Mass spectrometric analysis of four regioisomers of F2-isoprostanes formed by free radical oxidation of arachidonic acid

Moving forward with isoprostanes, neuroprostanes and phytoprostanes: where are we now?

Polyunsaturated fatty acids (PUFAs) are essential components in eukaryotic cell membrane. They take part in the regulation of cell signalling pathways and act as precursors in inflammatory metabolism. Beside these, PUFAs auto-oxidize through free radical initiated mechanism and release key products that have various physiological functions. These products surfaced in the early nineties and were classified as prostaglandin isomers or isoprostanes, neuroprostanes and phytoprostanes. Although these molecules are considered robust biomarkers of oxidative damage in diseases, they also contain biological activities in humans. Conceptual progress in the last 3 years has added more understanding about the importance of these molecules in different fields. In this chapter, a brief overview of the past 30 years and the recent scope of these molecules, including their biological activities, biosynthetic pathways and analytical approaches are discussed.

Development of an LC-ESI(-)-MS/MS method for the simultaneous quantification of 35 isoprostanes and isofurans derived from the major n3- and n6-PUFAs

Misregulation of oxidative and antioxidative processes in the organism - oxidative stress - contributes to the pathogenesis of different diseases, e.g. inflammatory or neurodegenerative diseases. Oxidative stress leads to autoxidation of polyunsaturated fatty acids giving rise to prostaglandin-like isoprostanes (IsoP) and isofurans (IsoF). On the one hand they could serve as biomarker of oxidative stress and on the other hand may act as lipid mediators, similarly as the enzymatically formed oxylipins. In the present paper we describe the development of an LC-ESI(-)-MS/MS method allowing the parallel quantification of 27 IsoP and 8 IsoF derived from 6 different PUFA (ALA, ARA, EPA, AdA, n6-DPA, DHA) within 12 min. The chromatographic separation was carried out on an RP-C18 column (2.1 × 150 mm, 1.8 μm) yielding narrow peaks with an average width at half maximum of 3.3-4.2 s. Detection was carried out on a triple quadrupole mass spectrometer operating in selected reaction monitoring mode allowing the selective detection of regioisomers. The limit of detection ranged between 0.1 and 1 nM allowing in combination with solid phase extraction the detection of IsoP and IsoF at subnanomolar concentrations in biological samples. The method was validated for human plasma showing high accuracy and precision. Application of the approach on the investigation of oxidative stress in cultured cells indicated a distinct pattern of IsoP and IsoF in response to reactive oxygen species which warrants further investigation. The described method is not only the most comprehensive approach for the simultaneous quantification of IsoP and IsoF, but it was also integrated in a targeted metabolomics method (Ostermann et al. (2015) Anal Bioanal Chem) allowing the quantification of in total 164 oxylipins formed enzymatically and non-enzymatically within 30.5 min.

Isoprostanes, neuroprostanes and phytoprostanes: An overview of 25years of research in chemistry and biology

Since the beginning of the 1990's diverse types of metabolites originating from polyunsaturated fatty acids, formed under autooxidative conditions were discovered. Known as prostaglandin isomers (or isoprostanoids) originating from arachidonic acid, neuroprostanes from docosahexaenoic acid, and phytoprostanes from α-linolenic acid proved to be prevalent in biology. The syntheses of these compounds by organic chemists and the development of sophisticated mass spectrometry methods has boosted our understanding of the isoprostanoid biology. In recent years, it has become accepted that these molecules not only serve as markers of oxidative damage but also exhibit a wide range of bioactivities. In addition, isoprostanoids have emerged as indicators of oxidative stress in humans and their environment. This review explores in detail the isoprostanoid chemistry and biology that has been achieved in the past three decades.

Angiotensin and mineralocorticoid receptor antagonism attenuates cardiac oxidative stress in angiotensin II-infused rats

Angiotensin II (Ang II) and aldosterone contribute to hypertension, oxidative stress and cardiovascular damage, but the contributions of aldosterone during Ang II-dependent hypertension are not well defined because of the difficulty to assess each independently. To test the hypothesis that during Ang II infusion, oxidative and nitrosative damage is mediated through both the mineralocorticoid receptor (MR) and angiotensin type 1 receptor (AT1), five groups of Sprague-Dawley rats were studied: (i) control; (ii) Ang II infused (80 ng/min × 28 days); (iii) Ang II + AT1 receptor blocker (ARB; 10 mg losartan/kg per day × 21 days); (iv) Ang II + mineralocorticoid receptor (MR) antagonist (Epl; 100 mg eplerenone/day × 21 days); and (v) Ang II + ARB + Epl (Combo; × 21 days). Both ARB and combination treatments completely alleviated the Ang II-induced hypertension, whereas eplerenone treatment only prolonged the onset of the hypertension. Eplerenone treatment exacerbated the Ang II-mediated increase in plasma and heart aldosterone 2.3- and 1.8-fold, respectively, while ARB treatment reduced both. Chronic MR blockade was sufficient to ameliorate the AT1-mediated increase in oxidative damage. All treatments normalized protein oxidation (nitrotyrosine) levels; however, only ARB and Combo treatments completely reduced lipid peroxidation (4-hydroxynonenal) to control levels. Collectively, these data suggest that receptor signalling, and not the elevated arterial blood pressure, is the principal culprit in the oxidative stress-associated cardiovascular damage in Ang II-dependent hypertension.

Electrospray ionization tandem mass spectrometry study of six isomeric cationic amphiphiles with ester/amide linker

RATIONALE

Isomeric cationic amphiphiles differing only in the orientation of the linker group have been demonstrated to possess dramatically changed gene transfer efficacies. Studies aimed at understanding structure-stability correlations of such isomeric cationic amphiphiles at the molecular level are yet to be undertaken. Such studies may throw significant new insights into the mechanistic origin on their contrasting bioactivities.

METHODS

Electrospray ionization mass spectrometry (ESI-MS) and multi-stage tandem mass spectrometric (MS(n)) experiments were performed on a LCQ ion trap mass spectrometer. The decomposition pathway was confirmed by high-resolution mass spectrometry data from a quadrupole time-of-flight (Q-TOF) mass spectrometer. Dissociation curves were drawn based on the intensities of precursor and product ions.

RESULTS

The collision-induced dissociation (CID) spectra of the M(+) ion of each isomeric pair showed distinct product ions (3 pairs). Normal esters (1 and 3) showed abundant product ions with a neighboring group participation (NGP) reaction and reverse esters (lipid 2 and 4) showed McLafferty rearrangement product ions. The spectra of a normal amide (5) and a reverse amide (6) are similar to that found in the corresponding ester, except for the absence of the McLafferty rearrangement in 6. Dissociation curves revealed that normal esters/amide decompose at lower energy than those of corresponding reverse esters/amide.

CONCLUSIONS

The lipids which easily decompose (flexible) show dramatically enhanced gene delivery capabilities and the lipids which decompose at higher collision energy (CE) values (rigid) are transfection incompetent.

