Determination of EETs using microbore liquid chromatography with fluorescence detection

Liquid-Chromatographic Methods for Carboxylic Acids in Biological Samples

Carboxyl-bearing low-molecular-weight compounds such as keto acids, fatty acids, and other organic acids are involved in a myriad of metabolic pathways owing to their high polarity and solubility in biological fluids. Various disease areas such as cancer, myeloid leukemia, heart disease, liver disease, and lifestyle diseases (obesity and diabetes) were found to be related to certain metabolic pathways and changes in the concentrations of the compounds involved in those pathways. Therefore, the quantification of such compounds provides useful information pertaining to diagnosis, pathological conditions, and disease mechanisms, spurring the development of numerous analytical methods for this purpose. This review article addresses analytical methods for the quantification of carboxylic acids, which were classified into fatty acids, tricarboxylic acid cycle and glycolysis-related compounds, amino acid metabolites, perfluorinated carboxylic acids, α-keto acids and their metabolites, thiazole-containing carboxylic acids, and miscellaneous, in biological samples from 2000 to date. Methods involving liquid chromatography coupled with ultraviolet, fluorescence, mass spectrometry, and electrochemical detection were summarized.

Methods of the Analysis of Oxylipins in Biological Samples

Oxylipins are derivatives of polyunsaturated fatty acids and due to their important and diverse functions in the body, they have become a popular subject of studies. The main challenge for researchers is their low stability and often very low concentration in samples. Therefore, in recent years there have been developments in the extraction and analysis methods of oxylipins. New approaches in extraction methods were described in our previous review. In turn, the old analysis methods have been replaced by new approaches based on mass spectrometry (MS) coupled with liquid chromatography (LC) and gas chromatography (GC), and the best of these methods allow hundreds of oxylipins to be quantitatively identified. This review presents comparative and comprehensive information on the progress of various methods used by various authors to achieve the best results in the analysis of oxylipins in biological samples.

Modern Methods of Sample Preparation for the Analysis of Oxylipins in Biological Samples

Oxylipins are potent lipid mediators derived from polyunsaturated fatty acids, which play important roles in various biological processes. Being important regulators and/or markers of a wide range of normal and pathological processes, oxylipins are becoming a popular subject of research; however, the low stability and often very low concentration of oxylipins in samples are a significant challenge for authors and continuous improvement is required in both the extraction and analysis techniques. In recent years, the study of oxylipins has been directly related to the development of new technological platforms based on mass spectrometry (LC–MS/MS and gas chromatography–mass spectrometry (GC–MS)/MS), as well as the improvement in methods for the extraction of oxylipins from biological samples. In this review, we systematize and compare information on sample preparation procedures, including solid-phase extraction, liquid–liquid extraction from different biological tissues.

P450 Eicosanoids and Reactive Oxygen Species Interplay in Brain Injury and Neuroprotection

Significance: Eicosanoids are endogenous lipid mediators that play important roles in brain function and disease. Acute brain injury such as that which occurs in stroke and traumatic brain injury increases the formation of eicosanoids, which, in turn, exacerbate or diminish injury. In chronic neurodegenerative diseases such as Alzheimer's disease and vascular dementia (VD), eicosanoid synthetic and metabolizing enzymes are altered, disrupting the balance between neuroprotective and neurotoxic eicosanoids. Recent Advances: Human and experimental studies have established the opposing roles of hydroxy- and epoxyeicosanoids and their potential utility as diagnostic biomarkers and therapeutic targets in neural injury. Critical Issues: A gap in knowledge remains in understanding the cellular and molecular mechanisms underlying the neurovascular actions of specific eicosanoids, such as specific isomers of epoxyeicosatrienoic (EETs) and hydroxyeicosatetraenoic acids (HETEs). Future Directions: EETs and HETEs exert their actions on brain cells by targeting multiple mechanisms, which include surface G-protein coupled receptors. The identification of high-affinity receptors for EETs and HETEs and their cellular localization in the brain will be a breakthrough in our understanding of these eicosanoids as mediators of cell-cell communications and contributors to brain development, function, and disease. Antioxid. Redox Signal. 28, 987-1007.

Endothelium-derived Relaxing Factors of Small Resistance Arteries in Hypertension

Endothelium-derived relaxing factors (EDRFs), including nitric oxide (NO), prostacyclin (PGI₂), and endothelium-derived hyperpolarizing factor (EDHF), play pivotal roles in regulating vascular tone. Reduced EDRFs cause impaired endothelium-dependent vasorelaxation, or endothelial dysfunction. Impaired endothelium-dependent vasorelaxation in response to acetylcholine (ACh) is consistently observed in conduit vessels in human patients and experimental animal models of hypertension. Because small resistance arteries are known to produce more than one type of EDRF, the mechanism(s) mediating endothelium-dependent vasorelaxation in small resistance arteries may be different from that observed in conduit vessels under hypertensive conditions, where vasorelaxation is mainly dependent on NO. EDHF has been described as one of the principal mediators of endothelium-dependent vasorelaxation in small resistance arteries in normotensive animals. Furthermore, EDHF appears to become the predominant endothelium-dependent vasorelaxation pathway when the endothelial NO synthase (NOS3)/NO pathway is absent, as in NOS3-knockout mice, whereas some studies have shown that the EDHF pathway is dysfunctional in experimental models of hypertension. This article reviews our current knowledge regarding EDRFs in small arteries under normotensive and hypertensive conditions.

