Prooxidant role of vitamin E in copper induced lipid peroxidation

Tobacco exposure in adults and children with proteinuric glomerulopathies: a NEPTUNE cohort study

BACKGROUND

Tobacco exposure has been recognized as a risk factor for cardiovascular disease (CVD) and progression of kidney disease. Patients with proteinuric glomerulopathies are at increased risk for cardiovascular morbidity and mortality. Multiple studies have linked tobacco exposure to CVD and chronic kidney disease, but the relationships between smoking and proteinuric glomerulopathies in adults and children have not been previously explored.

METHODS

Data from the Nephrotic Syndrome Study Network (NEPTUNE), a multi-center prospective observational study of participants with proteinuric glomerulopathies, was analyzed. 371 adults and 192 children enrolled in NEPTUNE were included in the analysis. Self-reported tobacco exposure was classified as non-smoker, active smoker, former smoker, or exclusive passive smoker. Baseline serum cotinine levels were measured in a sub-cohort of 178 participants.

RESULTS

The prevalence of active smokers, former smokers and exclusive passive smoking among adults at baseline was 14.6%, 29.1% and 4.9%, respectively. Passive smoke exposure was 16.7% among children. Active smoking (reference non-smoking) was significantly associated with greater total cholesterol among adults (β 17.91 95% CI 0.06, 35.76, p = 0.049) while passive smoking (reference non-smoking) was significantly associated with greater proteinuria over time among children (β 1.23 95% CI 0.13, 2.33, p = 0.03). Higher cotinine levels were associated with higher baseline eGFR (r = 0.17, p = 0.03).

CONCLUSION

Tobacco exposure is associated with greater risk for CVD and worse kidney disease outcomes in adults and children with proteinuric glomerulopathies. Preventive strategies to reduce tobacco exposure may help protect against future cardiovascular and kidney morbidity and mortality in patients with proteinuric glomerulopathies.

Vitamin C and E Supplements Enhance the Antioxidant Capacity of Erythrocytes Obtained from Aged Rats

BACKGROUND/AIMS

The main purpose of the present study was to investigate the effects of vitamin C and E supplements on the antioxidant capacity of erythrocytes obtained from young and aged rats.

METHODS

Male Wistar rats aged 3 and 24 months were used. Vitamins C and E were injected at doses of 200 mg/kg (day) intraperitoneally in young and aged groups. The antioxidant capacity, oxidant stress parameters, and deformability of red blood cells collected from different age stages were evaluated. An in vitro oxidation system was constructed to explore the mechanisms of antioxidant capacity change in the vitamin treatment groups.

RESULTS

Treatment with vitamins C and E can effectively restore the antioxidant capacity and deformability of red blood cells (RBCs) in aged rats. Under in vitro oxidative conditions, an age-dependent decline in the influx rate of L-cysteine was observed. This was significantly improved following treatment with vitamins C and E.

CONCLUSION

We present evidence of an improvement in the antioxidant capacity of RBCs by treatment with vitamins C and E in aged rats. These observations also suggest that treatment with vitamins C and E improves glutathione synthesis by enhancing the influx rate of L-cysteine through the modification of membrane proteins and lipids.

A theoretical study on antioxidant activity of ferulic acid and its ester derivatives

Phenolic compounds play a very crucial role as antioxidant that can prevent various diseases caused by free radicals in human body. Although, lots of natural phenolic compounds having antioxidant activity are available nowadays, the modeling of compounds with naturally available phenolics as building blocks is very important in order to get enhanced antioxidant activity. In this study, Ferulic acid (FA), one natural phenolic acid present in coffee, apples, orange, etc., is taken as building block and its ester derivatives with different alkyl groups are subjected to measure the antioxidant activity by using density functional theory (DFT). Various parameters like bond dissociation enthalpy (BDE), vertical ionization potential (IP[Formula: see text]), reactivity descriptors, metal chelation ability, etc. are used to measure the antioxidant activity. All the parameters suggest that the ester derivatives are superior antioxidants to the parent FA. Since FA has been reported to be present as esters in many herbs and plants, hence our study provides a route to study the structure activity relationship of this class of natural phenolics with antioxidant activity.

Effects of Low-Dose versus High-Dose γ-Tocotrienol on the Bone Cells Exposed to the Hydrogen Peroxide-Induced Oxidative Stress and Apoptosis

Oxidative stress and apoptosis can disrupt the bone formation activity of osteoblasts which can lead to osteoporosis. This study was conducted to investigate the effects of γ-tocotrienol on lipid peroxidation, antioxidant enzymes activities, and apoptosis of osteoblast exposed to hydrogen peroxide (H₂O₂). Osteoblasts were treated with 1, 10, and 100 μM of γ-tocotrienol for 24 hours before being exposed to 490 μM (IC₅₀) H₂O₂ for 2 hours. Results showed that γ-tocotrienol prevented the malondialdehyde (MDA) elevation induced by H₂O₂ in a dose-dependent manner. As for the antioxidant enzymes assays, all doses of γ-tocotrienol were able to prevent the reduction in SOD and CAT activities, but only the dose of 1 μM of GTT was able to prevent the reduction in GPx. As for the apoptosis assays, γ-tocotrienol was able to reduce apoptosis at the dose of 1 and 10 μM. However, the dose of 100 μM of γ-tocotrienol induced an even higher apoptosis than H₂O₂. In conclusion, low doses of γ-tocotrienol offered protection for osteoblasts against H₂O₂ toxicity, but itself caused toxicity at the high doses.

Liposomal Antioxidants for Protection against Oxidant-Induced Damage

Reactive oxygen species (ROS), including superoxide anion, hydrogen peroxide, and hydroxyl radical, can be formed as normal products of aerobic metabolism and can be produced at elevated rates under pathophysiological conditions. Overproduction and/or insufficient removal of ROS result in significant damage to cell structure and functions. In vitro studies showed that antioxidants, when applied directly and at relatively high concentrations to cellular systems, are effective in conferring protection against the damaging actions of ROS, but results from animal and human studies showed that several antioxidants provide only modest benefit and even possible harm. Antioxidants have yet to be rendered into reliable and safe therapies because of their poor solubility, inability to cross membrane barriers, extensive first-pass metabolism, and rapid clearance from cells. There is considerable interest towards the development of drug-delivery systems that would result in the selective delivery of antioxidants to tissues in sufficient concentrations to ameliorate oxidant-induced tissue injuries. Liposomes are biocompatible, biodegradable, and nontoxic artificial phospholipid vesicles that offer the possibility of carrying hydrophilic, hydrophobic, and amphiphilic molecules. This paper focus on the use of liposomes for the delivery of antioxidants in the prevention or treatment of pathological conditions related to oxidative stress.

