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

Use of ESR and HPLC to follow the anaerobic reaction catalysed by lipoxygenases

The measurement of the 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL) consumption by using ESR allows to follow the anaerobic reaction between linoleic acid (LH) and its 13-hydroperoxide (LOOH) catalysed by lipoxygenase. During this reaction, two types of radicals are initially obtained, alkyl (L) and alkoxyl (LO) radicals which formed two types of adducts (LT and OLT) with TEMPOL as characterised by HPLC. The stoichiometry of the adduct formation is two mole of TEMPOL consumed for one mole of LH and one mole of LOOH. Using ESR, the kinetic parameters and the mechanism of the anaerobic reaction have been determined at pH 6.5 for three different lipoxygenases, soybean, horse bean and wheat and compared to the values obtained at pH 9 for soybean lipoxygenase. Wheat lipoxygenase is very weakly active compared to the other enzymes. An uncompetitive inhibition of the anaerobic reaction catalysed by soybean and horse bean lipoxygenases was observed with 2,6-di-tert-butyl-4-methylphenol (BHT).

Characterization of free radicals formed from COX-catalyzed DGLA peroxidation

Like arachidonic acid (AA), dihomo-γ-linolenic acid (DGLA) is a 20-carbon ω-6 polyunsaturated fatty acid and a substrate of cyclooxygenase (COX). Through free radical reactions, COX metabolizes DGLA and AA to form well-known bioactive metabolites, namely, the 1 and 2 series of prostaglandins (PGs1 and PGs2), respectively. Unlike PGs2, which are viewed as proinflammatory, PGs1 possess anti-inflammatory and anticancer activities. However, the mechanisms linking the PGs to their bioactivities are still unclear, and radicals generated in COX-DGLA have not been detected. To better understand PG biology and determine whether different reactions occur in COX-DGLA and COX-AA, we have used LC/ESR/MS with a spin trap, α-(4-pyridyl-1-oxide)-N-tert-butyl nitrone (POBN), to characterize the carbon-centered radicals formed from COX-DGLA in vitro, including cellular peroxidation. A total of five types of DGLA-derived radicals were characterized as POBN adducts: m/z 266, m/z 296, and m/z 550 (same as or similar to COX-AA) and m/z 324 and m/z 354 (exclusively from COX-DGLA). Our results suggest that C-15 oxygenation to form PGGs occurs in both COX-DGLA and COX-AA; however, C-8 oxygenation occurs exclusively in COX-DGLA. This new finding will be further investigated for its association with various bioactivities of PGs, with potential implications for inflammatory diseases.

Characterization of novel radicals from COX-catalyzed arachidonic acid peroxidation

The peroxidation of arachidonic acid (AA) catalyzed by cyclooxygenase (COX) is a well-known free radical-mediated process that forms many bioactive products. Because of a lack of appropriate methodologies, however, no comprehensive structural evidence has been found previously for the formation of COX-mediated and AA-derived free radicals. Here we have used a combination of LC/ESR and LC/MS with a spin trap, alpha-[4-pyridyl-1-oxide]-N-tert-butylnitrone (POBN), to characterize the carbon-centered radicals formed from COX-catalyzed AA peroxidation in vitro, including cellular peroxidation in human prostate cancer cells (PC-3). Three types of radicals with numerous isomers were trapped by POBN as ESR-active peaks and MS-active ions of m/z 296, 448, and 548, all stemming from PGF(2)-type alkoxyl radicals. One of these was a novel radical centered on the carbon-carbon double bond nearest the PGF ring, caused by an unusual beta-scission of PGF(2)-type alkoxyl radicals. The complementary nonradical product was 1-hexanol, another novel beta-scission product, instead of the more common aldehyde. The characterization of these novel products formed from in vitro peroxidation provides a new mechanistic insight into COX-catalyzed AA peroxidation in cancer biology.

Characterization of novel radicals from COX-catalyzed arachidonic acid peroxidation

The peroxidation of arachidonic acid (AA) catalyzed by cyclooxygenase (COX) is a well-known free radical-mediated process that forms many bioactive products. Because of a lack of appropriate methodologies, however, no comprehensive structural evidence has been found previously for the formation of COX-mediated and AA-derived free radicals. Here we have used a combination of LC/ESR and LC/MS with a spin trap, alpha-[4-pyridyl-1-oxide]-N-tert-butylnitrone (POBN), to characterize the carbon-centered radicals formed from COX-catalyzed AA peroxidation in vitro, including cellular peroxidation in human prostate cancer cells (PC-3). Three types of radicals with numerous isomers were trapped by POBN as ESR-active peaks and MS-active ions of m/z 296, 448, and 548, all stemming from PGF(2)-type alkoxyl radicals. One of these was a novel radical centered on the carbon-carbon double bond nearest the PGF ring, caused by an unusual beta-scission of PGF(2)-type alkoxyl radicals. The complementary nonradical product was 1-hexanol, another novel beta-scission product, instead of the more common aldehyde. The characterization of these novel products formed from in vitro peroxidation provides a new mechanistic insight into COX-catalyzed AA peroxidation in cancer biology.

