Formation of hepoxilin A4, B4 and the corresponding trioxilins from 12(S)-hydroperoxy-5,8,10,14,17-icosapentaenoic acid

Molecular insights into lipoxygenases for biocatalytic synthesis of diverse lipid mediators

Oxylipins derived mainly from C20- and C22-polyunsaturated fatty acids (PUFAs), termed lipid mediators (LMs), are essential signalling messengers involved in human physiological responses associated with homeostasis and healing process for infection and inflammation. Some LMs involved in the resolution of inflammation and infection are termed specialized pro-resolving mediators (SPMs), which are generated by human M2 macrophages or polymorphonuclear leukocytes and have the potential to protect and treat hosts from bacterial and viral infections by phagocytosis activation. Lipoxygenases (LOXs) biosynthesize regio- and stereoselective LMs. Thus, understanding the regio- and stereoselectivities of LOXs for PUFAs at a molecular level is important for the biocatalytic synthesis of diverse LMs. Here, we elucidate the catalytic mechanisms and discuss regio- and stereoselectivities and their changes of LOXs determined by insertion direction and position of the substrate and oxygen at a molecular level for the biosynthesis of diverse human LMs. Recently, the biocatalytic synthesis of PUFAs to human LMs or analogues has been conducted using microbial LOXs. Such microbial LOXs involved in the biosynthesis of LMs are expected to exert significantly higher activity and stability than human LOXs. Diverse regio- and stereoselective LOXs can be obtained from microorganisms, which represent a wealth of genomic sources. We reconstruct the biosynthetic pathways of LOX-catalyzed LMs in humans and other organisms. Furthermore, we suggest the effective methods of biocatalytic synthesis of diverse human LMs from PUFAs or glucose by using microbial LOXs, increasing the stability and activity of LOXs, combining the reactions of LOXs, and constructing metabolic pathways.

Human lipoxygenase isoforms form complex patterns of double and triple oxygenated compounds from eicosapentaenoic acid

Lipoxygenases (ALOX) are lipid peroxidizing enzymes that catalyze the biosynthesis of pro- and anti-inflammatory lipid mediators and have been implicated in (patho-)physiological processes. In humans, six functional ALOX isoforms exist and their arachidonic acid oxygenation products have been characterized. Products include leukotrienes and lipoxins which are involved in the regulation of inflammation and resolution. Oxygenation of n3-polyunsaturated fatty acids gives rise to specialized pro-resolving mediators, e.g. resolvins. However, the catalytic activity of different ALOX isoforms can lead to a multitude of potentially bioactive products. Here, we characterized the patterns of oxygenation products formed by human recombinant ALOX5, ALOX15, ALOX15B and ALOX12 from eicosapentaenoic acid (EPA) and its 18-hydroxy derivative 18-HEPE with particular emphasis on double and triple oxygenation products. ALOX15 and ALOX5 formed a complex mixture of various double oxygenation products from EPA, which include 5,15-diHEPE and various 8,15-diHEPE isomers. Their biosynthetic mechanisms were explored using heavy oxygen isotopes (H₂¹⁸O, ¹⁸O₂ gas) and three catalytic activities contributed to product formation: i) fatty acid oxygenase activity, ii) leukotriene synthase activity, iii) lipohydroperoxidase activity. For ALOX15B and ALOX12 more specific product patterns were identified, which was also the case when these enzymes reacted in concert with ALOX5. Several double oxygenated compounds were formed from 18-HEPE by ALOX5, ALOX15B and ALOX12 including previously identified resolvins (RvE2, RvE3), while formation of triple oxygenation products, e.g. 5,17,18-triHEPE, required ALOX5. Taken together our data show that EPA can be converted by human ALOX isoforms to a large number of secondary oxygenation products, which might exhibit bioactivity.

