A systematic DFT study of structure and electronic properties of titanium dioxide.

DFT functionals are of paramount importance for an accurate electronic and structural description of transition metal systems. In this work, a systematic analysis using some well-known and commonly used DFT functionals is performed. A comparison of the structural and energetic parameters calculated with the available experimental data is made in order to find the adequate functional for an accurate description of the TiO2 bulk and surface of both anatase and rutile structures. In the absence of experimental data on the surface energy, the theoretical predictions obtained using the high-accuracy HSE06 functional were used as a reference to compare against the surface energy values calculated with the other DFT functionals. A clear improvement in the electronic description of both anatase and rutile was observed by introducing the Hubbard U correction term to PBE, PW91, and OptPBE functionals. The OptPBE-U4 functional was found to offer a good compromise between accurately describing the structural and electronic properties of titania.

Statistical Inference of Rate Constants in Chemical and Biochemical Reaction Networks Using an "Inverse" Event-Driven Kinetic Monte Carlo Method.

The use of rate models for networks of stochastic reactions is frequently used to comprehend the macroscopically observed dynamic properties of finite size reactive systems as well as their relationship to the underlying molecular events. Τhis particular approach usually stumbles on parameter derivation associated with stochastic kinetics, a quite demanding procedure. The present study incorporates a novel algorithm, which infers kinetic parameters from the system's time evolution, manifested as changes in molecular species populations. The proposed methodology reconstructs distributions required to infer kinetic parameters of a stochastic process pertaining to either a simulation or experimental data. The suggested approach accurately replicates rate constants of the stochastic reaction networks, which have evolved over time by event-driven Monte Carlo (MC) simulations using the Gillespie algorithm. Furthermore, our approach has been successfully used to estimate rate constants of association and dissociation events between molecular species developing during molecular dynamics (MD) simulations. We certainly believe that our method will be remarkably helpful for considering the macroscopic characteristic molecular roots related to stochastic physical and biological processes.

Prediction of Adsorption and Diffusion of Shale Gas in Composite Pores Consisting of Kaolinite and Kerogen using Molecular Simulation.

Natural gas production from shale formations is one of the most recent and fast growing developments in the oil and gas industry. The accurate prediction of the adsorption and transport of shale gas is essential for estimating shale gas production capacity and improving existing extractions. To realistically represent heterogeneous shale formations, a composite pore model was built from a kaolinite slit mesopore hosting a kerogen matrix. Moreover, empty slabs (2, 3, and 4 nm) were added between the kerogen matrix and siloxane surface of kaolinite. Using Grand-Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations, the adsorption and diffusion of pure methane, pure ethane, and a shale gas mixture were computed at various high pressures (100, 150, and 250 atm) and temperature of 298.15 K. The addition of an inner slit pore was found to significantly increase the excess adsorption of methane, as a pure component and in the shale gas mixture. The saturation of the composite pore with methane was observed to be at a higher pressure compared to ethane. The excess adsorption of carbon dioxide was not largely affected by pressure, and the local number density profile showed its strong affinity to kerogen micropores and the hydroxylated gibbsite surface under all conditions and pore widths. Lateral diffusion coefficients were found to increase with increasing the width of the empty slab inside the composite pore. Statistical errors of diffusion coefficients were found to be large for the case of shale gas components present at low composition. A larger composite pore configuration was created to investigate the diffusion of methane in different regions of the composite pore. The calculated diffusion coefficients and mean residence times were found to be indicative of the different adsorption mechanisms occurring inside the pore.

The phase behaviour of cetyltrimethylammonium chloride surfactant aqueous solutions at high concentrations: an all-atom molecular dynamics simulation study.

We explore the phase behaviour of aqueous solutions of the cetyltrimethyl ammonium chloride (CTAC) surfactant and in particular the transition from the micellar phase (L1) to the hexagonal columnar phase (H1) by employing all-atom (AA) molecular dynamics (MD) simulations for six CTAC concentrations in the range of 34.1 wt% to 70.5 wt%, at the temperature of 318 K and pressure of 1 atm. For the concentrations considered, we examine the spontaneous occurrence of the H1 phase by testing a number of plausible values for the linear density (molecules per unit length) along the cylindrical columns. Using large simulation cells and starting from random initial configurations, the MD simulations demonstrate that the micellar phase occurs for concentrations up to 50.0 wt%, with CTAC molecules self-assembling into a mixture of spherical and rod-like micelles. At even higher concentrations, the system self-organizes into the H1 phase in accordance with the available experimental data. For the analysis of the MD trajectories, we devise a clustering algorithm based on Voronoi tesselation which enables (a) the thorough characterization of the shape and structure of both molecules and assemblies, and (b) the investigation of the positional and orientational order in the system that are further scrutinised using radial pair correlation functions and X-ray diffraction patterns. Our work paves the way for the investigation of the phase behaviour at high concentrations of other surfactants.