Ultra-pressure liquid chromatography/tandem mass spectrometry targeted profiling of arachidonic acid and eicosanoids in human colorectal cancer

Cumulative evidence shows that eicosanoids such as prostaglandins, leukotrienes, thromboxanes and hydroxy eicosatetraenoic acids play an important role in associating inflammation with human colorectal cancer (CRC). In this study an ultra-pressure liquid chromatography/tandem mass spectrometry (UPLC/MS/MS) method was developed and validated for the targeted profiling of eight relevant eicosanoids and the major metabolic precursor, arachidonic acid (AA), in human colon. Multiple reaction monitoring (MRM) experiments were performed in negative electrospray ionization mode. The metabolites were separated using a C(18) column consisting of 1.7 µm ethylene-bridged hybrid particles (100 × 2.1 mm i.d.) and gradient elution (50 to 95% of solvent B) with a mobile phase comprising water (0.1% formic acid) [solvent A] and acetonitrile (0.1% formic acid) [solvent B] at a flow rate of 0.4 mL/min. The analysis time for each sample was 5.5 min. Our UPLC/MS/MS method demonstrated satisfactory validation results in terms of selectivity, sensitivity, matrix effect, linearity, extraction efficiency, intra- and inter-day precision, accuracy and autosampler stability. The method was applied for the clinical profiling of matched pairs of cancerous and normal colon mucosae obtained from eight colorectal cancer patients. Endogenous levels of AA and selected eicosanoids such as prostaglandin E(2) (PGE(2)), prostacyclin (PGI(2)) [assayed as its stable hydrolytic product 6-keto-prostaglandin(1α) (6-k PGF(1α))] and 12-hydroxy-5Z,8Z,10E,14Z-eicosatetraenoic acid (12-HETE) were found to be significantly different (p <0.05; paired t-test) between cancerous and normal mucosae.

A LC-MS-based method for quantification of biomarkers from serum of allergic rats

Allergies are highly complex disorders with clinical manifestations ranging from mild oral, gastrointestinal, recurrent wheezing, and cutaneous symptoms to life-threatening systemic conditions. The levels of arachidonic acid, eicosanoids, histamine, organic acids and valine are considered to have a variety of physiological functions in connection with allergies. In this research, we have developed a RP-LC/MS method to separate and quantitate six different potential endogenous biomarkers, including leukotrieneB₄ (LTB₄), prostaglandinD₂ (PGD₂), arachidonic acid (AA), histamine (HI), lactic acid (LA) and valine (VAL), from serum of rats with ovalbumin (OVA)-induced allergy and normal rats, and the discrepancies between the model group and the control group were compared. The separation was performed on a Prevail C₁₈ column (250 mm × 4.6 mm, 5 μm) with a gradient elution of acetonitrile with 0.1% formic acid (v/v) and 10 mM ammonium formate (adjusted to pH 4.0 with formic acid) at a flow rate of 0.5 mL min⁻¹ The method was validated and shown to be sensitive, accurate (recovery values 76.16–92.57%) and precise (RSD < 10% for all compounds) with a linear range over several orders of magnitude. The method was successfully applied to rat serum and shown to be indicative of the endogenous levels of biomarkers within the rat body. The analysis of the biomarkers can provide insight into the allergic mechanisms associated with related diseases.

Epoxyeicosatrienoic acids and endothelium-dependent responses

Epoxyeicosatrienoic acids (EETs) are cytochrome P450 metabolites of arachidonic acid that are produced by the vascular endothelium in response to agonists such as bradykinin and acetylcholine or physical stimuli such as shear stress or cyclic stretch. In the vasculature, the EETs have biological actions that are involved in the regulation of vascular tone, hemostasis, and inflammation. In preconstricted arteries in vitro, EETs activate calcium-activated potassium channels on vascular smooth muscle and the endothelium causing membrane hyperpolarization and relaxation. These effects are observed in a variety of arteries from experimental animals and humans; however, this is not a universal finding in all arteries. The mechanism of EET action may vary. In some arteries, EETs are released from the endothelium and are transferred to the smooth muscle where they cause potassium channel activation, hyperpolarization, and relaxation through a guanine nucleotide binding protein-coupled mechanism or transient receptor potential (TRP) channel activation. In other arteries, EETs activate TRP channels on the endothelium to cause endothelial hyperpolarization that is transferred to the smooth muscle by gap junctions or potassium ion. Some arteries use a combination of mechanisms. Acetylcholine and bradykinin increase blood flow in dogs and humans that is inhibited by potassium channel blockers and cytochrome P450 inhibitors. Thus, the EETs are endothelium-derived hyperpolarizing factors mediating a portion of the relaxations to acetylcholine, bradykinin, shear stress, and cyclic stretch and regulate vascular tone in vitro and in vivo.

Vascular pharmacology of epoxyeicosatrienoic acids

Epoxyeicosatrienoic acids (EETs) are cytochrome P450 metabolites of arachidonic acid that are produced by the vascular endothelium in responses to various stimuli such as the agonists acetylcholine (ACH) or bradykinin or by shear stress which activates phospholipase A(2) to release arachidonic acid. EETs are important regulators of vascular tone and homeostasis. In the modulation of vascular tone, EETs function as endothelium-derived hyperpolarizing factors (EDHFs). In models of vascular inflammation, EETs attenuate inflammatory signaling pathways in both the endothelium and vascular smooth muscle. Likewise, EETs regulate blood vessel formation or angiogenesis by mechanisms that are still not completely understood. Soluble epoxide hydrolase (sEH) converts EETs to dihydroxyeicosatrienoic acids (DHETs) and this metabolism limits many of the biological actions of EETs. The recent development of inhibitors of sEH provides an emerging target for pharmacological manipulation of EETs. Additionally, EETs may initiate their biological effects by interacting with a cell surface protein that is a G protein-coupled receptor (GPCR). Since GPCRs represent a common target of most drugs, further characterization of the EET receptor and synthesis of specific EET agonists and antagonist can be used to exploit many of the beneficial effects of EETs in vascular diseases, such as hypertension and atherosclerosis. This review will focus on the current understanding of the contribution of EETs to the regulation of vascular tone, inflammation, and angiogenesis. Furthermore, the therapeutic potential of targeting the EET pathway in vascular disease will be highlighted.

