Three-dimensional structure of low density lipoproteins by electron cryomicroscopy

Iron-catalyzed oxidation of Trp residues in low-density lipoprotein

The mechanisms of oxidation of low-density lipoproteins (LDLs) are not well defined, but epidemiological and experimental studies suggest that iron-catalyzed processes may contribute to atherogenesis. The aim of this study was to test the hypothesis that iron-catalyzed oxidations of LDLs in vitro produce diagnostic biomarkers of oxidation of the apolipoprotein that could be applied to studies in vivo. LDLs were oxidized in the presence of Fe2+, EDTA, and ascorbic acid for up to 40 h. Following delipidation and trypsin digestion, the peptides were separated by HPLC, with four peaks detected at 365 nm, whereas none were observed in peptides from unoxidized LDLs. The peptides were identified by MALDI-QTOF mass spectrometry as IVQILP(W+4) EQNEQVK, IYSL(W+4)EHSTK, FEGLQE(W+4)EGK, and YH(W+4)EHTGLTLR, with (W+4) rather than the W residues of the unoxidized protein. The mass gains (+4 increase in m/z in tryptophan, W) and absorbance at 365 nm indicate kynurenines, which were trypsin-releasable peptides that are on the surface of LDL particles. All four peptides thus characterized share the sequence of WE. The preferential oxidation of W residues in WE sequences suggest contributions from the C-proximate glutamate residues in chelation of the iron species, thereby influencing site selectivities of oxidation. These kynurenine-containing peptides might serve as biomarkers of iron-mediated oxidations in vivo.

Three-dimensional cryoEM reconstruction of native LDL particles to 16Å resolution at physiological body temperature

Background

Low-density lipoprotein (LDL) particles, the major carriers of cholesterol in the human circulation, have a key role in cholesterol physiology and in the development of atherosclerosis. The most prominent structural components in LDL are the core-forming cholesteryl esters (CE) and the particle-encircling single copy of a huge, non-exchangeable protein, the apolipoprotein B-100 (apoB-100). The shape of native LDL particles and the conformation of native apoB-100 on the particles remain incompletely characterized at the physiological human body temperature (37°C).

Methodology/Principal Findings

To study native LDL particles, we applied cryo-electron microscopy to calculate 3D reconstructions of LDL particles in their hydrated state. Images of the particles vitrified at 6°C and 37°C resulted in reconstructions at ∼16 Å resolution at both temperatures. 3D variance map analysis revealed rigid and flexible domains of lipids and apoB-100 at both temperatures. The reconstructions showed less variability at 6°C than at 37°C, which reflected increased order of the core CE molecules, rather than decreased mobility of the apoB-100. Compact molecular packing of the core and order in a lipid-binding domain of apoB-100 were observed at 6°C, but not at 37°C. At 37°C we were able to highlight features in the LDL particles that are not clearly separable in 3D maps at 6°C. Segmentation of apoB-100 density, fitting of lipovitellin X-ray structure, and antibody mapping, jointly revealed the approximate locations of the individual domains of apoB-100 on the surface of native LDL particles.

Conclusions/Significance

Our study provides molecular background for further understanding of the link between structure and function of native LDL particles at physiological body temperature.

Immuno-electron cryo-microscopy imaging reveals a looped topology of apoB at the surface of human LDL

A single copy of apoB is the sole protein component of human LDL. ApoB is crucial for LDL particle stabilization and is the ligand for LDL receptor, through which cholesterol is delivered to cells. Dysregulation of the pathways of LDL metabolism is well documented in the pathophysiology of atherosclerosis. However, an understanding of the structure of LDL and apoB underlying these biological processes remains limited. In this study, we derived a 22 Å-resolution three-dimensional (3D) density map of LDL using cryo-electron microscopy and image reconstruction, which showed a backbone of high-density regions that encircle the LDL particle. Additional high-density belts complemented this backbone high density to enclose the edge of the LDL particle. Image reconstructions of monoclonal antibody-labeled LDL located six epitopes in five putative domains of apoB in 3D. Epitopes in the LDL receptor binding domain were located on one side of the LDL particle, and epitopes in the N-terminal and C-terminal domains of apoB were in close proximity at the front side of the particle. Such image information revealed a looped topology of apoB on the LDL surface and demonstrated the active role of apoB in maintaining the shape of the LDL particle.

Enhancing the contrast of ApoB to locate the surface components in the 3D density map of human LDL

A 26 Å resolution map of the structure of human low-density lipoprotein (LDL) was obtained from electron cryomicroscopy and single-particle image reconstruction. The structure showed a discoidal-shaped LDL particle with high-density regions mainly distributed at the edge of the particle and low-density regions at the flat surface that covers the core region. To determine the chemical components that correspond to these density regions and to delineate the distribution of protein and phospholipid located at the particle surface at the resolution of the map, we used Mono-Sulfo-NHS-Undecagold labeling to increase preferentially the contrast of the apolipoprotein B component on the LDL particle. In the three-dimensional map from the image reconstruction of the undecagold-labeled LDL particles, the high-density region from the undecagold label was distributed mainly at the edge of the particle, and lower density regions were found at the flat surfaces that cover the neutral lipid core. This suggests that apolipoprotein B mainly encircles LDL at the edge of the particle and the phospholipid monolayers are located at the flat surfaces, which are parallel to the cholesterol ester layers in the core and may interact with the core lipid layers through the acyl chains.

