beta-Carotene: interactions with alpha-tocopherol and ascorbic acid in microsomal lipid peroxidation

beta-Carotene, alpha-tocopherol, and ascorbic acid were tested for their ability to inhibit, enhance, or react synergistically with O(2) (15, 150, 760 torr) and, 2,2'-azobis (2-amidino-propane) dihydrochloride (AAPH) or 1,1'-azobis (cyclohexane-carbonitrile) (ACCN) in isolated rat liver microsomes. beta-Carotene did not protect against lipid peroxidation, i.e., malondialdehyde (MDA) formation, in microsomal samples incubated at 37 degrees C with aqueous soluble AAPH at all added beta-carotene concentrations and oxygen tensions. More MDA (16%, p < 0.001) was produced at 15 torr of O(2,) and 160 nmol/mg protein of beta-carotene compared to respective vehicle control. Individually, alpha-tocopherol and ascorbic acid exhibited antioxidant protection (ascorbic acid &z.Gt; alpha-tocopherol); however, a mixture of both compounds was no more protective than ascorbic acid alone. beta-Carotene demonstrated a concentration-dependent antioxidant affect at 15 torr O(2) (p < 0.01); but a prooxidant effect at higher O(2) at 150 and 760 torr (>57%, p < 0.001) by lipid-soluble ACCN. alpha-Tocopherol exhibited concentration-dependent inhibitory effects on microsomal MDA formation at all oxygen tensions, but was most effective under 150 torr. Ascorbic acid demonstrated a concentration-dependent antioxidant effect only at 150 torr. ACCN-induced lipid peroxidation was no greater for the combination of the three compounds than ascorbic acid added alone. Thus, antioxidant or prooxidant activities for beta-carotene, alpha-tocopherol, and ascorbic acid in microsomal suspensions are related to O(2) tension, solubility, antioxidant concentrations and are governed by complex interactions. Differences between AAPH- and ACCN-induced lipid peroxidation are related to differences in lipid solubility.     

Effects of beta-carotene and alpha-tocopherol on radical-initiated peroxidation of microsomes.

Rat liver microsomal membranes were exposed to either beta-nicotinamide adenine dinucleotide phosphate (NADPH), adenosine 5'-diphosphate (ADP), and Fe+3 or to azocompounds, and the antioxidant activities of beta-carotene and alpha-tocopherol were studied. Lipid peroxidation was monitored either by malondialdehyde (MDA) formation in the thiobarbituric acid assay at 535 nm or by hydroperoxide formation at 234 nm, after high-pressure liquid chromatography (HPLC) separation of phospholipid hydroperoxides. The radical initiators, water-soluble 2,2'-azobis(2-amidinopropane) (AAPH) and lipid-soluble 2,2'-azobis(2,4-dimethylvaleronitrile (AMVN), when thermally decomposed at 37 degrees C under air, produced a constant rate of lipid peroxidation in microsomes and lag times inversely related to their concentrations. Using 25 mM AAPH, beta-carotene suppressed lipid peroxidation at a concentration of 50 nmol/mg protein; using 24 mM AMVN, an inhibition of MDA formation was observed at a concentration of only 5 nmol/mg protein. Inhibition by beta-carotene did not produce a clearly defined lag phase. During AAPH-induced lipid peroxidation, beta-carotene was consumed linearly, and high levels of the antioxidant were still present at the end of 45 min of incubation. Using NADPH/ADP/Fe+3, protection by beta-carotene was observed at 10 nmol/mg protein. alpha-Tocopherol effectively suppressed both MDA and hydroperoxide formation in a dose-dependent manner when either NADPH/ADP/Fe+3 or azocompounds were used. These effects were observed at very low concentrations of the added alpha-tocopherol, ranging from 2 to 3 nmol/mg protein. When the lag times were measurable (AAPH and AMVN), they were directly proportional to the concentration of alpha-tocopherol and revealed the presence of endogenous antioxidants in the microsomal membranes. Different temporal relationships between the loss of alpha-tocopherol and lipid peroxidation were observed in relation to the prooxidant used. A substantial depletion of about 70% of endogenous alpha-tocopherol preceded the propagation phase when induced by the azocompounds, while only 20% of antioxidant disappeared at the beginning of the peroxidation when induced by NADPH/ADP/Fe+3. Although our results show that both beta-carotene and alpha-tocopherol suppress the peroxidation of microsomal membranes, their antioxidant efficacy is influenced by several factors, including the type of radical initiator involved and the site and rate of radical production.     