An (1)O2 route to γ-hydroxyalkenal phospholipids by vitamin E-induced fragmentation of hydroperoxydiene-derived endoperoxides

Biologically active phospholipids that incorporate an oxidatively truncated acyl chain terminated by a γ-hydroxyalkenal are generated in vivo. The γ-hydroxyalkenal moiety protrudes from lipid bilayers like whiskers that serve as ligands for the scavenger receptor CD36, fostering endocytosis, e.g., of oxidatively damaged photoreceptor cell outer segments by retinal pigmented endothelial cells. They also covalently modify proteins generating carboxyalkyl pyrroles incorporating the ε-amino group of protein lysyl residues. We postulated that γ-hydroxyalkenals could be generated, e.g., in the eye, through fragmentation of hydroperoxy endoperoxides produced in the retina through reactions of singlet molecular oxygen with polyunsaturated phospholipids. Since phospholipid esters are far more abundant in the retina than free fatty acids, we examined the influence of a membrane environment on the fate of hydroperoxy endoperoxides. We now report that linoleate hydroperoxy endoperoxides in thin films and their phospholipid esters in biomimetic membranes fragment to γ-hydroxyalkenals, and fragmentation is stoichiometrically induced by vitamin E. The product distribution from fragmentation of the free acid in the homogeneous environment of a thin film is remarkably different from that from the corresponding phospholipid in a membrane. In the membrane, further oxidation of the initially formed γ-hydroxyalkenal to a butenolide is disfavored. A conformational preference for the γ-hydroxyalkenal, to protrude from the membrane into the aqueous phase, may protect it from oxidation induced by lipid hydroperoxides that remain buried in the lipophilic membrane core.

Fibrinogen is a co-antioxidant that supplements the vitamin E analog trolox in a model system

OBJECTIVE

It appears that the atherosclerotic plaque is a prooxidant environment where some molecules that are normally antioxidants, including vitamins C and E, may act as prooxidants that contribute to atherosclerosis by oxidizing LDL. Some molecules can act as co-antioxidants to eliminate this prooxidant effect by recycling or other mechanisms of supplementation. Fibrinogen and other acute phase proteins found in the plaque are antioxidants. We hypothesized that fibrinogen can act as a co-antioxidant to supplement vitamin E thereby eliminating its oxidative effect under prooxidant conditions. We tested a model system for this hypothesis using the vitamin E analogue Trolox in a cell free system.

METHODS

LDL was oxidized using 5 umol/l copper. Antioxidant conditions were achieved by adding the antioxidants immediately with LDL, while prooxidant conditions were created by adding antioxidants after a 40 min delay. Oxidation was monitored as the lag phase at 234 nm.

RESULTS

Under antioxidant conditions, the protective effect of fibrinogen and Trolox combined together were about equal to the sum of the anitioxidant effects of each alone (additive), while under prooxidant conditions the combined protection was 54-200% greater (synergistic). These effects were different than those of vitamin C with Trolox in that under antioxidant conditions fibrinogen and Trolox were additive while vitamin C and Trolox showed strong synergistic effects, and in that unlike vitamin C and Trolox fibrinogen showed no prooxidant tendencies under prooxidant reaction conditions.

CONCLUSIONS

The data indicated that fibrinogen did act as a co-antioxidant to supplement Trolox and eliminate its prooxidant effect, most probably, by directly quenching the phenoxyl radical, because unlike vitamin C, fibrinogen did not appear to recycle vitamin E. But fibrinogen may act as a universal antioxidant, since unlike Trolox and vitamin C, it showed little tendency toward becoming a prooxidant.

Chemical reactivities and physical effects in comparison between tocopherols and tocotrienols: physiological significance and prospects as antioxidants

Vitamin E is a generic term for all tocopherol and tocotrienol derivatives. The most abundant and active form of vitamin E isoforms in vivo is alpha-tocopherol, but recently the roles of other forms of vitamin E have received renewed attention. In this review, we summarize the differences among alpha-, beta-, gamma-, delta-tocopherols and tocotrienols specifically regarding the following points; (i) their radical-scavenging efficacies and chemical reactivity with metal ions in solution, (ii) their physical effects at the liposomal membrane interior, and (iii) their protective effects against cell toxicity. Moreover, the physiological significance and future prospects for using vitamin E, especially tocotrienols, for the prevention and treatment of disease are discussed.

Exercise and immobilization in aging animals: the involvement of oxidative stress and NF-kappaB activation

In the early 1980s, the concept of threshold of age in exercise and aging was proposed. In several studies it was shown that subjecting young animals to short periods of moderate to intense exercise improved the biochemical and morphological status of their skeletal muscles. This was not the case for old animals subjected to the same exercise regimens. Thus, by measuring several muscle energy-providing enzymes as well as antioxidant enzymes it was demonstrated that their levels and activities increased in young animals postexercise, while in old animals reduced activity of these enzymes was found on completion of the training. However, old animals that started training in young and middle age were still capable of improving their muscle condition as a result of exercise, as long as the onset of training was below a specific age threshold. In the following years, it was shown that intense physical exercise in young humans and animals is accompanied by elevation of oxidative stress parameters in muscles and other organs. Specifically, strenuous training of animals led to increased protein oxidation as measured by protein carbonyl accumulation in muscles, which could be attenuated by the administration of vitamin E. Nuclear factor kappaB (NF-kappaB) is a redox-sensitive transcription factor responsive to closely related reactive oxygen species (ROS) and reactive nitrogen species (RNS) redox cascades. Its involvement in exercise and immobilization has been demonstrated in several studies, indicating that these conditions may lead to inflammatory responses and to oxidative damage to tissues. Indeed, recent studies have revealed that NF-kappaB is involved in inflammatory responses that may result in muscle protein degradation. Additional studies have also demonstrated that the pattern and type of the NF-kappaB activation pathway vary between muscles of young and old animals subjected to limb immobilization for several weeks. This indicates that NF-kappaB may play a crucial role in the regulation of both inflammatory processes and protein turnover and degradation in muscles of old animals. Thus, the modulation of NF-kappaB activity in muscles of old animals by specific inhibitors may provide a means to retard muscle damage and protein degradation under conditions of immobilization.

Evaluation of the ability of antioxidants to counteract lipid oxidation: Existing methods, new trends and challenges

Oxidative degradation of lipids, especially that induced by reactive oxygen species (ROS), leads to quality deterioration of foods and cosmetics and could have harmful effects on health. Currently, a very promising way to overcome this is to use vegetable antioxidants for nutritional, therapeutic or food quality preservation purposes. A major challenge is to develop tools to assess the antioxidant capacity and real efficacy of these molecules. Many rapid in vitro tests are now available, but they are often performed in dissimilar conditions and different properties are thus frequently measured. The so-called 'direct' methods, which use oxidizable substrates, seem to be the only ones capable of measuring real antioxidant power. Some oxidizable substrates correspond to molecules or natural extracts exhibiting biological activity, such as lipids, proteins or nucleic acids, while others are model substrates that are not encountered in biological systems or foods. Only lipid oxidation and direct methods using lipid-like substrates will be discussed in this review. The main mechanisms of autoxidation and antioxidation are recapitulated, then the four components of a standard test (oxidizable substrate, medium, oxidation conditions and antioxidant) applied to a single antioxidant or complex mixtures are dealt with successively. The study is focused particularly on model lipids, but also on dietary and biological lipids isolated from their natural environment, including lipoproteins and phospholipidic membranes. Then the advantages and drawbacks of existing methods and new approaches are compared according to the context. Finally, recent trends based on the chemometric strategy are introduced as a highly promising prospect for harmonizing in vitro methods.