A combination study of spin-trapping, LC/ESR and LC/MS on carbon-centred radicals formed from lipoxygenase-catalysed peroxidation of eicosapentaenoic acid

Increased evidence from animal and in vitro cellular research indicates that the metabolism of eicosapentaenoic acid (EPA) can inhibit carcinogenesis in many cancers. Free radical-mediated peroxidation is one of many possible mechanisms to which EPA's anti-cancer activity has been attributed. However, no direct evidence has been obtained for the formation of any EPA-derived radicals. In this study, a combination of LC/ESR and LC/MS was used with alpha-[4-pyridyl 1-oxide]-N-tert-butyl nitrone to identify the carbon-centred radicals that are formed in lipoxygenase-catalysed EPA peroxidation. Of the numerous EPA-derived radicals observed, the major products were those stemming from beta-scission of 5-, 15- and 18-EPA-alkoxyl radicals. By means of an internal standard in LC/MS, this study also quantified each radical adduct in all its redox forms, including an ESR-active form and two ESR-silent forms. The comprehensive profile of EPA's radical formation provides a starting point for ongoing research in defining the biological effects of radicals generated from EPA peroxidation.

LC/ESR/MS study of spin trapped carbon-centred radicals formed from in vitro lipoxygenase-catalysed peroxidation of gamma-linolenic acid

Gamma-linolenic acid (GLA) has been reported as a potential anti-cancer and anti-inflammatory agent and has received substantial attention in cancer care research. One of the many proposed mechanisms for GLA biological activity is free radical-mediated lipid peroxidation. However, no direct evidence has been obtained for the formation of GLA-derived radicals. In this study, a combination of LC/ESR and LC/MS was used with alpha-[4-pyridyl-1-oxide]-N-tert-butyl nitrone (POBN) to profile the carbon-centred radicals that are generated in lipoxygenase-catalysed GLA peroxidation. A total of four classes of GLA-derived radicals were characterized including GLA-alkyl, epoxyallylic, dihydroxyallylic radicals and a variety of carbon-centred radicals stemming from the beta-scissions of GLA-alkoxyl radicals. By means of an internal standard in LC/MS, one also quantified each radical adduct in all its redox forms, including an ESR-active form and two ESR-silent forms. The results provided a good starting point for ongoing research in defining the possible biological effects of radicals generated from GLA peroxidation.

A novel protocol to identify and quantify all spin trapped free radicals from in vitro/in vivo interaction of HO(.-) and DMSO: LC/ESR, LC/MS, and dual spin trapping combinations

When dimethyl sulfoxide (DMSO) is oxidized via hydroxyl radical (HO(.-)), it forms methyl radicals ((.-)CH(3)) that can be spin trapped and detected by electron spin resonance (ESR). This ESR spin trapping technique has been widely used in many biological systems to indicate in vivo HO(.-) formation. However, we recently reported that (.-)CH(3) might not be the only carbon-centered radical that was trapped and detected by ESR from in vivo DMSO oxidation. In the present study, newly developed combination techniques consisting of dual spin trapping (free radicals trapped by both regular and deuterated alpha-[4-pyridyl 1]-N-tert-butyl nitrone, d(0)/d(9)-POBN) followed by LC/ESR and LC/MS were used to characterize and quantify all POBN-trapped free radicals from the interaction of HO(.-) and DMSO. In addition to identifying the two well-known free radicals, (.-)CH(3) and (.-)OCH(3), from this interaction, we also characterized two additional free radicals, (.-)CH(2)OH and (.-)CH(2)S(O)CH(3). Unlike ESR, which can measure POBN adducts only in their radical forms, LC/MS identified and quantified all three redox forms, including the ESR-active radical adduct and two ESR-silent forms, the nitrone adduct (oxidized adduct) and the hydroxylamine (reduced adduct). In the bile of rats treated with DMSO and POBN, the ESR-active form of POBN/(.-)CH(3) was not detected. However, with the addition of the LC/MS technique, we found approximately 0.75 microM POBN/(.-)CH(3) hydroxylamine, which represents a great improvement in radical detection sensitivity and reliability. This novel protocol provides a comprehensive way to characterize and quantify in vitro and in vivo free radical formation and will have many applications in biological research.

Characterization of the initial carbon-centered pentadienyl radical and subsequent radicals in lipid peroxidation: identification via on-line high performance liquid chromatography/electron spin resonance and mass spectrometry

The previously reported combination of an on-line high-performance liquid chromatography (LC)/electron spin resonance (ESR) system with mass spectrometric analysis (MS) created a unique technique to identify a variety of lipid-derived radicals ((.)L(d)) formed from in vitro lipid peroxidation (Iwahashi et al. [20]). To improve the sensitivity, resolution, and reliability of this method for in vitro and in vivo studies, we have investigated the effects of mobile phase pH, modifiers, and columns on the chromatographic separation of linoleic acid-derived radical adducts. Using tetrahydrofuran (THF) and 0.1% glacial acetic acid (HOAc) in an H(2)O/acetonitrile (ACN) mobile phase greatly increased the resolution and retention reproducibility of lipid radical adducts in LC/ESR. In addition, these modifications allowed the elimination of an ESR tuning problem and the synchronization of UV and ESR detection of radical adducts in on-line LC/ESR, neither of which had been possible previously. Analyte purity was therefore increased, thus increasing the reliability of radical detection via on-line LC/ESR as well as radical identification via MS analysis. For the first time, POBN adducts of linoleic carbon-centered pentadienyl radicals (L(.)) were detected and identified. The optimization of chromatography in the LC/ESR and MS combination provided a reliable and sensitive way for the detection and identification of expected radical adducts in vitro and in vivo.

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.

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.

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.