Molecular Dynamics Simulation of Pure n-Alkanes and Their Mixtures at Elevated Temperatures Using Atomistic and Coarse-Grained Force Fields.

The properties of higher n-alkanes and their mixtures is a topic of significant interest for the oil and chemical industry. However, the experimental data at high temperatures are scarce. The present study focuses on simulating n-dodecane, n-octacosane, their binary mixture at a n-dodecane mole fraction of 0.3, and a model mixture of the commercially available hydrocarbon wax SX-70 to evaluate the performance of several force fields on the reproduction of properties such as liquid densities, surface tension, and viscosities. Molecular dynamics simulations over a broad temperature range from 323.15 to 573.15 K were employed in examining a broad set of atomistic molecular models assessed for the reproduction of experimental data. The well-established united atom TraPPE (TraPPE-UA) was compared against the all atom optimized potentials for liquid simulations (OPLS) reparametrization for long n-alkanes, L-OPLS, as well as Lipid14 and MARTINI force fields. All models qualitatively reproduce the temperature dependence of the aforementioned properties, but TraPPE-UA was found to reproduce liquid densities most accurately and consistently over the entire temperature range. TraPPE-UA and MARTINI were very successful in reproducing surface tensions, and L-OPLS was found to be the most accurate in reproducing the measured viscosities as compared to the other models. Our simulations show that these widely used force fields originating from the world of biomolecular simulations are suitable candidates in the study of n-alkane properties, both in the pure and mixture states.

Scaling Laws for the Conformation and Viscosity of Ring Polymers in the Crossover Region around Me from Detailed Molecular Dynamics Simulations.

We present results from detailed, atomistic molecular dynamics (MD) simulations of pure, strictly monodisperse linear and ring poly(ethylene oxide) (PEO) melts under equilibrium and nonequilibrium (shear flow) conditions. The systems examined span the regime of molecular weights (Mw) from sub-Rouse (Mw < Me) to reptation (Mw ∼ 10 Me), where Me denotes the characteristic entanglement molecular weight of linear PEO. For both PEO architectures (ring and linear), the predicted chain center-of-mass self-diffusion coefficients DG as a function of PEO Mw are in remarkable agreement with experimental data. From the flow simulations under shear, we have extracted and analyzed the zero-shear viscosity of ring and linear PEO melts as a function of Mw.

Unscrambling micro-solvation of -COOH and -NH groups in neat dimethyl sulfoxide: insights from 1H-NMR spectroscopy and computational studies.

Dimethyl sulfoxide (DMSO) has a significant, multi-faceted role in medicine, pharmacy, and biology as well as in biophysical chemistry and catalysis. Its physical properties and impact on biomolecular structures still attract major scientific interest, especially the interactions of DMSO with biomolecular functional groups. In the present study, we shed light on the "isolated" carboxylic (-COOH) and amide (-NH) interactions in neat DMSO via1H NMR studies along with extensive theoretical approaches, i.e. molecular dynamics (MD) simulations, density functional theory (DFT), and ab initio calculations, applied on model compounds (i.e. acetic and benzoic acid, ethyl acetamidocyanoacetate). Both experimental and theoretical results show excellent agreement, thereby permitting the calculation of the association constants between the studied compounds and DMSO molecules. Our coupled MD simulations, DFT and ab initio calculations, and NMR spectroscopy results indicated that complex formation is entropically driven and DMSO molecules undergo multiple strong interactions with the studied molecules, particularly with the -COOH groups. The combined experimental and theoretical techniques unraveled the interactions of DMSO with the most abundant functional groups of peptides (i.e. peptide bonds, side chain and terminal carboxyl groups) in high detail, providing significant insights on the underlying thermodynamics driving these interactions. Moreover, the developed methodology for the analysis of the simulation results could serve as a template for future thermodynamic and kinetic studies of similar systems.

Thermophysical properties of imidazolium tricyanomethanide ionic liquids: experiments and molecular simulation.

