Resveratrol inhibition of lipid peroxidation

To define the molecular mechanism(s) of resveratrol inhibition of lipid peroxidation we have utilized model systems that allow us to study the different reactions involved in this complex process. Resveratrol proved (a) to inhibit more efficiently than either Trolox or ascorbate the Fe2+ catalyzed lipid hydroperoxide-dependent peroxidation of sonicated phosphatidylcholine liposomes; (b) to be less effective than Trolox in inhibiting lipid peroxidation initiated by the water soluble AAPH peroxyl radicals; (c) when exogenously added to liposomes, to be more potent than alpha-tocopherol and Trolox, in the inhibition of peroxidation initiated by the lipid soluble AMVN peroxyl radicals; (d) when incorporated within liposomes, to be a less potent chain-breaking antioxidant than alpha-tocopherol; (e) to be a weaker antiradical than alpha-tocopherol in the reduction of the stable radical DPPH*. Resveratrol reduced Fe3+ but its reduction rate was much slower than that observed in the presence of either ascorbate or Trolox. However, at the concentration inhibiting iron catalyzed lipid peroxidation, resveratrol did not significantly reduce Fe3+, contrary to ascorbate. In their complex, our data indicate that resveratrol inhibits lipid peroxidation mainly by scavenging lipid peroxyl radicals within the membrane, like alpha-tocopherol. Although it is less effective, its capacity of spontaneously entering the lipid environment confers on it great antioxidant potential.     

Antioxidative effect of α-tocopherol incorporation into lecithin liposomes on ascorbic acid-Fe2+-induced lipid peroxidation

The antioxidative effect of α-tocopherol incorporated into lecithin liposomes was studied. Lipid peroxidation of liposome membranes, assayed as malondialdehyde production, was catalyzed by ascorbic acid and Fe²⁺. The peroxidation reaction, which did not involve the formation of singlet oxygen, superoxide, hydrogen peroxide, or a hydroxyl radical, was inhibited by α-tocopherol and a model compound of α-tocopherol, 2,2,5,7,8-pentamethyl-6-hydroxy-chroman (TMC), but not by phytol, α-tocopherylquinone, or α-tocopheryl acetate. One mole of α-tocopherol completely prevented peroxidation of about 100 moles of polyunsaturated fatty acid. Decrease in membrane fluidity by lipid peroxidation, estimated as increase of fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH) embedded in the membrane, was also inhibited by α-tocopherol and TMC, reflecting their antioxidant functions. Cholesterol did not act as an antioxidant, even when incorporated in large amount into the liposome membranes, but it increased the antioxidative efficiency of α-tocopherol. When a mixture of liposomes with and without α-tocopherol was incubated with Fe²⁺ and ascorbic acid, α-tocopherol did not protect the liposomes not containing α-tocopherol from peroxidation. However, preincubation of the mixture, or addition of Triton X-100 allowed the α-tocopherol to prevent peroxidation of the liposomes not containing α-tocopherol. In contrast, in similar experiments, liposomes containing TMC prevented peroxidation of those without TMC without preincubation. Tocopherol in an amount so small as to exhibit only a slight antioxidative effect was oxidized when incorporated in egg lecithin liposomes, but it mostly remained unoxidized when incorporated in dipalmitoyllecithin liposomes, indicating that oxygen activated by ascorbic acid-Fe²⁺ does not oxidize α-tocopherol directly. Thus, decomposition of α-tocopherol may be caused by its interaction with peroxy and/or alkoxyl radicals generated in the process of lipid peroxidation catalyzed by Fe²⁺ and ascorbic acid.     