Biotransformation of polyunsaturated fatty acids to bioactive hepoxilins and trioxilins by microbial enzymes

Hepoxilins (HXs) and trioxilins (TrXs) are involved in physiological processes such as inflammation, insulin secretion and pain perception in human. They are metabolites of polyunsaturated fatty acids (PUFAs), including arachidonic acid, eicosapentaenoic acid and docosahexaenoic acid, formed by 12-lipoxygenase (LOX) and epoxide hydrolase (EH) expressed by mammalian cells. Here, we identify ten types of HXs and TrXs, produced by the prokaryote Myxococcus xanthus, of which six types are new, namely, HXB₅, HXD₃, HXE₃, TrXB₅, TrXD₃ and TrXE₃. We succeed in the biotransformation of PUFAs into eight types of HXs (>35% conversion) and TrXs (>10% conversion) by expressing M. xanthus 12-LOX or 11-LOX with or without EH in Escherichia coli. We determine 11-hydroxy-eicosatetraenoic acid, HXB₃, HXB₄, HXD₃, TrXB₃ and TrXD₃ as potential peroxisome proliferator-activated receptor-γ partial agonists. These findings may facilitate physiological studies and drug development based on lipid mediators.

Hepoxilins (HXs) and trioxilins (TrXs) are lipid metabolites with roles in inflammation and insulin secretion. Here, the authors discover a prokaryotic source of HXs and TrXs, identify the biosynthetic enzymes and heterologously express HXs and TrXs in E. coli.

ω-3 Tear Film Lipids Correlate With Clinical Measures of Dry Eye

PURPOSE

ω-3 and ω-6 polyunsaturated fatty acids modulate inflammatory processes throughout the body through distinct classes of lipid mediators that possess both proinflammatory and proresolving properties. The purpose of this cross-sectional study was to explore the relationship between lipid profiles in human tears and dry eye (DE) symptoms and signs.

METHODS

Forty-one patients with normal eyelid and corneal anatomy were prospectively recruited from a Veterans Administration Hospital over 18 months. Symptoms and signs of DE were assessed, and tear samples was analyzed by mass spectrometry-based lipidomics. Statistical analyses comparing the relationship between tear film lipids and DE included Pearson/Spearman correlations and t-tests.

RESULTS

Arachidonic acid (AA), docosahexaenoic acid (DHA), and eicosapentaenoic acid (EPA) were present in more than 90% of tear film samples. The ratio of ω-6 (AA) to ω-3 (DHA+EPA) fatty acids was correlated with multiple measures of tear film dysfunction (tear breakup time, Schirmer 2 scores, and corneal staining; all P < 0.05). Arachidonic acid-derived prostaglandin E2 was detected in the majority of samples and correlated with low tear osmolarity, meibomian gland plugging, and corneal staining.

CONCLUSIONS

Both ω-3 and ω-6 lipid circuits are activated in the human tear film. The ratio of ω-6:ω-3 tear lipids is elevated in DE patients in proportion to the degree of tear film dysfunction and corneal staining. Metabolic deficiency of ω-3 tear film lipids may be a driver of chronic ocular surface inflammation in DE.

Vascular hepoxilin and trioxilins mediate vasorelaxation through TP receptor inhibition in mouse arteries

AIM

12/15-lipoxygenase (12/15-LO) metabolizes arachidonic acid (AA) into several vasoactive eicosanoids. In mouse arteries, we previously characterized the enzyme's 15-LO metabolites 12(S)-hydroxyeicosatetraenoic acid (HETE), 15-HETE, hydroxyepoxyeicosatrienoic acids (HEETAs) and 11,12,15-trihydroxyeicosatrienoic acids (11,12,15-THETAs) as endothelium-derived relaxing factors. However, the observed 12-LO metabolites remained uncharacterized. The purpose of this study was to determine the structure and biological functions of eicosanoids generated by the enzyme's 12-LO activity.

METHODS

Metabolites extracted from aortas of C57BL/6 male mice were separated using a series of reverse and normal phase chromatographic steps and identified as hepoxilin A₃ , trioxilin A₃ and trioxilin C₃ by mass spectrometry. Activities of these natural compounds were tested on isometric tension and intracellular calcium release. The role of thromboxane (TP) receptor was determined in HEK293 cells overexpressing TPα receptor (TPα -HEK).

