Action of quinolinic and indolinonic aminoxyls as radical-scavenging antioxidants.

The kinetic studies on the actions of quinolinic and indolinonic aminoxyls in the oxidation of lipid peroxidation induced by free radicals were carried out to evaluate their antioxidant activity. These aminoxyls showed a similar reactivity toward peroxyl radical with alpha-tocopherol. The antioxidant efficacies of aminoxyls against oxidation of methyl linoleate in homogeneous solution were smaller than that of alpha-tocopherol. Hydroxylamine, a reduced form of aminoxyl, possessed a comparative antioxidant efficacy with alpha-tocopherol and was capable of suppressing the consumption of alpha-tocopherol. Aminoxyls showed more potent antioxidant activity than alpha-tocopherol against the oxidation of methyl linoleate micelles induced by peroxyl radical or by a combination of copper ion and hydrogen peroxide. These results suggest that quinolinic and indolinonic aminoxyls may act as potent antioxidants against lipid peroxidation, especially in the presence of a good reductant which reduces aminoxyl radicals to hydroxylamines.     

Synergistic interaction between the probucol phenoxyl radical and ascorbic acid in inhibiting the oxidation of low density lipoprotein.

Chain-breaking antioxidants such as butylated hydroxytoluene, alpha-tocopherol, and probucol have been shown to decrease markedly the oxidative modification of low density lipoprotein (LDL). Their mechanism of action appears to involve scavenging of LDL-lipid peroxyl radicals. The purpose of this study was to investigate the occurrence of radical reactions produced during oxidation of LDL and LDL-containing probucol initiated by lipoxygenase or copper. In addition, we have investigated the possibility of a synergistic interaction between ascorbate and probucol in inhibiting the oxidation of LDL. Incubation of LDL-containing probucol and lipoxygenase produced a composite electron spin resonance (ESR) spectrum due to the endogenous alpha-tocopheroxyl radical and probucol-derived phenoxyl radical. The spectral assignment was further verified by chemical oxidation of alpha-tocopherol and probucol. In the presence of ascorbic acid, these radicals in the LDL particle were reduced to their parent compounds with concomitant formation of the ascorbate radical. In both the peroxidation of linoleic acid and the copper-initiated peroxidation of LDL, the antioxidant activity of probucol was significantly increased by low (3-6 microM) concentrations of ascorbate. The probucol-dependent inhibition of LDL oxidation was enhanced in the presence of ascorbic acid. We conclude that the reaction between the phenoxyl radical of probucol and ascorbate results in a synergistic enhancement of the antioxidant capacity of these two compounds and speculate that such reactions could play a role in maintaining the antioxidant status of LDL during oxidative stress in vivo.     

17beta-estradiol affects in vivo the low density lipoprotein composition, particle size, and oxidizability.

The aim of this study was to explore the possible modifications induced by 17beta-estradiol (E(2)) in vivo on low-density lipoprotein (LDL) lipid composition, particle size, and oxidizability. For this purpose, women were recruited from an in vitro fertilization program, ranging their plasma E(2) levels from less than 12 pg/ml to more than 2000 pg/ml at the end of the treatment. The LDL lipid constituents were analyzed by thin layer chromatography and image analysis, and the LDL diameter was calculated from the lipid data. The results showed that high plasma E(2) levels were associated with smaller LDL particles, with lower amounts of free and esterified cholesterol and an increased relative content of alpha-tocopherol. The hormonal treatment produced a remodelation of the LDL acyl composition, rendering a lipoprotein enriched in saturated fatty acids, with a poorer polyunsaturated fatty acid content. These alterations in the physicochemical properties of LDL paralleled changes in the susceptibility of LDL to in vitro oxidation induced by both Cu(2+) and the peroxyl radical generator, 2,2'-azobis (2-amidinopropane), these changes being mainly reflected in a reduced maximum oxidation rate. The in vivo changes in the physicochemical properties of LDL induced by E(2) could explain some of the antiatherogenic actions of estrogens.     

