Antioxidants in relation to lipid peroxidation

Oxidative stress: from basic research to clinical application

The occurrence of reactive oxygen species, known as pro-oxidants, is an attribute of normal aerobic life. The steady-state formation of pro-oxidants is balanced by a similar rate of their consumption by antioxidants that are enzymatic and/or nonenzymatic. "Oxidative stress" results from imbalance in this pro-oxidant-antioxidant equilibrium in favor of the pro-oxidants. A number of diseases are associated with oxidative stress, being the basis of antioxidant therapy. Current evidence in clinical research does not show unequivocal distinction between causal or associative relationships of pro-oxidants to the disease process.

UVA-induced lipid peroxidation in cultured human fibroblasts

The UVA irradiation of cultured human fibroblasts leads to the formation and to the release of thiobarbituric acid-reactive substances in the supernatant. The major thiobarbituric acid-reactive substance is identified by fluorescence spectroscopy and HPLC, as malondialdehyde or malondialdehyde-forming substances under the thiobarbituric acid assay conditions. Malondialdehyde formation strongly suggests a UVA-induced lipid peroxidation. Lipid peroxidation is also supported by the inhibitory effect of D,L-alpha-tocopherol, the well-known chain breaking antioxidant, by the additional malondialdehyde formation in the dark after the photooxidative stress and by membrane damage revealed by lactate dehydrogenase leakage.

Inhibition of lipid peroxidation by ubiquinol in submitochondrial particles in the absence of vitamin E

The relationship between the antioxidant effects of reduced coenzyme Q10 (ubiquinol, UQH2) and vitamin E (alpha-tocopherol) was investigated in beef heart submitochondrial particles in which lipid peroxidation was initiated by incubation with ascorbate + ADP-Fe3+. These effects were examined after extraction of coenzyme Q10 (UQ-10) and vitamin E from the particles and reincorporation of the same components alone or in combination. The results show that UQH2 efficiently inhibits lipid peroxidation even when vitamin E is absent. It is concluded that UQH2 can inhibit lipid peroxidation directly, without the mediation of vitamin E.

Radiation inactivation of ion channels formed by gramicidin A. Protection by lipid double bonds and by alpha-tocopherol

The conductance induced by the channel-forming peptide gramicidin A in lipid membranes is reduced by many orders of magnitude on exposure of the membrane and its aqueous environment to ionizing radiation. This results from an interaction of free radicals of water radiolysis with the tryptophan residues of gramicidin A. The sensitivity of the ion channels towards irradiation is strongly reduced in the presence of either vitamin E or of highly unsaturated lipids. An increase of the D37 dose up to a factor of 50 was found. The phenomena are interpreted via a reduction of the effective concentration of free radicals (such as OH.) in the membrane by reaction with unsaturated fatty acid residues or with vitamin E.

Synthesis of 6-phosphatidyl-L-ascorbic acid by phospholipase D

Phospholipase D (EC 3.1.4.4) of Streptomyces species was found to catalyze transphosphatidylation to L-ascorbic acid from phosphatidylcholine (PC) in a biphasic reaction system. The product was identified as 1,2-diacyl-sn-glycero-3-phospho-6'-L-ascorbic acid (PA-AsA) by mass spectrometry and nuclear magnetic resonance spectroscopy. The optimal pH of transphosphatidylation was 4.5 and the rate of PA-AsA formation increased as concentrations of L-ascorbic acid increased. The conversion of PC to PA-AsA was greater than 80%. PA-AsA was found to be more resistant to hydrolysis by phospholipase D than was PC.

Sex differences in postischemic neuronal necrosis in gerbils

Twenty-four hour postischemic neuronal necrosis was compared in male vs. female Mongolian gerbils subjected to a 3-h period of severe incomplete hemispheric ischemia produced by unilateral carotid occlusion. The incidence of stroke-prone males was 42.9% versus 26.7% for the females. Among the stroke-prone animals, the males displayed significantly greater neuronal necrosis at 24 h after ischemia compared to the females in the cerebral cortex and CA1 region of the hippocampus. In the CA1 region of the stroke-prone males, only 2.0% of the normal neuronal population remained by 24 h compared to 36.8% in the stroke-prone females (p less than 0.02). In the cerebral cortex, the males had only 19.9% of normal versus 58.2% in the females (p less than 0.05). In a second series of mechanistic experiments, no differences in cortical blood flow (CBF) were disclosed between preselected male and female stroke-prone animals before, during, or for 2 h after ischemia. As with the CBF, the extent of cortical extracellular hypocalcia during ischemia did not differ significantly. However, the degree of postischemic recovery of cortical extracellular calcium was significantly better in the females from 30 min to 2 h after reperfusion. In the same experiments, hemispheric vitamin E levels were measured at the 2 h time point as an index of postischemic brain lipid peroxidation. No difference in baseline vitamin E levels was observed between male and female sham-operated gerbils. In the males subjected to 3 h of ischemia plus 2 h of reperfusion, the hemispheric vitamin E decreased by 43.5% compared to the sham-operated males.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipid peroxidation in brain and lungs from mice exposed to hyperoxia

Lipid peroxidation was measured in brains and lungs from mice which had been exposed to 100% O2 at 445 kPa, 515 kPa and 585 kPa for 30 min. These treatments produced varying degrees of convulsive activity and negligible to moderate lung damage. Lipid peroxidation was estimated by the thiobarbituric acid method and by analysis of Schiff bases. Lipid peroxidation did not increase in either lungs or brains after hyperbaric oxygen exposure, nor was there any difference between tissues from control or hyperoxic mice in the degree of lipid peroxidation which occurred during in vitro incubation of homogenates at 37 degrees in air for 150 min. Similarly, no increased lipid peroxidation was seen in lung tissue from mice exposed to normobaric 100% O2 for either 60 or 72 hr.