Vascular endothelial function in health and diseases

The vascular endothelium constitutes approximately 1% of body mass (1kg) and has a surface area of approximately 5000m(2). The endothelium is a multifunctional endocrine organ strategically placed between the vessel wall and the circulating blood, and has a key role in vascular homeostasis. The endothelium is both a target for and mediator of cardiovascular disease. The endothelium releases several relaxing and constricting factors, which can affect vascular homeostasis. Endothelial dysfunction, whether caused by physical injury or cellular damage, leads to compensatory responses that alter the normal homeostatic properties of the endothelium. In this review, we summarized some physiological aspects of endothelial function and then we discussed endothelial dysfunction during some pathological conditions.

Direct eicosanoid profiling of the hypoxic lung by comprehensive analysis via capillary liquid chromatography with dual online photodiode-array and tandem mass-spectrometric detection

Eicosanoids are arachidonic acid-derived mediators, with partly contradictory, incompletely elucidated actions. Thus, epoxyeicosatrienoic acids (EETs) are controversially discussed as putative vasodilatative endothelium-derived hyperpolarizing factors in the cardiovascular compartment but reported as vasoconstrictors in the lung. Inconsistent findings concerning eicosanoid physiology may be because previous methods were lacking sensitivity, identification reliability, and/or have focused on special eicosanoid groups only, ignoring the overall mediator context, and thus limiting the correlation accuracy between autacoid formation and bioactivity profile. Therefore, we developed an approach which enables the simultaneous assessment of 44 eicosanoids, including all representatives of the arachidonic acid cascade, i.e., cytochrome P450, lipoxygenase, cyclooxygenase products, and free isoprostanes as in vivo markers of oxidative stress, in one 50-minute chromatographic run. The approach combines (i) source-specific sample extraction, (ii) rugged isocratic and high-sensitivity capillary liquid-chromatographic separation, and (iii) reliable dual online photodiode-array and electrospray ionization tandem mass-spectrometric identification and quantitation. High sensitivity with limits of quantification in the femtogram range was achieved by use of capillary columns with typical high peak efficiency, due to small inner diameters, and virtually complete substance transfer to the mass spectrometer, due to flow rates in the low microliter range, instead of large inner diameter columns with low chromatographic signal and only partial analyte transfer employed by previous methods. This expeditious, global and sensitive technique provides the prerequisite for new, accurate insights regarding the physiology of specific mediators, for example EETs, in the context of all relevant vasoactive autacoids under varying conditions of oxidative stress by direct comparison of all eicosanoid generation profiles. Indeed, application of comprehensive "eicoprofiling" to hypoxically ventilated rabbit lungs revealed at a glance the enhanced biosynthesis of free EETs in the overall mediator generation context, thus suggesting their hypothetical contribution to hypoxic pulmonary vasoconstriction.

Development and validation of a high-performance liquid chromatography-electrospray mass spectrometry method for the simultaneous determination of 23 eicosanoids

Inflammation is implicated in the pathogenesis of a number of diseases, including cardiovascular disease. Current research is focused on developing assays to search for biomarkers for inflammation. Eicosanoids are the oxidative metabolites of arachidonic acid (eicosatetraenoic acid, AA), a long chain polyunsaturated fatty acid common in Western diets. AA can be oxidized by one of three pathways to form prostaglandins (PGs), leukotrienes (LTs), or a number of hydroxyl and epoxy compounds. These eicosanoids have a variety of physiological functions, including regulating inflammation. We have developed a method utilizing LC-MS to separate and quantitate 23 different eicosanoids from all the three oxidative pathways. The eicosanoids were separated using a gradient elution of acetonitrile with 0.1% formic acid (v/v) and water with 0.1% formic acid (v/v) at a flow rate of 1 mL/min with a Symmetry C18 column (250 mm x 4.6 mm). Deuterated eicosanoids were used as internal standards for quantitation. Mass spectrometric detection was carried out using an Agilent 1100-series LC-MSD with an electrospray ionization interface. Electrospray ionisation (ESI) mass spectra were acquired using negative ionization and selective ion monitoring. The method was validated and shown to be sensitive (LOQ at pg levels for most compounds), accurate (recovery values 75-120%) and precise (R.S.D.<20 for all compounds) with a linear range over several orders of magnitude. The method was applied to rat kidney tissue and shown to be indicative of the eicosanoid levels within a specific organ. The analysis of eicosanoids can provide insight into the inflammatory mechanisms associated with cardiovascular disease.

Quantitative profiling of epoxyeicosatrienoic, hydroxyeicosatetraenoic, and dihydroxyeicosatetraenoic acids in human intrauterine tissues using liquid chromatography/electrospray ionization tandem mass spectrometry

A reversed-phase liquid chromatography negative ion electrospray tandem mass spectrometry (LC/ESI-MS/MS) method was developed and validated to quantify a range of physiologically relevant eicosanoids, including 5,6-, 8,9-, 11,12-, and 14,15-epoxyeicosatrienoic acids (EETs); 5-, 8-, 9-, 12-, and 15-hydroxyeicosatetraenoic acids (HETEs), and 5,6-, 8,15-, and 12,20-dihydroxyeicosatetraenoic acids (DiHETEs) in human intrauterine tissues. A solid-phase extraction method was employed to extract the eicosanoids, and gradient LC separation was performed on a Kromasil C(18) column. Mass spectrometric detection was performed by multiple reaction monitoring over a 31-min run time. The calibration curves were linear over the range of 4-400pmol/g tissue, and the intra- and interday precision and accuracy were within a coefficient of variation of 2.0 to 27.4% and 4.6 to 17.9%, respectively. The lower limit of quantitation was 1.0pmol/g tissue. The method was applied successfully to the characterization and quantitation of eicosanoids in the different compartments of human intrauterine tissues. Our results demonstrate significantly greater amounts of HETEs than of either the EETs or DiHETEs (P<0.001), irrespective of tissue type. Specifically, the metabolite 12-HETE was significantly more abundant (P<0.001) than all other HETEs. Of the EET metabolites, 5,6-EET predominated (P<0.001). A significant negative correlation between EETs and HETEs for all tissues (rho=-0.390, P<0.001) was identified, implying a biological feedback mechanism between these two arachidonate metabolite classes.