Cell-permeable, mitochondrial-targeted, peptide antioxidants

Cellular oxidative injury has been implicated in aging and a wide array of clinical disorders including ischemia-reperfusion injury; neurodegenerative diseases; diabetes; inflammatory diseases such as atherosclerosis, arthritis, and hepatitis; and drug-induced toxicity. However, available antioxidants have not proven to be particularly effective against many of these disorders. A possibility is that some of the antioxidants do not reach the relevant sites of free radical generation, especially if mitochondria are the primary source of reactive oxygen species (ROS). The SS (Szeto-Schiller) peptide antioxidants represent a novel approach with targeted delivery of antioxidants to the inner mitochondrial membrane. The structural motif of these SS peptides centers on alternating aromatic residues and basic amino acids (aromatic-cationic peptides). These SS peptides can scavenge hydrogen peroxide and peroxynitrite and inhibit lipid peroxidation. Their antioxidant action can be attributed to the tyrosine or dimethyltyrosine residue. By reducing mitochondrial ROS, these peptides inhibit mitochondrial permeability transition and cytochrome c release, thus preventing oxidant-induced cell death. Because these peptides concentrate >1000-fold in the inner mitochondrial membrane, they prevent oxidative cell death with EC50 in the nM range. Preclinical studies support their potential use for ischemia-reperfusion injury and neurodegenerative disorders. Although peptides have often been considered to be poor drug candidates, these small peptides have excellent "druggable" properties, making them promising agents for many diseases with unmet needs.

The protection of bioenergetic functions in mitochondria by new synthetic chromanols

alpha-Tocopherol is the most important lipophilic antioxidant of the chromanol type protecting biomembranes from lipid peroxidation (LPO). Therefore, alpha-tocopherol and its derivatives are frequently used in the therapy or prevention of oxygen radical-derived diseases. In the present study, novel chromanol-type antioxidants (twin-chromanol, cis- and trans-oxachromanol) as well as the well-known short-chain analogue of alpha-tocopherol, pentamethyl-chromanol, were tested for their antioxidative potency in rat heart mitochondria (RHM). Our experiments revealed that the bioenergetic parameters of mitochondria were not deteriorated in the presence of chromanols (up to 50 nmol/mg protein). Exposure of RHM to cumene hydroperoxide and Fe2+ (final concentrations 50 microM each), inducing LPO, significantly affected their bioenergetic parameters which were determined in the presence of glutamate and malate (substrates of mitochondrial complex I). Alterations of the bioenergetic parameters were partially prevented in a concentration-dependent manner by preincubating RHM with antioxidants before adding the radical-generating system. In the lower concentration range, twin-chromanol turned out to be more efficient than pentamethyl-chromanol, both being far more protective than cis- and trans-oxachromanol. Measurement of protein-bound SH groups and thiobarbituric acid-reactive substances revealed that this protective effect was due to their antioxidative action. Furthermore, HPLC measurements of alpha-tocopherol and alpha-tocopheryl quinone in rat liver mitochondria demonstrated an alpha-tocopherol-sparing effect of twin-chromanol. In conclusion, new chromanol-type antioxidants, especially twin-chromanol, were able to improve bioenergetic and biochemical parameters of mitochondria exposed to oxidative stress.

Oxidative Susceptibility of Unfractionated Serum or Plasma: Response to Antioxidants in Vitro and to Antioxidant Supplementation

Background: The susceptibility of plasma lipids to oxidation is thought to be a factor contributing to atherogenic risk. Various groups have studied the in vitro oxidizability of isolated LDL and examined the effects of conventional antioxidants. The drawbacks associated with the isolation of LDL for evaluation of in vitro oxidizability, however, have limited the application of this measurement in large-scale studies.

Methods: We developed and evaluated an assay that can be used to directly assess the oxidative susceptibility of unfractionated serum or plasma lipids, obviating the need for isolation of lipoprotein fractions. Oxidative conditions were initiated in vitro with cuprous chloride and 2,2′-azobis(2-amidinopropane) hydrochloride. The effects of antioxidants added in vitro, and as an oral supplement, were monitored by conjugated diene formation.

Results: The addition of ascorbic acid (0–50 μmol/L) in vitro elicited a dose-dependent protective effect, increasing the lag time to oxidation (P <0.001). In contrast, α-tocopherol demonstrated prooxidant behavior at increasing concentrations (0–50 μmol/L), although we observed a decrease in the maximum rate of oxidation. Our findings are supported by the results from plasma samples of participants in a randomized antioxidant (vitamins C and E) intervention study after acute ischemic stroke. The group receiving vitamins C and E for 14 days showed an increased lag time to plasma lipid oxidation in vitro compared with the nonsupplemented group (P <0.05).

Conclusion: The susceptibility of unfractionated plasma or serum lipids to oxidation in vitro offers an alternative to LDL for evaluating the efficacy of antioxidant regimens.

Effect of vitamin E on glutathione-dependent enzymes

Reactive oxygen species and various electrophiles are involved in the etiology of diseases varying from cancer to cardiovascular and pulmonary disorders. The human body is protected against damaging effects of these compounds by a wide variety of systems. An important line of defense is formed by antioxidants. Vitamin E (consisting of various forms of tocopherols and tocotrienols) is an important fat-soluble, chain-breaking antioxidant. Besides working as an antioxidant, this compound possesses other functions with possible physiological relevance. The glutathione-dependent enzymes form another line of defense. Two important enzymes in this class are the free radical reductase and glutathione S-transferases (GSTs). The GSTs are a family of phase II detoxification enzymes. They can catalyze glutathione conjugation with various electrophiles. In most cases the electrophiles are detoxified by this conjugation, but in some cases the electrophiles are activated. Antioxidants do not act in isolation but form an intricate network. It is, for instance, known that vitamin E, together with glutathione (GSH) and a membrane-bound heat labile GSH-dependent factor, presumably an enzyme, can prevent damaging effects of reactive oxygen species on polyunsaturated fatty acids in biomembranes (lipid peroxidation). This manuscript reviews the interaction between the two defense systems, vitamin E and glutathione-dependent enzymes. On the simplest level, antioxidants such as vitamin E have protective effects on glutathione-dependent enzymes; however, we will see that reality is somewhat more complicated.

Comparative study on the action of tocopherols and tocotrienols as antioxidant: chemical and physical effects

alpha-Tocopherol is known as the most abundant and active form of vitamin E homologues in vivo, but recently the role of other forms of vitamin E has received renewed attention. The antioxidant properties were compared for alpha-, beta-, gamma- and delta-tocopherols and tocotrienols. The following results were obtained: (1). the corresponding tocopherols and tocotrienols exerted the same reactivities toward radicals and the same antioxidant activities against lipid peroxidation in solution and liposomal membranes; (2). tocopherols gave more significant physical effect than tocotrienols on the increase in rigidity at the membrane interior; (3). tocopherols and tocotrienols showed similar mobilities within the membranes, but tocotrienols were more readily transferred between the membranes and incorporated into the membranes than tocopherols; (4). alpha-tocopherol and alpha-tocotrienol, but not the other forms, reduced Cu(II) to give Cu(I) together with alpha-tocopheryl and alpha-tocotrienyl quinones, respectively and exerted prooxidant effect in the oxidation of methyl linoleate in SDS micelles.