Vitamin E derivatives as new potent inhibitors of microsomal lipid peroxidation

Several synthetic Vitamin E derivatives are strong inhibitors of lipid peroxidation induced in rat liver microsomes either chemically by ferrous ions and ascorbate or enzymatically by NADPH and carbon tetrachloride. The relative activities of these inhibitors are consistent with their intrinsic antioxidant properties, as peroxyl radicals scavengers. Among them, a 3,4-dihydro-6-hydroxy-2H-1-naphtopyran with IC50 around 0.08 microM is one of the most potent yet known inhibitor of lipid peroxidation.

Preservation of the endogenous antioxidants in low density lipoprotein by ascorbate but not probucol during oxidative modification

Several lines of evidence indicate that the oxidative modification of low density lipoproteins (LDL) may provide an important link between plasma LDL and the genesis of the atherosclerotic lesion. Ascorbate is an important water-soluble, chain-breaking antioxidant in humans. Probucol, a lipid-soluble antioxidant drug has been shown to retard the progression of atherosclerosis. The aim of the present study was to compare the effects of probucol and physiologic levels of ascorbate on the oxidative modification of LDL in both a cell-free (2.5 microM Cu++ in phosphate-buffered saline) and cellular system (human monocyte macrophages in Ham's F-10 medium). Both ascorbate and probucol inhibited the oxidative modification of LDL in both systems to a similar degree as evidenced by the thiobarbituric acid-reacting substance activity, electrophoretic mobility, and degradation by macrophages. However, whereas co-incubation with physiologic levels of ascorbate resulted in a substantial preservation of the alpha-tocopherol, gamma-tocopherol, and beta-carotene of the LDL, probucol in concentrations ranging from 10 to 80 microM failed to protect these antioxidants. Thus, in addition to being as potent as probucol in inhibiting the oxidation of LDL, ascorbate in contrast preserves the endogenous antioxidants in the LDL.

Cytotoxicity of linoleic acid peroxide, malondialdehyde and 4-hydroxynonenal towards human fibroblasts

Lipid peroxidation occurs during oxidative stress and leads to the formation of various active compounds. However, controversy remains about its importance in the events leading to cell death. One approach to estimate their role in cell death would be to test the toxicity of oxidative products generated during the stress. In this work, three of these products were incubated with human fibroblasts and their toxicities were compared. The three compounds tested are: linoleic acid peroxide (LOOH), malondialdehyde (MDA) and 4-hydroxynonenal (HNE). Three cellular parameters were assayed: viability, DNA synthesis estimated by thymidine incorporation and protein synthesis measured by leucine incorporation. Protection against cellular damages was also tested adding alpha-tocopherol in the culture medium. The results showed that the peroxide was more toxic than HNE and much more than MDA. The possibility of initiation and propagation of the free radical chain reaction could explain this highest toxicity. The fibroblasts seem to be protected by alpha-tocopherol against LOOH. These effects emphasize the crucial role of this lipophilic antioxidant to protect cells against peroxidation damages.

Effect of antioxidants on oxidative modification of LDL

Human low density lipoprotein (LDL) with a molecular mass of 2.5 million contains on average 1300 molecules of polyunsaturated fatty acids (PUFAs) bound in the different lipid classes. The predominant antioxidant in LDL is alpha-tocopherol, with an average of 6 molecules in each LDL particle. The other substances with potential antioxidant activity are: gamma-tocopherol, beta-carotene, alpha-carotene, lycopene, cryptoxanthin, cantaxanthin, phytofluene and ubiquinol-10. Each is present in amounts of only 1/20th to 1/300th of that of alpha-tocopherol. If LDL is exposed to oxidative conditions (Cu++ ions, macrophages) a lag phase precedes the oxidation of PUFAs. During the lag phase the antioxidants disappear with alpha-tocopherol the first to go and beta-carotene the last. The lag phase, which can readily be determined, is an index of the oxidation resistance of LDL. If LDL is loaded with vitamin E in vitro its oxidation resistance increases linearly with its alpha-tocopherol content according to the equation, y = kx+a. The same relationship is applicable if the alpha-tocopherol content of LDL is increased by taking oral vitamin E. Daily doses of 150, 225, 800 and 1200 IU RRR-alpha-tocopherol increased the LDL alpha-tocopherol on average to 138%, 158%, 144% and 215% of the initial value, the oxidation resistance being increased to 118%, 156%, 135% and 175%, respectively. The efficiency of vitamin E-dependent (= k) and the vitamin independent (= a) oxidation resistance seem to be subject specific with strong individual variation.(ABSTRACT TRUNCATED AT 250 WORDS)