RESULTS

All identified vascular 12-LO metabolites were biologically active. In mouse mesenteric arteries, trioxilin A₃ , C₃ and hepoxilin A₃ (3 μm) relaxed arteries constricted with the thromboxane mimetic, U46619-constricted arteries (maximum relaxations of 78.9 ± 3.2, 29.7 ± 4.6, 82.2 ± 5.0 and 88.0 ± 2.4% respectively), but not phenylephrine-constricted arteries. In TPα-HEK cells, trioxilin A₃ , C₃ and hepoxilin A₃ (10 μm) inhibited U46619 (10 nM)-induced increases in intracellular calcium by 53.0 ± 7.2%, 32.8 ± 5.0% and 37.9 ± 13.5% respectively. In contrast, trioxilin B₃ and hepoxilin B₃ were not synthesized in arteries and exhibited little biological activity.

CONCLUSION

Trioxilin A₃ and C₃ and hepoxilin A₃ are endogenous vascular relaxing factors. They are not endothelium-derived hyperpolarizing factors but mediate vascular relaxation by inhibiting TP agonist-induced increases in intracellular calcium. Thus, they regulate vascular homeostasis by acting as endogenous TP antagonists.

Versatility of tandem mass spectrometry for focused analysis of oxylipids

Liquid chromatography-tandem mass spectrometry (LC-MS/MS) in the multiple reaction monitoring (MRM) scan mode has been the primary MS method applied for the target identification of specific and minor oxylipids in complex matrices, such as eicosanoids and docosanoids, which are potent lipid mediators derived from polyunsaturated fatty acid oxygenation. However, the high specificity of MRM can limit the detection of species with m/z MRM transitions not covered by the method. In addition to MRM, tandem-quadrupole mass analyzers enable other experiments to be conducted, by fragmenting ions via collision-induced dissociation process (CID). This paper presents the potential of tandem mass spectrometry for the focused analysis of oxylipids. We have successfully developed an LC-MS/MS method for the identification of precursor ions of m/z 115, a diagnostic product ion of 5-hydroxy- and 5-epoxy-fatty acids. As a proof of concept, the developed method was used to discover several oxylipids oxidized at C5 derived from arachidonic acid (C20 : 4) oxygenation in a hypothalamus rat extract that were not identified using the target MRM methodology. The proposed focused MS/MS-based approach in a tandem mass analyzer has proven to be a powerful strategy to accelerate the identification of oxylipids with structural similarities and assist the field of lipidomic research.

Epoxide hydrolases: their roles and interactions with lipid metabolism

The epoxide hydrolases (EHs) are enzymes present in all living organisms, which transform epoxide containing lipids by the addition of water. In plants and animals, many of these lipid substrates have potent biologically activities, such as host defenses, control of development, regulation of inflammation and blood pressure. Thus the EHs have important and diverse biological roles with profound effects on the physiological state of the host organisms. Currently, seven distinct epoxide hydrolase sub-types are recognized in higher organisms. These include the plant soluble EHs, the mammalian soluble epoxide hydrolase, the hepoxilin hydrolase, leukotriene A4 hydrolase, the microsomal epoxide hydrolase, and the insect juvenile hormone epoxide hydrolase. While our understanding of these enzymes has progressed at different rates, here we discuss the current state of knowledge for each of these enzymes, along with a distillation of our current understanding of their endogenous roles. By reviewing the entire enzyme class together, both commonalities and discrepancies in our understanding are highlighted and important directions for future research pertaining to these enzymes are indicated.

