Synthesis of six epoxyketooctadecenoic acid (EKODE) isomers, their generation from nonenzymatic oxidation of linoleic acid, and their reactivity with imidazole nucleophiles

Determining the structure of protein-bound ceramides, essential lipids for skin barrier function

Protein-bound ceramides, specialized ceramides covalently bound to corneocyte surface proteins, are essential for skin permeability barrier function. However, their exact structure and target amino acid residues are unknown. Here, we found that epoxy-enone (EE) ceramides, precursors of protein-bound ceramides, as well as their synthetic analog, formed stable conjugates only with Cys among nucleophilic amino acids. NMR spectroscopy revealed that the β-carbon of the enone was attached by the thiol group of Cys via a Michael addition reaction. We confirmed the presence of Cys-bound EE ceramides in mouse epidermis by mass spectrometry analysis of protease-digested epidermis samples. EE ceramides were reversibly released from protein-bound ceramides via sulfoxide elimination. We found that protein-bound ceramides with reversible release properties accounted for approximately 60% of total protein-bound ceramides, indicating that Cys-bound EE ceramides are the predominant protein-bound ceramides. Our findings provide clues to the molecular mechanism of skin barrier formation by protein-bound ceramides.

Alpha-Tocopherol, a Powerful Molecule, Leads to the Formation of Oxylipins in Polyunsaturated Oils Differently to the Temperature Increase: A Detailed Study by Proton Nuclear Magnetic Resonance of Walnut Oil Oxidation

Lipid oxidation causes food degradation and the formation of toxic compounds. Therefore, the addition to foods of compounds able to avoid, delay or minimize this degradative process is a commonly used strategy. Nevertheless, neither the identity of most of the formed compounds in this complex process nor the way in which their formation is affected by the strategy used are well known. In this context, the effect the temperature increase and the enrichment level in alpha-tocopherol on the evolution of the walnut oil oxidation, as a model of an oil rich in polyunsaturated omega-6 acyl groups, submitted to storage conditions, are tackled by ¹H NMR. The study has allowed knowing the degradation kinetic of both the oil acyl groups and alpha-tocopherol, the identification of a very high number of oxylipins and the kinetic of their formation. The temperature increase accelerates the formation of all oxylipins, favouring the formation of hydroperoxy conjugated E,E-dienes and related derivatives versus that of the Z,E-isomers. The enrichment in alpha-tocopherol accelerates the formation of hydroperoxy conjugated Z,E-dienes and related derivatives, and delays in relation to the formation of the former that of the E,E-isomers and related derivatives, hindering, to a certain extent, the formation of the latter in line with the enrichment level.

Anticancer and Anti-Neuroinflammatory Constituents Isolated from the Roots of Wasabia japonica

Wasabi (Wasabia japonica (Miq.) Matsum.) is a pungent spice commonly consumed with sushi and sashimi. From the roots of this plant, a new 2-butenolide derivative (1) and 17 previously reported compounds (2–18) were isolated and structurally characterized. Their chemical structures were characterized based on the conventional NMR (¹H and ¹³C, COSY, HSQC, and HMBC) and HRESIMS data analysis. All of these phytochemicals (1–18) were evaluated for their antiproliferative effects on the four human tumor cell lines (A549, SK-OV-3, SK-MEL-2, and MKN-1), for their inhibitory activity on nitric oxide (NO) production in lipopolysaccharide (LPS)-activated BV-2 microglia cells, and for their nerve growth factor (NGF)-releasing effect from C6 glioma cells. Among the isolated compounds, compound 15 showed powerful antiproliferative activities against A549 and SK-MEL-2 cell lines with IC₅₀ values of 2.10 and 9.08 μM, respectively. Moreover, the new compound 1 exhibited moderate NO inhibition activity with IC₅₀ value of 45.3 μM.

