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

Copper-binding anticancer peptides from the piscidin family: an expanded mechanism that encompasses physical and chemical bilayer disruption

In the search for novel broad-spectrum therapeutics to fight chronic infections, inflammation, and cancer, host defense peptides (HDPs) have garnered increasing interest. Characterizing their biologically-active conformations and minimum motifs for function represents a requisite step to developing them into efficacious and safe therapeutics. Here, we demonstrate that metallating HDPs with Cu²⁺ is an effective chemical strategy to improve their cytotoxicity on cancer cells. Mechanistically, we find that prepared as Cu²⁺-complexes, the peptides not only physically but also chemically damage lipid membranes. Our testing ground features piscidins 1 and 3 (P1/3), two amphipathic, histidine-rich, membrane-interacting, and cell-penetrating HDPs that are α-helical bound to membranes. To investigate their membrane location, permeabilization effects, and lipid-oxidation capability, we employ neutron reflectometry, impedance spectroscopy, neutron diffraction, and UV spectroscopy. While P1-apo is more potent than P3-apo, metallation boosts their cytotoxicities by up to two- and seven-fold, respectively. Remarkably, P3-Cu²⁺ is particularly effective at inserting in bilayers, causing water crevices in the hydrocarbon region and placing Cu²⁺ near the double bonds of the acyl chains, as needed to oxidize them. This study points at a new paradigm where complexing HDPs with Cu²⁺ to expand their mechanistic reach could be explored to design more potent peptide-based anticancer therapeutics.

Antioxidant and Pro-Oxidant Activities of Melatonin in the Presence of Copper and Polyphenols In Vitro and In Vivo

Melatonin is a well-documented antioxidant. Physicochemical analysis using the density functional theory suggests that melatonin is a copper chelating agent; however, experimental evidence is still in demand. The present study investigated the influence of melatonin on reactive oxygen species (ROS) generated from polyphenol autoxidation in the presence of copper. Surprisingly, we found that melatonin paradoxically enhanced ROS formation in a redox system containing low concentrations of copper and quercetin (Que) or (−)-epigallocatechin-3-gallate (EGCG), due to reduction of cupric to cuprous ion by melatonin. Addition of DNA to this system inhibited ROS production, because DNA bound to copper and inhibited copper reduction by melatonin. When melatonin was added to a system containing high concentrations of copper and Que or EGCG, it diminished hydroxyl radical formation as expected. Upon addition of DNA to high concentrations of copper and Que, this pro-oxidative system generated ROS and caused DNA damage. The DNA damage was not prevented by typical scavengers of hydroxyl radical DMSO or mannitol. Under these conditions, melatonin or bathocuproine disulfonate (a copper chelator) protected the DNA from damage by chelating copper. When melatonin was administered intraperitoneally to mice, it inhibited hepatotoxicity and DNA damage evoked by EGCG plus diethyldithiocarbamate (a copper ionophore). Overall, the present study demonstrates the pro-oxidant and antioxidant activities of melatonin in the redox system of copper and polyphenols. The pro-oxidant effect is inhibited by the presence of DNA, which prevents copper reduction by melatonin. Interestingly, in-vivo melatonin protects against copper/polyphenol-induced DNA damage probably via acting as a copper-chelating agent rather than a hydroxyl radical scavenger. Melatonin with a dual function of scavenging hydroxyl radical and chelating copper is a more reliable DNA guardian than antioxidants that only have a single function of scavenging hydroxyl radical.

Chromones with lipoprotein oxidation inhibitory activity from an endophytic fungus Alternaria brassicae JS959 derived from Vitex rotundifolia

Chemical investigation of the ethyl acetate extract of an endophytic fungus, Alternaria brassicae JS959 derived from a halophyte, Vitex rotundifolia, led to the isolation of a new chromone, (2'S)-2-(2-acetoxypropyl)-7-hydroxy-5-methylchromone (1), along with sixteen known compounds: a chromone (2), twelve benzopyranones (3-14) and three perylenequinones (15-17). The chemical structures of the isolated compounds were identified by extensive spectroscopic data analysis including 1D, 2D NMR, HRESIMS, and optical rotation. Of these compounds, 1 and 2 showed inhibitory activity on Cu²⁺‒induced low density lipoprotein (LDL) and high density lipoprotein (HDL) oxidation in human blood plasma. The results suggest that metabolites of endophytic microbes could provide the basis for developing treatments for heart disease.

Peridinin Is an Exceptionally Potent and Membrane-Embedded Inhibitor of Bilayer Lipid Peroxidation

Antilipoperoxidant protein dysfunction is associated with many human diseases, suggesting that bilayer lipid peroxidation may contribute broadly to pathogenesis. Small molecule inhibitors of this membrane-localized chemistry could in theory enable better understanding and/or treatment of such diseases, but currently available compounds have important limitations. Many biological questions thus remain unanswered, and clinical trials have largely been disappointing. Enabled by efficient, building block-based syntheses of three atypical carotenoid natural products produced by microorganisms that thrive in environments of extreme oxidative stress, we found that peridinin is a potent inhibitor of nonenzymatic bilayer lipid peroxidation in liposomes and in primary human endothelial cells. We also found that peridinin blocks monocyte-endothelial cell adhesion, a key step in atherogenesis. A series of frontier solid-state NMR experiments with a site-specifically 13C-labeled isotopolog synthesized using the same MIDA boronate building block-based total synthesis approach revealed that peridinin is completely embedded within and physically spans the hydrophobic core of POPC membranes, maximizing its effective molarity at the site of the targeted lipid peroxidation reactions. Alternatively, the widely used carotenoid astaxanthin is significantly less potent and was found to primarily localize extramembranously. Peridinin thus represents a promising and biophysically well-characterized starting point for the development of small molecule antilipoperoxidants that serve as more effective biological probes and/or therapeutics.

