Human low density lipoprotein as a target of hypochlorite generated by myeloperoxidase

Free Radical Lipid Peroxidation Induced by Reactive Halogen Species

The review is devoted to the mechanisms of free radical lipid peroxidation (LPO) initiated by reactive halogen species (RHS) produced in mammals, including humans, by heme peroxidase enzymes, primarily myeloperoxidase (MPO). It has been shown that RHS can participate in LPO both in the initiation and branching steps of the LPO chain reactions. The initiation step of RHS-induced LPO mainly involves formation of free radicals in the reactions of RHS with nitrite and/or with amino groups of phosphatidylethanolamine or Lys. The branching step of the oxidative chain is the reaction of RHS with lipid hydroperoxides, in which peroxyl and alkoxyl radicals are formed. The role of RHS-induced LPO in the development of human inflammatory diseases (cardiovascular and neurodegenerative diseases, cancer, diabetes, rheumatoid arthritis) is discussed in detail.

Oxidatively modified low-density lipoproteins are potential mediators of proteasome inhibitor resistance in multiple myeloma

Proteasome inhibitor (PI) therapy has improved the survival of multiple myeloma (MM) patients. However, inevitably, primary or acquired resistance to PIs leads to disease progression; resistance mechanisms are unclear. Obesity is a risk factor for MM mortality. Oxidized LDL (OxLDL), a central mediator of atherosclerosis that is elevated in metabolic syndrome (co-occurrence of obesity, insulin resistance, dyslipidemia and hypertension), has been linked to an increased risk of solid cancers and shown to stimulate pro-oncogenic/survival signaling. We hypothesized that OxLDL is a mediator of chemoresistance and evaluated its effects on MM cell killing by PIs. OxLDL potently suppressed the ability of the boronic acid-based PIs bortezomib (BTZ) and ixazomib, but not the epoxyketone-based PI carfilzomib, to kill human MM cell lines and primary cells. OxLDL suppressed BTZ-induced inhibition of proteasome activity and induction of pro-apoptotic signaling. These cytoprotective effects were abrogated when lipid hydroperoxides (LOOHs) associated with OxLDL were enzymatically reduced. We also demonstrated the presence of OxLDL in the MM bone marrow microenvironment as well as numerous granulocytes and monocytes capable of cell-mediated LDL oxidation through myeloperoxidase. Our findings suggest that OxLDL may be a potent mediator of boronic acid-based PI resistance, particularly for MM patients with metabolic syndrome, given their elevated systemic levels of OxLDL. LDL cholesterol-lowering therapy to reduce circulating OxLDL, and pharmacologic targeting of LOOH levels or resistance pathways induced by the modified lipoprotein, could deepen the response to these important agents and offer clinical benefit to MM patients with metabolic syndrome.

The Role of Halogenative Stress in Atherogenic Modification of Low-Density Lipoproteins

This review discusses formation of reactive halogen species (RHS) catalyzed by myeloperoxidase (MPO), an enzyme mostly present in leukocytes. An imbalance between the RHS production and body's ability to remove or neutralize them leads to the development of halogenative stress. RHS reactions with proteins, lipids, carbohydrates, and antioxidants in the content of low-density lipoproteins (LDLs) of the human blood are described. MPO binds site-specifically to the LDL surface and modifies LDL properties and structural organization, which leads to the LDL conversion into proatherogenic forms captured by monocytes/macrophages, which causes accumulation of cholesterol and its esters in these cells and their transformation into foam cells, the basis of atherosclerotic plaques. The review describes the biomarkers of MPO enzymatic activity and halogenative stress, as well as the involvement of the latter in the development of atherosclerosis.

Tripping up Trp: Modification of protein tryptophan residues by reactive oxygen species, modes of detection, and biological consequences

Proteins comprise a majority of the dry weight of a cell, rendering them a major target for oxidative modification. Oxidation of proteins can result in significant alterations in protein molecular mass such as breakage of the polypeptide backbone and/or polymerization of monomers into dimers, multimers, and sometimes insoluble aggregates. Protein oxidation can also result in structural changes to amino acid residue side chains, conversions that have only a modest effect on protein size but can have widespread consequences for protein function. There are a wide range of rate constants for amino acid reactivity, with cysteine, methionine, tyrosine, phenylalanine, and tryptophan having the highest rate constants with commonly encountered biological oxidants. Free tryptophan and tryptophan protein residues react at a diffusion-limited rate with hydroxyl radical and also have high rate constants for reactions with singlet oxygen and ozone. Although oxidation of proteins in general and tryptophan residues specifically can have effects detrimental to the health of cells and organisms, some modifications are neutral, whereas others contribute to the function of the protein in question or may act as a signal that damaged proteins need to be replaced. This review provides a brief overview of the chemical mechanisms by which tryptophan residues become oxidized, presents both the strengths and the weaknesses of some of the techniques used to detect these oxidative interactions, and discusses selected examples of the biological consequences of tryptophan oxidation in proteins from animals, plants, and microbes.

Nicotine-enhanced oxidation of low-density lipoprotein and its components by myeloperoxidase/H2O2/Cl- system

In this study, the effect of nicotine on the LDL oxidation by the MPO/H2O2/Cl- system and the effect of HOCl on LDL and some of its components, such as methyl linoleate, vitamin E and the amino acid tryptophan were explored. Nicotine, in micromolar concentrations, enhanced the tryptophan oxidation, either present in LDL or free, in solution. Nicotine also decreased the formation of conjugated dienes and oxygen consumption in a methyl linoleate / HOCl system, and there was evidence to suggest an increase in chlorohydrin formation. Acceleration of the vitamin E oxidation by HOCl was also observed in the presence of nicotine. These data show that the interaction of nicotine and HOCl can promote significant biochemical modifications in LDL particle and some of its components involved in the pathogenesis of cardiovascular and other diseases.