NADPH-dependent inhibition of lipid peroxidation in rat liver microsomes.

Microsomal NADPH-driven electron transport is known to initiate lipid peroxidation by activating oxygen in the presence of iron. This pro-oxidant effect can mask an antioxidant function of NADPH-driven electron transport in microsomes via vitamin E recycling from its phenoxyl radicals formed in the course of peroxidation. To test this hypothesis we studied the effects of NADPH on the endogenous vitamin E content and lipid peroxidation induced in liver microsomes by an oxidation system independent of iron: an azo-initiator of peroxyl radicals, 2,2'-azobis (2,4-dimethylvaleronitrile), (AMVN), in the presence of an iron chelator deferoxamine. We found that under conditions NADPH: (i) inhibited lipid peroxidation; (ii) this inhibitory effect was less pronounced in microsomes from vitamin E-deficient rats than in microsomes from normal rats; (iii) protected vitamin E from oxidative destruction; (iv) reduced chromanoxyl radicals of vitamin E homologue with a 6-carbon side-chain, chromanol-alpha-C-6. Thus NADPH-driven electron transport may function both to initiate and/or inhibit lipid peroxidation in microsomes depending on the availability of transition metal catalysts.     

Antioxidant effect of ethanol toward in vitro peroxidation of human low-density lipoproteins initiated by oxygen free radicals

This study was designed to evaluate the effect of ethanol on the peroxidation of human low-density lipoprotein (LDL) initiated by oxygen free radicals (O(2)(.-) and (.)OH in the absence of ethanol; O(2)(.-) and ethanol-derived peroxyl radicals, RO(2)(.), in the presence of ethanol) generated by gamma radiolysis. Initial radiolytic yields as determined by several markers of lipid peroxidation [i.e. decrease in endogenous antioxidants alpha-tocopherol and beta-carotene, formation of conjugated dienes and of thiobarbituric acid-reactive substances (TBARS)] were determined in 3 g liter(-1) LDLs (expressed as total LDL concentration) in the absence of ethanol or its presence at six different concentrations (0.42-17 x 10(-2) mol liter(-1)). Ethanol acted as an antioxidant by decreasing the rate of consumption of LDL endogenous antioxidants and the yields of formation of lipid peroxidation products, and by delaying the onset of the propagation phase for conjugated dienes and TBARS. With regard to the different markers studied, except for alpha-tocopherol and beta-carotene consumption, the effect of ethanol did not appear to be dependent on its concentration. Indeed, (.)OH were scavenged by ethanol at the lowest ethanol concentration (0.42 x 10(-2) mol liter(-1)), leading to RO(2)(.). These RO(2)(.) resulted in lower radiation-induced yields related to endogenous antioxidant consumption or to formation of lipid peroxidation products (for example, approximately 10% of RO(2)(.) oxidized LDLs from TBARS). Thus, under our in vitro conditions, ethanol behaved as an antioxidant when added to the LDL solutions. This should be taken into account in the reported antioxidant activity of wine. This is also of interest when lipophilic compounds have to be added as ethanolic solutions to LDLs to evaluate in vitro their antioxidant activity toward LDL peroxidation.     

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.     

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

The antioxidant efficacy of alpha-carotene and comparison with beta-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 alpha- and beta-carotenes, and the chain breaking antioxidant alpha-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, alpha-carotene retarded PCOOH formation better than beta-carotene. Similarly, lutein was a better antioxidant than is zeaxanthin. But lutein and zeaxanthin were more effective antioxidants than alpha- and beta-carotenes, respectively. After 1 h of incubation of the carotenoid with AMVN, alpha-, beta-carotene, lutein and zeaxanthin limited PCOOH formation by 77%, 68%, 85% and 82%, respectively, while alpha-tocopherol elicited 90% reduction. AMVN incubated with EYPC for 2 h induced the formation of TBARS compared to control (P < 0.001). alpha-Carotene significantly suppressed the TBARS formation by 78% whilst beta-carotene, lutein, zeaxanthin and alpha-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 alpha-carotene is a better antioxidant than is beta-carotene in phosphatidyl choline vesicles. It may, therefore, be useful in limiting free radical mediated peroxidative damage against membrane phospholipids in vivo.     

