Temperature-dependent 13C nuclear magnetic resonance studies of human serum low density lipoproteins

Title: Human Serum/Plasma Glycoprotein Analysis by 1H-NMR, an Emerging Method of Inflammatory Assessment

Several studies suggest that variations in the concentration of plasma glycoproteins can influence cellular changes in a large number of diseases. In recent years, proton nuclear magnetic resonance (1H-NMR) has played a major role as an analytical tool for serum and plasma samples. In recent years, there is an increasing interest in the characterization of glycoproteins through 1H-NMR in order to search for reliable and robust biomarkers of disease. The objective of this review was to examine the existing studies in the literature related to the study of glycoproteins from an analytical and clinical point of view. There are currently several techniques to characterize circulating glycoproteins in serum or plasma, but in this review, we focus on 1H-NMR due to its great robustness and recent interest in its translation to the clinical setting. In fact, there is already a marker in H-NMR representing the acetyl groups of the glycoproteins, GlycA, which has been increasingly studied in clinical studies. A broad search of the literature was performed showing a general consensus that GlycA is a robust marker of systemic inflammation. The results also suggested that GlycA better captures systemic inflammation even more than C-reactive protein (CRP), a widely used classical inflammatory marker. The applications reviewed here demonstrated that GlycA was potentially a key biomarker in a wide range of diseases such as cancer, metabolic diseases, cardiovascular risk, and chronic inflammatory diseases among others. The profiling of glycoproteins through 1H-NMR launches an encouraging new paradigm for its future incorporation in clinical diagnosis.

1H NMR spectroscopy quantifies visibility of lipoproteins, subclasses, and lipids at varied temperatures and pressures

NMR-based quantification of human lipoprotein (sub)classes is a powerful high-throughput method for medical diagnostics. We evaluated select proton NMR signals of serum lipoproteins for elucidating the physicochemical features and the absolute NMR visibility of their lipids. We separated human lipoproteins of different subclasses by ultracentrifugation and analyzed them by 1H NMR spectroscopy at different temperatures (283-323 K) and pressures (0.1-200 MPa). In parallel, we determined the total lipid content by extraction with chloroform/methanol. The visibility of different lipids in the 1H NMR spectra strongly depends on temperature and pressure: it increases with increasing temperatures but decreases with increasing pressures. Even at 313 K, only part of the lipoprotein is detected quantitatively. In LDL and in HDL subclasses HDL2 and HDL3, only 39%, 62%, and 90% of the total cholesterol and only 73%, 70%, and 87% of the FAs are detected, respectively. The choline head groups show visibilities of 43%, 75%, and 87% for LDL, HDL2, and HDL3, respectively. The description of the NMR visibility of lipid signals requires a minimum model of three different compartments, A, B, and C. The thermodynamic analysis of compartment B leads to melting temperatures between 282 K and 308 K and to enthalpy differences that vary for the different lipoproteins as well as for the reporter groups selected. In summary, we describe differences in NMR visibility of lipoproteins and variations in biophysical responses of functional groups that are crucial for the accuracy of absolute NMR quantification.

High Resolution NMR Spectroscopy as a Structural and Analytical Tool for Unsaturated Lipids in Solution

Mono- and polyunsaturated lipids are widely distributed in Nature, and are structurally and functionally a diverse class of molecules with a variety of physicochemical, biological, medicinal and nutritional properties. High resolution NMR spectroscopic techniques including 1H-, 13C- and 31P-NMR have been successfully employed as a structural and analytical tool for unsaturated lipids. The objective of this review article is to provide: (i) an overview of the critical 1H-, 13C- and 31P-NMR parameters for structural and analytical investigations; (ii) an overview of various 1D and 2D NMR techniques that have been used for resonance assignments; (iii) selected analytical and structural studies with emphasis in the identification of major and minor unsaturated fatty acids in complex lipid extracts without the need for the isolation of the individual components; (iv) selected investigations of oxidation products of lipids; (v) applications in the emerging field of lipidomics; (vi) studies of protein-lipid interactions at a molecular level; (vii) practical considerations and (viii) an overview of future developments in the field.

