Separation and identification of F2-isoprostane regioisomers and diastereomers by novel liquid chromatographic/mass spectrometric methods

Isoprostanes are isomers of prostaglandins that are generated from free radical-initiated autoxidation of arachidonic acid. Quantification of F(2)-isoprostanes is regarded as the "gold standard" to assess oxidative stress in various human diseases. There are 32 possible racemic isoprostane isomers that exist as four sets of regioisomers. Each regioisomer is composed of eight diastereomers. We report liquid chromatographic/mass spectrometric methods to separate and identify F(2)-isoprostane stereoisomers. These methods have been applied to the analysis of F(2)-isoprostanes derived from tissues of rats exposed to an oxidative stress and are useful to assess the relative formation of various regioisomers and stereoisomers generated in vitro and in vivo. The delineation of the more abundant isomers formed will allow for studies to examine the biological relevance of selected compounds in vivo.

Lipid-Soluble 3-Pyridinol Antioxidants Spare α-Tocopherol and Do Not Efficiently Mediate Peroxidation of Cholesterol Esters in Human Low-Density Lipoprotein

Because alpha-tocopherol (alpha-TOH) mediates the peroxidation of cholesterol-esterified lipids in human low-density lipoprotein (LDL) in vitro and has displayed disappointing results against the onset and development of atherosclerosis, it may not be appropriate for use as a therapeutic in the prevention and/or treatment of the disease. Herein are described the experimental results demonstrating that 3-pyridinols spare alpha-TOH, do not efficiently mediate lipid peroxidation, and protect lipoprotein tryptophan residues in human LDL.

Nonacid cleavable detergents applied to MALDI mass spectrometry profiling of whole cells

Although cleavable detergents were first synthesized a number of years ago, they have only recently been successfully applied to problems involving biological molecules. Recent reports have demonstrated that these compounds are useful for applications involving both 2D PAGE and mass spectrometry. However, most cleavable surfactants have utilized acid-labile functional groups to affect cleavage. In applications where extreme pH is required, acid cleavable detergents have limited usefulness. We report the synthesis of fluoride cleavable silane compounds and photolabile cinnamate esters as cleavable detergents having alternative cleavage chemistries than previously reported cleavable detergents. These compounds were applied to whole cell analysis using MALDI mass spectrometry, and it was demonstrated that their use results in an increase in the number of proteins analyzed by increasing protein solubility.

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.

Peroxidation of Polyunsaturated Fatty Acid Methyl Esters Catalyzed by N-Methyl Benzohydroxamic Acid: A New and Convenient Method for Selective Synthesis of Hydroperoxides and Alcohols

A new catalytic system, based on N-methyl benzohydroxamic acid (NMBHA), was developed for the selective synthesis of lipid hydroperoxides and alcohols by aerobic oxidation of lipid precursors. The reactions were carried out under mild conditions with air at atmospheric pressure and 37 degrees C. The products were isolated in much better yields (50-60%) as compared to previously reported procedures. NMBHA can be recovered at the end of the reaction and recycled. The putative reactive intermediate, a nitroxyl radical, readily abstracts a hydrogen atom from polyunsaturated lipid precursors such as linoleate esters, and NMBHA itself is an excellent hydrogen atom donor, thus conferring high selectivity to oxidations catalyzed by this reagent.

Identification and analysis of products formed from phospholipids in the free radical oxidation of human low density lipoproteins

Phospholipids reside in the surface layer of LDLs and constitute approximately 20-25% of the particle by weight. We report a study of the primary products generated from the most abundant molecular species of phosphatidylcholines present in LDL during in vitro free radical oxidations. The 13-hydroperoxides of 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphocholine (PLPC) and 1-stearoyl-2-linoleoyl-sn-glycero-phosphocholine (SLPC) and the 15-hydroperoxides of 1-palmitoyl-2-arachidonyl-sn-glycero-3-phosphocholine (PAPC) and 1-stearoyl-2-arachidonoyl-sn-glycero-phosphocholine (SAPC) were found to increase in a time-dependent manner and in significant amounts even in the presence of alpha-tocopherol. Phospholipid alcohols also formed during the course of the oxidations. Early in the LDL oxidations, while alpha-tocopherol was still present, the thermodynamically favored trans,trans products of PLPC and SLPC were found to form in significantly larger quantities than those formed from cholesteryl linoleate. Additionally, quantities of PAPC 11-hydroperoxide (11-OOH) decreased over time relative to PAPC 15-OOH, even while alpha-tocopherol was still present in the oxidation, presumably as a result of further oxidation of PAPC 11-OOH to form cyclic peroxide oxidation products. These results suggest that alpha-tocopherol is more closely associated with the inner cholesteryl ester-rich hydrophobic core of an LDL particle and is not as effective as an antioxidant in the outer phospholipid layer as it is in the lipid core.

