Kinetic analysis of copper-induced peroxidation of LDL

New insights into the kinetic and thermodynamic evaluations of lipid peroxidation

Simultaneous evaluations over the whole practical range of peroxidation, including the initiation and propagation phases, provide more informative and reliable data than single-parameter analyses being mostly employed only over the course of the initiation phase. Besides an overview on the dominant mechanisms governing the initiation and propagation phases, this article highlights a number of unifying parameters that represent inclusively the two phases. Then, the reliable method to calculate induction period and critical reverse micelle concentration of lipid hydroperoxides as the two interstitial parameters when transitioning from the initiation to the propagation phase is reviewed. Next, a reconsidered form of the conventional methodology on the kinetics of chain-breaking antioxidants is presented. After that, the Arrhenius kinetic and thermodynamic Eyring-Polanyi parameters calculated from the initiation, composite, and decomposition rate constants are compared in order to assess oxidative stabilities. Finally, shelf-life predictions based on a number of proposed end-points of peroxidation are addressed.

Redox control of chondrocyte differentiation and chondrogenesis

Chondrogenesis involves the recruitment and migration of mesenchymal cells, mesenchymal condensation, and chondrocyte differentiation and hypertrophy. Multiple factors precisely regulate chondrogenesis. Recent studies have demonstrated that the redox status of chondrocytes plays an essential role in the regulation of chondrocyte differentiation and chondrogenesis. Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are important factors that change the intracellular redox status. Physiological levels of ROS/RNS act as intracellular signals in chondrocytes, and oxidative stress impairs the metabolism of chondrocytes. Under physiological conditions, the balance between ROS/RNS production and elimination ensures that redox-sensitive signalling proteins function correctly. The redox homeostasis of chondrocytes ensures that they respond appropriately to endogenous and exogenous stimuli. This review focuses on the redox regulation of key signalling pathways and transcription factors that control chondrogenesis and chondrocyte differentiation. Additionally, the mechanism by which ROS/RNS regulate signalling proteins and transcription factors in chondrocytes is also reviewed.

Deuterium kinetic isotope effect (DKIE) in copper-induced LDL peroxidation: Interrelated effects of on inhibition and propagation

LDL peroxidation plays a major role in many physiological and pathophysiological processes. The mechanisms of LDL peroxidation induced by transition metal ions have therefore been studied intensively. It has been proposed that the mechanism involves free radical production that occurs via decomposition of hydroperoxides. This, in turn, requires the cleavage of O-H bonds. Cleavage of O-D bond is slower and comparison of the kinetics in D₂O to the kinetics in H₂O is therefore a straightforward way to test this aspect of the alleged mechanism. The kinetics of peroxidation exhibit marked deuterium kinetic isotope effects at all the stages of oxidation under all the studied conditions. We found that the rate of propagation of copper-induced peroxidation is a monotonically decreasing function of D₂O fraction in D₂O/H₂O mixtures. The only elementary reaction that involves "exchangeable" hydrogen at this stage is copper-induced decomposition of conjugated hydroperoxides. Therefore, we conclude that the latter step is rate-limiting reaction including cleavage of oxygen-hydrogen bond of hydroperoxide. The lag preceding rapid peroxidation exhibits a biphasic dependence on the fraction of D₂O. This may be understood on the basis of the effect of substituting hydrogen atoms by deuterium. Specifically, such substitution is expected to decrease both the rate of initiation of peroxidation and the potency of the antioxidant. We interpret our results in terms of the effects of isotopic substitution on the rates of the reactions that involve the abstraction of "exchangeable" hydrogen atoms of OH groups in tocopherol and hydroperoxides.

Paradoxical effects of SAA on lipoprotein oxidation suggest a new antioxidant function for SAA

Oxidative stress and inflammation, which involve a dramatic increase in serum amyloid A (SAA) levels, are critical in the development of atherosclerosis. Most SAA circulates on plasma HDL particles, altering their cardioprotective properties. SAA-enriched HDL has diminished anti-oxidant effects on LDL, which may contribute to atherogenesis. We determined combined effects of SAA enrichment and oxidation on biochemical changes in HDL. Normal human HDLs were incubated with SAA, oxidized by various factors (Cu²⁺, myeloperoxidase, H₂O₂, OCl^-), and analyzed for lipid and protein modifications and biophysical remodeling. Three novel findings are reported: addition of SAA reduces oxidation of HDL and LDL lipids; oxidation of SAA-containing HDL in the presence of OCl^- generates a covalent heterodimer of SAA and apoA-I that resists the release from HDL; and mild oxidation promotes spontaneous release of proteins (SAA and apoA-I) from SAA-enriched HDL. We show that the anti-oxidant effects of SAA extend to various oxidants and are mediated mainly by the unbound protein. We propose that free SAA sequesters lipid hydroperoxides and delays lipoprotein oxidation, though much less efficiently than other anti-oxidant proteins, such as apoA-I, that SAA displaces from HDL. These findings prompt us to reconsider the role of SAA in lipid oxidation in vivo.

The effect of compartmentalization on the kinetics of transition metal ions-induced lipoprotein peroxidation

In a previous study, we proposed characterizing the typically observed kinetic profiles of transition metal ion-induced lipid peroxidation in terms of a limited number of characteristic time-points. These time-points can be derived from experimental time-dependencies and be presented in terms of rate constants and concentrations as calculated based on mechanistic considerations. The critical part of that analysis was that we had to assume that the experimental system behaves as if it is homogeneous, i.e., as if the reaction occurs in a solution. In spite of the uncertainties due to the latter assumption, we obtained a reasonable agreement between the experimental data and the theoretically predicted dependencies, which supports our theoretical treatment. Yet, several previous findings could not have been explained in terms of our ('quasi-homogeneous') model, indicating that the model is valid not under all conditions. One example is that under certain conditions, rapid peroxidation occurs prior to complete consumption of LDL-associated tocopherol. This can be attributed to compartmentalization of residual tocopherol, namely, after the onset of propagation, part of the LDL particles contain tocopherol, whereas in the other, tocopherol-depleted particles, the PUFA may undergo rapid LOOH-accelerated peroxidation only if they contain at least two hydroperoxides molecules per particle. In the present investigation, we show that the results of all our kinetic studies can be understood if we consider compartmentalization. Specifically, for any given composition of the particles (LDL and/or HDL), the kinetic results may be governed by the distribution and rate of exchange of antioxidants and hydroperoxides between particles. Our analysis is of special importance for systems containing more than one population of lipoprotein particles.

Citrulline Supplementation Induces Changes in Body Composition and Limits Age-Related Metabolic Changes in Healthy Male Rats

BACKGROUND

Aging is associated with profound metabolic disturbances, and citrulline may be of use to limit them.

OBJECTIVE

The aim of this work was to evaluate the long-term effect of citrulline supplementation on metabolism in healthy aged rats.

METHODS

Twenty-month-old male rats were randomly assigned to be fed (ad libitum) for 12 wk with either a citrulline-enriched diet (1 g ⋅ kg(-1) ⋅  d(-1)) or a standard diet [rendered isonitrogenous by addition of nonessential amino acids (NEAAs)]. Motor activity and muscle strength were measured, body composition was assessed, and muscle metabolism (protein structure, mitochondrial exploration, and transductional factors) and lipid metabolism (lipoprotein composition and sensitivity to oxidative stress) were explored.

