Peroxidation and phospholipase A2 hydrolytic susceptibility of liposomes consisting of mixed species of phosphatidylcholine and phosphatidylethanolamine.

The relationship between lipid peroxidation and phospholipase A2 (PLA2) hydrolytic activity was studied using unilamellar vesicles (liposomes) as model membranes. Hydrolytic specificity was examined using vesicles prepared with pure bovine heart phosphatidylcholine (PC), bovine heart phosphatidylethanolamine (PE), or mixtures of these phospholipids, using two preparative procedures, i.e., sonication or extrusion. Lipid peroxidation was induced by incubating vesicles with cumene hydroperoxide and hematin at 37 degrees C. Determinations of the extent of peroxidation by means of diene conjugate content derived from second derivative spectra or by polarographic measurement of oxygen consumption rates provided a basis for comparing the extent of peroxidation of each phospholipid species to their subsequent hydrolysis by PLA2 (from Crotalus adamanteus). The extent of hydrolysis was determined through the release of arachidonic acid from either PC or PE. The PE distribution among the outer vs. inner leaflet of the membrane bilayer was nearly equal in sonicated vesicles, whereas most of the phospholipid was incorporated into the inner leaflet in extruded vesicles. The proportion of PE found in the inner leaflet progressively increased as the ratio of PE to PC increased in both sonicated and extruded vesicle preparations. Lipid peroxidation had no effect on PE distribution under the conditions examined. There was a clear preference for PC peroxidation for all vesicle compositions tested and PC was preferentially hydrolyzed by PLA2. This effect is proposed to result from a perturbation of membrane structure following peroxidation with assimilation of PC into PLA2-susceptible domains whereas PE peroxidation and hydrolysis is less affected in mixed PC/PE vesicles. Lipid peroxidation imposes an additional hydrolytic susceptibility over the effects exerted through the mixing of these phospholipids which is based on structural changes rather than formation of specific substrates for PLA2.

Perturbation in liver mitochondrial Ca2+ homeostasis in experimental iron overload: a possible factor in cell injury.

The functional state of isolated mitochondria and specifically the integrity of the inner membrane, were investigated in the liver of rats made siderotic by dietary supplementation with carbonyl iron. The concentration of iron in the hepatic tissue increased progressively up to nearly 40 days and reached a steady-state level. When the iron content reached a threshold value (higher than 90 nmol/mg protein) the occurrence of in vivo lipid peroxidation in the mitochondrial membrane was detected. This process did not result in gross alterations in the mitochondrial membrane, as indicated by electron microscopy, phosphorylative capability and membrane potential measurements. On the contrary, the induction of lipoperoxidative reaction appeared to be associated with the activation of Ca2+ release from mitochondria. This was shown to occur as a consequence of rather subtle modifications in the inner membrane structure via a specific efflux route, which appeared to be linked to the oxidation level of mitochondrial pyridine nucleotides. The induction of this Ca2+ release from iron-treated mitochondria resulted in enhancement of Ca2+ cycling, a process which dissipates energy to reaccumulate into mitochondria the released Ca2+. The perturbation in mitochondrial Ca2+ homeostasis reported here may be a factor in the onset of cell damage in this experimental model of hepatic iron overload.

Non-random peroxidation of different classes of membrane phospholipids in live cells detected by metabolically integrated cis-parinaric acid.