LOX-1: a multiligand receptor at the crossroads of response to danger signals

PURPOSE OF REVIEW

LOX-1 is a multiligand receptor implicated in endothelial dysfunction and atherosclerosis, although it was originally identified as an oxidized LDL receptor. In this review, the roles of various LOX-1 ligands and their interaction with LOX-1 are discussed to understand the pathophysiological significance of LOX-1.

RECENT FINDINGS

LOX-1 knockout mice showed resistance of endothelium-dependent vasorelaxation against oxidized LDL and retardation of atherosclerosis progression. LOX-1 ligand reduction in mice also attenuated atherosclerosis progression. In a human cohort study, high concentration of apoB-containing LOX-1 ligands predicted the incidence of cardiovascular disease. Furthermore, modified HDL, which existed in high concentration in the plasma of coronary artery disease patients, was found to induce impairment of endothelial nitric oxide release via LOX-1. In addition to lipoproteins, LOX-1 was found to work as a C-reactive protein receptor providing a scaffold for the activation of the complement system.

SUMMARY

LOX-1 is a unique molecule among the sensors of danger signals. LOX-1 is not only sensing danger signals such as modified LDL and heat shock protein, but also scaffolding other danger sensors including C-reactive protein and C1q, and directly commanding responses to danger signals by working as a cell adhesion molecule. Via these functions, LOX-1 might work as a surveillance molecule of vascular homeostasis.

Free and Peptide-bound DOPA Can Inhibit Initiation of Low Density Lipoprotein Oxidation

Hydroxyl radicals have been shown to convert free tyrosine to 3,4-dihydroxyphenyl-alanine (DOPA) which has reducing properties. During protein or peptide oxidation such reducing species are also formed from tyrosine residues. Free DOPA or peptide-bound DOPA (PB-DOPA) is able to promote radical-generating events, facilitating the damage of biomolecules such as nucleic acids. Radical induced lipid oxidation in low density lipoprotein (LDL) transforms the lipoprotein into an atherogenic particle. As PB-DOPA has been found in atherosclerotic plaques, we tested the ability of free and PB-DOPA to influence LDL oxidation. Free DOPA, in contrast to tyrosine had strong inhibitory action on both, the copper-ion initiated and metal ion independent (AAPH-induced) lipid oxidation. Free DOPA also inhibited LDL oxidation induced by the copper transport protein ceruloplasmin. To test if PB-DOPA was also able to inhibit LDL oxidation, DOPA residues were generated enzymatically in the model peptides insulin and tyr-tyr-tyr, respectively. PB-DOPA formation substantially increased the ability of both molecules to inhibit LDL oxidation by copper or AAPH. We hypothesize that DOPA-peptides and -proteins may have the potential to act as efficacious antioxidants in the atherosclerotic plaque.

Cholesterol, Linoleic Acid or/and Tyrosine Yield Different Spectra of Products when Oxidized Alone or in a Mixture: Studies in Various Oxidative Systems

Identification of reliable biomarkers for oxidative stress for the prediction of the early development of pathological conditions is essential. The detection of biomarkers for oxidative stress such as degradation products of polyunsaturated fatty acid (PUFA), oxysterols, and oxidized proteins, as indicators of oxidative stress are in use, but suffers from insufficient specificity, accuracy and reliability. The overall aim of the present study was to develop new markers which will not only provide information about the presence and level of oxidative stress in biological systems but also on the type of reactive oxygen species (ROS) involved and their metabolic consequences. In the first stage of the study, we compared the level and type of oxidized products formed when different ROS were applied onto three major biomolecules, i.e. cholesterol, linoleic acid (LH) and tyrosine, representing sterols, PUFA and protein, when each compounds was exposed alone or in a mixture to the ROS [copper ions, 2,2-azobis(2-amidinopropane) dihydrochloride (AAPH) and hypochlorous acid (HOCl)]. It was found that different types of oxidants resulted in the formation of different types of oxidation products. Furthermore, oxidation pattern differs when the substrates (cholesterol, PUFA or amino acid) were present alone or in a mixture. As biological systems such as lipoproteins and cell membranes are composed of the above studied molecules, the need for simultaneous detection of the major oxidized products is requires for better characterization of the oxidative stress outcome.

High-density lipoproteins, inflammation and oxidative stress

Plasma levels of HDL (high-density lipoprotein)-cholesterol are strongly and inversely correlated with atherosclerotic cardiovascular disease. Both clinical and epidemiological studies have reported an inverse and independent association between serum HDL-cholesterol levels and CHD (coronary heart disease) risk. The cardioprotective effects of HDLs have been attributed to several mechanisms, including their involvement in the reverse cholesterol transport pathway. HDLs also have antioxidant, anti-inflammatory and antithrombotic properties and promote endothelial repair, all of which are likely to contribute to their ability to prevent CHD. The first part of this review summarizes what is known about the origins and metabolism of HDL. We then focus on the anti-inflammatory and antioxidant properties of HDL and discuss why these characteristics are cardioprotective.