HTHQ (1-O-hexyl-2,3,5-trimethylhydroquinone), an anti-lipid-peroxidative compound: its chemical and biochemical characterizations

Recently, it has become apparent that reactive oxygen species (ROS) play many important roles in biological systems. For example, relationships between many diseases, such as cancer, cardiac infarction and arteriosclerosis, and ROS have been found. It is also well known that anti-oxidative agents scavenge ROS in biological systems, which in turn prevents ROS-related diseases. In our previous efforts to develop effective anti-oxidative compounds, we found that 1-O-hexyl-2,3,5-trimethylhydroquinone (HTHQ), which is a hydroquinone monoalkyl ether, is a potent anti-oxidative agent. Here, the scavenging activities of HTHQ against ROS, such as superoxide anion radicals, hydroxyl radicals, t-butyl peroxyl radicals and singlet oxygens, were examined by the ESR (electron spin resonance)-spin trapping method. Among ROS, HTHQ scavenged t-butyl peroxyl radicals most effectively (IC₅₀=0.31±0.04 mM), showing approximately twice the activity of a well-known lipophilic anti-oxidant, d,l-α-tocopherol (IC₅₀=0.67±0.06 mM), as measured by IC₅₀ values defined as the 50% inhibition concentration of the generated ROS. In addition, a relatively stable ESR spectrum of free radicals due to HTHQ was observed during the reaction of HTHQ and t-butyl peroxyl radicals, indicating a direct reaction of HTHQ and t-butyl peroxyl radicals. The free radicals due to HTHQ were more stable than those derived from d,l-α-tocopherol under the same conditions examined. On the basis of these results, we evaluated anti-lipid-peroxidative activity of HTHQ in three systems involving micelles, liposomes and rat liver microsomes. HTHQ exhibited a similar anti-oxidative activity to that of d,l-α-tocopherol against lipid peroxidation in linolate micelles initiated by addition of Fe²⁺. On the other hand, HTHQ exhibited approximately 4.8-fold higher anti-lipid-peroxidation activity than that of d,l-α-tocopherol against the peroxidation in phosphatidylcholine liposomes initiated by addition of Fe²⁺. Furthermore, HTHQ scavenged the lipid peroxides at a rate approximately 150 times higher than that of d,l-α-tocopherol against Fe³⁺-ADP-induced lipid peroxidation in rat liver microsomes, indicating that the anti-lipid-peroxidation activity of HTHQ might be substantially elevated in biological systems in comparison with that of d,l-α-tocopherol. Based on these results, we suggest that HTHQ reacts directly with peroxyl radicals, such as t-butyl peroxyl radicals and peroxides of linolate micelles, liposomes and microsomes, by scavenging them to form stable free radicals. The resulting free radicals are presumed to be reduced by several reducing mechanisms in biological systems similarly to those of d,l-α-tocopherol, and then the lipid-peroxidation reactions will be terminated. In conclusion, HTHQ was found to be a potent anti-lipid-peroxidative compound and its anti-oxidation activity to be extremely elevated in biological systems, such as that of liver microsomes via the generation of stable free radicals. We propose that HTHQ is a potent anti-oxidative agent for use in future treatments for lipid-peroxide relevant diseases.     

Enhancement of peroxidase-induced lipid peroxidation by indole-3-acetic acid: effect of antioxidants

Summary

Indole-3-acetic acid (IAA) enhanced the peroxidase-induced lipid peroxidation in phosphatidylcholine liposomes, as measured by loss of fluorescence of cis-parinaric acid. α-Tocopherol or β-carotene in the lipid phase or ascorbate or Trolox in the aqueous phase inhibited the loss of fluorescence induced by the peroxidase + IAA system, but glutathione had only a small inhibitory effect. The peroxyl radical formed by one-electron oxidation of IAA, followed by decarboxylation and reaction with oxygen, is suggested to act as the initiator of lipid peroxidation. The protection by ascorbate or Trolox is explained by the reactivity of these compounds with the IAA indolyl radical, as shown by pulse radiolysis experiments, whereas the weak effect of glutathione agrees with its low reactivity towards the IAA-derived peroxyl radical and its precursors.     

