Different mechanisms are progressively recruited to promote Cu(II) reduction by isolated human low-density lipoprotein undergoing oxidation

Milk thistle extracts inhibit the oxidation of low-density lipoprotein (LDL) and subsequent scavenger receptor-dependent monocyte adhesion

Silymarin encompasses a group of flavonolignans that are extracted from Silybum marianum (Asteraceae) fruits. The silymarins have previously been reported to lower low-density lipoprotein (LDL) levels associated with high-fat diets. The present study reports the efficacy of the silymarins in inhibiting oxidized low-density lipoprotein (oxLDL) generation and subsequent scavenger receptor (SR) mediated monocyte adherence to oxLDL. The flavonolignans that comprise silymarin include silichristin (SC), silidianin (SD), silibinin (SBN), and isosilibinin (IS). These flavonolignans (300 microM) lowered oxLDL generation, measured by the thiobarbituric acid-reacting substances (TBARS) assay, by 60.0, 28.1, 60.0, and 30.1%, respectively. SBN treatment of LDL in the presence of copper sulfate (CuSO 4) resulted in a significant dose-dependent inhibition of monocyte adhesion. Inhibition was paralleled by a decrease in binding of anti-oxLDL antibodies recognized by U937 monocyte Fc gamma receptors (FcgammaR). These results showed that silymarin and SBN, likely through antioxidant and free radical scavenging mechanisms of action, inhibit the generation of oxLDL and oxidation-specific neoepitopes recognized by SR and FcgammaR expressed on monocytes/macrophages.

Ligand dependence in the copper-catalyzed oxidation of hydroquinones

Transition metal-mediated oxidation of hydroquinones is an important physiologic reaction, and copper(II) effectively catalyzes the reaction in phosphate-buffered saline (PBS). Studies reported herein in phosphate buffer alone demonstrate that copper(II) is an ineffective catalyst in the absence of coordinating ligands, but that 1,10-phenanthroline and histamine facilitate the copper(II)-mediated oxidation of hydroquinone and its 2,5- and 2,6-di-tert-butyl analogs to the corresponding benzoquinones. The high concentration of chloride in PBS is the key element that allows copper(II) to work in this system. Although the bis-bathocuproine disulfonate complex of Cu(II), (BC)(2)Cu(II), is a strong stoichiometric oxidant, stoichiometric amounts of copper(II) in the presence of ligands other than BC oxidize hydroquinones very slowly under anaerobic conditions. Thus, the rapid copper(II)-catalyzed reaction operating aerobically does not involve a simple ping-pong reduction of copper(II) to copper(I) by hydroquinone and reoxidation of copper(I) by O(2).

Polyhydroxylated 2-styrylchromones as potent antioxidants

Four polyhydroxylated 2-styrylchromones, structurally related to flavones and cinnamic acid, have been studied. An SC derivative with OH groups only at positions 3' and 4' on the styryl moiety and another SC bearing an additional OH group at position 5 on the benzopyrone ring were more potent inhibitors of the Cu2+-induced peroxidation of LDL than the flavonoid quercetin. Fluorescence and absorption spectroscopies suggested that one LDL particle may bind 40 SC molecules. A pulse radiolysis study in pH 7 buffered micellar solutions of neutral TX100 and positively charged CTAB demonstrated that one-electron oxidation by *Br2-, *O2- and tryptophan radicals (8Trp) depends strongly on the micellar microenvironment. All SCs were readily oxidized by *O2- in CTAB micelles (rate constants: 6-18 x 10(6) M(-1) s(-1)). In TX100 micelles only the SC derivative with OH groups in position 3' and 4' reacted with *O2- (rate constant: 1.1 x 10(6) M(-1)s(-1)). In CTAB, electron transfer to *Trp radicals was observed for all SCs with rate constants > or =3.2 x 10(7) M(-1) s(-1). In TX100 micelles, this reaction occurred solely with the derivative bearing OH groups only at positions 3' and 4'.

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.

Biological activities ofPrunella vulgarisextract

The organic fraction (OF; 25.7% w/w of rosmarinic acid) of Prunella vulgaris (total extract) was found to exhibit the following: scavenging activity on diphenylpicrylhydrazyl radical (DPPH), inhibition of in vitro human LDL Cu(II)-mediated oxidation, protection of rat mitochondria and rat hepatocytes exposed to either tert-butyl hydroperoxide, or to Cu(II) and Fe(III) ions. OF also showed a potential to inhibit rat erythrocyte haemolysis and it reduced the production of LTB(4) in bovine PMNL generated by the 5-lipoxygenase pathway. Other observations included antiproliferative effects against HaCaT cells and mouse epidermal fibroblasts and a moderate OF antimicrobial activity on gram-positive bacteria. Rosmarinic, caffeic and 3-(3,4-dihydroxyphenyl)lactic acids exhibited less potent activity than the plant extract in all bioassays. The antioxidative, antimicrobial, together with antiviral effects offer good prospects for the medicinal applications of P. vulgaris.