A liquid chromatography/mass spectrometric method for simultaneous analysis of arachidonic acid and its endogenous eicosanoid metabolites prostaglandins, dihydroxyeicosatrienoic acids, hydroxyeicosatetraenoic acids, and epoxyeicosatrienoic acids in rat brain tissue

A sensitive, specific, and robust liquid chromatography/mass spectrometric (LC/MS) method was developed and validated that allows simultaneous analysis of arachidonic acid (AA) and its cyclooxygenase, cytochrome P450, and lipoxygenase pathway metabolites prostaglandins (PGs), dihydroxyeicosatrienoic acids (DiHETrEs), hydroxyeicosatetraenoic acids (HETEs) and epoxyeicosatrienoic acids (EETs), including PGF(2alpha), PGE(2), PGD(2), PGJ(2), 14,15-DiHETrE, 11,12-DiHETrE, 8,9-DiHETrE, 5,6-DiHETrE, 20-HETE, 15-HETE, 12-HETE, 9-HETE, 8-HETE, 5-HETE, 14,15-EET, 11,12-EET, 8,9-EET, and 5,6-EET in rat brain tissues. Deuterium labeled PGF(2alpha)-d(4), PGD(2)-d(4), 15(S)-HETE-d(8), 14,15-EET-d(8), 11,12-EET-d(8), 8,9-EET-d(8), and AA-d(8) were used as internal standards. Solid phase extraction was used for sample preparation. A gradient LC/MS method using a C18 column and electrospray ionization source under negative ion mode was optimized for the best sensitivity and separation within 35 min. The method validation, including LC/MS instrument qualification, specificity, calibration model, accuracy, precision (without brain matrix and with brain matrix), and extraction efficiency were performed. The linear ranges of the calibration curves were 2-1000 pg for PGs, DiHETrEs, HETEs, and EETs, 10-2400 pg for PGE(2) and PGD(2), and 20-2000 ng for AA, respectively.

Identification and quantification of metabolites of arachidonic acid from cultures of endothelial cells by HPLC-MS2

Epoxyeicosatrienoic acids (EETs) and hydroxyeicosatetraenoic acids (HETEs) are oxidative products of arachidonic acid, some of which participate in the regulation of vascular tone. Little is known about the production of EETs and HETEs in cultures of endothelial cells. This paper reports an assay for the simultaneous quantification of isomers of EETs and HETEs from endothelial cell culture supernatants by employing solid-phase extraction and liquid chromatography-mass spectrometry. The method enabled measurement of 5,6-EET, 8,9-EET, 11,12-EET, 14,15-EET, 5-HETE, 8-HETE, 11-HETE, 12-HETE and 15-HETE. The metabolites were chromatographically separated by reversed-phase HPLC and identified by negative ESI tandem mass spectrometry and this method was used to investigate the metabolism of arachidonic acid with an endothelial cell line. For quantification, the sum of signal intensities of characteristic fragment ions was used. The detection limits for 5,6-EET and of other EET and HETE isomers were 2.0, 0.64 and 8 ng ml(-1) culture medium, respectively. The precision of the method was determined with spiked culture medium (three concentrations, n = 5) and the average RSD ranged from 6.0 to 24.2%. The dynamic range was 0.6-23.5 ng ml(-1) culture medium for EETs and 8.0-200 ng ml(-1) for HETEs. Arachidonic acid was mainly metabolised to HETEs with product levels ranging from 59.3 to 460 ng 10(-6) cells. The median of 8,9-EET and 14,15-EET was 14.5 and 17.7 ng 10(-6) cells, respectively, whereas 5,6-EET and 11,12-EET were below 2 ng 10(-6) cells in a 5-min incubation assay at a 30 microM arachidonic acid substrate concentration.

Action of epoxyeicosatrienoic acids on cellular function

Epoxyeicosatrienoic acids (EETs), which function primarily as autocrine and paracrine mediators in the cardiovascular and renal systems, are synthesized from arachidonic acid by cytochrome P-450 epoxygenases. They activate smooth muscle large-conductance Ca(2+)-activated K(+) channels, producing hyperpolarization and vasorelaxation. EETs also have anti-inflammatory effects in the vasculature and kidney, stimulate angiogenesis, and have mitogenic effects in the kidney. Many of the functional effects of EETs occur through activation of signal transduction pathways and modulation of gene expression, events probably initiated by binding to a putative cell surface EET receptor. However, EETs are rapidly taken up by cells and are incorporated into and released from phospholipids, suggesting that some functional effects may occur through a direct interaction between the EET and an intracellular effector system. In this regard, EETs and several of their metabolites activate peroxisome proliferator-activated receptor alpha (PPARalpha) and PPARgamma, suggesting that some functional effects may result from PPAR activation. EETs are metabolized primarily by conversion to dihydroxyeicosatrienoic acids (DHETs), a reaction catalyzed by soluble epoxide hydrolase (sEH). Many potentially beneficial actions of EETs are attenuated upon conversion to DHETs, which do not appear to be essential under routine conditions. Therefore, sEH is considered a potential therapeutic target for enhancing the beneficial functions of EETs.