Quantitative assays of lipid peroxidation in intact, living cells are essential for evaluating oxidative damage from various sources and for testing the efficacy of antioxidant interventions. We report a novel method based on the use of cis-parinaric acid (PnA) as a reporter molecule for membrane lipid peroxidation in intact mammalian cells. Using four different cell lines (human leukemia HL-60, K562 and K/VP.5 cells, and Chinese hamster ovary (CHO) fibroblasts), we developed a technique to metabolically integrate PnA into all major classes of membrane phospholipids, i.e., phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol and cardiolipin, that can be quantified by HPLC with fluorescence detection. Integrated PnA constituted less than 1% of lipid fatty acid residues, suggesting that membrane structure and characteristics were not significantly altered. Low concentrations (20-40 microM) of tert-butyl hydroperoxide (t-BuOOH) caused selective oxidation of PnA residues in phosphatidylserine and phosphatidylethanolamine of K562 cells and K/VP.5 cells while cell viability was unaffected. At higher t-BuOOH concentrations (exceeding 100 microM), however, a progressive, random oxidation of all major phospholipid classes occurred and was accompanied by significant cell death. In HL-60 cells, phosphatidylethanolamine, phosphatidylserine and cardiolipin were sensitive to low concentrations of t-BuOOH, while phosphatidylcholine and phosphatidylinositol were not affected. Phosphatidylinositol was the only phospholipid that responded to the low concentrations of t-BuOOH in CHO cells. At high t-BuOOH concentrations, again, all phospholipid classes underwent extensive oxidation. All phospholipids were nearly equally affected by peroxidation induced by a initiator of peroxyl radicals, 2,2'-azobis-(2,4-dimethylvaleronitrile) AMVN), in K562 cells. In gamma-irradiated (4-128 Gy) CHO cells, phosphatidylserine was the most affected phospholipid class (34% peroxidation) followed by phosphatidylinositol (24% peroxidation) while the other three phospholipid classes were apparently unaffected. Since loss of PnA fluorescence is a direct result of irreparable oxidative loss of its conjugated double bond system, the method described allows for selective and sensitive monitoring of oxidative stress in live cells without interference from cell repair mechanisms.

Conjugated dienes detected in tissue lipid extracts by second derivative spectrophotometry.

By studying lipid peroxidation induced by tetrachloromethane in rat liver microsomal PUFA, it has recently been shown that the primary products formed are conjugated diene hydroperoxides having either cis, trans (c,t) or trans,trans (t,t) stereochemistry. Both c,t and t,t hydroperoxidienes present distinct absorbances at 242 nm and 233 nm, respectively. The reaction is kinetically controlled in relation to the total H-atom donating ability of the cell environment. These results have been confirmed in vivo and in vitro experiments performed under different experimental conditions. The need for a precise and objective method to detect conjugated diene signals, the inherent difficulties with current techniques, and the availability of new spectrophotometric techniques have led us to devise a new method based on the second derivatization of the spectrum.

Inhibition of peroxynitrite dependent DNA base modification and tyrosine nitration by the extra virgin olive oil-derived antioxidant hydroxytyrosol.

Hydroxytyrosol is one of the o-diphenolic compounds in extra virgin olive oil and has been suggested to be a potent antioxidant. The superoxide radical (O2*-) and nitric oxide (NO*) can react very rapidly to form peroxynitrite (ONOO ), a reactive tissue damaging species thought to be involved in the pathology of several chronic diseases. Hydroxytyrosol was highly protective against the peroxynitrite-dependent nitration of tyrosine and DNA damage by peroxynitrite in vitro. Given that extra virgin olive oil is consumed daily by many humans, hydroxytyrosol derived from this diet could conceivably provide a defense against damage by oxidants in vivo. The biological activity of hydroxytyrosol in vivo will depend on its intake, uptake and access to cellular compartments.

MPTP fails to induce lipid peroxidation in vivo.

It has been speculated that the conversion of MPTP to MPP+ destroys dopaminergic neurons by promoting the generation of hydroxyl radicals and causes lipid peroxidation. The results obtained in the present work indicate that the primary products of lipid peroxidation are not detectable in MPTP treated animals and thus other mechanisms besides lipid peroxidation should be considered to explain the cytotoxicity of this neurotoxin.

Methyl mercury enhances [3H]diazepam binding in different areas of the rat brain.

Three days after the acute oral administration of methyl mercury (MeHg), a 27-60% increase in the total number of binding sites for [3H]diazepam was seen in the retina and different areas of the rat brain, with no change, except in the retina, in the apparent dissociation constant for its ligand. In contrast, MeHg failed to change [3H]spiroperidol and [3H]GABA binding in the same areas. Moreover, MeHg decreased cyclic GMP content in the cerebellar cortex. The various possible mechanisms involved in the action of MeHg on benzodiazepine binding are discussed.

A novel approach to study linoleic acid autoxidation: importance of simultaneous detection of the substrate and its derivative oxidation products.