Antioxidant and pro-oxidant effects of lycopene in comparison with beta-carotene on oxidant-induced damage in Hs68 cells

Lycopene has become a focal point in recent research following clinical trials that suggest that beta-carotene may promote lung cancer in smokers. Because lycopene only differs from beta-carotene in lacking the beta-ionone structure, and beta-carotene is known to have pro-oxidant activity in vitro, we sought to determine whether lycopene might also have pro-oxidant activity in vitro in comparison with beta-carotene. Human foreskin fibroblasts (Hs68 cells) were first enriched with 10 and 20 μM lycopene or beta-carotene for 1 hr followed by incubation with various oxidants. Lipid peroxidation was measured as thiobarbituric acid-reactive substances (TBARS) released into the medium and DNA damage was measured as formation of comet and 8-hydroxy-2'-deoxyguanosine. The results showed that lycopene at 20 μM significantly decreased levels of TBARS induced by ferric nitrilotriacetate (Fe/NTA) but enhanced levels of TBARS induced by a lipid-soluble radical generator (2,2'-azobis[2,4-dimethylvaleronitrile]; AMVN). Both the antioxidant and pro-oxidant effects of lycopene tended to be dose-dependent. beta-Carotene at 20 μM did not significantly decrease TBARS induced by Fe/NTA but significantly increased TBARS induced by AMVN. Lipid peroxidation induced by a water-soluble radical generator 2,2'-azobis(2-amidinopropane)dihydrochloride was not significantly affected by either lycopene or beta-carotene. Neither lycopene nor beta-carotene affected DNA damage or changes in cell morphology induced by any of the three oxidants tested. The present study in Hs68 cells demonstrates that lycopene can be either an antioxidant or a pro-oxidant depending on the oxidants used, and that lycopene and beta-carotene behave similarly under the in vitro oxidative conditions. Although it is unclear whether lycopene may have pro-oxidant activity in vivo, our results caution that it may be premature to undertake clinical trials with lycopene.     

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.     

Characterization of Large Unilamellar Vesicles as Models for Studies of Lipid Peroxidation Initiated by Azocompounds

The aim of this work was to characterize large unilamellar vesicles (LUVETs) prepared by a hand-driven extrusion device in order to use them for studies of lipid peroxidation and antioxidant activity. Vesicle structure and size were examined by electron microscopy. Lipid and antioxidant content was determined before and after the extrusion procedure. Then LUVETs were subjected to autoxidation initiated by both the lipid-soluble 2,2'-azobis(2,4-dimethylvaleronitrile) and the water-soluble 2,2'-azobis(2-amidinopropane hydrochloride) azocompounds. The results demonstrated that: i) LUVETs prepared with lipid concentrations ranging between 25 and 150 mM were essentially unilamellar and reasonably homogeneous, with an average diameter of 90 nm; ii) the phospholipid, cholesterol and antioxidant amounts retained by filters were about 10-15%; iii) LUVETs were suitable for autoxidation studies initiated by the water-soluble azocompound both in the absence and presence of antioxidants. The lipid-soluble azocompound could be used only at low concentrations and its vesicle content had to be determined since part of the initiator was not incorporated into the lipid bilayer. These data suggest that LUVETs seem to be recommended for studies of lipid peroxidation and antioxidant activity.     

The inhibition of radical-initiated peroxidation of microsomal lipids by both alpha-tocopherol and beta-carotene.

Rat liver microsomal lipids in hexane solution were exposed to the lipid-soluble radical initiator, azobis-isobutyronitrile (AIBN), and the antioxidant activities of alpha-tocopherol and beta-carotene have been compared. Lipid peroxidation was monitored both by conjugated diene formation at 233 nm, and by malondialdehyde (MDA) formation in the thiobarbituric acid assay at 535 nm. Diene formation was continuous for at least 120 min in the presence of 85 micrograms/ml lipid and 4 mM AIBN. Both alpha-tocopherol and beta-carotene acted as chain-breaking antioxidants, suppressing lipid peroxidation and producing an induction period at concentrations as low as 0.5 and 8 microM, respectively. When both of these lipid-soluble antioxidants were present together, the oxidation was strongly suppressed and the induction period was the sum of the individual antioxidants, alpha-Tocopherol and beta-carotene also inhibited MDA generation. In the presence of 170 micrograms/ml lipid and 8 mM AIBN, beta-carotene exhibited an IC50 of 1.1 microM and inhibited completely at 15 microM. Using beta-carotene, an induction period was observed, although much less pronounced than with alpha-tocopherol. Furthermore, beta-carotene inhibited MDA production in a concentration-dependent manner and exhibited an IC50 of 50 microM. In addition, added beta-carotene delayed the radical-initiated destruction of the endogenous alpha-tocopherol and gamma-tocopherol in this system.     