A role for 5,6-epoxyeicosatrienoic acid in calcium entry by de novo conformational coupling in human platelets

A major pathway for Ca(2+) entry in non-excitable cells is activated following depletion of intracellular Ca(2+) stores. A de novo conformational coupling between elements in the plasma membrane (PM) and Ca(2+) stores has been proposed as the most likely mechanism to activate this capacitative Ca(2+) entry (CCE) in several cell types, including platelets. Here we report that a cytochrome P450 metabolite, 5,6-EET, might be a component of the de novo conformational coupling in human platelets. In these cells, 5,6-EET induces divalent cation entry without having any detectable effect on Ca(2+) store depletion. 5,6-EET-induced Ca(2+) entry was sensitive to the CCE blockers 2-APB, lanthanum, SKF-96365 and nickel and impaired by incubation with anti-hTRPC1 antibody. Ca(2+) entry stimulated by low concentrations of thapsigargin, which selectively depletes the dense tubular system and induces EET production, was impaired by the cytochrome P450 inhibitor 17-ODYA, which has no effect on CCE mediated by depletion of the acidic stores using 2,5-di-(tert-butyl)-1,4-hydroquinone. We have found that 5,6-EET-induced Ca(2+) entry requires basal levels of H(2)O(2), which might maintain a redox state favourable for this event. Finally, our results indicate that 5,6-EET induces the activation of tyrosine kinase proteins and the reorganization of the actin cytoskeleton, which might provide a support for the transport of portions of the Ca(2+) store towards the PM to facilitate de novo coupling between IP(3)R type II and hTRPC1 detected by coimmunoprecipitation. We propose that the involvement of 5,6-EET in TG-induced coupling between IP(3)R type II and hTRPC1 and subsequently CCE is compatible with the de novo conformational coupling in human platelets.

14,15-Dihydroxyeicosatrienoic acid activates peroxisome proliferator-activated receptor-alpha

Epoxyeicosatrienoic acids (EETs), lipid mediators synthesized from arachidonic acid by cytochrome P-450 epoxygenases, are converted by soluble epoxide hydrolase (SEH) to the corresponding dihydroxyeicosatrienoic acids (DHETs). Originally considered as inactive degradation products of EETs, DHETs have biological activity in some systems. Here we examined the capacity of EETs and DHETs to activate peroxisome proliferator-activated receptor-alpha (PPARalpha). We find that among the EET and DHET regioisomers, 14,15-DHET is the most potent PPARalpha activator in a COS-7 cell expression system. Incubation with 10 microM 14,15-DHET produced a 12-fold increase in PPARalpha-mediated luciferase activity, an increase similar to that produced by the PPARalpha agonist Wy-14643 (20 microM). Although 10 microM 14,15-EET produced a threefold increase in luciferase activity, this was abrogated by the SEH inhibitor dicyclohexylurea. 14-Hexyloxytetradec-5(Z)-enoic acid, a 14,15-EET analog that cannot be converted to a DHET, did not activate PPARalpha. However, PPARalpha was activated by 2-(14,15-epoxyeicosatrienoyl)glycerol, which was hydrolyzed and the released 14,15-EET converted to 14,15-DHET. COS-7 cells incorporated 14,15-[3H]DHET from the medium, and the cells also retained a small amount of the DHET formed during incubation with 14,15-[3H]EET. Binding studies indicated that 14,15-[3H]DHET binds to the ligand binding domain of PPARalpha with a Kd of 1.4 microM. Furthermore, 14,15-DHET increased the expression of carnitine palmitoyltransferase 1A, a PPARalpha-responsive gene, in transfected HepG2 cells. These findings suggest that 14,15-DHET, produced from 14,15-EET by the action of SEH, may function as an endogenous activator of PPARalpha.

Rapid and simple determination of epoxyeicosatrienoic acids in rabbit renal artery by reversed-phase HPLC with fluorescence detection

A liquid chromatographic method with fluorescence detection coupled with a solid-phase extraction was applied to the rapid determination of epoxyeicosatrienoic acids (EETs) in the rabbit renal artery. The EETs were extracted with an acetonitrile from renal artery homogenate and concentrated by a solid-phase extraction method. The concentrated EETs were reacted directly with a 6, 7-dimethoxy-1-methyl-2 (1H)-quinoxalinone-3-propionyl-carboxylic acid (DMEQ) hydrazide and separated by a reversed-phase HPLC with eluting a combination of a step-wise and a gradient of a mixture of methanol and water. The content of EETs in the renal arteries was significantly greater in the 0.5% cholesterol fed rabbits than in control rabbits. It is suggested that hyperchlesterolemia increases the production of EETs in the rabbit renal artery.

Determination of bioactive eicosanoids in brain tissue by a sensitive reversed-phase liquid chromatographic method with fluorescence detection

Arachidonic acid (AA) is metabolized to prostaglandins (PGs) via cyclooxygenases (COX) catalysis, and to epoxyeicosatrienoic acids (EETs), dihydroxyeicosatrienoic acids (DiHETrEs), and hydroxyeicosatetraenoic acids (HETEs) via cytochrome P450 (CYP450) enzymes. A reliable and robust fluorescence based HPLC method for these eicosanoids was developed. A new selective reverse-phase solid phase extraction (SPE) procedure was developed for PG, DiHETrEs, HETE, and EETs of interest from rat cortical brain tissue. The eicosanoids were derivatized with 2-(2,3-naphthalimino)ethyl-trifluoromethanesulphonate (NE-OTf), followed by separation and quantification at high sensitivity using reverse-phase HPLC with fluorescent detection, and further identified via LC/MS. The derivatization was studied and optimized to obtain reproducible reactions. Various PGs, DiHETrEs, HETEs, EETs, and AA were sensitively detected and baseline resolved simultaneously. LC/MS under positive electrospray ionization selected ion monitoring (SIM) mode was developed to further identify the peaks of these eicosanoids in cortical brain tissue. The method was applied in the traumatic brain injured rat brain.