Triggering of inflammatory response by myeloperoxidase-oxidized LDL

The oxidation theory proposes that LDL oxidation is an early event in atherosclerosis and that oxidized LDL contributes to atherogenesis in triggering inflammation. In contrast to the copper-modified LDL, there are few studies using myeloperoxidase-modified LDL (Mox-LDL) as an inflammation inducer. Our aim is to test whether Mox-LDL could constitute a specific inducer of the inflammatory response. Albumin, which is the most abundant protein in plasma and which is present to an identical concentration of LDL in the intima, was used for comparison. The secretion of IL-8 by endothelial cells (Ea.hy926) and TNF-alpha by monocytes (THP-1) was measured in the cell medium after exposure of these cells to native LDL, native albumin, Mox-LDL, or Mox-albumin. We observed that Mox-LDL induced a 1.5- and 2-fold increase (ANOVA; P < 0.001) in IL-8 production at 100 microg/mL and 200 microg/mL, respectively. The incubation of THP-1 cells with Mox-LDL (100 microg/mL) increased the production of TNF-alpha 2-fold over the control. Native LDL, albumin, and Mox-albumin showed no effect in either cellular types. The myeloperoxidase-modified LDL increase in cytokine release by endothelial and monocyte cells and by firing both local and systemic inflammation could induce atherogenesis and its development.

Antioxidant activity of cleomiscosins A and C isolated fromAcer okamotoanum

Phytochemical investigation of Acer okamotoanum leaf and twig led to the isolation of two coumarinolignans, cleomiscosin A (1) and cleomiscosin C (2). Here, we found that 2 dose-dependently inhibits LDL oxidation mediated by either catalytic copper ions (Cu2+) or free radicals generated with the azo compound 2,2'-azobis-(2-amidinopropane)dihydro-chloride (AAPH) with IC50s of 29.5 and 11.9 microM, respectively. By electrophoretic analysis, we also observed that 2 protects apolipoprotein B-100 (apoB-100) against Cu2+-induced fragmentation (65.3% inhibition at 5 microM). Furthermore, fluorescence analyses clearly indicated that both 1 and 2 protect against the oxidative modification of apoB-100 induced by either Cu2+ or HOCl (1, IC50s of 13.4 and 8.1 microM, respectively; 2, IC50s of 23.6 and 3.9 microM, respectively). These findings suggest that 1 and 2 could be beneficial in preventing LDL oxidation in atherosclerotic lesions.

Modified Low-Density Lipoproteins and High-Density Lipoproteins

It has long been known that the oxidative state of the various plasma lipoproteins modulates platelet aggregability, thereby contributing to atherogenesis. Low-density lipoprotein (LDL), occurring in vivo both in the native and oxidised forms, interacts directly with platelets, by binding to specific receptors. While the identity of the receptors for native LDL and some subfractions of high-density lipoproteins (HDL) remains disputed, apoE-containing HDL(2) binds to LRP8. The nature of these interactions as well as the distinction between candidate receptor proteins was elucidated using covalently modified apolipoproteins, which pointed to the participation of apolipoproteins in high affinity binding. However, the platelet effects initiated by binding of native lipoproteins remain controversial. Some of this ambiguity can be traced to the fact that native LDL inevitably undergoes substantial oxidisation upon modification, including by radiolabelling. The platelet-activating effects provoked by oxidised LDL are irrefutable, but many details remain unknown. The role of CD36 in platelet binding by oxidised LDL is well established, although additional receptors may exist. Much less is known about the interaction of oxidised HDL with platelets, since platelet activation was observed in some, but not all studies. Various frequently applied in vitro oxidation methods produce modified lipoprotein species that may not be relevant in vivo. Based on the reported modifications obtained by in vitro oxidation of LDL, early investigations focused mainly on the formation and the eventual effects of oxidised lipids. More recently, alterations to lipoproteins performed using hypochloric acid and myeloperoxidase redirected the attention to the role of modified apoproteins in triggering platelet responses.

Modification of low-density lipoprotein by myeloperoxidase-derived oxidants and reagent hypochlorous acid

Substantial evidence supports the notion that oxidative processes contribute to the pathogenesis of atherosclerosis and coronary heart disease. The nature of the oxidants that give rise to the elevated levels of oxidised lipids and proteins, and decreased levels of antioxidants, detected in human atherosclerotic lesions are, however, unclear, with multiple species having been invoked. Over the last few years, considerable data have been obtained in support of the hypothesis that oxidants generated by the heme enzyme myeloperoxidase play a key role in oxidation reactions in the artery wall. In this article, the evidence for a role of myeloperoxidase, and oxidants generated therefrom, in the modification of low-density lipoprotein, the major source of lipids in atherosclerotic lesions, is reviewed. Particular emphasis is placed on the reactions of the reactive species generated by this enzyme, the mechanisms and sites of damage, the role of modification of the different components of low-density lipoprotein, and the biological consequences of such oxidation on cell types present in the artery wall and in the circulation, respectively.

Oxidative stress markers after laparoscopic and open cholecystectomy

BACKGROUND

Surgical injury is associated with oxidative stress, often due to ischemia/reperfusion injury. During laparoscopy, increased intra-abdominal pressure caused by pneumoperitoneum may cause splanchnic ischemia followed by reperfusion due to deflation. We measured several markers of oxidative stress in patients undergoing laparoscopic cholecystectomy (LC) versus open cholecystectomy (OC) to find if these surgical procedures result in different patterns of oxidative stress.

METHODS

This prospective study enrolled 43 patients with symptomatic cholelithiasis, of whom 21 underwent open, and 22 laparoscopic, cholecystectomy. Twenty healthy adults comprised the control group. Total antioxidant status (TAS), superoxide dismutase (SOD), endogenous peroxide level (POX), oxidized low density lipoprotein (oLDL) autoantibodies (oLAb), and neopterin were measured preoperatively and on postoperative days 1, 3, and 7.