RESULTS

Compared with the NEAA-treated group, citrulline supplementation was associated with lower mortality (0% vs. 20%; P = 0.05), 9% higher lean body mass (P < 0.05), and 13% lower fat mass (P < 0.05). Compared with the NEAA-treated group, citrulline-treated rats had greater muscle mass (+14-48% depending on type of muscle; P < 0.05 for tibialis, gastrocnemius, and plantaris). Susceptibility to oxidation of lipoproteins, as measured by the maximal concentration of 7-ketocholesterol after copper-induced VLDL and LDL oxidation, was lower in citrulline-treated rats than in NEAA-treated rats (187 ± 8 μmol/L vs. 243 ± 7 μmol/L; P = 0.0005).

CONCLUSIONS

Citrulline treatment in male aged rats favorably modulates body composition and protects against lipid oxidation and, thus, emerges as an interesting candidate to help prevent the aging process.

Reactive oxygen species, heat stress and oxidative-induced mitochondrial damage. A review

In recent years there has been enormous interest in researching oxidative stress. Reactive oxygen species (ROS) are derived from the metabolism of oxygen as by-products of cell respiration, and are continuously produced in all aerobic organisms. Oxidative stress occurs as a consequence of an imbalance between ROS production and the available antioxidant defence against them. Nowadays, a variety of diseases and degenerative processes such as cancer, Alzheimer's and autoimmune diseases are mediated by oxidative stress. Heat stress was suggested to be an environmental factor responsible for stimulating ROS production because of similarities in responses observed following heat stress compared with that occurring following exposure to oxidative stress. This manuscript describes the main mitochondrial sources of ROS and the antioxidant defences involved to prevent oxidative damage in all the mitochondrial compartments. It also deals with discussions concerning the cytotoxic effect of heat stress, mitochondrial heat-induced alterations, as well as heat shock protein (HSP) expression as a defence mechanism.

Skeletal hybridization and PfRIO-2 kinase modeling for synthesis of α-pyrone analogs as anti-malarial agent

The pharmacophoric hybridization and computational design approach were applied to generate a novel series of α-pyrone analogs as plausible anti-malarial lead candidate. A putative active site in flexible loop close to wing-helix domain of PfRIO2 kinase was explored computationally to understand the molecular basis of ligand binding. All the synthesized molecules (3a-g) exhibited in vitro antimalarial activity. Oxidative stress induced by 3a-d were calculated and found to be significantly higher in case of 3b. Therefore, 3b, which shown most significant result was identified as promising lead for further SAR study to develop potent anti-malarials.

Kinetic modeling of low density lipoprotein oxidation in arterial wall and its application in atherosclerotic lesions prediction

Oxidation of low-density lipoprotein (LDL) is one of the major factors in atherogenic process. Trapped oxidized LDL (Ox-LDL) in the subendothelial matrix is taken up by macrophage and leads to foam cell generation creating the first step in atherosclerosis development. Many researchers have studied LDL oxidation using in vitro cell-induced LDL oxidation model. The present study provides a kinetic model for LDL oxidation in intima layer that can be used in modeling of atherosclerotic lesions development. This is accomplished by considering lipid peroxidation kinetic in LDL through a system of elementary reactions. In comparison, characteristics of our proposed kinetic model are consistent with the results of previous experimental models from other researches. Furthermore, our proposed LDL oxidation model is added to the mass transfer equation in order to predict the LDL concentration distribution in intima layer which is usually difficult to measure experimentally. According to the results, LDL oxidation kinetic constant is an important parameter that affects LDL concentration in intima layer so that existence of antioxidants that is responsible for the reduction of initiating rates and prevention of radical formations, have increased the concentration of LDL in intima by reducing the LDL oxidation rate.

Antioxidant protective effect of flavonoids on linoleic acid peroxidation induced by copper(II)/ascorbic acid system

Antioxidants are compounds that can delay or inhibit lipid oxidation. The peroxidation of linoleic acid (LA) in the absence and presence of Cu(II) ion-ascorbate combinations was investigated in aerated and incubated emulsions at 37°C and pH 7. LA peroxidation induced by copper(II)-ascorbic acid system followed first order kinetics with respect to hydroperoxides concentration. The extent of copper-initiated peroxide production in a LA system assayed by ferric thiocyanate method was used to determine possible antioxidant and prooxidant activities of the added flavonoids. The effects of three different flavonoids of similar structure, i.e. quercetin (QR), morin (MR) and catechin (CT), as potential antioxidant protectors were studied in the selected peroxidation system. The inhibitive order of flavonoids in the protection of LA peroxidation was: morin>catechin≥quercetin, i.e. agreeing with that of formal reduction potentials versus NHE at pH 7, i.e. 0.60, 0.57 and 0.33V for MR, CT, and QR, respectively. Morin showed antioxidant effect at all concentrations whereas catechin and quercetin showed both antioxidant and prooxidant effects depending on their concentrations. The structural requirements for antioxidant activity in flavonoids interestingly coincide with those for Cu(II)-induced prooxidant activity, because as the reducing power of a flavonoid increases, Cu(II)-Cu(I) reduction is facilitated that may end up with the production of reactive species. The findings of this study were evaluated in the light of structure-activity relationships of flavonoids, and the results are believed to be useful to better understand the actual conditions where flavonoids may act as prooxidants in the preservation of heterogeneous food samples containing traces of transition metal ions.

Structure/function relationships of apolipoprotein a-I mimetic peptides: implications for antiatherogenic activities of high-density lipoprotein

RATIONALE

Apolipoprotein (apoA)-I mimetic peptides are a promising type of anti-atherosclerosis therapy, but how the structural features of these peptides relate to the multiple antiatherogenic functions of HDL is poorly understood.

OBJECTIVE

To establish structure/function relationships of apoA-I mimetic peptides with their antiatherogenic functions.

METHODS AND RESULTS

Twenty-two bihelical apoA-I mimetic peptides were investigated in vitro for the capacity and specificity of cholesterol efflux, inhibition of inflammatory response of monocytes and endothelial cells, and inhibition of low-density lipoprotein (LDL) oxidation. It was found that mean hydrophobicity, charge, size of hydrophobic face, and angle of the link between the helices are the major factors determining the efficiency and specificity of cholesterol efflux. The peptide with optimal parameters was more effective and specific toward cholesterol efflux than human apoA-I. Charge and size of hydrophobic face were also the major factors affecting antiinflammatory properties, and the presence of cysteine and histidine residues was the main factor determining antioxidant properties. There was no significant correlation between capacities of the peptides to support individual functions; each function had its own optimal set of features.

CONCLUSIONS

None of the peptides was equally effective in all the antiatherogenic functions tested, suggesting that different functions of HDL may have different mechanisms and different structural requirements. The results do suggest, however, that rationalizing the design of apoA-I mimetic peptides may improve their therapeutic value and may lead to a better understanding of mechanisms of various antiatherogenic functions of HDL.