The importance of lipid solubility in antioxidants and free radical generating systems for determining lipoprotein proxidation.

The oxidative modification of low-density lipoprotein (LDL) plays an important role in atherosclerosis. Protecting LDL from oxidation has been shown to reduce the risk of coronary heart disease. In this study, we compared the protective effects of two lipophilic antioxidants (vitamin E and lazaroid) with two hydrophilic antioxidants (trolox and vitamin C) in the presence of several different free radical generating systems. Vitamin E (IC50 = 5.9 microM) and lazaroid (IC50 = 5.0 microM) were more effective in inhibiting lipid peroxidation caused by a Fe-ADP free radical generating system than vitamin C (IC50 = 5.2 x 10(3) microM) and trolox (IC5 = 1.2 x 10(3) microM). Preincubation of lipoproteins with a lipophilic antioxidant increased the protective effect against various free radicals. Preincubation with hydrophilic antioxidants did not have an effect. We also tested the efficacy of the antioxidants when the free radicals were generated within the lipid or the aqueous environment surrounding the LDL. For this purpose, we used the peroxyl generating azo-compounds AMVN (2,2'-azobis(2,4-dimethylvaleronitrile)) and AAPH (2,2'azobis(2-amidinopropane) dihydrochloride). All of the antioxidants tested were more effective against free radicals generated in a water soluble medium than they were against free radicals generated in a lipid environment. In conclusion, our data demonstrate that lipid solubility is an important factor for both the antioxidant and the free radical generating systems in determining the extent of lipid peroxidation in LDL. Our data also demonstrate that antioxidant efficacy in one set of experimental conditions may not necessarily translate into a similar degree of protection in another set of conditions where lipophilicity is a variable.     

Antioxidant Activities of Some Extracts of Thymus zygis

The antioxidant activities of methanol and ethyl ether extracts obtained from Thymus zygis, collected during the flowering or non-flowering period, were evaluated and compared. To investigate this potential, extracts were tested on their capacity to react with diphenyl-picrylhydrazyl (DPPH) in a homogeneous medium, and to inhibit Fe²⁺/ascorbate-induced membrane lipid peroxidation, as estimated by the formation of thiobar-bituric acid-reactive substances (TBARS). Although methanol extracts reduce DPPH radicals more efficiently than ethyl ether extracts, suggesting a potent radical scavenger activity, the ethyl ether extracts were found to be most active in inhibiting lipid peroxidation in sarcoplasmic reticulum (SR) membranes. In addition, both extracts present peroxyl and superoxide radical scavenging activities. Peroxyl radicals were generated by the water soluble 2, 2A-azobis(2-amidinopropane) dihydrochloride (AAPH) azoinitiator, and the scavenging activities of the extracts were measured by the inhibition of cis-parinaric acid (PnA) fluorescence decay in SR. Superoxide radicals were generated either by an enzymatic or a non-enzymatic system, and the scavenger ability was evaluated by the inhibition of nitrob-lue tetrazolium reduction. Methanolic extracts are more potent as scavengers of peroxyl and super oxide radicals than the ethyl ether extracts. Apparently, there is a relationship between antioxidant potency and the total phenolic groups content in each extract.     

Distinct temporal relation among oxygen uptake, malondialdehyde formation, and low-level chemiluminescence during microsomal lipid peroxidation

An oxystat system was employed in conjunction with a single-photon counting apparatus for simultaneous monitoring of oxygen uptake, oxidative decomposition of membrane lipids, and occurrence of electronically excited species during microsomal lipid peroxidation. During NADPH/ADP-iron-promoted lipid peroxidation at a steady state oxygen partial pressure (pO2) of 30 mm Hg, complex time relationships among oxygen uptake, malondialdehyde (MDA) formation, and low-level chemiluminescence were observed. While the first two parameters occurred nearly simultaneously, low-level chemiluminescence occurred with a significant delay. A decrease of the steady state pO2 to 3 mm Hg led to significant increases of the lag phases of all three parameters and a further enhancement of the time displacement of low-level chemiluminescence in relation to oxygen uptake and MDA formation. At a pO2 of 0.5 mm Hg, the lowest pO2 maintained during this study, no low-level chemiluminescence was observed while oxygen uptake and MDA formation were still detected. In contrast, during NADPH/CCl4-promoted lipid peroxidation at a pO2 of 0.5 mm Hg a sudden drastic rise of low-level chemiluminescence accompanying oxygen uptake and MDA formation was observed. At pO2 between 0.5 and 3 mm Hg all three parameters occurred nearly concomitantly during the entire incubation. At pO2 levels above 3 mm Hg all three parameters showed principally the same behavior. However, the respective maxima of low-level chemiluminescence were reached with some delay. The present observations support the assumption that the decomposition of membrane lipid peroxyl radicals to MDA and the formation of electronically excited species proceed via different pathways. The time displacement between oxygen uptake and MDA formation, on the one hand, and low-level chemiluminescence, on the other hand, depends on the type of initiating radical system and on the steady state pO2 level. It is suggested that the differences are due to distinct subsets (chemical or spatial) of secondary peroxyl radicals in the membrane.     