Lipid oxidation inactivates the anticoagulant function of protein Z-dependent protease inhibitor (ZPI)

Lipid oxidation due to oxidative stress plays an important role in the pathogenesis of inflammatory and thrombotic cardiovascular diseases. Several findings suggest that lipid peroxidation can alter the function of coagulation proteins and contribute to a hypercoagulable state, but the molecular mechanisms are unclear. Here, we report that oxidized phospholipids suppress the anticoagulant function of the serpin, protein Z-dependent protease inhibitor (ZPI), a specific inhibitor of membrane-associated factor Xa (FXa) that requires protein Z (PZ), phospholipid, and calcium as cofactors. We found that this suppression arises from a diminished ability of the oxidized membrane to function as a cofactor to promote ZPI inhibition of membrane-bound FXa, due fully or in part to the susceptibility of the bound ZPI-PZ complex to oxidative inactivation. Surprisingly, free ZPI was also susceptible to inactivation by oxidized membrane vesicles in the absence of calcium. Oxidized vesicles containing both phosphatidylserine and polyunsaturated fatty acids were required to promote inactivation of the ZPI-PZ complex or free ZPI, indicating that binding of the PZ-complexed or free ZPI to peroxide-modified phospholipid vesicles mediates the inactivation. Heparin protected the ZPI-PZ complex and free ZPI from inactivation, suggesting that blocking the heparin-binding site on ZPI interferes with ZPI binding to lipid or to PZ. This was confirmed by direct lipid-binding experiments. Native PAGE indicated that oxidization induced dissociation of the ZPI-PZ complex and increased the negative charge of ZPI. We conclude that compromised ZPI anticoagulant function could contribute to thrombus initiation and growth in oxidative stress-induced cardiovascular diseases.

Effects of cyclodextrins on the structure of LDL and its susceptibility to copper-induced oxidation

Cyclodextrins (CDs) have long been widely used as drug/food carriers and were recently developed as drugs for the treatment of diseases (e.g. Niemann-Pick C1 and cancers). It is unknown whether cyclodextrins may influence the structure of low-density lipoprotein (LDL), its susceptibility to oxidation, and atherogenesis. In this study, four widely used cyclodextrins including α-CD, γ-CD, and two derivatives of β-CD (HPβCD and MβCD) were recruited. Interestingly, agarose gel electrophoresis (staining lipid and protein components of LDL with Sudan Black B and Coomassie brilliant blue, respectively but simultaneously) shows that cyclodextrins at relatively high concentrations caused disappearance of the LDL band and/or appearance of an additional protein-free lipid band, implying that cyclodextrins at relatively high concentrations can induce significant electrophoresis-detectable lipid depletion of LDL. Atomic force microscopy (AFM) detected that MβCD (as a representative of cyclodextrins) induced size decrease of LDL particles in a dose-dependent manner, further confirming the lipid depletion effects of cyclodextrins. Moreover, the data from agarose gel electrophoresis, conjugated diene formation, MDA production, and amino group blockage of copper-oxidized LDL show that cyclodextrins can impair LDL susceptibility to oxidation. It implies that cyclodextrins probably help to inhibit atherogenesis by lowering LDL oxidation.

D-penicillamine prevents ram sperm agglutination by reducing the disulphide bonds of a copper-binding sperm protein

Head-to-head agglutination of ram spermatozoa is induced by dilution in the Tyrode's capacitation medium with albumin, lactate and pyruvate (TALP) and ameliorated by the addition of the thiol d-penicillamine (PEN). To better understand the association and disassociation of ram spermatozoa, we investigated the mechanism of action of PEN in perturbing sperm agglutination. PEN acts as a chelator of heavy metals, an antioxidant and a reducing agent. Chelation is not the main mechanism of action, as the broad-spectrum chelator ethylenediaminetetraacetic acid and the copper-specific chelator bathocuproinedisulfonic acid were inferior anti-agglutination agents compared with PEN. Oxidative stress is also an unlikely mechanism of sperm association, as PEN was significantly more effective in ameliorating agglutination than the antioxidants superoxide dismutase, ascorbic acid, α-tocopherol and catalase. Only the reducing agents cysteine and DL-dithiothreitol displayed similar levels of non-agglutinated spermatozoa at 0 h compared with PEN but were less effective after 3 h of incubation (37 °C). The addition of 10 µM Cu(2+) to 250 µM PEN + TALP caused a rapid reversion of the motile sperm population from a non-agglutinated state to an agglutinated state. Other heavy metals (cobalt, iron, manganese and zinc) did not provoke such a strong response. Together, these results indicate that PEN prevents sperm association by the reduction of disulphide bonds on a sperm membrane protein that binds copper. ADAM proteins are possible candidates, as targeted inhibition of the metalloproteinase domain significantly increased the percentage of motile, non-agglutinated spermatozoa (52.0% ± 7.8) compared with TALP alone (10.6% ± 6.1).

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.