Non-enzymatic lipid oxidation products in biological systems: assessment of the metabolites from polyunsaturated fatty acids

Metabolites of non-enzymatic lipid peroxidation of polyunsaturated fatty acids notably omega-3 and omega-6 fatty acids have become important biomarkers of lipid products. Especially the arachidonic acid-derived F2-isoprostanes are the classic in vivo biomarker for oxidative stress in biological systems. In recent years other isoprostanes from eicosapentaenoic, docosahexaenoic, adrenic and α-linolenic acids have been evaluated, namely F3-isoprostanes, F4-neuroprostanes, F2-dihomo-isoprostanes and F1-phytoprostanes, respectively. These have been gaining interest as complementary specific biomarkers in human diseases. Refined extraction methods, robust analysis and elucidation of chemical structures have improved the sensitivity of detection in biological tissues and fluids. Previously the main reliable instrumentation for measurement was gas chromatography-mass spectrometry (GC-MS), but now the use of liquid chromatography-tandem mass spectrometry (LC-MS/MS) and immunological techniques is gaining much attention. In this review, the types of prostanoids generated from non-enzymatic lipid peroxidation of some important omega-3 and omega-6 fatty acids and biological samples that have been determined by GC-MS and LC-MS/MS are discussed.

Antimicrobial lipids from the hemolymph of brachyuran crabs

The potential of marine crabs as a source of biologically active products is largely unexplored. In the present study, antimicrobial activity of the hemolymph (plasma) and hemocytes (plasma cells) of six brachyuran crabs was investigated against 16 pathogenic strains. Among the 16 strains tested maximum zone of inhibition was recorded in the hemolymph of Hyas araneus against Shigella flexineri. Interestingly Staphylococcus aureus and Salmonella typhi were susceptible to all the hemolymph and hemocytes samples. Likewise, the highest zone of inhibition was exhibited by both hemolymph and hemocytes samples against Vibrio cholerae. On the basis of TLC, 1HNMR, and 13CNMR it may be concluded that the antimicrobial activity in the hemolymph extract is due to the presence of lipids. This observation is further supported by the ESI-MS of the methanolic extract of hemolymph of H. araneus. ESI-MS shows cluster of peaks in the region m/z 445 to m/z 491 due to lysoglycerolipids/glycerides and cluster of signals between m/z 216 and 246, due to fatty acids/esters present in the sample.

Beyond prostaglandins--chemistry and biology of cyclic oxygenated metabolites formed by free-radical pathways from polyunsaturated fatty acids

Polyunsaturated fatty acids (PUFAs) are important constituents in all organisms. They fulfil many functions, ranging from modulating the structure of membranes to acting as precursors of physiologically important molecules, such as the prostaglandins, which for a long time were the most prominent cyclic PUFA metabolites. However, since the beginning of the 1990s a large variety of cyclic metabolites have been discovered that form under autoxidative conditions in vivo to a much larger extent than do prostaglandins. These compounds--isoprostanes, neuroprostanes, phytoprostanes, and isofurans--proved subsequently to be ubiquitous in nature. They display a wide range of biological activities, and isoprostanes have become the currently most reliable indicators of oxidative stress in humans. In a relatively short time, the structural variety, properties, and applications of the autoxidatively formed cyclic PUFA derivatives have been uncovered.

HPLC–Atmospheric Pressure Chemical Ionization MS/MS for Quantification of 15-F2t-Isoprostane in Human Urine and Plasma

Background: Quantification of F2-Isoprostanes is considered a reliable index of the oxidative stress status in vivo and is valuable in the diagnosis and monitoring of a variety of diseases. Because of complex and lengthy sample preparation procedures, current chromatography/mass spectrometry and immunoassays are impractical for measuring larger numbers of samples. Thus, we developed and validated a semiautomated high-throughput HPLC tandem mass spectrometry assay for the quantification of F2-Isoprostane F2t in human urine and plasma.

Methods: After protein precipitation (500 μL methanol/zinc sulfate added to 500 μL plasma), samples were injected into the HPLC system and extracted online. The extracts were then back-flushed onto the analytical column and detected with an atmospheric pressure chemical ionization-triple quadrupole mass spectrometer monitoring the deprotonated molecular ions [M-H]− of 15-F2t-IsoP (m/z = 353→193) and the internal standard 15-F2t-IsoP-d4 (m/z = 357→197).

Results: In human urine, the assay was linear from 0.025 to 80 μg/L and in human plasma from 0.0025 to 80 μg/L (r2>0.99). Interday accuracy and precision for concentrations above the lower limit of quantification were <10%. Concentrations of 15-F2t-IsoP in urine of 16 healthy individuals ranged from 55–348 ng/g creatinine. In 16 plasma samples from healthy individuals, free 15-F2t-IsoP was detectable in all samples and concentrations were 3–25 ng/L.

Conclusions: Our assay meets all predefined method performance criteria, allows for analysis of >80 samples/day, and has sufficient sensitivity for quantifying 15-F2t-IsoP concentrations in plasma and urine from healthy individuals. It is, thus, suitable for clinical routine monitoring and the analysis of samples from larger clinical trials.

Glycolipids from the red alga Chondria armata (Kütz.) Okamura

Three distinct fractions containing polar glycolipids (PF(1-3)) were isolated from the chloroform soluble fraction of crude methanolic extract of red alga Chondria armata (Kütz.) Okamura on gel chromatography over Sephadex LH20. Their structure was elucidated by multidimensional nuclear magnetic resonance (NMR) techniques like 1H, 1H correlation spectroscopy (COSY), 1H, 1H total COSY (TOCSY), 1H, 13C heteronuclear multiple quantum coherence (HMQC), and 1H, 13C heteronuclear multiple bond correlation (HMBC) complemented by electrospray ionization mass spectrometry (ESI-MS) in the positive ion mode. The coupling constant of the anomeric proton in 1H NMR spectrum and sign of rotation indicated an exclusive configuration of the sugar molecules in the glycerolipids. Major glycolipids were identified as (2R)-2-O-(5,8,11,14-eicosatetranoyl)-3-O-alpha-d-galactopyranosyl-sn-glycerol (GL2), its pentacetate (GL1), and (2R)-1-O-(palmitoyl)-2-O-(5,8,11, 14,17-eicosapentanoyl)-3-O-beta-d-galactopyranosyl-sn-glycerol (GL3). Each was methanolysed to give the same galactosylglycerol which on ESI-MS provided a pseudomolecular ion at m/z 309 representing deacylated glycolipid with the sodiated sugar moiety. Additionally, six minor glycolipids were also identified on the basis of ESI-MS. These include a 1,2-di-O-acyl-3-O-(acyl-6'-galactosyl)-glycerol (GL1a), sulfonoglycolipids 2-O-palmitoyl-3-O-(6'-sulfoquinovopyranosyl)-glycerol (GL2a) and its ethyl ether derivative (GL2b), 1-oleoyl-2-palmitoyl-3-O-galactosyl glycerol (GL3a), and 1,2-diacyl phosphatidyl glycerol (GL3b). GL1, GL1a, and GL2b are new to the literature. The novelty of the remaining identified compounds lies in the diversity of their fatty acid composition. Antimicrobial properties of these glycolipids against pathogens were evaluated. The yeast Candida albicans and the bacteria Klebsiella sp. were as sensitive as the standard Nystatin and antibiotic Streptomycin against PF3. Considerable activity was expressed by the same metabolite against the fungus Cryptococcus neoformans as compared to the control. Weak activity against the bacteria Shigella flexineri and Vibrio cholerae and the fungus Aspergillus fumigatus was also observed. Fraction PF2 was weakly active against some strains whereas all of them were resistant to its acetyl derivative PF1. Antimicrobial activity of glycolipids is being reported here for the first time.

