A search for oxygen-centered free radicals in the lipoxygenase/linoleic acid system

How hydrogen peroxide is metabolized by oxidized cytochrome c oxidase

In the absence of external electron donors, oxidized bovine cytochrome c oxidase (CcO) exhibits the ability to decompose excess H2O2. Depending on the concentration of peroxide, two mechanisms of degradation were identified. At submillimolar peroxide concentrations, decomposition proceeds with virtually no production of superoxide and oxygen. In contrast, in the millimolar H2O2 concentration range, CcO generates superoxide from peroxide. At submillimolar concentrations, the decomposition of H2O2 occurs at least at two sites. One is the catalytic heme a3-CuB center where H2O2 is reduced to water. During the interaction of the enzyme with H2O2, this center cycles back to oxidized CcO via the intermediate presence of two oxoferryl states. We show that at pH 8.0 two molecules of H2O2 react with the catalytic center accomplishing one cycle. In addition, the reactions at the heme a3-CuB center generate the surface-exposed lipid-based radical(s) that participates in the decomposition of peroxide. It is also found that the irreversible decline of the catalytic activity of the enzyme treated with submillimolar H2O2 concentrations results specifically from the decrease in the rate of electron transfer from heme a to the heme a3-CuB center during the reductive phase of the catalytic cycle. The rates of electron transfer from ferrocytochrome c to heme a and the kinetics of the oxidation of the fully reduced CcO with O2 were not affected in the peroxide-modified CcO.

Edaravone inhibits lipid peroxidation in neonatal hypoxic-ischemic rats: An in vivo microdialysis study

The occurrence of hypoxia-ischemia (HI) during early fetal or neonatal stages of an individual leads to the damaging of immature neurons resulting in behavioral and psychological dysfunctions. Free radical-mediated lipid peroxidation is the main cause of neurotoxicity including neonatal brain damage. Edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one) is a novel anti-oxidant agent and the drug of choice in the treatment of acute ischemic brain disorders in adult patient. The purpose of this study is to determine the direct effects of edaravone in inhibiting the lipid peroxidation production in the neonatal rat brains during hypoxic-ischemic insult by electron paramagnetic resonance (EPR) spectoroscopy and in vivo brain microdialysis. Seven-day-old Wistar rats were subjected to left common carotid artery ligation and a probe was inserted in the rat hippocampus. Edaravone (5, 50, or 100 microM) or saline was perfused with a spin trap agent (alpha-(4-pyridyl-N-oxide)-N-tert-butylnitrone; POBN) before, during and after hypoxia (1h of 8% O2 exposure) and then analyzed by EPR. Edaravone (100 microM) did not show any EPR evidence of POBN adduct formation during and after hypoxic-ischemic insult. However, the EPR signal increased, but not significantly during the hypoxic period in the hypoxic and edaravone 50 microM-treated groups compared to control. Edaravone at 5 microM significantly increased the EPR signals compared to control. This study shows that edaravone directly and dose-dependently inhibited the formation of lipid free radicals produced during hypoxic-ischemic insult in the neonatal rat brain. These results suggest that edaravone is able to attenuate neuronal damage in the rat neonatal brain by inhibiting the formation of lipid radicals.

Generation of lipid radicals in the hippocampus of neonatal rats after acute hypoxic-ischemic brain damage

Free radical-mediated lipid peroxidation has been strongly suggested to be the main cause of neuronal toxicity in the rat brain, including neonatal brain damage. The primary objective of this experiment was to see if the generation of free radicals occurred in the acute phase of ischemic-hypoxic insult in neonatal rats, by electron paramagnetic resonance (EPR) spectroscopy and in vivo brain microdialysis. A spin trap agent, alpha-(4-pyridyl-N-oxide)-N-tert-butylnitrone was perfused through a probe in the hippocampus before and after hypoxia and then an analysis was performed by EPR. From the EPR analysis of spin adduct in the dialysates, we obtained the EPR spectrum of six line spectra for which the hyperfine coupling constants corresponded to those of the EPR signal from the lipoxygenase/linoleic acid (LPX/LA), a lipid radical generating system, increased transiently just after hypoxia. The results of our in vivo study show the lipid peroxidation of the neuronal membrane to progress during neonatal ischemic-hypoxic insult. We hypothesize that an increased formation of lipid radicals may participate in the cascade of reactions leading to neuronal damage in the hippocampus following ischemic-hypoxic insult in neonatal rats.