Effect of Copper on Fatty Acid Profiles in Non- and Semifermented Teas Analyzed by LC-MS-Based Nontargeted Screening

Unsaturated fatty acids are well-known precursors of aroma compounds, which are considered important for green tea quality. Due to the known copper-induced oxidation of unsaturated fatty acids and the broad variability of the amount of copper present in tea infusions, this paper investigates the influence of copper, added at a nontoxic concentration (300 μM) to non- and semifermented teas, on the degradation of fatty acids and fatty acid hydroperoxides thereof. The abundance of fatty acids in green and oolong tea was determined by means of a nontargeted approach applying high-resolution MS/MS. As a result, most of the fatty acids in green and oolong tea were already oxidized prior to copper addition. Addition of 300 μM CuSO4 to the oolong tea sample resulted in a decrease of 13-hydroperoxy-9Z,11E-octadecadienoic acid, an important flavor precursor, from 0.12 ± 0.02 to 0.05 ± 0.01 μM (p = 0.035), and other oxidized fatty acids decreased as well. However, copper-induced degradation of oxidized fatty acids was less pronounced in green tea compared to oolong tea, most likely due to the formation of copper complexes with low-molecular-weight compounds as evidenced by electron paramagnetic resonance spectroscopy.

Association between copper levels and myocardial infarction: a meta-analysis

BACKGROUND AND AIMS

There are conflicting reports as to the correlation between copper (Cu) levels and myocardial infarction (MI). The purpose of the present study is to clarify the association between Cu levels and MI.

METHODS

We searched articles in Pubmed and the Chinese Journal Full-text Database published as of October 2014. A meta-analysis was used to pool estimates of the standardized mean difference (SMD) with 95% confidence interval (CI).

RESULTS

Pooled analysis indicated that subjects with MI had higher serum Cu levels than healthy controls (SMD = 1.629, 95%CI = [1.027, 2.232], Z = 5.30, p < 0.001). Further subgroup analysis stratified by ethnicity indicated that subjects with MI had higher serum Cu levels than healthy controls among members of the Asian race (SMD = 2.191, 95%CI = [1.401, 2.981], Z = 5.43, p < 0.001), but not among members of the Caucasian race (SMD = 0.411, 95%CI = [-0.030, 0.851], Z = 1.83, p = 0.068). The results obtained from hairs showed no association between MI and hair Cu levels (SMD = 0.338, 95% CI = [-0.171, 0.848], Z = 1.30, p = 0.193). But the subgroup analysis stratified by geological location indicated that subjects with MI had higher hair Cu levels than healthy controls in Pakistan (SMD = 0.785, 95% CI = [0.587, 0.983], Z = 10.29, p < 0.001), but not in India (SMD = -9.028, 95% CI = [-10.747, -7.309], Z = 7.77, p < 0.001).

CONCLUSION

This meta-analysis indicates a significant association between high serum Cu levels and MI. However, the subgroup analysis found that there was significant effect modification of Cu levels by ethnicity. Thus, we suggest that a trans-regional multicenter study is needed to obtain better understanding of causal relationships between Cu and MI in different human races.

Kinetic study of low density lipoprotein oxidation by copper

Oxidation of Low Density Lipoprotein (LDL) is regarded as a key event in the development of atherosclerosis. The aim of this study was to investigate effect of various copper concentrations on LDL oxidation kinetic profile as a mechanism in atherosclerosis process. LDL was isolated from plasma and its oxidation with copper was investigated by monitoring the formation of conjugated dienes. Based on time course of the formation of conjugated diene was observed at concentrations of 0.5 to 10 µM copper, represented the conventional kinetics of LDL oxidation with an inhibition period followed by a propagation phase. In contrast, at concentrations of 20 to 50 µM copper, LDL oxidation proceeded after a negligibly short lag-time followed by a distinct propagation phase. At lower copper concentrations of about 0.5 µM, LDL oxidation can be combined in 4 consecutive oxidation phase. The increasing copper concentration (to 10 µM) lowered the first propagation and shortened the seconded inhibition period until they melted into one apparent kinetic phase. But in copper concentrations of about 20 to 50 µM, increasing copper concentration increased the first propagation and the second inhibition but lowered the second propagation phase. The results of this investigation on the copper dependence of the oxidation kinetics suggest that LDL contains two different copper binding sites. Copper bound to the low affinity binding sites with molar ratio of 200 to 500 of copper / LDL. These ions bound to the high affinity binding sites with molar ratio of copper / LDL of 5 to 100.

An interference study of the copper-induced plasma oxidizability test

OBJECTIVES

The aim of this study is to investigate the effect of cristaloid cardioplegic fluid (CCF) and its contents on the susceptibility of plasma to copper-catalyzed lipid peroxidation test.

DESIGN AND METHODS

The plasma pool was divided into eight groups. Equal volumes of CCF or one of its contents were added to each group of the plasma pool. The accumulation of conjugated diene (CD) by copper-induced oxidation was monitored for a period of 5 h. Thiobarbituric acid-reactive substances (TBARS) formed during the incubation of plasma with copper was also measured.

RESULTS

It was found that, the production of CD and TBARS were inhibited and the lag time had increased, when the plasma was mixed with CCF or its contents.

CONCLUSIONS

As a result, we conclude that that the susceptibility of plasma to copper-induced lipid peroxidation is interfered by CCF. The chloride ions, which major content of CCF, may play an important role on this effect.

Effects of oral contraception with ethinylestradiol and drospirenone on oxidative stress in women 18-35 years old

BACKGROUND

Oral contraceptives (OCs) with estrogens and progestins may affect oxidative stress (OS) status.

STUDY DESIGN

A group of 32 women using oral contraceptives (OCU) containing 0.03 mg ethinylestradiol and 3 mg drospirenone have been compared to a matched control group of 30 noncontraception users (NCU). Blood levels of antioxidants, trace elements and markers of lipid peroxidation were assessed by biochemical methods. A microarray analysis of whole blood mRNA levels of 200 genes involved in OS-dependant pathway was also performed.