Protective activity of plicatin B against human LDL oxidation induced in metal ion-dependent and -independent processes. Experimental and theoretical studies

Oxidation of low-density lipoproteins (LDL) is thought to be a major factor in the pathophysiology of atherosclerosis. Natural antioxidants have been shown to protect LDL from oxidation and to inhibit atherogenic developments in animals. Structurally related prenylated pterocarpans, erybraedin C and bitucarpin A, and the prenylchalcone plicatin B were examined for their ability to inhibit LDL oxidation in vitro. The kinetic profile of peroxidation is characterized by the lag time of oxidation (t(lag)), the maximal rate of oxidation (V(max)) and the maximal accumulation of oxidation products (OD(max)). Specific variation of the set of kinetic parameters by antioxidants may provide important information about the mechanism of inhibitory action of a given compound. At equimolar concentrations (1 microM) the prenylated derivatives tested were found to inhibit 1 microM copper sulphate-induced oxidation of LDL (50 microg protein/ml) in accordance with the following order of activity: plicatin B>erybraedin Cbitucarpin A. Structural aspects, such as hydrogen-donating substituents, their number and arrangement in the aromatic ring moieties, and the prenyl and methoxy substituents, were investigated in order to explain the findings obtained. It is well known that the antioxidant activity of flavonoids is believed to be caused by a combination of transition metal chelation and free-radical-scavenging activities. To investigate these differences we comparatively studied the protective mechanism of plicatin B in copper-dependent or -independent LDL oxidation. The latter was mediated by 2,2'-azo-bis-(2-amidinopropane) dihydrochloride (ABAP). We measured the formation of conjugated dienes (OD(234 nm)). Plicatin B (0.2-1.5 microM) delayed the Cu(2+) (1 microM) promoted oxidation as conjugate diene formation (t(lag)) of the LDL by 45.2-123.5 min and reduced V(max) by 0.46-0.29 microM/min. In the ABAP (0.2mM) promoted LDL oxidation t(lag) increased by 67.2-110.2 min through plicatin B (0.5-2.5 microM). In experiments in which Cu(2+) concentrations increased (0.5 - 3 microM) and the amount of plicatin B (1 microM) was maintained constant, a significant decrease in t(lag) and an increase in V(max) was observed. In this study plicatin B appeared to exhibit a mixed mechanism, interfering with the formation of the radicals by chelating copper involved in the initiation/propagation reaction, but also by scavenging free hydroperoxyl radicals resulting from ABAP thermolysis. In addition, theoretical analysis indicated that plicatin B preferentially established the chelating complex with Cu(2+), because its affinity value is notably higher (by a factor of 5) than that for Cu(+).

Lipid Peroxidation in the Presence of Albumin, Inhibitory and Prooxidative Effects

Oxidative modifications of LDL are involved in atherogenesis. Previously we have developed a simple assay to evaluate the susceptibility of lipids to copper-induced peroxidation in the relatively natural milieu of unfractionated serum in the presence of excess citrate. Based on our previous results we have proposed that the inducer of peroxidation in our optimized assay is a copper-citrate complex. Recent investigations indicate that under certain conditions a copper-albumin complex may induce peroxidation of ascorbate. Two different complexes may be formed in albumin-containing systems (e.g. serum) namely 1:1 and 2:1 copper-albumin complexes. The aim of the present work was to evaluate the possibility that at least one of these complexes may be responsible for the induction of peroxidation of lipids in lipidic systems containing copper and albumin, including our optimized assay. Towards this end, we have investigated the dependence of copper-induced peroxidation on the concentration of added albumin in lipidic systems in the absence and presence of citrate. In all the systems investigated in this study (PLPC liposomes, LDL, HDL and mixtures of HDL and LDL) we found that at low concentrations of free copper (e.g. in the presence of excess citrate) the 2:1 copper-albumin complex is redox-active and that this complex is the major contributor to the initiation of lipid peroxidation in these systems and in our optimized assay. The possible relevance of the induction of peroxidation in vivo by the latter complex has yet to be studied.

Redox regulation of cell survival

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) play important roles in regulation of cell survival. In general, moderate levels of ROS/RNS may function as signals to promote cell proliferation and survival, whereas severe increase of ROS/RNS can induce cell death. Under physiologic conditions, the balance between generation and elimination of ROS/RNS maintains the proper function of redox-sensitive signaling proteins. Normally, the redox homeostasis ensures that the cells respond properly to endogenous and exogenous stimuli. However, when the redox homeostasis is disturbed, oxidative stress may lead to aberrant cell death and contribute to disease development. This review focuses on the roles of key transcription factors, signal-transduction pathways, and cell-death regulators in affecting cell survival, and how the redox systems regulate the functions of these molecules. The current understanding of how disturbance in redox homeostasis may affect cell death and contribute to the development of diseases such as cancer and degenerative disorders is reviewed. We also discuss how the basic knowledge on redox regulation of cell survival can be used to develop strategies for the treatment or prevention of those diseases.

Copper-induced peroxidation of phosphatidylserine-containing liposomes is inhibited by nanomolar concentrations of specific antioxidants

Copper-induced peroxidation of liposomal palmitoyllinoleoyl-phosphatidylcholine (PLPC) is inhibited by alpha-tocopherol at micromolar concentrations. In our previous study we found that when the liposomes contain phosphatidylserine (PS), nanomolar concentrations of Toc were sufficient to inhibit peroxidation. In an attempt to gain understanding of the origin of this extreme antioxidative potency, we tested the antioxidative potency of 36 additional antioxidants and the dependence of their potency on the presence of PS in the liposomes. The results of these studies reveal that only 11 of the tested antioxidants possess similar antioxidative potency to that of Toc. These include trolox, butylated hydroxytoluene (BHT), curcumin, nordihydroguaiaretic acid (NDGA), diethylstilbestrol (DES), 2 of the 13 tested flavonoids (luteolin and 7,3',4'-trihydroxyflavone; T-414), alpha-naphthol, 1,5-, 1,6- and 1,7-dihydroxynaphthalenes (DHNs). Propyl gallate (PG), methyl syringate, rosmarinic acid, resveratrol, other flavonoids, as well as beta-naphthol, 1,2-, 1,3-, 1,4-, 2,3-, 2,6-, and 2,7-DHNs were either moderately antioxidative or pro-oxidative. For liposomes made of PLPC (250 microM) and PS (25 microM) the "lag" preceding copper-induced peroxidation (5 microM copper) was doubled upon addition of 30-130nM of the "super-active" antioxidants. We propose that the mechanism responsible for the extreme antioxidative potency against copper-induced peroxidation in PS-containing liposomes involves replenishment of the antioxidant in a ternary PS-copper-antioxidant complex. Based on structure-activity relationship of the 37 tested antioxidants, the "super-antioxidative potency" is attributed to the recycling of relatively stable semiquinone or semiquinone-like radicals.

Antiatherogenic activity of extracts of Argania spinosa L. pericarp: beneficial effects on lipid peroxidation and cholesterol homeostasisThis article is one of a selection of papers published in this special issue (part 1 of 2) on the Safety and Efficacy of Natural Health Products

Prevention of lipoprotein oxidation by natural compounds may prevent atherosclerosis via reducing early atherogenesis. In this study, we investigated for the first time the beneficial properties of methanolic extract of argania pericarp (MEAP) towards atherogenesis by protecting human low-density lipoprotein (LDL) against oxidation while promoting high-density lipoprotein (HDL)-mediated cholesterol efflux. By measuring the formation of malondialdehyde (MDA) and conjugated diene as well as the lag phase and the progression rate of lipid peroxidation, the MEAP was found to possess an inhibitory effect. In addition, MEAP reduced the rate of disappearance of α-tocopherol as well as the apoB electrophoretic mobility in a dose-dependent manner. These effects are related to the free radical scavenging and copper-chelating effects of MEAP. In terms of cell viability, MEAP has shown a cytotoxic effect (0–40 μg/mL). Incubation of3H-cholesterol-loaded J774 macrophages with HDL in the presence of increasing concentrations of MEAP enhanced HDL-mediated cholesterol efflux independently of ABCA1 receptor pathways. Our findings suggest that argania seed pericarp provides a source of natural antioxidants that inhibit LDL oxidation and enhance cholesterol efflux and thus can prevent development of cardiovascular diseases.