Simultaneous resolution of underivatized regioisomers and stereoisomers of arachidonate epoxides by capillary electrophoresis

cis-Epoxyeicosatrienoic acids (EETs) and their hydrolysis products (threo-DHETs) have been proposed to be endothelial-dependent hyperpolarizing factors (EDHFs) which upregulate blood flow when tissue perfusion is impaired. Various EET regioisomers and enantiomers are formed from arachidonate by inducible cytochrome P450 epoxygenase isoforms, and tissue EET profiles may vary with diet, time, and disease. Because EET actions and metabolism may be regio- and stereospecific, convenient methods to measure profiles of EET isomers in tissues are needed. In the current studies, we describe two simple capillary electrophoretic methods for resolving EETs. The first method involves capillary electrophoresis with a mixture of neutral and anionic beta-cyclodextrins, which in one step baseline-resolves underivatized EET regioisomers and their enantiomers. Low picogram amounts of EET enantiomers were identified based on migration times and UV spectra. The method was also used to assess the antipode purity of EET standards, and to determine murine hepatic levels of EET enantiomers. The second method involves capillary electrochromatography, which also baseline-resolves underivatized EET and DHET regioisomers in one step. We conclude that in EET assays the major advantages of capillary electrophoresis over reversed-phase HPLC are improved peak efficiency, sensitivity, and resolution, plus precise coelution of deuterated and nondeuterated EETs.

Comparison of vasodilatory properties of 14,15-EET analogs: structural requirements for dilation

Epoxyeicosatrienoic acids (EETs) are endothelium-derived eicosanoids that activate potassium channels, hyperpolarize the membrane, and cause relaxation. We tested 19 analogs of 14,15-EET on vascular tone to determine the structural features required for activity. 14,15-EET relaxed bovine coronary arterial rings in a concentration-related manner (ED(50) = 10(-6) M). Changing the carboxyl to an alcohol eliminated dilator activity, whereas 14,15-EET-methyl ester and 14,15-EET-methylsulfonimide retained full activity. Shortening the distance between the carboxyl and epoxy groups reduced the agonist potency and activity. Removal of all three double bonds decreased potency. An analog with a Delta8 double bond had full activity and potency. However, the analogs with only a Delta5 or Delta11 double bond had reduced potency. Conversion of the epoxy oxygen to a sulfur or nitrogen resulted in loss of activity. 14(S),15(R)-EET was more potent than 14(R),15(S)-EET, and 14,15-(cis)-EET was more potent than 14,15-(trans)-EET. These studies indicate that the structural features of 14,15-EET required for relaxation of the bovine coronary artery include a carbon-1 acidic group, a Delta8 double bond, and a 14(S),15(R)-(cis)-epoxy group.

Human coronary endothelial cells convert 14,15-EET to a biologically active chain-shortened epoxide

Cytochrome P-450 epoxygenase-derived epoxyeicosatrienoic acids (EETs) play an important role in the regulation of vascular reactivity and function. Conversion to the corresponding dihydroxyeicosatrienoic acids (DHETs) by soluble epoxide hydrolases is thought to be the major pathway of EET metabolism in mammalian vascular cells. However, when human coronary artery endothelial cells (HCEC) were incubated with (3)H-labeled 14,15-EET, chain-shortened epoxy fatty acids, rather than DHET, were the most abundant metabolites. After 4 h of incubation, 23% of the total radioactivity remaining in the medium was converted to 10,11-epoxy-hexadecadienoic acid (16:2), a product formed from 14,15-EET by two cycles of beta-oxidation, whereas only 15% was present as 14,15-DHET. Although abundantly present in the medium, 10,11-epoxy-16:2 was not detected in the cell lipids. Exogenously applied (3)H-labeled 10,11-epoxy-16:2 was neither metabolized nor retained in the cells, suggesting that 10,11-epoxy-16:2 is a major product of 14,15-EET metabolism in HCEC. 10,11-Epoxy-16:2 produced potent dilation in coronary microvessels. 10,11-Epoxy-16:2 also potently inhibited tumor necrosis factor-alpha-induced production of IL-8, a proinflammatory cytokine, by HCEC. These findings implicate beta-oxidation as a major pathway of 14,15-EET metabolism in HCEC and provide the first evidence that EET-derived chain-shortened epoxy fatty acids are biologically active.

Capillary electrophoresis of cytochrome P-450 epoxygenase metabolites of arachidonic acid. 1. Resolution of regioisomers

The essential fatty acid arachidonate is oxidized by cytochrome P-450 epoxygenases to four epoxyeicosatrienoic acids (EETs): 14,15-, 11,12-, 8,9-, and 5,6-EETs. Each of the four EET regioisomers and their hydrolysis products (DHETs) has multiple paracrine and autocrine functions and may also potently dilate blood vessels and activate potassium channels. The present work describes a method to resolve EETs and DHETs by capillary electrophoresis (CE) using trimethyl-beta-cyclodextrin and CH3CN as buffer additives. While stored at 25 degrees C, most of the EET and DHET regioisomers remained intact when suspended in alkaline vehicle. However, under these same conditions, 5,6-EET rapidly broke down to a lactone and was slowly converted to 5,6-DHET. When subjected to CE, the EET and DHET regioisomers were baseline resolved (R > or = 1.3); 10 pg of an EET or a DHET regioisomer was readily detectable at 194 nm. In addition, the UV spectra were regiospecific and identical to those obtained during HPLC except that an additional, weak absorption occurred at 235 nm. Together, the high-sensitivity, high-resolution, and differential UV spectra permitted the identification and quantification of EETs in phospholipids isolated from murine liver. Thus, CE was successfully used for the trace analysis of eicosanoids.

Cytochrome p-450 epoxygenase metabolites of docosahexaenoate potently dilate coronary arterioles by activating large-conductance calcium-activated potassium channels