EPR evidence of hydroxyl radical generation as an initiator of lipid peroxidation in amyloid β-protein-stimulated PC12 cells

Recent data from several groups suggest that the primary mechanism of amyloid beta-protein (Abeta) neurotoxicity may be mediated by free radicals. To evaluate this hypothesis, our aim is to make the mechanism of Abeta neurotoxicity clear, especially in the formation of free radicals. In this study, rat pheochromocytoma (PC12) cells were exposed to Abeta25-35 and confirmed free radical generations using two kinds of spin trap agents, 5,5-dimethyl-1-pyrroline-N-oxide; DMPO and alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone; POBN. DMPO spin adduct revealed that hydroxyl radical (OH), while POBN spin adduct identified a lipid radical (L) as electron paramagnetic resonance (EPR) evidence of lipid peroxidation in the process of cell damage by Abeta25-35 exposure. An Abeta cytotoxicity assay also was performed by using WST-8 reduction system and histochemical analysis. These analyses showed cell damage induced by Abeta. This study provides EPR evidence that Abeta neurotoxicity is derived from hydrogen abstraction from polyunsaturated lipid acid by hydroxyl radical as a cause of lipid peroxidation.

Nitric oxide and hydroxyl radicals initiate lipid peroxidation by NMDA receptor activation

In this experiment, we used direct electron paramagnetic resonance (EPR) spectra to measure lipid peroxidation by hydroxyl radical (.OH), nitric oxide (.NO) and lipid radical (.L). NMDA-receptor associated lipid peroxidation is thought to act through .OH in induction of neurotoxicity. The origin of .OH generation was found to arise mainly from peroxynitrite anion produced from O(2)(-) and .NO rather than from Fenton's reaction. This study verified that .OH generation from interactive reactions between .NO and O(2)(-) initiates NMDA-induced lipid peroxidation of PC12 cells.

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

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

The roles played by crucial free radicals like lipid free radicals, nitric oxide, and enzymes NOS and NADPH in CCl(4)-induced acute liver injury of mice

Mice were administered a single dose of carbon tetrachloride (CCl(4)) to induce acute liver injury. We found that lactate dehydrogenase (LDH) and glutamic pyruvic transaminase (GPT) levels in serum, as well as the level of thiobarbituric acid reaction substances (TBARS) in liver homogenate increased significantly in a manner both dose dependent and time dependent after CCl(4) administration. Such results suggest that the liver is susceptible to CCl(4) treatment and that lipid peroxidation is associated with CCl(4)-induced liver injury. The spin-trapping electron paramagnetic resonance (EPR) method was used to detect nitric oxide (NO) level in liver. The chemiluminescence method was also employed to measure the NO(2)(-)/NO(3)(-) concentration in serum. The NO levels in liver tissues and NO(2)(-)/NO(3)(-) concentration in serum were found to decrease significantly both in a dose-dependent manner and in time course after CCl(4) treatment. The nitric oxide synthase (NOS) II activity in the liver, in contrast, was found to increase significantly. Our study suggests that not only should the expression of NOS be analyzed but NO organ and blood concentration must be measured in the study of diseases involving nitric oxide. L-arginine treatment had no significant effect on the liver function of CCl(4)-treated mice. It was found that NO donor sodium nitroprusside (SNP; 50 or 100 microg/kg) treatment resulted in decreases of LDH, GPT, and TBARS levels, leading to a protective effect on CCl(4)-treated mice. On the other hand, N(G)-nitro-L-arginine methyl ester (L-NAME, 100 or 300 mg/kg) treatment caused more severe liver damage. Moreover, we have found in an in vitro EPR study that SNP could scavenge lipid peroxyl radical LOO&z.rad;. The above results together suggest that NO may protect CCl(4)-induced liver injury through scavenging lipid radical, inhibiting the lipid peroxidation chain reaction. On the basis of our analysis, we put forth two explanations for the stated discrepancy between NOS II and NO production: (i) NO was used up gradually in terminating lipid peroxidation and (ii) NADPH was depleted (on the basis of correlation evidence only).

Beta-carotene 15,15'-dioxygenase activity is responsive to copper and iron concentrations in rat small intestine

OBJECTIVE

Previous in vitro studies have suggested that beta-carotene 15,15'-dioxygenase is an iron-dependent enzyme. However, in vivo, it is difficult to alter iron tissue concentration by varying dietary iron because of homeostatic control. On the other hand, an interaction between iron and copper has been shown, i.e., copper-deficiency results in an increase of iron in rat liver. Therefore, we hypothesized that intestinal iron concentration could be increased by copper-deficiency. Our objective was to examine the effects of iron as affected by dietary copper on beta-carotene 15,15'-dioxygenase activity in the small intestine.

METHODS

Weanling male Sprague-Dawley rats (40 to 45g) were divided into four dietary groups: two copper-adequate groups (6.0 microg Cu/g diet) and two copper-deficient groups (0.6 microg Cu/g) combined with either normal iron (44 microg Fe/g) or high iron (87 microg Fe/g). Iron and copper concentrations were determined by atomic absorption spectrophotometry and the dioxygenase activity by reverse phase HPLC.

RESULTS

Intestinal copper concentration was significantly reduced (40%) by the consumption of the copper-deficient diets, but intestinal iron was not changed by doubling dietary iron in rats fed either copper-adequate or copper-deficient diets. However, as hypothesized, the two copper-deficient groups exhibited higher intestinal iron concentration (> or =137%, p<0.001) than the copper-adequate controls. In addition, intestinal beta-carotene 15,15'-dioxygenase activity was increased by 27% and 106%, respectively, for copper-deficient rats fed either normal or high iron diets, compared to the respective copper-adequate controls (p<0.01). The dioxygenase activity was not significantly affected by dietary iron in either copper-adequate or copper-deficient groups. Finally, the enzyme activity was positively correlated (r=0.67, p<0.0001) with iron concentration and negatively correlated (r=-0.49, p<0.01) with copper concentration in small intestine.