Influence of high levels of vitamin E on semen parameters of cocks

This study was an attempt to find whether the reproductive performance of cocks would be influenced by oral administration of different amounts of dietary vitamin E over a long period of time. For that purpose 60 cocks were divided into five dietary groups of 12 animals each, and supplemented with 0 (control group), 100, 1,000, 10,000 or 20,000 IU alpha-tocopherol/kg diet, respectively, over a period of 12 months. The effect on semen parameters and biochemical parameters measured in pooled semen samples and the weight of the testes were determined. The weight of testes decreased with increasing amounts of supplemented vitamin E. Volume, pH, colour, consistency and motility were not influenced by the diets, but density of ejaculate (sperm/microl), total amount of spermatozoa and morphology of sperm were significantly lowered by increasing amounts of supplemented vitamin E. The alpha-tocopherol concentration in ejaculates increased significantly in relation to the diet whereas phospholipid content and thiobarbituric acid-reactive substances (TBARS) of the semen samples decreased significantly with increasing vitamin E supplementation. The reproductive performance of cocks was negatively influenced by high doses of vitamin E although decreased TBARS indicated rising oxidative defence.

Intestinal motility disorder induced by free radicals: a new model mimicking oxidative stress in gut

Literature data suggest that the inflamed intestine may be subjected to a considerable oxidative stress. Therefore, the aim of the present study was to simulate the oxidative stress in the gastrointestinal tract and to explore its effect on intestinal motility. This was attained by treating isolated segments from the rabbit jejunum and from the guinea pig ileum with 2,2'-Azobis (2-amidinopropane) dihydrochloride (ABAP), which generates peroxyl radicals by thermal decomposition. Treatment of intestinal segments with ABAP reduced the muscarinic cholinergic response to acetylcholine in both preparations and induced a dose-dependent inhibition of the spontaneous contractions in the jejunum, also in the presence of tetrodotoxin. ABAP was found to inhibit the contractile response induced by BaCl(2) in guinea pig ileum preparations. This effect was not dose-dependent and it was reversed by Bay-K 8644, which activates voltage operated L-type calcium channels. The rapid and reversible effects of ABAP suggest that it might directly affect L-type calcium channels before lipoperoxidation induction. In conclusion, the results of the present study show that ABAP could be a useful tool to simulate early contractility dysfunctions mediated by oxidative stress.

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.

Opposite roles of selenium-dependent glutathione peroxidase-1 in superoxide generator diquat- and peroxynitrite-induced apoptosis and signaling

Oxidative injuries including apoptosis can be induced by reactive oxygen species (ROS) and reactive nitrogen species (RNS) in aerobic metabolism. We determined impacts of a selenium-dependent glutathione peroxidase-1 (GPX1) on apoptosis induced by diquat (DQ), a ROS (superoxide) generator, and peroxynitrite (PN), a potent RNS. Hepatocytes were isolated from GPX1 knockout (GPX1-/-) or wild-type (WT) mice, and treated with 0.5 mm DQ or 0.1-0.8 mm PN for up to 12 h. Loss of cell viability, high levels of apoptotic cells, and severe DNA fragmentation were produced by DQ in only GPX1-/- cells and by PN in only WT cells. These two groups of cells shared similar cytochrome c release, caspase-3 activation, and p21(WAF1/CIP1) cleavage. Higher levels of protein nitration were induced by PN in WT than GPX1-/- cells. Much less and/or slower cellular GSH depletion was caused by DQ or PN in GPX1-/- than in WT cells, and corresponding GSSG accumulation occurred only in the latter. In conclusion, it is most striking that, although GPX1 protects against apoptosis induced by superoxide-generator DQ, the enzyme actually promotes apoptosis induced by PN in murine hepatocytes. Indeed, GSH is a physiological substrate for GPX1 in coping with ROS in these cells.

A critical overview of the chemistry of copper-dependent low density lipoprotein oxidation: roles of lipid hydroperoxides, alpha-tocopherol, thiols, and ceruloplasmin

The mechanisms by which low-density lipoprotein (LDL) particles undergo oxidative modification to an atherogenic form that is taken up by the macrophage scavenger-receptor pathway have been the subject of extensive research for almost two decades. The most common method for the initiation of LDL oxidation in vitro involves incubation with Cu(II) ions. Although various mechanisms have been proposed to explain the ability of Cu(II) to promote LDL modification, the precise reactions involved in initiating the process remain a matter of contention in the literature. This review provides a critical overview and evaluation of the current theories describing the interactions of copper with the LDL particle. Following discussion of the thermodynamics of reactions dependent upon the decomposition of preexisting lipid hydroperoxides, which are present in all crude LDL preparations, attention is turned to the more difficult (but perhaps more physiologically-relevant) system of the hydroperoxide-free LDL particle. In both systems, the key role of alpha-tocopherol is discussed. In addition to its protective, radical-scavenging action, alpha-tocopherol can also behave as a prooxidant via its reduction of Cu(II) to Cu(I). Generation of Cu(I) greatly facilitates the decomposition of lipid hydroperoxides to chain-carrying radicals, but the mechanisms by which the vitamin promotes LDL oxidation in the absence of preformed hydroperoxides remain more speculative. In addition to the so-called tocopherol-mediated peroxidation model, in which polyunsaturated fatty acid oxidation is initiated by the alpha-tocopheroxyl radical (generated during the reduction of Cu(II) by alpha-tocopherol), an evaluation of the role of the hydroxyl radical is provided. Important interactions between copper ions and thiols are also discussed, particularly in the context of cell-mediated LDL oxidation. Finally, the mechanisms by which ceruloplasmin, a copper-containing plasma protein, can bring about LDL modification are discussed. Improved understanding of the mechanisms of LDL oxidation by copper ions should facilitate the establishment of any physiological role of the metal in LDL modification. It will also assist in the interpretation of studies in which copper systems of LDL oxidation are used in vitro to evaluate potential antioxidants.

The role of vitamin E in atherogenesis: linking the chemical, biological and clinical aspects of the disease

Atherosclerosis is a disease involving both oxidative modifications and disbalance of the immune system. Vitamin E, an endogenous redox-active component of circulating lipoproteins and (sub)cellular membranes whose levels can be manipulated by supplementation, has been shown to play a role in the initiation and progression of the disease. Recent data reveal that the activities of vitamin E go beyond its redox function. Moreover, it has been shown that vitamin E can exacerbate certain processes associated with atherogenesis. In this essay we review the role of biology of atherosclerosis, and suggest that these two facets decide the clinical manifestation and outcome of the disease.

Exacerbating effect of vitamin E supplementation on DNA damage induced in cultured human normal fibroblasts by UVA radiation

The effects of vitamin E supplementation were evaluated in cultured human normal fibroblasts exposed to ultraviolet A radiation (320-380 nm) (UVA). Cells were incubated in medium containing alpha-tocopherol, alpha-tocopherol acetate or the synthetic analog Trolox for 24 h prior to UVA exposure. DNA damage in the form of frank breaks and alkali-labile sites, collectively termed single-strand breaks (SSB), was assayed by the technique of single cell gel electrophoresis (comet assay), immediately following irradiation or after different repair periods. The generation of hydrogen peroxide (H2O2) and superoxide ion (O2.-) was measured by flow cytometry through the oxidation of indicators into fluorescent dyes. It was observed that pretreatment of cells with any form of vitamin E resulted in an increased susceptibility to the photoinduction of DNA SSB and in a longer persistence of damage, whereas no significant change was observed in the production of H2O2 and O2.- reactive oxygen species, compared to untreated controls. These findings indicate that in human normal fibroblasts, exogenously added vitamin E exerts a promoting activity on DNA damage upon UVA irradiation and might lead to increased cytotoxic and mutagenic risks.