RESULTS

Levels of zinc, vitamin E and antibodies to oxidized low-density lipoproteins (LDLs) were not significantly different between the two groups. On the other hand, significant increases in the mean levels of lipid peroxides (+176%, p<.001), oxidized LDLs (+145%, p<.002), copper (+103%, p<.001), Cu/Zn ratio (+100%, p<.001) and a significant decrease in the mean level of beta-carotene (-41%, p<.01) were observed in the OCU compared to NCU. There was a highly significant positive correlation between the lipid peroxide levels and the copper-to-zinc ratio. From the 200 genes tested by microarray, one coding for HSP70 was significantly up-regulated (log(2) fold change=+ 0.45, p<.02) and one coding for inducible nitric oxide synthase significantly down-regulated (log(2) fold change=-0.24, p<.05) in the OCU compared to the NCU.

CONCLUSIONS

The recently introduced combination of ethinylestradiol and drospirenone induced the heightening of lipid peroxidation correlated with high levels of copper, a situation that could be associated with increased cardiovascular risk.

Medicinal plants of Cree communities (Québec, Canada): antioxidant activity of plants used to treat type 2 diabetes symptoms

Traditional medicines (TM) used to treat symptoms of diabetes by two Cree communities were assessed for their free radical scavenging activity using the stable 1,1-diphenyl-2-picrylhydrazyl radical, and their ability to protect human low-density lipoprotein from Cu2+-mediated oxidation by measuring lag time before the appearance of conjugated dienes and formation of thiobarbituric acid-reactive substances. Water-soluble phenolic content was also measured. A total of 20 medicinal plants from Whapmagoostui and 16 from Mistissini were compared with 16 extracts of plants that were not used medicinally. Medicinal plant extracts, particularly those from Larix laricina, displayed high antioxidant activity, comparable with ascorbic acid, Trolox, and the known antioxidant flavonoids quercetin, epicatechin, catechin. Extracts of Pinaceae and Ericaceae contained the highest levels of phenolics. Factors such as season and area of plant collection, as well as the plant part from which the extract was derived, affected antioxidant activity. Positive correlations were found between the established traditional knowledge of Cree Elders and Healers and the antioxidant activity for medicinal plants used in Mistissini (r = 0.3134; p = 0.058) and in Whapmagoostui (r = 0.5165; p = 0.001). Significant correlations between phenolic content were also seen with the existing ethnobotanical data (r = 0.5015; p = 0.003) and bioassays (r = 0.4178; p = 0.003). These results indicate that a clear majority of plants used by the Cree are excellent sources of antioxidants.

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.

Probabilistic kinetic model of slow oxidation of low-density lipoprotein. 3. Hydroperoxide-free initiation

A theoretical model is presented that explains slow copper-induced oxidation of low-density lipoprotein in the systems free of seeded hydroperoxydes. The model is based on the probabilistic kinetic theory, modified to take into account different radical generation rates by oxidized and reduced forms of the metal ions. It is shown that the initiation and progression of the LDL oxidation can take place in any LDL dispersion by metal-induced oxidation of alpha-tocopherol, one of the constituents of LDL, and formation of slowly reacting alpha-tocopheroxyl radical. Selected values of several adjustable parameters define actual temporal profiles of the quantities defining the oxidation process.

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.

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.

Anti- and pro-oxidant effects of quercetin in copper-induced low density lipoprotein oxidation. Quercetin as an effective antioxidant against pro-oxidant effects of urate

We recently reported that, depending on its concentration, urate is either a pro- or an antioxidant in Cu(2+)-induced low-density lipoprotein (LDL) oxidation. We also previously demonstrated an antioxidant synergy between urate and some flavonoids in the Cu(2+)-induced oxidation of diluted serum. As a result, the effect of the flavonoid quercetin on the Cu(2+)-induced oxidation of isolated LDL has been studied either in the presence or absence of urate. We demonstrate that, like urate, quercetin alone, at low concentration, exhibits a pro-oxidant activity. The pro-oxidant behavior depends on the Cu(2+) concentration but it is not observed at high Cu(2+) concentration. When compared with urate, the switch between the pro- and the antioxidant activities occurs at much lower quercetin concentrations. As for urate, the pro-oxidant character of quercetin is related to its ability to reduce Cu(2+) with the formation of semioxidized quercetin and Cu(+) with an expected yield larger than that obtained with urate owing to a more favorable redox potential. It is also shown that the pro-oxidant activity of urate can be inhibited by quercetin. An electron transfer between quercetin and semioxidized urate leading to the repair of urate could account for this observation as suggested by recently published pulse radiolysis data. It is anticipated that the interactions between quercetin-Cu(2+)-LDL and urate, which are tightly controlled by their respective concentration, determine the balance between the pro- and antioxidant behaviors. Moreover, as already observed with other antioxidants, it is demonstrated that quercetin alone behaves as a pro-oxidant towards preoxidized LDL.

Nitric oxide inhibits prooxidant actions of uric acid during copper-mediated LDL oxidation

Interactions between uric acid and physiologically relevant fluxes of nitric oxide ((?)NO) during copper-mediated low-density lipoprotein (LDL) oxidation were evaluated. In the absence of (?)NO, a dual pro- and antioxidant action of uric acid was evident: low concentrations of uric acid enhanced lipid oxidation and alpha-tocopherol consumption, while its protective role was observed at higher concentrations. The prooxidant effects of uric acid were mostly related to its copper-reducing ability to form Cu(+), an initiator of lipid oxidation processes. While the prooxidant action of uric acid was completely inhibited by (?)NO, the antioxidant action of (?)NO was slightly counterbalanced by uric acid. Enhancement of alpha-tocopherol consumption by uric acid was inhibited in the presence of (?)NO while additive antioxidant effects between (?)NO and uric acid were observed in conditions where uric acid spared alpha-tocopherol. Altogether, these results suggest that in the artery wall, the (?)NO/uric acid pair may exert antioxidant actions on LDL, even if increased amounts of redox active copper were available at conditions favoring prooxidant activities of uric acid.