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.

Synthesis and Reactivity of Some 6-Substituted-2,4-dimethyl-3-pyridinols, a Novel Class of Chain-Breaking Antioxidants

The synthesis and study of a series of 6-substituted-2,4-dimethyl-3-pyridinols having interesting antioxidant properties is reported. The general synthetic strategy leading to the compounds involved a low-temperature aryl bromide-to-alcohol conversion as the last step. 2,4-dimethyl-3-pyridinol (1a), 2,4,6-trimethyl-3-pyridinol (1b), and 2,4-dimethyl-6-(dimethylamino)-3-pyridinol (1d) were thus prepared from the corresponding 3-bromopyridine precursor. The methoxy derivative 2,4-dimethyl-6-(methoxy)-3-pyridinol (1c) was also prepared by an alternate route via a Baeyer-Villiger reaction on the substituted benzaldehyde precursor. Novel bicyclic pyridinols 2 and 3 required prior construction of the ring structure. Thus, 2 was prepared by the use of a 6-step intramolecular Friedel-Crafts strategy, and 3 required an 11-step sequence with a thermolytic intramolecular inverse-demand Diels-Alder reaction between a pyrimidine ring and an alkyne as the key step. Basicities of the pyridinols approached physiological pH with increasing electron density in the ring. Pyridinols 1a-d were found to be indefinitely stable to air oxidation while 2 and 3 decomposed upon extended exposure to the atmosphere. The reactivities of the pyridinols toward chain-carrying peroxyl radicals in homogeneous organic solution were examined by studying the kinetics of radical-initiated styrene autoxidations under controlled conditions. These experiments revealed that some of the newly synthesized pyridinols are the most effective phenolic chain-breaking antioxidants reported to date.

Mass spectrometry of intracellular and membrane proteins using cleavable detergents

Detergents have been used to enhance the solubility of hydrophobic biomolecules for decades. Despite the widespread use of detergents in biochemistry, the presence of these molecules often complicates further analysis by mass spectrometry. This study presents a solution to this problem by outlining a method utilizing a novel cleavable detergent, 3-[3-(1,1-bisalkyloxyethyl)pyridin-1-yl]propane-1-sulfonate (PPS). This detergent can be used to extract protein contained within the interior of the cell by disrupting cell membranes. Once the proteins are free from the cell, PPS also assists in protein solubilization by shielding the hydrophobic regions of the newly extracted protein from unfavorable interactions with water. The added advantage of PPS over conventional detergents such as sodium dodecyl sulfate or n-octylglucoside is that the detergent properties that interfere with MALDI mass spectrometry can be eliminated prior to analysis. PPS was found to improve sensitivity in MALDI analyses of both soluble proteins and membrane proteins without degrading spectral quality. The virtues of this strategy were applied to whole cell extracts. Analysis of these extracts resulted in an overall increase in both the number of peaks observed and overall signal intensity.

Autoxidative transformation of chiral omega6 hydroxy linoleic and arachidonic acids to chiral 4-hydroxy-2E-nonenal

Recently, we established that 13S-hydroperoxy-linoleic acid is converted to 4S-hydroperoxy-nonenal (4S-HPNE) during autoxidation, implicating hydrogen abstraction from C-8 as an initiating step [Schneider, C., et al. (2001) J. Biol. Chem. 275, 20831-20838]. On the basis of the proposed mechanism, an equivalent initiating reaction could occur from the corresponding 13S-hydroxy acid. Herein, we examined the outcome of autoxidation reactions of the omega6 hydroxy fatty acids, 13S-hydroxyoctadecadienoic acid and 15S-hydroxyeicosatetraenoic acid, as compared with reactions of the corresponding hydroperoxy substrates. Autoxidation of the hydroxy starting materials (37 degrees C, dry film) yielded 4-hydroxy-nonenal (4-HNE) as a prominent polar metabolite (and not the 4-hydroperoxide), whereas the hydroperoxide starting materials gave rise to 4-HPNE. Stereochemical analysis showed that the optical purity of the 4-hydroxy group of 4-HNE precisely matched the optical purity of the 15S- or 13S-hydroxy group of the starting fatty acid substrate (98 and 90% S, respectively). The hydroperoxide 15S-HPETE (98% 15S) gave rise to 4S-HPNE, also with retention of optical purity (98% 4S). The preservation of stereochemical configuration provides evidence that aldehyde formation does not involve participation of the hydro(pero)xy group and indicates a similar mechanism for the formation of 4-HNE and 4-HPNE during autoxidation of omega6 hydro(pero)xy fatty acids. Our results establish, moreover, that omega6 hydroxy fatty acids are potential precursors of reactive cytotoxic aldehydes in biological systems.