Free radical recycling and intramembrane mobility in the antioxidant properties of alpha-tocopherol and alpha-tocotrienol

d-Alpha-tocopherol (2R,4'R,8'R-Alpha-tocopherol) and d-alpha-tocotrienol are two vitamin E constituents having the same aromatic chromanol "head" but differing in their hydrocarbon "tail": tocopherol with a saturated and toctrienol with an unsaturated isoprenoid chain. d-Alpha-tocopherol has the highest vitamin E activity, while d-alpha-tocotrienol manifests only about 30% of this activity. Since vitamin E is considered to be physiologically the most important lipid-soluble chain-breaking antioxidant of membranes, we studied alpha-tocotrienol as compared to alpha-tocopherol under conditions which are important for their antioxidant function. d-Alpha-tocotrienol possesses 40-60 times higher antioxidant activity against (Fe2+ + ascorbate)- and (Fe2+ + NADPH)-induced lipid peroxidation in rat liver microsomal membranes and 6.5 times better protection of cytochrome P-450 against oxidative damage than d-alpha-tocopherol. To clarify the mechanisms responsible for the much higher antioxidant potency of d-alpha-tocotrienol compared to d-alpha-tocopherol, ESR studies were performed of recycling efficiency of the chromanols from their chromanoxyl radicals. 1H-NMR measurements of lipid molecular mobility in liposomes containing chromanols, and fluorescence measurements which reveal the uniformity of distribution (clusterizations) of chromanols in the lipid bilayer. From the results, we concluded that this higher antioxidant potency of d-alpha-tocotrienol is due to the combined effects of three properties exhibited by d-alpha-tocotrienol as compared to d-alpha-tocopherol: (i) its higher recycling efficiency from chromanoxyl radicals, (ii) its more uniform distribution in membrane bilayer, and (iii) its stronger disordering of membrane lipids which makes interaction of chromanols with lipid radicals more efficient. The data presented show that there is a considerable discrepancy between the relative in vitro antioxidant activity of d-alpha-tocopherol and d-alpha-tocotrienol with the conventional bioassays of their vitamin activity.

Free radical damage to proteins: the influence of the relative localization of radical generation, antioxidants, and target proteins

Free radicals were generated at known rates in the aqueous phase (by means of 2,2'-azobis (2-amidinopropane) dihydrochloride [AAPH]) and in a membranous (lipid) phase (by means of 2,2'-azobis (2,4-dimethylvaleronitrile [AMVN]). A soluble protein (bovine serum albumin: BSA), and membranes of lysed mitochondria containing radioactively labeled monoamine oxidase (MAO), were exposed to the resultant radical fluxes. Antioxidants were added to the system, either in the aqueous phase (Trolox) or in a liposomal membrane phase (alpha-tocopherol). Protein damage was assessed as tryptophan oxidation and conformational changes in tryptophan fluorescence of the soluble protein, BSA, and as fragmentation of both BSA and monoamine oxidase. Radicals generated in the aqueous phase, by AAPH, were effective in damaging BSA and MAO. Radicals generated within the liposome membrane phase (by AMVN) were less effective against BSA than those deriving from AAPH. Liposomal AMVN radicals could damage MAO, present in a separate membranous phase, though again, less effectively than could AAPH-derived radicals. BSA could be protected by Trolox, the aqueous soluble antioxidant, but hardly by tocopherol itself. Damage to MAO was limited by Trolox, and also by the hydrophobic antioxidant, tocopherol. Damaging reactions due to radicals generated in a membrane phase were significantly accelerated when the membrane was peroxidizable (soybean phosphatidylcholine) rather than nonperoxidizable (saturated dimyristoyl phosphatidylcholine). Thus lipid radicals also played some role in protein damage in these systems. BSA was attacked similarly in the presence or absence of liposomes by AAPH. Correspondingly, BSA could inhibit the peroxidation of liposomes induced by AAPH and less efficiently that induced by AMVN.(ABSTRACT TRUNCATED AT 250 WORDS)

Another cholesterol hypothesis: cholesterol as antioxidant

Current emphasis on cholesterol as agency if not cause of human atherosclerosis and subsequent cardiovascular disease ignores the essentiality of cholesterol in life processes. Additionally ignored is the ubiquitous presence of low levels of oxidized cholesterol derivatives (oxysterols) in human blood and select tissues, oxysterols also implicated in atherosclerosis. Whereas such oxysterols may be regarded putatively as agents injurious to the aorta, an alternative view of some of them is here proposed: that B-ring oxidized oxysterols of human blood represent past interception of blood and tissue oxidants in vivo by cholesterol as an ordinary aspect of oxygen metabolism. Such interception and subsequent efficient hepatic metabolism of oxysterols so formed, with biliary secretion and fecal excretion, constitute as in vivo antioxidant system. Whether cholesterol, oxysterols, oxidized lipoproteins, or oxidants in blood, singly or in concert, cause or exacerbate human atherosclerosis remains to be understood.