Free radical scavenging and antioxidant effects of lactate ion: an in vitro study.

Divergent literature data are found concerning the effect of lactate on free radical production during exercise. To clarify this point, we tested the pro- or antioxidant effect of lactate ion in vitro at different concentrations using three methods: 1) electron paramagnetic resonance (EPR) was used to study the scavenging ability of lactate toward the superoxide aion (O(2)(-).) and hydroxyl radical (.OH); 2) linoleic acid micelles were employed to investigate the lipid radical scavenging capacity of lactate; and 3) primary rat hepatocyte culture was used to study the inhibition of membrane lipid peroxidation by lactate. EPR experiments exhibited scavenging activities of lactate toward both O(2)(-). and.OH; lactate was also able to inhibit lipid peroxidation of hepatocyte culture. Both effects of lactate were concentration dependent. However, no inhibition of lipid peroxidation by lactate was observed in the micelle model. These results suggested that lactate ion may prevent lipid peroxidation by scavenging free radicals such as O(2)(-). and.OH but not lipid radicals. Thus lactate ion might be considered as a potential antioxidant agent.     

Prooxidant activity of β-carotene under 100% oxygen pressure in rat liver microsomes

The effects of the partial pressure of oxygen (pO₂ on antioxidant efficiency of β-carotene in inhibiting 2,2′-azobis(2-amidinopropane) (AAPH)-induced lipid peroxidation are investigated in rat liver microsomal membranes. The rate of peroxyl radicals generated by thermolysis of AAPH at 37°C is markedly higher at 150 than 760 mm Hg pO₂. At 150 mm Hg pO₂ β-carotene acts as an antioxidant, inhibiting 2,2′-azobis(2-amidinopropane) (AAPH)-induced Malondialdehyde (MDA) formation. At 760 mm Hg pO₂, it loses its antioxidant activity and shows a prooxidant effect, increasing lipid peroxidation products, -Tocopherol prevents the prooxidant effect of β-carotene in a dose-dependent manner. Our data provide the first evidence of a prooxidant effect of β-carotene under 100% oxygen pressure in a biological membrane model and point out the existence of cooperative interactions between β-carotene and -tocopherol.     

Antioxidant Activities of Some Extracts of Thymus zygis

The antioxidant activities of methanol and ethyl ether extracts obtained from Thymus zygis, collected during the flowering or non-flowering period, were evaluated and compared. To investigate this potential, extracts were tested on their capacity to react with diphenyl-picrylhydrazyl (DPPH) in a homogeneous medium, and to inhibit Fe²⁺/ascorbate-induced membrane lipid peroxidation, as estimated by the formation of thiobar-bituric acid-reactive substances (TBARS). Although methanol extracts reduce DPPH radicals more efficiently than ethyl ether extracts, suggesting a potent radical scavenger activity, the ethyl ether extracts were found to be most active in inhibiting lipid peroxidation in sarcoplasmic reticulum (SR) membranes. In addition, both extracts present peroxyl and superoxide radical scavenging activities. Peroxyl radicals were generated by the water soluble 2, 2A-azobis(2-amidinopropane) dihydrochloride (AAPH) azoinitiator, and the scavenging activities of the extracts were measured by the inhibition of cis-parinaric acid (PnA) fluorescence decay in SR. Superoxide radicals were generated either by an enzymatic or a non-enzymatic system, and the scavenger ability was evaluated by the inhibition of nitrob-lue tetrazolium reduction. Methanolic extracts are more potent as scavengers of peroxyl and super oxide radicals than the ethyl ether extracts. Apparently, there is a relationship between antioxidant potency and the total phenolic groups content in each extract.     

