Antioxidant effect of FeAOX-6 on free radical species produced during iron catalyzed breakdown of tert-butyl hydroperoxide

There is a body of evidences demonstrating, in biological systems, a cooperative interaction between tocopherols and carotenoids. FeAOX-6 is a novel antioxidant that combines the chroman head of alpha-tocopherol and a fragment of the isoprenyl chain of lycopene. We have tested its antioxidant effect on different radical species generated in a chemical system, where peroxyl, alkoxyl and methyl radicals are generated by the ferrous ion-mediated decomposition of tert-butyl hydroperoxide. We found that FeAOX-6 has the same effectiveness of alpha-tocopherol in quenching peroxyl radical with no contribution by lycopene. The antioxidant activity of FeAOX-6 on alkoxyl and methyl radicals is comparable to that of the equimolar mixture of the parent compounds. Lycopene is able to quench alkoxyl radical, while it has no effect on peroxyl radical, showing a different antioxidant activity compared to other carotenoids, such as beta-carotene and lutein.     

Antioxidant activity of carotenoids: An electron-spin resonance study on β-carotene and lutein interaction with free radicals generated in a chemical system

β-Carotene is thought to be a chain-breaking antioxidant, even though we have no information about the mechanism of its antioxidant activity. Using electron-spin resonance (ESR) spectroscopy coupled to the spin-trapping technique, we have studied the effect of β-carotene and lutein on the radical adducts of the spin-trap PBN (N-t -butyl-α-phenylnitrone) generated by the metal-ion breakdown of different tert -butyl hydroperoxide (t BOOH) concentrations in methylene chloride. The peroxyl radical, along with an oxidation product of PBN (the PBNOx), trapped at room temperature from the breakdown of high concentration of t BOOH (1 M), were quenched by β-carotene or lutein, in competition with the spin-trapping agent. However, carotenoids were not able to quench the alkoxyl and methyl radicals generated in the reaction carried out in the presence of low t BOOH concentration (1 mM). The reaction between carotenoids and the peroxyl radical was also carried out in the absence of the spin trap, at 77 K: Under these different experimental conditions, we did not detect any radical species deriving from carotenoids. In the same system, a further evidence of the peroxyl radical quenching by β-carotene and lutein was obtained. The antioxidant activity of vitamin E was also tested, for comparison with the carotenoids. In the presence of α-tocopherol, peroxyl and alkoxyl radicals were quenched, and the tocopheroxyl radical was detected. Our data provide the first direct evidence that carotenoids quench peroxyl radicals. Under our experimental conditions, we did not detect any carotenoid radical species that could derive from the interaction with the peroxyl radical. The radical-trapping activity of β-carotene and lutein demonstrated in this chemical reaction contributes to our understanding carotenoid antioxidant action in biological systems. © 1998 John Wiley & Sons, Inc. J Biochem Toxicol 12: 299–304, 1998     

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.     

Design,synthesis, and antioxidant activity of FeAOX-6, a novel agent deriving from a molecular combination of the chromanyl and polyisoprenyl moieties

Several lines of evidence suggest potential benefits by a combination of carotenoids and tocopherols in chronic diseases. Therefore, we have designed FeAOX-6, a novel antioxidant that combines into a single molecule the chroman head of tocopherols and a fragment of lycopene, consisting of a polyisoprenyl sequence of four conjugated double bonds. The ability of FeAOX-6 in inhibiting lipid peroxidation and reactive oxygen species (ROS) production induced by different sources of free radicals (t-BOOH, AAPH, and H2O2) in arachidonic acid solution and in isolated thymocytes was investigated. Its antioxidant efficiency was also compared with that of alpha-tocopherol, lycopene, and a mixture of the two antioxidants. The results strongly suggest that FeAOX-6 can act as a potent antioxidant in our models, by inhibiting malondialdehyde production and ROS generation in a dose- and a time-dependent manner. In the cell model, the compound also provides a higher antioxidant capacity than alpha-tocopherol and lycopene, alone or in combination, suggesting the possibility of an oxidative intramolecular cooperation.     

Bactericidal activity of alkyl peroxyl radicals generated by heme-iron-catalyzed decomposition of organic peroxides.