Vitamin E and the peroxidizability of erythrocyte membranes in neonates

We showed the increased susceptibility of neonatal biomembranes to oxidation by a kinetic analysis using an azo compound as a free-radical initiator and red blood cell (RBC) ghosts as a model membrane. When the RBC ghosts were oxidized, oxygen consumption was suppressed during the induction period in which membrane tocopherol was consumed at a constant rate, while increased oxygen uptake was observed after the tocopherol was exhausted. The total tocopherol content was similar in cord, maternal, and adult RBC ghosts, and there were no differences in the induction period (tinh) among the three types of ghosts. While the oxygen uptake rate during the induction period (Rinh) was similar in cord and adult ghosts, the rate in the subsequent phase (Rp) was considerably faster in the cord ghosts. Fatty acid analysis in the membrane lipids showed that the active bisallylic hydrogen (active H) content was greater in cord ghosts than in adult ghosts. The active H content closely correlated with the Rp, but did not with the Rinh. The kinetic chain length (KCL), i.e., the ratio of the rate of propagation to that of initiation, was calculated from Rp and tocopherol consumption rate and KCL values were higher in cord ghosts than in adult ghosts. The faster Rp and the higher KCL of the cord ghosts were attributable to a greater active H content rather than to the tocopherol content.

Free radical-induced liver injury. II. Effects of intraperitoneally administered 2,2'-azobis(2-amidinopropane) dihydrochloride on the fatty acid profiles of hepatic triacylglycerol and phospholipids

Liver injury induced by the radical initiator, 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH) and carbon tetrachloride (CCI4) was examined by the analysis of lipids in the liver of rats. Increased triacylglycerol (TAG) was found in the liver within 24 hr following injection of these drugs. In AAPH-treated and CCI4-treated rats, it was 2.1 and 1.8 times that in the controls, respectively. TAG-palmitate and -oleate were found at particularly increased levels, while polyunsaturated fatty acid profiles of hepatic phospholipids were essentially the same for the treated and untreated rats. It is evident from these findings that radical initiators cause no decrease in polyunsaturated fatty acids in hepatic lipids, but accumulate TAG in the liver. Such a condition is the equivalent of liver injury in the rats in whose diets vitamin E has long been deficient.

Effect of additives on the inactivation of lysozyme mediated by free radicals produced in the thermolysis of 2,2'-azo-bis-(2-amidinopropane)

The inactivation of lysozyme caused by the radicals produced by thermolysis of 2,2'-azo-bis-2-amidinopropane can be prevented by the addition of different compounds that can react with the damaging free radicals. Compounds of high reactivity (propyl gallate, Trolox, cysteine, albumin, ascorbate, and NADH) afford almost total protection until their consumption, resulting in well-defined induction times. The number of radicals trapped by each additive molecule consumed ranges from 3 (propyl gallate) to 0.12 (cysteine). This last value is indicative of chain oxidation of the inhibitor. Uric acid is able to trap nearly 2.2 radicals per added molecule, but even at large (200 microM) concentrations, a residual inactivation of the enzyme is observed, which may be caused by urate-derived radicals. Compounds of lower reactivity (tryptophan, Tempol, hydroquinone, desferrioxamine, diethylhydroxylamine, methionine, histidine, NAD+ and tyrosine) only partially decrease the lysozyme inactivation rates. For these compounds, we calculated the concentration necessary to reduce the enzyme inactivation rate to one half of that observed in the absence of additives. These concentrations range from 9 microM (tryptophan and Tempol) to 5 mM (NAD+).

Effect of exposure of various oxidants on rat liver and intestinal microsomes--a comparative study

Rat liver and intestinal microsomes were exposed to various free radical generating systems and their effect were assessed by studying different parameters such as formation of malonaldehyde (MDA) and conjugated diene, arachidonic acid depletion and alteration in protein thiol groups and tocopherol levels. These studies revealed that liver being highly vulnerable tissue showed all the effects of free radical attack whereas intestinal microsomes were resistant to most oxidants except iron independent generation of free radicals using 2-2'-azobis (2-amidinopropane) dihydrochloride (ABAP). Intestinal microsomes were found to contain considerable amount of non-esterified fatty acids in total lipid fraction as compared to liver microsomes and iron-fatty acid complex may be incapable of participating in peroxidation. In vitro measurement of hydroxyl radical generation showed that intestinal microsomes were incapable of generating these active species. These results suggest that iron dependent free radical mediated lipid peroxidation might not occur in intestinal epithelial cells.