Dipicolinic acid prevents the copper-dependent oxidation of low density lipoprotein

Effect of dipicolinic acid (pyridine 2,6-dicarboxylic acid) and pyridine compounds on the copper-dependent oxidation of human low density lipoprotein was analyzed in relation to the inhibition of copper reduction. Dipicolinic acid inhibited copper-dependent LDL oxidation completely, but the LDL oxidation was slightly inhibited by pyridine compounds with one carboxyl group at 2 or 6-position. Reduction of copper by LDL itself and ascorbate was inhibited completely by dipicolinic acid, but only partially by picolinic acid, quinolinic acid and isocinchomeronic acid with 2- or 6-carboxylic group. Pyridine compounds without 2- or 6-carboxyl group did not show any inhibitory effect on the LDL oxidation and the copper reduction. Protective effect of dipicolinic acid on the LDL oxidation was closely correlated with the copper-reducing activity. Dipicolinic acid shows an antioxidant action by the formation of a chelation complex with copper. This may have implications in understanding mechanisms of preventing LDL oxidation during the early phase of atherosclerosis.

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.

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 and genomic stability in mammals

As the free ion and in the form of some complexes, there is no doubt that copper can promote damage to cellular molecules and structures through radical formation. At the same time, and perhaps as a consequence, mammals have evolved means of minimizing levels of free copper ions and destructive copper complexes that enter the organism and its cells. These means include tight binding of copper ions to protein carriers and transporters; direct exchange of copper between protein carriers, transporters, and cuproenzymes; and mobilization of secretory mechanisms and excretory pathways, as needed. As a consequence, normally, and except under certain genetic conditions, copper is likely to be benign to most mammals and not responsible for genomic instability, including fragmentation of and/or alterations to DNA, induction of mutations or apoptosis, or other toxic events. Indeed, cuproenzymes are important members of the antioxidant system of the organism.

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.

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.

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

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

Copper-mediated toxicity of 2,4,5-trichlorophenol: biphasic effect of the copper(I)-specific chelator neocuproine

The lipophilic copper(I)-specific chelator neocuproine has been frequently used as an inhibitor of copper-mediated damage in biological systems. In this communication we report that the copper-mediated toxicity of 2,4,5-trichlorophenol is markedly potentiated by neocuproine at levels which are near-stoichiometric with respect to the copper concentration but is inhibited at higher concentrations. However, no potentiation was observed when neocuproine was substituted by bathocuproinedisulfonic acid, a negative charged ligand with essentially the same copper-binding characteristics as neocuproine. We found that the potentiation by neocuproine was due to the formation of a lipophilic copper complex, while the inhibition by bathocuproinedisulfonic acid was due to the formation of a hydrophilic one. Caution in the use of neocuproine to study copper-mediated toxicity is advised.

The kinetics of copper-induced LDL oxidation depend upon its lipid composition and antioxidant content

Copper promotes oxidation of human low-density lipoprotein (LDL) through molecular mechanisms that are still under investigation. We employed native human LDL, phospholipid-containing delipidated LDL ghosts, or trilinolein-reconstituted, phospholipid-containing LDL to investigate both LDL oxidation and the associated process of copper reduction. Both LDL ghosts and trilinolein-reconstituted LDL were devoid of antioxidants and were extremely susceptible to AAPH-induced oxidation but, paradoxically, were rather resistant to copper-mediated oxidation. The dynamic reduction of Cu(II) to Cu(I) was quantitatively decreased in LDL ghosts and in trilinolein-reconstituted LDL, also lacking the initial rapid reduction and the subsequent inhibition phases, due to the absence of endogenous antioxidants. Conversely, the rate of copper reduction was linear and likely due to lipid peroxides, either already present or formed during copper-induced oxidation. We suggest that copper undergoes redox transitions in LDL by utilizing reducing equivalents originating from endogenous antioxidants and/or from lipid peroxides in the LDL lipid core.

Cu(I) availability paradoxically antagonizes antioxidant consumption and lipid peroxidation during the initiation phase of copper-induced LDL oxidation

The incubation of isolated human low-density lipoprotein (LDL) with Cu(II) promoted extensive oxidation of both the lipid and protein moieties of the lipoprotein particle. When the Cu(II) to LDL molar ratio was equal or higher than 50, the removal of Cu(I) formed by the contemporary presence of the Cu(I) chelator bathocuproine disulphonate (BC) markedly accelerated the formation of end-products of lipid peroxidation. Moreover, the initial rate of Cu(II)-induced consumption of either endogenous antioxidants in LDL or free alpha-tocopherol in suspension was increased in the presence of BC, thus indicating that the continuous removal of Cu(I) enhanced both antioxidant consumption and LDL oxidation promoted by copper. Furthermore, the direct addition of Cu(I), together with Cu(II), to a suspension of isolated LDL efficiently delayed the onset of extensive lipid peroxidation and decreased the rate of antioxidant consumption. The latter effect, however, was detectable exclusively on LDL-associated alpha-tocopherol and not on alpha-tocopherol in suspension, thus suggesting that the competition between Cu(I) and Cu(II) was taking place at discrete sites associated with the LDL particle. Finally, the inclusion of Cu(I) in the incubation medium of LDL already undergoing extensive peroxidation did not inhibit but rather markedly stimulated the rate of peroxidation. Although apparently in contrast with the common view that Cu(I) and not Cu(II) is the real trigger species of LDL oxidation in the copper model, the results reported here indicate that the availability of Cu(I) during the initiation phase of LDL oxidation promoted by copper antagonizes both antioxidant consumption and the onset of extensive oxidation.