Human LDL core cholesterol ester packing: three-dimensional image reconstruction and SAXS simulation studies

Human LDL undergoes a reversible thermal order-disorder phase transition associated with the cholesterol ester packing in the lipid core. Structural changes associated with this phase transition have been shown to affect the resistance of LDL to oxidation in vitro studies. Previous electron cryo-microscopy studies have provided image evidence that the cholesterol ester is packed in three flat layers in the core at temperatures below the phase transition. To study changes in lipid packing, overall structure and particle morphology in three dimensions (3D) subsequent to the phase transition, we cryo-preserved human LDL at a temperature above phase transition (53°C) and examined the sample by electron microscopy and image reconstruction. The LDL frozen from 53°C adopted a different morphology. The central density layer was disrupted and the outer two layers formed a "disrupted shell"-shaped density, located concentrically underneath the surface density of the LDL particle. Simulation of the small angle X-ray scattering curves and comparison with published data suggested that this disrupted shell organization represents an intermediate state in the transition from isotropic to layered packing of the lipid. Thus, the results revealed, with 3D images, the lipid packing in the dynamic process of the LDL lipid-core phase transition.

Morphology and structure of lipoproteins revealed by an optimized negative-staining protocol of electron microscopy

Plasma lipoprotein levels are predictors of risk for coronary artery disease. Lipoprotein structure-function relationships provide important clues that help identify the role of lipoproteins in cardiovascular disease. The compositional and conformational heterogeneity of lipoproteins are major barriers to the identification of their structures, as discovered using traditional approaches. Although electron microscopy (EM) is an alternative approach, conventional negative staining (NS) produces rouleau artifacts. In a previous study of apolipoprotein (apo)E4-containing reconstituted HDL (rHDL) particles, we optimized the NS method in a way that eliminated rouleaux. Here we report that phosphotungstic acid at high buffer salt concentrations plays a key role in rouleau formation. We also validate our protocol for analyzing the major plasma lipoprotein classes HDL, LDL, IDL, and VLDL, as well as homogeneously prepared apoA-I-containing rHDL. High-contrast EM images revealed morphology and detailed structures of lipoproteins, especially apoA-I-containing rHDL, that are amenable to three-dimensional reconstruction by single-particle analysis and electron tomography.

Automated multi-model reconstruction from single-particle electron microscopy data

Biological macromolecules can adopt multiple conformational and compositional states due to structural flexibility and alternative subunit assemblies. This structural heterogeneity poses a major challenge in the study of macromolecular structure using single-particle electron microscopy. We propose a fully automated, unsupervised method for the three-dimensional reconstruction of multiple structural models from heterogeneous data. As a starting reference, our method employs an initial structure that does not account for any heterogeneity. Then, a multi-stage clustering is used to create multiple models representative of the heterogeneity within the sample. The multi-stage clustering combines an existing approach based on Multivariate Statistical Analysis to perform clustering within individual Euler angles, and a newly developed approach to sort out class averages from individual Euler angles into homogeneous groups. Structural models are computed from individual clusters. The whole data classification is further refined using an iterative multi-model projection-matching approach. We tested our method on one synthetic and three distinct experimental datasets. The tests include the cases where a macromolecular complex exhibits structural flexibility and cases where a molecule is found in ligand-bound and unbound states. We propose the use of our approach as an efficient way to reconstruct distinct multiple models from heterogeneous data.

Model of human low-density lipoprotein and bound receptor based on cryoEM

Human plasma low-density lipoproteins (LDL), a risk factor for cardiovascular disease, transfer cholesterol from plasma to liver cells via the LDL receptor (LDLr). Here, we report the structures of LDL and its complex with the LDL receptor extracellular domain (LDL.LDLr) at extracellular pH determined by cryoEM. Difference imaging between LDL.LDLr and LDL localizes the site of LDLr bound to its ligand. The structural features revealed from the cryoEM map lead to a juxtaposed stacking model of cholesteryl esters (CEs). High density in the outer shell identifies protein-rich regions that can be accounted for by a single apolipoprotein (apo B-100, 500 kDa) leading to a model for the distribution of its alpha-helix and beta-sheet rich domains across the surface. The structural relationship between the apo B-100 and CEs appears to dictate the structural stability and function of normal LDL.

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.

Disruption of human plasma high-density lipoproteins by streptococcal serum opacity factor requires labile apolipoprotein A-I

Human plasma high-density lipoproteins (HDL), the primary vehicle for reverse cholesterol transport, are the target of serum opacity factor (SOF), a virulence determinant of Streptococcus pyogenes that turns serum opaque. HDL comprise a core of neutral lipidscholesteryl esters and some triglyceridesurrounded by a surface monolayer of cholesterol, phospholipids, and specialized proteins [apolipoproteins (apos) A-I and A-II]. A HDL is an unstable particle residing in a kinetic trap from which it can escape via chaotropic, detergent, or thermal perturbation. Recombinant (r) SOF catalyzes the transfer of nearly all neutral lipids of approximately 100,000 HDL particles (D approximately 8.5 nm) into a single, large cholesteryl ester-rich microemulsion (CERM; D > 100 nm), leaving a new HDL-like particle [neo HDL (D approximately 5.8 nm)] while releasing lipid-free (LF) apo A-I. CERM formation and apo A-I release have similar kinetics, suggesting parallel or rapid consecutive steps. By using complementary physicochemical methods, we have refined the mechanistic model for HDL opacification. According to size exclusion chromatography, a HDL containing nonlabile apo A-I resists rSOF-mediated opacification. On the basis of kinetic cryo-electron microscopy, rSOF (10 nM) catalyzes the conversion of HDL (4 microM) to neo HDL via a stepwise mechanism in which intermediate-sized particles are seen. Kinetic turbidimetry revealed opacification as a rising exponential reaction with a rate constant k of (4.400 +/- 0.004) x 10(-2) min(-1). Analysis of the kinetic data using transition state theory gave an enthalpy (DeltaH()), entropy (DeltaS(++)), and free energy (DeltaG()) of activation of 73.9 kJ/mol, -66.87 J/K, and 94.6 kJ/mol, respectively. The free energy of activation for opacification is nearly identical to that for the displacement of apo A-I from HDL by guanidine hydrochloride. We conclude that apo A-I lability is required for HDL opacification, LF apo A-I desorption is the rate-limiting step, and nearly all HDL particles contain at least one labile copy of apo A-I.