RESULTS

POX values decreased significantly on postoperative day 1 in the OC (P<0.01), but not in the LC, group. On postoperative day 7, POX values were higher than preoperatively in both groups (P<0.01) with no difference between the LC and OC groups. Significant postoperative elevations of oLAb and neopterin levels were observed only on postoperative day 7 in the OC group. There were no changes of oLAb and neopterin levels in the LC group. TAS and SOD levels did not change after either LC or OC.

CONCLUSION

Cholecystectomy, either open or laparoscopic, caused only moderate oxidative stress. Open cholecystectomy caused changes of oLAb and neopterin, suggesting more severe oxidative stress, and a possible role of reactive oxygen species in the healing of the laparotomic wound.

A colorimetric method for detection and quantification of chlorinating activity of hemeperoxidases

A method of detecting and assaying the halogenating activity of hemeperoxidases using the colored substrate, thionin, is reported here. In the presence of suitable amounts of peroxide and chloride, chloroperoxidase chlorinates thionin and bleaches the intense color of this substrate. The kinetics was quite similar to that of the established monochlorodimedone assay. Therefore, the thionin assay can be taken as an index of chlorinating activity. This reaction affords an escape from background problems and allows easy processing of large volumes of samples.

Hypochlorous acid scavenging properties of local Mediterranean plant foods

Oxidative modification of low density lipoprotein (LDL) is involved in the pathogenesis of atherosclerosis and coronary heart disease, which are low in the Mediterranean area possibly due to a high dietary proportion of plant foods. Ethanolic extracts were prepared from more than 120 Mediterranean edible plants collected in remote areas (which maintain their traditional diet) and their antioxidant potential was studied. Extracts derived from Agaricus campestris, Cynara cardunculus, Thymus pulegioides, and Vicia faba were subjected to further analysis in this study. The extracts' potential to scavenge the DPPH radical (2,2-diphenyl-1-picrylhydrazyl radical) and hypochlorous acid (HOCl), as well as their antioxidant capacity, was comparable to the those obtained for standard antioxidants (e.g., quercetin, Trolox). Myeloperoxidase (MPO) catalyzes the production of the highly chlorinating and oxidizing agent HOCl, which reacts with the LDL apoprotein moiety, leading to the derivatization of its aminoacidic residues. Coincubation with extracts significantly prevented HOCl-induced modification of the LDL residue tryptophan, whereas higher concentrations were required to retard lysine damage. Moreover, the extracts inhibited MPO-catalyzed guaiacol oxidation in a concentration-dependent manner in a cell-free assay but, in contrast, did not affect MPO activity in isolated human neutrophils. MPO is also known to facilitate nitric dioxide oxidation. The formation of 3-nitrotyrosine was significantly lower in bovine endothelial aortic cells incubated with C. cardunculus or T. pulegioides. In synthesis, our study shows that local Mediterranean plant foods prevent HOCl toxicity in vitro and, thus, suggests further mechanisms responsible for the reported health-beneficial effect of the Mediterranean diet.

Estrogen, neutrophils and oxidation

The potential role of estrogens in the prevention of cardiovascular disease (CVD) is still under debate. Previous studies from our laboratory have shown that estradiol may act as a pro oxidant at physiological concentrations, enhancing peroxidase-mediated oxidation of low density lipoprotein (LDL). In the present study, we show that physiological concentrations of estradiol enhance fMLP-mediated neutrophil degranulation and oxidative stress markers. For example, 10 nM estradiol increased myeloperoxidase (MPO), elastase, and superoxide release by 19.9 +/- 9.6% (p = 0.006), 16.3 +/- 5.2% (p = 0.09), and 36.1 +/- 19.5% (p = 0.05), respectively. The enhancement of neutrophil degranulation by estradiol resulted in an increase in the formation of LDL oxidation markers such as conjugated dienes and thiobarbituric acid-reactive substances (20.7 +/- 7.2%, p = 0.04). Thus, estradiol can act as a pro oxidant, promoting neutrophil degranulation as well as reacting with MPO to enhance the oxidation of LDL. This mechanism supports our hypothesis that oxidative stress may be beneficial towards the prevention of CVD both by promoting plasma oxidation of LDL, with its subsequent clearance by the liver, as well as by inducing a threshold antioxidant defense in the arteries. Our study also suggests that estradiol by promoting oxidation in the plasma is beneficial in preventing CVD.

Class B scavenger receptors CD36 and SR-BI are receptors for hypochlorite-modified low density lipoprotein

The presence of HOCl-modified epitopes inside and outside monocytes/macrophages and the presence of HOCl-modified apolipoprotein B in atherosclerotic lesions has initiated the present study to identify scavenger receptors that bind and internalize HOCl-low density lipoprotein (LDL). The uptake of HOCl-LDL by THP-1 macrophages was not saturable and led to cholesterol/cholesteryl ester accumulation. HOCl-LDL is not aggregated in culture medium, as measured by dynamic light scattering experiments, but internalization of HOCl-LDL could be inhibited in part by cytochalasin D, a microfilament disrupting agent. This indicates that HOCl-LDL is partially internalized by a pathway resembling phagocytosis-like internalization (in part by fluid-phase endocytosis) as measured with [14C]sucrose uptake. In contrast to uptake studies, binding of HOCl-LDL to THP-1 cells at 4 degrees C was specific and saturable, indicating that binding proteins and/or receptors are involved. Competition studies on THP-1 macrophages showed that HOCl-LDL does not compete for the uptake of acetylated LDL (a ligand to scavenger receptor class A) but strongly inhibits the uptake of copper-oxidized LDL (a ligand to CD36 and SR-BI). The binding specificity of HOCl-LDL to class B scavenger receptors could be demonstrated by Chinese hamster ovary cells overexpressing CD36 and SR-BI and specific blocking antibodies. The lipid moiety isolated from the HOCl-LDL particle did not compete for cell association of labeled HOCl-LDL to CD36 or SR-BI, suggesting that the protein moiety of HOCl-LDL is responsible for receptor recognition. Experiments with Chinese hamster ovary cells overexpressing scavenger receptor class A, type I, confirmed that LDL modified at physiologically relevant HOCl concentrations is not recognized by this receptor.