Stable analogs of 13‑hydroxy-9,10-trans-epoxy-(11E)-octadecenoate (13,9-HEL), an oxidized derivative of linoleic acid implicated in the epidermal skin barrier

Hydroxy-epoxy- and trihydroxy derivatives of linoleic acid are proposed to play an essential function in formation of the mammalian skin permeability barrier, which could account for the essential nature of its precursor, linoleic acid. Recent literature suggests that a specific oxidized enone derivative of LA esterified in ceramides facilitates binding to proteins, potentially serving a structural role in formation of the epidermal skin barrier. However, it is still to be established if other linoleic acid derivatives are also required for skin barrier formation, and whether the essential role is performed exclusively by an esterified, structural lipid or as an unesterified, labile signaling lipid, or by some combination of these derivatives. Progress in this domain is limited by lack of availability of hydroxy‑epoxy-and trihydroxy- and octadecenoate derivatives of linoleic acid and related compounds, and challenges in maintaining them in the unesterified lipid pool. Here we describe methods for the total synthesis of hydroxy‑epoxy-octadecenoate derivatives of linoleic acid (HEL¹), and stable analogs that are designed to be resistant to inactivation by: (a) acylation/esterification (thus trapping these lipids in the free acid pool), (b) dehydrogenation, and (c) analogs combining both modifications. We further provide a total synthesis of corresponding hydroxy‑epoxy- derivatives of sebaleic acid (a regioisomer of linoleic acid present in skin), and of small molecule scaffolds containing the allylic and non-allylic epoxide 7-carbon substructures shared by both families of hydroxy‑epoxy-and trihydroxy- octadecenoates. Finally, we demonstrate that 2,2-dimethyl analogs of hydroxy‑epoxy-and trihydroxy- octadecenoates are resistant to esterification with an in vitro assay and thus provide a novel template for stabilizing labile, bioactive lipids as free acids by preventing acylation/esterification.

Synthesis of α,β-unsaturated epoxy ketones utilizing a bifunctional sulfonium/phosphonium ylide

Herein, a new protocol for rapid synthesis of α,β-unsaturated epoxy ketones utilizing a bifunctional sulfonium/phosphonium ylide is described. This approach comprises two sequential chemoselective reactions between sulfonium and phosphonium ylides and two distinct aldehydes, which allows for the rapid construction of a variety of unsymmetric α,β-unsaturated epoxy ketones. This methodology allows the rapid construction of the core reactive functionality of a family of lipid peroxidation products, the epoxyketooctadecenoic acids, but can be further broadly utilized as a useful synthon for the synthesis of natural products, particularly those derived from oxidized fatty acids. Accordingly, a protocol utilizing this approach to synthesize the epoxyketooctadecenoic acid family of molecules is described.

1H NMR Study of the In Vitro Digestion of Highly Oxidized Soybean Oil and the Effect of the Presence of Ovalbumin

Oxidized lipids containing a wide variety of potentially toxic compounds can be ingested through diet. However, their transformations during digestion are little known, despite this knowledge being essential in understanding their impact on human health. Considering this, the in vitro digestion process of highly oxidized soybean oil, containing compounds bearing hydroperoxy, aldehyde, epoxy, keto and hydroxy groups, among others, is studied by ¹H nuclear magnetic resonance. Lipolysis extent, oxidation occurrence and the fate of oxidation products both present in the undigested oil and formed during digestion are analyzed. Furthermore, the effect during digestion of two different ovalbumin proportions on all the aforementioned issues is also addressed. It is proved that polyunsaturated group bioaccessibility is affected by both a decrease in lipolysis and oxidation occurrence during digestion. While hydroperoxide level declines throughout this process, epoxy-compounds, keto-dienes, hydroxy-compounds, furan-derivatives and n-alkanals persist to a great extent or even increase. Conversely, α,β-unsaturated aldehydes, especially the very reactive and toxic oxygenated ones, diminish, although part of them remains in the digestates. While a low ovalbumin proportion hardly affects oil evolution during digestion, at a high level it diminishes oxidation and reduces the concentration of potentially bioaccessible toxic oxidation compounds.

Roseabol A, a New Peptaibol from the Fungus Clonostachys rosea

A new 11 amino acid linear peptide named roseabol A (1) and the known compound 13-oxo-trans-9,10-epoxy-11(E)-octadecenoic acid (2) were isolated from the fungus Clonostachys rosea. Combined NMR and MS analysis revealed that roseabol A (1) contained amino acid residues characteristic of the peptaibol family of peptides such as isovaline, α-aminoisobutyric acid, hydroxyproline, leucinol, and an N-terminal isovaleric acid moiety. The amino acid sequence was established by a combination of NMR studies and tandem MS fragmentation analyses, and the absolute configurations of the constituent amino acids of 1 were determined by the advanced Marfey’s method. Compound 2 showed inhibitory activity against Merkel cell carcinoma, a rare and difficult-to-treat type of skin cancer, with an IC₅₀ value of 16.5 μM.