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.

Integral apolipoproteins increase surface-located triacylglycerol in intact native apoB-100-containing lipoproteins

High-resolution NMR was used to measure the presence and quantity of triacylglycerol (TAG) in the surface of intact native apolipoprotein B-100-containing lipoprotein particles that are made by chickens in response to estrogen treatment and that in hens are deposited in yolk follicles (VLDLy). Integration of 13C NMR resonances shows that intact VLDLy particles contain more surface TAG (5.1 +/- 0.6 mol%, 6.7 +/- 0.8 weight %) than predicted by apolipoprotein-free models using similarly acyl-heterogenous TAG. Change in downfield chemical shift values of surface to core TAG in VLDLy was 0.8 ppm compared with 1.3 ppm in vesicles prepared with purified egg phosphatidylcholine and TAG isolated from the VLDLy, indicating that reduced surface TAG hydration may contribute to the resistance to lipase hydrolysis characteristic of this lipoprotein species. Apolipoprotein-mediated changes in surface lipid composition and lipid hydration provide possible general mechanisms for selectivity in lipoprotein substrate characteristics.

Identification and localization of two distinct microenvironments for the diacylglycerol component of lipophorin particles by 13C NMR

13C nuclear magnetic resonance spectroscopy of lipoproteins, isolated from the insect Manduca sexta, has been employed to probe the microenvironment of diacylglycerol (DG), their major neutral lipid component. Natural abundance 13C NMR spectra of high density lipophorin exhibited several well-separated resonances derived from its lipid moiety, including those for the carbonyl carbon atoms of phospholipid and DG fatty acyl chains in the region of 175-180 ppm. To verify the assignment of the DG acyl chain carbonyl carbon resonances, di[1-13C]oleoylglycerol high density lipophorin was isolated after instilling a bolus of tri[1-13C]oleoylglycerol into the midgut of larvae fed a fat-free diet. 13C NMR spectra of the isolated lipoprotein revealed a specific and dramatic enrichment of resonances at 175.5 ppm. Expansion of this region revealed two resonances separated by 0.08 ppm. These were assigned as 1,2- and 1,3- isomers of DG, the latter presumably arising from spontaneous acyl chain migration of 1,2-DG following lipoprotein isolation. On the basis of compositional and structural analysis of this lipoprotein, it is postulated that these DG species are localized predominantly in the hydrophobic core of the particle. By contrast, natural abundance 13C NMR spectra of the DG-rich, low density lipophorin (LDLp) subspecies revealed two additional resonances, separated by 0.2 ppm, that were tentatively assigned as 1,2- and 1,3-DG present at the surface of the particle. The verify this assignment, experiments employing phospholipase C, to convert lipophorin surface associated phospholipid into DG, were performed.(ABSTRACT TRUNCATED AT 250 WORDS)

Esterified cholesterol and triglyceride are present in plasma membranes of Chinese hamster ovary cells

The chemical composition of highly purified plasma membrane preparations from a series of malignant Chinese hamster ovary (CHO) cell lines were undertaken to ascertain if neutral lipid, including cholesteryl ester and triacylglycerol, were present. Triacylglycerols (33-41 nmol/mg total lipid) and cholesteryl ester (226-271 nmol/mg) were measured in the plasma membranes and differences in the chemical composition of these membranes recorded. The most significant difference was a gradual decrease in the level of free cholesterol from wild type (312 +/- 7 nmol/mg total plasma membrane lipid), Pod RII-6 (268 +/- 64 nmol/mg total plasma membrane lipid), Col R-22 (243 +/- 39 nmol/mg total plasma membrane lipid) to EOT (204 +/- 20 nmol/mg total plasma membrane lipid), with a concomitant increase in the degree of saturation of the cholesteryl ester fatty acids, particularly palmitic acid. No statistically significant differences were apparent in the chemical composition of the whole cells in this series. The one-dimensional (1D) 1H-NMR spectra of the four malignant cell lines showed a gradation in intensity of lipid resonances, in the order of wild type, Pod RII-6, Col R-22 and EOT, with EOT having the strongest lipid spectrum. Interestingly, the increase in acyl-chain signal intensities in the 1H-NMR spectra of this series of CHO cells and emergence of signals from cholesterol and/or cholesteryl ester, coincide with alterations in the amount of free cholesterol and the degree of saturation of the fatty-acyl chain of the esterified cholesterol in the plasma membranes. It is our hypothesis that, together, cholesteryl ester and triacylglycerol form domains in the plasma membrane and that when the cholesteryl ester has a largely saturated fatty acid content, the lipids are in isotropic liquid phase and hence visible by NMR.