In this paper we have proposed a novel approach for studying the reaction of lipid oxidation by using the simplest chemical system available. Neat linoleic acid was incubated for 24 hours at 37 degrees C in the air. The course of lipid oxidation was followed by measuring simultaneously by HPLC with a diode array detector 1) linoleic acid decrease, 2) the products formed by radical attack, namely four hydroperoxy-octadeca-dienoic acid (HPODE) isomers, two c,t (c,t) and two trans,trans (t,t). 3) the byproducts formed by HPODE degradations, the four oxo-octadeca-dienoic acid (oxo-ODE) isomers. In HPODEs the presence of conjugated diene chromophore was confirmed by second derivative spectrophotometry. c,t HPODEs were also identified for their positional isomerism, while for t,t molecules the lack of suitable reference compound makes unfeasible the identification of their positional isomerism. As in the case of the latter two c,t and two t,t oxo-ODE isomers were characterized. This simple system appears to be useful for studying the activity exherted by lipophilic molecules that, like alpha-tocopherol, may act as antioxidants and/or as hydrogen atom donating molecules. The presence of alpha-tocopherol in different concentration for 24 hours in the reaction environment, shifts the reaction of linoleic acid autoxidation towards different byproduct formations. From the results obtained it is evident that alpha-tocopherol acts as hydrogen atom donor at all concentration tested, shifting the reaction toward a prevalent formation of c,t isomer of both HPODEs and oxo-ODEs. At concentration lower than 40 nmoles, when the ratio between alpha-tocopherol and linoleic acid was 1:100, the reaction of autoxidation is strongly inhibited, while at higher concentration alpha-tocopherol acted as a prooxidant. In these experimental conditions, alpha-tocopherylquinone was spectrophotometrically identified as the predominant oxidation product of alpha-tocopherol.

On the hepatotoxicity of 1,1,2,2-tetrachloroethane.

Intoxication of male and female mice with a single dose (300 or 600 mg/kg) of 1,1,2,2-tetrachloroethane (TTCE) resulted in significant decreases in cytochrome P-450 (to 58-73% of the control) and NADPH-cytochrome (P-450) c-reductase (to 29-35% of the control) in hepatic microsomes. This was accompanied by an alteration of mixed function monooxygenases stemming from the marked reduction (to 20-64% of the control) of several oxidative activities to selected substrates towards different P-450 isozymes (classes IA1, IA2, IIB1, IIE1 and IIIA). As phase II markers, epoxide hydrolase (approximately 35% loss), UDP-glucuronosyl transferase (approximately 42% loss) and to a lesser extent glutathione S-transferase (approximately 17% loss) were all affected. Also, the activity of delta-aminolevulinic (ALA) synthetase was decreased (approximately 57% of the control). On the contrary, heme oxygenase activity was increased (up to 35%) at the maximal dose tested. The decrease of P-450-function may be explained in terms of an alteration in the rate of heme biosynthesis and degradation, provoking a loss of heme content (approximately 33%) as well as of the direct inactivation of both P-450 and reductase. Because of increasing evidence on the involvement of free radical intermediates in the case of toxicity of haloalkanes, electron spin resonance spectroscopy (ESR) spin-trapping in vivo techniques were used to characterize the possible free radical species involved in the observed liver damage. The results obtained with the spin-trap N-benzylidene-2-methylpropylamine N-oxide (phenyl t-butylnitrone, PBN) provide evidence for the formation and trapping of the CHCl2CHCl free radicals. The detection of conjugated diene signals by means of second-derivative spectrophotometry, have enabled us to show that in vivo lipid peroxidation may be one of the main mechanisms responsible for TTCE hepatotoxicity.

An improved and simple method for determining diene conjugation in autoxidized polyunsaturated fatty acids.

The autoxidation of linolenic acid was produced at room temperature in a rotating apparatus by passing a stream of dry air over a thin layer of polyunsaturated fatty acid (PUFA). During these experiments, volatile thiobarbituric acid positive product(s), were recovered. The extent of fatty acid autoxidation was measured either (a) by the production of malonaldehyde (MDA) or (b) by the height of the peak at 233 nm of the second derivative spectrum, corresponding to the hidden maximum at 235 nm in the normal absorbance spectra of autoxidized PUFA. The identification of the conjugated double bond structure, arising from the autoxidation of linolenic acid was further confirmed by 1H-NMR spectrometric determinations.