CONCLUSIONS

Intestinal beta-carotene 15,15'-dioxygenase may be an iron-dependent enzyme sensitive to copper status in vivo.

The role of gut-derived bacterial toxins and free radicals in alcohol-induced liver injury

Previous research from this laboratory using a continuous enteral ethanol (EtOH) administration model demonstrated that Kupffer cells are pivotal in the development of EtOH-induced liver injury. When Kupffer cells were destroyed using gadolinium chloride (GdCl₃ ) or the gut was sterilized with polymyxin B and neomycin, early inflammation due to EtOH was blocked. Anti-tumour necrosis factor (TNF)-α antibody markedly decreased EtOH-induced liver injury and increased TNF-mRNA. These findings led to the hypothesis that EtOH-induced liver injury involves increases in circulating endotoxin leading to activation of Kupffer cells. Pimonidazole, a nitro-imidazole marker, was used to detect hypoxia in downstream pericentral regions of the lobule. Following one large dose of EtOH or chronic enteral EtOH for 1 month, pimonidazole binding was increased significantly in pericentral regions of the liver lobule, which was diminished with GdCl₃ . Enteral EtOH increased free radical generation detected with electron spin resonance (ESR). These radical species had coupling constants matching α-hydroxyethyl radical and were shown conclusively to arise from EtOH based on a doubling of the ESR lines when ¹³ C-EtOH was given. α-Hydroxyethyl radical production was also blocked by the destruction of Kupffer cells with GdCl₃ . It is known that females develop more severe EtOH-induced liver injury more rapidly and with less EtOH than males. Female rats on the enteral protocol exhibited more rapid injury and more widespread fatty changes over a larger portion of the liver lobule than males. Plasma endotoxin, ICAM-1, free radical adducts, infiltrating neutrophils and transcription factor NFκB were approximately two-fold greater in livers from females than males after 4 weeks of enteral EtOH treatment. Furthermore, oestrogen treatment increased the sensitivity of Kupffer cells to endotoxin. These data are consistent with the hypothesis that Kupffer cells participate in important gender differences in liver injury caused by ethanol.

EPR kinetic studies of the LDL oxidation process driven by free radicals

Oxidation is widely assumed to play a causal role in the pathogenesis of atherosclerosis. Oxidative modification of low density lipoprotein (LDL) can be followed by analyzing the lag phase of the conjugated diene formation at 234 nm in LDL exposed to Cu2+. This procedure is restricted to isolated LDL fractions. To make this assay applicable to different biological systems, the present paper introduces a method to determine the time course of lipid peroxidation by measuring the EPR signal intensity and thereby the concentration of the radicals formed. Stable radical spin adducts were generated using the spin trap PBN (N-tert.-butyl-alpha-phenylnitrone) and were detected by EPR spectroscopy. Comparing the specific formation of radicals and the generation of conjugated dienes as measured by UV absorbance revealed analogous lag, propagation and decomposition phases.

In vivo analysis of hydrogen peroxide and lipid radicals in the striatum of rats under long-term administration of a neuroleptic

It has been hypothesized that free radicals play a causative role in tardive dyskinesia, which is an inveterate movement disorder caused by chronic administration of neuroleptics. To verify this hypothesis, rats were reared while being regularly treated with water containing a neuroleptic, haloperidol (HPD), for 1 year (HPD group). The changes in the striatal hydrogen peroxide content of the rats in the HPD and control groups were measured by using a Pt-disk microelectrode while the animals were in a freely moving state following intraperitoneal administration of HPD (HPD challenge). We also performed electron spin resonance (ESR) detection of lipid radicals in the striatum before the HPD challenge. HPD challenge led to significant elevation of the intrastriatal hydrogen peroxide in all animals, but the elevation in the HPD group was smaller than that in the control group. However, in the HPD group, marked ESR signals of intrastriatal lipid radicals were observed. We think that these results support the hypothesis on the role of free radicals in tardive dyskinesia.

Generation of lipid radicals in the hippocampal extracellular space during kainic acid-induced seizures in rats

We report direct electron spin resonance (ESR) evidence of extracellular free radical formation during kainic acid-induced seizures obtained using in vivo brain microdialysis in freely moving rats. Saline solution containing the spin trap agent alpha-(4-pyridyl-N-oxide)-N-tert-butylnitrone was perfused through the hippocampus. ESR analysis of the dialysate samples revealed a six-line spectra, for which the hyperfine coupling constants corresponded to those of the ESR signal from the lipoxygenase/linoleic acid system, a lipid radical generating system. This result is direct evidence that lipid peroxidation of the neuronal membrane progresses during seizure activity. Increased formation of lipid radicals may participate in the cascade of reactions leading to neuronal damage in the hippocampus following kainic acid-induced seizure activity.