The low-viscous tricyanomethanide ([TCM](-))-based ionic liquids (ILs) are gaining increasing interest as attractive fluids for a variety of industrial applications. The thermophysical properties (density, viscosity, surface tension, electrical conductivity and self-diffusion coefficient) of the 1-alkyl-3-methylimidazolium tricyanomethanide [Cnmim][TCM] (n = 2, 4 and 6-8) IL series were experimentally measured over the temperature range from 288 to 363 K. Moreover, a classical force field optimized for the imidazolium-based [TCM](-) ILs was used to calculate their thermodynamic, structural and transport properties (density, surface tension, self-diffusion coefficients, viscosity) in the temperature range from 300 to 366 K. The predictions were directly compared against the experimental measurements. The effects of anion and alkyl chain length on the structure and thermophysical properties have been evaluated. In cyano-based ILs, the density decreases with increasing molar mass, in contrast to the behavior of the fluorinated anions, being in agreement with the literature. The contribution per -CH2- group to the increase of the viscosity presents the following sequence: [PF6](-) > [BF4](-) > [Tf2N](-) > [DCA](-) > [TCB](-) > [TCM](-). [TCM](-)-based ILs show lower viscosity than dicyanamide ([DCA](-))- and tetracyanoborate ([TCB](-))-based ILs, while the latter two exhibit a crossover which depends both on temperature and the alkyl chain length of the cation. The surface tension of the investigated ILs decreases with increasing alkyl chain length. [C2mim][TCM] shows an outlier behavior compared to other members of the homologous series. The surface enthalpies and surface entropies for all the studied systems have been calculated based on the experimentally determined surface tensions. The relationship between molar conductivity and viscosity was analyzed using the Walden rule. The experimentally determined self-diffusion coefficients of the cations are in good agreement with the molecular simulation predictions, in which a decrease of the self-diffusion of the cations with increasing alkyl chain length is observed with a simultaneous increase in viscosity and for the longer alkyl lengths the anion becomes more mobile than the cation.

Elucidation of the binding mechanism of renin using a wide array of computational techniques and biological assays.

We investigate the binding mechanism in renin complexes, involving three drugs (remikiren, zankiren and enalkiren) and one lead compound, which was selected after screening the ZINC database. For this purpose, we used ab initio methods (the effective fragment potential, the variational perturbation theory, the energy decomposition analysis, the atoms-in-molecules), docking, molecular dynamics, and the MM-PBSA method. A biological assay for the lead compound has been performed to validate the theoretical findings. Importantly, binding free energy calculations for the three drug complexes are within 3 kcal/mol of the experimental values, thus further justifying our computational protocol, which has been validated through previous studies on 11 drug-protein systems. The main elements of the discovered mechanism are: (i) minor changes are induced to renin upon drug binding, (ii) the three drugs form an extensive network of hydrogen bonds with renin, whilst the lead compound presented diminished interactions, (iii) ligand binding in all complexes is driven by favorable van der Waals interactions and the nonpolar contribution to solvation, while the lead compound is associated with diminished van der Waals interactions compared to the drug-bound forms of renin, and (iv) the environment (H2O/Na(+)) has a small effect on the renin-remikiren interaction.

Probing micro-solvation in "numbers": the case of neutral dipeptides in water.

How many solvent molecules and in what way do they interact directly with biomolecules? This is one of the most challenging questions regarding a deep understanding of biomolecular functionalism and solvation. We herein present a novel NMR spectroscopic study, achieving for the first time the quantification of the directly interacting water molecules with several neutral dipeptides. Our proposed method is supported by both molecular dynamics simulations and density functional theory calculations, advanced analysis of which allowed the identification of the direct interactions between solute-solvent molecules in the zwitterionic L-alanyl-L-alanine dipeptide-water system. Beyond the quantification of dipeptide-water molecule direct interactions, this NMR technique could be useful for the determination and elucidation of small to moderate bio-organic molecular groups' direct interactions with various polar solvent molecules, shedding light on the biomolecular micro-solvation processes and behaviour in various solvents.

Efficient enzymatic preparation of hydroxycinnamates in ionic liquids enhances their antioxidant effect on lipoproteins oxidative modification.