Antioxidant effect of bovine serum albumin on membrane lipid peroxidation induced by iron chelate and superoxide

Albumin is supposed to be the major antioxidant circulating in blood. This study examined the prevention of membrane lipid peroxidation by bovine serum albumin (BSA). Lipid peroxidation was induced by the exposing of enzymatically generated superoxide radicals to egg yolk phosphatidylcholine liposomes incorporating lipids with different charges in the presence of chelated iron catalysts. We used three kinds of Fe³⁺-chelates, which initiated reactions that were dependent on membrane charge: Fe³⁺-EDTA and Fe³⁺-EGTA catalyzed peroxidation in positively and negatively charged liposomes, respectively, and Fe³⁺-NTA, a renal carcinogen, catalyzed the reaction in liposomes of either charge. Fe³⁺-chelates initiated more lipid peroxidation in liposomes with increased zeta potentials, followed by an increase of their availability for the initiation of the reaction at the membrane surface. BSA inhibits lipid peroxidation by preventing the interaction of iron chelate with membranes, followed by a decrease of its availability in a charge-dependent manner depending on the iron-chelate concentration: one is accompanied and the other is unaccompanied by a change in the membrane charge. The inhibitory effect of BSA in the former at high concentrations of iron chelate would be attributed to its electrostatic binding with oppositely charged membranes. The inhibitory effect in the latter at low concentrations of iron chelate would be caused by BSA binding with iron chelates and keeping them away from membrane surface where lipid peroxidation is initiated. Although these results warrant further in vivo investigation, it was concluded that BSA inhibits membrane lipid peroxidation by decreasing the availability of iron for the initiation of membrane lipid peroxidation, in addition to trapping active oxygens and free radicals.     

Formation of α-tocopherol radical and recycling of α-tocopherol by ascorbate during peroxidation of phosphatidylcholine liposomes: An electron paramagnetic resonance study

The events accompanying the inhibitory effect of α-tocopherol and/or ascorbate on the peroxidation of soybean L-α-phosphatidylcholine liposomes, which are an accepted model of biological membranes, were investigated by electron paramagnetic resonance, optical and polarograpic methods. The presence of α-tocopherol radical in the concentration range 10^?8–10^?7 M was detected from its EPR spectrum during the peroxidation of liposomes, catalysed by the Fe³⁺-triethylnetatramine complex. The α-tocopherol radical, generated in the phosphatidylcholine bilayer, is accessible to ascorbic acid, present in the aqueous phase at physiological concentrations. Ascorbic acid regenerates from it the α-tocopherol itself. A kinetic rate constant of about 2·10⁵ M^?·s^?1 was estimated from the reaction as it occurs under the adopted experimental conditions. The scavenging effect of α-tocopherol on lipid peroxidation is maintained as long a ascorbic acid is present.     

Action of 6-amino-3-pyridinols as novel antioxidants against free radicals and oxidative stress in solution,plasma, and cultured cells

Free radical-mediated lipid peroxidation has been implicated in the pathogenesis of various diseases. Lipid peroxidation products are cytotoxic and they modify proteins and DNA bases, leading eventually to degenerative disorders. Various synthetic antioxidants have been developed and assessed for their capacity to inhibit lipid peroxidation and oxidative stress induced by free radicals. In this study, the capacity of novel 6-amino-2,4,5-trimethyl-3-pyridinols for scavenging peroxyl radicals, inhibiting plasma lipid peroxidation in vitro, and preventing cytotoxicity induced by glutamate, 6-hydroxydopamine, 1-methyl-4-phenylpyridium (MPP⁺ ), and hydroperoxyoctadecadienoic acid was assessed. It was found that they exerted higher reactivity toward peroxyl radicals and more potent activity for inhibiting the above oxidative stress than α-tocopherol, the most potent natural antioxidant, except against the cytotoxicity induced by MPP⁺. These results suggest that the novel 6-amino-3-pyridinols may be potent antioxidants against oxidative stress.     

Capacity of fucoxanthin for scavenging peroxyl radicals and inhibition of lipid peroxidation in model systems

Abstract

Carotenoids act as physiological antioxidant by scavenging reactive-free radicals as well as quenching singlet oxygen. Fucoxanthin is one of the abundant carotenoids found in edible brown seaweeds. The assessment of radical scavenging capacity of carotenoids has been the subject of extensive studies, which, however, gave inconsistent results. In the present study, the capacity of fucoxanthin for scavenging peroxyl radicals, chain carrying species of lipid peroxidation, was assessed quantitatively by measuring the effect of α-tocopherol on the decay of fucoxanthin induced by peroxyl radicals. It was found that α-tocopherol was 7.1 times more reactive than fucoxanthin in heptane solution, but interestingly fucoxanthin exerted 1.6 times higher reactivity than α-tocopherol in methanol solution. In SDS micelles, the relative reactivity of fucoxanthin and α-tocopherol depended on the site of peroxyl radical formation. The efficacy of lipid peroxidation inhibition by fucoxanthin was much less than that of α-tocopherol.     