Clinical pharmacology and therapeutic use of antioxidant vitamins

The clinical use of antioxidants has gained considerable interest during the last decade. It was suggested from epidemiological studies that diets high in fruits and vegetables might help decrease the risk of cardiovascular disease. Therefore, supplements of vitamins C and E were applied through protocols aimed to prevent diseases such as atherosclerosis, preeclampsia or hypertension, thought to be mediated by oxidative stress. Despite the biological properties of these vitamins could account for an effective protection, as shown by several clinical and experimental studies, their efficacy remains controversial in the light of some recent clinical trials and meta-analyses. However, the methodology of these studies, criteria for selection of patients, the uncertain extent of progression of the disease when initiating supplementation, the lack of mechanistic studies containing basic scientific aspects, such as the bioavailability, pharmacokinetic properties, and the nature of the antioxidant sources of vitamins, could account for the inconsistency of the various clinical trials and meta-analyses assessing the efficacy of these vitamins to prevent human diseases. This review presents a survey of the clinical use of antioxidant vitamins E and C, proposing study models based on the biological effects of these compounds likely to counteract the pathophysiological mechanisms able to explain the structural and functional organ damage.

Acute and subchronic copper treatments alter extracellular nucleotide hydrolysis in zebrafish brain membranes

Copper is a divalent cation with physiological importance since deficiency of copper homeostasis can cause serious neurological diseases. ATP is an important signalling molecule stored at nerve endings and its inactivation is promoted by ecto-nucleotidases. In this study, we verified the effect of acute and subchronic copper treatments on ecto-nucleotidase activities in zebrafish brain membranes. Treatment with copper sulfate (15 microg/L) during 24h inhibited ATP hydrolysis (16%), whereas ADP and AMP hydrolysis were not altered. Nevertheless, a 96-h exposure with the copper concentration mentioned above inhibited NTPDase (31% and 42% for ATP and ADP hydrolysis, respectively) and ecto-5'-nucleotidase (40%) activities. NTPDase1, NTPDase2_mg and NTPDase2_mv transcripts were decreased after copper exposures during 24 and 96 h. Subchronic copper treatment also reduced the NTPDase2_mq and ecto-5'-nucleotidase expression. In vitro assays demonstrated that NTPDase activities were reduced after copper exposure during 40 min. ATP hydrolysis was inhibited at 0.25, 0.5 and 1mM (13%, 31% and 48%, respectively) and ADP hydrolysis also had a significant decrease at these same copper concentrations (41%, 63% and 68%, respectively). In contrast to the subchronic exposure, no significant changes on ecto-5'-nucleotidase were observed after in vitro assays. Lineweaver-Burk plots suggested that both inhibitory effects on nucleotide hydrolysis may occur in a non-competitive manner. Altogether, these findings indicate that copper is able to promote distinct changes on ecto-nucleotidases after in vivo and in vitro treatments and, consequently, it could control the nucleotide and nucleoside levels, modulating the purinergic signalling.

Peroxidation of liposomal lipids

Free radicals, formed via different mechanisms, induce peroxidation of membrane lipids. This process is of great importance because it modifies the physical properties of the membranes, including its permeability to different solutes and the packing of lipids and proteins in the membranes, which in turn, influences the membranes' function. Accordingly, much research effort has been devoted to the understanding of the factors that govern peroxidation, including the composition and properties of the membranes and the inducer of peroxidation. In view of the complexity of biological membranes, much work was devoted to the latter issues in simplified model systems, mostly lipid vesicles (liposomes). Although peroxidation in model membranes may be very different from peroxidation in biological membranes, the results obtained in model membranes may be used to advance our understanding of issues that cannot be studied in biological membranes. Nonetheless, in spite of the relative simplicity of peroxidation of liposomal lipids, these reactions are still quite complex because they depend in a complex fashion on both the inducer of peroxidation and the composition and physical properties of the liposomes. This complexity is the most likely cause of the apparent contradictions of literature results. The main conclusion of this review is that most, if not all, of the published results (sometimes apparently contradictory) on the peroxidation of liposomal lipids can be understood on the basis of the physico-chemical properties of the liposomes. Specifically: (1) The kinetics of peroxidation induced by an "external" generator of free radicals (e.g. AAPH) is governed by the balance between the effects of membrane properties on the rate constants of propagation (k (p)) and termination (k (t)) of the free radical peroxidation in the relevant membrane domains, i.e. in those domains in which the oxidizable lipids reside. Both these rate constants depend similarly on the packing of lipids in the bilayer, but influence the overall rate in opposite directions. (2) Peroxidation induced by transition metal ions depends on additional factors, including the binding of metal ions to the lipid-water interface and the formation of a metal ions-hydroperoxide complex at the surface. (3) Reducing agents, commonly regarded as "antioxidants", may either promote or inhibit peroxidation, depending on the membrane composition, the inducer of oxidation and the membrane/water partitioning. All the published data can be explained in terms of these (quite complex) generalizations. More detailed analysis requires additional experimental investigations.

Oxidation of liposomal cholesterol and its effect on phospholipid peroxidation

Lipid peroxidation is believed to play an important role in the pathogenesis of many diseases. Much research has therefore been devoted to peroxidation of different lipids in biomembranes and in model systems (liposomes) of different compositions. Yet, in spite of the relative simplicity of the liposomes, the existing literature is insufficient to reach definite conclusions regarding basic questions including the susceptibility of cholesterol to oxidation, its effect on the peroxidation of polyunsaturated phospholipids such as palmitoyllinoleoylphosphatidylcholine (PLPC) and how cholesterol influences the effect of water-soluble antioxidants such as urate on the peroxidation. The aim of the present study was to clarify these issues. Its major findings are that: (i) AAPH-induced peroxidation of cholesterol is slow and independent of the peroxidation of PLPC. In turn, AAPH-induced peroxidation of PLPC is not affected by cholesterol, independent of the presence of urate in the system. (ii) Cholesterol is not susceptible to copper-induced oxidation, but its inclusion in PLPC liposomes affects the peroxidation of PLPC, slowing down the initial stage of oxidation but promoting later stages. (iii) Addition of urate accelerates copper-induced peroxidation of PLPC in the absence of cholesterol, whereas in cholesterol-containing liposomes it inhibits PLPC oxidation. We attribute the complexity of the observed kinetics to the known cholesterol-induced rigidization of liquid crystalline bilayers.