Roles of Lipid Peroxidation-Derived Electrophiles in Pathogenesis of Colonic Inflammation and Colon Cancer

Redox stress is a common feature of gut disorders such as colonic inflammation (inflammatory bowel disease or IBD) and colorectal cancer (CRC). This leads to increased colonic formation of lipid-derived electrophiles (LDEs) such as 4-hydroxynonenal (4-HNE), malondialdehyde (MDA), trans, trans-2,4-decadienal (tt-DDE), and epoxyketooctadecenoic acid (EKODE). Recent research by us and others support that treatment with LDEs increases the severity of colitis and exacerbates the development of colon tumorigenesis in vitro and in vivo, supporting a critical role of these compounds in the pathogenesis of IBD and CRC. In this review, we will discuss the effects and mechanisms of LDEs on development of IBD and CRC and lifestyle factors, which could potentially affect tissue levels of LDEs to regulate IBD and CRC development.

Saegusa Oxidation of Enol Ethers at Extremely Low Pd-Catalyst Loadings under Ligand-free and Aqueous Conditions: Insight into the Pd(II)/Cu(II)-Catalyst System

A highly efficient and practical Pd(II)/Cu(OAc)₂-catalyst system of Saegusa oxidation, which converts enol ethers to the corresponding enals with a number of diverse substrates at extremely low catalyst loadings (500 mol ppm) under ligand-free and aqueous conditions, is described. Its synthetic utility was demonstrated by large-scale applications of the catalyst system to important nature molecules. This work allows Saegusa oxidation to become a highly practical approach to preparing enals and also suggests new insight into the Pd(II)/Cu(II)-catalyst system for dehydrogenation of carbonyl compounds and decreasing Pd-catalyst loadings.

The lipid peroxidation product EKODE exacerbates colonic inflammation and colon tumorigenesis

Oxidative stress is emerging as an important contributor to the pathogenesis of colorectal cancer (CRC), however, the molecular mechanisms by which the disturbed redox balance regulates CRC development remain undefined. Using a liquid chromatography–tandem mass spectrometry-based lipidomics, we found that epoxyketooctadecenoic acid (EKODE), which is a lipid peroxidation product, was among the most dramatically increased lipid molecules in the colon of azoxymethane (AOM)/dextran sodium sulfate (DSS)-induced CRC mice. This is, at least in part, due to increased oxidative stress in colon tumors, as assessed by analyzing gene expression of oxidative markers in AOM/DSS-induced CRC mice and human CRC patients in the Cancer Genome Atlas (TCGA) database. Systemic, short-time treatment with low-dose EKODE increased the severity of DSS-induced colitis, caused intestinal barrier dysfunction and enhanced lipopolysaccharide (LPS)/bacterial translocation, and exacerbates the development of AOM/DSS-induced CRC in mice. Furthermore, treatment with EKODE, at nM doses, induced inflammatory responses via JNK-dependent mechanisms in both colon cancer cells and macrophage cells. Overall, these results demonstrate that the lipid peroxidation product EKODE is an important mediator of colonic inflammation and colon tumorigenesis, providing a novel mechanistic linkage between oxidative stress and CRC development.

Oxylipins Associated to Current Diseases Detected for the First Time in the Oxidation of Corn Oil as a Model System of Oils Rich in Omega-6 Polyunsaturated Groups. A Global, Broad and in-Depth Study by 1H NMR Spectroscopy

For the first time, an important number of oxylipins have been identified and quantified in corn oil submitted to mild oxidative conditions at each time of their oxidation process. This oil can be considered as a model system of edible oils rich in polyunsaturated omega-6 groups. The study was carried out using 1H nuclear magnetic resonance spectroscopy (1H NMR), which does not require chemical modification of the sample. These newly detected oxylipins include dihydroperoxy-non-conjugated-dienes, hydroperoxy-epoxy-, hydroxy-epoxy- and keto-epoxy-monoenes as well as E-epoxy-monoenes, some of which have been associated with several diseases. Furthermore, the formation of other functional groups such as poly-formates, poly-hydroxy and poly-ether groups has also been proven. These are responsible for the polymerization and increased viscosity of the oil. Simultaneously, monitoring of the formation of well-known oxylipins, such as hydroperoxy-, hydroxy-, and keto-dienes, and of different kinds of oxygenated-alpha,beta-unsaturated aldehydes such as 4-hydroperoxy-, 4-hydroxy-, 4-oxo-2E-nonenal and 4,5-epoxy-2E-decenal, which are also related to different degenerative diseases, has been carried out. The provided data regarding the compounds identification and their sequence and kinetics of formation constitute valuable information for future studies in which lipid oxidation is involved, both in food and in other scientific fields.