Ceruloplasmin and cardiovascular disease

Transition metal ion-mediated oxidation is a commonly used model system for studies of the chemical, structural, and functional modifications of low-density lipoprotein (LDL). The physiological relevance of studies using free metal ions is unclear and has led to an exploration of free metal ion-independent mechanisms of oxidation. We and others have investigated the role of human ceruloplasmin (Cp) in oxidative processes because it the principal copper-containing protein in serum. There is an abundance of epidemiological data that suggests that serum Cp may be an important risk factor predicting myocardial infarction and cardiovascular disease. Biochemical studies have shown that Cp is a potent catalyst of LDL oxidation in vitro. The pro-oxidant activity of Cp requires an intact structure, and a single copper atom at the surface of the protein, near His(426), is required for LDL oxidation. Under conditions where inhibitory protein (such as albumin) is present, LDL oxidation by Cp is optimal in the presence of superoxide, which reduces the surface copper atom of Cp. Cultured vascular endothelial and smooth muscle cells also oxidize LDL in the presence of Cp. Superoxide release by these cells is a critical factor regulating the rate of oxidation. Cultured monocytic cells, when activated by zymosan, can oxidize LDL, but these cells are unique in their secretion of Cp. Inhibitor studies using Cp-specific antibodies and antisense oligonucleotides show that Cp is a major contributor to LDL oxidation by these cells. The role of Cp in lipoprotein oxidation and atherosclerotic lesion progression in vivo has not been directly assessed and is an important area for future studies.

Ascorbic acid enhances 17 beta-estradiol-mediated inhibition of oxidized low density lipoprotein formation

Postmenopausal women who use estrogen appear to be protected from coronary heart disease (CHD). Studies have demonstrated that estrogen can lower low-density lipoprotein (LDL) levels and the antioxidant activity of 17 beta-estradiol can prevent the oxidation of this LDL. Ascorbic acid is regarded as a major hydrophylic antioxidant, however, its impact on the prevention of CHD has yet to be clearly demonstrated. Modified low density lipoprotein (LDL(-)) is an important marker of LDL oxidation in vivo, since it contributes to the oxidative susceptibility of low density lipoprotein, and at physiological levels displays pro-inflammatory and cytotoxic properties. Previously we showed that women taking estrogen replacement therapy have lower LDL(-) levels along with lower predisposition of the LDL to oxidize. In this study, we evaluated the potential action of 17 beta-estradiol (E(2)) in combination with ascorbic acid (AA) measured on the basis of LDL oxidative susceptibility in vitro and in the presence of cultured cells. High concentrations of E(2) were able to inhibit LDL oxidation, whereas in the presence of ascorbic acid nano- to picomolar levels of E(2) were sufficient to suppress LDL oxidation (P<0.05). Preconditioning male aortic endothelial cells (RAEC) with 5 ng/ml of E(2) (E(2)RAEC) reduced the formation of LDL(-) (P<0.005), and a more extensive inhibition was found in the presence of AA (P<0.0001). Interestingly, E(2) enhanced the uptake of LDL in the absence or presence of AA, however, this was not seen for the uptake of LDL(-). These results provide the first evidence that ascorbic acid can enhance the antioxidant effect of E(2) by preventing LDL oxidation by copper ions or cells. The cytoprotective and antiatherogenic effect of E(2) appears to involve a reduction in the extent of oxidized LDL formation and uptake. The enhanced activity of E(2) in the presence of ascorbate indicates that the antioxidant and antiatherosclerosis activity of E(2) may occur at concentrations within the physiological range.

High levels of dietary vitamin E do not replace cellular glutathione peroxidase in protecting mice from acute oxidative stress

Our objective was to determine whether high levels of dietary vitamin E replaced the protection of the Se-dependent cellular glutathione peroxidase (GPX1) against paraquat- or diquat-induced acute oxidative stress in mice. Two experiments were conducted using GPX1 knockout [GPX1(-/-)] mice and wild-type (WT) mice (n = 78/group). In Experiment 1, mice were fed torula yeast-based, Se-adequate (0.4 mg/kg as sodium selenite) diets + 0, 75, 750 or 7,500 mg all-rac-alpha-tocopheryl acetate for 5 wk before an intraperitoneal injection of 50 mg paraquat/kg body weight. In Experiment 2, mice were fed the diet + 0 or 750 mg all-rac-alpha-tocopheryl acetate for 5 wk and were killed 1 or 3 h after an injection of diquat at 12, 24 or 48 mg/kg. In Experiment 1, all mice died of the injection and there were 8- to 15-fold differences (P < 0.001) in survival times between the GPX1(-/-) and the WT mice. Although increasing tocopheryl acetate from 0 to 750 mg/kg extended the survival time of the GPX1(-/-) mice for 2 h (P = 0.06), the highest tocopheryl acetate level resulted in a decrease (P < 0.05) in survival time in the WT mice. The vitamin E-deficient GPX1(-/-) mice had the highest concentration of hepatic thiobarbituric acid reacting substances. In Experiment 2, the diquat-induced formation of hepatic F(2)-isoprostanes was accelerated (P < 0.05) by vitamin E deficiency and was also affected by the GPX1 knockout. Diquat produced much greater (P < 0.01) dose-dependent increases in plasma alanine transaminase (ALT) activities in the GPX1(-/-) than in the WT mice. Hepatic phospholipid hydroperoxide GPX activities were decreased (P < 0.05) by the diquat injection only in the vitamin E-deficient GPX1(-/-) mice. Despite a potent inhibition of hepatic lipid peroxidation, high levels of dietary vitamin E do not replace the protection of GPX1 against the paraquat-induced lethality or the diquat-induced plasma ALT activity increase in mice.

Ceruloplasmin copper induces oxidant damage by a redox process utilizing cell-derived superoxide as reductant

Oxidative damage by transition metals bound to proteins may be an important pathogenic mechanism. Ceruloplasmin (Cp) is a Cu-containing plasma protein thought to be involved in oxidative modification of lipoproteins. We have previously shown that Cp increased cell-mediated low-density lipoprotein (LDL) oxidation by a process requiring cell-derived superoxide, but the underlying chemical mechanism(s) is (are) unknown. We now show that superoxide reduction of Cp Cu is a critical reaction in cellular LDL oxidation. By bathocuproine disulfonate (BCS) binding and by superoxide utilization, we showed that exogenous superoxide reduces a single Cp Cu atom, the same Cu required for LDL oxidation. The Cu atom remained bound to Cp during the redox cycle. Three avenues of evidence showed that vascular cells reduce Cp Cu by a superoxide-dependent process. The 2-fold higher rate of Cp Cu reduction by smooth muscle cells (SMC) compared to endothelial cells (EC) was consistent with their relative rates of superoxide release. Furthermore, Cp Cu reduction by cells was blocked by Cu,Zn superoxide dismutase (SOD1). Finally, the level of superoxide produced by EC and SMC was sufficient to cause the amount of Cu reduction observed. An important role of Cp Cu reduction in LDL oxidation was suggested by results showing that SOD1 inhibited Cp Cu reduction and LDL oxidation by SMC with equal potency, while tumor necrosis factor-alpha stimulated both processes. In summary, these results show that superoxide is a critical cellular reductant of divalent transition metals involved in oxidation, and that protein-bound Cu is a substrate for this reaction. The role of these mechanisms in oxidative processes in vivo has yet to be defined.