Role of oxidized low-density lipoprotein in renal disease

Accelerated atherosclerosis is often observed in patients with chronic renal failure. In the present review we summarize and discuss the recent literature on the pathogenic role of low-density lipoproteins modified by oxidative processes in atherosclerosis and the possible role in renal diseases. Pathogenetically, the oxidation of low-density lipoproteins is considered to be a key event in the development of atherosclerosis, in part by causing enhanced uptake of lipids by macrophages. In addition, oxidation of low-density lipoproteins exerts cytotoxic, proinflammatory and immunogenic properties, all of which could potentially contribute to the development and progression of atherosclerosis.

An improved method for the sensitive monitoring of low density lipoprotein modification by myeloperoxidase

The aim of this investigation was to compare an improved fluorometric method with an UV absorbance assay for their ability to monitor low density lipoprotein (LDL) modification by myeloperoxidase (MPO) and to evaluate determining factors influencing the modification of LDL. Using absorbance at 234 nm to study the kinetics of LDL aggregation, and a native fluorescence assay for protein oxidation, we found that all components of the MPO/H2O2/Cl- system may have rate determining effects on LDL modification. While the lipoprotein modification rate correlated positively with enzyme concentration, variation of the concentration of H2O2 had a biphasic effect on the maximal rate of LDL modification with both methods. Furthermore, a positive association was found between the maximal rate of LDL modification and the acidity of the medium, with a pathophysiologically relevant optimal rate at a slightly acidic pH of 5-6, but hardly any modification above pH 6.8. In summary, both methods provide simple and useful tools for the continuous monitoring of LDL modification by the MPO/H2O2/Cl- system, but the more sensitive fluorometric method is preferable, since it allows the application of experimental conditions which are much closer to the situation in vivo.

Growing significance of myeloperoxidase in non-infectious diseases

Myeloperoxidase (MPO) is a glycoprotein released by activated polymorphonuclear neutrophils, which takes part in the defense of the organism through production of hypochlorous acid (HOCl), a potent oxidant. Since the discovery of MPO deficiency, initially regarded as rare and restricted to patients suffering from severe infections, MPO has attracted clinical attention. The development of new technologies allowing screening for this defect has permitted new advances in the comprehension of underlying mechanisms. Apart from its implications for host defense, the expression of MPO restricted to myeloid precursors makes MPO mRNA a good marker of acute myeloid leukemia. In addition, during the last few years, involvement of MPO has been described in numerous diseases such as atherosclerosis, lung cancer, Alzheimer's disease and multiple sclerosis. Both strong oxidative activity and MPO genetic polymorphism have been involved. This review summarizes the broad range of diseases involving MPO and points out the possible use of this protein as a new clinical marker and a future therapeutic target.

Kinetics of lipid peroxidation in mixtures of HDL and LDL, mutual effects

In view of the proposed central role of LDL oxidation in atherogenesis and the established role of HDL in reducing the risk of atherosclerosis, several studies were undertaken to investigate the possible effect of HDL on LDL peroxidation. Since these investigations yielded contradictory results, we have conducted systematic kinetic studies on the oxidation in mixtures of HDL and LDL induced by different concentrations of copper, 2, 2'-azo bis (2-amidinopropane) hydrochloride (AAPH) and myeloperoxidase (MPO). These studies revealed that oxidation of LDL induced either by AAPH or MPO is inhibited by HDL under all the studied conditions, whereas copper-induced oxidation of LDL is inhibited by HDL at low copper/lipoprotein ratio but accelerated by HDL at high copper/lipoprotein ratio. The antioxidative effects of HDL are only partially due to HDL-associated enzymes, as indicated by the finding that reconstituted HDL, containing no such enzymes, inhibits peroxidation induced by low copper concentration. Reduction of the binding of copper to LDL by competitive binding to the HDL also contributes to the antioxidative effect of HDL. The acceleration of copper-induced oxidation of LDL by HDL may be attributed to the hydroperoxides formed in the "more oxidizable" HDL, which migrate to the "less oxidizable" LDL and enhance the oxidation of the LDL lipids induced by bound copper. This hypothesis is supported by the results of experiments in which native LDL was added to oxidizing lipoprotein at different time points. When the native LDL was added prior to decomposition of the hydroperoxides in the oxidizing lipoprotein, the lag preceding oxidation of the LDL was much shorter than the lag observed when the native LDL was added at latter stages, after the level of hydroperoxides became reduced due to their copper-catalyzed decomposition. The observed dependence of the interrelationship between the oxidation of HDL and LDL on the oxidative stress should be considered in future investigations regarding the oxidation of lipoprotein mixtures.