Regiochemistry of Neuroprostanes Generated from the Peroxidation of Docosahexaenoic Acid in Vitro and in Vivo

Isoprostanes (IsoPs) are isomers of prostaglandins that are generated from the free radical-initiated peroxidation of arachidonic acid (C20.4 omega-6). IsoPs exert potent bioactivity and are regarded as the "gold standard" to assess oxidative stress in various human diseases. Analogously, autoxidation of docosahexaenoic acid (DHA, C22.6 omega-3) generates an array of IsoP-like compounds that are termed neuroprostanes (NPs). A major class of NPs identified in vitro and in vivo contains F-type prostane rings and are know as F4-NPs. A number of different F4-NP regioisomers are formed from the peroxidation of DHA. Among the eight possible regioisomeric groups, we hypothesize that 4- and 20-series NPs are generated in greater amounts than other classes because the precursors that lead to regioisomers other than those of the 4- and 20-series can be further oxidized to form novel dioxolane-IsoP-like compounds, analogous to those generated from arachidonate. Various mass spectrometric approaches, including electron capture atmospheric pressure chemical ionization mass spectrometry, were utilized to analyze NPs formed in vitro and in vivo based on their characteristic fragmentation in the gas phase. Experimental results were consistent with our hypothesis that 4- and 20-series NP regioisomers are preferentially generated. The discovery of regioselectivity in the formation of NPs will allow studies of the biological activities of NPs to focus on the more abundantly generated compounds to determine their role in modulating the pathophysiological consequences of DHA oxidation and oxidant stress.

New insights regarding the autoxidation of polyunsaturated fatty acids

Free radical-initiated autoxidation of polyunsaturated fatty acids (PUFAs) has been implicated in numerous human diseases, including atherosclerosis and cancer. This review covers the free radical mechanisms of lipid oxidation and recent developments of analytical techniques to analyze the lipid oxidation products. Autoxidation of PUFAs generates hydroperoxides as primary oxidation products, and further oxidation leads to cyclic peroxides as secondary oxidation products. Characterization of these oxidation products is accomplished by several mass spectrometric techniques. Ag+ coordination ion spray mass spectrometry has proven to be a powerful tool to analyze the intact lipid peroxides. Monocyclic peroxides, bicyclic endoperoxides, serial cyclic peroxides, and a novel class of endoperoxides (dioxolane-isoprostane peroxides) have been identified from the oxidation of arachidonate. Electron capture atmospheric pressure chemical ionization mass spectrometry has been applied to study lipid oxidation products after derivatization. All eight possible diastereomeric isoprostanes are observed from the oxidation of a single hydroperoxide precursor. 5- and 15-series isoprostanes are more abundant than the 8- and 12-series because the precursors that lead to 8- and 12-series compounds can undergo further oxidation and form dioxolane-isoprostane peroxides. Furthermore, formation of isoprostanes from 15-hydroperoxyeicosatetraenoate occurs from beta-fragmentation of the corresponding peroxyl radical to generate a pentadienyl radical rather than a "dioxetane" intermediate, as previously suggested.

Total synthesis of isoprostanes: discovery and quantitation in biological systems

Isoprostanes (iP's), a new class of natural products isomeric with prostaglandins, are formed as the result of free radical oxygenation of polyunsaturated fatty acids. We have identified these iP's and developed analytical methodology to measure them in biological fluids. The approach we took, which led to the discovery and measurement of iP's, is as follows: (1) based on some biochemical and chemical considerations, we proposed possible structures for these isoprostanes; (2) we performed the total syntheses of some of these iP's, in particular Groups III through VI, and used them as markers for their discovery in biological fluids and developed a GC/MS and an LC/MS methodologies based on iPF2alpha-III, iPF2alpha-VI, and 8,12-iso-iPF2alpha-VI; (3) with the help of these assays, we measured elevated levels of iP's in Alzheimer's disease and atherosclerosis.

Identification of a novel class of endoperoxides from arachidonate autoxidation

Free radical-initiated lipid autoxidation in low density lipoprotein (LDL) has been implicated in the pathogenesis of atherosclerosis. Oxidation of the lipid components of LDL leads to a complex mixture of hydroperoxides, bicyclic endoperoxides, monocyclic peroxides, and serial cyclic peroxides. The oxidation compounds and/or their decomposition products can modify protein components, which may lead to various diseases. A novel class of peroxides (termed dioxolane-isoprostanes) having a bicyclic endoperoxide moiety characteristic of the isoprostanes and a dioxolane peroxide functionality in the same molecule was identified in the product mixture formed from in vitro autoxidation of cholesteryl arachidonate. The same products are also detected in in vitro oxidized LDL. Various mass spectrometric techniques have been applied to characterize these new peroxides. The structure of these compounds has also been confirmed by independent synthesis. We reason, based on the free radical mechanism of the transformation, that only the 12- and 8-peroxyl radicals (those leading to 12-HPETE and 8-HPETE) of arachidonate can form these new peroxides. We also suggest that the formation of these peroxides provides a rationale to explain the fact that 5- and 15-series isoprostanes are formed in preference to 8- and 12-series. Furthermore, series of other isoprostanes, such as dioxolane A(2), D(2), E(2), etc., can be derived from the dioxolane-isoprostane peroxides. These findings offer further insights into the oxidation products of arachidonate and the opportunity to study their potential biological relevance.