Generation of Probucol Radicals and Their Reduction by Ascorbate and Dihydrolipoic Acid in Human Low Density Lipoproteins

Probucol, 4.4′-[(1-methylethylidene)bis(thio)]bis-[2,6-bis(1.1-dimethyl)phenol], is a lipid regulating drug whose therapeutic potential depends on its antioxidant properties. Probucol and x-tocopherol were quantitatively compared in their ability to scavenge peroxyl radicals generatcd by the thermal decomposition of the lipid-soluble azo-initiator 2,2′-azo-bis(2,4-dimethyl-valeronitrile), AMVN, in dioleoylphos-phatidylcholine (DOPC) liposomes. Probucol showed 15-times lower peroxyl radical scavenging efficiency than x-tocopherol as measured by the effects on AMVN-induced luminol-dependent chemiluminescence. We suggest that probucol cannot protect x-tocopherol against its loss in the course of oxidation, although probucol is known to prevent lipid peroxidation in membranes and lipoproteins. In human low density lipoproteins (LDL) ESR signals of the probucol phenoxyl radical were detected upon incubation with lipoxygenase + linolenic acid or AMVN. Ascorbate was shown to reduce probucol radicals. Dihydro-lipoic acid alone was not able to reduce the probucol radical but in the presence of both ascorbate and dihydrolipoic acid a synergistic effect of a stepwise reduction was observed. This resulted from ascorbate-dependent reduction of probucol radicals and dihydrolipoic acid-dependent reduction of ascorbyl radicals. The oxidized form of dihydrolipoic acid, thioctic acid, did not affect probucol radicals either in the presence or in the absence of ascorbate.     

Clofibric acid or diethylmaleate supplemented diet decrease blood pressure in DOCA-salt treated male Sprague-Dawley rats - relation with liver antioxidant status

The effects of ascorbate and a-tocopherol as antioxidants and as co-operative factors against NADPH-dependent lipid peroxidation in human placental mitochondria have been studied. The addition of ascorbate at low concentration (up to 50 M) to the NADPH-generating system resulted in increasing lipid peroxidation and Fe³⁺ to Fe²⁺ reduction. High concentration of ascorbate (150 M), which produced maximal rate of ascorbate-dependent lipid peroxidation, was found to inhibit almost completely NADPH-dependent lipid peroxidation by maintaining too much iron in its reduced form. Either stimulatory or inhibitory effect of ascorbate on NADPH-dependent lipid peroxidation depends on the appropriate Fe³⁺/Fe²⁺ ratio. -Tocopherol caused a decrease of NADPH-dependent lipid peroxidation, inhibiting completely this process at 150 M concentration. The inhibitory effect of -tocopherol increased rapidly with the increasing ascorbate concentration, almost complete inhibition of NADPH-dependent lipid peroxidation being obtained at 25 M -tocopherol and 50 M ascorbate. This strong inhibitory combined effect of -tocopherol and ascorbate was independent of the Fe³⁺/Fe²⁺ ratio, as a-tocopherol is not able to reduce Fe³⁺ to Fe²⁺ under the conditions employed. These findings suggest that antioxidant effects of ascorbate in placental mitochondria are mediated by recycling of a-tocopherol rather than by strong reduction of Fe³⁺ to Fe²⁺. On the basis of the results obtained, we assume that adequate concentrations of a-tocopherol and ascorbate in placental tissue may prevent the release of lipid peroxide from placental mitochondria and therefore could be protective against the development of preeclampsia.     