Biocatalytic lipophilization of hydroxycinnamic acids was performed in several BF(4)(-) and PF(6)(-) imidazolium ionic liquids using immobilized lipases. The influence of various reaction parameters on the performance of the biocatalytic process was pointed out, using as model reaction the esterification of ferulic acid. The biocatalytic lipophilization strongly depended on the ion composition of ionic liquids used. Conversions and initial reaction rates were significantly higher in PF(6)(-) as compared with BF(4)(-) ionic liquids and commonly used organic solvents. The high enzyme stability and the relative solubility of substrate versus product in PF(6)(-) ionic liquids can account for the improved synthesis of lipophilic ferulates. These lipophilic derivatives, when used at a concentration of up to 400-fold lower than the parental compound, efficiently inhibited the oxidation of isolated LDL, HDL and total serum in vitro. Moreover, it has been shown for the first time that the lipophilic ferulates improve the antioxidant efficiency of the HDL(3c) towards LDL in vitro oxidation.

Effect of a synthetic peptide corresponding to residues 313 to 320 of the alphaIIb subunit of the human platelet integrin alphaIIbbeta3 on carotid artery thrombosis in rabbits.

The platelet integrin receptor alpha(IIb)beta(3) plays a critical role in thrombosis. We have shown previously that the octapeptide YMESRADR, corresponding to sequences 313 to 320 of the human alpha(IIb) subunit, inhibits human platelet activation and fibrinogen binding to alpha(IIb)beta(3), possibly interacting with the ligand. We investigated the effect of YMESRADR on electrically induced carotid artery thrombosis in New Zealand white rabbits. Peptide was administered via the femoral vein, starting 60 min before and continuing for 90 min after the electrical stimulation. Carotid blood flow was monitored for 90 min after the electrical stimulation. The peptide effects on platelet aggregation, in vitro and ex vivo, and on various coagulation, bleeding, and hemostatic parameters were evaluated. YMESRADR significantly inhibited rabbit platelet aggregation in vitro in a dose-dependent manner. It is important that peptide administration in vivo, at doses ranging from 3 to 15 mg/kg, prolonged the duration of the patency of the carotid artery, and no artery occlusion was observed until the end of the study (90 min after electrical stimulation). Furthermore, YMESRADR administration reduced platelet aggregation ex vivo and thrombus weight; however, these reductions reached statistical significance, compared with the control group, at the peptide doses of 12 and 15 mg/kg. YMESRADR did not affect any coagulation parameter studied and the hemostatic response observed in control animals. Thus, YMESRADR represents a novel antiplatelet agent that can inhibit thrombus formation effectively and carotid artery occlusion without causing hemorrhagic complications in a rabbit model of arterial thrombosis.

Short- and long-term elevation of autoantibody titers against oxidized LDL in patients with acute coronary syndromes. Role of the lipoprotein-associated phospholipase A2 and the effect of atorvastatin treatment.

Oxidized low-density lipoprotein (oxLDL) is immunogenic while oxidized phospholipids (oxPL) formed on oxLDL and lysophosphatidylcholine (lyso-PC) generated during LDL oxidation through the hydrolysis of oxPL by the lipoprotein-associated phospholipase A(2) (Lp-PLA(2)), significantly contribute to oxLDL immunogenicity. We determined the autoantibody titers against various forms of mildly oxLDL in patients with acute coronary syndromes without persistent elevation of the ST segment (NSTE-ACS) and with undetectable serum levels of lipoprotein (a). Moreover, the effect of early atorvastatin administration on these autoantibody titers was evaluated. From the 133 consecutive NSTE-ACS patients, 55 were eligible for the study. Thirty-four received atorvastatin (group A), whereas 21 did not received any hypolipidemic therapy (group B). Two forms of copper-oxidized LDL were prepared at the end of propagation or decomposition phase (oxLDL(P) or oxLDL(D), respectively). Similar types of oxLDL were prepared after previous inactivation of the endogenous Lp-PLA(2) [oxLDL(-)]. In group B, autoantibody titers of IgG class against oxLDL(P) and oxLDL(D) were elevated at 1 month of follow-up to reach the baseline values 3 months afterwards. By contrast the titers against oxLDL(-)(P) and oxLDL(-)(D) increased at 1 month of follow-up and remained elevated for up to 6 months of follow-up. Atorvastatin treatment prevented the elevation of autoantibody titers against all forms of oxidized LDL. We conclude that a short-term immune response against oxLDL(P) and oxLDL(D) (enriched in lyso-PC) and a chronic immune response against oxLDL(-)(P) and oxLDL(-)(D) (enriched in oxPL) are observed after an NSTE-ACS, suggesting an important role of the LDL-associated Lp-PLA(2) in modulating these immune responses. Early atorvastatin treatment prevents both immune responses; however, the clinical significance of this effect remains to be established.