Oxidative status and oral contraceptive. Its relevance to platelet abnormalities and cardiovascular risk

Oral contraceptive (OC) use is a risk for thrombogenic events. This paper reviews effects of OC on oxidative status, coagulation, and platelet activity. Complicating effects of cardiovascular risk factors such as smoking, diabetes, hyperpidemia, and hypertension, are discussed. From these data we conclude that: 1. OC use modifies slightly but significantly the oxidative status in women and in animals by decreasing in plasma and blood cells the antioxidant defenses (vitamins and enzymes). 2. The changes in the oxidative status are related to an increase in plasma lipid peroxides apparently responsible for the hyperaggregability and possibly the imbalance in clotting factors associated with the OC-induced prethrombotic state. 3. These effects of OC appear to be increased by a high intake of polyunsaturated fat and counteracted by supplements of vitamin E. 4. The risk factors acting synergistically with OC, have all been shown to increase platelet reactivity. In addition, smoking, diabetes, and, to some extent, dyslipidemia are associated with an increased level of lipid peroxides and concomitant changes in the antioxidant defenses that can be additive to those induced by OC. Thus, free radicals and lipid peroxidation could be the underlying mechanism in the predisposition to thrombosis induced by most risk factors in OC users. 5. Results of epidemiologic and experimental studies in this field will be concordant only when diet and natural antioxidants will be systematically taken into consideration.

Free radical-induced liver injury. I. Effects of dietary vitamin E deficiency on triacylglycerol level and its fatty acid profile in rat liver

Effects of dietary vitamin E deficiency on the fatty acid compositions of total lipids and phospholipids were studied in several tissues of rats fed a vitamin E-deficient diet for 4, 6, and 9 months. No significant differences were observed between the vitamin E deficiency and controls except in the fatty acid profiles of liver total lipids. Triacylglycerol (TAG) accumulation was found in the liver of rats fed a vitamin E-deficient diet. The levels of TAG-palmitate and -oleate increased particularly in the liver from such animals. The fatty acid compositions of hepatic phospholipids were not affected by the diet. Increased TAG observed in the liver of rats fed a vitamin E-deficient diet was restored to normal when the diet was supplemented with 20 mg alpha-tocopheryl acetate/kg diet. These findings indicate that dietary vitamin E deficiency causes TAG accumulation in the liver and that the antioxidant, vitamin E, is capable of preventing free radical-induced liver injury.

An in vitro model to test relative antioxidant potential: ultraviolet-induced lipid peroxidation in liposomes

Since antioxidants have been shown to play a major role in preventing some of the effects of aging and photoaging in skin, it is important to study this phenomenon in a controlled manner. This was accomplished by developing a simple and reliable in vitro technique to assay antioxidant efficacy. Inhibition of peroxidation by antioxidants was used as a measure of relative antioxidant potential. Liposomes, high in polyunsaturated fatty acids (PUFA), were dispersed in buffer and irradiated with ultraviolet (UV) light. Irradiated liposomes exhibited a significantly higher amount of hydroperoxides than liposomes containing antioxidants in a dose- and concentration-dependent manner. Lipid peroxidation was determined spectrophotometrically by an increase in thiobarbituric acid reacting substances. To further substantiate the production of lipid peroxides, gas chromatography was used to measure a decrease in PUFA substrate. In order of decreasing antioxidant effectiveness, the following results were found among lipophilic antioxidants: BHA greater than catechin greater than BHT greater than alpha-tocopherol greater than chlorogenic acid. Among hydrophilic antioxidants, ascorbic acid and dithiothreitol were effective while glutathione was ineffective. In addition, ascorbic acid was observed to act synergistically with alpha-tocopherol, which is in agreement with other published reports on the interaction of these two antioxidants. Although peroxyl radical scavengers seem to be at a selective advantage in this liposomal/UV system, these results demonstrate the validity of this technique as an assay for measuring an antioxidant's potential to inhibit UV-induced peroxidation.

Recycling and antioxidant activity of tocopherol homologs of differing hydrocarbon chain lengths in liver microsomes

Tocopherols (vitamin E) function as inhibitors of lipid peroxidation in biomembranes by donating a hydrogen atom to the chain propagating lipid radicals, thus giving rise to chromanoxyl radicals of the antioxidant. We have shown that alpha-tocopherol homologs differing in the lengths of their hydrocarbon side chains (alpha-Cn) manifest strikingly different antioxidant potencies in membranes. The antioxidant activity of tocopherol homologs during (Fe2+ + ascorbate)- or (Fe2+ + NADPH)-induced lipid peroxidation in rat liver microsomes increased in the order alpha-tocopherol (alpha-C16) less than alpha-C11 less than alpha-C6 less than alpha-C1. Chromanoxyl radicals generated from alpha-tocopherol and its more polar homologs by an enzymatic oxidation system (lipoxygenase + linolenic acid) can be recycled in rat liver microsomes by NAD-PH-dependent electron transport or by ascorbate. The efficiency of recycling increased in the same order: alpha-tocopherol (alpha-C16) less than alpha-C11 less than alpha-C6 less than alpha-C1. Thus the high efficiency of regeneration of short-chain homologs of vitamin E may account for their high antioxidant potency.

Antioxidant activity of 6-phosphatidyl-L-ascorbic acid

The antioxidant activity of 6-phosphatidyl-L-ascorbic acid was investigated in a homogeneous solution and a liposomal suspension. In an apolar solvent, one molecule of 6-phosphatidyl-L-ascorbic acid could trap one peroxyl radical. When 6-phosphatidyl-L-ascorbic acid was included in multilamellar liposomes of egg yolk phosphatidylcholine, it could retard more effectively the aqueous peroxyl-induced peroxidation of phosphatidylcholine than L-ascorbic acid.