(+)-Catechin as antioxidant: mechanisms preventing human plasma oxidation and activity in red wines

We evaluated the antioxidant effect of (+)-catechin (CTCH), in the presence of physiological antioxidant levels of ascorbic acid (AA), alpha-tocopherol (AT) and beta-carotene (BC), in human plasma oxidised with AAPH. Following a five-hour incubation, the formation of lipid oxidation products (TBARS) was almost doubled, and the concentrations of lipid soluble antioxidants were 10 to 30% from the initial levels. In these conditions, AA was consumed within the first hour of incubation. The addition of CTCH prevented AT and BC depletion and TBARS formation, but had no effect on AA consumption. When the kinetics of oxidation were analysed CTCH oxidation preceded lipid soluble antioxidant depletion, but no consumption of CTCH was associated to AA oxidation. Considering that CTCH could contribute to the antioxidant activity of red wine, we first characterised both the antioxidant capacity and CTCH content of several wines. The wines with highest content of CTCH and antioxidant activity were also the most effective in preventing AAPH-mediated oxidation of plasma vitamin E. Results support the idea that CTCH could have a role as a physiological antioxidant in human plasma, and that CTCH of wine could contribute to the antioxidant status of human plasma.     

The 21-aminosteroid inhibitors of lipid peroxidation: reactions with lipid peroxyl and phenoxy radicals

The 21-aminosteroids U74006F and U74500A have been examined for their ability to scavenge the lipid peroxyl (LOO.) and phenoxy (PhO.) radicals. Lipid peroxidation was followed by measuring the formation of linoleic acid hydroperoxide (LOOH; 18:200H) from linoleic acid during incubations in methanol at 37 degrees C. Initiation of lipid peroxidation was by the radical generator 2,2'-azobis(2,4-dimethylvaleronitrile; AMVN), which under the conditions employed, initiated LOOH formation at a constant rate of 22 microM/h with a kinetic chain length of 21. Alpha-tocopherol (alpha TC) nearly completely blocked the chain reaction by scavenging LOO., reducing its formation to that essentially attributable to initiation alone. The average inhibition rate constant kinh for alpha TC at 37 degrees C was calculated as 4.9 x 10(5) M-1 sec-1. U74006F or U74500A also inhibited LOOH formation, reducing its rate to a constant fraction of control in a concentration dependent manner. U74500A was a more potent scavenger of LOO. than U74006F; however, both compounds were considerably less potent than alpha TC based upon their respective kinh's at 37 degrees C. Similarly, alpha TC, U74006F and U74500A scavenged PhO.. As seen with LOO. scavenging, alpha TC was orders of magnitude more reactive toward PhO. than either 21-aminosteroid as judged by their respective second order rate constants (k2). Both U74006F and U74500A were degraded during their reaction with LOO. or PhO. to as yet uncharacterized product(s). The data indicate that while the 21-aminosteroids can scavenge lipid radicals, their activity in this regard is less than expected based upon their ability to inhibit iron dependent lipid peroxidation.     

Carotenoids are degraded by free radicals but do not affect lipid peroxidation in unilamellar liposomes under different oxygen tensions

It has been questioned whether carotenoids can act as antioxidants in biological membranes. Biological membranes can be modeled for studies of lipid peroxidation using unilamellar liposomes. Both carotenoid depletion and lipid peroxidation were increased with increasing oxygen tension in unilamellar liposomes. Carotenoids in such liposomes were found to be very sensitive to degradation by free radicals generated from iron and 2,2'-azobis(2-amidinopropane) dihydrochloride, but they were not protective against lipid peroxidation. Lycopene and beta-carotene were more sensitive to free radical attack than lutein, zeaxanthin, and beta-cryptoxanthin.     

Efficient radical trapping at the surface and inside the phospholipid membrane is responsible for highly potent antiperoxidative activity of the carotenoid astaxanthin

The effects of the carotenoids beta-carotene and astaxanthin on the peroxidation of liposomes induced by ADP and Fe(2+) were examined. Both compounds inhibited production of lipid peroxides, astaxanthin being about 2-fold more effective than beta-carotene. The difference in the modes of destruction of the conjugated polyene chain between beta-carotene and astaxanthin suggested that the conjugated polyene moiety and terminal ring moieties of the more potent astaxanthin trapped radicals in the membrane and both at the membrane surface and in the membrane, respectively, whereas only the conjugated polyene chain of beta-carotene was responsible for radical trapping near the membrane surface and in the interior of the membrane. The efficient antioxidant activity of astaxanthin is suggested to be due to the unique structure of the terminal ring moiety.