Circulatory lipid transport: lipoprotein assembly and function from an evolutionary perspective

Circulatory transport of neutral lipids (fat) in animals relies on members of the large lipid transfer protein (LLTP) superfamily, including mammalian apolipoprotein B (apoB) and insect apolipophorin II/I (apoLp-II/I). Latter proteins, which constitute the structural basis for the assembly of various lipoproteins, acquire lipids through microsomal triglyceride transfer protein (MTP)--another LLTP family member--and bind them by means of amphipathic structures. Comparative research reveals that LLTPs have evolved from the earliest animals and additionally highlights the structural and functional adaptations in these lipid carriers. For instance, in contrast to mammalian apoB, the insect apoB homologue, apoLp-II/I, is post-translationally cleaved by a furin, resulting in their appearance of two non-exchangeable apolipoproteins in the insect low-density lipoprotein (LDL) homologue, high-density lipophorin (HDLp). An important difference between mammalian and insect lipoproteins relates to the mechanism of lipid delivery. Whereas in mammals, endocytic uptake of lipoprotein particles, mediated via members of the LDL receptor (LDLR) family, results in their degradation in lysosomes, the insect HDLp was shown to act as a reusable lipid shuttle which is capable of reloading lipid. Although the recent identification of a lipophorin receptor (LpR), a homologue of LDLR, reveals that endocytic uptake of HDLp may constitute an additional mechanism of lipid delivery, the endocytosed lipoprotein appears to be recycled in a transferrin-like manner. Binding studies indicate that the HDLp-LpR complex, in contrast to the LDL-LDLR complex, is resistant to dissociation at endosomal pH as well as by treatment with EDTA mimicking the drop in Ca(2+) concentration in the endosome. This remarkable stability of the ligand-receptor complex may provide a crucial key to the recycling mechanism. Based on the binding and dissociation capacities of mutant and hybrid receptors, the specific binding interaction of the ligand-binding domain of the receptor with HDLp was characterized. These structural similarities and functional adaptations of the lipid transport systems operative in mammals and insects are discussed from an evolutionary perspective.

A method for the alignment of heterogeneous macromolecules from electron microscopy

We propose a feature-based image alignment method for single-particle electron microscopy that is able to accommodate various similarity scoring functions while efficiently sampling the two-dimensional transformational space. We use this image alignment method to evaluate the performance of a scoring function that is based on the Mutual Information (MI) of two images rather than one that is based on the cross-correlation function. We show that alignment using MI for the scoring function has far less model-dependent bias than is found with cross-correlation based alignment. We also demonstrate that MI improves the alignment of some types of heterogeneous data, provided that the signal-to-noise ratio is relatively high. These results indicate, therefore, that use of MI as the scoring function is well suited for the alignment of class-averages computed from single-particle images. Our method is tested on data from three model structures and one real dataset.

Low density lipoproteins as circulating fast temperature sensors

Background

The potential physiological significance of the nanophase transition of neutral lipids in the core of low density lipoprotein (LDL) particles is dependent on whether the rate is fast enough to integrate small (±2°C) temperature changes in the blood circulation.

Methodology/Principal Findings

Using sub-second, time-resolved small-angle X-ray scattering technology with synchrotron radiation, we have monitored the dynamics of structural changes within LDL, which were triggered by temperature-jumps and -drops, respectively. Our findings reveal that the melting transition is complete within less than 10 milliseconds. The freezing transition proceeds slowly with a half-time of approximately two seconds. Thus, the time period over which LDL particles reside in cooler regions of the body readily facilitates structural reorientation of the apolar core lipids.

Conclusions/Significance

Low density lipoproteins, the biological nanoparticles responsible for the transport of cholesterol in blood, are shown to act as intrinsic nano-thermometers, which can follow the periodic temperature changes during blood circulation. Our results demonstrate that the lipid core in LDL changes from a liquid crystalline to an oily state within fractions of seconds. This may, through the coupling to the protein structure of LDL, have important repercussions on current theories of the role of LDL in the pathogenesis of atherosclerosis.

Reconstituting initial events during the assembly of apolipoprotein B-containing lipoproteins in a cell-free system

The synthesis of apolipoprotein B (apoB) dictates the formation of chylomicrons and very low-density lipoproteins, two major lipoprotein precursors in the human plasma. Despite its biological significance, the mechanism of the assembly of these apoB-containing lipoproteins remains elusive. An essential obstacle is the lack of systems that allow fine dissection of key components during assembly, including nascent apoB peptide, lipids in defined forms, chaperones, and microsomal triglyceride transfer protein (MTP). In this study, we used a prokaryotic cell-free expression system to reconstitute early events in the assembly of apoB-containing lipoprotein that involve the N-terminal domains of apoB. Our study shows that N-terminal domains larger than 20.5% of apoB (B20.5) have an intrinsic ability to remodel vesicular phospholipid bilayers into discrete protein-lipid complexes. The presence of appropriate lipid substrates during apoB translation plays a pivotal role for successful lipid recruitment, and similar lipid recruitment fails to occur if the lipids are added posttranslationally. Cotranslational presence of MTP can dramatically promote the folding of B6.4-20.5 and B6.4-22. Furthermore, apoB translated in the presence of MTP retains its phospholipid recruitment capability posttranslationally. Our data suggest that during the synthesis of apoB, the N-terminal domain has a short window for intrinsic phospholipid recruitment, the time frame of which is predetermined by the environment where apoB synthesis occurs. The presence of MTP prolongs this window of time by acting as a chaperone. The absence of either proper lipid substrate or MTP may result in the improper folding of apoB and, consequently, its degradation.