Stopped-flow investigation of antioxidant activity of estrogens in solution

A kinetic study of the reaction between estrogens (female hormone) and substituted phenoxyl radical has been performed, as a model for the reactions of estrogens with lipid peroxyl radical in biological systems. The rates of reaction of estrogens (estrone 1, estradiol 2, 2-methoxyestrone 3, 3-methoxyestrone 4, and 2-hydroxyestrone 5) with substituted phenoxyl radical in benzene have been determined spectrophotometrically, using stopped-flow technique. The second-order rate constants, k2, obtained are 84 M-1.s-1 for 1, 138 M-1.s-1 for 2, 520 M-1.s-1 for 3, less than 10(-4) M-1.s-1 for 4, and 2.6 X 10(5) M-1.s-1 for 5 at 25.0 degrees C. 2-Hydroxyestrone 5 was found to be 2.9-times more active than alpha-tocopherol, which has the highest antioxidant activity among natural tocopherols. The order of magnitude of k2 value (1 less than 2 less than 3 less than alpha-Toc less than 5) is in agreement with that of in vitro tests of their antioxidant activities, as measured by the inhibition of lipid peroxidation. Further, similar measurements have been performed for the reaction between the above estrogens 1-5 and tocopheroxyl 6 in benzene solution. It was found that the estrogens having an OH group at the aromatic ring have an ability to regenerate the tocopheroxyl 6 to tocopherol. Especially, the 2-hydroxyestrone 5 showed about three orders of magnitude higher reactivity than ascorbic acid.

Structure-activity relationships of polymethoxyflavones and other flavonoids as inhibitors of non-enzymic lipid peroxidation

Polymethoxylated flavones and C-glycosyl derivatives isolated from medicinal plants besides other flavonoid compounds were studied for their influence on lipid peroxidation induced by FeSO4+ cysteine in rat liver microsomes. A number of hydroxyflavones (e.g. luteolin); C-glycosyl-flavones (e.g. orientin); methoxyflavones (e.g. gardenin D) and flavonols (e.g. datiscetin), as well as the flavanol leucocyanidol and the biflavone amentoflavone behaved as inhibitors of non-enzymic lipid peroxidation. Structure-activity relationships were established and it was observed that the structural features for active polyhydroxylated compounds were different from those of polymethoxylated flavones, antiperoxidative flavonoids possessing a high lipophilicity.

Comparative study of oxygen toxicity in human fibroblasts and endothelial cells

The resistance of human pulmonary fibroblasts (WI-38) and human umbilical vein endothelial cells to oxygen toxicity (1 atm O2) was compared. Endothelial cells were more sensitive than fibroblasts. They contained also less antioxidant enzymes except for SOD: respectively 132%, 96%, 70%, 59%, and 21% of the SOD, GSH peroxidase, GSH reductase, catalase, and G6PD content of fibroblasts. However, they contained 1.81-fold more GSH than fibroblasts. Their lower content of antioxidant enzymes can explain their higher sensitivity to oxygen. The efficiency of natural antioxidant molecules and enzymes in the protection of cells incubated 3 days under 1 atm O2 was studied. alpha-tocopherol added in the culture medium led to a significant protection, contrary to the result for ascorbic acid. Microinjection of catalase, SOD, and GSH peroxidase directly into the cells was also tested: the protection was concentration dependent for both types of cells but SOD did not protect the endothelial cells. Lower activities of the other enzymes were needed to achieve protection of the endothelial cells, compared to fibroblasts. Since endothelial cells were also shown to display lower antioxidant enzyme activities, it can be hypothesized that their content is optimized for survival in physiological conditions.

Stopped-flow kinetic study of the regeneration reaction of tocopheroxyl radical by reduced ubiquinone-10 in solution

Kinetic study of the reaction between tocopheroxyl (vitamin E radical) and ubiquinol-10 (reduced ubiquinone, n = 10) has been performed. The rates of reaction of ubiquinol with alpha-tocopheroxyl 1 and seven kinds of alkyl substituted tocopheroxyl radicals 2-8 in solution have been determined spectrophotometrically, using a stopped-flow technique. The result shows that the rate constants decrease as the total electron-donating capacity of the alkyl substituents on the aromatic ring of tocopheroxyls increases. For the tocopheroxyls with two alkyl substituents at ortho positions (C-5 and C-7), the second-order rate constants, k1, obtained vary in the order of 10(2), and decrease predominantly, as the size of two ortho-alkyl groups (methyl, ethyl, isopropyl and tert-butyl) in tocopheroxyl increases. On the other hand, the reaction between tocopheroxyl and ubiquinone-10 (oxidized ubiquinone) has not been observed. The result indicates that ubiquinol-10 regenerates tocopherol by donating a hydrogen atom of the 1-OH and/or 4-OH group to the tocopheroxyl radical. For instance, the k1 values obtained for alpha-tocopheroxyl are 3.74 x 10(5) M-1.s-1 and 2.15 x 10(5) M-1.s-1 in benzene and ethanol solution at 25 degrees C, respectively. The above reaction rates, k1, obtained were compared with those of vitamin C with alpha-tocopheroxyl reported by Packer et al. (k2 = 1.55 x 10(6) M-1.s-1) and Scarpa et al. (k2 = 2 x 10(5) M-1.s-1), which is well known as a usual regeneration reaction of tocopheroxyl in biomembrane systems. The result suggests that ubiquinol-10 also regenerates the tocopheroxyl to tocopherol and prevents lipid peroxidation in various tissues and mitochondria.