Rabbit aorta converts 15-HPETE to trihydroxyeicosatrienoic acids: potential role of cytochrome P450

Previous work showed that rabbit aorta metabolizes arachidonic acid via 15-lipoxygenase to 15-hydroperoxyeicosatetraenoic acid (15-HPETE), which undergoes an enzymatic rearrangement to 11-hydroxy-14,15-epoxyeicosatrienoic acid (11-H-14,15-EETA) and 15-hydroxy-11,12-epoxyeicosatrienoic acid (15-H-11,12-EETA). Hydrolysis of the epoxy group results in the formation of 11,14,15- and 11,12,15-trihydroxyeicosatrienoic acids (THETAs). Endothelial cells have several heme-containing enzymes including cytochromes P450 (CYP), nitric oxide synthase (eNOS), and prostacyclin (PGI(2)) synthase that catalyze the rearrangement of 15-HPETE to HEETAs. Incubation of arachidonic acid and 15-lipoxygenase, or 15-HPETE with rabbit aortic microsomes or rat liver microsomes, a rich source of CYP, resulted in the formation of a product that comigrated with THETAs and HEETAs on HPLC. Immunoblot analysis showed the presence of CYP2C8 and CYP2J2 in aortic tissue and when CYP2J2 or CYP2C8 was incubated with arachidonic acid and 15-lipoxygenase, the major products were 11,12,15- and 11,14,15-THETAs. Incubation of purified hematin, CYP2C11, eNOS or PGI(2) synthase enzymes with arachidonic acid and 15-lipoxygenase produced a different pattern of metabolites from rabbit aortic microsomes. Clotrimazole, a non-specific CYP inhibitor, and ebastine and terfenadone, specific CYP2J2 inhibitors, blocked the ability of aortic microsomes to produce THETAs while specific inhibitors of PGI(2) synthase, eNOS or CYP2C8/2C9 had no effect on THETA production. We suggest that a CYP, possibly CYP2J2, may function as the hydroperoxide isomerase converting 15-HPETE to HEETAs in rabbit vascular tissue. Further hydrolysis of the epoxy group of the HEETAs results in the formation of 11,12,15- and 11,14,15-THETAs. The HEETAs and THETAs are both vasodilators and may function as important regulators of vascular tone.

Hepoxilins and trioxilins in barnacles: an analysis of their potential roles in egg hatching and larval settlement

The barnacle life cycle has two key stages at which eicosanoids are believed to be involved in cellular communication pathways, namely the hatching of nauplii and the settlement of cypris larvae. Barnacle egg-hatching activity has previously been reported to reside in a variety of eicosanoids, including 8-hydroxyeicosapentaenoic acid and a number of tri-hydroxylated polyunsaturated fatty acid derivatives, the trioxilins. The production of the eicosapentaenoic acid metabolite trioxilin A4 (8,11,12-trihydroxy-5,9,14,17-eicosatetraenoic acid) by the barnacles Balanus amphitrite and Elminius modestus was confirmed using a combination of high-performance liquid chromatography and gas chromatography, both linked to mass spectrometry. In addition, both species also generated trioxilin A3 (8,11,12-trihydroxy-5,9,14-eicosatrienoic acid; an arachidonic acid-derived product), 8,11,12-trihydroxy-9,14,17-eicosatrienoic acid (a omega3 analogue of trioxilin A3; derived from omega3 arachidonic acid) and 10,13,14-trihydroxy-4,7,11,16,19-docosapentaenoic acid (a docosahexaenoic acid-derived product). In contrast to earlier reports, trioxilin A3 had no E. modestus egg-hatching activity at any of the concentrations tested (10(-9)-10(-6) mol l(-1)). The unstable epoxide precursor hepoxilin A3, however, caused significant levels of hatching at 10(-6) mol l(-1). Furthermore, the stable hepoxilin B3 analogue PBT-3 stimulated hatching at 10(-7) mol l(-1). Neither trioxilin A3, hepoxilin A3 or PBT-3 at 0.25-30 micromol l(-1) served as settlement cues for B. amphitrite cypris larvae.