To clarify the nature of cytocidal molecular species among the radicals generated in the iron-catalyzed reactions of peroxides (ROOH), we examined the cytocidal effects of these radicals against gram-positive and gram-negative bacteria in the presence or absence of various radical scavengers. Three organic peroxides, t-butyl hydroperoxide (t-BuOOH), methyl ethyl ketone peroxide (MEKOOH), and cumene hydroperoxide, were used. Each radical generated from these peroxides was identified and quantitated by electron spin resonance (ESR) spin trapping with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). The major cytotoxic radical species generated in the mixtures of various peroxides and heme iron, especially methemoglobin, metmyoglobin, or hemin, was the alkyl peroxyl radical (ROO.). Strong bactericidal action against gram-positive bacteria was observed in the peroxide-heme iron system, especially in the case of t-BuOOH and MEKOOH. Killing curves for gram-positive bacteria showed an initial lag period, which may indicate the multihit/multitarget kinetics of cell killing. When the diethylenetriamine pentaacetic acid (DTPA)-Fe2+ complex was used as a catalyst for decomposition of various peroxides, alkyl, alkoxyl, and alkyl peroxyl radicals were identified by spin-trapping analysis. However, study of the time course of alkyl peroxyl radical production in the DTPA-Fe2+ complex system revealed that radical species generated in this system were very short lived: a maximal level was achieved within 1 min and then declined sharply, and no bactericidal activity was observed after 10 min. In contrast, the alkyl peroxyl radical level generated by the organic peroxide-heme iron system remained high for 30 min or longer. The generation of alkyl peroxyl radicals quantified by ESR correlated quite well with the bactericidal effect of the system of peroxide plus iron. In addition, bactericidal activity was completely inhibited by the addition of the spin trap DMPO, as well as of other various radical scavengers (alpha-tocopherol and L-ascorbic acid), into the peroxide-heme iron system, but this effect was not observed with superoxide dismutase, beta-carotene, dimethyl sulfoxide, diphenylamine, or butylated hydroxyltoluene. In view of these results, it is assumed that alkyl peroxyl radicals are the potent molecular species that are cytotoxic against bacteria, whereas alkoxyl radicals (RO.) generated in this system do not affect bacterial viability.     

Betacyanins as phenol antioxidants. Chemistry and mechanistic aspects of the lipoperoxyl radical-scavenging activity in solution and liposomes

Reaction kinetics of betanin and its aglycone betanidin towards peroxyl radicals generated from the azo-initiated oxidation of methyl linoleate in methanol and of a heterogeneous aqueous/soybean phosphatidylcholine liposomal system were studied by monitoring formation of linoleic acid hydroperoxides and consumption of the pigments. Betanin was a weak retarder in methanol and an effective chain breaking antioxidant in the liposomal model, indicating that kinetic solvent effects and partition in lipid bilayers may affect its activity. Betanidin behaved as a chain terminating antioxidant in both models. Kinetic parameters characterizing peroxyl radical-scavenging activity showed that betanidin was more effective than betanin, in terms of both radical-scavenging rate constant and stoichiometric factor, with effectiveness of the same order as vitamin E under comparable conditions. Products identified by spectrophotometric and HPLC techniques indicated reaction of the glucose-substituted monophenol and ortho-diphenol moieties of betanin and betanidin, respectively, and suggested mechanisms of the antioxidant activity. Either betanin or betanidin incorporated in liposomes with α-tocopherol had additive effects, supporting partition of the pigments in the bilayer and lipoperoxyl radical reduction.     

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 α-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 α-tocopherol; (e) to be a weaker antiradical than α-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 α-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.     

Tocotrienol prevents AAPH-induced neurite degeneration in neuro2a cells

Abstract

Objectives

Reactive oxygen species induce neurite degeneration before inducing cell death. However, the degenerative mechanisms have not yet been elucidated. While tocotrienols have a known neuroprotective function, the underlying mechanism remains unclear and may or may not involve antioxidant action. In this study, we hypothesize that free radical-derived membrane injury is one possible mechanism for inducing neurite degeneration. Therefore, we examined the potential neuroprotective effect of tocotrienols mediated through its antioxidant activity.

Methods

Mouse neuroblastoma neuro2a cells were used to examine the effect of the water-soluble free radical generator 2,2′-azobis(2-methylpropionamide) dihydrochloride (AAPH) on neurite dynamics. After 24 hours of AAPH treatment, cell viability, neurite number, and the number of altered neurites were measured in the presence or absence of α-tocotrienol.

Results

Treatment of neuro2a cells with a low concentration of AAPH induces neurite degeneration, but not cell death. Treatment with 5 µM α-tocotrienol significantly inhibited neurite degeneration in AAPH-treated neuro2a cells. Furthermore, morphological changes in AAPH-treated neuro2a cells were similar to those observed with colchicine treatment.

Conclusions

α-Tocotrienol may scavenge AAPH-derived free radicals and alkoxyl radicals that are generated from AAPH-derived peroxyl radicals on cell membranes. Therefore, α-tocotrienol may have a neuroprotective effect mediated by its antioxidant activity.     