Determination of isoprostanes in urine samples from Alzheimer patients using porous graphitic carbon liquid chromatography–tandem mass spectrometry

F2-isoprostanes (F2-iPs) comprise four classes of isomers produced non-enzymatically by free radical attack on arachidonic acid, a component of the cell membrane. This paper describes a new method for the quantification of F2-isoprostanes in urine samples from thoroughly diagnosed Alzheimer's disease (AD) patients. The sample pretreatment consisted of liquid extraction of 900 microl urine with diethyl ether, its subsequent evaporation, and finally, reconstitution in 50 microl water. Of this, 20 microl was injected into a HPLC system with a 15 mm x 1 mm porous graphitic carbon column coupled to a triple quadrupole mass spectrometer running in negative electrospray ionization mode. The F2-isoprostanes were separated in 15 min using a linear solvent gradient comprising water, methanol, acetonitrile and ammonium hydroxide at a pH of 9.5. The average recovery obtained was approximately 75%. The limit of detection (3S/N) was calculated for iPF2alpha-III to be 0.7 pg injected on column, corresponding to 0.1 nM. The average level of iPF2alpha was 241 +/- 163 pg/mg creatinine in the urine samples from AD patients (average +/- standard deviation). The corresponding control values were 216 +/- 101 pg/mg creatinine, i.e. no statistically significant difference was noticed. No correlation pattern specific to Alzheimer's disease was revealed by principal component analysis of the isoprostane peaks obtained either. The results from this study support earlier findings that levels of peripheral isoprostanes are not increased in patients with Alzheimer's disease.

Nitrotyrosine and 8-isoprostane formation indicate free radical-mediated injury in hearts of patients subjected to cardioplegia

OBJECTIVE

Myocardial ischemia and reperfusion induced by cardioplegic arrest subjects the heart to free radical-mediated stress. The purpose of our study was to investigate the effect of cardioplegia-induced ischemia and reperfusion on myocardial formation and distribution of (1) nitrotyrosine as an indicator for peroxynitrite-mediated tissue injury resulting from increased nitric oxide release and (2) 8-isoprostane as an indicator for oxygen-derived free radical-mediated lipid peroxidation.

METHODS

In 10 patients undergoing coronary artery operations (64 +/- 6 [mean +/- SD] years, 3 women and 7 men) subjected to cardiopulmonary bypass and intermittent cold blood cardioplegia, we collected transmural left ventricular biopsy specimens before and at the end of cardiopulmonary bypass. Specimens were cut at 10 micro m and subjected to immunocytochemical staining against the nitric oxide-producing enzyme constitutive nitric oxide synthase, cyclic guanosine monophosphate (intracellular second messenger of nitric oxide), nitrotyrosine, and 8-isoprostane by using polyclonal antibodies. For global left ventricular function determination, we measured the fractional area of contraction using transesophageal echocardiography.

RESULTS

Nitric oxide synthase activity in cardiac myocytes increased from 34 +/- 10 gray units before cardiopulmonary bypass to 47 +/- 12 gray units at the end of bypass (P =.015), and all hearts showed increased cyclic guanosine monophosphate content in both myocytes and endothelial cells at the end of bypass. The number of nitrotyrosine-positive capillaries increased from 36 +/- 29/mm(2) before bypass to 82 +/- 47/mm(2) at the end of bypass (P =.012), and 8-isoprostane-positive capillaries increased from 92 +/- 72/mm(2) before bypass to 209 +/- 108/mm(2) at the end of bypass (P =.005). The fractional area of contraction was 53% +/- 12% before bypass and 56% +/- 12% after bypass (P =.47) but was slightly decreased to 45% +/- 14% at 4 hours after bypass (P =.121).

CONCLUSIONS

Our data show that cardioplegia-induced myocardial ischemia and reperfusion is associated with nitrotyrosine and 8-isoprostane formation mainly in the coronary endothelium, indicating injury mediated by both peroxynitrite and oxygen-derived free radicals. Because nitric oxide synthase activation was accompanied with increased cyclic guanosine monophosphate, these data suggest that direct effects of nitric oxide on cardiac myocytes, as well as nitric oxide-mediated coronary endothelial injury, might contribute to injury associated with cardioplegia and cardiopulmonary bypass.

Derivatization of F2-isoprostanes with 1-pyrenyldiazomethane and their subsequent determination by fluorescence high-performance liquid chromatography

F(2)-isoprostanes are produced during free radical oxidation of cell phospholipids and are considered reliable biomarkers of oxidative stress. Currently, mass spectroscopy is the method of choice to detect F(2)-isoprostanes. However, due to numerous isomeric forms, analysis of F(2)-isoprostanes using MS detectors requires preliminary chromatographic separation. The current study was undertaken to develop a method of HPLC separation and quantification of different isomers of prostaglandin F(2alpha), including 8-iso-PGF(2alpha), following derivatization with 1-pyrenyldiazomethane (PDAM) to a highly fluorescent 1-pyrenylmethyl ester. HPLC separation and quantification of 1-pyrenylmethyl esters of PGF(2alpha) isomers at picogram level are complicated by numerous interfering products of the degradation of 1-pyrenyldiazomethane. A procedure of derivatization and purification was developed to reduce these interfering contaminants. The procedure of derivatization includes sorption of PGF(2alpha) isomers from solution in a hexane:ethyl acetate mixture (10:1) on a cellulose support prepared in the form of small (4 x 4mm) filter paper squares. Bound PGF(2alpha) isomers are derivatized by 1-pyrenyldiazomethane dissolved in the same hexane:ethyl acetate mixture. During subsequent washing of the cellulose squares by the hexane:ethyl acetate mixture (10:1), fluorescent derivatives of PGF(2alpha) remain bound to cellulose while a significant portion of the contaminants are washed out. The 1-pyrenylmethyl esters of PGF(2alpha) can be quantitatively extracted from cellulose by an ethyl acetate:methanol (1:1) mixture. The next step in eliminating interference is a solid-phase extraction on silica cartridges using ethyl acetate for application of the sample and an ethyl acetate:methanol (1:1) mixture for elution. Final purification is achieved by normal-phase HPLC with wet ethyl acetate as the mobile phase. This chromatographic method displays remarkable resolution in the separation of different PGF(2alpha) isomers and can be used not only for sample purification but also for pre-MS separation. The purified 1-pyrenylmethyl esters of PGF(2alpha) were quantitatively analyzed by reverse-phase HPLC with fluorescent detection, with a detection limit of 5-10 pg.

Mass spectrometric analysis of leukotriene A4 and other chemically reactive metabolites of arachidonic acid

The biosynthesis of prostaglandins and leukotrienes proceeds through the formation of chemically reactive intermediates leukotriene A4 (LTA4) and prostaglandin H2 (PGH2) which in aqueous solutions have chemical half-lives of 3 s and 3 min, respectively. Prostacyclin (PGI2) is another chemically reactive prostanoid that has a chemical half-life of 3-4 min. The recent development of reversed phase HPLC stationary phases that are stable to elevated pH (pH 10-12) without significant column damage has permitted direct analysis of these acid-sensitive eicosanoids. Using electrospray ionization, molecular anions [M - H]- of these compounds were observed at m/z 317 for LTA4 and m/z 351 for both PGH2 and PGI2. The mechanism of formation of ions derived from collisional activation of LTA4 was studied using stable isotope labeled and chemical analogs of LTA4 and found to involve formation of highly conjugated anions at m/z 261 and 163. The collisional activation of the molecular anion of PGH2 yielded a product ion spectrum identical to that observed for the isomeric prostaglandins PGE2 and PGD2. However, it was possible to baseline separate PGE2, PDG2, and PGH2 by reversed phase HPLC using basic HPLC mobile phases. The collisional activation of PGI2 led to a family of abundant ions including highly conjugated carbon-centered and oxygen-centered radical species (m/z 245 and 205) likely derived from the attack of the carboxylate anion on the cyclic enolether of PGI2 as well as the most abundant product ion (m/z 215) which formed following loss of neutral hexanal and water. The structures of these product ions were consistent with high resolution measurements measured in a quadrupole time-of-flight mass spectrometer.