Probabilistic kinetic model of slow oxidation of low-density lipoprotein: II. Experiments

Theoretical probabilistic kinetic model has been applied to describe the measurements of several oxidation markers as a function of time, during slow oxidation of low-density lipoprotein (LDL). It has been demonstrated that such a process could be described as tocopherol-mediated peroxidation (TMP), initiated and sustained by the action of copper ions, present in LDL in trace amounts. In that process concentration of alpha-tocopherol remains essentially unaltered. Tocopherol and copper ions act as catalysts, oscillating between the oxidized and reduced states. The fitting of the theoretical model to the experimental data resulted in determination of the numerical values for the kinetic parameters. It has been found that the parameter values used for the fitting of the data collected for a number of samples from various donors differ rather little. The kinetic chain length of 1.3 (in presence of co-antioxidants) and 2.9 (in the absence of co-antioxidants) is shorter than found by others. The difference probably comes from the much lower concentration of copper ions in our systems (about 0.1 ion per LDL particle).

Neutrophil myeloperoxidase chlorinates and nitrates soy isoflavones and enhances their antioxidant properties

Soy isoflavones and other polyphenolics have a number of potentially important beneficial effects on the pro-oxidant aspects of chronic inflammation. The impact of inflammatory cell-specific metabolism of polyphenolics, which can include halogenation and nitration, on the properties of these compounds has not been examined. Using either human neutrophils or differentiated human leukemia cells (HL-60) stimulated with phorbol ester to elicit a respiratory burst, the hypothesis that local generation of reactive oxygen and nitrogen species may metabolize and modify the biological properties of the soy isoflavones was examined. Coincubation of the stimulated cells with genistein or daidzein had no effect on the respiratory burst. Medium from stimulated cells in the presence of the isoflavones and NO(2)(-) increased the inhibition of copper-induced LDL oxidation. Mass spectrometry analysis of this medium revealed that monochlorinated, dichlorinated, and nitrated isoflavones, formed through a myeloperoxidase-dependent mechanism, were present. The consumption of genistein in the presence of cells was both extensive and rapid with > 95% of the genistein converted to either the chlorinated or nitrated metabolites within 30 min. Chemically synthesized 3'-chlorogenistein and 3'-chlorodaidzein increased the inhibition of LDL oxidation by approximately 4-fold and 2-fold over genistein and daidzein, respectively. These results lead to the hypothesis that inflammatory cell-specific metabolism of polyphenolics can modify the properties of these compounds at the local site of inflammation.

Antioxidant properties of low molecular weight phenols present in the mediterranean diet

The antioxidant capacity of low molecular weight phenols found in olive fruit and in virgin olive oil has been investigated. The radical scavenging activity of some of the investigated phenols is higher than that of the most used antioxidants, and among them, 3,4- or 2,5-dihydroxyl phenols are also able to chelate copper ions leading to chelates that are only slightly active in the promotion of free radical reactions. The ability of tested phenols to reduce Cu(II) and their activity-structure relationships was also studied, showing that their reducing capacity is connected to the presence of a specific ligand of the reduced ion. The number of reduced ions per mole of phenol is lower than that calculated for some flavonols and isoflavones so exerting a lower prooxidant action. This fact may be important in vivo when free transition metal ions are involved in oxidation processes.

Mechanisms by which cysteine can inhibit or promote the oxidation of low density lipoprotein by copper

Oxidised low density lipoprotein (LDL) may play a role in atherogenesis. We have investigated some of the mechanisms by which the thiol cysteine and the disulphide cystine can influence the oxidation of LDL by copper ions. Cysteine or cystine (100 microM) inhibited the oxidation of native LDL by copper in a simple phosphate buffer. One of the mechanisms by which cysteine (or more likely its oxidation products in the presence of copper) and cystine inhibited LDL oxidation was by decreasing the binding of copper to LDL (97% inhibition). Cysteine, but not cystine, rapidly reduced Cu(2+) to Cu(+). This may help to explain the antioxidant effect of cysteine as it may limit the amount of Cu(2+) that is available to convert alpha-tocopherol in LDL into the prooxidant alpha-tocopherol radical. Cysteine (but not cystine) had a prooxidant effect, however, toward partially oxidised LDL in the presence of a low copper concentration, which may have been due to the rapid breakdown of lipid hydroperoxides in partially oxidised LDL by Cu(+) generated by cysteine. To prove that cysteine can cause the rapid breakdown of lipid hydroperoxides in LDL, we enriched LDL with lipid hydroperoxides using an azo initiator in the absence of copper. Cysteine, but not cystine, increased the rate of lipid hydroperoxide decomposition to thiobarbituric acid-reactive substances (TBARS) in the presence of copper.