Metabolism of 12-hydroperoxyeicosatetraenoic acid to vasodilatory trioxilin C3 by rabbit aorta

Arachidonic acid is metabolized by both the cyclooxygenase and lipoxygenase pathways by rabbit aorta. We investigated the metabolism of 12-hydroperoxyeicosatetraenoic acid by aortic homogenates and microsomes. Rabbit aortic homogenates were incubated in the presence of (14)C-arachidonic acid plus 12-lipoxygenase and analyzed by reversed-phase high-pressure liquid chromatography (HPLC). Under these experimental conditions, there was a (14)C-metabolite that migrated at 17.6 min. This (14)C-metabolite was not observed when aortic homogenates were incubated in the absence of 12-lipoxygenase. Similar results were obtained with aortic microsomes. Further analysis using a different HPLC solvent system resolved the (14)C-metabolite into a number of products. Gas chromatography/mass spectrometric (GC-MS) analysis of the major product (labeled peak 3) after conversion to the methyl ester-trimethylsilyl derivative showed two major compounds (compounds A and B) eluting at 13.99 and 14.14 min. The two compounds differed in the intensities of the 213 and 243 m/z ions with 243 being greater than 213 in compound A and the opposite in compound B (relative abundance 213 vs. 243; 100% vs. 43% for compound A and 5% vs. 100% for compound B). Based on the mass spectra, peak 3 contained two metabolites identified as the methyl ester-trimethylsilyl ether derivatives of 8,11,12-trihydroxyeicosatrienoic acid (trioxilin A(3)) and 8,9,12-trihydroxyeicosatrienoic acid (trioxilin C(3)). Biological activity of the mixture of two trioxilins isolated from aortic homogenates was tested in phenylephrine-precontracted aortas and found to produce concentration-dependent relaxations (maximal relaxation: 20.1+/-7.6%). Further testing with authentic trioxilin A(3) and C(3) revealed that trioxilin C(3) was the active metabolite (maximal relaxation: 16.6+/-1.3%). In conclusion, trioxilin C(3) acid was isolated and identified as a novel biologically active arachidonic acid metabolite formed by rabbit aorta when 12-lipoxygenase is supplied exogenously.

Docosahexaenoic acid causes accumulation of free arachidonic acid in rat pineal gland and hippocampus to form hepoxilins from both substrates

Hepoxilins (Hx) are biologically active metabolites of arachidonic acid (AA) formed regioselectively from 12(S)-HPETE by 'hepoxilin synthase'. Hx modulate synaptic neurotransmission in hippocampal CA1 neurons, and inhibit norepinephrine release in hippocampal slices. During the course of our studies we investigated whether docosahexaenoic acid (DHA) was a substrate for hepoxilin formation. We used two tissues, the pineal gland and hippocampal slices. Tissues were incubated alone or with AA (20 microg/ml) or DHA (20 microg/ml). After 60 min at 37 degrees C, samples were acid-extracted to convert Hx into their stable trioxilin (TrX) form and analyzed as the Me-TMSi derivatives by EI-GC/MS to determine the structures of the DHA metabolites, and as PFB-TMSi derivatives by GC/MS in the NICI mode using SIM to simultaneously quantify TrX products of the 3-series (derived from AA) monitored at m/z 569, while those of the 5-series (derived from DHA) were monitored at m/z 593. Results show good conversion of both substrate fatty acids by the rat pineal gland and hippocampal slices, into the 3-series (21.3 +/- 5.8 and 12.5 +/- 2.2 ng/microg protein, respectively) and 5-series TrX (12.3 +/- 2.7 and 2.9 +/- 0.4 ng/microg protein, respectively). Surprisingly though, experiments with DHA, in both tissues, also showed formation of TrX derived from endogenous AA (3-series) (10.4 +/- 8.3 and 3.1 +/- 2.1 ng/microg protein, respectively). These experiments demonstrate previously unreported actions of DHA causing the accumulation of AA, which is converted into hepoxilins. In order to prove that AA is accumulated during DHA stimulation of the tissue, we carried out separate experiments with hippocampal slices in which the neutral lipids and phospholipids were labeled with [14C]AA. DHA caused a time-dependent appearance of free [14C]AA which was released mostly from the TG pool. Measurement of the AA/DHA ratio in the TG pool by GC/MS further indicated that DHA is incorporated into the TG at the expense of AA. These results demonstrate that DHA competes with AA for acylation into the metabolically active TG fraction, and both fatty acids are converted into hepoxilins of the corresponding series.