Strategies for UHPLC-MS/MS-Based Analysis of Different Classes of Acyl Chain Oxidation Products Resulting from Thermo-Oxidation of Sitostanyl Oleate

Phytostanyl fatty acid esters, which are being added to a variety of foods owing to their cholesterol-lowering properties, are susceptible to thermally induced oxidation reactions. Using sitostanyl oleate as an example, we, in this study, aimed at developing strategies for the detection and the quantitation of acyl chain oxidation products (ACOPs). The elaborated ultra high performance liquid chromatography-mass spectrometry MS/MS-based approach involves (i) derivatizations of hydroxy and oxo groups, (ii) combined use of atmospheric pressure chemical ionization and electrospray ionization as ionization techniques, and (iii) establishment of selective multireaction monitoring transitions. These strategies allow the identification of a broad spectrum of nonpolar short-chain as well as polar short- and long-chain hydroxy, epoxy, and oxo ACOPs. Semiquantitative assessments showed that sitostanyl 9,10-epoxystearate was the predominating ACOP resulting from thermo-oxidation of sitostanyl oleate at 180 °C for 30 min. The elaborated analytical approach paves the way to narrow the existing knowledge gap on the fate of phytostanyl fatty acid esters upon heat treatments.

Impact of Cr(VI) on the oxidation of polyunsaturated fatty acids in Helianthus annuus roots studied by metabolomic tools

The impact of Cr(VI) in sunflower roots has been studied, focusing on the oxidation of polyunsaturated fatty acids. Plants were grown hydroponically in the presence of 0, 1.0, 5.0 and 25 mgCr L^-1. Methanolic root extracts were analyzed by capillary liquid chromatography coupled through negative electrospray ionization to a quadrupole-time of flight mass spectrometry (capHPLC-ESI-QTOF-MS). Using partial least squares algorithm, eighteen features strongly affected by Cr(VI) were detected and annotated as linoleic acid (LA), alpha-linolenic acid (ALA) and sixteen oxidation products containing hydroperoxy-, epoxy-, keto-, epoxyketo- or hydroxy-functionalities, all of them classified as oxylipins. Inspection of the MS/MS spectra acquired for features eluting at different retention times but assigned as a sole compound, confirmed isomers formation: three hydroperoxy-octadecadienoic acids (HpODE), two oxo-octadecadienoic acids (OxoODE) and four epoxyketo-octadecenoic acids (EKODE). Around 70% of metabolites in sunflower LA metabolic pathway were affected by Cr(VI) stress and additionally, four EKODE isomers not included in this pathway were found in the exposed roots. Among ALA-derived oxylipins, 13-epi-12-oxo-phytodienoic acid (OPDA) is of relevance, because of its participation in the activation of secondary metabolism. The abundances of all oxylipins were directly dependent on the Cr(VI) concentration in medium; furthermore, autooxidation of LA to HpODE isomers was observed after incubation with Cr(VI). These results point to the direct involvement of Cr(VI) in non-enzymatic oxidation of fatty acids; since oxylipins are signaling molecules important in plant defensive response, their synthesis under Cr(VI) exposure sustains the ability of sunflower to grow in Cr(VI)-contaminated environments.

Lipidomic profiling of targeted oxylipins with ultra-performance liquid chromatography-tandem mass spectrometry

Oxylipins are bioactive mediators that play diverse roles in (patho)physiology. We developed a sensitive and selective ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method for the simultaneous profiling of 57 targeted oxylipins derived from five major n-6 and n-3 polyunsaturated fatty acids (PUFAs) that serve as oxylipin precursors, including linoleic (LA), arachidonic (AA), alpha-linolenic (ALA), eicosapentaenoic (EPA), and docosahexaenoic (DHA) acids. The targeted oxylipin panel provides broad coverage of lipid mediators and pathway markers generated from cyclooxygenases, lipoxygenases, cytochrome P450 epoxygenases/hydroxylases, and non-enzymatic oxidation pathways. The method is based on combination of protein precipitation and solid-phase extraction (SPE) for sample preparation, followed by UPLC-MS/MS. This is the first methodology to incorporate four hydroxy-epoxy-octadecenoic acids and four keto-epoxy-octadecenoic acids into an oxylipin profiling network. The novel method achieves excellent resolution and allows in-depth analysis of isomeric and isobaric species of oxylipin extracts in biological samples. The method was quantitatively characterized in human plasma with good linearity (R = 0.990-0.999), acceptable reproducibility (relative standard deviation (RSD) < 20% for the majority of analytes), accuracy (67.8 to 129.3%) for all analytes, and recovery (66.8-121.2%) for all analytes except 5,6-EET. Ion enhancement effects for 28% of the analytes in tested concentrations were observed in plasma, but were reproducible with RSD < 17.2%. Basal levels of targeted oxylipins determined in plasma and serum are in agreement with those previously reported in literature. The method has been successfully applied in clinical and preclinical studies.