Lipid peroxyl radical intermediates in the peroxidation of polyunsaturated fatty acids by lipoxygenase. Direct electron spin resonance investigations

Lipid peroxyl radicals resulting from the peroxidation of polyunsaturated fatty acids by soybean lipoxygenase were directly detected by the method of rapid mixing, continuous-flow electron spin resonance spectroscopy. When air-saturated borate buffer (pH 9.0) containing linoleic acid or arachidonate acid was mixed with lipoxygenase, fatty acid-derived peroxyl free radicals were readily detected; these radicals have a characteristic g-value of 2.014. An organic free radical (g = 2.004) was also detected; this may be the carbon-centered fatty acid free radical that is the precursor of the peroxyl free radical. The ESR spectrum of this species was not resolved, so the identification of this free radical was not possible. Fatty acids without at least two double bonds (e.g. stearic acid and oleic acid) did not give the corresponding peroxyl free radicals, suggesting that the formation of bisallylic carbon-centered radicals precedes peroxyl radical formation. The 3.8-G doublet feature of the fatty acid peroxyl spectrum was proven (by selective deuteration) to be a hyperfine coupling due to a gamma-hydrogen that originated as a vinylic hydrogen of arachidonate. Arachidonate peroxyl radical formation was shown to be dependent on the substrate, active lipoxygenase, and molecular oxygen. Antioxidants are known to protect polyunsaturated fatty acids from peroxidation by scavenging peroxyl radicals and thus breaking the free radical chain reaction. Therefore, the peroxyl signal intensity from micellar arachidonate solutions was monitored as a function of the antioxidant concentration. The reaction of the peroxyl free radical with Trolox C was shown to be 10 times slower than that with vitamin E. The vitamin E and Trolox C phenoxyl radicals that resulted from scavenging the peroxyl radical were also detected.     

Vitamin E, antioxidant and nothing more

All of the naturally occurring vitamin E forms, as well as those of synthetic all-rac-alpha-tocopherol, have relatively similar antioxidant properties, so why does the body prefer alpha-tocopherol as its unique form of vitamin E? We propose the hypothesis that all of the observations concerning the in vivo mechanism of action of alpha-tocopherol result from its role as a potent lipid-soluble antioxidant. The purpose of this review then is to describe the evidence for alpha-tocopherol's in vivo function and to make the claim that alpha-tocopherol's major vitamin function, if not only function, is that of a peroxyl radical scavenger. The importance of this function is to maintain the integrity of long-chain polyunsaturated fatty acids in the membranes of cells and thus maintain their bioactivity. That is to say that these bioactive lipids are important signaling molecules and that changes in their amounts, or in their loss due to oxidation, are the key cellular events that are responded to by cells. The various signaling pathways that have been described by others to be under alpha-tocopherol regulation appear rather to be dependent on the oxidative stress of the cell or tissue under question. Moreover, it seems unlikely that these pathways are specifically under the control of alpha-tocopherol given that various antioxidants other than alpha-tocopherol and various oxidative stressors can manipulate their responses. Thus, virtually all of the variation and scope of vitamin E's biological activity can be seen and understood in the light of protection of polyunsaturated fatty acids and the membrane qualities (fluidity, phase separation, and lipid domains) that polyunsaturated fatty acids bring about.     

Lipid peroxidation and its inhibition in low density lipoproteins: quenching of cis-parinaric acid fluorescence.

The fluorescent polyunsaturated parinaric acid incorporated in LDL particles is highly sensitive to the concentration of peroxyl radicals in the aqueous medium, undergoing rapidly oxidative degradation, as detected by a quenching of fluorescence, without delay after radical generation in solution. Ascorbate, cysteine, and urate suppress the parinaric acid fluorescence decay promoted by peroxyl radicals generated at a constant rate (thermal decomposition of 2,2'-azo-bis(2-amidino-propane hydrochloride)) in a concentration-dependent manner. The chain-breaking efficiencies of these antioxidants are evaluated from the time interval (inhibition period) of parinaric acid protection from oxidative degradation. The results correlate with the inhibition periods of LDL oxidation as monitored by O2 consumption. Therefore, the sensitive and simple parinaric acid assay can be used as a semiquantitative screening test for the detection of potentially important water-soluble chain-breaking antioxidants. Conversely to O2 consumption, the absence of any initial lag phase of probe degradation attests to the sensitivity of the assay. An improved methodology based on second-derivative spectroscopy to follow the formation of conjugated diene isomers directly in the preparation without the need for lipid extraction also confirms the sensitivity of this assay. To assess the usefulness of parinaric acid assay, strong chain-breaking activities of caffeic and chlorogenic acids are reported.     