Hepoxilins: a review on their enzymatic formation, metabolism and chemical synthesis

This article reviews published evidence describing the enzymatic and nonenzymatic formation and the routes of metabolism of the hepoxilins. Also treated are the major approaches used for the chemical synthesis of these compounds and for some of their analogs.

Hepoxilins: a review on their cellular actions

This review is intended to summarize the biological actions of the hepoxilins reported to date. These actions appear to have, as their basis, changes in intracellular concentrations of ions including calcium and potassium ions as well as changes in second messenger systems. Recent evidence suggests that the biological actions of the hepoxilins may be receptor-mediated as indicated from data showing the existence of hepoxilin-specific binding proteins in the human neutrophil. Such evidence also implicates the association of G-proteins both in hepoxilin-binding as well as in hepoxilin action. The potential use of stable analogs of the hepoxilins is discussed as well as the directions in which this area is heading.

High-performance liquid chromatography-thermospray mass spectrometry of epoxy polyunsaturated fatty acids and epoxyhydroxy polyunsaturated fatty acids from an incubation mixture of rat tissue homogenate

A method for the analysis of epoxy polyunsaturated fatty acids (EpPUFAs) and epoxyhydroxy polyunsaturated fatty acids (EpHPUFAs) in rat tissue homogenate, with homo-gamma-linolenic acid (20:3, n - 6), arachidonic acid (20:4, n - 6), eicosapentaenoic acid (20:5, n - 3) or docosahexaenoic acid (22:6, n - 3) as a substrate, has been developed. Extraction with dichloromethane at pH 4-5 and concentration in the presence of pyridine were performed. Spectral analysis of chromatograms obtained with high-performance liquid chromatography-thermospray mass spectrometry showed the presence of EpPUFAs, EpHPUFAs and dihydroxy metabolites (DiHPUFAs) of EpPUFAs corresponding to each precursor fatty acid. On a selected-ion monitoring chromatogram, many EpPUFAs, EpHPUFAs and DiHPUFAs in an extract from an incubation mixture of each precursor fatty acid in aged rat tissue homogenate were detected simultaneously within 70 min. EpPUFAs and DiHPUFAs derived from 20:3 (n - 6) or 20:5 (n - 3) were detected in significant amounts. From these results, a highly active cytochrome P450 system or non-enzymic oxidative reactions in aged rat tissue homogenate were suggested.

12S,19- and 12S,20-dihydroxyeicosanoids: novel 12S-hydroxy-5,8-cis-10-trans-14-cis-eicosatetraenoic acid metabolites formed by hydroxylation and reduction in murine lymphocytes

Murine spleen cells and purified B lymphocytes oxidized arachidonic acid via the lipoxygenase pathway. The major metabolite of both the whole spleen and enriched B lymphocytes was 12S-hydroxy-5,8-cis-10-trans-14-cis-eicosatetraenoic acid. A novel metabolite was observed that did not have an absorbance from 210 to 400 nm, indicating the absence of a conjugated double bond system. The new metabolite was converted to the methyl ester, reduced by platinum oxide, derivatized to the trimethylsilyl ether, and analyzed by gas chromatography-mass spectrometry. A major and a minor component were observed in the analysis of the new compound. The major component had major diagnostic ions indicating the presence of hydroxyl groups at C-12 and C-19. The minor component had major diagnostic ions indicating the presence of hydroxyl groups at C-12 and C-20. The new metabolites are characterized as a mixture of 12S,19- and 12S,20-dihydroxyeicosanoids presumably formed by hydroxylation and reduction of one or more double bonds of 12S-hydroxy-5,8-cis-10-trans-14-cis-eicosatetraenoic acid. These metabolites were formed predominantly with whole spleen lymphocytes but could be detected at longer incubation times or by using 12S-hydroxy-5,8-cis-10-trans-14-cis-eicosatetraenoic acid as the starting substrate with highly enriched B lymphocytes.