High Resolution NMR Spectroscopy as a Structural and Analytical Tool for Unsaturated Lipids in Solution

Mono- and polyunsaturated lipids are widely distributed in Nature, and are structurally and functionally a diverse class of molecules with a variety of physicochemical, biological, medicinal and nutritional properties. High resolution NMR spectroscopic techniques including 1H-, 13C- and 31P-NMR have been successfully employed as a structural and analytical tool for unsaturated lipids. The objective of this review article is to provide: (i) an overview of the critical 1H-, 13C- and 31P-NMR parameters for structural and analytical investigations; (ii) an overview of various 1D and 2D NMR techniques that have been used for resonance assignments; (iii) selected analytical and structural studies with emphasis in the identification of major and minor unsaturated fatty acids in complex lipid extracts without the need for the isolation of the individual components; (iv) selected investigations of oxidation products of lipids; (v) applications in the emerging field of lipidomics; (vi) studies of protein-lipid interactions at a molecular level; (vii) practical considerations and (viii) an overview of future developments in the field.

The chloroplast membrane associated ceQORH putative quinone oxidoreductase reduces long-chain, stress-related oxidized lipids

Under oxidative stress conditions the lipid constituents of cells can undergo oxidation whose frequent consequence is the production of highly reactive α,β-unsaturated carbonyls. These molecules are toxic because they can add to biomolecules (such as proteins and nucleic acids) and several enzyme activities cooperate to eliminate these reactive electrophile species. CeQORH (chloroplast envelope Quinone Oxidoreductase Homolog, At4g13010) is associated with the inner membrane of the chloroplast envelope and imported into the organelle by an alternative import pathway. In the present study, we show that the recombinant ceQORH exhibits the activity of a NADPH-dependent α,β-unsaturated oxoene reductase reducing the double bond of medium-chain (C⩾9) to long-chain (18 carbon atoms) reactive electrophile species deriving from poly-unsaturated fatty acid peroxides. The best substrates of ceQORH are 13-lipoxygenase-derived γ-ketols. γ-Ketols are spontaneously produced in the chloroplast from the unstable allene oxide formed in the biochemical pathway leading to 12-oxo-phytodienoic acid, a precursor of the defense hormone jasmonate. In chloroplasts, ceQORH could detoxify 13-lipoxygenase-derived γ-ketols at their production sites in the membranes. This finding opens new routes toward the understanding of γ-ketols role and detoxification.

Lipoxygenases mediate the effect of essential fatty acid in skin barrier formation: a proposed role in releasing omega-hydroxyceramide for construction of the corneocyte lipid envelope

A barrier to water loss is vital to maintaining life on dry land. Formation of the mammalian skin barrier requires both the essential fatty acid linoleate and the two lipoxygenases 12R-lipoxygenase (12R-LOX) and epidermal lipoxygenase-3 (eLOX3), although their roles are poorly understood. Linoleate occurs in O-linoleoyl-ω-hydroxyceramide, which, after hydrolysis of the linoleate moiety, is covalently attached to protein via the free ω-hydroxyl of the ceramide, forming the corneocyte lipid envelope, a scaffold between lipid and protein that helps seal the barrier. Here we show using HPLC-UV, LC-MS, GC-MS, and (1)H NMR that O-linoleoyl-ω-hydroxyceramide is oxygenated in a regio- and stereospecific fashion by the consecutive actions of 12R-LOX and eLOX3 and that these products occur naturally in pig and mouse epidermis. 12R-LOX forms 9R-hydroperoxy-linoleoyl-ω-hydroxyceramide, further converted by eLOX3 to specific epoxyalcohol (9R,10R-trans-epoxy-11E-13R-hydroxy) and 9-keto-10E,12Z esters of the ceramide; an epoxy-ketone derivative (9R,10R-trans-epoxy-11E-13-keto) is the most prominent oxidized ceramide in mouse skin. These products are absent in 12R-LOX-deficient mice, which crucially display a near total absence of protein-bound ω-hydroxyceramides and of the corneocyte lipid envelope and die shortly after birth from transepidermal water loss. We conclude that oxygenation of O-linoleoyl-ω-hydroxyceramide is required to facilitate the ester hydrolysis and allow bonding of the ω-hydroxyceramide to protein, providing a coherent explanation for the roles of multiple components in epidermal barrier function. Our study uncovers a hitherto unknown biochemical pathway in which the enzymic oxygenation of ceramides is involved in building a crucial structure of the epidermal barrier.