Lipid peroxidation and molecular damage to polyunsaturated fatty acids in rat liver. Recognition of two classes of hydroperoxides formed under conditions in vivo.

The diene conjugates formed during the autoxidation of microsomal lipid extracts, and in endoplasmic reticulum in vivo after exposing rats to CCl4 have been examined by second derivative absorption spectrophotometry. Within a few minutes after administering CCl4 to a rat there is a pronounced signal in microsomal lipid extracts that is ascribed to the cis-trans diene conjugates of microsomal polyunsaturated fatty acids. Somewhat later a second signal becomes evident that is ascribed to trans-trans isomers. The appearance of the trans-trans isomer is very strongly suppressed by prior administration of vitamin E to the rat. It is concluded that the relative contents of cis-trans and trans-trans dienes in lipid extracts of tissue reflect the tissue contents of hydrogen donors as already established for model experiments with polyunsaturated fatty acids in vitro.

NMR spin-lattice relaxation times of intracellular Na-23 on rat livers and related lipid peroxidation following CCl4 intoxication.

Liver tissues were isolated from rats acutely intoxicated with carbon tetrachloride, and Na-23 NMR signals were analyzed to investigate the T1 relaxation times of intracellular sodium ions under pathological conditions in presence of the paramagnetic shift reagent (dysprosium tripolyphosphate). We studied the significant increase of T1 found in CCl4 treated rats with respect to controls, which was elsewhere demonstrated as being independent of cell necrosis. Evidence is given that neither fat accumulation nor proliferative processes affect the observed T1 lengthening. When T1 relaxation times were measured in the liver of vitamin E treated rats subsequently intoxicated with carbon tetrachloride, a significative shortening of T1 with respect to CCl4-intoxicated rats was observed. These results were discussed in terms of the antioxidant action exerted by vitamin E, taking into account that peroxidation of microsomal lipids is the key factor in the process of carbon tetrachloride induced liver injury. Furthermore, the observed T1 changes were discussed in terms of the interactions of Na+ with cell membranes and/or the occurrence of viscosity changes.

Biochemical changes in the rat cerebellar cortex elicited by chronic treatment with methyl mercury.

Long-term (20 days) treatment with methyl mercury (MeHg) increases the total number of benzodiazepine binding sites and decreases essentially the content of cyclic GMP in the cerebellar cortex. In contrast, this treatment fails to modify the content of GABA and cyclic AMP, GAD activity and GABA binding sites in the same brain area. The changes in cyclic GMP and benzodiazepine binding sites in the cerebellar cortex are discussed in relation to the motor disturbances associated with MeHg intoxication.

Recognition and quantitation of cis-vaccenic and eicosenoic fatty acids in olive oils by 13C nuclear magnetic resonance spectroscopy.

The presence of 11-cis monoenoic fatty acids was detected in olive oil samples by means of 13C nuclear magnetic resonance spectroscopy, and the positional isomery on the glycerol backbone was derived. The 11-cis vaccenic and eicosenoic fatty acid resonances were recognized and the amounts of the fatty acids quantified. For comparison purposes, a quantitative analysis was also made by gas chromatography.

Peroxidase-catalyzed oxidation of beta-carotene in HL-60 cells and in model systems: involvement of phenoxyl radicals.