Charged amino acid residues 997-1000 of human apolipoprotein B100 are critical for the initiation of lipoprotein assembly and the formation of a stable lipidated primordial particle in McA-RH7777 cells

We previously demonstrated that a portion, or perhaps all, of the residues between 931 and 1000 of apolipoprotein (apo) B100 are required for the initiation of apoB-containing particle assembly. Based on our structural model of the first 1000 residues of apoB (designated as apoB:1000), we hypothesized that this domain folds into a three-sided lipovitellin-like "lipid pocket" via a hairpin-bridge mechanism. We proposed that salt bridges are formed between four tandem charged residues 717-720 in the turn of the hairpin bridge and four tandem complementary residues 997-1000 located at the C-terminal end of the model. To identify the specific motif within residues 931 and 1000 that is critical for apoB particle assembly, apoB:956 and apoB:986 were produced. To test the hairpin-bridge hypothesis, the following mutations were made: 1) residues 997-1000 deletion (apoB:996), 2) residues 717-720 deletion (apoB:1000Delta717-720), and 3) substitution of charged residues 997-1000 with alanines (apoB:996 + 4Ala). Characterization of particles secreted by stable transformants of McA-RH7777 cells demonstrated the following. 1) ApoB:956 did not form stable particles and was secreted as large lipid-rich aggregates. 2) ApoB:986 formed both a lipidated particle that was denser than HDL(3) and large lipid-rich aggregates. 3) Compared with wild-type apoB:1000, apoB:1000Delta717-720 displayed the following: (i) significantly diminished capacity to form intact lipidated particles and (ii) increased propensity to form large lipid-rich aggregates. 4) In striking contrast to wild-type apoB:1000, (i) apoB:996 and apoB:996 + 4Ala were highly susceptible to intracellular degradation, (ii) only a small proportion of the secreted proteins formed stable HDL(3)-like lipoproteins, and (iii) a majority of the secreted proteins formed large lipid-rich aggregates. We conclude that the first 1000 amino acid residues of human apoB100 are required for the initiation of nascent apoB-containing lipoprotein assembly, and residues 717-720 and 997-1000 play key roles in this process, perhaps via a hairpin-bridge mechanism.

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.

The N-terminal domain of apolipoprotein B-100: structural characterization by homology modeling

Background

Apolipoprotein B-100 (apo B-100) stands as one of the largest proteins in humans. Its large size of 4536 amino acids hampers the production of X-ray diffraction quality crystals and hinders in-solution NMR analysis, and thus necessitates a domain-based approach for the structural characterization of the multi-domain full-length apo B.

Results

The structure of apo B-17 (the N-terminal 17% of apolipoprotein B-100) was predicted by homology modeling based on the structure of the N-terminal domain of lipovitellin (LV), a protein that shares not only sequence similarity with B17, but also a functional aspect of lipid binding and transport. The model structure was first induced to accommodate the six disulfide bonds found in that region, and then optimized using simulated annealing.

Conclusion

The content of secondary structural elements in this model structure correlates well with the reported data from other biophysical probes. The overall topology of the model conforms with the structural outline corresponding to the apo B-17 domain as seen in the EM representation of the complete LDL structure.

The physiological and molecular regulation of lipoprotein assembly and secretion

Triglycerides are insoluble in water and yet are transported at milligram per millilitre concentrations in the bloodstream. This is made possible by the ability of the liver and intestine to assemble lipid-protein emulsions (i.e. lipoproteins), which transport hydrophobic molecules. The assembly of triglyceride-rich lipoproteins requires the coordination of protein and lipid synthesis, which occurs on the cytoplasmic surface of the endoplasmic reticulum (ER), and their concerted assembly and translocation into the luminal ER secretory pathway as nascent lipoprotein particles. The availability of lipid substrate for triglyceride production and the machinery for lipoprotein assembly are highly sensitive to nutritional, hormonal, and genetic modulation. Disorders in lipid metabolism or an imbalance between lipogenesis and lipoprotein assembly can lead to hyperlipidemia and/or hepatic steatosis. We selectively review recently-identified machinery, such as transcription factors and nuclear hormone receptors, which provide new clues to the regulation of lipoprotein secretion.

Cross-correlation of common lines: a novel approach for single-particle reconstruction of a structure containing a flexible domain

We describe a novel approach to sorting class averages of a structure in multiple conformational states in order to generate 3D reconstructions that account for conformational variability present in the sample. The method assumes that the relative Euler angles between class averages are known, then uses a common lines approach to match any given class against a set of distinct conformations from a selected view of the structure. We show the effectiveness of the method both on model data and on an experimental dataset for which the conformational variability is limited to a defined region within the structure. During our studies of hepatitis C virus (HCV) internal ribosome entry site (IRES) interaction with the human translation initiation factor eIF3, we observed that the IRES RNA included a flexible region holding multiple conformations. While current classification methods were used to produce two-dimensional averages of the complex showing these different conformations, no method existed for relating these averages in three dimensions. Our approach overcame these limitations, giving us structural insight that was previously not possible.