Spin trapping of polyunsaturated fatty acid-derived peroxyl radicals: reassignment to alkoxyl radical adducts

Polyunsaturated fatty acid (PUFA) peroxyl radicals play a crucial role in lipid oxidation. ESR spectroscopy with the spin-trapping technique is one of the most direct methods for radical detection. There are many reports of the detection of PUFA peroxyl radical adducts; however, it has recently been reported that attempted spin trapping of organic peroxyl radicals at room temperature formed only alkoxyl radical adducts in detectable amounts. Therefore, we have reinvestigated spin trapping of the linoleic, arachidonic, and linolenic acid-derived PUFA peroxyl radicals. The slow-flow technique allowed us to obtain well-resolved ESR spectra of PUFA-derived radical adducts in a mixture of soybean lipoxygenase, PUFA, and the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO). However, interpretation of the ESR spectra was complicated by the overlapping of the PUFA-derived alkoxyl radical adduct spectra. In order to understand these spectra, PUFA-derived alkoxyl radical adducts were modeled by various alkoxyl radical adducts. For the first time, we synthesized a wide range of DMPO adducts with primary and secondary alkoxyl radicals. It was found that many ESR spectra previously assigned as DMPO/peroxyl radical adducts based on their close similarity to the ESR spectrum of the DMPO/superoxide radical adduct, in conjunction with their insensitivity to superoxide dismutase, are indeed alkoxyl radical adducts. We have reassigned the PUFA alkylperoxyl radical adducts to their corresponding alkoxyl radical adducts. Using hyperfine coupling constants of model DMPO/alkoxyl radical adducts, the computer simulation of DMPO/PUFA alkoxyl radical adducts was performed. It was found that the trapped, oxygen-centered PUFA-derived radical is a secondary, chiral alkoxyl radical. The presence of a chiral carbon atom leads to the formation of two diastereomers of the DMPO/PUFA alkoxyl radical adduct. Therefore, attempted spin trapping of the PUFA peroxyl radical by DMPO at room temperature leads to the formation of the PUFA alkoxyl radical adduct.     

Real Time Detection of Reactions Between Radicals of Lycopene and Tocopherol Homologues

Laser flash photolysis of lycopene in homogeneous chloroform solution together with tocopherol homolopes results in rapid formation of the lycopene radical cation and slower formation of tocopheroxyl radicals. Time-resolved detection by absorption spectroscopy of decay of the lycopene radical cation, of formation of the tocopheroxyl radicals, and of bleaching of lycopene has shown that a-tocopherol is able to reduce the lycopene radical cation and thereby partially regenerate lycopene on a ms timescale. In contrast, lycopene is able to reduce the δ-tocopheroxyl radical, whereas an equilibrium exists between the lycopene radical cation and β- or γ-tocopherol. The relative stability of these antioxidant radicals is hence: a-tocopheroxyl > lycopene radical cation ≈ β-tocopheroxyl - γ-tocopheroxyl > S-toco-pheroxyl.     

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.     

Oxygen radical chemistry of polyunsaturated fatty acids

Polyunsaturated fatty acids (PUFA) are readily susceptible to autoxidation. A chain oxidation of PUFA is initiated by hydrogen abstraction from allylic or bis-allylic positions leading to oxygenation and subsequent formation of peroxyl radicals. In media of low hydrogen-donating capacity the peroxyl radical is free to react further by competitive pathways resulting in cyclic peroxides, double bond isomerization and formation of dimers and oligomers. In the presence of good hydrogen donators, such as alpha-tocopherol or PUFA themselves, the peroxyl radical abstracts hydrogen to furnish PUFA hydroperoxides. Given the proper conditions or catalysts, the hydroperoxides are prone to further transformations by free radical routes. Homolytic cleavage of the hydroperoxy group can afford either a peroxyl radical or an alkoxyl radical. The products of peroxyl radicals are identical to those obtained during autoxidation of PUFA; that is, it makes no difference whether the peroxyl radical is generated in the process of autoxidation or from a performed hydroperoxide. Of particular interest is the intramolecular rearrangement of peroxyl radicals to furnish cyclic peroxides and prostaglandin-like bicyclo endoperoxides. Other principal peroxyl radical reactions are the beta-scission of O2, intermolecular addition and self-combination. Alkoxyl radicals of PUFA, contrary to popular belief, do not significantly abstract hydrogens, but rather are channeled into epoxide formation through intramolecular rearrangement. Other significant reactions of PUFA alkoxyl radicals are beta-scission of the fatty chain and possibly the formation of ether-linked dimers and oligomers. Although homolytic reactions of PUFA hydroperoxides have received the most attention, hydroperoxides are also susceptible to heterolytic transformations, such as nucleophilic displacement and acid-catalyzed rearrangement.     