New insights into the role of lipoprotein(a)-associated lipoprotein-associated phospholipase A2 in atherosclerosis and cardiovascular disease.

Lipoprotein(a) [Lp(a)] plays an important role in atherosclerosis. The biological effects of Lp(a) have been attributed either to apolipoprotein(a) or to its low-density lipoprotein-like particle. Lp(a) contains platelet-activating factor acetylhydrolase, an enzyme that exhibits a Ca2+-independent phospholipase A2 activity and is complexed to lipoproteins in plasma; thus, it is also referred to as lipoprotein-associated phospholipase A2. Substrates for lipoprotein-associated phospholipase A2 include phospholipids containing oxidatively fragmented residues at the sn-2 position (oxidized phospholipids; OxPLs). OxPLs may play important roles in vascular inflammation and atherosclerosis. Plasma levels of OxPLs present on apolipoprotein B-100 particles (OxPL/apolipoprotein B) are correlated with coronary artery, carotid, and peripheral arterial disease. Furthermore, OxPL/apolipoprotein B levels in plasma are strongly correlated with Lp(a) levels, are preferentially sequestered on Lp(a), and thus are potentially subjected to degradation by the Lp(a)-associated lipoprotein-associated phospholipase A2. The present review article focuses specifically on the characteristics of the lipoprotein-associated phospholipase A2 associated with Lp(a) and discusses the possible role of this enzyme in view of emerging data showing that OxPLs in plasma are preferentially sequestered on Lp(a) and may significantly contribute to the increased atherogenicity of this lipoprotein.

Autoantibody titers against OxLDL are correlated with Achilles tendon thickness in patients with familial hypercholesterolemia.

Achilles tendon xanthomas are associated with increased cardiovascular risk in patients with familial hypercholesterolemia (FH). Oxidized low density lipoprotein (OxLDL), the antibodies against OxLDL, and the LDL-associated phospholipase A(2) (Lp-PLA(2)) may play important roles in atherogenesis. We investigated the possible association between plasma levels of OxLDL, Lp-PLA(2) activity, and autoantibody titers against various types of mildly OxLDL with Achilles tendon thickness (ATT). ATT was determined by sonography in 80 unrelated heterozygous FH patients. Three different types of mildly OxLDL were prepared: OxLDL(L), OxLDL(P), and OxLDL(D), at the end of the lag, propagation, and decomposition phases of oxidation, respectively. Similar types of OxLDL were also prepared after inactivation of the LDL-associated Lp-PLA(2). These types were denoted OxLDL(-)(L), OxLDL(-)(P), and OxLDL(-)(D). FH patients exhibited significantly higher plasma OxLDL levels and serum IgG titers against OxLDL(P) and OxLDL(D) compared with 40 normolipidemic apparently healthy controls. ATT values were positively correlated with autoantibody titers against OxLDL(P) and OxLDL(D); however, in multiple regression analysis, ATT was independently associated only with the autoantibody titers against OxLDL(D). We conclude that the IgG autoantibody titers against OxLDL(D) but not OxLDL or Lp-PLA(2) may play an important role in the pathogenesis of Achilles tendon xanthomas in FH patients.

Investigation of the role of adjacent amino acids to the 313-320 sequence of the alphaIIb subunit on platelet activation and fibrinogen binding to alphaIIbbeta3.