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 alpha-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.     

Interaction and reactivity of urocanic acid towards peroxyl radicals

The capacity of urocanic acid to interact with peroxyl radicals has been evaluated in several systems: oxidation in the presence of a free radical source (2,2'-azobis(2-amidinopropane; AAPH), protection of phycocyanin bleaching elicited by peroxyl radicals, and Cu(II)- and AAPH-promoted LDL oxidation. The results indicate that both isomers (cis and trans) are mild peroxyl radical scavengers. For example, trans-urocanic acid is nearly 400 times less efficient than Trolox in the protection of the peroxyl radical promoted bleaching of phycocyanin. Regarding the removal of urocanic acid by peroxyl radicals, nearly 100 muM trans-urocanic acid is required to trap half of the produced radicals under the employed conditions (10 mM AAPH, 37 degrees C). Competitive experiments show that the cis-isomer traps peroxyl radicals 30% less efficiently than the trans-isomer. Given the high concentrations that trans-urocanic acid reaches in skin, its capacity to trap peroxyl radicals could contribute to the protection of the tissue towards ROS-mediated processes. Furthermore, both isomers, and particularly the cis-isomer, protect LDL from Cu(II)-induced oxidation.     

Protection of low density lipoprotein oxidation by the antioxidant agent IRFI005, a new synthetic hydrophilic vitamin E analogue

The oxidative modification of low density lipoprotein (LDL) is thought to be an important factor in the initiation and development of atherosclerosis. Antioxidants have been shown to protect LDL from oxidation and to inhibit atherosclerosis development in animals. Potent synthetic antioxidants are currently being tested, but they are not necessarily safe for human use. We here characterize the antioxidant activity of IRFI005, the active metabolite of Raxofelast (IRFI0016) that is a novel synthetic analog of vitamin E under clinical development, and demonstrate that it prevents oxidative modification of LDL. IFI005 inhibited the oxidative modification of LDL, measured through the generation of MDA, electrophoretic mobility and apo B100 fluorescence. During the oxidation process IRF1005 was consumed with the formation of the benzoquinone oxidation product. The powerful antioxidant activity of IRFI005 is at least in part mediated by a chain breaking mechanism as it is an efficient peroxyl radical scavenger with a rate constant k(IRFI005 + LOO(o)) of 1.8 X 10(6) M(-1)s(-1). 4. IRFI005 substantially preserved LDL-associated antioxidants, alpha-tocopherol and carotenoids, and when co-incubated with physiologic levels of ascorbate provoked a synergistic inhibition of LDL oxidation. Also the co-incubation of IRFI005 with Trolox caused a synergistic effect, and a lag phase in the formation of the trolox-benzoquinone oxidation product. A synergistic inhibition of lipid peroxidation was also demonstrated by co-incubating IRFI005 and alpha-tocopherol incorporated in linoleic acid micelles. These data strongly suggest that IRFI005 can operate by a recycling mechanism similar to the vitamin E/ascorbate sysem.     

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

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

The role of membrane phospholipids in oxidative phosphorylation. A hypothesis of LC-coupling

The role of lipid (pentadienyl) radicals and Mitchell's "C-elements" was analyzed. The function of the "C-elements" is necessary, but not enough for coupling of oxidation and phosphorylation. It was proposed that oxidative phosphorylation in the mitochondria membrane is the result of a direct coupling, involving participation of the "LC-elements" (where L and C reflect the inductive and capacitive character of the transformation device). The "L-elements" (the short pi-system) are lipid (pentadienyl) radicals formed in polyunsaturated fatty acids of phospholipids.     