Structural analysis of reconstituted lipoproteins containing the N-terminal domain of apolipoprotein B

Apolipoproteins play a central role in lipoprotein metabolism, and are directly implicated in cardiovascular diseases, but their structural characterization has been complicated by their structural flexibility and heterogeneity. Here we describe the structural characterization of the N-terminal region of apolipoprotein B (apoB), the major protein component of very low-density lipoprotein and low-density lipoprotein, in the presence of phospholipids. Specifically, we focus on the N-terminal 6.4-17% of apoB (B6.4-17) complexed with the phospholipid dimyristoylphosphatidylcholine in vitro. In addition to circular dichroism spectroscopy and limited proteolysis, our strategy incorporates nanogold-labeling of the protein in the reconstituted lipoprotein complex followed by visualization and molecular weight determination with scanning transmission electron microscopy imaging. Based on the scanning transmission electron microscopy imaging analysis of approximately 1300 individual particles where the B6.4-17 is labeled with nanogold through a six-His tag, most complexes contain either two or three B6.4-17 molecules. Circular dichroism spectroscopy and limited proteolysis of these reconstituted particles indicate that there are no large conformational changes in B6.4-17 upon lipoprotein complex formation. This is in contrast to the large structural changes that occur during apolipoprotein A-I-lipid interactions. The method described here allows a direct measurement of the stoichiometry and molecular weight of individual particles, rather than the average of the entire sample. Thus, it represents a useful strategy to characterize the structure of lipoproteins, which are not structurally uniform, but can still be defined by an ensemble of related patterns.

Theoretical model of human apolipoprotein B100 tertiary structure

Low density lipoprotein (LDL) particles are the main cholesterol carriers in human plasma. The organization of the particle, composed of apolar lipids and phospholipid monolayer stabilized by apolipoprotein B100 (apoB), is highly complex and still unknown. ApoB is an extremely large protein (4563 amino acids) and very little is known about its structure. A 3D model of the N-terminal region has been recently proposed and has provided interesting insights about the physico-chemical properties of the protein and putative interaction zones with lipids. In the present article, we propose the first tentative 3D modelling for most remaining residues. All predicted features emerging from the models are confronted with agreement to experimental data available. Using different up-to-date prediction methods, we decomposed the protein into eight domains and predicted 3D structure for each of them. The analysis of hydrophobic patches, polar regions, coupled with functional predictions based on the 3D models, gives new clues to understanding of the functional role of apoB. We suggest precise regions putatively involved in the lipid interactions, and discuss the position of apoB on the LDL particle. Finally, we propose relative organization of the domains, providing a shape quite compatible with the low resolution electron microscopy map.

Method of imaging low density lipoproteins by atomic force microscopy

This short paper reports a simple method to image low density lipoproteins (LDL) using atomic force microscopy (AFM). This instrument allows imaging of biological samples in liquid and presents the advantage of needing no sample preparation such as staining or fixation that may affect their general structure. Dimensions (diameter and height) of individual LDL particles were successfully measured. AFM imaging revealed that LDL have a quasi-spherical structure on the x and y axis with an oblate spheroid structure in the z axis (i.e., height). LDLs were found to have an average diameter of 23 +/- 3 nm. The obtained mean height was 10 +/- 2 nm.

Defining lipid-interacting domains in the N-terminal region of apolipoprotein B

Apolipoprotein B (apoB) is a nonexchangeable apolipoprotein that dictates the synthesis of chylomicrons and very low density lipoproteins. ApoB is the major protein in low density lipoprotein, also known as the "bad cholesterol" that is directly implicated in atherosclerosis. It has been suggested that the N-terminal domain of apoB plays a critical role in the formation of apoB-containing lipoproteins through the initial recruitment of phospholipids in the endoplasmic reticulum. However, very little is known about the mechanism of lipoprotein nucleation by apoB. Here we demonstrate that a strong phospholipid remodeling function is associated with the predicted alpha-helical and C-sheet domains in the N-terminal 17% of apoB (B17). Using dimyristoylphosphatidylcholine (DMPC) as a model lipid, these domains can convert multilamellar DMPC vesicles into discoidal-shaped particles. The nascent particles reconstituted from different apoB domains are distinctive and compositionally homogeneous. This phospholipid remodeling activity is also observed with egg phosphatidylcholine (egg PC) and is therefore not DMPC-dependent. Using kinetic analysis of the DMPC clearance assay, we show that the identified phospholipid binding sequences all map to the surface of the lipid binding pocket in the B17 model based on the homologous protein, lipovitellin. Since both B17 and microsomal triglyceride transfer protein (MTP), a critical chaperone during lipoprotein assembly, are homologous with lipovitellin, the identification of these phospholipid remodeling sequences in B17 provides important insights into the potential mechanism that initiates the assembly of apoB-containing lipoproteins.

Removal of divalent cations induces structural transitions in red clover necrotic mosaic virus, revealing a potential mechanism for RNA release

The structure of Red clover necrotic mosaic virus (RCNMV), an icosahedral plant virus, was resolved to 8.5 A by cryoelectron microscopy. The virion capsid has prominent surface protrusions and subunits with a clearly defined shell and protruding domains. The structures of both the individual capsid protein (CP) subunits and the entire virion capsid are consistent with other species in the Tombusviridae family. Within the RCNMV capsid, there is a clearly defined inner cage formed by complexes of genomic RNA and the amino termini of CP subunits. An RCNMV virion has approximately 390 +/- 30 Ca2+ ions bound to the capsid and 420 +/- 25 Mg2+ ions thought to be in the interior of the capsid. Depletion of both Ca2+ and Mg2+ ions from RCNMV leads to significant structural changes, including (i) formation of 11- to 13-A-diameter channels that extend through the capsid and (ii) significant reorganization within the interior of the capsid. Genomic RNA within native capsids containing both Ca2+ and Mg2+ ions is extremely resistant to nucleases, but depletion of both of these cations results in nuclease sensitivity, as measured by a significant reduction in RCNMV infectivity. These results indicate that divalent cations play a central role in capsid dynamics and suggest a mechanism for the release of viral RNA in low-divalent-cation environments such as those found within the cytoplasm of a cell.