Marrubium vulgare extract inhibits human-LDL oxidation and enhances HDL-mediated cholesterol efflux in THP-1 macrophage

The objective of the present study was to elucidate the beneficial properties of aqueous extracts of Marrubium vulgare (AEM) towards cardiovascular disease by protecting human-LDL against lipid peroxidation and promoting HDL-mediated cholesterol efflux. Human-LDL were oxidised by incubation with CuSO(4) in the presence of increased concentrations of AEM (0-100 microg/ml). LDL lipid peroxidation was evaluated by conjugated diene formation, vitamin E disappearance as well as LDL-electrophoretic mobility. HDL-mediated cholesterol efflux assay was carried out in human THP-1 macrophages. Incubation of LDL with AEM significantly prolonged the lag phase (P=0.014), lowered the progression rate of lipid peroxidation (P=0.004), reduced the disappearance of vitamin E and the electrophoretic mobility in a dose-dependent manner. Also, incubation of HDL with AEM significantly increased HDL-mediated cholesterol efflux from THP-1 macrophages implicating an independent ATP binding cassette A1 (ABCA1) pathways. Our findings suggest that M. vulgare provides a source of natural antioxidants, which inhibit LDL oxidation and enhance reverse cholesterol transport and thus can prevent cardiovascular diseases development. These antioxidant properties increase the anti-atherogenic potential of HDL.

Free radicals, metals and antioxidants in oxidative stress-induced cancer

Oxygen-free radicals, more generally known as reactive oxygen species (ROS) along with reactive nitrogen species (RNS) are well recognised for playing a dual role as both deleterious and beneficial species. The "two-faced" character of ROS is substantiated by growing body of evidence that ROS within cells act as secondary messengers in intracellular signalling cascades, which induce and maintain the oncogenic phenotype of cancer cells, however, ROS can also induce cellular senescence and apoptosis and can therefore function as anti-tumourigenic species. The cumulative production of ROS/RNS through either endogenous or exogenous insults is termed oxidative stress and is common for many types of cancer cell that are linked with altered redox regulation of cellular signalling pathways. Oxidative stress induces a cellular redox imbalance which has been found to be present in various cancer cells compared with normal cells; the redox imbalance thus may be related to oncogenic stimulation. DNA mutation is a critical step in carcinogenesis and elevated levels of oxidative DNA lesions (8-OH-G) have been noted in various tumours, strongly implicating such damage in the etiology of cancer. It appears that the DNA damage is predominantly linked with the initiation process. This review examines the evidence for involvement of the oxidative stress in the carcinogenesis process. Attention is focused on structural, chemical and biochemical aspects of free radicals, the endogenous and exogenous sources of their generation, the metal (iron, copper, chromium, cobalt, vanadium, cadmium, arsenic, nickel)-mediated formation of free radicals (e.g. Fenton chemistry), the DNA damage (both mitochondrial and nuclear), the damage to lipids and proteins by free radicals, the phenomenon of oxidative stress, cancer and the redox environment of a cell, the mechanisms of carcinogenesis and the role of signalling cascades by ROS; in particular, ROS activation of AP-1 (activator protein) and NF-kappaB (nuclear factor kappa B) signal transduction pathways, which in turn lead to the transcription of genes involved in cell growth regulatory pathways. The role of enzymatic (superoxide dismutase (Cu, Zn-SOD, Mn-SOD), catalase, glutathione peroxidase) and non-enzymatic antioxidants (Vitamin C, Vitamin E, carotenoids, thiol antioxidants (glutathione, thioredoxin and lipoic acid), flavonoids, selenium and others) in the process of carcinogenesis as well as the antioxidant interactions with various regulatory factors, including Ref-1, NF-kappaB, AP-1 are also reviewed.

HDL enhances oxidation of LDL in vitro in both men and women

Background

Oxidative modification of low-density lipoprotein (LDL) is a key event in the oxidation hypothesis of atherogenesis. Some in vitro experiments have previously suggested that high-density lipoprotein (HDL) co-incubated with LDL prevents Cu²⁺-induced oxidation of LDL, while some other studies have observed an opposite effect. To comprehensively clarify the role of HDL in this context, we isolated LDL, HDL₂ and HDL₃ from sera of 61 free-living individuals (33 women and 28 men).

Results

When the isolated LDL was subjected to Cu²⁺-induced oxidation, both HDL₂ and HDL₃ particles increased the rate of appearance and the final concentration of conjugated dienes similarly in both genders. Oxidation rate was positively associated with polyunsaturated fatty acid content of the lipoproteins in that it was positively related to the content of linoleate and negatively related to oleate. More saturated fats thus protected the lipoproteins from damage.

Conclusion

We conclude that in vitro HDL does not protect LDL from oxidation, but is in fact oxidized fastest of all lipoproteins due to its fatty acid composition, which is oxidation promoting.

Quantitative determination of low density lipoprotein oxidation by FTIR and chemometric analysis

This study was conducted to develop a quantitative FTIR spectroscopy method to measure LDL lipid oxidation products and determine the effect of oxidation on LDL lipid and protein. In vitro LDL oxidation at 37 degrees C for 1 h produced a range of conjugated diene (CD) (0.14-0.26 mM/mg protein) and carbonyl contents (0.9-3.8 microg/g protein) that were used to produce calibration sets. Spectra were collected from the calibration set and partial least squares regression was used to develop calibration models from spectral regions 4000-650, 3750-3000, 1720-1500, and 1180-935 cm(-1) to predict CD and carbonyl contents. The optimal models were selected based on their standard error of prediction (SEP), and the selected models were performance-tested with an additional set of LDL spectra. The best models for CD prediction were derived from spectral regions 4000-650 and 1180-935 cm(-1) with the lowest SEP of 0.013 and 0.013 mM/mg protein, respectively. The peaks at 1745 (cholesterol and TAG ester C=O stretch), 1710 (carbonyl C-O stretch), and 1621 cm(-1) (peptide C=O stretch) positively correlated with LDL oxidation. FTIR and chemometrics revealed protein conformational changes during LDL oxidation and provided a simple technique that has potential for rapidly observing structural changes in human LDL during oxidation and for measuring primary and secondary oxidation products.

Thyroid hormone (T3) and its acetic derivative (TA3) protect low-density lipoproteins from oxidation by different mechanisms

Triiodothyronine (T3) and triiodothyroacetic acid (TA3) are thyroid compounds that similarly protect low-density lipoprotein (LDL) against oxidation induced by the free radical generator 2,2'-azobis-[2-amidinopropane] dihydrochloride (AAPH). However, TA3 is more antioxidant than T3 on LDL oxidation induced by copper ions (Cu2+), suggesting that these compounds act by different mechanisms. Here we measured conjugated diene production kinetics during in vitro human LDL (50 mg LDL-protein per l) oxidation induced by various Cu2+ (0.5-4 microM) or AAPH (0.25-2 mM) concentrations in the presence of T3, TA3, butylated hydroxytoluene (BHT) (a free radical scavenger) or ethylenediaminetetracetic acid (EDTA) (a metal chelator). From the kinetics were estimated: length of the lag phase (Tlag), maximum velocity of conjugated diene production (Vmax), and maximum amount of generated dienes (Dmax). Thyroid compound effects on these oxidation parameters were compared to those of the controls BHT and EDTA. In addition we measured by atomic absorption spectrometry copper remaining in LDL after a 30 min incubation of LDL with Cu2+ and the compounds followed by extensive dialysis, i.e. copper bound to LDL. As expected, LDL-copper was decreased by EDTA in a concentration-dependent manner, whereas it was not affected by BHT. T3 increased LDL-copper whereas TA3 slightly decreased it. The whole data suggest that T3 and TA3 are free radical scavengers that also differently disturb LDL-copper binding, an essential step for LDL lipid peroxidation. The most likely mechanisms are that T3 induces new copper binding sites inside the LDL particle, increasing the LDL-copper amount but in a redox-inactive form, whereas TA3 blocks some redox-active copper binding sites highly implicated in the initiation and the propagation of lipid peroxidation. Alternatively, we also found that a little amount of copper is tightly bound in LDL, which may be essential for the propagation of lipid peroxidation induced by free radical generators.