Alpha-tocopherol induces oxidative damage to DNA in the presence of copper(II) ions

There is currently much interest in the possibility that dietary antioxidants may confer protection from certain diseases, such as atherosclerosis and cancer. The importance of alpha-tocopherol (vitamin E) as a biological antioxidant is widely recognized. However, pro-oxidant properties of alpha-tocopherol have been observed in chemical systems, and it has been reported that the vitamin can induce tumor formation and act as a complete tumor promotor in laboratory animals. In the present communication, we find that alpha-tocopherol can act as a potent DNA-damaging agent in the presence of copper(II) ions, using a simplified, in vitro model. alpha-Tocopherol was found to promote copper-dependent reactive oxygen species formation from molecular oxygen, resulting in DNA base oxidation and backbone cleavage. Neither alpha-tocopherol nor Cu(II) alone induced DNA damage. Bathocuproine, a Cu(I)-specific chelator, and catalase inhibited the DNA damage, whereas free hydroxyl radical scavengers did not. The order of DNA cleavage sites was thymine, cytosine > guanine residues. Examinations using an oxygen electrode and cytochrome c indicate that molecular oxygen was consumed in the reaction of alpha-tocopherol and Cu(II) and that superoxide was formed. Stoichiometry studies showed that two Cu(II) ions could be reduced by each alpha-tocopherol molecule. Electron spin resonance spin-trapping investigations were then used to demonstrate that hydrogen peroxide interacts with Cu(I) to generate the reactive species responsible for DNA damage, which is either the hydroxyl radical or a species of similar reactivity. These findings may be of relevance to the tumorigenic properties of the vitamin reported in the literature. However, further studies are required to establish the significance of these reactions under in vivo conditions.

Dynamics of vitamin E action against LDL oxidation

Vitamin E acts as an important antioxidant against oxidative modification of low density lipoprotein (LDL) which is accepted as an initial event in the pathogenesis of atherosclerosis. In spite of the numerous studies and reports, the action and role of vitamin E have not been fully elucidated yet. In this brief overview, the dynamics of action of vitamin E as an antioxidant have been discussed and it is emphasized that the total antioxidant potency is determined by the relative importance of many competing reactions which is determined by the reactivities and concentrations of substrates, radicals and antioxidant and by physical factors of the environment.

Passive smoking induces atherogenic changes in low-density lipoprotein

BACKGROUND

According to the American Heart Association, passive smoking is an important risk factor for coronary heart disease (CHD), but the mechanisms underlying this association are not fully understood. We studied the acute effect of passive smoking on the factors that influence the development of CHD: the antioxidant defense of human serum, the extent of lipid peroxidation, and the accumulation of LDL cholesterol in cultured human macrophages, the precursors of foam cells in atherosclerotic lesions.

METHODS AND RESULTS

Blood samples were collected during 2 ordinary working days from healthy, nonsmoking subjects (n=10) before and after (up to 5.5 hours) spending half an hour in a smoke-free area (day 1) or in a room for smokers (day 2). Passive smoking caused an acute decrease (1.5 hours after exposure) in serum ascorbic acid (P<.001) and in serum antioxidant defense (P<.001), a decreased capacity of LDL to resist oxidation (P<.01), and the appearance of increased amounts of lipid peroxidation end products in serum (P<.01). Finally, LDL isolated from subjects after passive smoking was taken up by cultured macrophages at an increased rate (P<.05).

CONCLUSIONS

Exposure of nonsmoking subjects to secondhand smoke breaks down the serum antioxidant defense, leading to accelerated lipid peroxidation, LDL modification, and accumulation of LDL cholesterol in human macrophages. These data provide the pathophysiological background for the recent epidemiological evidence about the increased CHD risk among passive smokers.

Copper ions promote peroxidation of low density lipoprotein lipid by binding to histidine residues of apolipoprotein B100, but they are reduced at other sites on LDL

Oxidized LDL is implicated in the pathogenesis of atherosclerosis. A widely studied model for oxidation of the lipid in LDL involves Cu2+. Recent studies suggest that Cu2+ may be reduced to Cu1+ by alpha-tocopherol to initiate LDL lipid peroxidation. LDL demonstrates binding sites for Cu2-, but the nature of these binding sites, as well their role in promoting Cu2+ reduction and lipid peroxidation, has not been established. In the current studies, we used diethylpyrocarbonate (DEPC) to modify the histidine residues of apolipoprotein B100, the major protein in LDL. First, we demonstrated that histidine residues were preferentially modified by DEPC under our experimental conditions. Then we monitored the kinetics of Cu(2+)-promoted oxidation of LDL and DEPC-modified LDL. In both cases, the progress curve of lipid peroxidation exhibited a lag phase and a propagation phase. However, when LDL was modified with DEPC, the length of the lag phase was prolonged whereas the rate of lipid peroxidation during the propagation phase was lower. Studies with LDL oxidized by 2,2'-azobis (2-amidinopropane) hydrochloride and phosphatidylcholine liposomes oxidized with hydroxyl radical established that DEPC was not acting simply as a nonspecific inhibitor of lipid peroxidation. DEPC treatment of LDL almost completely inhibited its ability to bind Cu2+. These observations suggest that peroxidation of the lipids in LDL can proceed with normal kinetics only when Cu2+ binds preferentially to sites on apolipoprotein B100 that contain histidine residues. We also compared the kinetics of Cu2+ reduction in the absence and presence of DEPC. There was no effect of DEPC modification on either the rate or extent of Cu2+ reduction by LDL. Therefore LDL is likely to contain a second class of binding sites for Cu2+ that does not involve histidine residues. Thus, LDL appears to contain at least two classes of Cu(2+)-binding sites: histidine containing sites, which are responsible in part for promoting lipid peroxidation during the propagation phase, and sites at which Cu2+ is reduced without binding to histidine.

The possible role of copper ions in atherogenesis: the Blue Janus

It has been proposed that the oxidative modification of low density lipoprotein (LDL) is a key event in human atherogenesis. Copper ions can catalyse the oxidative modification of LDL in vitro and there is some evidence that they may also participate in the oxidation of LDL within the arterial wall. However, copper ions also form an intrinsic constituent of superoxide dismutase and caeruloplasmin, enzymes that may be involved in preventing oxidative injury. Atherosclerotic lesions frequently contain considerable quantities of extracellular matrix molecules. These may contribute to the expansion of the arterial neointima, causing luminal narrowing. They may also play a beneficial role by stabilising the plaque. Copper is an essential component of lysyl oxidase, an enzyme involved in the biosynthesis of collagen, which is a major constituent of the extracellular matrix. The impact of alterations in body copper status on atherogenesis is therefore difficult to predict. Experimental and epidemiological data are conflicting and therefore do not provide a clear resolution of this issue. We have reviewed the biochemical and cellular effects of copper ions that may play a role in atherogenesis.