Iridium-catalyzed oxidative methyl esterification of primary alcohols and diols with methanol

Oxidative methyl esterification of primary alcohols and diols with methanol was successfully achieved, using acetone as a hydrogen acceptor, under the influence of an iridium complex combined with 2-(methylamino)ethanol (MAE) as catalyst.

Cardiolipin: characterization of distinct oxidized molecular species

Cardiolipin (CL) is a phospholipid predominantly found in the mitochondrial inner membrane and is associated structurally with individual complexes of the electron transport chain (ETC). Because the ETC is the major mitochondrial reactive oxygen species (ROS)-generating site, the proximity to the ETC and bisallylic methylenes of the PUFA chains of CL make it a likely target of ROS in the mitochondrial inner membrane. Oxidized cellular CL products, uniquely derived from ROS-induced autoxidation, could serve as biomarkers for the presence of the ROS and could help in the understanding of the mechanism of oxidative stress. Because major CL species have four unsaturated acyl chains, whereas other phospholipids usually have only one in the sn-2 position, characterization of oxidized CL is highly challenging. In the current study, we exposed CL, under aerobic conditions, to singlet oxygen (¹O₂), the radical initiator 2,2'-azobis(2-methylpropionamidine) dihydrochloride, or room air, and the oxidized CL species were characterized by HPLC-tandem mass spectrometry (MS/MS). Our reverse-phase ion-pair HPLC-MS/MS method can characterize the major and minor oxidized CL species by detecting distinctive fragment ions associated with specific oxidized species. The HPLC-MS/MS results show that monohydroperoxides and bis monohydroperoxides were generated under all three conditions. However, significant amounts of CL dihydroperoxides were produced only by ¹O₂-mediated oxidation. These products were barely detectable from radical oxidation either in a liposome bilayer or in thin film. These observations are only possible due to the chromatographic separation of the different oxidized species.

Nucleophilicities and Lewis basicities of imidazoles, benzimidazoles, and benzotriazoles

The kinetics of the reactions of some imidazoles, benzimidazoles and benzotriazoles with benzhydrylium ions (diarylcarbenium ions) have been studied photometrically in DMSO, acetonitrile, and aqueous solution at 20 degrees C. The resulting second-order rate constants have been used to determine the nucleophile-specific parameters N and s of these azoles according to the linear-free-energy relationship log k (20 degrees C) = s(N + E). With N = 11.47 (imidazole in acetonitrile), N = 10.50 (benzimidazole in DMSO), and N = 7.69 (benzotriazole in acetonitrile) these azoles are significantly less nucleophilic than previously characterized amines, such as DMAP (N = 14.95 in acetonitrile) and DABCO (N = 18.80 in acetonitrile). For some reactions of the 1-methyl substituted azoles with benzhydrylium ions equilibrium constants have been measured, which render a comparison of the Lewis basicities of these compounds. Substitution of the rate and equilibrium constants of these reactions into the Marcus equation yields the corresponding intrinsic barriers DeltaG(0)( not equal). From the ranking of DeltaG(0)( not equal) (imidazoles > pyridines > 1-azabicyclooctanes) one can derive that the reorganization energies for the reactions of imidazoles with electrophiles are significantly higher than those for the other amines and that imidazoles are less nucleophilic than pyridines and 1-azabicyclooctanes of comparable basicity.