Chronic depletion of glutathione (GSH) and minimal modification of LDL in vivo: its prevention by glutathione mono ester (GME) therapy

A decline in reduced glutathione (GSH) level is associated with aging and free radical mediated diseases. The objective of this study was to determine whether the chronic depletion of extra cellular GSH causes oxidative damage to the circulating macromolecules such as lipoproteins. Decreased concentrations of plasma glutathione, vitamin E and ascorbic acid were recorded in the rats treated with buthionine sulfoximine (BSO), a selective GSH inhibitor. In LDL isolated from BSO-treated animals, the concentration of malondialdehyde (MDA) and conjugated dienes were significantly increased (P<0.01), whereas the levels of vitamin E were decreased (P<0.01). The analysis of total and LDL cholesterol revealed significant changes between the control and experimental groups. Of interest, altered concentrations of lyso-phosphatidyl choline (Lyso-PC) and phosphatidyl choline (PC) were recorded from the BSO mediated minimally modified LDL. A negative correlation between LDL-BDC/MDA and its antioxidant capacity was noted. Upon in vitro oxidation with CuSO(4), the electrophoretic behavior of purified LDL-apoprotein-B on agarose gel showed an increased mobility in BSO-treated rats, indicative of in vivo modification of LDL to become susceptible for in vitro oxidation. The increased mobility of LDL (after in vitro oxidation) isolated from the BSO-treated animals correlates with a decrease in its amino groups, as determined by the trinitrobenzene sulfonic acid (TNBS) reactants. However, the mobility of LDL molecule was not altered due to BSO treatment in vivo. Interestingly, the minimal modification on LDL does not lead to any vascular damage in the dorsal aorta of the rats injected with BSO. The administration of glutathione monoester (GME), at a dose of 5 mmol/kg body weight, twice a day, for 30 days, to animals treated with l-buthionine-SR-sulfoximine (BSO, 4 mmol/kg body weight, twice a day, for 30 days) normalized the antioxidant status and prevented the minimal modifications on LDL. Thus, increasing the cellular GSH levels may trigger beneficial effects against oxidative stress.

Peroxidation of liposomal palmitoyllinoleoylphosphatidylcholine (PLPC), effects of surface charge on the oxidizability and on the potency of antioxidants

Peroxidation of membrane phospholipids is an important determinant of membrane function. Previously we studied the kinetics of peroxidation of the polyunsaturated fatty acid (PUFA) residues in model membranes (liposomes) made by sonication of palmitoyllinoleoylphosphatidylcholine (PLPC). Since most biomembranes are negatively-charged, we have now studied the effect of negative surface charge on the kinetics of peroxidation of liposomes made of PLPC and 9% of one of the negatively-charged phospholipids phosphatidylserine (PS) or phosphatidic acid (PA). Peroxidation was initiated by either CuCl2 or AAPH and continuously monitored spectrophotometrically. The following results were obtained: (i) The negative charge had only a slight effect on AAPH-induced peroxidation, but accelerated markedly copper-induced peroxidation of the liposomes, probably by increasing the binding of copper to the membrane surface. (ii) Ascorbic acid (AA) inhibited AAPH-induced but promoted copper-induced peroxidation in all the studied liposomes, probably by enhancing the production of free radicals upon reduction of Cu(II) to Cu(I). (iii) alpha-tocopherol (Toc) inhibited AAPH-induced peroxidation in all the studied liposomes, whereas the effect of tocopherol on copper-induced peroxidation varied from being pro-oxidative in PA-containing liposomes, to being extremely anti-oxidative in PS-containing liposomes, even at very low tocopherol concentrations. The significance of the latter unusual protective effect, which we attribute to recycling of tocopherol by a PS-Cu complex, requires further investigation.

Copper-mediated LDL oxidation by homocysteine and related compounds depends largely on copper ligation

Oxidation of low-density lipoprotein (LDL) is thought to be a major factor in the pathophysiology of atherosclerosis. Elevated plasma homocysteine is an accepted risk factor for atherosclerosis, and may act through LDL oxidation, although this is controversial. In this study, homocysteine at physiological concentrations is shown to act as a pro-oxidant for three stages of copper-mediated LDL oxidation (initiation, conjugated diene formation and aldehyde formation), whereas at high concentration, it acts as an antioxidant. The affinity for copper of homocysteine and related copper ligands homocysteine, cystathionine and djenkolate was measured, showing that at high concentrations (100 microM) under our assay conditions, they bind essentially all of the copper present. This is used to rationalise the behaviour of these ligands, which stimulate LDL oxidation at low concentration but generally inhibit it at high concentration. Albumin strongly reduced the effect of homocystine on lag time for LDL oxidation, suggesting that the effects of homocystine are due to copper binding. In contrast, copper binding does not fully explain the pro-oxidant behaviour of low concentrations of homocysteine towards LDL, which appears in part at least to be due to stimulation of free radical production. The likely role of homocysteine in LDL oxidation in vivo is discussed in the light of these results.

Probabilistic kinetic model of slow oxidation of low-density lipoprotein: I. Theory

The microscopic probabilistic model has been introduced to explain the kinetics of very slow oxidation of low-density lipoprotein (LDL) from human plasma. The LDL oxidation, carried out in very unfavorable conditions, is assumed to be initiated by the traces of the transition-metal ions associated with the lipoprotein. The substrates for the metal-ion attack are alpha-tocopherol and the pre-formed lipid hydroperoxide. The theory assumes oscillation of the metal ions and alpha-tocopherol from the oxidized to the reduced states. In this model alpha-tocopherol acts as a pro-oxidant. The entire oxidation process consists of rare bursts of events in individual LDL particles. The reactions within the particles are treated in terms of probabilities of individual active species to participate in a specified reaction. The circular flow of the radical reactions could be visualized as circular flow of microscopic probabilities. The empirical, macroscopic quantities are quantitatively related with the microscopic probabilities, determined by a set of five adjustable parameters. The differential equations describing the initial radical generation rate and the rates of change of concentration of oxygen, hydroperoxide, co-antioxidant and trapped radicals in an LDL system are numerically solved in a finite difference approach.

Contribution of HDL-apolipoproteins to the inhibition of low density lipoprotein oxidation and lipid accumulation in macrophages

High-density lipoprotein (HDL) is known as a protective factor against atherosclerosis. However, whether HDL-apolipoproteins (apo-HDL) contribute to the protection in arterial cells remains unclear. The localization patterns of human apolipoproteins in atherosclerotic arteries were determined using immunohistochemical examination. The results indicate that several apolipoproteins are retained in component cells of the coronary artery walls. To elucidate the possible roles of apo-HDL in the protection of atherosclerotic lesion formation, we investigated the effects of apo-HDL on the formation of conjugated diene (CD) in a cell-free system and thiobarbituric acid-reactive substances (TBARS) in the medium of a macrophage-mediated LDL oxidation system. The results showed that apo-HDL significantly exerted an inhibitory effect on LDL lipid oxidation in vitro. In addition, apo-HDL decreased cholesterol influx but enhanced cholesterol efflux from J774 macrophages in a dose-dependent manner. These results are consistent with the notion that there is reduced intracellular lipid accumulation in apo-HDL treated macrophages. These data provide a direct evidence for apo-HDL in protecting LDL from oxidative modification and in reducing the accumulation of cholesterol and lipid droplets by J774 macrophages.