Free radical-mediated chain oxidation of low density lipoprotein and its synergistic inhibition by vitamin E and vitamin C

The oxidation of human low density lipoprotein (LDL) initiated by free radical initiator and its inhibition by vitamin E and water-soluble antioxidants have been studied. It was found that the kinetic chain length was considerably larger than 1, suggesting that LDL was oxidized by a free radical chain mechanism. Vitamin E acted as a lipophilic chain-breaking antioxidant. Water-soluble chain-breaking antioxidants such as ascorbic acid and uric acid suppressed the oxidation of LDL initiated by aqueous radicals but they could not scavenge lipophilic radicals within LDL to break the chain propagation. Ascorbic acid acted as a synergistic antioxidant in conjunction with vitamin E.

Physiologic levels of ascorbate inhibit the oxidative modification of low density lipoprotein

Oxidatively modified low density lipoprotein (LDL) could contribute to the atherosclerotic process by its cytotoxic effect, uptake by the scavenger receptor and influence on monocyte and macrophage motility. The aim of the present study was to examine the effect of physiologic levels of alpha-tocopherol and ascorbate on Cu2(+)-induced oxidative modification of LDL. Whereas alpha-tocopherol had an inhibitory effect on the oxidative modification of LDL only for 5 h, as evidenced by the electrophoretic mobility and lipid peroxide content, ascorbate inhibited the oxidative modification of LDL for both 5 and 24 h. By inhibiting the oxidative modification of LDL, ascorbate prevented the uptake and degradation of oxidatively modified LDL by the scavenger-receptor mechanism of cultured human monocyte derived macrophages. It thus appears that in this cell-free system (2.5 microM Cu2+), ascorbate is a more potent antioxidant than alpha-tocopherol. These findings indicate that ascorbate in physiologic concentrations should inhibit the oxidate modification of LDL in vivo.

Flavonoids inhibit the oxidative modification of low density lipoproteins by macrophages

Low density lipoproteins (LDL) can be oxidatively modified in vitro by macrophages and certain other cell types so that macrophages will take them up much faster. This process may be important in the formation of cholesterol-laden foam cells derived from macrophages in atherosclerotic lesions. In this study, we have shown that certain flavonoids, plant constituents found in the diet, are potent inhibitors of the modification of 125I-labelled LDL by macrophages, with IC50 values in the micromolar range (e.g. morin and fisetin 1 microM; quercetin and gossypetin 2 microM). The potencies of individual flavonoids in inhibiting LDL modification did not correlate with their previously determined potencies as inhibitors of 5-lipoxygenase and cyclo-oxygenase. The modification of LDL by macrophages exhibits a lag period of about 4-6 hr before enhanced uptake is detected. During this time, there is a rapid depletion in its content of alpha-tocopherol (an endogenous antioxidant found in lipoproteins) followed by a large increase in the level of hydroperoxides. The flavonoids conserved the alpha-tocopherol content of LDL and delayed the onset of detectable lipid peroxidation. Flavonoids also inhibited the cell-free oxidation of LDL mediated by CuSO4. These findings raise the possibility that flavonoids may protect LDL against oxidation in atherosclerotic lesions and may therefore be natural anti-atherosclerotic components of the diet, although this will depend to a large extent on their pharmacokinetics.

Biokinetics of dietary RRR-alpha-tocopherol in the male guinea pig at three dietary levels of vitamin C and two levels of vitamin E. Evidence that vitamin C does not "spare" vitamin E in vivo

The net rates of uptake of "new" and loss of "old" 2R,4'R,8'R-alpha-tocopherol (RRR-alpha-TOH, which is natural vitamin E) have been measured in the blood and in nine tissues of male guinea pigs over an eight week period by feeding diets containing deuterium-labelled alpha-tocopheryl acetate (d6-RRR-alpha-TOAc). There was an initial two week "lead-in" period during which 24 animals [the "high" vitamin E (HE) group] received diets containing 36 mg of unlabelled (d0) RRR-alpha-TOAc and 250 mg of ascorbic acid per kg diet, while another 24 animals [the "low" vitamin E (LE) group] received diets containing 5 mg d0-RRR-alpha-TOAc and 250 mg ascorbic acid per kg diet. The HE group was then divided into three equal subgroups, which were fed diets containing 36 mg d6-RRR-alpha-TOAc and 5000 mg [the "high" vitamin C (HEHC) subgroup], 250 mg [the "normal" vitamin C (HENC) subgroup] and 50 mg [the "low" vitamin C (HELC) subgroup] ascorbic acid per kg diet. One animal from each group was sacrificed each week and the blood and tissues were analyzed for d0- and d6-RRR-alpha-TOH by gas chromatography-mass spectrometry. The LE group was similarly divided into three equal subgroups with animals receiving diets containing 5 mg d6-RRR-alpha-TOAc and 5,000 mg (LEHC), 250 mg (LENC) and 50 mg (LELC) ascorbic acid per kg diet with a similar protocol being followed for sacrifice and analyses. In the HE group the total (d0(-) + d6-) RRR-alpha-TOH concentrations in blood and tissues remained essentially constant over the eight week experiment, whereas in the LE group the total RRR-alpha-TOH concentrations declined noticeably (except in the brain, an organ with a particularly slow turnover of vitamin E). There were no significant differences in the concentrations of "old" d0-RRR-alpha-TOH nor in the concentrations of "new" d6-RRR-alpha-TOH found in any tissue at a particular time between the HEHC, HENC and HELC subgroups, nor between the LEHC, LENC and LELC subgroups. We conclude that the long-postulated "sparing" action of vitamin C on vitamin E, which is well documented in vitro, is of negligible importance in vivo in guinea pigs that are not oxidatively stressed in comparison with the normal metabolic processes which consume vitamin E (e.g., by oxidizing it irreversibly) or eliminate it from the body.(ABSTRACT TRUNCATED AT 400 WORDS)