Factors Influencing the Autoxidation of Fatty Acids: Effect of Olefin Geometry of the Nonconjugated Diene

Autoxidations of cis,cis, cis,trans, and trans,trans nonconjugated octadecadienoates and pentadecadienes were carried out in the presence of alpha-tocopherol to investigate the effect of olefin geometry on this oxidation process and provide insight into the factors that influence the autoxidation of fatty acids. We have found that as the trans character of the diene increases, the amount of O(2) trapping at the central (bis-allylic) position of the pentadienyl radical also increases. In addition, the rate constant for beta-fragmentation (k(beta) approximately 10(6) s(-1)) of the bis-allylic peroxyl radical decreased on going from the cis,cis to the trans,trans diene. We have also found that for the cis,trans nonconjugated dienes, there is a preference for trapping of the pentadienyl radical by O(2) at the transoid end, generating the cis,trans conjugated hydroperoxide as the major product.

Mechanism of beta-silyl diacyl peroxide decomposition: a mild and stereoselective synthesis of beta-silyl esters

A novel method for the formation of beta-silyl esters is presented. Mechanistic studies were carried out on the formation and decomposition of beta-silyl diacyl peroxides, showing that the decomposition pathway involves an ionic mechanism that is influenced by the presence of the beta-silyl moiety. These studies exclude a free radical decomposition pathway as evidenced by the absence of chemically induced dynamic nuclear polarization (CIDNP) during the reaction and a strong correlation of the resulting regioisomeric product distribution to sigma(+). This reaction allows for the formation of beta-silyl esters in 45-50% isolated yield with predictable regioselectivity and good to excellent diastereoselectivity. Studies demonstrate that ester products which are formed at benzylic centers have the erythro configuration, whereas ester products formed at alkyl centers have the threo configuration.

Lysine peroxycarbamates: free radical-promoted peptide cleavage

Strategies are reported that combine in one step a predictable chemical-based protein digestion with mass spectrometry. Lysine residue amino groups in peptides and proteins are modified by reaction with a peroxycarbonate derived from p-nitrophenol, and tert-butyl hydroperoxide. The peroxycarbonate reacts with lysine residues in peptides and proteins, and the resulting lysine peroxycarbamates undergo homolytic fragmentation under conditions of low-energy collision-induced dissociation (CID). Observed fragmentation of the peptides involves apparent free radical processes including Hofmann-Löffler-type rearrangements that lead to peptide chain fragmentation. Strategies for directed cleavage of peptides by free radical promoted processes are feasible, and such strategies may well simplify schemes for protein analysis.

Liquid chromatography coordination ion-spray mass spectrometry (LC?CIS?MS) of docosahexaenoate ester hydroperoxides

Coordination ion-spray mass spectrometry (CIS-MS) is a useful tool in the detection and identification of complex mixtures of cholesterol ester and phospholipid hydroperoxides. The methyl ester, cholesterol ester, and phospholipid hydroperoxides of docosahexaenoic acid were analyzed by LC-CIS-MS and their elution orders were identified. Their corresponding alcohols were also identified. The methyl hydroperoxydocosahexaenoate (HPDHE) elution order is 14, 17, 16, 13, 20, 11, 10, 4, 7, 8 while the methyl hydroxydocosahexaenoate (HDHE) elution order is 14 > or = 17, 16, 13, 11, 10, 20, 7, 8, 4. The cholesteryl HPDHE elution order is 14, 17, 16, 13, 20, 11 > or = 10, 4, 7, 8 and the cholesteryl HDHE elution order is 17, 14, 16, 13, 11, 20, 10, 7, 8, 4. The elution order of the 1-palmitoyl-2-docosahexaenoyl-sn-glycero-3-phosphatidylcholine (PDPC) hydroperoxides and alcohols is 20, 16, 17, 13, 14, 10, 11, 7, 8, 4.