Antioxidant Activity of Vitamin E and Trolox: Understanding of the Factors that Govern Lipid Peroxidation Studies In Vitro

Peroxidation of lipids is of significant interest owing to the evidence that peroxyl radicals and products of lipid peroxidation may be involved in the toxicity of compounds initiating a deteriorative reaction in the processing and storage of lipid-containing foods. In view of the significance of the antioxidant role of the dietary compound vitamin E and its water-soluble analogue Trolox in research of lipid-containing foods, it is desirable to determine more specifically how and where they operate its antioxidant activity in lipid membranes. In this study, unilamellar liposomes of phosphatidylcholine were used as membrane mimetic systems to estimate the antioxidant properties of vitamin E and Trolox and establish a relationship between their interactions with the membrane and their consequent antioxidant activity. Lipid peroxidation was initiated by the peroxyl radical (ROO^•) in lipid and aqueous media by the thermal decomposition of azocompounds and was assessed by the fluorescence intensity decay of the fluorescent probe diphenylhexatriene propionic acid. Results obtained showed that membrane lipoperoxidation is related not only to the scavenging characteristics of the compounds studied but also to their ability to interact with the lipid bilayers, and consequently liposomes provide additional information to that obtained currently from assays performed in aqueous buffer media.     

Inhibition of lipid peroxidation by S-nitrosoglutathione and copper

The antioxidant properties of S -nitrosoglutathione, a nitric oxide-derived product were studied in different experimental systems. By using the crocin bleaching test, S -nitrosoglutathione, in the presence of copper ions, shows an antioxidant capacity about six times higher than that of Trolox c and referable to the interception of peroxyl radicals by nitric oxide. Copper alone shows a modest inhibitory action, which is about seven times lower than that of Trolox c. S -nitrosoglutathione prevents lipid peroxidation induced by the well-known Fe 2+ /ascorbate system (IC 50 =450 μM) and the inhibitory effect is strongly reinforced by the presence of copper ions (IC 50 =6.5 μM). In addition, cumene hydroperoxide-induced lipid peroxidation is markedly decreased by S -nitrosoglutathione, provided that copper ions, maintained reduced by ascorbate, are present. Decomposition of S -nitrosoglutathione through metal catalysis and/or the presence of reducing agents and the consequent release of nitric oxide are of crucial importance for eliciting the antioxidant power. In this way, copper ions and/or reducing species with low antioxidant potency are able to promote the formation of an extremely strong antioxidant species such as nitric oxide.     

Prooxidant and antioxidant properties of Trolox C,analogue of vitamin E,in oxidation of low-density lipoprotein

Trolox C (Trolox), a water-soluble analogue of vitamin E lacking the phytyl chain, was investigated with respect to its effect on the oxidation of low-density lipoprotein (LDL). Trolox was added at different time points of LDL oxidation induced by Cu²⁺ and aqueous peroxyl radicals. In the case of Cu²⁺ -induced LDL oxidation, the effect of Trolox changed from antioxidant to prooxidant when added at later time points during oxidation; this transition occurred whenever α-tocopherol was just consumed in oxidizing LDL. Thus, in the case of Cu²⁺-dependent LDL oxidation, the presence of lipophilic antioxidants in the LDL particle is likely to be a prerequisite for the antioxidant activity of Trolox.

When oxidation was induced by peroxyl radicals, as a model of metal-independent oxidation, the effect of Trolox was always antioxidant, suggesting the importance of Cu²⁺/Cu⁺ redox-cycling in the prooxidant mechanism of Trolox. Our data suggest that, in the absence of significant amounts of lipophilic antioxidants, LDL becomes highly susceptible to oxidation induced by transition metals in the presence of aqueous reductants.     

Antioxidant mechanisms of isoflavones in lipid systems: paradoxical effects of peroxyl radical scavenging

Oxidation of lipids has been implicated in the pathophysiology of atherosclerosis. It has been suggested that scavenging of lipid peroxyl radicals contribute to the antiatherosclerotic effects of naturally occurring compounds such as the isoflavones. This group of polyphenolics includes genistein and is present in relatively high concentrations in food products containing soy. Soy isoflavones are capable of inhibiting lipoprotein oxidation in vitro and suppressing formation of plasma lipid oxidation products in vivo. However, key aspects of the antioxidant mechanisms remain unknown. In this study the antioxidant effects of genistein and other soy isoflavones on lipid peroxidation initiated by mechanistically diverse oxidants was investigated. Although isoflavones inhibited lipid peroxidation stimulated by both metal-dependent and independent processes, the concentration required for these effects were relatively high compared to those found in vivo. Interestingly, however, isoflavones were not consumed and remained in the native state over the time during which inhibition of lipid peroxidation was observed. This was also the case under conditions where synergistic inhibition of LDL oxidation was observed with ascorbate. Furthermore, in an oxidation system driven solely by peroxyl radicals, isoflavones were found to be relatively poor peroxyl radical scavengers. Consistent with the apparent lack of reactivity with lipid-derived oxidants, isoflavones were also relatively resistant to oxidation mediated by the potent oxidant peroxynitrite. The potential antioxidant mechanisms of isoflavones are discussed in the context of possible reactivities of isoflavone-derived phenoxyl radicals.     