EPR spin-trapping evidence for the direct, one-electron reduction of tert-butylhydroperoxide to the tert-butoxyl radical by copper(II): paradigm for a previously overlooked reaction in the initiation of lipid peroxidation

Lipid peroxidation is often initiated using Cu(II) ions. It is widely assumed that Cu(II) oxidizes preformed lipid hydroperoxides to peroxyl radicals, which propagate oxidation of the parent fatty acid via hydrogen atom abstraction. However, the oxidation of alkyl hydroperoxides by Cu(II) is thermodynamically unfavorable. An alternative means by which Cu(II) ions could initiate lipid peroxidation is by their one-electron reduction of lipid hydroperoxides to alkoxyl radicals, which would be accompanied by the generation of Cu(III). We have investigated by EPR spectroscopy, in conjunction with the spin trap 5,5-dimethyl-1-pyrroline N-oxide, the reactions of various Cu(II) chelates with tert-butylhydroperoxide. Spectra contained signals from the tert-butoxyl, methyl, and methoxyl radical adducts. In many previous studies, the signal from the methoxyl adduct has been assigned incorrectly to the tert-butylperoxyl adduct, which is now known to be unstable, releasing the tert-butoxyl radical upon decomposition. This either is trapped by 5,5-dimethyl-1-pyrroline N-oxide or undergoes beta-scission to the methyl radical, which either is trapped or reacts with molecular oxygen to give, ultimately, the methoxyl radical adduct. By using metal chelates that are known to be specific in either their oxidation or reduction of tert-butylhydroperoxide (the Cu(II) complex of bathocuproine disulfonic acid and the Fe(II) complex of diethylenetriaminepentaacetic acid, respectively) for comparison, we have been able to deduce, from the relative concentrations of the three radical adducts, that the Cu(II) complexes tested each reduce tert-butylhydroperoxide directly to the tert-butoxyl radical. These findings suggest that a previously overlooked reaction, namely the direct reduction of preformed lipid hydroperoxides to alkoxyl radicals by Cu(II), may be responsible for the initiation of lipid peroxidation by Cu(II) ions.

Prooxidant and antioxidant properties of human serum ultrafiltrates toward LDL: important role of uric acid

Oxidized LDL is present within atherosclerotic lesions, demonstrating a failure of antioxidant protection. A normal human serum ultrafiltrate of Mr below 500 was prepared as a model for the low Mr components of interstitial fluid, and its effects on LDL oxidation were investigated. The ultrafiltrate (0.3%, v/v) was a potent antioxidant for native LDL, but was a strong prooxidant for mildly oxidized LDL when copper, but not a water-soluble azo initiator, was used to oxidize LDL. Adding a lipid hydroperoxide to native LDL induced the antioxidant to prooxidant switch of the ultrafiltrate. Uric acid was identified, using uricase and add-back experiments, as both the major antioxidant and prooxidant within the ultrafiltrate for LDL. The ultrafiltrate or uric acid rapidly reduced Cu2+ to Cu+. The reduction of Cu2+ to Cu+ may help to explain both the antioxidant and prooxidant effects observed. The decreased concentration of Cu2+ would inhibit tocopherol-mediated peroxidation in native LDL, and the generation of Cu+ would promote the rapid breakdown of lipid hydroperoxides in mildly oxidized LDL into lipid radicals. The net effect of the low Mr serum components would therefore depend on the preexisting levels of lipid hydroperoxides in LDL. These findings may help to explain why LDL oxidation occurs in atherosclerotic lesions in the presence of compounds that are usually considered to be antioxidants.

Anti- and pro-oxidant effects of urate in copper-induced low-density lipoprotein oxidation

We reported earlier that urate may behave as a pro-oxidant in Cu2+-induced oxidation of diluted plasma. Thus, its effect on Cu2+-induced oxidation of isolated low-density lipoprotein (LDL) was investigated by monitoring the formation of malondialdehyde and conjugated dienes and the consumption of urate and carotenoids. We show that urate is antioxidant at high concentration but pro-oxidant at low concentration. Depending on Cu2+ concentration, the switch between the pro- and antioxidant behavior of urate occurs at different urate concentrations. At high Cu2+ concentration, in the presence of urate, superoxide dismutase and ferricytochrome c protect LDL from oxidation but no protection is observed at low Cu2+ concentration. The use of Cu2+ or Cu+ chelators demonstrates that both copper redox states are required. We suggest that two mechanisms occur depending on the Cu2+ concentration. Urate may reduce Cu2+ to Cu+, which in turn contributes to formation. The Cu2+ reduction is likely to produce the urate radical (UH.-). It is proposed that at high Cu2+ concentration, the reaction of UH.- radical with generates products or intermediates, which trigger LDL oxidation. At low Cu2+ concentration, we suggest that the Cu+ ions formed reduce lipid hydroperoxides to alkoxyl radicals, thereby facilitating the peroxidizing chain reaction. It is anticipated that these two mechanisms are the consequence of complex LDL-urate-Cu2+ interactions. It is also shown that urate is pro-oxidant towards slightly preoxidized LDL, whatever its concentration. We reiterate the conclusion that the use of antioxidants may be a two-edged sword.

Degradation of trilinolein by laccase enzymes

Laccase enzymes were investigated for their potential to catalyze the oxidation of trilinolein and methyl linoleate. This study demonstrates that laccase enzymes can oxidize unsaturated fatty acid esters and their associated lipids. The reaction products resulting from laccase-catalyzed reactions with trilinolein were analyzed using combined reversed-phase high-performance liquid chromatography and mass spectrometry via an atmospheric pressure chemical ionization source. The dominant oxidation products detected were monohydroperoxides, bishydroperoxides, and epoxides. This paper presents the first detailed investigation into the interaction between laccase enzymes and lipids containing unsaturated fatty acids.

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.