The mechanism of action of antioxidants against lipoprotein peroxidation, evaluation based on kinetic experiments

Peroxidation of blood lipoproteins is regarded as a key event in the development of atherosclerosis. Hence, attenuation of the oxidative modification of lipoproteins by natural and synthetic antioxidants in vivo is considered a possible way of prevention of cardiovascular disorders. The assessment of the susceptibility of lipoproteins to oxidation is commonly based on in vitro oxidation experiments. Monitoring of oxidation provides the kinetic profile characteristic for the given lipoprotein preparation. The kinetic profile of peroxidation is characterized by three major parameters: the lag preceding rapid oxidation, the maximal rate of oxidation (V(max)) and the maximal accumulation of oxidation products (OD(max)). Addition of antioxidants alters this pattern, affecting the kinetic parameters of oxidation. In particular, antioxidants may prolong the lag and/or decrease the V(max) and/or decrease the OD(max). Such specific variation of the set of kinetic parameters may provide important information on the mechanism of the inhibitory action of a given antioxidant (scavenging free radicals, metal-binding or other mechanisms). Numerous natural and synthetic compounds were reported to inhibit oxidation of lipoproteins. Based on the analysis of reported effects and theoretical considerations, we propose a simple protocol that relates the kinetic effects of a given antioxidant to the mechanism of its action.

Copper-induced peroxidation of liposomal palmitoyllinoleoylphosphatidylcholine (PLPC), effect of antioxidants and its dependence on the oxidative stress

In an attempt to deepen our understanding of the mechanisms responsible for lipoprotein peroxidation, we have studied the kinetics of copper-induced peroxidation of the polyunsaturated fatty acid residues in model membranes (small, unilamellar liposomes) composed of palmitoyllinoleoylphosphatidylcholine (PLPC). Liposomes were prepared by sonication and exposed to CuCl(2) in the absence or presence of naturally occurring reductants (ascorbic acid (AA) and/or alpha-tocopherol (Toc)) and/or a Cu(I) chelator (bathocuproinedisulfonic acid (BC) or neocuproine (NC)). The resultant oxidation process was monitored by recording the time-dependence of the absorbance at several wavelengths. The observed results reveal that copper-induced peroxidation of PLPC is very slow even at relatively high copper concentrations, but occurs rapidly in the presence of ascorbate, even at sub-micromolar copper concentrations. When added from an ethanolic solution, tocopherol had similar pro-oxidative effects, whereas when introduced into the liposomes by co-sonication tocopherol exhibited a marked antioxidative effect. Under the latter conditions, ascorbate inhibited peroxidation of the tocopherol-containing bilayers possibly by regeneration of tocopherol. Similarly, both ascorbate and tocopherol exhibit antioxidative potency when the PLPC liposomes are exposed to the high oxidative stress imposed by chelated copper, which is more redox-active than free copper. The biological significance of these results has yet to be evaluated.

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.

Oxidized low density lipoproteins downregulate LPS-induced platelet-activating factor receptor expression in human monocyte-derived macrophages: implications for LPS-induced nuclear factor-kappaB binding activity

Monocytes/macrophages play a key role in atherogenesis due to their inflammatory properties including formation of lipid mediators such as platelet-activating-factor (PAF). We investigated the effect of oxidized low-density lipoprotein (oxLDL) on lipopolysaccharide (LPS)-induced PAF receptor (PAF-R) expression in human macrophages and the implication of the nuclear factor (NF)-kappaB in this regulation. LPS-treatment (1 microg.mL(-1)) of macrophages increased PAF binding and PAF-R mRNA expression by 56% (P < 0.05) and twofold (P < 0.01), respectively. In contrast, highly oxidized low-density lipoprotein [ox24hLDL; 100 microg.mL(-1); thiobarbituric acid reacting substances: 31 +/- 3 nmol equiv. malondialdehyde (MDA).mg protein LDL-1] diminished PAF-R expression (-69%; P < 0.05) and mRNA level (- 45%; P < 0.01). LPS pretreatment induced the activated form of p65 in the nuclear compartment of macrophages (detected by Western blotting) and NF-kappaB binding activity (by electrophoretic mobility shift assay). Treatment of macrophages with ox24hLDL suppressed the LPS-induced binding of NF-kappaB to DNA. In addition, treatment of macrophages with lysophosphatidylcholine (2 and 10 microM), a major component of oxLDL, inhibited the LPS-induced NF-kappaB binding to DNA and reduced PAF binding by 30 and 70%, respectively. In conclusion, oxLDL may downregulate PAF-R expression in human macrophages by inhibiting LPS-induced NF-kappaB binding to DNA.

Measurement of copper-binding sites on low density lipoprotein

Copper is often used to oxidize low density lipoprotein (LDL) in experiments in vitro and is a candidate for oxidizing LDL in atherosclerotic lesions. The binding of copper ions to LDL is usually thought to be a prerequisite for LDL oxidation by copper, although estimates of LDL copper binding vary widely. We have developed and validated an equilibrium dialysis assay in a MOPS-buffered system to measure copper binding to LDL and have found 38.6+/-0.7 (mean+/-SEM, n=25) copper binding sites on LDL. The binding was saturated at a copper concentration of 10 micromol/L at LDL concentrations of up to 1 mg protein/mL. Copper-binding capacity increased progressively and markedly when LDL was oxidized to increasing extents. Chemical modification of histidyl and lysyl residues on apolipoprotein B-100 reduced the number of binding sites by 56% and 23%, respectively. As an example of the potential of this method to assess the effects of antioxidants on copper binding to LDL, we have shown that the flavonoids myricetin, quercetin, and catechin (but not epicatechin, kaempferol, or morin), at concentrations equimolar to the copper present (10 micromol/L), significantly decreased copper binding to LDL by 82%, 56%, and 20%, respectively.

The dose-dependent effect of copper-chelating agents on the kinetics of peroxidation of low-density lipoprotein (LDL)

Copper-induced peroxidation of lipoproteins involves continuous production of free radicals via a redox cycle of copper. Formation of Cu(I) during Cu(II)-induced peroxidation of LDL was previously demonstrated by accumulation of the colored complexes of Cu(I) in the presence of one of the Cu(I)-specific chelators bathocuproine (BC) or neocuproine (NC). All the studies conducted thus far employed high concentrations of these chelators (chelator/Cu(II) > 10). Under these conditions, at low copper concentrations the chelators prolonged the lag preceding oxidation, whereas at high copper concentrations the chelators shortened the lag. In an attempt to gain understanding of these non-monotonic effects, we have studied systematically the peroxidation of LDL (0.1 microM, 50 microg protein/mL) at varying concentrations of NC or BC over a wide range of concentrations of the chelators and copper. These studies revealed that: (i) At copper concentrations of 5 microM and below, NC prolonged the lag in a monotonic, dose-dependent fashion typical for other complexing agents. However, unlike with other chelators, the maximal rate of oxidation was only slightly reduced (if at all). (ii) At copper concentrations of 15 microM and above, the addition of about 20 microM NC or BC resulted in prolongation of the lag, but this effect became smaller at higher concentrations of the chelators, and at yet higher concentrations the lag became much shorter than that observed in the absence of chelators. Throughout the whole range of NC concentrations, the maximal rate of peroxidation increased monotonically upon increasing the NC concentration. (iii) Unlike in the absence of chelators, the prooxidative effect of copper did not exhibit saturation with respect to copper, up to copper concentrations of 30 microM. Based on these results we conclude that the copper-chelates can partition into the hydrophobic core of LDL particles and induce peroxidation by forming free radicals within the core. This may be significant with respect to the understanding of the possible mechanisms of peroxidation by chelated transition metals in vivo.