Diets enriched in docosahexaenoic acid, a major n-3 fatty acid in fish oil, have hypotensive properties. One mechanism that can lower blood pressure is the direct dilation of arterioles by docosahexaenoic metabolites. Vascular endothelium contains cytochrome P-450 epoxygenases that transform the n-6 fatty acid arachidonate into epoxyeicosatrienoic acids (EETs), potent dilators of coronary arterioles and activators of large-conductance calcium-activated potassium (BK(Ca)) channels. To test whether analogous activations occur for docosahexaenoate, we compared the potency of docosahexaenoate and its five cytochrome P-450 epoxygenase metabolites, epoxydocosapentaenoates (EDPs), in dilating porcine coronary arterioles (<135 microm in diameter) precontracted with endothelin. The five EDP regioisomers had dilation EC(50) values ranging from 0.5 to 24 pM (n = 5-6). In contrast, the EDP hydrolysis product 13,14-dihydroxydocosapentaenoic acid (13,14-DHDP) had an EC(50) value of 30 +/- 22 nM (n = 7), whereas docosahexaenoate only dilated vessels at > or =1.0 microM (n = 7). Using patch-clamp techniques in the inside-out configuration, we determined that the 13,14-EDP regioisomer potently activated (EC(50) value of 6.6 +/- 0.6 pM; n = 5) BK(Ca) channels in myocytes from the porcine coronary arterioles. Moreover, 13,14-EDP potently activated BK(Ca) channels in myocytes from rat coronary small arteries (150-300 microm in diameter); with an EC(50) value of 2.2 +/- 0.6 pM (n = 7), 13,14-EDP was 1000-fold more potent than EETs in activating BK(Ca) channels. We conclude that EDPs potently dilate coronary microvessels and are the most potent fatty epoxides known to activate BK(Ca) channels in coronary smooth muscle cells. Both actions may contribute to the hypotensive effects of dietary fish oils.

The simultaneous quantification of cytochrome P450 dependent linoleate and arachidonate metabolites in urine by HPLC-MS/MS

A method for the simultaneous quantification of urinary linoleic and arachidonic acid derived epoxides and diols, as well as the arachidonate omega hydroxylated product has been developed. The method employs negative mode electrospray ionization and HPLC with tandem mass spectroscopy for quantification. Odd chain length saturated epoxy and dihydroxy fatty acids are used as analytical surrogates resulting in linear calibrations (r (2) > or = 0.9995). Standard addition analyses showed that matrix effects do not prevent these surrogates from yielding reliable quantitative results. Using 4 ml urine aliquots at a final extract volume of 100 micro l and injecting 10 micro l, method detection limits and limits of quantification were < or =0.5 and 1.5 nM, respectively. The sensitivity for dihydroxy lipids was from 3- to 10-fold greater than the corresponding epoxy fatty acid. Shot to shot run times of 31 min were achieved. Rodent and human urine analyses indicated the method sensitivity is sufficient for general research applications. In addition, diurnal fluctuations in linoleate and arachidonate derived metabolites were observed in human subjects.

CYP4A mRNA, protein, and product in rat lungs: novel localization in vascular endothelium

The vasodilatory effect of 20-hydroxyeicosatetraenoic acid (20-HETE) on lung arteries is opposite to the constrictor effect seen in cerebral and renal vessels. These observations raise questions about the cellular localization of 20-HETE-forming isoforms in pulmonary arteries and other tissues. Using in situ hybridization, we demonstrate for the first time CYP4A (a family of cytochrome P-450 enzymes catalyzing formation of 20-HETE from the substrate arachidonic acid) mRNA in pulmonary arterial endothelial and smooth muscle cells, bronchial smooth muscle and bronchial epithelial cells, type I epithelial cells, and macrophages in adult male rat lungs. Moreover, we detect CYP4A protein in rat pulmonary arteries and bronchi as well as cultured endothelial cells. Finally, we identify endogenously formed 20-HETE by using fluorescent HPLC techniques, as well as the capacity to convert arachidonic acid into 20-HETE in pulmonary arteries, bronchi, and endothelium. These data show that 20-HETE is an endogenous product of several pulmonary cell types and is localized to tissues that optimally position it to modulate physiological functions such as smooth muscle tone or electrolyte flux.

Cytochrome P450 pathways of arachidonic acid metabolism

Cytochrome P450s metabolize arachidonic acid to hydroxyeicosatetraenoic acids and epoxyeicosatrienoic acids. These eicosanoids are formed in a tissue and cell-specific manner and have numerous biological functions. Of major interest are the opposing actions of hydroxyeicosatetraenoic and epoxyeicosatrienoic acids within the vasculature. Regio- and stereoisomeric epoxyeicosatrienoic acids have potent vasodilatory properties while 20-hydroxyeicosatetraenoic acid is a potent vasoconstrictor. Both effects are mediated through actions on large-conductance Ca2+-activated K+ channels. Cytochrome P450-derived eicosanoids are also important in the regulation of ion transport, and have recently been shown to influence a number of fundamental biological processes including cellular proliferation, apoptosis, inflammation, and hemostasis. The formation of these functionally relevant eicosanoids is tightly controlled by the expression and activity of the cytochrome P450 epoxygenases and hydroxylases. In addition, soluble epoxide hydrolase catalyzes the hydrolysis of epoxyeicosatrienoic acids to dihydroxyeicosatrienoic acids, and the activity of this enzyme is a critical determinant of tissue epoxyeicosatrienoic and dihydroxyeicosatrienoic acid levels. The intracellular balance between epoxyeicosatrienoic, dihydroxyeicosatrienoic and hydroxyeicosatetraenoic acids influences the biological response to these eicosanoids and alterations in their levels have recently been associated with certain pathological conditions. The involvement of the cytochrome P450-derived eicosanoids in a wide array of biological functions and the observation that levels are altered in pathological conditions suggest that the enzymes involved in the formation and degradation of these fatty acids may be novel therapeutic targets.