Mass spectrometric characterization of protein modification by the products of nonenzymatic oxidation of linoleic acid

Autoxidation of linoleic acid (LA) enhanced by Fe(II)/ascorbate generates unsaturated hydroperoxides which undergo further oxidative evolution resulting in a mixture of electrophiles, including epoxyketooctadecenoic acid and dienones with intact C-18 chains as well as oxidative cleavage products such as 4-hydroxy-2(E)-nonenal (HNE), 4-oxo-2(E)-nonenal (ONE), 2(E)-octenal, 9-hydroxy-12-oxo-10(E)-dodecenoic acid, 9,12-dioxo-10(E)-dodecenoic acid, and 11-oxoundec-9(E)-enoic acid. Mass spectrometric (MALDI-TOF-MS and LC-ESI-MS/MS) studies have been performed following incubation of the model protein beta-lactoglobulin with LA, Fe(II), and ascorbate, which identified adducts of these electrophiles with three different protein nucleophiles. Deuterium labeled linoleic acid 17,17,18,18,18-d(5)-(9Z,12Z)-octadeca-9,12-dienoic acid (d(5)-LA) was synthesized to facilitate the detection and characterization of the protein modifications by mass spectrometry. Reduction by NaBH(4) served to trap reversible adducts and to quantify the number of reducible functional groups in each adduct. This study, which mimics the distribution of reactive lipid peroxidation products generated by a continuous low level flux of reactive oxygen species present in vivo under conditions of oxidative stress, confirms that many irreversibly formed adducts previously identified following exposure of model proteins to pure electrophilic modifiers such as HNE and ONE are also generated during in situ oxidation of LA. These adducts include HNE-His Michael adducts (MA), ONE-Lys 4-ketoamide, ONE-Lys pyrrolinone, and a Cys/His-ONE-Lys pyrrole cross-link. However, reversibly formed adducts, such as the HNE-Lys Schiff base, are not present at detectable levels. The isotopic labeling allowed less commonly identified mirror-image adducts derived from the carboxy terminus of LA to be identified. A novel 2-octenoic acid-His MA was discovered.

Charge-derivatized amino acids facilitate model studies on protein side-chain modifications by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

The alpha-amino groups of histidine and lysine were derivatized with p-carboxylbenzyltriphenylphosphonium to form the pseudo dipeptides, PHis and PLys, which can be sensitively detected by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) due to the fixed positive charge of the phosphonium group. Detection limits of PHis and PLys by MALDI-TOFMS were both 30 fmol with a signal-to-noise ratio of 5:1. These pseudo dipeptides were excellent surrogates for His- or Lys-containing peptides in model reactions mimicking proteins with reactive electrophiles, prominently those generated by peroxidation of polyunsaturated fatty acids including 4-hydroxy-2(E)-nonenal (HNE), 4-oxo-2(E)-nonenal (ONE), 2(E)-octenal, and 2(E)-heptenal. An air-saturated solution of linoleic acid (d0:d5 = 1:1) was incubated in the presence of Fe(II) and ascorbate with these two pseudo dipeptides, and the reaction products were characterized by MALDI-TOFMS and liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS). By using PHis and PLys, the previously reported ONE-derived His-furan adduct was detected along with evidence for a cyclic alpha,beta-unsaturated ketone. A dimer formed from ONE was found to react with PHis through Michael addition. Alkenals were found to form two novel adducts with PLys. 2(E)-Octenoic acid-His Michael adduct and N(epsilon)-pentanoyllysine were identified as potential protein side-chain adducts modified by products of linoleic acid peroxidation. In addition, when PHis or PLys and AcHis or BocLys were exposed to the linoleic acid peroxidation, an epoxy-keto-ocatadecenoic acid mediated His-His cross-link was detected, along with the observation of a His-ONE/9,12-dioxo-10-dodecenoic acid-Lys derived pyrrole cross-link.

Activation of the antioxidant response element by specific oxidized metabolites of linoleic acid

Linoleic acid is required for normal mammalian health and development, but is also prone to oxidation, yielding metabolites with biological effects. We screened linoleic acid, other fatty acids, and some of their derivatives and found that an epoxy-keto derivative of linoleic acid (but neither linoleic acid itself nor others of its oxidation products) strongly activates the antioxidant response element (ARE) in IMR-32 neuroblastoma cells and cerebro-cortical neurons. The active compound, 12,13-epoxy-9-keto-10(trans)-octadecenoic acid (EKODE), induces the expression of ARE-regulated cytoprotective genes such as NQO1 at the transcript and protein levels. EKODE requires transcription factor NRF2 and PI3-kinase for ARE activity. The results suggest that specific oxidation products of linoleic acid may initiate responses that lessen damage caused by oxidative stress.