Effects of hypochlorous acid on unsaturated phosphatidylcholines

Effects of hypochlorous acid and of the myeloperoxidase-hydrogen peroxide-chloride system on mono- and polyunsaturated phosphatidylcholines were analyzed by means of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). Chlorohydrins and glycols were detected as main products according to the characteristic shift of molecular masses. Mainly mono-chlorohydrins result upon the incubation of HOCl/(-)OCl with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, whereas only traces of mono-glycols were detected. 1-Palmitoyl-2-linoleoyl-sn-glycero-3-phosphocholine yielded a complex mixture of products. Mono-chlorohydrins and glycols dominated only at short incubation, while bis-chlorohydrins as well as products containing one chlorohydrin and one glycol moiety appeared after longer incubation. Similarly, a complex product mixture resulted upon incubation of 1-stearoyl-2-arachidonoyl-sn-glycero-3-phosphocholine with hypochlorous acid. Additionally, tris-chlorohydrins, products with two chlorohydrin and one glycol moiety, as well as lysophosphatidylcholines and fragmentation products of the arachidonoyl side chain were detectable. Mono-chlorohydrins of 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine were detected after the incubation of the latter phospholipid with the myeloperoxidase-hydrogen peroxide-chloride system at pH 6.0. These chlorohydrins were not observed in the absence of chloride, hydrogen peroxide, or myeloperoxidase as well as in the presence of methionine, taurine, or sodium azide. Thus, mono-chlorohydrins in 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine produced by hypochlorous acid from the myeloperoxidase-hydrogen peroxide-chloride system can also be detected by means of MALDI-TOF MS.

Hypochlorite induces the formation of LDL(-), a potentially atherogenic low density lipoprotein subspecies

Oxidation of low density lipoprotein (LDL) induced by hypochlorous acid (HOCl) leading to LDL(-), a minimally oxidized subspecies of LDL, was investigated. LDL(-) is characterized by its greater electronegativity and oxidative status, and is found in plasma in vivo. Its concentration was found to be elevated under conditions that predispose humans to atherosclerosis. We found that HOCl also converts LDL rapidly to an even more oxidized state, identified as LDL(2-), which is more electronegative than LDL(-). After milder oxidation for short durations, formation of LDL(-) takes place while less LDL(2-) is formed. Under these conditions, addition of methionine not only suppressed further oxidation of LDL but also favored the formation of LDL(-) over LDL(2-), possibly by removing chloramines at lysyl residues of LDL. The presence of lipoprotein-deficient plasma did not prevent HOCl-mediated conversion of LDL to more electronegative species. It is concluded that the HOCl-mediated conversion of LDL into more electronegative species might be physiologically relevant.

Therapeutic potential of dietary phase 2 enzyme inducers in ameliorating diseases that have an underlying inflammatory component

Many diseases associated with ageing have an underlying oxidative stress and accompanying inflammatory component, for example, Alzheimer's disease or atherosclerosis. Reviewed in this manuscript are: the role of oxidative stress in activating the transcription factor nuclear factor kappa B (NFκB), the role of NFκB in activating pro-inflammatory gene transcription, strong oxidants produced by cells, anti-oxidant defense systems, the central role of phase 2 enzymes in the anti-oxidant defense, dietary phase 2 enzyme inducers and evidence that dietary phase 2 enzymes decrease oxidative stress. It is likely that a diet containing phase 2 enzyme inducers may ameliorate or even prevent diseases that have a prominent inflammatory component to them. Research should be directed into the potential therapeutic effects of dietary phase 2 enzyme inducers in ameliorating diseases with an underlying oxidative stress and inflammatory component to them.Key words: Alzheimer's disease, atherosclerosis, diet, glutathione, inflammation, stroke.

Kinetics of chlorination of monochlorodimedone by myeloperoxidase

The phagocyte-derived enzyme myeloperoxidase has been recently implicated in the pathogenesis of atherosclerosis, because it catalyzes the reaction of hydrogen peroxide with chloride ions to give the highly toxic oxidant hypochlorous acid. The aim of this study was to determine the dependence of this reaction on the concentration of hydrogen peroxide and of the enzyme by means of the photometric monochlorodimedone assay. The initial rate of hypochlorous acid formation increased less than proportionally with increasing myeloperoxidase concentrations. Variation of the concentration of hydrogen peroxide had a biphasic effect, with an optimal concentration of hydrogen peroxide. Above this concentration enzyme destruction is apparently predominant. The progress curves of hypochlorous acid formation showed two distinct maxima. It was concluded that hypochlorous acid not only reacts with monochlorodimedone but also with the amino groups of myeloperoxidase to form intermediary chloramines that may further chlorinate monochlorodimedone. This was supported by the kinetics in the presence of the amino compound glycine, a competitive substrate for chlorination by hypochlorous acid. In the presence of high concentrations of glycine the progress curve rises continuously, yielding a greatly increased concentration of chlorinating species, either hypochlorous acid or chloramines. We concluded that glycine protects myeloperoxidase against hypochlorous acid-induced self-destruction.

Role of oxidized low-density lipoprotein in the atherosclerosis of uremia

Lipoprotein oxidation is involved in the genesis of atherosclerosis. In chronic renal failure (CRF), oxidative stress is enhanced because of an imbalance between pro-oxidant and antioxidant systems. Oxidative modifications of low-density lipoproteins (LDLs) occur not only at the level of lipid moiety, but also of protein moiety. We have shown that oxidation of LDL by hypochlorous acid (HOCl) in vitro, reflecting increased myeloperoxidase activity in vivo, leads to modifications of apoliproteins such that the latter in turn are capable of triggering macrophage nicotinamide adenine dinucleotide phosphate-oxidase activation. These oxidative changes of LDL protein moiety, if shown to occur to a significant extent in uremic patients in vivo, may represent an important alternative pathway in the pathogenesis of atheromatous lesions.

