental values of this heparin lie somewhere in between those of HA and LA heparins. It appears that there are no appreciable differences in the physico-chemical properties, including conformation, among the heparins in H2O at 25 degrees C.

Reassembly of low-density lipoproteins

The methodologies described here for the selective and sequential reassembly of model LDL particles, although in many instances still in the developmental stages, will undoubtedly provide a basis on which further advances in LDL reassembly will be made. Reassembled LDL complexes of defined lipids and apoB provide well-defined model systems in which to study the molecular interactions and structural organization of LDL, including the lipid-lipid interactions in the particle core, the lipid-lipid and lipid-protein interactions which determine the surface organization and protein conformation, and the interactions between the core and surface components. These reassembled LDL complexes should serve as important models to study the delivery of isotopically labeled lipids with differing physical properties to cells in order to investigate the metabolic complexity of intracellular LDL catabolism and its relationship to positive cholesterol balance and atherogenesis.

Comparative study of the lipid dynamics in the surface layer of porcine and human high density lipoprotein subclasses by spin labeling

In order to obtain information on the determinants of the lipid dynamics in the high density lipoproteins (HDL), we have compared the structural properties of human HDL subclasses with porcine HDL artificially subdivided into density subfractions corresponding to those of human HDL. Four different positional isomers of spin labeled fatty acids and spin labeled androstanol experienced more restricted motion in porcine HDL than in the human HDL2 and HDL3 subclasses. The differences in the spin label motion could not be accounted for on the basis of the differences in the chemical composition of the lipoproteins examined. They are, however, most probably due to the specific properties of the interactions between lipids and proteins that differ among the lipoproteins.

The structural stability of low-density lipoprotein. A kinetic X-ray scattering study of its interaction with arterial proteoglycans

The structural stability of human plasma low-density lipoproteins (LDL) has been studied kinetically by means of dynamic X-ray scattering techniques. At 37 degrees C, rapid deterioration of the lipoproteins present in the insoluble complex which is formed on mixing solutions of LDL and human arterial proteoglycans is observed. This is evidenced by the progressive blurring of the X-ray scattering spectra. At 4 degrees C the X-ray patterns from the insoluble complex show a single reflection centered at 34 A-1, which is characteristic of pure cholesteryl esters organized in a smectic phase. This behaviour contrasted with the high stability exhibited by free LDL at 37 degrees C. Our results show that, in the form of insoluble complex, the lipoprotein molecules are rapidly disrupted and that the neutral lipids segregated with formation of a lipid phase. This suggests that LDL particles become destabilized by their interaction with arterial proteoglycans.

Structure of the cholesteryl ester core of human plasma low density lipoproteins: selective deuteration and neutron small-angle scattering

The structural arrangement of cholesteryl esters in human plasma low density lipoproteins (LDL) has been studied by selective deuteration and neutron small-angle scattering. LDL were labeled by in vitro exchange with two different kinds of deuterated cholesteryl esters, one labeled in the fatty acyl chain (cholesteryl myristate-d27) and the other in the branched side chain of cholesterol (cholesteryl-25,26,27-d7 oleate). Neutron scattering data from deuterated and protonated LDL were compared to identify the locations of the fatty acyl and cholesterol side chain moieties. Below the thermotropic transition, radii of gyration of 60 A and 70 A were obtained for these two domains, respectively, indicating that the cholesteryl nuclei are situated more distantly from the center than the fatty acyl chains. At 37 degrees C, above the thermotropic transition of the cholesteryl esters in LDL, both parts have similar radii of gyration of approximately 56 A. This information is used in a discussion of possible structural models for the apolar lipid core of LDL.

Structural studies of plasma lipoproteins

These studies with neutron small-angle scattering were designed to probe the location of the apoprotein of HDL on recombinant particles formed with phospholipids. The use of isotopic substitution with deuterium in the headgroup of the phospholipid allows the positions of the molecular components to be probed in more detail. These initial experiments point to the direction future structural studies must take. Reconstitution techniques together with compositional modification of native lipoproteins must be coupled with a combination of high accuracy x-ray and neutron scattering studies, the latter employing isotopic substitution with deuterium to highlight specific areas of the structure. This approach will undoubtedly provide a higher resolution picture of the molecular organization of plasma lipoproteins.

A fluorescence study of apolipoprotein localization in relation to lipids in serum low density lipoproteins

Fluorescence energy transfer studies were carried out on low density lipoproteins (LDL) containing pyrene, in order to investigate their structure. The results indicate that almost all of the LDL tryptophan residues are located in the same surroundings near the surface of the particle and are immersed in the lipid phase 10-20 A below the lipid/water interface. The data do not support a model of protein spikes protruding from the particle surface. Such spikes have been observed in LDL preparations only after long-term storage.

Neonatal umbilical cord blood lipoproteins. Isolation and characterization of intermediate density and low density lipoproteins

Intermediate density lipoproteins (IDL) (d = 1.006 to 1.019 g/ml) and low density lipoproteins (LDL) (d = 1.019 to 1.063 g/ml) were isolated from human umbilical cord blood plasma by sequential ultracentrifugation. The concentration, chemical and apolipoprotein composition, size and size distribution of the neonatal IDL and LDL for both sexes were determined. The IDL and LDL from the neonates showed no sex-related differences in composition or concentration. The IDL and LDL were lower in concentration and differed in composition of concentration. The IDL and LDL were lower in concentration and differed in composition with regard to each other and with regard to the comparable adult fractions. The apolipoprotein (apo) composition showed only the high molecular weight form of apo B present in the IDL, while the LDL showed the presence of two lower molecular weight forms of apo B in addition to the high molecular weight form, along with appreciable amounts of apo E and apo A-l. The size distribution of the neonatal IDL and LDL showed a constant pattern, with peaks at approximately 300 A for IDL and 257 and 244 A for neonatal LDL. The alterations in composition, size and size distribution, as well as the lower concentrations present in neonate, point to differences between the neonate and the adult in the metabolism of lipoproteins with a density of 1.006 to 1.063 g/ml.