Studies on protective mechanisms of four components of green tea polyphenols against lipid peroxidation in synaptosomes

The comparison of the protective effects of four components of "green tea polyphenols' (GTP) - (-)-epigallocatechin gallate, EGCG; (-)-epicatechin gallate, ECG; (-)epigallocatechin, EGC; and (-)epicatechin, EC - against iron-induced lipid peroxidation in synaptosomes showed that: (1) the inhibitory effects of those compounds on TBA reactive materials from lipid peroxidation decreased in the order of EGCG > ECG > EGC > EC; (2) the scavenging effects of those compounds on lipid free radicals produced by lipid peroxidation could be classified as follows: ECG > EGCG > EC > EGC. Furthermore, we investigated the iron-chelating activity and the free radical scavenging activity of those compounds as their protective mechanisms against lipid peroxidation in synaptosomes. As for the iron-chelating activity, the ratio of EGC, EGCG, ECG or EC to iron(III) was 3:2, 2:1, 2:1 and 3:1, respectively. The hydroxyl radical (HO) scavenging activity of those compounds was investigated in a photolysis of the H2O2 system. It was found that their ability to scavenge hydroxyl radicals decreased in the order of ECG > EC > EGCG >> EGC. It was also found that they could scavenge lipid free radicals in the lecithin/lipoxidase system and their scavenging activity was classified as follows: ECG > EGCG >> EGC > EC. Moreover, we found that their antioxidant active positions were different from each other and the stability of the semiquinone free radicals produced by those compounds in NaOH solution decreased in the order of EGCG > ECG >> EC. The results indicated that the ability of those compounds to protect synaptosomes from the damage of lipid peroxidation initiated by Fe2+/Fe3+ was dependent not only on their iron-chelating activity and free-radical scavenging activity, but also on the stability of their semiquinone free radicals.

Electron spin resonance assessment of susceptibility of roe deer (Capreolus capreolus) and red deer (Cervus elaphus) to oilseed rape (Brassica napus) poisoning

Ex vivo studies have been carried out on roe and red deer erythrocytes using electron spin resonance spectroscopy. Free radical formation in cells challenged with the brassica-derived haemolysin, dimethyldisulphide, was measured using spin trapping techniques. Significantly greater amounts of radical were trapped in the roe deer cells which may relate to differences in the antioxidant profile of the two genotypes. Results suggest that roe deer have a greater risk of developing oilseed rape poisoning than red deer.

Superoxide generation by lipoxygenase in the presence of NADH and NADPH

The ability of soybean lipoxygenase to mediate NAD(P)H oxidation and concomitant superoxide generation in the presence of linoleic acid was examined. At optimum pH of 8.3, lipoxygenase oxidized both NADH and NADPH in the presence of 700 microM linoleic acid. The oxidation of NAD(P)H was biphasic. The initial rates of NADH and NADPH oxidation were 130 and 140 nmoles/min/nmole of enzyme respectively and the corresponding final rates were 344 and 350 nmoles/min/nmole of enzyme. The apparent Km values calculated for NADH and NADPH oxidation were 13 microM and 117 microM respectively. NAD(P)H oxidation was accompanied by the reduction of either ferricytochrome c or nitroblue tetrazolium (NBT) which can be abolished (approx. 85%) by superoxide dismutase (SOD) suggesting the generation of superoxide anion radicals. Under optimal conditions, the rates of superoxide generation, measured as the SOD-inhibitable reduction of ferricytochrome c, were 325 and 235 nmoles/min/nmole of enzyme for NADH and NADPH respectively. Under identical experimental conditions, the SOD-inhibitable NBT reduction rates were 308 and 210 nmoles/min/nmole of enzyme for NADH and NADPH respectively. Both NADH and NADPH could be regenerated after oxidation using the appropriate dehydrogenases. These results strongly suggest that lipoxygenase not only generates lipid hydroperoxides but can also generate superoxide via oxidation of pyridine nucleotides and may, therefore, significantly contribute to oxidative stress in cells.

Cell fatty acid composition affects free radical formation during lipid peroxidation

Lipid-derived free radicals generated from intact human U937 monocytes exposed to iron-induced oxidative stress were detected by electron paramagnetic resonance (EPR) with the spin trap alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN). Lipid radical formation was enhanced when the cells were enriched with n-3 or n-6 polyunsaturated fatty acids. Computer simulation indicated that at least two POBN spin adducts were formed, having spectral characteristics consistent with carbon-centered radicals (aN = 15.9 G and aH = 2.6 G; aN = 15.1 G and aH = 2.8 G). These alkyl radicals are probably formed by beta-scission of alkoxyl radicals. POBN spin adduct formation correlated with ethane generation. Addition of ascorbate to the assay medium greatly increased the radical signal intensity. Although radical generation was cell dependent and POBN spin adducts were observed in cell homogenates, the adducts formed by the intact cells were detected only in the extracellular medium. These findings indicate that the extent of lipid radical formation in response to oxidative stress can be influenced by changes in the polyunsaturated fatty acid composition of the cell lipids and suggest the possibility that carbon-centered lipi radicals may interact with extracellular structures.