Molecular determination by electron microscopy of the actin filament end structure

In eukaryotic cells, actin filaments play various crucial roles by altering their spatial and temporal distributions in the cell. The distribution of actin filaments is regulated by the binding of end-binding proteins, including capping protein (CapZ in muscle), the Arp2/3 complex, gelsolin, formin and tropomodulin, to the end of the actin filament. In order to determine the nature of these regulations, structural elucidations of actin filament-end-binding protein complexes are crucially important. Here, we have developed new procedures on the basis of single-particle analysis to determine the structure of the end of actin filaments from electron micrographs. In these procedures, the polarity of the actin filament image, as well as the azimuth orientation and the axial position of each actin protomer within a short stretch near the filament end, were determined accurately. This improved both the stability and accuracy of the structural determination dramatically. We tested our procedures by reconstructing structures from simulated filament images, which were obtained from 24 model structures for the actin-CapZ complex. These model structures were generated by random docking of the atomic structure of CapZ to the barbed end of an atomic model of the actin filament. Of the 24 model structures, 23 were recovered correctly by the present procedures. We found that our analysis was robust against local aberrations of the helical twist near the end of the actin filament. Finally, the procedures were applied successfully to determine the structure of the actin-CapZ complex from real cryo-electron micrographs of the complex. This is the first method for elucidating the detailed 3D structures at the end of the actin filament.

Binding of the low-density lipoprotein by streptococcal collagen-like protein Scl1 of Streptococcus pyogenes

Several bacterial genera express proteins that contain collagen-like regions, which are associated with variable (V) non-collagenous regions. The streptococcal collagen-like proteins, Scl1 and Scl2, of group A Streptococcus (GAS) are members of this 'prokaryotic collagen' family, and they too contain an amino-terminal non-collagenous V region of unknown function. Here, we use recombinant rScl constructs, derived from several Scl1 and Scl2 variants, and affinity chromatography to identify Scl ligands present in human plasma. First, we show that Scl1, but not Scl2, proteins from different GAS serotypes bind the same ligand identified as apolipoprotein B (ApoB100), which is a major component of the low-density lipoprotein (LDL). Scl1 binding to purified ApoB100 and LDL is specific and concentration-dependent. Furthermore, the non-collagenous V region of the Scl1 protein is responsible for LDL/ApoB100 binding because only those rScls, constructed by domain swapping, which contain the V region from Scl1 proteins, were able to bind to ApoB100 and LDL ligands, and this binding was inhibited by antibodies directed against the Scl1-V region. Electron microscopy images of Scl1-LDL complexes showed that the globular V domain of Scl1 interacted with spherical particles of LDL. Importantly, live M28-type GAS cells absorbed plasma LDL on the cell surface and this binding depended on the surface expression of the Scl1.28, but not Scl2.28, protein. Phylogenetic analysis showed that the non-collagenous globular domains of Scl1 and Scl2 evolved independently to form separate lineages, which differ in amino acid sequence, and these differences may account for the variations in binding patterns of Scl1 and Scl2 proteins. Present studies provide insight into the structure-function relationship of the Scl proteins and also underline the importance of lipoprotein binding by GAS.

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–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.

Structure and physiologic function of the low-density lipoprotein receptor

The low-density lipoprotein receptor (LDLR) is responsible for uptake of cholesterol-carrying lipoprotein particles into cells. The receptor binds lipoprotein particles at the cell surface and releases them in the low-pH environment of the endosome. The focus of the current review is on biochemical and structural studies of the LDLR and its ligands, emphasizing how structural features of the receptor dictate the binding of low-density lipoprotein (LDL) and beta-migrating forms of very low-density lipoprotein (beta-VLDL) particles, how the receptor releases bound ligands at low pH, and how the cytoplasmic tail of the LDLR interfaces with the endocytic machinery.

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.

The LDL receptor: how acid pulls the trigger

The low-density lipoprotein receptor normally carries lipoprotein particles into cells, and releases them upon delivery to the low pH milieu of the endosome. Recent structural and functional studies of the receptor, combined with the plethora of prior knowledge about normal receptor function and the effects of disease-associated mutations that cause familial hypercholesterolemia, reveal a detailed molecular model for how the acidic environment of the endosome triggers the release of bound lipoprotein particles. Remarkably, the receptor dynamically interconverts between open (ligand-active) and closed (ligand-inactive) conformations in response to pH, relying on a specific arrangement of fixed and flexible interdomain connections to facilitate efficient binding and release of its lipoprotein ligands.

Characterization of native and oxidized human low-density lipoproteins by the Z-scan technique

The nonlinear optical response of human normal and oxidized by Cu2+ low-density lipoproteins particles (LDL), were investigated by the Z-scan technique as a function of temperature and concentration of LDL particles. The Z-scan signals increase linearly with concentration of normal LDL particles, following the usual Beer-Lambert law in a broad range of concentrations. The oxidized LDL particles do not show nonlinear optical response. On the other hand, normal LDL increases its nonlinear optical response as a function of temperature. These behaviors can be attributed to an absorbing element that is modified by the oxidative process. Contrarily, changes in the physical state of the cores and conformation of the ApoB100 protein due to an increase in temperature seems to enhance their nonlinear optical properties. This tendency is not due to aggregation of particles. The main contribution to the nonlinear optical response of normal LDL particles comes from the phospholipid fraction of the particles.