Formation of isoprostane bicyclic endoperoxides from the autoxidation of cholesteryl arachidonate

Autoxidation of polyunsaturated fatty acids and esters leads to a complex mixture containing hydroperoxides and cyclic peroxides. Prostaglandin bicyclic endoperoxides have been detected from the autoxidation of cholesteryl arachidonate by LC-MS and GC-MS techniques. All four possible types (I-IV) of bicyclic endoperoxides have been found starting from different regioisomeric hydroperoxides of cholesteryl arachidonate. Furthermore, the stereochemistry of Type IV bicyclic endoperoxides has been determined by conversion to pentafluorobenzyl (PFB) ester, trimethylsilyl (TMS) derivatives, and comparison with synthetic standards by the use of GC-MS. All eight possible diastereomers of the derivatized isoprostanes were observed and were separated by gas chromatography. The bicyclic endoperoxides with the two alkyl chains syn on the cyclopentane ring were formed preferentially to those with anti configuration, a result anticipated from earlier work. Substantial amounts of the anti-substituted isoprostanes, including PGF(2)(alpha), were, however, observed in the product mixture.

Cardiovascular responses to the isoprostanes iPF(2alpha)-III and iPE(2)-III are mediated via the thromboxane A(2) receptor in vivo

BACKGROUND

Isoprostanes (iPs) are free radical-catalyzed products of arachidonic acid that reflect lipid peroxidation in vivo. Several iPs exert biological effects in vitro and may contribute to the functional consequences of oxidant stress. For example, iPF(2alpha)-III (8-iso PGF(2alpha)) and iPE(2)-III modulate platelet function and vascular tone. Although these effects are blocked by antagonists of the receptor (TP) for the cyclooxygenase product thromboxane A(2), it has been speculated that the iPs may activate a receptor related to, but distinct from, the TP.

METHODS AND RESULTS

Transgenic mice (TPOEs) were generated in which the TP-beta isoform was under the control of the preproendothelin promoter. They overexpressed TP-beta in the vasculature but not in platelets and exhibited an exaggerated pressor response to infused iPF(2alpha)-III compared with wild-type mice. This was blocked by TP antagonism. The platelet response to the iP was unaltered in TPOEs compared with wild-type mice. By contrast, both the pressor response to iPF(2alpha)-III and its effects on platelet function were abolished in mice lacking the TP gene. This was also true of the effects of infused iPE(2)-III on mean arterial pressure and platelet aggregation.

CONCLUSIONS

Both iPF(2alpha)-III and iPE(2)-III exert their effects on platelet function and vascular tone in vivo by acting as incidental ligands at membrane TPs rather than via a distinct iP receptor. Activation of TPs by iPs may be of importance in syndromes in which cyclooxygenase activation and oxidant stress coincide, such as in atherosclerosis and reperfusion after tissue ischemia.

Quantitative high performance liquid chromatography/tandem mass spectrometric analysis of the four classes of F(2)-isoprostanes in human urine

Isoprostanes (iPs) are free radical catalyzed prostaglandin isomers. Analysis of individual isomers of PGF(2alpha)-F(2)-iPs-in urine has reflected lipid peroxidation in humans. However, up to 64 F(2)-iPs may be formed, and it is unknown whether coordinate generation, disposition, and excretion of F(2)-iPs occurs in humans. To address this issue, we developed methods to measure individual members of the four structural classes of F(2)-iPs, using liquid chromatography/tandem mass spectrometry (LC/MS/MS), in which sample preparation is minimized. Authentic standards of F(2)-iPs of classes III, IV, V, and VI were used to identify class-specific ions for multiple reaction monitoring. Using iPF(2alpha)-VI as a model compound, we demonstrated the reproducibility of the assay in human urine. Urinary levels of all F(2)-iPs measured were elevated in patients with familial hypercholesterolemia. However, only three of eight F(2)-iPs were elevated in patients with congestive heart failure, compared with controls. Paired analyses by GC/MS and LC/MS/MS of iPF(2alpha)-VI in hypercholesterolemia and of 8, 12-iso-iPF(2alpha)-VI in congestive heart failure were highly correlated. This approach will permit high throughput analysis of multiple iPs in human disease.

F(2)-isoprostanes: sensitive and specific non-invasive indices of lipid peroxidation in vivo

Isoprostanes are members of a complex family of lipids, isomers of the conventional enzymatically derived prostaglandins (PG), which are produced in vivo primarily, if not exclusively, by a free radical-catalyzed peroxidation of polyunsaturated fatty acids. Most of the work has been focused upon a group of isomers of the enzyme-derived PGF(2alpha), called F(2)-isoprostanes (F(2)-iPs). Because of their mechanism of formation, chemical stability and the rapid development of sensitive methods for their measurement, they have the attraction as non-invasive indices of oxidant stress in vivo. Altered generation of F(2)-iPs has been reported in a variety of clinical settings putatively associated with oxidant stress. These include atherosclerosis, chronic obstructive pulmonary disease and Alzheimer's disease. Furthermore, the measurement of specific F(2)-iPs may provide a sensitive biochemical basis for rational dose-selection of natural and synthetic inhibitor of lipid peroxidation. Although F(2)-iPs possess biological activities in vitro and in vivo, much remains to be learned about their role and as mediators of the cellular effects of lipid peroxidation and oxidant stress.

Alcohol-induced generation of lipid peroxidation products in humans

To address the hypothesis that elevated blood alcohol increases systemic oxidant stress, we measured urinary excretion of isoprostanes (iPs), free radical-catalyzed products of arachidonic acid. Ten healthy volunteers received acute doses of alcohol (Everclear-R) or placebo under randomized, controlled, double-blind conditions. Urinary iPF2a-III increased in a time- and dosage-dependent manner after dosing with alcohol, with the peak urinary iPF2a-III excretion correlating with the rise in blood alcohol. To determine whether oxidant stress was associated with alcohol-induced liver disease (ALD), we then studied the excretion of iP in individuals with a documented history of alcohol-induced hepatitis or alcohol-induced chronic liver disease (AC). Both urinary iPF2a-III and urinary iPF2a-VI were markedly increased in patients with acute alcoholic hepatitis. In general, urinary iPF2a-III was significantly elevated in cirrhotic patients, relative to controls, but excretion was more pronounced when cirrhosis was induced by alcohol than by hepatitis C. Excretion of iPF2a-VI, as well as 4-hydroxynonenal and the iPF2a-III metabolite, 2,3-dinor-5, 6-dihydro-iPF2a-III, was also increased in AC. Vitamin C, but not aspirin, reduced urinary iPs in AC. Thus, vasoactive iPs, which serve as indices of oxidant stress, are elevated in the urine in both acute and chronic ALD. Increased generation of iPs by alcohol in healthy volunteers is consistent with the hypothesis that oxidant stress precedes and contributes to the evolution of ALD.