Interactions between nitric oxide and lipid oxidation pathways: implications for vascular disease

Nitric oxide ((.)NO) signaling pathways and lipid oxidation reactions are of central importance in both the maintenance of vascular homeostasis and the progression of vascular disease. Because both of these pathways involve free radical species that can also react together at extremely fast rates, convergent interactions between these pathways are expected. Biochemical and cell biology studies have defined multiple interactions of (.)NO with oxidizing lipids that could lead to either vascular protection or potentiation of inflammatory vascular injury. For example, low levels of (.)NO generated by endothelial nitric oxide synthase can terminate propagating lipid radicals and inhibit lipoxygenases, reactions that would be protective. Alternatively, if generated at elevated levels, for example, after inducible nitric oxide synthase expression in inflammation, (.)NO can be converted to prooxidant species, such as peroxynitrite (ONOO(-)) and nitrogen dioxide ((.)NO(2)), that can potentiate inflammatory injury to vascular cells. Finally, both enzymatic and nonenzymatic lipid oxidation reactions can influence (.)NO bioactivity by directly scavenging (.)NO or altering the induction and catalytic activity of nitric oxide synthase enzymes. In this review, we summarize the biochemical interactions between (.)NO and lipid oxidation reactions and discuss the recognized and potential roles of these reactions in the vasculature.

Comparison of HOCl traps with myeloperoxidase inhibitors in prevention of low density lipoprotein oxidation

In this study, the production of the highly toxic oxidant hypochlorous acid (HOCl) by the phagocytic enzyme myeloperoxidase (MPO) was quantitated and the concomitant alterations of low density lipoprotein (LDL) were analyzed in view of the potential role of LDL in atherosclerosis. Using the monochlorodimedone assay, it was found that HOCl is produced in micromolar concentrations. The kinetics of the decrease of tryptophan fluorescence appeared to be a sensitive method to monitor LDL alterations under near in vivo conditions. Therefore, this method was used to subsequently compare the effectiveness of MPO inhibitors that block production of HOCl with compounds that act as HOCl traps. The efficiency of MPO inhibitors to prevent LDL damage increased in the series benzohydroxamic acid < salicylhydroxamic acid < 3-amino-1,2,4-triazole < sodium azide < potassium cyanide < p-hydroxy-benzoic acid hydrazide, while for the HOCl traps the protective efficiency increased in the series glycine < taurine < methionine. We conclude that HOCl traps may have high potential therapeutic impact in vivo due to their low toxicity, although high concentrations of them would have to reach sites of inflammation. In contrast, only low concentrations of a specific MPO inhibitor would be required to irreversibly inhibit the enzyme.

Modification of protein moiety of human low density lipoprotein by hypochlorite generates strong platelet agonist

Conflicting reports exist about the effects of mildly or extensively oxidized low density lipoproteins (LDLs) on the reactivity of human platelets. This platelet response is mainly caused by modification of the protein and lipid moiety, giving rise to very differently modified species with hardly predictable properties. The aim of this study was to prepare oxidized LDL with modifications essentially restricted to the protein moiety and to determine the eventual platelet responses. We treated LDL at 0 degrees C for 10 minutes with a 60- to 1000-fold molar excess of sodium hypochlorite in borate buffer in the presence of the radical scavenger butylated hydroxytoluene. Under these conditions, neither fragmentation of apolipoprotein B-100 nor formation of LDL aggregates was observed, and lipid oxidation products did not exceed the amount present in untreated LDLs. The degree of modification and the respective effects on platelet function were highly reproducible. Hypochlorite-modified LDLs act as strong platelet agonists, inducing morphological changes, dense granule release, and irreversible platelet aggregation. The evoked platelet effects are completely suppressed by inhibitors of the phosphoinositide cycle but not by EDTA or acetylsalicylic acid. Most likely, these effects are transmitted via high-affinity binding to a single class of sites, which does not recognize native or acetylated LDL. Obviously, modified lysines, and the secondary lipid modifications derived from them, are not essential for this interaction. We conclude that bioactive oxidized lipids are not directly involved in the stimulation of platelets by hypochlorite-modified LDLs.

Kinetics of tryptophan oxidation in plasma lipoproteins by myeloperoxidase-generated HOCl

The relative susceptibility of the apoprotein components of human lipoproteins [high-density lipoprotein (HDL) and low-density lipoprotein (LDL)] and their subclasses to oxidation by the myeloperoxidase/H2O2/Cl- system in vitro was studied by measuring the decrease in rate of tryptophan fluorescence. Whereas the lipoprotein-modification rate showed a saturation type of dependence on the concentration of myeloperoxidase, a biphasic dependence on the concentration of the lipoproteins was found. High concentrations of H2O2 were also found to inhibit tryptophan oxidation in LDL but to a lesser extent in HDL. The optimal rate of LDL and HDL modification was observed at pH 6.0. HDL was modified much more rapidly than LDL, which may be due to differences in size and different relative contents of protein and lipids per particle. No differences in rates of modification of LDL subclasses were observed, when the assays were standardized to equal LDL protein concentrations, but, when standardized to equal particle mass, an optimum at subclass 8 was found, which is probably due to differences in apolipoprotein B-100 conformation. It was concluded that HDL may have a beneficial effect in retarding LDL modification in inflammatory processes.

Oxidation of LDL by myeloperoxidase and reactive nitrogen species: reaction pathways and antioxidant protection

Oxidative modification of low density lipoprotein (LDL) appears to play an important role in atherogenesis. Although the precise mechanisms of LDL oxidation in vivo are unknown, several lines of evidence implicate myeloperoxidase and reactive nitrogen species, in addition to ceruloplasmin and 15-lipoxygenase. Myeloperoxidase generates a number of reactive species, including hypochlorous acid, chloramines, tyrosyl radicals, and nitrogen dioxide. These reactive species oxidize the protein, lipid, and antioxidant components of LDL. Modification of apolipoprotein B results in enhanced uptake of LDL by macrophages with subsequent formation of lipid-laden foam cells. Nitric oxide synthases produce nitric oxide and, under certain conditions, superoxide radicals. Numerous other sources of superoxide radicals have been identified in the arterial wall, including NAD(P)H oxidases and xanthine oxidase. Nitric oxide and superoxide readily combine to form peroxynitrite, a reactive nitrogen species capable of modifying LDL. In this review, we examine the reaction pathways involved in LDL oxidation by myeloperoxidase and reactive nitrogen species and the potential protective effects of the antioxidant vitamins C and E.