The functionalisation of saturated hydrocarbons--XXX. Model studies on the mechanism of some oxygenases

In harmony with our studies on the activation of hydrocarbons by Gif chemistry, we have, in the first part of this paper, studied the mechanism of the lipoxygenase enzymes using soybean lipoxygenase as a model. We have shown with trimethyl phosphite that no free radical is released by the enzyme. In a second part, we have studied the mechanism of the alpha-ketoglutarate dependent enzymes and shown evidence for a mechanism involving the reduction of an intermediate hydroperoxide by the alpha-ketoglutaric acid.

Lack of protection of PBN in isolated heart during ischemia and reperfusion: implications for radical scavenging mechanism

We evaluated the ability of alpha-phenyl-tert-butyl nitrone (PBN) to trap free radicals and to protect the rat myocardium during ischemia and reperfusion. Isolated bicarbonate buffer-perfused hearts (n = 8) were subjected to 20 min global ischemia (37 degrees C) followed by reperfusion with 0.4 to 4.0 mM PBN. Coronary effluent containing the PBN adduct was extracted in toluene. Electron spin resonance analysis of the toluene extract revealed a PBN-hydroxyl adduct. To verify this assignment, a Fenton system was used to generate an authentic PBN-hydroxyl adduct (n = 8), which yielded the same ESR spectra as the reperfusion-derived adduct. The structure of the adduct formed in the Fenton system was confirmed by gas chromatography-mass spectrometry. The ESR parameters of the PBN-hydroxyl adduct were exquisitely sensitive to solvent polarity during extraction of the adduct. Extraction of an authentic PBN-hydroxyl adduct into chloroform, chloroform:methanol, and toluene closely matched the ESR parameters obtained during reperfusion of ischemic myocardium in other animal models. To determine whether PBN could confer any protective effect during ischemia or reperfusion, hearts (n = 8/group) were subjected to 35 min global ischemia at 37 degrees C with the St. Thomas' II cardioplegic solution followed by 30 min reperfusion. Percent recovery (mean +/- SEM) of developed pressure, rate pressure product, and leakage of lactate dehydrogenase during reperfusion in control hearts were 58 +/- 3%, 48 +/- 4% and 3.2 +/- 0.5 IU/15 min/g wet wt. PBN at a concentration of 0.4 mM or 4.0 mM when present either during ischemia alone or reperfusion alone did not exert any effect upon recovery of developed pressure, rate pressure product or post-ischemic enzyme leakage. We conclude that PBN fails to improve contractile recovery and reduce enzyme leakage during reperfusion of myocardium subjected to global ischemia.

Site-specific trapping of reactive species in low-density lipoprotein oxidation: biological implications

Abundant data suggest that the oxidative modification of low-density lipoprotein is mediated by lipid-derived free radicals and aldehydes derived from them. In this report we have addressed the site-specific aspects of low-density lipoprotein modification. To this end, both water-soluble and lipid-soluble spin traps (i.e., diamagnetic organic molecules containing nitroso or nitrone functional groups) were used. Radical adducts were detected by electron spin resonance-spin trapping technique. Biochemical indices of low-density lipoprotein modification were thiobarbituric acid reactive substances formation, electrophoretic mobility and macrophage-mediated uptake of oxidized low-density lipoprotein. Results from this study have shown that the lipophilic spin trap, alpha-phenyl-tert-butyl-N-nitrone, traps a primary low-density lipoprotein lipid-derived radical, while also inhibiting the total oxidative modification in a dose-dependent manner. The more hydrophilic analog, i.e., alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone, appeared to trap the secondary alkyl radicals and did not exert any inhibitory effect on oxidative modification of low-density lipoprotein. The lipophilic nitroso spin trap, 2-methyl-2-nitroso propane, which traps a lipid-derived radical, inhibited the low-density lipoprotein modification as did the water-soluble nitroso analog, 2-hydroxymethyl-2-nitroso propane. However, the water-soluble nitroso analog did not trap the lipid radical. The inhibitory effect of 2-hydroxymethyl-2-nitroso propane was tentatively attributed to trapping of aldehydes. It is conceivable that spin traps can inhibit the oxidative modification of low-density lipoprotein by trapping of the lipid radicals as well as trapping aldehydes formed from lipid peroxidation.

Adaptation of the blood antioxidant defence mechanisms of sheep with a genetic lesion resulting in low red cell glutathione concentrations

Finnish Landrace sheep with a genetic lesion which results in restricted cysteine transport across the red cell membrane have total glutathione concentrations in their red blood cells that are approximately 40% of those in normal sheep of the same breed. However, dimethyldisulphide-challenged red blood cells from both phenotypes produce an ESR-spin adduct at similar rates. The resistance of the low glutathione phenotype red cells to oxidant challenge is reflected by increases in the activities of antioxidant enzymes. Sheep with a genotypic disorder in cysteine transport may be a suitable model for studying the genetic expression of antioxidant enzymes in response to oxidant loads.