An improved method for the measurement of urinary and plasma F2-isoprostanes using gas chromatography-mass spectrometry

We have developed an improved method for the measurement of F2-isoprostanes using stable isotope dilution capillary gas chromatography/electron capture negative ionization mass spectrometry (GC-ECNI-MS). The F2-isoprostane family consists of a series of chemically stable prostaglandin F2 (PGF2)-like compounds generated during peroxidation of arachidonic acid in phospholipids. There is evidence that measurement of F2-isoprostanes represents a reliable and useful index of lipid peroxidation and oxidant stress in vivo. Furthermore, 8-epi-PGF2alpha, which is one of the more abundant F2-isoprostanes, is biologically active, being a potent mitogen and vasoconstrictor of rat and rabbit lung and kidney, as well as a partial agonist of platelet aggregation. Measurement of F2-isoprostanes in biological samples is complex and has involved methods which utilize multiple chromatographic steps, including separation by thin-layer chromatography, leading to poor sample recovery. We now present an improved method for the measurement of plasma and urinary F2-isoprostanes using a combination of silica and reverse-phase extraction cartridges, high-performance liquid chromatography (HPLC), and GC-ECNI-MS. Different approaches to the derivatization of the F2-isoprostanes prior to GC-ECNI-MS are also addressed. The overall recovery of F2-isoprostanes is improved (approx 70% for urine) and the within and between assay reproducibility is 6.7% (n = 23) and 3.7% (n = 3), respectively. The mean urinary excretion of F2-isoprostanes in eight healthy males was 365 +/- 5 pmol/mmol creatinine and in three smokers 981 +/- 138 pmol/mmol creatinine. The mean total (free + esterified) plasma F2-isoprostane concentration was 952 +/- 38 pmol/liter, with a within and between assay reproducibility of 8% (n = 13) and 5.6% (n = 3), respectively. This improved method for the measurement of F2-isoprostanes represents a significant advance in terms of the rapidity and yield in the purification of biological samples. The inclusion of HPLC separation enables improved analysis of F2-isoprostanes by GC-MS. This methodology will assist in defining the role of F2-isoprostanes as in vivo markers of oxidant stress in clinical and experimental settings.

Demyelination: the role of reactive oxygen and nitrogen species

This review summarises the role that reactive oxygen and nitrogen species play in demyelination, such as that occurring in the inflammatory demyelinating disorders multiple sclerosis and Guillain-Barré syndrome. The concentrations of reactive oxygen and nitrogen species (e.g. superoxide, nitric oxide and peroxynitrite) can increase dramatically under conditions such as inflammation, and this can overwhelm the inherent antioxidant defences within lesions. Such oxidative and/or nitrative stress can damage the lipids, proteins and nucleic acids of cells and mitochondria, potentially causing cell death. Oligodendrocytes are more sensitive to oxidative and nitrative stress in vitro than are astrocytes and microglia, seemingly due to a diminished capacity for antioxidant defence, and the presence of raised risk factors, including a high iron content. Oxidative and nitrative stress might therefore result in vivo in selective oligodendrocyte death, and thereby demyelination. The reactive species may also damage the myelin sheath, promoting its attack by macrophages. Damage can occur directly by lipid peroxidation, and indirectly by the activation of proteases and phospholipase A2. Evidence for the existence of oxidative and nitrative stress within inflammatory demyelinating lesions includes the presence of both lipid and protein peroxides, and nitrotyrosine (a marker for peroxynitrite formation). The neurological deficit resulting from experimental autoimmune demyelinating disease has generally been reduced by trial therapies intended to diminish the concentration of reactive oxygen species. However, therapies aimed at diminishing reactive nitrogen species have had a more variable outcome, sometimes exacerbating disease.

Increased formation of distinct F2 isoprostanes in hypercholesterolemia

BACKGROUND

F2 isoprostanes are stable, free radical-catalyzed products of arachidonic acid that reflect lipid peroxidation in vivo.

METHODS AND RESULTS

Specific assays were developed by use of mass spectrometry for the F2 isoprostanes iPF2alpha-III and iPF2alpha-VI and arachidonic acid (AA). Urinary excretion of the 2 F2 isoprostanes was significantly increased in hypercholesterolemic patients, whereas substrate AA in urine did not differ between the groups. iPF2alpha-III (pmol/mmol creatinine) was elevated (P<0.0005) in homozygous familial hypercholesterolemic (HFH) patients (85+/-5. 5; n=38) compared with age- and sex-matched normocholesterolemic control subjects (58+/-4.2; n=38), as were levels of iPF2alpha-VI (281+/-22 versus 175+/-13; P<0.0005). Serum cholesterol correlated with urinary iPF2alpha-III (r=0.41; P<0.02) and iPF2alpha-VI (r=0. 39; P<0.03) in HFH patients. Urinary excretion of iPF2alpha-III (81+/-10 versus 59+/-4; P<0.05) and iPF2alpha-VI (195+/-18 versus 149+/-20; P<0.05) was also increased in moderately hypercholesterolemic subjects (n=24) compared with their controls. Urinary excretion of iPF2alpha-III and iPF2alpha-VI was correlated (r=0.57; P<0.0001; n=106). LDL iPF2alpha-III levels (ng/mg arachidonate) were elevated (P<0.01) in HFH patients (0.32+/-0.08) compared with controls (0.09+/-0.02). The concentrations of iPF2-III in LDL and urine were significantly correlated (r=0.42; P<0.05) in HFH patients.

CONCLUSIONS

Asymptomatic patients with moderate and severe hypercholesterolemia have evidence of oxidant stress in vivo.

Linoleic acid peroxidation--the dominant lipid peroxidation process in low density lipoprotein--and its relationship to chronic diseases

Modern separation and identification methods enable detailed insight in lipid peroxidation (LPO) processes. The following deductions can be made: (1) Cell injury activates enzymes: lipoxygenases generate lipid hydroperoxides (LOOHs), proteases liberate Fe ions--these two processes are prerequisites to produce radicals. (2) Radicals attack any activated CH2-group of polyunsaturated fatty acids (PUFAs) with about a similar probability. Since linoleic acid (LA) is the most abundant PUFA in mammals, its LPO products dominate. (3) LOOHs are easily reduced in biological surroundings to corresponding hydroxy acids (LOHs). LOHs derived from LA, hydroxyoctadecadienoic acids (HODEs), surmount other markers of LPO. HODEs are of high physiological relevance. (4) In some diseases characterized by inflammation or cell injury HODEs are present in low density lipoproteins (LDL) at 10-100 higher concentration, compared to LDL from healthy individuals.