The platelet integrin receptor alphaIIbbeta3 plays a critical role in thrombosis and haemostasis by mediating interactions between platelets and several ligands, primarily fibrinogen. We have previously shown that the synthetic peptide YMESRADRKLAEVGRVYLFL corresponding to residues 313-332 of alphaIIb, is a potent inhibitor of platelet aggregation and fibrinogen binding to alphaIIbbeta3, interacting with fibrinogen rather than the receptor. Furthermore, we have demonstrated that the biological activities of the above peptide are due to the sequence YMESRADR, which corresponds to residues 313-320. By using new synthetic peptide analogues we investigated the structural characteristics responsible for the biological activity of YMESRADR as well the possible influence of the adjacent amino acids on the peptide's biological potency. According to our results, the synthetic octapeptide YMESRADR, is a potent inhibitor of platelet aggregation and P-selectin expression. Furthermore, YMESRADR inhibits fibrinogen binding but it does not significantly influence the binding of PAC-1 to ADP-activated platelets. The inhibitory potency of YMESRADR was gradually diminished by deleting the YMES sequence from the amino terminus and prolonging the carboxyl terminus of this peptide with the KLAE sequence. Extension of YMESRADR towards the amino terminus with the GAPL sequence (GAPLYMESRADR) does not modify the biological activity of YMESRADR. Furthermore, extension of GAPLYMESRADR at its carboxy terminus with the KLAE sequence (GAPLYMESRADRKLAE) significantly diminished its biological potency. Substitution of E315 with D significantly enhances antiaggregatory potency and completely abolishes the inhibitory effect on P-selectin expression. Importantly, the D315-containing peptides inhibit to a similar extent both fibrinogen and PAC-1 binding to activated alphaIIbbeta3 in contrast to the E315-containing peptide which only inhibits fibrinogen binding. In conclusion, the present study suggests that the YMESRADR sequence 313-320 of alphaIIb, is an important functional region of the insert connecting the beta2 and beta3 antiparallel beta-strands of the W5 blade of the alphaIIb subunit. Structural changes significantly modify the biological properties of this region.

Lipoprotein(a) as a cardiovascular risk factor.

Evidence for the role of lipoprotein(a) (Lp[a]) in atherosclerosis and thrombosis has considerably increased over the past few years. Therefore, Lp(a) is currently classified as an emerging lipid risk factor for cardiovascular disease. High Lp(a) plasma levels carried in particles with small-sized apolipoprotein(a) isoforms are associated with preclinical vascular changes, cardiovascular disease and the mode of presentation of coronary artery disease (acute coronary syndromes). However, randomized clinical trials with an emphasis on agents that specifically lower plasma Lp(a) do not exist. At present, screening for increases in Lp(a) in the general population is not recommended. The measurement of Lp(a) may be of value in individuals with an increased risk of cardiovascular disease, particularly in patients with high low-density lipoprotein cholesterol plasma levels, since a high Lp(a) concentration in such subjects further increases the risk of coronary heart disease.

Reduced PAF-acetylhydrolase activity associated with Lp(a) in patients with coronary artery disease.

Lipoprotein(a) [Lp(a)] may be an independent risk factor for coronary artery disease (CAD). Lp(a) is enriched in platelet activating factor acetylhydrolase (PAF-AH), an enzyme which hydrolyzes and inactivates platelet activating factor (PAF) and oxidized phospholipids that are implicated in atherogenesis. We determined the mass and catalytic properties of the Lp(a)-associated PAF-AH in 28 CAD patients in relation to the LDL-associated enzyme ones. Results were then compared to those of 30 control subjects and 16 unrelated patients with primary hypercholesterolemia (Type IIA dyslipidemia) before and after atorvastatin therapy. The mass, the specific activity and kinetic constants of the Lp(a)-associated PAF-AH were significantly lower in CAD patients compared to those of either controls or hypercholesterolemic patients, a phenomenon not observed for LDL-associated PAF-AH. The enzyme specific activity and kinetic constants were significantly increased after removal of apo(a) from Lp(a) by reductive cleavage, which was not found in the control population, suggesting that the apo(a) moiety of Lp(a) from CAD patients may play an important role in the observed lower catalytic efficiency of PAF-AH. The reduced PAF-AH mass and specific activity on Lp(a) is a feature characteristic of this lipoprotein in CAD patients and may lead to a diminished capability of Lp(a) to degrade proinflammatory phospholipids. The consequences of this phenomenon as regards the pathophysiological role of Lp(a) in atherosclerosis remain to be established.

Effect of lipoprotein (a) on platelet activation induced by platelet-activating factor: role of apolipoprotein (a) and endogenous PAF-acetylhydrolase.