Direct Measurement of Nitric Oxide and Oxygen Partitioning into Liposomes and Low Density Lipoprotein

Nitric oxide (*NO) has been proposed to play a relevant role in modulating oxidative reactions in lipophilic media like biomembranes and lipoproteins. Two factors that will regulate *NO reactivity in the lipid milieu are its diffusion and solubility, but there is no data concerning the actual diffusion (D) and partition coefficients (KP) of *NO in biologically relevant hydrophobic phases. Herein, a "equilibrium-shift" method was designed to directly determine the *NO and O2 partition coefficients in liposomes and low density lipoprotein (LDL) relative to water. It was found that *NO partitions 4.4- and 3.4-fold in liposomes and LDL, respectively, whereas O2 behaves similarly with values of 3.9 and 2.9, respectively. In addition, actual diffusion coefficients in these hydrophobic phases were determined using fluorescence quenching and found that *NO diffuses approximately 2 times slower than O2 in the core of LDL and 12 times slower than in buffer (DNOLDL=3.9 x 10(-6) cm2 s(-1),DO2LDL=7.0 x 10(-6) cm2 s(-1),DNObuffer=DO2buffer=4.5 x 10(-5) cm2 s(-1)). The influence of *NO and O2 partitioning and diffusion in membranes and lipoproteins on *NO reaction with lipid radicals and auto-oxidation is discussed. Particularly, the 3-4-fold increase in O2 and *NO concentration within biological hydrophobic phases provides quantitative support for the idea of an accelerated auto-oxidation of *NO in lipid-containing structures, turning them into sites of enhanced local production of oxidant and nitrosating species.

Preferred orientations of LDL in vitreous ice indicate a discoid shape of the lipoprotein particle

The structure of the human low-density lipoprotein (LDL) was analyzed in vitreous ice using cryo-electron microscopy (cryo-EM). In relatively thick cryo-EM preparations, random orientation of LDL particles produced various types of projections on the microscope screen, including circular projections with a high-density ring and rectangular projections with two high-density bands. However, in especially thin preparations, preferred, non-random orientations of the LDL particle produced only circular projections of the lipoprotein structure. In preparations with high LDL concentrations, ordered two-dimensional arrays, including hexagonal arrangements of circular projections and short stacks of rectangular projections, were observed. These observations are consistent with a discoid shape of the LDL particle, and suggest that surface tension forces may influence orientation of the LDL disc in thin aqueous films. Face-on orientation of LDL in especially thin cryo-EM preparations may explain earlier difficulties in identifying discoid features of the lipoprotein particle, and illustrates that some caution is warranted when attempts are made to reconstruct the three-dimensional structure of LDL from cryo-electron micrographs.

MR and fluorescent imaging of low-density lipoprotein receptors

RATIONALE AND OBJECTIVES

Over-expression of low-density lipoprotein receptors (LDLRs) occurs in many types of malignancies and is related to the requirement for lipids for rapid proliferation of the tumors. On the other hand, LDLRs that are unable to bind LDL are found on hepatocytes of patients with familial hypercholesterolemia (FH), a genetic disease that leads to premature atherosclerosis and death. The highly selective binding of LDL to LDLR makes these particles ideal carriers of therapeutic and diagnostic contrast agents into the targeted cells. The objectives of this paper are to examine whether a prototype contrast agent (PTIR267) with dual detection properties is suitable for labeling of LDL particles for in vivo detection of LDLR by magnetic resonance imaging (MRI) and for in vitro monitoring of cellular localization by confocal fluorescence microscopy.

MATERIALS AND METHODS

PTIR267 is a lipophilic GdDTPA derivative conjugated to a fluorescent dye. The conjugated dye molecule makes the probe sufficiently water soluble to allow labeling of LDL by a brief incubation of LDL with PTIR267 dissolved in PBS at 37 degrees C (mole ratio LDL: PTIR267 = 0.09:1). The molar relaxivity of PTIR267 in saline is 26 mM(-1)s(-1). Specific LDLR-mediated uptake of PTIR267-labeled LDL was demonstrated in vitro by confocal fluorescence imaging of B16 melanoma cells using confocal fluorescence imaging. In vivo uptake of PTIR267-labeled LDL by a subcutaneously implanted B16 melanoma in mice leads to 30% decrease in longitudinal relaxation time (T(1)) in the tumor. In vivo uptake of PTIR267-labeled LDL leads to 70% decrease in T(1) in a normal C57BL/6 mouse liver; however, in the liver of LDL receptor gene knockout (LDLr-/-) mice with C57BL/6 background, only 12% decrease in T(1) is observed.

CONCLUSIONS

The dual fluorescence and MR imaging properties of PTIR267, combined with the ease of LDL labeling, suggest that it will be a useful tool for optimization of LDLR-targeted cancer diagnosis or therapy and for monitoring the efficacy of gene therapy of FH.

Design and Synthesis of Novel N-Acetylgalactosamine-Terminated Glycolipids for Targeting of Lipoproteins to the Hepatic Asialoglycoprotein Receptor

A novel glycolipid has been prepared that contains a cluster glycoside with an unusually high affinity for the asialoglycoprotein receptor (ASGPr) and a bile acid moiety that mediates stable incorporation into lipidic particles. The glycolipid spontaneously associated with low-density lipoproteins (LDL) and high-density lipoproteins (HDL) within human and murine plasma, and loading of lipoproteins with this glycolipid resulted in an efficient dose-dependent recognition and uptake of LDL and HDL by the liver (and not by spleen) upon intravenous injection into wild-type mice. Preinjection with asialoorosomucoid largely inhibited the uptake, establishing that both HDL and LDL were selectively recognized and processed by the ASGPr on liver parenchymal cells. Finally, repeated intravenous administration of the glycolipid to hyperlipidemic LDL receptor-deficient mice evoked an efficient and persistent cholesterol-lowering effect. These results indicate that the glycolipid may be a promising alternative for the treatment of hyperlipidemic patients who do not respond sufficiently to current cholesterol-lowering therapies.