Phenol antioxidant quantity and quality in foods: fruits

The free and bound phenols have been measured in 20 fruits commonly consumed in the American diet. Phenols were measured colorimetrically using the Folin-Ciocalteu reagent with catechin as the standard after correction for ascorbic acid contribution. On a fresh weight basis, cranberry had the highest total phenols, and was distantly followed by red grape. Free and total phenol quality in the fruits was analyzed by using the inhibition of lower density lipoprotein oxidation promoted by cupric ion. Ascorbate had only a minor contribution to the antioxidants in fruits with the exception of melon, nectarine, orange, white grape, and strawberry. The fruit extracts' antioxidant quality was better than the vitamin antioxidants and most pure phenols, suggesting synergism among the antioxidants in the mixture. Using our assay, fruits had significantly better quantity and quality of phenol antioxidants than vegetables. Fruits, specifically apples and cranberries, have phenol antioxidants that can enrich lower density lipoproteins and protect them from oxidation. The average per capita consumption of fruit phenols in the U.S. is estimated to be 255 mg/day of catechin equivalents.

Essential role of ICAM-1 in mediating monocyte adhesion to aortic endothelial cells

Monocyte-endothelial cell interactions have been implicated in the pathogenesis of a number of vascular diseases that target arterial and aortic endothelium, including atherosclerosis. Many different adhesion molecules, such as intercellular adhesion molecule (ICAM)-1, are thought to mediate monocyte binding to endothelial cells during the development of these diseases. However, conflicting results have been reported regarding the specific role of ICAM-1 in these events. In this study, we used a genetic approach to determine the contribution of ICAM-1 in mediating monocyte adhesion to mouse aortic endothelial cells (MAEC) derived from both wild-type and ICAM-1(-/-) mice. Treatment of wild-type MAEC with oxidized low-density lipoprotein significantly induced both WEHI 274.1 and whole blood monocyte adhesion, whereas similarly treated ICAM-1(-/-) MAEC showed a complete inhibition of monocyte binding. Dose-response treatment with tumor necrosis factor-alpha also increased monocyte adhesion to wild-type MAEC, but significant adhesion was only observed at higher doses for ICAM-1(-/-) MAEC. These data demonstrate a crucial role for ICAM-1-mediated monocyte-endothelial cell interactions in response to specific stimuli involved in inflammatory vascular diseases.

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.

Apolipoprotein A-I promotes the formation of phosphatidylcholine core aldehydes that are hydrolyzed by paraoxonase (PON-1) during high density lipoprotein oxidation with a peroxynitrite donor

High density lipoprotein (HDL) is rich in polyunsaturated phospholipids that are sensitive to oxidation. However, the effect of apolipoprotein A-I and paraoxonase-1 (PON-1) on phosphatidylcholine oxidation products has not been identified. We subjected native HDL, trypsinized HDL, and HDL lipid suspensions to oxidation by the peroxynitrite donor, 3-morpholinosydnonimine. HDL had a basal level of phosphatidylcholine mono- and di-hydroperoxides that increased to a greater extent in HDL, compared with either trypsinized HDL or HDL lipid alone. Phosphatidylcholine core aldehydes, which were present in small amounts, increased 10-fold during oxidation of native HDL, compared with trypsinized HDL (p = 0.004), and 4-fold compared with HDL lipid suspensions (p = 0.0021). In addition, the content of lysophosphatidylcholine increased 300% during oxidation of native HDL, but only 80 and 25%, respectively, during oxidation of trypsinized HDL and HDL lipid suspensions. Phosphatidylcholine isoprostanes accumulated in comparable amounts during the oxidation of all three preparations. Incubation of apolipoprotein A-I with 1-palmitoyl-2-linoleoyl glycerophosphocholine proteoliposomes in the presence of 3-morpholinosydnonimine or apoAI with phosphatidylcholine hydroperoxides resulted in a significant increase in phosphatidylcholine core aldehydes with no formation of lysophosphatidylcholine. We propose that apolipoprotein A-I catalyzes a one-electron oxidation of alkoxyl radicals. Purified PON-1 hydrolyzed phosphatidylcholine core aldehydes to lysophosphatidylcholine. We conclude that, upon HDL oxidation with peroxynitrite, apolipoprotein AI increases the formation of phosphatidylcholine core aldehydes that are subsequently hydrolyzed by PON1.

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.

Characterization of peroxynitrite-oxidized low density lipoprotein binding to human CD36

Peroxynitrite-mediated oxidation may be an important physiological mechanism for oxidation of low density lipoprotein (LDL), however, the molecular basis for the interaction of peroxynitrite oxidized LDL (OxLDL) with scavenger receptors such as CD36, has not been characterized. In this study, we compared the biochemical characteristics and receptor binding of LDL that was oxidized using: (1) Cu2+, a standard method of oxidizing LDL in vitro; and (2) 3-morpholinosydnonimine (SIN-1), a source of peroxynitrite. Both methods of oxidation caused an increase in electrophoretic migration of LDL, but greater mobility was observed with Cu2+-OxLDL. In addition, greater fragmentation of apolipoprotein B was observed following Cu2+ oxidation than after SIN-1 oxidation. The levels of lipid peroxides and thiobarbituric acid reactive substances were similar after 20 h of oxidation by both methods, although the time-course was distinct. Cu2+ and SIN-1-OxLDL bound specifically to the macrophage scavenger receptor CD36 with high affinity. Binding of the 20 h SIN-1 treated LDL to CD36 was comparable to a 4 h Cu2+ modified LDL. The binding of Cu2+ and SIN-1-OxLDL to CD36 was similar under different biochemical conditions and modifications of the receptor, suggesting that OxLDL particles, generated by either method, bind to the same domain of CD36. The results demonstrate that SIN-1 produced an oxidized LDL particle that binds specifically to CD36 and suggests that peroxynitrite OxLDL may represent a more physiologically relevant model than Cu2+-OxLDL for studying the interactions of OxLDL with cells and lipoprotein receptors in vitro.