OBJECTIVE: Lipoprotein (a) [Lp(a)] is considered an atherogenic lipoprotein, which is also implicated in thrombosis. Lp(a) binds to platelets and modulates the effects of various platelet agonists. Platelet activating factor (PAF) is a potent platelet agonist degraded and inactivated by PAF-acetylhydrolase (PAF-AH), which in plasma is associated with lipoproteins. Lp(a) is enriched in PAF-AH, thus a functional characteristic of this lipoprotein is its capability to hydrolyze and inactivate PAF. In the present study, we investigated the effect of Lp(a) on PAF-induced platelet activation. The potential roles of the apo(a) moiety and especially of the PAF-AH content of Lp(a) on the above effect were also addressed. METHODS: Lp(a) was isolated by affinity chromatography from plasma of apparently healthy fasting donors with serum Lp(a) concentrations >/=20 mg/dl. Reduced Lp(a) [Lp(a-)] was prepared by incubation of Lp(a) with dithiothreitol (DTT), whereas inactivation of Lp(a)-associated PAF-AH was performed by incubation of Lp(a) with pefabloc [pefa-Lp(a)]. Platelet-rich plasma (PRP) or washed platelets were prepared from peripheral venous blood samples of normolipidemic, apparently healthy fasting donors with their serum Lp(a) levels lower than 0.8 mg/dl. The surface expression of the platelet integrin-receptor alpha(IIb)beta3 and the fibrinogen binding to the activated alpha(IIb)beta3 was studied by flow cytometry. RESULTS: Lp(a), at doses higher than 20 microg/ml, inhibits PAF-induced platelet activation in a dose-dependent manner. Pefa-Lp(a), lacking PAF-AH activity, exhibited a similar to Lp(a) inhibitory effect. Importantly, the Lp(a) inhibitory effect was not influenced by the apo(a) isoform size, whereas Lp(a-) was a more potent inhibitor compared to Lp(a). Similarly to PAF, Lp(a) inhibits platelet aggregation induced by ADP or Calcium ionophore A23187. Lp(a), pefa-Lp(a) or Lp(a-) effectively inhibited PAF- or ADP-induced surface expression of alphaIIbbeta3, the Lp(a-) being more potent compared to Lp(a) or to pefa-Lp(a). Finally, Lp(a) significantly inhibited fibrinogen binding to platelets activated with PAF. CONCLUSIONS: Lp(a) inhibits PAF-induced platelet activation in a non-specific manner. The Lp(a)-associated PAF-AH does not play any important role in this effect, whereas the apo(a) moiety of Lp(a) significantly reduces its inhibitory effect. The inhibition of alpha(IIb)beta3 activation and fibrinogen binding to the activated platelets may represent the major mechanism by which Lp(a) inhibits PAF-induced platelet aggregation.

Lipoprotein (a) levels and apolipoprotein (a) isoform size in patients with subclinical hypothyroidism: effect of treatment with levothyroxine.

The increased risk for ischemic heart disease (IHD) associated with subclinical hypothyroidism (SH) has been partly attributed to dyslipidemia. There is limited information on the effect of SH on lipoprotein (a) [Lp(a)], which is considered a significant predictor of IHD. Serum Lp(a) levels are predominantly regulated by apolipoprotein [apo(a)] gene polymorphisms. The aim of our study was to evaluate the Lp(a) levels and apo(a) phenotypes in patients with SH compared to healthy controls as well as the influence of levothyroxine substitution therapy on Lp(a) values in relation to the apo(a) isoform size. Lp(a) levels were measured in 69 patients with SH before and after restoration of a euthyroid state and in 83 age- and gender-matched healthy controls. Apo(a) isoform size was determined by sodium dodecyl sulfate (SDS) agarose gel electrophoresis followed by immunoblotting and development via chemiluminescence. Patients with SH exhibited increased Lp(a) levels compared to controls (median value 10.6 mg/dL vs. 6.0 mg/dL, p = 0.003]), but this was not because of differences in the frequencies of apo(a) phenotypes. There was no association between thyrotropin (TSH) and Lp(a) levels in patients with SH. In subjects with either low (LMW; 25 patients and 28 controls) or high (HMW; 44 patients and 55 controls) molecular weight apo(a) isoforms, Lp(a) concentrations were higher in patients than in the control group (median values 26.9 mg/dL vs. 21.8 mg/dL, p = 0.02 for LMW, and 6.0 mg/dL versus 3.3 mg/dL, p < 0.001 for HMW). Levothyroxine treatment resulted in an overall reduction of Lp(a) levels (10.6 mg/dL baseline vs. 8.9 mg/dL posttreatment, p = 0.008]). This effect was mainly evident in patients with LMW apo(a) isoforms associated with high baseline Lp(a) concentrations (median values 26.9 mg/dL vs. 23.2 mg/dL pretreatment and posttreatment, respectively; p = 0.03). In conclusion, even though a causal effect of thyroid dysfunction on Lp(a) was not clearly demonstrated in patients with SH, levothyroxine treatment is beneficial, especially in patients with increased baseline Lp(a) levels and LMW apo(a) isoforms.