IPF2alpha-I: an index of lipid peroxidation in humans

Isoprostanes are prostaglandin isomers produced from arachidonic acid by a free radical-catalyzed mechanism. Urinary excretion of 8-iso-prostaglandin F2alpha, an isomer of the PGG/H synthase (cyclooxygenase or COX) enzyme product, prostaglandin F2alpha (PGF2alpha), has exhibited promise as an index of oxidant stress in vivo. We have developed a quantitative method to measure isoprostane F2alpha-I, (IPF2alpha-I) a class I isomer (8-iso-PGF2alpha is class IV), using gas chromatography/mass spectrometry. IPF2alpha-I is severalfold as abundant in human urine as 8-iso-PGF2alpha, with mean values of 737 +/- 20.6 pg/mg creatinine. Both isoprostanes are formed in a free radical-dependent manner in low density lipoprotein oxidized by copper in vitro. However, IPF2alpha-I, unlike 8-iso-PGF2alpha, is not formed in a COX-dependent manner by platelets activated by thrombin or collagen in vitro. Similarly, COX inhibition in vivo has no effect on IPF2alpha-I. Neither serum IPF2alpha-I, an index of cellular capacity to generate the isoprostane, nor urinary excretion of IPF2alpha-I, an index of actual generation in vivo, is depressed by aspirin or indomethacin. In contrast, both serum thromboxane B2 and urinary excretion of its 11-dehydro metabolite are depressed by the COX inhibitors. Although serum 8-iso-PGF2alpha formation is substantially depressed by COX inhibitors, urinary excretion of the compound is unaffected. Urinary IPF2alpha-I is elevated in cigarette smokers compared with controls (1525 +/- 180 versus 740 +/- 40 pg/mg creatinine; P < 0.01) and is highly correlated with urinary 8-iso-PGF2alpha (r = 0.9; P < 0.001). Urinary IPF2alpha-I is a novel index of lipid peroxidation in vivo, which can be measured with precision and sensitivity. It is an abundant F2-isoprostane formed in a free radical- but not COX-dependent manner. Although 8-iso-PGF2alpha may be formed as a minor product of COX, this pathway contributes trivially, if at all, to levels in urine. Urinary excretion of both isoprostanes is elevated in cigarette smokers.

Eicosanoids and isoeicosanoids: indices of cellular function and oxidant stress

Arachidonic acid (AA) is an unsaturated fatty acid constituent of the phospholipid domain of cell membranes. It is subject to release via mobilization of phospholipases, particularly a cytoplasmic phospholipase A2. Thereafter, it may be metabolized by at least two cyclooxygenase (COX) isoforms to prostaglandins and related compounds, via lipoxygenases to leukotrienes and via p450-catalyzed metabolism to epoxyeicosatrienoic acids. Collectively, these bioactive lipids are termed eicosanoids. All of these lipids express potent bioactivity in vitro. Clinical studies have already demonstrated the importance of COX and lipoxygenase (LOX) products in human disease. The generation of models of COX, LOX and prostaglandin receptor gene inactivation is likely to broaden our insight into the importance of these compounds in vivo. Crystallization of the biosynthetic enzymes is likely to facilitate the development of highly specific inhibitors, as is the case already for COX-2. AA possesses intrinsic biological properties. It is also subject to free radical attack, generating isomeric eicosanoid species, the isoeicosanoids. These compounds may also express biological activity in vitro, although their importance in vivo is unclear. Development of specific assays for these compounds in urine suggests their utility as noninvasive indices of oxidant stress in vivo.

Increased formation of the isoprostanes IPF2alpha-I and 8-epi-prostaglandin F2alpha in acute coronary angioplasty: evidence for oxidant stress during coronary reperfusion in humans

BACKGROUND

The role of oxidant stress in cardiac ischemia/reperfusion injury in humans remains controversial. This is due, in part, to the limitations of available indices of oxidant stress in vivo. Isoprostanes are stable, free radical-catalyzed products of arachidonic acid. We assessed their formation in patients undergoing coronary reperfusion via percutaneous transluminal coronary angioplasty (PTCA).

METHODS AND RESULTS

We developed specific, mass spectrometry assays for two structurally distinct F2 isoprostanes, 8-epi-PGF2alpha and IPF2alpha-I. Urine samples for isoprostane determination were collected in patients undergoing coronary arteriography (n=11), elective PTCA (n=15), and angiography after thrombolysis for acute myocardial infarction (MI) (n=10). Urinary levels (pmol/mmol creatinine) of both isoprostanes were markedly increased from baseline in the first 6 hours after PTCA for acute MI (105+/-17.8 versus 230+/-66 for 8-epi-PGF2alpha [P=.009] and 466+/-91 versus 833+/-153 for IPF2alpha-I [P=.001]) and returned toward preprocedural values by 24 hours (122+/-18 for 8-epi-PGF2alpha and 457+/-102 for IPF2alpha-I). There was a slight increase in urinary 8-epi-PGF2alpha levels (64.7+/-9.5 versus 84.9+/-10.6; P=.02) after diagnostic coronary arteriography and elective PTCA (88.7+/-7.5 versus 114.3+/-16.1; P=.01). A striking correlation was observed (r=.68, P<.0001; n=33) between urinary 8-epi-PGF2alpha and IPF2alpha-I levels in patients receiving thrombolytic agents for acute MI.

CONCLUSIONS

Urinary F2 isoprostane levels are elevated in patients after treatments resulting in reperfusion for acute MI. These findings provide evidence consistent with increased oxidant stress in vivo in this setting. Measurement of urinary isoprostanes may offer a noninvasive approach to the assessment of oxidant stress and the efficacy of antioxidant therapies in these syndromes.