Absence of an effect of vitamin E on protein and lipid radical formation during lipoperoxidation of LDL by lipoxygenase

Low-density lipoprotein (LDL) oxidation is the primary event in atherosclerosis, and LDL lipoperoxidation leads to modifications in apolipoprotein B-100 (apo B-100) and lipids. Intermediate species of lipoperoxidation are known to be able to generate amino acid-centered radicals. Thus, we hypothesized that lipoperoxidation intermediates induce protein-derived free radical formation during LDL oxidation. Using DMPO and immuno-spin trapping, we detected the formation of protein free radicals on LDL incubated with Cu²⁺ or the soybean lipoxidase (LPOx)/phospholipase A₂ (PLA₂). With low concentrations of DMPO (1 mM), Cu²⁺ dose-dependently induced oxidation of LDL and easily detected apo B-100 radicals. Protein radical formation in LDL incubated with Cu²⁺ showed maximum yields after 30 min. In contrast, the yields of apo B-100 radicals formed by LPOx/PLA₂ followed a typical enzyme-catalyzed kinetics that was unaffected by DMPO concentrations of up to 50 mM. Furthermore, when we analyzed the effect of antioxidants on protein radical formation during LDL oxidation, we found that ascorbate, urate, and Trolox dose-dependently reduced apo B-100 free radical formation in LDL exposed to Cu²⁺. In contrast, Trolox was the only antioxidant that even partially protected LDL from LPOx/PLA₂. We also examined the kinetics of lipid radical formation and protein radical formation induced by Cu²⁺ or LPOx/PLA₂ for LDL supplemented with α-tocopherol. In contrast to the potent antioxidant effect of α-tocopherol on the delay of LDL oxidation induced by Cu²⁺, when we used the oxidizing system LPOx/PLA₂, no significant protection was detected. The lack of protection of α-tocopherol on the apo B-100 and lipid free radical formation by LPOx may explain the failure of vitamin E as a cardiovascular protective agent for humans.     

Action of DCFH and BODIPY as a Probe for Radical Oxidation in Hydrophilic and Lipophilic Domain

Fluorogenic probes such as 2',7'-dichlorofluorescin (DCFH) have been extensively used to detect oxidative events and to measure antioxidant capacity. At the same time, however, the inherent drawbacks of these probes such as non-specificity towards oxidizing species have been pointed out. The present study was carried out to analyze the action and dynamics of 4, 4-difluoro-5-(4-phenyl-1,3-butadienyl)-4-bora-3a,4a-diaza-s-indacene-3-undecanoic acid (BODIPY) and DCFH as a fluorescent probe in the free radical-mediated lipid peroxidation in homogeneous solution, aqueous suspensions of liposomal membranes and LDL and plasma. The rate constant for the reaction of BODIPY with peroxyl radicals was estimated as 6.0×10₃ M_-1s_-1, which makes BODIPY kinetically an inefficient probe especially in the presence of potent radical-scavenging antioxidants such as tocopherols, but a convenient probe for lipid peroxidation. On the other hand, the reactivity of DCFH toward peroxyl radicals was as high as Trolox, a water-soluble analogue of α-tocopherol. Thus, DCFH is kinetically more favored probe than BODIPY and could scavenge the radicals within lipophilic domain as well as in aqueous phase. The partition coefficients for BODIPY and DCFH were obtained as 4.57 and 2.62, respectively. These results suggest that BODIPY may be used as an efficient probe for the free radical-mediated oxidation taking place in the lipophilic domain, especially after depletion of α-tocopherol, while it may not be an efficient probe for detection of aqueous radicals.     