Reduction of Cu(II) by lipid hydroperoxides: implications for the copper-dependent oxidation of low-density lipoprotein

The Cu(II)-promoted oxidation of lipids is a lipid hydroperoxide (LOOH)-dependent process that has been used routinely to assess the oxidizability of low-density lipoprotein (LDL) in human subjects. Metal-dependent redox reactions, including those mediated by copper, have been implicated in the pathogenesis ofatherosclerosis. Despite its widespread use and possible biological significance, key elements of the mechanism are not clear. For example, although it is evident that copper acts as a catalyst, which implies a redox cycle between the Cu(II) and Cu(I) redox states, the reductants remain uncertain. In LDL these could include alpha-tocopherol, amino acid residues on the protein and LOOH. However, both alpha-tocopherol and amino acid residues are probably consumed before the most rapid phase of lipid peroxidation occurs, suggesting that another reductant must be donating electrons to Cu(II), the most likely candidate being LOOH. This role has been disputed, since LDLs nominally devoid of LOOH are still capable of reducing Cu(II) to Cu(I) and thermodynamic calculations for this reaction are not favourable. Direct investigation of the role of LOOH as reductant has not been reported and in the present study, using simple lipid systems and LDL, we have re-examined this issue using the Cu(I) chelator bathocuproine. We have shown that Cu(II) may promote lipid peroxidation in liposomes, which do not contain either protein or alpha-tocopherol, and that this is associated with reduction to Cu(I). The data also indicate that an equilibrium between free Cu(II) and LOOH exists, which only in the presence of an oxidizable substrate, i.e. unsaturated fatty acids, is shifted towards formation of Cu(I) and lipid-derived peroxyl radicals. We propose that reduction of Cu(II) by LOOH is a necessary component in sustaining the propagation of lipid peroxidation and that the formation of peroxyl radicals and their products in a lipid environment is sufficient to overcome thermodynamic barriers to the reaction.

The effect of albumin on copper-induced LDL oxidation

In an attempt to gain deeper understanding of the mechanism or mechanisms responsible for the protective effect of serum albumin against Cu(2+)-induced peroxidation of low density lipoprotein (LDL), we have examined the influence of the concentrations of bovine serum albumin (BSA), Cu2+ and LDL on the kinetics of peroxidation. Since the common method of monitoring the oxidation by continuous recording of the absorbance of conjugated dienes at 234 nm cannot be used at high BSA-concentrations because of the intensive absorption of BSA, we have monitored the time-dependent increase of absorbance at 245 nm. At this wavelength, conjugated dienes absorb intensely, whereas the background absorbance of BSA is low. Using this method, as well as the TBARS assay for determination of malondialdehyde, over a large range of BSA concentrations, we show that in many cases the influence of BSA on the kinetics of oxidation can be compensated for by increasing the concentration of copper. This reconciles the apparent contradiction between previously published data. Detailed studies of the kinetic profiles obtained under different conditions indicate that binding of Cu2+ to albumin plays the major role in its protective effect while other mechanisms contribute much less than copper binding. This conclusion is consistent with the less pronounced effect of BSA on the oxidation induced by the free radical generator AAPH. It is also shown that the copper-albumin complex is capable of inducing LDL oxidation, although the kinetics of the latter process is very different from that of copper-induced oxidation. Nevertheless, when compared to copper induced oxidation at similar concentration of the oxidation-promotor, the kinetics of oxidation induced by copper-albumin complex is very different and is consistent with a tocopherol mediated peroxidation, characteristic under low radical flux. Similar kinetics was observed for copper-induced oxidation only at much lower copper concentrations.

Requirement for, promotion, or inhibition by alpha-tocopherol of radical-induced initiation of plasma lipoprotein lipid peroxidation

alpha-Tocopherol (alpha-TOH), generally regarded as the most important lipid-soluble, chain-breaking antioxidant in human plasma, can also be a pro-oxidant in isolated low-density lipoprotein (LDL) (Bowry V. W.; Stocker R. J. Am. Chem. Soc. 115:6029-6044; 1993). Here we examined whether this pro-oxidant activity of alpha-TOH is of more general relevance. We compared the oxidizability of lipid hydroperoxide-free, in vivo or in vitro alpha-TOH-depleted LDL and high-density lipoprotein (HDL), as well as plasma reconstituted with alpha-TOH-depleted lipoproteins, with that of the corresponding native and alpha-TOH-supplemented samples, using water- and lipid-soluble peroxyl radicals (ROO.), hydroxyl radicals (.OH), Cu2+, the transition metal-containing Ham's F-10 medium, soybean 15-lipoxygenase, and horseradish peroxidase as oxidants. Lipoprotein and plasma oxidizability was assessed by the loss of cholesteryl esters and alpha-TOH and the accumulation of hydroperoxides of cholesteryl esters and phospholipids. Compared to native LDL, HDL, and plasma, the in vivo and in vitro alpha-TOH-depleted counterparts were highly resistant to peroxidation initiation by all oxidants when used at mild radical flux conditions. Wherever tested, the oxidizability of isolated LDL decreased proportionally with decreasing alpha-TOH content. Initiation of LDL lipid oxidation by lipoxygenase and Cu2+ (even up to Cu2+:LDL ratio of 20:1) had an absolute requirement for alpha-TOH. Oxidation of reconstituted plasma with ROO. showed that in the absence of the vitamin, plasma lipids were largely resistant to oxidation, whereas bilirubin and urate oxidized more rapidly. Replenishing the in vitro depleted LDL with alpha-TOH, but not with alpha-tocopherol acetate, fully restored its original content of vitamin E and its oxidizability. Similarly, dietary supplementation with alpha-TOH restored the vitamin content and oxidizability of the in vivo alpha-TOH-depleted lipoproteins and plasma obtained from a patient with familial isolated vitamin E deficiency. Under high fluxes of ROO. and .OH, the activity of alpha-TOH in LDL switched from pro- to anti-oxidant, with the switching point for .OH observed at a lower radical flux than that for ROO.. Together, our results show that alpha-TOH generally makes lipoproteins more reactive towards radical oxidants; this can result in a pro-oxidant activity depending on the specific oxidation conditions.