Functional characterization of the ocular prostaglandin f2alpha (PGF2alpha) receptor. Activation by the isoprostane, 12-iso-PGF2alpha

Prostaglandin F2alpha (PGF2alpha) is a product of cyclooxygenase-catalyzed metabolism of arachidonic acid. Recently, PGF2alpha analogs have been hypothesized to reduce intraocular pressure via relaxation of the ciliary muscle. To investigate the molecular basis of PGF2alpha receptor (FP) activation in the eye, we cloned the FP from a human ciliary body (hcb) cDNA library. The open reading frame of the hcb-FP cDNA was identical to the uterine FP cDNA. The hcb-FP appeared to be predominantly membrane-localized, as visualized by an FP-specific peptide antibody, and coupled to inositol phosphate formation when stably expressed in HEK 293 cells. Interestingly, the hcb-FP could also be activated by the F2 isoprostane, 12-iso-PGF2alpha, in addition to its cognate ligand, PGF2alpha. 12-iso-PGF2alpha was less potent (EC50 = 5 microM) than PGF2alpha (EC50 = 10 nM) in generating inositol phosphates via the hcb-FP in HEK 293 cells. Both ligands also stimulated mitogenesis in NIH 3T3 cells. Although 12-iso-PGF2alpha caused a dose-dependent activation of the FP, it failed to activate the recombinant human prostacyclin receptor and caused only minimal activation of the thromboxane receptor isoforms stably expressed in HEK 293 cells. Four additional F2 isoprostanes, 8-iso-PGF2alpha, IPF2alpha-I, IPF2alpha-III, and 9beta,11beta-PGF2, caused trivial, or no, activation of the FP. Consistent with these observations, only PGF2alpha and 12-iso-PGF2alpha caused rapid homologous desensitization of FP and also exhibited cross-desensitization, with PGF2alpha resulting in a maximum of approximately 60% desensitization. The human FP may thus be activated specifically, by the free radical-catalyzed F2 isoprostane, 12-iso-PGF2alpha, in addition to the cyclooxygenase product, PGF2alpha. Incidental receptor activation by isoprostanes may complement the actions of PGF2alpha in clinical syndromes where oxidant stress and augmented prostaglandin biosynthesis coincide.

Evidence for the formation of F3-isoprostanes during peroxidation of eicosapentaenoic acid

8-Epi PGF2alpha, a potent vasocontrictor, is a specific product of non-enzymatic peroxidation of arachidonic acid. It seems likely that similar products could arise from other polyunsaturated fatty acids (PUFAs) and might be useful biomarkers of their peroxidation in vivo. This was investigated using eicosapentaenoic acid (EPA). EPA liposomes (1 mg/ml PBS) were exposed at 37 degrees C to either 2,2'-azobis-(2-amidinopropane) dichloride (AAPH) or copper ions at final concentrations of 1 mM and 10 microM, respectively. Sample processing involved solid-phase extraction on a C18-followed by an NH2 cartridge. After conversion to pentafluorobenzyl ester/trimethylsilyl derivatives, F3-isoprostanes were analysed by negative ion-chemical ionisation mass spectrometry (GC-MS/NICI) using tetradeuterated PGF2alpha (PGF2-d4) as the internal standard. Quantitative analysis was carried out by selected ion monitoring of the carboxylated anion [M-180] at m/z 567 and 573 for the PGF3-like compounds and PGF2-d4, respectively. EPA oxidised by AAPH or by copper ions gave rise to a family of F3-isoprostanes with 8-epi PGF3alpha as a minor product. Formation of F3-isoprostanes correlated well with other indices of lipid peroxidation (hydroperoxides and thiobarbituric acid reactive substances). The possibility of analysing specific lipid peroxidation products from individual fatty acids should facilitate nutritional and biomedical studies.

Novel indices of oxidant stress in cardiovascular disease: specific analysis of F2-isoprostanes

The development of methods to measure specific isoprostanes affords a unique opportunity to investigate both the role of oxidant stress as a mechanism of disease in vivo and to select rational doses of putative antioxidant drugs and vitamins for evaluation in human disease. The ability to measure these compounds directly in situ at the site of their formation, to immunolocalize them to target cells in atherosclerotic plaque and other tissues (61) and to assess their biosynthesis non-invasively in urine promises to elucidate the role of lipid peroxidation in cardiovascular disease.

Identification and relative quantitation of F2-isoprostane regioisomers formed in vivo in the rat

F2-isoprostanes are a complex mixture of isomers formed in four regioisomeric family types by free radical-initiated oxidation of arachidonic acid present in membrane phospholipids. F2-isoprostanes isolated from the livers of rats treated with carbon tetrachloride were separated by initial reverse phase HPLC and detected using electrospray ionization mass spectrometry with the characteristic loss of 44 u (C2H4O) from the common 1,3-diol cyclopropane ring found in these eicosanoids. Collision induced decomposition of the carboxylate anions from the separated F2-isoprostanes formed abundant ions characteristic for regioisomers of Type I (m/z 115), Type III (m/z 127), and Type IV (m/z 193), which made possible characterization of these three family subtypes by LC/MS/MS. Capillary GC/MS was employed to further identify the F2-isoprostane regioisomers using electron ionization mass spectrometry and to obtain characteristic mass spectra of the pentafluorobenzyl ester trimethylsilyl ether derivatives. Quantitation of the F2-isoprostanes separated by both reverse-phase HPLC and capillary GC/MS was carried out using negative ion chemical ionization mass spectrometry. The most abundant isomers identified were Type I and IV regioisomers constituting 33 and 25% of the total products, respectively. As expected, the Type II and III regioisomer products were of less abundance. Over 45 F2-isoprostanes could be separated in this complex mixture, suggesting random production of each regioisomeric subtype in this in vivo model.

Nonenzymatic free radical-catalyzed generation of thromboxane-like compounds (isothromboxanes) in vivo

The isoprostanes (IsoPs) are novel bioactive prostaglandin-like compounds produced in vivo by free radical-catalyzed peroxidation of arachidonyl-containing lipids. Previously, we have identified IsoPs containing F-type and D- and E-type prostane rings that are formed by reduction and rearrangement of IsoP endoperoxide intermediates, respectively. We now explore whether thromboxane B2 (TxB2)-like compounds, termed B2-isothromboxanes (B2-IsoTxs), are formed by rearrangement of IsoP endoperoxides. Detection of these compounds was carried out using a stable isotope dilution mass spectrometric assay originally developed for quantification of cyclooxygenase-derived TxB2. Incubations of arachidonic acid with Fe/ADP/ascorbate for 30 min in vitro generated a series of peaks representing putative B2-IsoTx at levels of 62.4 +/- 21.0 ng/mg arachidonate. Using various chemical modification and derivatization approaches, it was determined that these compounds contained hemiacetal ring structures and two double bonds, as would be expected for B2-IsoTx. Analysis of the compounds by electron ionization mass spectrometry yielded multiple mass spectra similar to those of TxB2. B2-IsoTxs are also formed esterified to phospholipids; oxidation of arachidonyl-containing phosphatidylcholine in vitro followed by hydrolysis resulted in the release of large amounts of these compounds. To explore whether B2-IsoTxs are also formed in vivo, a well characterized animal model of lipid peroxidation consisting of orogastric administration of CCl4 to rats was used. Levels of B2-IsoTx esterified in lipids in the liver increased 41-fold from 2.5 +/- 0.5 to 102 +/- 30 ng/g of liver. In addition, circulating levels of free compounds increased from undetectable (<5 pg/ml) to 185 +/- 30 pg/ml after CCl4, a 37-fold increase. Thus, we have provided evidence that IsoTxs constitute another novel class of eicosanoids produced in vivo nonenzymatically by free radical-catalyzed lipid peroxidation. These studies thus expand our understanding of products of lipid peroxidation formed in vivo from the free radical-catalyzed peroxidation of arachidonic acid.