Fatty acid radical formation in rats administered oxidized fatty acids: in vivo spin trapping investigation

We report in vivo evidence for fatty acid-derived free radical metabolite formation in bile of rats dosed with spin traps and oxidized polyunsaturated fatty acids (PUFA). When rats were dosed with the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and oxidized PUFA, the DMPO thiyl radical adduct was formed due to a reaction between oxidized PUFA and/or its metabolites with biliary glutathione. In vitro experiments were performed to determine the conditions necessary for the elimination of radical adduct formation by ex vivo reactions. Fatty acid-derived radical adducts of alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN) were detected in vivo in bile samples collected into a mixture of iodoacetamide, desferrioxamine, and glutathione peroxidase. Upon the administration of oxidized 13C-algal fatty acids and 4-POBN, the EPR spectrum of the radical adducts present in the bile exhibited hyperfine couplings due to 13C. Our data demonstrate that the carbon-centered radical adducts observed in in vivo experiments are unequivocally derived from oxidized PUFA. This in vivo evidence for PUFA-derived free radical formation supports the proposal that processes involving free radicals may be the molecular basis for the previously described cytotoxicity of dietary oxidized PUFA.

Detection of free radicals during brain ischemia and reperfusion by spin trapping and microdialysis

Extracellular free radicals were detected in rat striatal perfusate samples by intracerebral microdialysis coupled to the spin trapping technique. Five Sprague-Dawley rats were subjected to 30 min of global ischemia followed by reperfusion; throughout the experimental period the intrastriatal dialysing probe was perfused with Ringer's solution containing the spin trap agent pyridyl-N-oxide-t-butylnitrone (100 mM) together with the iron chelating agent diethylentriaminepentacetic acid (100 microM). A radical adduct occurred during ischemia and early reperfusion, but not in basal conditions; the spin adduct was characterized as a carbon centered radical, consistent with the presence of an oxidative attack on membrane lipids. The direct evidence of the formation of free radicals supports the hypothesis that free radicals play a role in the pathogenesis of the histological damage during brain ischemia.

Isolation and characterization of the initial radical adduct formed from linoleic acid and alpha-(4-pyridyl 1-oxide)-N-tert-butylnitrone in the presence of soybean lipoxygenase

The spin trapping agent alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN) was used to trap the initial radical formed from [U-14C]linoleic acid in the reaction with soybean lipoxygenase. By using low levels of enzyme and relatively short incubation times it was possible to avoid the formation of secondary oxidation products and polymers. The adduct was extracted after methyl esterification, and isolated by a combination of open column chromatography on silicic acid and high pressure liquid chromatography on Spherisorb S5 CN with non-aqueous solvents. The 1:1 POBN-linoleate adduct was characterized by UV, IR and ESR spectra of the appropriate HPLC column fraction, by the ratio of the UV absorption to 14C content, and by mass spectrometry of the reduced (hydroxylamine) form. The results indicated that POBN trapped a linoleic acid carbon-centered radical such that POBN was attached to the fatty acid chain at C-13 or C-9 (two isomers), the linoleate double bonds having become conjugated in the process. The exact locations of the bridges in the two isomers were only tentatively determined. There was no evidence for the presence of oxygen-bridged adducts. The trapped linoleoyl radical adduct provides evidence for the production of a free radical as part of the enzymatic mechanism of soybean lipoxygenase.

Cytotoxicity of fatty acid oxygenase activation in rat basophilic leukemia cells

Apart from the generation of potent inflammatory mediators, the effects of fatty acid oxygenase activation, per se, on the host cell have not been well-delineated. Fatty acid oxygenases were activated in rat basophilic leukemia cells (RBL-1) by incubating them for 2-4 hr with 33-300 microM of arachidonic acid (AA) or linoleic acid (LA). As a control, the cells were incubated with one of two analogs of these fatty acids which are not oxygenase substrates: eicosatetraynoic acid or linoelaidic acid. Effects of oxygenase activation on cell viability were monitored by an assay for mitochondrial function. Cytotoxicity occurred in incubations with exogenous AA or LA in direct proportion to the substrate concentration but was not found in the control incubations or in incubations with the principal monohydroxylated AA products, 5-, 15-, and 12-HETE. Nordihydroguaiaretic acid (80 microM) and alpha-tocopherol (100 microM) significantly decreased the cell death observed during incubations with AA or LA. It is concluded that extensive oxygenase activation can result in cell death from intermediates produced proximal to the stable monohydroxylated derivatives.