ESR study of a biological assay on whole blood: antioxidant efficiency of various vitamins

This study deals with the activity of various vitamins against the radical-mediated oxidative damage in human whole blood. We have used a biological method that allows both the evaluation of plasma and that of red blood cell resistance against the free radicals induced by 2,2′-azobis (2-amidinopropane) hydrochloride (AAPH). Spin trapping measures using mainly 5-(diethoxyphosphoryl)-5-methyl-1-pyrolline N-oxide nitrone (DEPMPO) were carried out under several conditions to identify the free radicals implicated in this test. Only the oxygenated-centred radical generated from AAPH was found highly reactive to initiate red blood cell lysis. With DEPMPO only alkoxyl radicals were observed and no evidence was found for alkylperoxyl radicals. The antioxidant activity of several lipid- and water-soluble vitamins has been assessed by the biological assay and through two chemical methods. We have noticed high antioxidant activities for tocopherols (in the order δ>γ>α) in the biological test but not through chemical methods. At 1 μM, the δ-tocopherol efficiency in inhibiting radical-induced red blood cell hemolysis was three times as high as the α-tocopherol efficiency. For β-carotene no significant activity even in whole blood was shown. Highly surprising antioxidant activities were observed for acid folic and pyridoxine, compared to ascorbic acid. At 10 μM, the effectiveness of folic acid was almost three times as high as vitamin C. The biological test seems clinically more relevant than most other common assays because it can detect several classes of antioxidants.     

Molecular modeling of the reactive configuration of peroxidized lipid and α-tocopherol in the membrane

The mutual arrangement of a phospholipid molecule containing a peroxyl radical and a molecule of membrane-acting antioxidant α-tocopherol (vitamin E) in the lipid bilayer has been studied by molecular dynamics simulation. The geometry of molecules in the membrane is revealed at which the hydrogen atom can be transferred from the exocyclic hydroxyl of α-tocopherol to the peroxyl lipid radical. It is shown that, under equilibrium conditions, the peroxidized fatty acid segment rises nearer to the polar surface of the membrane, while α-tocopherol submerges into the hydrophobic part of the lipid bilayer.     

Electron spin resonance study of the role of vitamin E and vitamin C in the inhibition of fatty acid oxidation in a model membrane

The neutral α-tocopheroxyl radicals, generated in monolayers on silica gel containing α-tocopherol and partly autoxidised methyl linoleate at 90°C, were detected and identified by ESR spectroscopy. Addition of ascorbic acid to the monolayer resulted in the complete quenching of the α-tocopheroxyl radical spectrum. This lends support to the view that ascorbate transfers hydrogen to α-tocopheroxyl radicals thus regenerating α-tocopherol.     

Studies on the photolytic breakdown of hydroperoxides and peroxidized fatty acids by using electron spin resonance spectroscopy. Spin trapping of alkoxyl and peroxyl radicals in organic solvents.

Spin trapping using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) has been used to detect and distinguish between the carbon-centred, alkoxyl, and peroxyl radicals produced during the photolytic decomposition of hydroperoxides. Photolysis of tert-butyl and cumene hydroperoxides, and peroxidized fatty acids, in toluene, with low levels of u.v. light, is shown to lead to the initial production of alkoxyl radicals by homolysis of the oxygen-oxygen bond. Subsequent reaction of these radicals with excess hydroperoxide leads, by hydrogen abstraction, to the production of peroxyl radicals that can be detected as their corresponding adducts with the spin trap. Subsequent breakdown of these adducts produces alkoxyl radicals and a further species that is believed to be the oxidized spin-trap radical 5,5-dimethyl-1-pyrrolidone-2-oxyl. No evidence was obtained at low hydroperoxide concentrations, with either the cumene or lipid alkoxyl radicals, for the occurrence of beta-scission reactions; the production of low levels of carbon-centred radicals is believed to be due to the alternative reactions of hydrogen abstraction, ring closure, and/or 1,2 hydrogen shifts. Analogous experiments with 3,3,5,5-tetramethyl-1-pyrroline N-oxide (TMPO) led only to the trapping of alkoxyl radicals with no evidence for peroxyl radical adducts, this is presumably due to a decreased rate of radical addition because of increased steric hindrance.     