Characterization of the core and surface of human plasma lipoproteins. A study based on the use of five fluorophores

The physical properties of the core and the surface of five classes of human plasma lipoproteins were investigated using five fluorescent probes. The location of the fluorescence probes in the lipoprotein assembly was determined using collisional quenching and resonance energy transfer. The fluorophores monitor different regions of the lipoproteins, as shown by fluorescence quenching. Diphenylhexatriene (DPH) and methyl trans-parinaric acid (MTPA), which are apolar molecules, are localized mainly in the lipoprotein core. Their distribution into the surface is dependent upon the volume ratio of the hydrophobic part of the envelope and the core. The polar fluorophores, trimethylaminodiphenylhexatriene (TMADPH), hydroxycoumarin (HC) and trans-parinaric acid (TPA) are anchored in the glycerol skeleton region of the surface monolayer with the fluorophore group of HC in the headgroup region of the phospholipids. We determined the temperature-dependent steady-state fluorescence anisotropy (r) of these fluorophores in the four major classes of lipoproteins: VLDL, LDL, HDL2, HDL3 and in abnormal HDL from abetalipoproteinemia patients (HDLab). The hydrophobic probes, DPH and MTPA, reported the r values in the lipoproteins in the following order: LDL greater than HDL2 greater than HDL3 much greater than VLDL. This order correlates with the triglyceride-to-cholesterol ester (TG/CE) ratio in the core of lipoproteins. The polar probes HC, TPA and TMADPH reported the r value in a different order: HDL2, HDL3 greater than or equal to LDL much greater than VLDL. This is compatible with the decreasing order of the protein to lipid ratio in the envelope of these lipoproteins. HDLab was investigated by three fluorescent probes: DPH, TMADPH and HC. The anisotropy of DPH in HDLab was larger than that of either HDL2 or HDL3 in normal donors, probably due to the smaller TG/CE ratio in HDLab. The lower r values reported by HC and TMADPH for HDLab are not fully understood and may be related to other factors such as acyl chains composition. The characterization of lipoproteins by fluorescence depolarization using probes of known location in the lipoprotein assembly is very sensitive and may be used to report deviation from the norm.

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.

High-resolution 1H NMR spectral signature from human atheroma

Coronary artery disease due to atherosclerosis takes the lives of approximately 550,000 Americans each year--an enormous toll. Put in economic terms, the cost to the United States alone has been estimated to exceed 60 billion dollars annually. We have found that well-resolved proton (1H) NMR spectra can be obtained from human atheroma (fatty plaque), despite its macroscopic solid appearance. The fraction of the total spectral intensity corresponding to the sharp 1H NMR signals is temperature dependent and approaches unity at body temperature (37 degrees C). Studies of the total lipids extracted from atheroma and cholesteryl esters were conducted to identify the chemical and physical origin of the spectral signature. The samples were characterized through assignment of their chemical shifts and by measurement of their T1 and T2 relaxation times as a function of magnetic field strength. The results suggest that the relatively sharp 1H NMR signals from human atheroma (excluding water) are due to a mixture of cholesteryl esters, whose liquid-crystalline to isotropic fluid phase transition is near body temperature. Preliminary applications to NMR imaging of human atheroma are reported, which demonstrate early fatty plaque formation within the wall of the aorta. These findings offer a basis for noninvasive imaging by NMR to monitor early and potentially reversible stages of human atherogenesis.