The Antioxidative Effect of Fullerenes during the Peroxidation of Methyl Linoleate in Toluene

The antioxidative effect of fullerenes C₆₀ and C₇₀ was examined by measuring the inhibition of methyl linoleate (MeL) peroxidation in toluene initiated by 2,2′-azobis(2,4-dimethylvaleronitrile) (AMVN). The fullerenes retarded the formation of MeL hydroperoxides and lowered the rate of propagation. The reaction rates of fullerenes with AMVN-derived peroxyl radicals were much higher than that of MeL. These results indicate that fullerenes can act as retarders of lipid peroxidation, though their activity is low compared with that of α-tocopherol.     

The antioxidative effect of fullerenes during the peroxidation of methyl linoleate in toluene

The antioxidative effect of fullerenes C(60) and C(70) was examined by measuring the inhibition of methyl linoleate (MeL) peroxidation in toluene initiated by 2,2'-azobis(2,4-dimethylvaleronitrile) (AMVN). The fullerenes retarded the formation of MeL hydroperoxides and lowered the rate of propagation. The reaction rates of fullerenes with AMVN-derived peroxyl radicals were much higher than that of MeL. These results indicate that fullerenes can act as retarders of lipid peroxidation, though their activity is low compared with that of α-tocopherol.     

The multiple effects of ethylenediaminetetraacetate in several model lipid peroxidation systems

Experiments were performed which illustrate the various ways EDTA can influence lipid peroxidation. Either detergent-dispersed linoleate, or liposomes made from extracted microsomal phospholipids were utilized as substrates for peroxidation. Peroxidation was accomplished using Fe²⁺ or Fe³⁺. In systems utilizing Fe²⁺, EDTA chelation facilitated Fe²⁺ autoxidation which in turn caused peroxidation of detergent-dispersed linoleate. Peroxidation was not initiated during EDTA-Fe²⁺ autoxidation when the substrate lipids were in a liposomal configuration. Systems utilizing Fe³⁺ required an enzyme (either xanthine oxidase or NADPH-cytochrome P₄₅₀ reductase) to reduce the iron for peroxidative activity. EDTA chelation of Fe³⁺ enhanced the xanthine oxidase and NADPH-cytochrome P₄₅₀ reductase-catalyzed peroxidation of detergent-dispersed linoleate, presumably by facilitating the reduction of Fe³⁺. Catalase and mannitol inhibited both EDTA-Fe²⁺- and EDTA-Fe³⁺-dependent lipid peroxidation. EDTA-Fe³⁺ was not capable of initiating peroxidation of phospholipid liposomes following enzymatic reduction by either enzyme, but ADP-chelated iron effectively initiated liposomal peroxidation in similar systems. With xanthine oxidase-catalyzed peroxidation of liposomes with ADP-Fe³⁺, the inclusion of EDTA-Fe³⁺ caused a modest enhancement of activity. EDTA-Fe³⁺ greatly stimulated NADPH-cytochrome P₄₅₀ reductase-catalyzed peroxidation of liposomes with ADP-Fe³⁺. In contrast, the addition of EDTA, rather than EDTA-Fe³⁺ inhibited the liposomal peroxidation catalyzed by either enzyme with ADP-Fe³⁺ when the EDTA concentration exceeded the concentration of Fe³⁺.     

α-Tocopherol mediated peroxidation in the copper (II) and met myoglobininduced oxidation of human low density lipoprotein: The influence of lipid hydroperoxides

The principal antioxidant in human LDL, α-tocopherol, is converted to the α-tocopheroxyl radical after reaction with peroxyl radicals or Cu²⁺, and, if it does not terminate with peroxyl radicals, could initiate lipid peroxidation; a phenomenon called ‘tocopherol mediated peroxidation’. Only in the presence of Cu²⁺ and low levels of lipid hydroperoxides was an α-tocopherol dependent decrease in the resistance of LDL to oxidation detected. This suggests that tocopherol mediated peroxidation will probably not contribute significantly as a pro-oxidant process in those individuals most at risk of developing atherosclerosis through an oxidative mechanism.