Photoactivation of phthalocyanine-loaded low density lipoproteins induces a local oxidative stress that propagates to human erythrocytes: protection by caffeic acid

Toxic effects imposed to human erythrocytes by low density lipoproteins carrying phthalocyanines used in photodynamic therapy (PDT) of tumors are described. This study was aimed at evaluating cytotoxic effects induced by reactive species produced locally in photosensitizer-loaded lipoproteins and further transferred to the cells. The experimental set up designed to examine these interactions starts with the loading of human plasma with the photosensitizer, the subsequent rapid purification and dialysis of the LDL fraction and incubation with human erythrocytes. This experimental model was assessed by following leakage of endogenous K+ from cells, electrochemical detection of oxygen, spectroscopic determination of conjugated dienes, phthalocyanine, SH groups and hemoglobin, analysis of fatty acids by gas chromatography and identification of a-tocopherol by HPLC. Photosensitizer-loaded lipoproteins become more susceptible to oxidation, exhibiting shorter lag phases of lipid oxidation, higher rates of oxidation and increased loss of endogenous alpha-tocopherol when challenged with peroxyl radicals and copper, as compared with native lipoproteins from the same plasma sample. Incubation of photosensitized lipoproteins with erythrocytes under light (>560 nm) results in a sigmoidal efflux of K+ followed by hemolysis. The phenolic antioxidant caffeic acid inhibits lipoprotein oxidation induced by peroxyl radicals, either in native or photosensitizer-loaded fractions, delays hemolysis of erythrocytes and partially prevents membrane loss of SH groups in ghosts, but not the efflux of K+. Mechanistically, a chain lipid peroxidation reaction does not participate in the toxic effects to cells but a specific pool of membrane SH groups sensitive to caffeic acid is likely to be involved. This study suggests that an oxidative stress occurring locally in phthalocyanine-loaded low density lipoproteins may further induce cytotoxic effects by targeting specific SH groups at the cell membrane level. The physiological relevance of these findings and the beneficial use of antioxidants are discussed in the context of PDT.     

In vitro oxidation of alpha-tocopherol (vitamin E) in human platelets upon incubation with unsaturated fatty acids, diamide and superoxide

Incubation of human blood platelets in vitro in Tyrode solution with unsaturated fatty acids, diamide or superoxide (generated in situ) resulted in the oxidation of tocopherol in the platelets. Arachidonate concentrations of (3-5).10(-4) M caused a 50% decrease in platelet alpha-tocopherol. The addition of saturated fatty acids or platelet-active substances such as ADP, dibutyryl cyclic AMP, and some prostaglandins, or peroxidizing agents such as hydrogen peroxide and tert-butylhydroperoxide to the incubation medium did not cause any change in platelet tocopherol content. During incubations of platelets with arachidonate, malonaldehyde as well as alpha-tocopherolquinone were produced. The latter was also produced during incubations with diamide or superoxide. The oxidation of tocopherol induced by unsaturated fatty acids may be one factor responsible for the well-known increase in dietary vitamin E requirements induced by polyunsaturated fatty acids. The oxidative consumption of tocopherol in the membranes could be expected to take place during localized release of oxidants such as superoxide and polyunsaturated fatty acids during normal biological function (e.g., phagocytosis) or pathological processes (e.g., ischemia). Tocopherol utilization is kept low probably by the regeneration of the compound by vitamin C and/or the preferential utilization of the other biological antioxidants.     

Superoxide and peroxyl radical generation from the reduction of polyunsaturated fatty acid hydroperoxides by soybean lipoxygenase.

Soybean lipoxygenase is shown to catalyze the breakdown of polyunsaturated fatty acid hydroperoxides to produce superoxide radical anion as detected by spin trapping with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). In addition to the DMPO/superoxide radical adduct, the adducts of peroxyl, acyl, carbon-centered, and hydroxyl radicals were identified in incubations containing linoleic acid and lipoxygenase. These DMPO radical adducts were observed just prior to the system becoming anaerobic. Only a carbon-centered radical adduct was observed under anaerobic conditions. The superoxide radical production required the presence of fatty acid substrates, fatty acid hydroperoxides, active lipoxygenase, and molecular oxygen. Superoxide radical production was inhibited when nordihydroguaiaretic acid, butylated hydroxytoluene, or butylated hydroxyanisole was added to the incubation mixtures. We propose that polyunsaturated fatty acid hydroperoxides are reduced to form alkoxyl radicals and that after an intramolecular rearrangement, the resulting hydroxyalkyl radical reacts with oxygen, forming a peroxyl radical which subsequently eliminates superoxide radical anion.     