Orientational order of cholesteryl oleate in low-density lipoprotein observed by 2H-NMR

Cholesteryl oleate, selectively deuterated at various positions along the acyl chain, has been incorporated into fresh human serum low-density lipoprotein (LDL2). Temperature-dependent 2H-NMR spectra were recorded between 15 and 45 degrees C. For deuterons at C-2' and C-5' of the acyl chain, two 2H-NMR spectral components, a broad and a narrow signal, are observed. This is interpreted as reflecting the coexistence of two cholesteryl ester regions in the LDL2 core which possess different degrees of order. The C-2H bond order parameters, SCD, are approx. 0.12-0.20 for the more ordered region and approx. 0.04-0.06 for the less ordered region. Longitudinal relaxation times, T1, of deuterated cholesteryl oleate are found to increase between C-8' and the terminal -C2H3 group, which is consistent with an increased rate of chain motion toward the free ends of the ester acyl chains.

Thermotropic transitions of lipids in porcine low density lipoproteins

The order----disorder thermal transition in porcine low density lipoprotein-1 (LDL1) and low density lipoprotein-2 (LDL2) has been studied by differential scanning calorimetry (DSC), 1H-NMR and ESR spin probing. The results by different methods are in general agreement in that the thermal transition in LDL1 takes place at about 32 degrees C. For LDL2 there are two overlapping transition regions, one centered at about 25.5 degrees C and the other at about 33 degrees C. The nature of the two transitions in LDL2 has been discussed.

The temperature dependence of the fluorescence decay of low-density lipoproteins

The decay of the intrinsic tryptophan fluorescence of low-density lipoproteins (LDL) is measured using a picosecond laser system with an excitation wavelength of 295 nm. The emission wavelengths were observed at 320, 340 and 360 nm. The measurements were performed in the temperature range 10-45 degrees C. The fluorescence decay data are analysed in terms of: (i) three exponentials; (ii) a continuous distribution of exponentials; and (iii) the average decay time. The results indicate that the tryptophans are in a non-polar environment and that the fluorescence is not affected by phase transitions found to occur in the lipid core of the LDL molecule.

Physical properties of cholesteryl esters

Cholesteryl esters, the intracellular storage form and intravascular transport form of cholesterol, can exist in crystal, liquid crystal and liquid states. The physical state of cholesteryl esters at physiologic temperatures may be a determinant of their pathogenicity. This review has surveyed saturated aliphatic cholesteryl esters of chain length 1 to 24 carbons and a series of medium-chained unsaturated cholesteryl esters from chain lengths 14 to 24 carbons. A systematic study of transition temperatures by polarizing microscopy and enthalpies by differential scanning calorimetry has provided unifying concepts concerning the phase behavior as a function of chain length and unsaturation. Neat cholesteryl esters show chain-length dependence of transition temperature and enthalpy of both the crystal and liquid crystal transitions. Double bond position along the fatty acyl chain affected stability of the liquid crystal phases; a smectic phase was not observed for any cholesteryl ester with a double bond more proximal than delta 9. 13C NMR spectroscopy in the isotropic liquid phase has provided evidence suggesting a balance of ring-ring vs. chain-chain interactions as a determinant for isotropic liquid----cholesteric vs. isotropic liquid----smectic transitions. Specifically, anisotropic molecular motions of the steroid ring are greater for cholesteryl esters forming a cholesteric phase than a smectic phase from the melt. Chain-chain interactions apparently predominate in smectic phase formation. The X-ray diffraction patterns of cholesteryl esters as a function of chain length reveal several isostructural series and known single crystal data are presented. A chain length depending on the periodicity of the smectic phase is observed which may be different for saturated vs. unsaturated esters. In summary, the phase behavior of cholesteryl ester molecules is complex and cannot be determined a priori from the phase behavior of component cholesterol and fatty acid. The data presented here should provide insight into the biological behavior of this lipid class.