Antiodidant activity of thiosulfinates derived from garlic

Abstract

Garlic extract significantly inhibited the oxidation of methyl linoleate in homogeneous acetonitrile solution, whereas the antioxidant effect of allicin-free garlic extract, prepared by removing allicin by prepared by removing allicin by preparative HPLC, was much lower than that of the garlic extract. These results suggest that the antioxidant properties are mostly attributed to the presence of allicin in the garlic extract. Allicin a major component of the thiosulfinates in garlic extract, was found to be effective for inhibiting methyl linoleate oxidation, but its efficiency was less than that of α-tocopherol. Next, the reactivity of allicin toward the peroxyl radical, which is a chain-propagating species, was investigated by direct ESR detection. The addition allicin to 2,2′-azobis(2,4-dimethylvaleronitrile)-peroxyl radical solution caused the signal intensity of the peroxyl radical to dose-dependently decrease, indicating that allicin is capable of scavenging the the peroxyl radical and acting as an antioxidant. Finally, we studied the structure–anioxidant activity relationship for thiosulfinates and suggested that the combination of the allyl group (–CH₂CH=CH₂) and the –S(O)S– group is necessary for the antioxidant action of thiosulfinates in the garlic extract. In addition, one of the two possible combinations, –S(O)S–CH₂CH=CH₂, was found to make a much larger contribution to the antioxidant activity of the thiosulfinates than the other, CH₂=CH–CH₂–S(O)S–.     

Antioxidant effect of capsaicin on lipid peroxidation in homogeneous solution,micelle dispersions and liposomal membranes

Abstract

The antioxidant activity of capsaicin (CAP) was measured in the oxidation of methyl linoleate (ML) in homogeneous solution, of ML micelles in aqueous dispersions and also of soybean phosphatidylcholine liposomal membrane, and was compared to that of -tocopherol (-TOH) which is one of the most important antioxidants in vivo. The reactivity of CAP toward galvinoxyl (a model phenoxyl radical) in acetonitrile solution was found to be much smaller than that of -TOH, suggesting that the radical scavenging activity of CAP is much weaker than that of -TOH. In fact, in homogeneous acetonitrile solution where the antioxidant activity is determined primarily by the chemical activity of the antioxidant toward peroxyl radicals, CAP inhibited the oxidation of ML much less efficiently than -TOH and a clear induction period was not observed. The antioxidant activity of CAP was found to be about 60 times smaller than that of -TOH in homogeneous solution. However, in micelle oxidation, the difference in antioxidant activity of the two antioxidants was much smaller than in homogeneous solution. Furthermore, in the membrane, CAP inhibited the oxidation almost as effectively as -TOH. These results suggest that CAP can act as an antioxidant in the biomembrane.     

Antioxidant activity of palm oil carotenes in peroxyl radical-mediatedperoxidation of phosphatidyl choline liposomes

Abstract

The antioxidant efficacy of α-carotene and comparison with β-carotene in multilamellar liposomes prepared from egg yolk phosphatidyl choline (EYPC) exposed to the lipid soluble 2,2′-azobis (2,4-dimethyl valeronitrile) (AMVN) was investigated. Lipid peroxidation was measured as thiobarbituric acid reacting substances (TBARS)at 532 nm or as hydroperoxide formation at 234 nm after separation of phosphatidyl choline hydroperoxide (PCOOH) by high-pressure liquid chromatography (HPLC). Lutein and zeaxanthin, the hydroxyl derivatives of α- and β-carotenes, and the chain breaking antioxidant α-tocopherol were also included in the study.AMVN being a lipid soluble, non polar azo initiator penetrates into the hydrophobic interior of the phospholipid bilayer, forming peroxyl radicals which peroxidate the phospholipid leading to PCOOH accumulation. All the carotenoids tested at 1 mol% relative to EYPC significantly suppressed the formation of PCOOH compared to control samples.In this system, α-carotene retarded PCOOH formation better than β-carotene. Similarly, lutein was a better antioxidant than is zeaxanthin. But lutein and zeaxanthin were more effective antioxidants than α- and β-carotenes, respectively. After 1 h of incubation of the carotenoid with AMVN, α-, β-carotene, lutein and zeaxanthin limited PCOOH formation by 77%, 68%, 85%and 82%, respectively, while α-tocopherol elicited 90%reduction.AMVN incubated with EYPC for 2 h induced the formation of TBARS compared to control (P <0.001). α-Carotene significantly suppressed the TBARS formation by 78% whilst β-carotene, lutein, zeaxanthin and α-tocopherol elicited 60%, 91%and 80% reductions, respectively. Increasing the concentration of the carotenoid >1 mol% to EYPC did not significantly increase protection of the membrane against free radical attack.Our findings suggest that α-carotene is a better antioxidant than is β-carotene in phosphatidyl choline vesicles. It may, therefore, be useful in limiting free radical mediated peroxidative damage against membrane phospholipids _{in vivo}.