Interaction and reactivity of urocanic acid towards peroxyl radicals

Abstract

The capacity of urocanic acid to interact with peroxyl radicals has been evaluated in several systems: oxidation in the presence of a free radical source (2,2′-azobis(2-amidinopropane; AAPH), protection of phycocyanin bleaching elicited by peroxyl radicals, and Cu(II)- and AAPH-promoted LDL oxidation. The results indicate that both isomers (cis and trans) are mild peroxyl radical scavengers. For example, trans-urocanic acid is nearly 400 times less efficient than Trolox in the protection of the peroxyl radical promoted bleaching of phycocyanin. Regarding the removal of urocanic acid by peroxyl radicals, nearly 100 μM trans-urocanic acid is required to trap half of the produced radicals under the employed conditions (10 mM AAPH, 37°C). Competitive experiments show that the cis-isomer traps peroxyl radicals ~30% less efficiently than the trans-isomer. Given the high concentrations that trans-urocanic acid reaches in skin, its capacity to trap peroxyl radicals could contribute to the protection of the tissue towards ROS-mediated processes. Furthermore, both isomers, and particularly the cis-isomer, protect LDL from Cu(II)-induced oxidation.     

A method to measure the oxidizability of both the aqueous and lipid compartments of plasma.

The lipophilic radical initiator (MeO-AMVN) and the fluorescent probe C11BODIPY581/591 (BODIPY) were used to measure the lipid compartment oxidizability of human plasma. Aqueous plasma oxidizability was initiated by the aqueous peroxyl radical generator, AAPH, and 2',7'-dichlorodihydrofluorescein (DCFH) was employed as the marker of the oxidative reaction. The distribution in aqueous and lipid compartments of the two radical initiators was determined by measuring the rate of consumption of the plasma hydrophilic and lipophilic endogenous antioxidants. In the presence of AAPH (20 mM), the order of consumption was: ascorbic acid > alpha-tocopherol > uric acid > beta-carotene, indicating a gradient of peroxyl radicals from the aqueous to the lipid phase. When MeO-AMVN was used (2mM), beta-carotene was consumed earlier than uric acid and almost at the same time as alpha-tocopherol, reflecting the diffusion and activation of MeO-AMVN in the lipophilic phase. The rate of BODIPY oxidation (increase in green fluorescence) significantly increased after the depletion of endogenous alpha-tocopherol and beta-carotene, whereas it was delayed for 180 min when AAPH was used instead of MeO-AMVN. The measurement of lipid oxidation in plasma was validated by adding to plasma the two lipophilic antioxidants, alpha-tocopherol and beta-carotene, whose inhibitory effects on BODIPY oxidation were dependent on the duration of the preincubation period and hence to their lipid diffusion. DCFH oxidation induced by AAPH only began after uric acid, the main hydrophilic plasma antioxidant, was consumed. In contrast, when MeO-AMVN was used, DCFH oxidation was delayed for 120 min, indicating its localization in the aqueous domain. In summary, the selective fluorescence method reported here is capable of distinguishing the lipophilic and hydrophilic components of the total antioxidant capacity of plasma.     

Pulse radiolysis study of the interaction of retinoids with peroxyl radicals

Vitamin A (retinol) and its derivatives-retinal and retinoic acid-are known for their ability to inhibit lipid peroxidation. Antioxidant actions of retinoids have been attributed to chain-breaking by scavenging of peroxyl radicals. Based on chemical analysis of retinoic acid degradation products formed during microsomal lipid peroxidation, it was previously suggested that retinoids interact with peroxyl radicals forming free carbon-centered radical adducts. However, it can be argued that such a mode of antioxidant action of retinoids is not sufficient to fully explain their effectiveness at inhibiting lipid peroxidation, which in many systems is comparable to, or even exceeds, that of alpha-tocopherol. In order to elucidate the mechanism of interaction of retinoids with peroxyl radicals, (trichloromethyl)peroxyl radical was generated by pulse radiolysis, and its interactions with retinoids solubilized in Triton X-100 micelles were followed by kinetic absorption spectroscopy. All retinoids--retinol, retinal, and retinoic acid--interacted with the peroxyl radical, and at least two transient products were detected. One of these products, absorbing at 590 nm, was identified as retinoid cation radical. Therefore, we postulate that, apart from formation of radical adducts, retinoids may also scavenge peroxyl radicals by electron transfer.     