The role of copper reduction by alpha-tocopherol in low-density lipoprotein oxidation

The oxidation of lipoproteins is thought to be an important early step in atherogenesis. The measurement of lipid peroxidation in low-density lipoprotein (LDL) challenged with Cu2+ has become a widespread test to determine the "susceptibility" of LDL to oxidation. The determination of lag time to oxidation is thought to be a measure of the total antioxidant capacity of the LDL. However, we and others have failed to observe any correlation between lag time and the LDL content of its major lipid antioxidant, alpha-tocopherol. In fact, several studies now suggest a pro-oxidant role for tocopherol under some conditions of LDL oxidation. In the present study we sought to determine if there was a relationship between Cu2+ reduction by LDL and kinetic parameters of LDL oxidation. LDL (0.3 mmol/l cholesterol, approximately 0.1 mg protein/ml) was incubated at 30 degrees C with 2 microM Cu2+ and the formation of conjugated dienes measured over a 4-h period. Using neocuproine, an indicator molecule that specifically complexes Cu+ but not Cu2+, the reduction of Cu2+ by LDL was monitored. The final Cu concentration in these assays was 100 microM and neocuproine 750 microM. Cu+ formation was measured by absorbance at 454 nm. A strong negative correlation was observed between copper reduction by LDL and lag time to oxidation (r = -0.66, p < .005, n = 16). Further experiments showed that (1) LDL was able to reduce Cu2+ to Cu+ in a time and concentration-dependent manner; (2) blocking of free -SH groups on LDL apoprotein B by preincubation with dithionitrobenzoic acid (DTNB) had no significant effect on the rate and extent of Cu2+ reduction; (3) consumption of tocopherol in LDL undergoing oxidation with Cu was very rapid (rate = 6 x 10(-10) M s(-1)). When Cu+ formed during incubation with LDL was complexed with neocuproine, there was significant inhibition of LDL oxidation, as indicated by lipid peroxide formation and mobility on agarose gel electrophoresis. Surprisingly, tocopherol consumption was even more rapid in the presence of neocuproine, consistent with a shift in Cu2+/Cu+ equilibrium and faster reduction of Cu2+ by alpha-tocopherol. These results indicate that under these conditions tocopherol is a major reducing agent in LDL, converting Cu2+ to Cu+, and therefore, may play an important role in promoting LDL oxidation. However, there was no correlation between LDL tocopherol content and reduction of Cu2+. Examination of the time course of Cu2+ reduction in tocopherol enriched and depleted LDL indicates that tocopherol may determine Cu reduction at early time points but that the eventual capacity of LDL to reduce Cu may depend on more complex interactions between tocopherol and other LDL components.

A comparison of the kinetics of low-density lipoprotein oxidation initiated by copper or by azobis (2-amidinopropane)

This article describes the kinetics of low density lipoprotein (LDL) oxidation catalyzed by azobis (2-amidinopropane) dihydrochloride, ABAP, or by copper. The LDLs were isolated from nonhuman primates fed diets enriched in one of three types of fatty acids: saturated fatty acids, monounsaturated fatty acids, predominantly, oleic acid, or polyunsaturated fatty acids, predominantly linoleic acid. Oxidation was followed by monitoring the formation of conjugated diene hydroperoxides from polyunsaturated fatty acids (PUFA). For both copper and ABAP-initiated oxidation, the rate of LDL oxidation depended on the concentrations of initiator, PUFA, and LDL. Except for the dependence on PUFA concentration the rate of LDL oxidation was not directly influenced by the fatty acid composition of the LDL particle. The two initiators had very different dependence on initiator concentration. Because LDL particles are essentially small, lipid-rich droplets, the kinetic descriptions of LDL oxidation assumed: (1), that there was only one chain per particle, and (2) that the radical chain was terminated when a second radical either entered or was formed in the particle. When two LDL samples having very different lag times were mixed, the oxidation profile was bimodal. This finding demonstrated that the oxidation of native LDL particles was independent of the oxidation state of the other native LDL particles in solution, i.e., LDL particles do not rapidly exchange radicals, for example, hydroperoxyl radicals. Oxidation initiated by ABAP was proportional to [ABAP]0.5, suggesting that hydroperoxyl radical recombination between the lipid hydroperoxyl radical and the ABAP-hydroperoxyl radical was the chain-terminating step. The reciprocal of the rate of copper oxidation was linearly related to the reciprocal copper concentration, demonstrating that the binding of copper to LDL was necessary to initiate oxidation. This binding constant showed considerable variability among LDL samples. The kinetic descriptions of LDL oxidation reflect the differences in the mechanisms of initiation and termination.

Antioxidant and prooxidant behavior of flavonoids: structure-activity relationships

The antioxidant and prooxidant behavior of flavonoids and the related activity-structure relationships were investigated in this study using the oxygen radical absorbance capacity assay. Three different reactive species were used in the assay: 2,2'-azobis(2-amidino-propane) dihydrochloride, a peroxyl radical generator; Cu(2+)-H2O2, mainly a hydroxyl radical generator; and Cu2+, a transition metal. Flavonoids including flavones, isoflavones, and flavanones acted as antioxidants against peroxyl and hydroxyl radicals and served as prooxidants in the presence of Cu2+. Both the antioxidant and the copper-initiated prooxidant activities of a flavonoid depend upon the number of hydroxyl substitutions in its backbone structure, which has neither antioxidant nor prooxidant action. In general, the more hydroxyl substitutions, the stronger the antioxidant and prooxidant activities. The flavonoids that contain multiple hydroxyl substitutions showed antiperoxyl radical activities several times stronger than Trolox, an alpha-to copherol analogue. The single hydroxyl substitution at position 5 provides no activity, whereas the di-OH substitution at 3' and 4' is particularly important to the peroxyl radical absorbing activity of a flavonoid. The conjugation between rings A and B does not affect the antioxidant activity but is very important for the copper-initiated prooxidant action of a flavonoid. The O-methylation of the hydroxyl substitutions inactivates both the antioxidant and the prooxidant activities of the flavonoids.

Alpha-tocopherol as a reductant for Cu(II) in human lipoproteins. Triggering role in the initiation of lipoprotein oxidation

Initiation of lipid peroxidation by Cu(II) requires reduction of Cu(II) to Cu(I) as a first step. It is unclear, however, whether this reaction occurs in the course of lipoprotein oxidation. It is also unknown which reductant, if any, can drive the reduction of Cu(II) in this case. We found that Cu(II) was rapidly reduced to Cu(I) by all major human lipoproteins (high, low, and very low density lipoproteins (HDL, LDL, and VLDL), and chylomicrons). Cu(II)-reducing activity was associated with a lipid moiety of the lipoproteins. The rates of Cu(II) reduction by different lipoproteins were similar when the lipoproteins were adjusted to similar alpha-tocopherol concentrations. Enriching lipoproteins with alpha-tocopherol considerably increased the rate of CU(II) reduction. CU(II) reduction by alpha-tocopherol-deficient LDL isolated from a patient with familial inherited vitamin E deficiency was found to occur much slower in comparison with LDL isolated from a donor with a normal plasma level of alpha-tocopherol. Initial rate of CU(II) reduction by alpha-tocopherol-deficient LDL was found to be zero. Enriching LDL with ubiquinol-10 to concentrations close to those of alpha-tocopherol did not influence the reaction rate. When LDL was treated with ebselen to eliminate preformed lipid hydroperoxides, the reaction rate was also not changed significantly. CU(II) reduction was accompanied by a consumption of lipoprotein alpha-tocopherol and accumulation of conjugated dienes in the samples. Increasing alpha-tocopherol content in lipoproteins slightly decreased the rate of conjugated diene accumulation in LDL and HDL and considerably increased it in VLDL. The results suggest that alpha-tocopherol plays a triggering role in the lipoprotein oxidation by CU(II), providing its initial step as follows: alpha TocH + CU(II) --> alpha Toc. + Cu(I) + H+. This reaction appears to diminish or totally eliminate the antioxidative activity of alpha-tocopherol in the course of lipoprotein oxidation.