Lipid oxidation enhances the function of activated protein C

Although lipid oxidation products are usually associated with tissue injury, it is now recognized that they can also contribute to cell activation and elicit anti-inflammatory lipid mediators. In this study, we report that membrane phospholipid oxidation can modulate the hemostatic balance. Oxidation of natural phospholipids results in an increased ability of the membrane surface to support the function of the natural anticoagulant, activated protein C (APC), without significantly altering the ability to support thrombin generation. Lipid oxidation also potentiated the ability of protein S to enhance APC-mediated factor Va inactivation. Phosphatidylethanolamine, phosphatidylserine, and polyunsaturation of the fatty acids were all required for the oxidation-dependent enhancement of APC function. A subgroup of thrombotic patients with anti-phospholipid antibodies specifically blocked the oxidation-dependent enhancement of APC function. Since leukocytes are recruited and activated at the thrombus or sites of vessel injury, our findings suggest that after the initial thrombus formation, lipid oxidation can remodel the membrane surface resulting in increased anticoagulant function, thereby reducing the thrombogenicity of the thrombus or injured vessel surface. Anti-phospholipid antibodies that block this process would therefore be expected to contribute to thrombus growth and disease.

Pulmonary surfactant proteins A and D are potent endogenous inhibitors of lipid peroxidation and oxidative cellular injury

The lung is composed of a series of branching conducting airways that terminate in grape-like clusters of delicate gas-exchanging airspaces called pulmonary alveoli. Maintenance of alveolar patency at end expiration requires pulmonary surfactant, a mixture of phospholipids and proteins that coats the epithelial surface and reduces surface tension. The surfactant lining is exposed to the highest ambient oxygen tension of any internal interface and encounters a variety of oxidizing toxicants including ozone and trace metals contained within the 10 kl of air that is respired daily. The pathophysiological consequences of surfactant oxidation in humans and experimental animals include airspace collapse, reduced lung compliance, and impaired gas exchange. We now report that the hydrophilic surfactant proteins A (SP-A) and D (SP-D) directly protect surfactant phospholipids and macrophages from oxidative damage. Both proteins block accumulation of thiobarbituric acid-reactive substances and conjugated dienes during copper-induced oxidation of surfactant lipids or low density lipoprotein particles by a mechanism that does not involve metal chelation or oxidative modification of the proteins. Low density lipoprotein oxidation is instantaneously arrested upon SP-A or SP-D addition, suggesting direct interference with free radical formation or propagation. The antioxidant activity of SP-A maps to the carboxyl-terminal domain of the protein, which, like SP-D, contains a C-type lectin carbohydrate recognition domain. These results indicate that SP-A and SP-D, which are ubiquitous among air breathing organisms, could contribute to the protection of the lung from oxidative stresses due to atmospheric or supplemental oxygen, air pollutants, and lung inflammation.

Direct Evidence for apo B-100-Mediated Copper Reduction: Studies with Purified apo B-100 and Detection of Tryptophanyl Radicals

Copper binding to apolipoprotein B-100 (apo B-100) and its reduction by endogenous components of low-density lipoprotein (LDL) represent critical steps in copper-mediated LDL oxidation, where cuprous ion (Cu(I)) generated from cupric ion (Cu(II)) reduction is the real trigger for lipid peroxidation. Although the copper-reducing capacity of the lipid components of LDL has been studied extensively, we developed a model to specifically analyze the potential copper reducing activity of its protein moiety (apo B-100). Apo B-100 was isolated after solubilization and extraction from size exclusion-HPLC purified LDL. We obtained, for the first time, direct evidence for apo B-100-mediated copper reduction in a process that involves protein-derived radical formation. Kinetics of copper reduction by isolated apo B-100 was different from that of LDL, mainly because apo B-100 showed a single phase-exponential kinetic, instead of the already described biphasic kinetics for LDL (namely alpha-tocopherol-dependent and independent phases). While at early time points, the LDL copper reducing activity was higher due to the presence of alpha-tocopherol, at longer time points kinetics of copper reduction was similar in both LDL and apo B-100 samples. Electron paramagnetic resonance studies of either LDL or apo B-100 incubated with Cu(II), in the presence of the spin trap 2-methyl-2-nitroso propane (MNP), indicated the formation of protein-tryptophanyl radicals. Our results supports that apo B-100 plays a critical role in copper-dependent LDL oxidation, due to its lipid-independent-copper reductive ability.

Total antioxidant capacity as a tool to assess redox status: critical view and experimental data

The measure of antioxidant capacity (AC) considers the cumulative action of all the antioxidants present in plasma and body fluids, thus providing an integrated parameter rather than the simple sum of measurable antioxidants. The capacity of known and unknown antioxidants and their synergistic interaction is therefore assessed, thus giving an insight into the delicate balance in vivo between oxidants and antioxidants. Measuring plasma AC may help in the evaluation of physiological, environmental, and nutritional factors of the redox status in humans. Determining plasma AC may help to identify conditions affecting oxidative status in vivo (e.g., exposure to reactive oxygen species and antioxidant supplementation). Moreover, changes in the plasma AC after supplementation with galenic antioxidants or with antioxidant-rich foods may provide information on the absorption and bioavailability of nutritional compounds. Consequently, this review discusses the rationale, interpretation, confounding factors, measurement limits, and human applications of the measure of plasma AC.