Correlation of low-density lipoprotein modification by myeloperoxidase with hypochlorous acid formation

Myeloperoxidase is an enzyme in phagocytes which catalyzes several redox reactions. A major product is hypochlorous acid which appears to be important in inflammatory processes such as atherosclerosis. The aim of this study was to investigate whether the kinetics of low-density lipoprotein modification by the myeloperoxidase/hydrogen peroxide/chloride system in vitro conform to the established kinetics of hypochlorous acid formation and to compare the results with known in vivo data. The absorbance at 234 nm was applied to study the kinetics of the modification of low-density lipoprotein. Variation of the concentration of low-density lipoprotein, hydrogen peroxide, and chloride, respectively, had a biphasic effect on the maximal rate of low-density lipoprotein modification. Increasing the substrates up to certain threshold levels resulted in increased modification, however, further increases caused inhibition of low-density lipoprotein modification. The inhibitory effect of higher low-density lipoprotein concentrations might be relevant, since these concentrations occur in the human aortic intima. Furthermore, a positive correlation was found between the maximal rate of low-density lipoprotein modification and the acidity of the medium. In summary, low-density lipoprotein modification is affected by the myeloperoxidase/hydrogen peroxide/chloride system in a similar manner to hypochlorous acid production. We conclude that myeloperoxidase, which has been detected in atherosclerotic lesions, is able to modify low-density lipoprotein into the form which is taken up by macrophages in an uncontrolled manner.

Pathways of phospholipid oxidation by HOCl in human LDL detected by LC-MS

A wealth of evidence now indicates that low-density lipoprotein (LDL) must be modified to promote atherosclerosis, and that this may involve oxidants released by phagocytes. Many studies of oxidative damage in atherosclerosis previously have concentrated on damage by nonhalogenated oxidants, but HOCl is a highly toxic oxidant produced by myeloperoxidase in phagocytes, which is also likely to be important in the disease pathogenesis. Currently some controversy exists over the products resulting from reaction of HOCl with LDL lipids, in particular regarding whether predominantly chlorohydrins or lipid peroxides are formed. In this study LC-MS of phosphatidylcholines in human LDL treated either with HOCl or the myeloperoxidase system was used as a specific method to detect chlorohydrin and peroxide formation simultaneously, and with comparable sensitivity. Chlorohydrin products from lipids containing oleic, linoleic and arachidonic acids were detected, but no hydroperoxides of linoleoyl or arachidonoyl lipids could be observed. This study provides the first direct evidence that lipid chlorohydrins rather than peroxides are the major products of HOCl- or myeloperoxidase-treated LDL phospholipids. This in turn provides important information required for the study of oxidative damage in vivo which will allow the type and source of oxidants involved in the pathology of atherosclerosis to be investigated.

Differential oxidation of apolipoprotein E isoforms and interaction with phospholipids

Accumulation of oxidized proteins has been demonstrated in the brain of patients suffering from Alzheimer's disease (AD). Among the proteins found in cerebral amyloid deposits, apolipoprotein (apo) E is a polymorphic protein which one specific isoform, apo E4, has been widely associated with AD. Apo E may be linked with AD by its isoform-specific interaction with lipids or other proteins in amyloid plaques. Using the myeloperoxidase oxidative system, we report that oxidation of the three recombinant apo E isoforms is differential (as estimated using immunoblot and high-performance liquid chromatography analysis), with apo E4 being more susceptible than apo E3, which in turn is much more susceptible than apo E2. In addition, susceptibility to thrombin proteolysis is reduced when apo E is oxidized, and oxidation of apo E decreases its incorporation into phospholipid discs by approximately 50%. Oxidation of apo E may contribute to inefficient lipid recycling in the brain, particularly regarding apo E4 and E3. Our results link and strengthen both the E4 allele linkage with AD and the role of protein oxidation in AD. The cerebral mechanisms underlying apo E oxidation and/or myeloperoxidase functions in vivo remain to be assessed.

Oxidized low-density lipoprotein induces macrophage respiratory burst via its protein moiety: A novel pathway in atherogenesis?

Oxidized low-density lipoproteins (oxLDL) play a crucial role in atherogenesis mainly via their capacity to bind and to activate macrophages. However, the role of the protein LDL moiety in this process is not yet established. In this study, human LDL were exposed to hypochlorous acid (HOCl), a selective protein oxidant, or copper sulfate (CuSO(4)), a major lipid oxidant, and tested for their capacity to activate the NADPH-oxidase of human THP-1- and U937-derived macrophages as measured by lucigenin chemiluminescence (CL). Compared to native LDL which had no effect, HOCl-oxLDL triggered potent CL responses in both U937 and THP-1 cells but only when these were fully differentiated into macrophages by phorbol myristate acetate. In contrast, Cu-oxLDL only triggered a moderate CL response of U937 cells and had little effect on THP-1 cells. While delipidation did not affect HOCl-oxLDL-induced CL response it abolished that induced by Cu-oxLDL. Interestingly, U937 cells showed higher CL responses to both types of oxLDL than THP-1 cells, a finding which could be related to their higher expression of the scavenger receptor CD36. Taken together these results strongly support the role of the protein moiety in oxLDL-induced macrophage activation.