Cu2+ -induced low density lipoprotein peroxidation is dependent on the initial O2 concentration: an O2 consumption study

Atherosclerotic plaques form in the arterial intima, where low density lipoprotein (LDL) is thought to be oxidatively modified at sites which may contain catalytic amounts of copper in the presence of low O2 tension. We have investigated O2 consumption during LDL peroxidation induced by Cu2+ ions in vitro and found two phases: a lag phase followed by a phase of rapid O2 consumption. The length of the lag phase was dependent on Cu2+ and on initial O2 concentrations; increasing either decreased the lag time; however, LDL. concentration had no effect. LDL-induced Cu2+ reduction, however, was not affected by low initial O2 concentrations, suggesting that O2 is not required for LDL-mediated reduction of Cu2+. Following the lag phase, O2 consumption was dependent upon LDL or initial O2 concentrations; Cu2+ concentrations had little effect, suggesting that the propagation phase is more dependent on the presence of LDL lipids and O2 as substrates for the reaction. In summary, LDL peroxidation takes place in the presence of Cu2+ at low O2 tension; however, the reaction is dependent upon initial O2 concentrations; increases shorten the lag phase and accelerate O2 consumption.

Kinetic analysis of copper-induced peroxidation of HDL, autoaccelerated and tocopherol-mediated peroxidation

Comparison of the kinetic profiles of copper-induced peroxidation of HDL and LDL at different copper concentrations reveals that under all the studied experimental conditions HDL is more susceptible to oxidation than LDL. The mechanism responsible for HDL oxidation is a complex function of the copper/HDL ratio and of the tocopherol content of the HDL. At high copper concentrations, the kinetic profiles were similar to those observed for LDL oxidation, namely, relatively rapid accumulation of oxidation products, via an autoaccelerated, noninhibited mechanism, was preceded by an initial "lag phase." Under these conditions, the maximal peroxidation rate (V(max)) of HDL and LDL depended similarly on the molar ratio of bound copper/lipoprotein. Analysis of this dependency in terms of the binding characteristics of copper to lipoprotein, yielded similar dissociation constant (K = 10(-6) M) but different maximal binding capacities for the two lipoproteins (8 Cu(+2)/HDL as compared to 17 Cu(+2)/LDL). Given the size difference between HDL and LDL, these results imply that the maximal surface density of bound copper is at least 2-fold higher for HDL than for LDL. This difference may be responsible for the higher susceptibility of HDL to copper-induced oxidation in the presence of high copper concentrations. At relatively low copper concentrations, the kinetic profile of HDL oxidation was biphasic, similar to but more pronounced than the biphasic kinetics observed for the oxidation of LDL lipids at the same concentration of copper. Our results are consistent with the hypothesis that the first phase of rapid oxidation occurs via a tocopherol-mediated-peroxidation (TMP) mechanism. Accordingly, enrichment of HDL with tocopherol resulted in enhanced accumulation of hydroperoxides during the first phase of copper-induced oxidation. Notably, the maximal accumulation during the first phase decreased upon increasing the ratio of bound copper/HDL. This behavior can be predicted theoretically for peroxidation via a TMP mechanism, in opposition to autoaccelerated peroxidation. The possible pathophysiological significance of these findings is discussed.

Interaction of hyaluronic acid-linked phosphatidylethanolamine (HyPE) with LDL and its effect on the susceptibility of LDL lipids to oxidation

The amphiphilic polysaccharide hyaluronic acid-linked phosphatidylethanolamine (HyPE), synthesized by covalently binding dipalmitoyl-phosphatidylethanolamine (DPPE) to short chain hyaluronic acid (mol. wt. approximately = 30 000), interacts with low-density lipoproteins (LDL), to form a 'sugar-decoration' of the LDL surface. This results in an increase in the apparent size of the LDL particles, as studied by photon correlation spectroscopy, and in broadening of the 1H NMR signals of the LDL's phospholipids. Experiments conducted with fluorescently-labeled HyPE indicate that the interaction of HyPE with LDL involves incorporation of the hydrocarbon chains of this amphiphilic polysaccharide into the outer monolayer of the LDL. This interaction also inhibits the copper-induced oxidation of the LDL polyunsaturated fatty acids, avoiding oxidation altogether when the concentration of HyPE is higher than a tenth of the concentration of the LDL's phospholipids. This can not be attributed to competitive binding of copper by HyPE. We propose that the protection of LDL lipids against copper-induced oxidation is due to formation of a sugar network around the LDL.

Copper-induced LDL peroxidation: interrelated dependencies of the kinetics on the concentrations of copper, hydroperoxides and tocopherol

Excessive uptake of oxidized low density lipoprotein plays a role in the onset of atherosclerosis. Lipid-associated antioxidants, the most abundant of which is tocopherol (vitamin E), are therefore believed to have anti-atherogenic properties. By contrast, hydroperoxides enhance the peroxidation of low density lipoprotein. We demonstrate that none of these compounds markedly affect the maximal rate of oxidation of low density lipoprotein, whereas the lag preceding rapid oxidation is prolonged by tocopherol but shortened by hydroperoxides. The corresponding 'prolongation' and 'shortening' can be compensated by each other in low density lipoprotein preparations enriched with both these compounds. The dependence of the balance between the effects of tocopherol and hydroperoxides on the copper concentration indicates that the antioxidative effect of vitamin E increases with the oxidative stress.

Higher cholesterol in human LDL is associated with the increase of oxidation susceptibility and the decrease of antioxidant defence: experimental and simulation data

Increased low-density lipoprotein (LDL) cholesterol is a recognized risk factor for atherosclerosis. There is also strong evidence that oxidatively modified LDL initiates the development of this pathological process and the administration of antioxidants might have a protective effect. However, the appropriate trials did not provide completely consistent results. We found in this study that the oxidation kinetics and also the antioxidant effectiveness are different depending on the cholesterol content in LDL. Higher cholesterol in LDL causes an acceleration of its oxidation as well as an increase of resistance to the antioxidative effect of ascorbic acid. In searching for a theoretical background of this dual impact of cholesterol in LDL, computer simulation of LDL oxidation was used. It was found that the pre-existing level of lipid hydroperoxides together with the total amount of oxidizable lipid substrate associated with the cholesterol level in LDL were satisfactory prerequisites for a best fit to the experimental data. In conclusion, this study provides at least a partial explanation for some failures to arrest, by administration of antioxidants, the progression of atherosclerosis in animal and human hypercholesterolemia.