The role of free radicals in brassica-induced anaemia of sheep: an ESR spin trapping study

The formation of reactive free radical species in sheep erythrocytes challenged with dimethyldisulphide, a brassica-derived haemolysin, has been investigated by electron spin resonance spectroscopy using the spin trap alpha-(4-pyridyl 1-oxide)-N-tert-butylnitrone. Erythrocytes exposed to this agent undergo a burst of free radical activity as demonstrated by the appearance of a spin adduct. The results suggest that haemolytic anaemia which can occur in sheep grazing forage brassicas is a consequence of oxidative stress.

Radical adducts of nitrosobenzene and 2-methyl-2-nitrosopropane with 12,13-epoxylinoleic acid radical, 12,13-epoxylinolenic acid radical and 14,15-epoxyarachidonic acid radical. Identification by h.p.l.c.-e.p.r. and liquid chromatography-thermospray-m.s

Linoleic acid-derived radicals, which are formed in the reaction of linoleic acid with soybean lipoxygenase, were trapped with nitrosobenzene and the resulting radical adducts were analysed by h.p.l.c.-e.p.r. and liquid chromatography-thermospray-m.s. Three nitrosobenzene radical adducts (peaks I, II and III) were detected; these gave the following parent ion masses: 402 for peak I, 402 for peak II, and 386 for peak III. The masses of peaks I and II correspond to the linoleic acid radicals with one more oxygen atom [L(O).]. The radicals are probably carbon-centred, because the use of 17O2 did not result in an additional hyperfine splitting. Computer simulation of the peak I radical adduct e.p.r. spectrum also suggested that the radical is carbon-centred. The peak I radical was also detected in the reaction of 13-hydroperoxylinoleic acid with FeSO4. From the above results, peak I is probably the 12,13-epoxylinoleic acid radical. An h.p.l.c.-e.p.r. experiment using [9,10,12,13-2H4]linoleic acid suggested that the 12,13-epoxylinoleic acid radical is a C-9-centred radical. Peak II is possibly an isomer of peak I. Peak III, which was observed in the reaction mixture without soybean lipoxygenase, corresponds to a linoleic acid radical (L.). The 12,13-epoxylinoleic acid radical, 12,13-epoxylinolenic acid radical and 14,15-epoxyarachidonic acid radical were also detected in the reactions of linoleic acid, linolenic acid and arachidonic acid respectively, with soybean lipoxygenase using nitrosobenzene and 2-methyl-2-nitrosopropane as spin-trapping agents.

Isolation and identification of alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone radical adducts formed by the decomposition of the hydroperoxides of linoleic acid, linolenic acid, and arachidonic acid by soybean lipoxygenase

alpha-(4-Pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN) radical adducts, which are formed in the reactions of soybean lipoxygenase with linoleic acid, arachidonic acid, and linolenic acid, were isolated using HPLC-ESR spectroscopy. Both linoleic acid and arachidonic acid gave one radical adduct, whereas in the case of linolenic acid, two radical adducts were isolated. These radical adducts all showed virtually identical uv spectra with lambda max at 292 and 220 nm in hexane. The absence of absorbance with lambda max at 234 nm indicates that a conjugated diene structure is not contained in these radical adducts. The mass spectra of the radical adducts formed from linoleic and arachidonic acids were identical and contained a molecular ion of m/z 264, consistent with the trapping of the pentyl radical by 4-POBN. Indeed, authentic 4-POBN pentyl radical adduct obtained from the reaction between pentylhydrazine and 4-POBN gave the same mass spectrum as the product obtained from the reaction of linoleic acid and arachidonic acid with 4-POBN. The two 4-POBN radical adducts formed in the linolenic acid reaction were shown by mass spectrometry to be isomers of pentenyl radicals. The 4-POBN-pentyl radical adduct was also detected in the reaction mixture of 13-hydroperoxy-linoleic acid, soybean lipoxygenase, and 4-POBN, indicating that the pentyl radical and pentenyl radical are formed by the decomposition of the hydroperoxides.

Histamine release from rat mast cells induced by metabolic activation of polyunsaturated fatty acids into free radicals

Polyunsaturated fatty acids (PUFA: arachidonic and linoleic acid) release histamine from isolated purified rat serosal mast cells only in the presence of oxidizing systems such as phenobarbital-induced rat liver microsomes, prostaglandin-H-synthetase (PHS) or soybean lipoxygenase. The release of mast cell histamine by activated PUFA has a long time-course and the electron microscopical features are consistent with an exocytotic secretion in the case of arachidonic acid and cell lysis in the case of linoleic acid. The phenomenon is associated with a significant increase in malonyldialdehyde (MDA) and conjugated diene generation, suggesting a relationship between histamine release and membrane lipid peroxidation. The secretion of histamine was inhibited by anti-free radical interventions such as D-mannitol, reduced glutathione and alpha-tocopherol. Some cyclooxygenase and lipoxygenase inhibitors, cimetidine and carnitine derivatives, are differentially active in the inhibition of mast cell histamine release by activated arachidonic acid. These results suggest that free radical derivatives of PUFA, generated by metabolic activation, trigger mast cell histamine release.