A synopsis of the process of lipid peroxidation since the discovery of the essential fatty acids

Eighty years ago, Burr and Burr, introduced for the first time the concept of essential fatty acids. Now is very well known that requirements for polyunsaturated fatty acids PUFAs can not be met by de novo metabolic processes within mammalian tissues. Animals are absolutely dependent on plants for providing the two major precursors of the n-6 and n-3 fatty acids, C18:2n-6; linoleic and C18:3n-3; α-linolenic acids. In animal tissues these precursors are transformed to fatty acids containing three to six double bonds. During the last four decades the interest in polyunsaturated fatty acids has augmented manifolds, and the number of published studies is rising each year. The current impetus for this interest has been mainly the observation that PUFAs and their metabolites have several physiological roles including: energy provision, membrane structure, cell signaling and regulation of gene expression. In addition the observation that PUFAs are targets of lipid peroxidation opens a new important area of investigation. Melatonin, the main secretory product of the pineal gland, efficiently scavenges both the hydroxyl and peroxyl radicals counteracting lipid peroxidation in biological membranes. In addition the two key pineal biochemical functions, lipoxygenation and melatonin synthesis may be synergistically regulated by the status of n-3 essential fatty acids. At the retina level, free radicals may preferentially react with the membrane polyunsaturated fatty acids leading to the release of lipoperoxide radicals. These lipoperoxides can induce oxidative stress linked to membrane lysis, damage to neuronal membranes may be related to alteration of visual function.     

Formation of active oxygen species and lipid peroxidation induced by hypochlorite.

Hypochlorite or its acid, hypochlorous acid, may exert both beneficial and toxic effects in vivo. In order to understand the role and action of hypochlorite, the formation of active oxygen species and its kinetics were studied in the reactions of hypochlorite with peroxides and amino acids. It was found that tert-butyl hydroperoxide and methyl linoleate hydroperoxide reacted with hypochlorite to give peroxyl and/or alkoxyl radicals with little formation of singlet oxygen in contrast to hydrogen peroxide, which gave singlet oxygen exclusively. Amino acids and ascorbate reacted with hypochlorite much faster than peroxides. Free radical-mediated lipid peroxidation of micelles and membranes in aqueous suspensions was induced by hypochlorite, the chain initiation being the decomposition of hydroperoxides by hypochlorite. It was suppressed efficiently by ebselen which reduced hydroperoxides and by alpha-tocopherol, which broke chain propagation, but less effectively by hydrophilic antioxidants present in the aqueous phase. Cysteine suppressed the oxidation, but it was poorer antioxidant than alpha-tocopherol. Ascorbate also exerted moderate antioxidant capacity, but it acted as a synergist with alpha-tocopherol. Taken together, it was suggested that the primary target of hypochlorite must be sulfhydryl and amino groups in proteins and that the lipid peroxidation may proceed as the secondary reaction, which is induced by radicals generated from sulfenyl chlorides and chloramines.     

Mode of action of sesame lignans in protecting low-density lipoprotein against oxidative damage in vitro

We investigated the antioxidant properties of sesaminol, a major component of sesame oil, on the oxidative modification of human low-density lipoprotein (LDL) in vitro. Sesaminol inhibited the Cu2+-induced lipid peroxidation in LDL in a concentration-dependent manner with an IC50 36.0 +/- 10.0 nM. Sesaminol was a more effective scavenger than either alpha-tocopherol or probucol in reducing the peroxyl radicals derived from 2,2'-azobis (2-amidinopropane) dihydrochloride (AAPH) in aqueous solution. In addition, as determined by the secondary products of lipid peroxidation identified by using immunochemical methods, sesaminol completely inhibited the formation of 4-hydroxy-nonenal (4-HNE)- and malondialdehyde (MDA)-adducts in a concentration-dependent manner. Probucol and alpha-tocopherol at the same concentration exhibited a lesser inhibitory effect. Our findings suggest that sesaminol is a potentially effective antioxidant that can protect LDL against the oxidation.