Combined data from LDL composition and size measurement are compatible with a discoid particle shape

The size of LDL is usually reported as particle diameter, with the implicit assumption that it is a spherical particle. On the other hand, data obtained by cryoelectron microscopy and crystallographic analysis suggest that LDL shape may be discoid. We have investigated LDL particle geometry by combining data on LDL lipid composition with size measurement. The mean LDL diameter of 160 samples was measured by high-performance gel-filtration chromatography (HPGC), and particle volume was calculated from its lipid composition. Assuming a spherical shape, diameters calculated from volume correlated poorly with values obtained by HPGC (R(2) = 0.36). Assuming a discoid shape, particle height was calculated from volume and HPGC diameter. Diameter (20.9 +/- 0.5 nm) and height (12.1 +/- 0.8 nm) were not significantly related to each other (r = 0.14, P = 0.09) and accounted for 23% and 77%, respectively, of the variation in particle volume. In multivariate regression models, LDL core lipids were the main determinants of height (R(2) = 0.83), whereas free cholesterol in the shell, which contributes only 5-9% to LDL mass, was the main determinant of diameter (R(2) = 0.54). We conclude that combined data from composition and size measurements are compatible with a discoid particle shape and propose a structural model for LDL in which free cholesterol plays a major role in determining particle shape and diameter.

Hydrophobic nature of rat lymph chylomicrons

BACKGROUND

A typical molecular structure of a lipoprotein is composed of hydrophobic lipids at the core and hydrophilic apolipoprotein side chains and lipid head groups at the surface. Some of the hydrophobic characteristics of rat lymph chylomicrons were investigated.

METHODS

Thoracic duct was cannulated and lymph was collected overnight. Chylomicrons (>100 nm) were isolated by ultracentrifugation at 4 x 10(6)xg min. Since particle aggregation is a characteristic of hydrophobic nature of lipoproteins, as an index of aggregation, the turbidity generated by vortexing and storage of chylomicrons was measured spectrophotometrically at 680 nm. We also assessed the ability of chylomicrons to interact with five different hydrophobic interaction chromatography (HIC) media.

RESULTS

Neither shaking nor prolonged storage at 4 degrees C produced an increase in the optical density of chylomicron solution indicating no aggregation took place. Typical elution profiles of chylomicrons through octyl, phenyl (high substance) and butyl sepharose columns showed two peaks. Peak I material emerged with 4 mol/l NaCl in a position corresponding to the void volume and peak II material eluted with water. Phenyl sepharose (high performance) media exhibited the maximum binding strength towards chylomicrons among the five different media. In the case of phenyl sepharose (low substance) column, an additional material was eluted with 3 mol/l NaCl between peaks I and II. These results indicate the heterogeneity of chylomicron surface hydrophobicity.

CONCLUSION

Since particle aggregation is a characteristics of hydrophobicity of lipoproteins and believed to be an underlying cause of atherosclerosis, fractionation of lipoproteins by hydrophobic interaction chromatography may introduce a new approach into the assessment of lipoprotein atherogeneicity.

Recognition and separation of single particles with size variation by statistical analysis of their images

Macromolecules may occupy conformations with structural differences that cannot be resolved biochemically. The separation of mixed molecular populations is a pressing problem in single-particle analysis. Until recently, the task of distinguishing small structural variations was intractable, but developments in cryo-electron microscopy hardware and software now make it possible to address this problem. We have developed a general strategy for recognizing and separating structures of variable size from cryo-electron micrographs of single particles. The method uses a combination of statistical analysis and projection matching to multiple models. Identification of size variations by multivariate statistical analysis was used to do an initial separation of the data and generate starting models by angular reconstitution. Refinement was performed using alternate projection matching to models and angular reconstitution of the separated subsets. The approach has been successful at intermediate resolution, taking it within range of resolving secondary structure elements of proteins. Analysis of simulated and real data sets is used to illustrate the problems encountered and possible solutions. The strategy developed was used to resolve the structures of two forms of a small heat shock protein (Hsp26) that vary slightly in diameter and subunit packing.

Effect of gangliosides on the copper-induced oxidation of human low-density lipoproteins

The role of gangliosides in the copper-induced oxidative modification of human low-density lipoprotein (LDL) was studied focusing on the early stage of LDL oxidation in which the concentration of conjugated dienes increases only weakly. The changes in the protein and lipid component were followed using fluorescence spectroscopy. The results indicate that binding of gangliosides to LDL causes slower destruction of tryptophan fluorescence and suppresses cross-linking between the reactive groups of the protein and the products of lipid peroxidation. The protective role of gangliosides could be assigned to their interference with the lipid-protein interaction in the LDL particle, which might be important for the maintenance of the native plasma antioxidant status in vivo.

The study of structural accessibility of free thiol groups in human low-density lipoproteins

The experimental evidence for the apolipoprotein B100 (apoB) domain structuring in low-density lipoprotein (LDL) was investigated focusing on the accessibility of free thiol groups. Three different spectroscopic methods were combined with the biochemical perturbations of LDL particle. The spectrophotometric method was adapted for LDL and the exposure of free thiols was analyzed in the native LDL and LDL exposed to sequential denaturation. The results indicate that 24-h denaturation does not expose all free thiols in LDL. Using thiol-specific spin labeling and electron paramagnetic resonance spectroscopy (EPR), different populations of labeled thiols were resolved. The comparison of the EPR spectra of native LDL and LDL with selectively blocked thiol groups revealed significant difference in the respective hyperfine splittings. The phenomenon can arise due to different polarity and/or mobility of the nitroxides in the microenvironments of spin label binding sites of these two LDL samples. The results indicate that nine thiol groups in apoB are distributed in different domains of LDL: two are more exposed, two are buried deeply in the lipid matrix of the particle and the rest are located in hydrophobic parts of this extremely complex protein-lipid assembly. These observations provide experimental support for the emerging theoretical models of apoB.

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.