Spin trapping of free radicals and lipid peroxidation in microsomal preparations from malignant hyperthermia susceptible pigs

Microsomes were prepared from livers of malignant hyperthermia susceptible (MHS) or resistant (MHR) pigs. On incubation with the spin trap alpha-(4-pyridyl-l-oxide)-N-tert-butylnitrone (4-POBN), the microsomes from MHS pigs produced a characteristic electron spin resonance (ESR) signal at a greater rate than those from MHR pigs. Increased formation in the incubations of thiobarbituric acid reactive substances (TBARS) by the microsomes of the MHS pigs indicated an enhanced susceptibility to free radical-mediated lipid peroxidation. These results provide further evidence that MHS pigs have an antioxidant abnormality which may contribute to the fatal MH response. However the nature of the abnormality is unclear. The enhanced formation of unstable free radicals and indices of lipid peroxidation was not due to decreased vitamin E concentration or glutathione peroxidase activity in the microsomes. Furthermore, fatty acid profiles were similar in microsomes from MHS and MHR pigs indicating similar amounts of potential substrate for TBARS formation.

Spin trapping of lipid-derived radicals in liposomes

Electron-spin resonance-spin trapping has been used to detect lipid-derived radicals in liposomes. Using the lipid-soluble spin trap 2-methyl-nitrosopropane (MNP), we have detected both the lipid and hydrogen-atom spin adducts in liposomes composed of a fully saturated phospholipid (dimyristoylphosphatidylcholine, DMPC) with various mol fractions of unsaturated phospholipid (1-palmitoyl-2-arachidonoylphosphatidylcholine, PAPC) or fatty acid (arachidonic acid, AA). The lipid-derived spin adduct formed during autoxidation of liposomes was separated by thin-layer chromatography and found to co-migrate with the product(s) formed by direct addition of MNP to the corresponding unsaturated lipid or fatty acid. Both the MNP-PAPC and MNP-AA spin adducts showed some restriction of rotational motion when in the liposome bilayer (rotational correlation times 0.72 and 0.69.10(-9) s, respectively), and nitrogen hyperfine coupling constants (14.94-14.96 G) consistent with a hydrophobic localization. Radical versus non-radical mechanisms of spin adduct formation during liposome autoxidation were separated using alpha-tocopherol as a radical scavenger. The utility of nitroso spin traps in trapping of radicals in liposomes is discussed.

Separation of polyunsaturated fatty acid radicals by high-performance liquid chromatography with electron spin resonance and ultraviolet detection

In the reaction of soybean lipoxygenase (EC 1.13.11.12) with polyunsaturated fatty acids such as linoleic, linolenic and arachidonic acids, some radical species were detected using the electron spin resonance (ESR) spin-trapping technique. The radical species derived from the three polyunsaturated fatty acids were not distinguishable because the ESR spectra of the spin adducts of nitrosobenzene with their three radical species showed no difference in their hyperfine splittings. To overcome this defect of the spin-trapping technique, these spin-adducts were separated by employing high-performance liquid chromatography (HPLC) combined with ESR spectroscopy. The spin adducts were eluted from a C18 reversed-phase column in the order linolenic acid, linoleic acid and arachidonic acid. The half-lives of the spin adducts separated by HPLC-ESR were determined as linoleic acid 600 min, linolenic acid 360 min and arachidonic acid 160 min. The use of an ultraviolet detector together with the HPLC-ESR technique resulted in a 500-fold increase in sensitivity in the detection of the radical species.

Singlet oxygen production by biological systems

Singlet oxygen (1 delta g) is a highly reactive, short-lived intermediate which readily oxidizes a variety of biological molecules. The biochemical production of singlet oxygen has been proposed to contribute to the destructive effects seen in a number of biological processes. Several model biochemical systems have been shown to produce singlet oxygen. These systems include the peroxidase-catalyzed oxidations of halide ions, the peroxidase-catalyzed oxidations of indole-3-acetic acid, the lipoxygenase-catalyzed oxidation of unsaturated long chain fatty acids and the bleomycin-catalyzed decomposition of hydroperoxides. Results from these model systems should not be uncritically extrapolated to living systems. Recently, however, an intact cell, the human eosinophil, was shown to generate detectable amounts of singlet oxygen. This result suggests that singlet oxygen may be shown to be a significant biochemical intermediate in a few biological processes.

Applications of electron spin resonance spectroscopy to the identification of radicals produced during lipid peroxidation

Electron spin resonance (ESR) spectroscopy, which is the only commonly available method for directly detecting free radicals in biological systems, has now been quite extensively used to study radicals produced by metabolism of xenobiotic chemicals and the interaction of such species with lipid molecules. This review examines a variety of different xenobiotic systems and tissues and summarises the information obtained from these studies, with particular reference to the elucidation of the nature of the radicals involved in the initiation and propagation of lipid peroxidation.

Spin trapping: ESR parameters of spin adducts

Spin trapping has become a valuable tool for the study of free radicals in biology and medicine. The electron spin resonance hyperfine splitting constants of spin adducts of interest in this area are tabulated. The entries also contain a brief comment on the source of the radical trapped.