Quantification of F-ring isoprostane-like compounds (F4-neuroprostanes) derived from docosahexaenoic acid in vivo in humans by a stable isotope dilution mass spectrometric assay

Lipid peroxidation has been implicated in the pathophysiological sequelae of human neurodegenerative disorders. It is recognized that quantification of lipid peroxidation is best assessed in vivo by measuring a series of prostaglandin (PG) F2-like compounds termed F2-isoprostanes (IsoPs) in tissues in which arachidonic acid is abundant. Unlike other organs, the major polyunsaturated fatty acid (PUFA) in the brain is docosahexaenoic acid (DHA, C22:6 omega-6), and this fatty acid is particularly enriched in neurons. We have previously reported that DHA undergoes oxidation in vitro and in vivo resulting in the formation of a series of F2-IsoP-like compounds termed F4-neuroprostanes (F4-NPs). We recently chemically synthesized one F4-NP, 17-F4c-NP, converted it to an 18O-labeled derivative, and utilized it as an internal standard to develop an assay to quantify endogenous production of F4-NPs by gas chromatography (GC)/negative ion chemical ionization (NICI) mass spectrometry (MS). The assay is highly precise and accurate. The lower limit of sensitivity is approximately 10 pg. Levels of F4-NPs in brain tissue from rodents were 8.7 +/- 2.0 ng/g wet weight (mean +/- S.D.). Levels of the F4-NPs in brains from normal humans were found to be 4.9 +/- 0.6 ng/g (mean +/- S.D.) and were 2.1-fold higher in affected regions of brains from humans with Alzheimer's disease (P = 0.02). Thus, this assay provides a sensitive and accurate method to assess oxidation of DHA in animal and human tissues and will allow for the further elucidation of the role of oxidative injury to the central nervous system in association with human neurodegenerative disorders.

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.

Formation of prostaglandins E2 and D2 via the isoprostane pathway: a mechanism for the generation of bioactive prostaglandins independent of cyclooxygenase

It has heretofore been assumed that the cyclooxygenases (COXs) are solely responsible for peostaglandin (PG) synthesis in vivo. An important structural feature of PGH2 formed by COX is the trans-configuration of side chains relative to the prostane ring. Previously, we reported that a series of PG-like compounds termed isoprostanes (IsoPs) are formed in vivo in humans from the free radical-catalyzed peroxidation of arachidonate independent of COX. A major difference between these compounds and PGs is that IsoPs are formed from endoperoxide intermediates, the vast majority of which contain side chains that are cis relative to the prostane ring. In addition, unlike the formation of eicosanoids from COX, IsoPs are formed as racemic mixtures because they are generated nonenzymatically. IsoPs containing E- and D-type prostane rings (E2/D2-IsoPs) are one class of IsoPs formed, and we have reported previously that one of the major IsoPs generated is 15-E2t-IsoP (8-iso-PGE2). Unlike PGE2, 15-E2t-IsoP is significantly more unstable in buffered solutions in vitro and undergoes epimerization to PGE2. Analogously, the D-ring IsoP (15-D2c-IsoP) would be predicted to rearrange to PGD2. We now report that compounds identical in all respects to PGE2 and PGD2 and their respective enantiomers are generated in vivo via the IsoP pathway, presumably by epimerization of racemic 15-E2t-IsoP and 15-D2c-IsoP, respectively. Racemic PGE2 and PGD2 were present esterified in phospholipids derived from liver tissue from rats exposed to oxidant stress at levels of 24 +/- 16 and 37 +/- 12 ng/g of tissue, respectively. In addition, racemic PGs, particularly PGD2, were present unesterified in urine from normal animals and humans and represented up to 10% of the total PG detected. Levels of racemic PGD2 increased 35-fold after treatment of rats with carbon tetrachloride to induce oxidant stress. In this setting, PGD2 and its enantiomer generated by the IsoP pathway represented approximately 30% of the total PGD2 present in urine. These findings strongly support the contention that a second pathway exists for the formation of bioactive PGs in vivo that is independent of COX.