14,15-Dihydroxyeicosatrienoic acid relaxes bovine coronary arteries by activation of K(Ca) channels

Epoxyeicosatrienoic acids (EETs) cause vascular relaxation by activating smooth muscle large conductance Ca(2+)-activated K(+) (K(Ca)) channels. EETs are metabolized to dihydroxyeicosatrienoic acids (DHETs) by epoxide hydrolase. We examined the contribution of 14,15-DHET to 14,15-EET-induced relaxations and characterized its mechanism of action. 14,15-DHET relaxed U-46619-precontracted bovine coronary artery rings but was approximately fivefold less potent than 14,15-EET. The relaxations were inhibited by charybdotoxin, iberiotoxin, and increasing extracellular K(+) to 20 mM. In isolated smooth muscle cells, 14,15-DHET increased an iberiotoxin-sensitive, outward K(+) current and increased K(Ca) channel activity in cell-attached patches and inside-out patches only when GTP was present. 14,15-[(14)C]EET methyl ester (Me) was converted to 14,15-[(14)C]DHET-Me, 14,15-[(14)C]DHET, and 14,15-[(14)C]EET by coronary arterial rings and endothelial cells but not by smooth muscle cells. The metabolism to 14,15-DHET was inhibited by the epoxide hydrolase inhibitors 4-phenylchalcone oxide (4-PCO) and BIRD-0826. Neither inhibitor altered relaxations to acetylcholine, whereas relaxations to 14,15-EET-Me were increased slightly by BIRD-0826 but not by 4-PCO. 14,15-DHET relaxes coronary arteries through activation of K(Ca) channels. Endothelial cells, but not smooth muscle cells, convert EETs to DHETs, and this conversion results in a loss of vasodilator activity.

What is new in endothelium-derived hyperpolarizing factors?

The chemical identification and functional characterization of endothelium-derived hyperpolarizing factors varies depending on vascular size, vascular bed and species. Three major candidates are the epoxyeicosatrienoic acids, cytochrome P450 metabolites of arachidonic acid, potassium ion and hydrogen peroxide. Additionally, electrical coupling through myoendothelial gap junctions serves to conduct electrical changes from the endothelium to the smooth muscle and may mediate or propagate hyperpolarization. Endothelium-derived hyperpolarizing factors are important mediators of vascular relaxation most specifically in resistance sized arteries where they regulate tissue blood flow. The release of the factors is modulated by a number of influences including agonist stimulation, shear stress, estrogen and disease. This article reviews the latest studies concerning the characterization of endothelium-derived hyperpolarizing factors, the mechanisms of factor release and alterations of the factors.

Liquid chromatographic-electrospray ionization-mass spectrometric analysis of cytochrome P450 metabolites of arachidonic acid

Arachidonic acid (AA) can be metabolized by cytochrome P450 (CYP) enzymes to many biologically active compounds including 5,6-, 8,9-, 11,12-, and 14,15-epoxyeicosatrienoic acids (EETs), their corresponding dihydroxyeicosatrienoic acids (DHETs), and 20-hydroxyeicosatetraenoic acid (20-HETE). These eicosanoids are potent regulators of vascular tone. We developed a liquid chromatography-electrospray ionization-mass spectrometry method to simultaneously determine 5,6-, 8,9-, 11,12-, and 14,15-EETs; 5,6-, 8,9-, 11,12-, and 14,15-DHETs; and 20-HETE. [2H8]EETs, [2H8]DHETs, and [2H2]20-HETE were used as internal standards. These compounds are readily separated on a C18 reverse-phase column using water:acetonitrile with 0.005% acetic acid as a mobile phase. The internal standards, [2H8]EETs, [2H8]DHETs, and [2H2]20-HETE, eluted slightly faster than the natural eicosanoids. The samples were ionized by electrospray with fragmentor voltage of 120 V and detected in a negative mode. The negative ion detection gave a lower background than the positive ion detection for these compounds. These eicosanoids exhibited high abundance of the ions corresponding to [M - 1]-. The m/z = 319, 337, and 319 ions were used for quantitation of EETs, DHETs, and 20-HETE, respectively. The detection limits using selected ion monitoring of these compounds are about 1 pg per injection. The position of functional groups and water content of mobile phase had a significant effect on the sensitivity of detection. Water content of 40% was found to give maximal sensitivity. The method was used to determine EETs, DHETs, and 20-HETE in bovine coronary artery endothelial cells, dog plasma, rat astrocytes, and rat kidney microsome samples.

Optimized thiol derivatizing reagent for the mass spectral analysis of disubstituted epoxy fatty acids

A novel procedure is described for the derivatization of fatty acid epoxides in the presence of their corresponding diols. The acidic character of 2,3,5,6-tetrafluorobenzenethiol promotes favorable mass fragmentation of linoleate and arachidonate derived epoxide derivatives and reduces alkene isomerization to a manageable side reaction, eliminated through the addition of a thiol scavenger. After silylation, regioisomeric mixtures of epoxy- and dihydroxylipids are simultaneously detected and discriminated using gas chromatography with electron impact mass spectral detection. Silylated hydroxysulfanyloctadecanoids yielded instrumental detection limits of 5 pg/microl, sufficient sensitivity for the quantification of endogenous epoxylipids.

Identifying endothelium-derived hyperpolarizing factor: recent approaches to assay the role of epoxyeicosatrienoic acids

Investigation of endothelial regulation of vascular reactivity and tone has led to the discovery of chemical mediators such as nitric oxide (NO) and prostacyclin (PGI2). Evidence has emerged indicating another as yet unidentified hyperpolarizing agent (endothelium-derived hyperpolarizing factor or EDHF) that is different from NO and PGI2 and exerts it effects through calcium-activated potassium channels (KCa). Previous studies to identify EDHF have been carried out using inhibitors that block NOS and COX before application of KCa channel and/or muscarinic receptor antagonists. Such pharmacological manipulation has complicated interpretation of results, clearly pointing to the need for altered approaches to verify previous studies. Evidence has emerged that potential EDHF candidates vary with vessel size, species and tissue beds, indicating that there may be more than one EDHF. To date, the most commonly described and best characterized of them all are a set of arachidonic acid metabolites, epoxyeicosatrienoic acids (EETs). These compounds are synthesized both intra- and extravascularly. Until recently, methodology to detect EETs in the microvasculature has been tedious and expensive, limiting the experimentation that is necessary to confirm EETs as an EDHF. This review describes state-of-the-art methods for assaying EETs in biological samples, after summarizing evidence for EETs as an EDHF and introducing emerging concepts of the role of extravascular EETs in linking neuronal activity to localized blood flow during functional hyperemia.