How to characterize a biological antioxidant

An antioxidant is a substance that, when present at low concentrations compared to those of an oxidizable substrate, significantly delays or prevents oxidation of that substrate. Many substances have been suggested to act as antioxidants in vivo, but few have been proved to do so. The present review addresses the criteria necessary to evaluate a proposed antioxidant activity. Simple methods for assessing the possibility of physiologically-feasible scavenging of important biological oxidants (superoxide, hydrogen peroxide, hydroxyl radical, hypochlorous acid, haem-associated ferryl species, radicals derived from activated phagocytes, and peroxyl radicals, both lipid-soluble and water-soluble) are presented, and the appropriate control experiments are described. Methods that may be used to gain evidence that a compound actually does function as an antioxidant in vivo are discussed. A review of the pro-oxidant and anti-oxidant properties of ascorbic acid that have been reported in the literature leads to the conclusion that this compound acts as an antioxidant in vivo under most circumstances.

Bile acids: antioxidants or enhancers of peroxidation depending on lipid concentration

Using three different assay systems, we have discovered a heretofore unrecognized antioxidant property of bile acids at physiological concentrations. Bile acids inhibit peroxidation of the polyunsaturated lipid, linoleic acid, and of the highly fluorescent protein phycoerythrin. In part, the antioxidant activity results from scavenging of peroxyl radicals by direct oxidation of the bile acids. The most abundant products of the reaction of cholate and chenodeoxycholate with peroxyl radicals were studied in detail and shown to be the keto derivatives formed by oxidation of the 7 alpha-hydroxyl groups. Paradoxically, at linoleate concentrations higher than 1-2 mM, glycocholate up to approximately 10-14 mM enhances lipid peroxidation and inhibits only at higher concentrations. These findings may prove important in understanding the etiology of certain disease states of the biliary tract and intestine where lipid peroxidation may be involved and in providing a rationale for the positive epidemiological correlation between high lipid intake and higher fecal bile acid output and colon cancer.

Importance of a threshold for error accumulation in cell degenerative processes. I. Modulation of the threshold in a model of free radical-induced cell degeneration

Antioxidant enzymes (catalase, superoxide dismutase and glutathione peroxidase) have been injected into human fibroblasts exposed to 2 atm O2 in order to test if the threshold of oxidative damage versus antioxidant defenses could be modulated and if the damage remains reversible beyond the threshold. Cell damage was estimated by thymidine incorporation and cell survival curves. The proportion of dividing cells, measured by thymidine incorporation, rapidly decreased after O2 incubation: no cells could divide after 15 h of hyperoxia. However, cells incubated for a short time and injected with a high concentration of any of the three enzymes divided like non-oxygen-incubated cells: the enzymes could protect the cells against their loss of division potential. However, when cells were incubated for a longer period and/or when the injected enzyme concentration was lower, cells were either less or not protected and could no longer divide. These results suggest the presence of a threshold for the oxidative damage which cannot be totally repaired and which impairs the cell division; this threshold can, however, be modulated by supplementation of antioxidant enzymes, glutathione peroxidase being the most efficient.

Decreases in tissue levels of ubiquinol-9 and -10, ascorbate and alpha-tocopherol following spinal cord impact trauma in rats

Generation of free radicals and subsequent lipid peroxidation have been proposed to contribute to delayed tissue damage following traumatic spinal cord injury (SCI). Ubiquinols (reduced coenzyme Q), ascorbate (vitamin C), and alpha-tocopherol (vitamin E) are endogenous antioxidants; decreases in tissue levels of these compounds may, therefore, reflect ongoing oxidative reactions. In the present studies, alterations in tissue levels of ubiquinol-9 and -10, ascorbate, and alpha-tocopherol were examined after SCI of varying severity in the rat. Levels of alpha-tocopherol did not change significantly after injury. Ascorbate and ubiquinol levels were decreased after trauma. Changes in tissue levels of ubiquinol, but not ascorbate reflected the degree of trauma. Thus, ubiquinol levels may provide a useful marker of the oxidative component of the secondary injury response.