Biosynthesis, catabolism, and biological properties of HPETEs, hydroperoxide derivatives of arachidonic acid

The oxygenation of arachidonic acid by lipoxygenases results in the formation of HPETEs (hydroperoxyeicosatetraenoic acids), the first products of the LOX pathway. These compounds are short lived and are catabolised into various families of more stable compounds of which the HETEs, hepoxilins, lipoxins and leukotrienes have been identified so far. The development of new techniques have helped to identify and understand the structures of various HPETEs and only recently the biological effects of HPETEs and their various catabolites are being unraveled. Although lipoxygenases are ubiquitous, not all tissues possess the same spectrum of lipoxygenase enzymes. Hence different HPETEs can be formed in different tissues. Recent studies have revealed that HPETEs or products derived from them possess a diversity of important biological properties including the regulation of electrolyte flux and eicosanoid and corticosterone syntheses, release of histamine, regulation of oocyte maturation and release of various reproductive hormones. HPETEs appear to be involved in some pathological conditions viz, skin psoriasis, Clarkson's disease, nerve injury and spinal cord ischemia. These novel eicosanoids are associated with the release of insulin as well as renin. Recently HPETEs have been suggested to act as second messengers in the Aplysia sensory neurons and its catabolite, hepoxilin, has been demonstrated to have effects on mammalian hippocampal neurons. The purpose of this review is to provide a brief summary of the formation of the HPETEs and the various families of compounds derived from them as well as the various types of biological activities for these products described so far.

Formation and metabolism of hepoxilin A3 by the rat brain

Incubation of homogenates of the rat cerebral cortex with arachidonic acid led to the appearance of hepoxilin A3, analysed as its stable trihydroxy derivative, trioxilin A3, by high resolution gas chromatography/electron impact mass spectrometry. Using the stable deuterium isotope dilution technique, it is estimated that the cerebral cortex generates 5.0 +/- 0.2 ng/mg protein of hepoxilin A3. The formation of this product was stimulated by the addition of exogenous arachidonic acid (12.9 +/- 1.5 ng/mg protein) and blocked by boiling of the tissue. Addition of the dual cyclooxygenase/lipoxygenase inhibitor BW 755C at a concentration of 75 microM did not result in a blockade of hepoxilin formation. Three other regions were also tested for their ability to form hepoxilin A3 upon stimulation with exogenous arachidonic acid, i.e. median eminence, 11.7 +/- 1.6 ng/mg protein, pituitary, 12.3 +/- 0.7 ng/mg protein; pons, 26.6 +/- 0.2 ng/mg protein. In a separate study, 14C-labelled hepoxilin A3 was transformed into 14C-labelled trioxilin A3 by homogenates of the rat whole brain, demonstrating the presence of epoxide hydrolases in the CNS which utilise the hepoxilins as substrates. This is the first demonstration of the occurrence of the hepoxilin pathway in the central nervous system.

In vivo formation of hepoxilin A3 in the rat

Bolus intravenous injection of arachidonic acid (10 mg/kg) in the rat led to the appearance of hepoxilin A3 in the circulation. The product was assayed as the Me t-BDMSi derivative of its stable trihydroxy product trioxilin A3, by capillary gas chromatography-electron impact mass spectrometry using the stable deuterium isotope dilution technique. Hepoxilin A3, was undetected in blood samples taken prior to the injection of arachidonic acid, but rapidly appeared (4.62 +/- 1.3 ng/ml blood, n = 3) within 1 minute after injection of arachidonic acid. The plasma concentration of insulin increased by 36% over the same period after injection of arachidonic acid. These experiments demonstrate for the first time the formation of this new class of insulin secretagogues in vivo and their temporal correlation with plasma insulin concentrations in vivo.