Recent studies provide extensive evidence for the importance of carotenoids in protecting against oxidative stress associated with a number of diseases. In particular, reactions of carotenoids with phenoxyl radicals generated by peroxidase-catalyzed one-electron metabolism of phenolic compounds may represent an important antioxidant function of carotenoids. To further our understanding of the antioxidant mechanisms of carotenoids, we used in the present work two different phenolic compounds, phenol and a polar homologue of vitamin E (2,2,5,7,8-pentamethyl-6-hydroxychromane, PMC), as representatives of two different types of phenols to study reactions of their respective phenoxyl radicals with carotenoids in cells and in model systems. We found that phenoxyl radicals of PMC did not oxidize beta-carotene in either HL-60 cells or in model systems with horseradish peroxidase (HRP)/H2O2. In contrast, the phenoxyl radicals generated from phenol (by native myeloperoxidase in HL-60 cells or HRP/H2O2 in model systems) effectively oxidized beta-carotene and other carotenoids (canthaxanthin, lutein, lycopene). One-electron reduction of the phenoxyl radical by ascorbate (assayed by electron spin resonance-detectable formation of semidehydroascorbyl radicals) prevented HRP/H2O2-induced oxidation of beta-carotene. PMC, but not phenol, protected beta-carotene against oxidation induced by a lipid-soluble azo-initiator of peroxyl radicals. No adducts of peroxidase/phenol/H2O2-induced beta-carotene oxidation intermediates with phenol were detected by high-performance liquid chromatography-mass spectrometry analysis of the reaction mixture. Since carotenoids are essential constituents of the antioxidant defenses in cells and biological fluids, their depletion through the reaction with phenoxyl radicals formed from endogenous, nutritional and environmental phenolics, as well as phenolic drugs, may be an important factor in the development of oxidative stress.

Rise of hepatic glutathione concentration induced in rats by chronic lead nitrate treatment. Its role in aflatoxin B1 intoxication.

Liver concentration of the reduced form of glutathione is increased by chronic lead nitrate administration. Hepatic glutathione content is lowered by acute aflatoxin B1 intoxication while in lead-pretreated rats this decrement is not evident. The administration of lead nitrate which increases liver glutathione also decreases acute aflatoxin B1 toxicity. This provides additional evidence that, in the liver, glutathione concentration plays a role in controlling acute aflatoxin B1 toxicity.

Enhanced interfacial catalysis and hydrolytic specificity of phospholipase A2 toward peroxidized phosphatidylcholine vesicles.

The hydrolytic action of phospholipase A2 was examined with unilamellar vesicles composed of soybean phosphatidylcholine in terms of the calcium dependency of the enzyme and substrate specificity following lipid peroxidation. Experiments were performed under conditions where enzyme:substrate ratios were low, specifically in the range of one to five enzyme molecules for every 10 vesicle particles. Accordingly, low hydrolytic activities were found where less than 15% of the phospholipids were hydrolyzed under the various conditions of time, enzyme:substrate ratios, calcium concentrations, and extent of peroxidation utilized. Vesicle peroxidation increased the Ca2+ binding potential to a degree comparable to addition of the anionic phospholipid, dioleoylphosphatidic acid (DOPA). A remarkable similarity was found between the binding profiles for Ca2+ and phospholipase A2 activity; however, enzyme activity toward oxidized vesicles was beyond the increases observed for Ca2+ binding. Under conditions where approximately 5% of the phospholipids were peroxidized the effective Ca2+ concentration required for half-maximal activity was less than one-half that required for unoxidized vesicles. Peroxidation of vesicle phospholipids markedly increased the rate and extent of hydrolysis, even in the presence of DOPA or deoxycholate. Deoxycholate is known to induce vesicle fusion such that a larger proportion of enzyme is associated with a fewer number of enlarged vesicles. Using a dual isotope technique to measure hydrolysis of oxidized vs unoxidized phospholipids and covesicle preparations to study enzyme binding and activity, a significantly greater apparent intervesicle exchange of enzyme was found after peroxidation of vesicles with more than a twofold hydrolytic specificity toward the oxidized phospholipids. We postulate that a combination of structural and Ca2+ binding affinity changes are produced in membranes following lipid peroxidation which evoke an additive effect on PLA2 activity. Although oxidized phospholipids may serve as activators of phospholipase A2 by presenting the interface in a form where Ca2+ and enzyme binding and/or specific activity are increased, an additional and important factor appears to involve membrane fusion or vesicle-vesicle interactions. This process facilitates enzyme activity through the replenishment of substrates wherein the otherwise limited interaction of enzyme and substrate is overcome by more rapid or extensive vesicle fusion which increases access to the phospholipids available in the preparation.