Secondary structure and thermal behavior of trypsin-treated low-density lipoproteins from human serum, studied by circular dichroism

Low-density lipoproteins (LDL) were prepared from the serum of normolipidemic men on normal diets with or without supplemental beta-carotene. LDL were subjected to limited hydrolysis (5 h at 37 degrees C) with trypsin (enzyme:protein, 1:40 w/w), and their digested products separated by gel filtration. The trypsin-treated LDL contained about 80% of the original protein and essentially all of the original lipids of native LDL. The circular dichroic spectrum of trypsin-treated LDL below 240 nm resembled that of native LDL, except that the magnitudes of the ellipticity were smaller, corresponding to 25 and 33% helical content, respectively. The lower content of helix in trypsin-treated LDL suggests that certain helical regions in apolipoprotein B are sensitive to tryptic attack; however, a major portion of the helical structure of the apolipoprotein is resistant. The thermal stability of helix in trypsin-treated LDL resembled that of native LDL, suggesting that removal of the trypsin-accessible regions of the apolipoprotein has little or no effect on the forces stabilizing the remaining helices. Data on the induced circular dichroism of beta-carotene, an intrinsic probe of the neutral lipid core, showed a reduced transition temperature for cholesteryl esters after trypsin treatment. This finding suggests that the trypsin-accessible regions of apolipoprotein B may influence the fluidity of the core.

Cholesteryl myristate conformation in liquid crystalline mesophases determined by neutron scattering

The possible involvement of cholesteryl ester states in the development and persistence of atherosclerosis and the transport and storage of cholesteryl esters has led to questions concerning the organization and conformation of cholesteryl ester molecules in both pure phases and membranes. The experiments we report here were designed to measure the distance between the center of mass of the fatty acyl terminal methyl group and the center of mass of the three-carbon branched terminus of the cholesterol moiety at the opposite end of the molecule. The distance obtained is thus a gauge of cholesteryl ester conformation through the conformational range from a completely extended conformation to a U-shaped conformation. Neutron scattering experiments on partially deuterated samples of pure cholesteryl myristate in the crystalline, smectic, cholesteric, and isotropic phases indicate that the molecule is extended in each of these states. A discussion of specific molecular models consistent with these results and extension of these conclusions to other cholesteryl esters is included.

Structural organization of free and esterified cholesterol in human high density lipoproteins. A 100.6 MHz 13C NMR study

The structural organization of free and esterified cholesterol in human high density lipoproteins has been studied by high-field 1H and 13C NMR. The measurements are consistent with free cholesterol being present in at least two different environments. Part of the free cholesterol is oriented in the outer surface layer of the high density lipoprotein particle in contact with phospholipid or apoprotein, or both. The rest is probably present in the liquid, hydrophobic core of the HDL particle.

Atherogenic diets and neutral-lipid organization in plasma low density lipoproteins

The plasma low density lipoproteins (LDL) of rhesus monkeys fed 3 atherogenic diets exhibited thermal transitions at temperatures much higher (37--43 degrees C) than those observed in control animals or in normal humans (20--33 degrees C). The same differences were noted in the neutral lipids (cholesteryl esters and triglycerides) which were isolated from the respective lipoproteins. In particular, the difference in thermal properties between the normal and abnormal LDLs was attributable to subtle differences in their cholesteryl ester compositions (mainly an increase in the saturated and monounsaturated fatty acid moieties), with altered triglyceride contents playing only a minor role. Thus, at body temperature, the hyperlipidemia that follows the administration of atherogenic diets is associated with a high degree of order of the neutral lipids in the core of the LDL particle. This, in turn, may be related to the atherogenicity of the abnormal lipoprotein species.