Formation of 8,15-dihydroxy eicosatetraenoic acidvia 15- and 12-lipoxygenases in fish gill

Rainbow trout (Oncorhynchus mykiss) brain cells biosynthesize novel docosahexaenoic acid-derived resolvins and protectins-Mediator lipidomic analysis

Docosahexaenoic acid (DHA; C22:6 n-3) is an abundant fatty acid in fish phospholipids. In the present study, we employed liquid chromatography-ultraviolet spectrometry-tandem mass spectrometry and dissociated rainbow trout (Oncorhynchus mykiss) brain cells to determine whether fish utilize endogenous DHA to produce the recently uncovered novel lipid mediators termed the resolvins and protectins, generated by mammalian cells [Serhan CN, Hong S, Gronert K, et al. Resolvins: a family of bioactive products of omega-3 fatty acid transformation circuits initiated by aspirin treatment that counter proinflammation signals. J Exp Med 2002; 196:1025-37; Hong S, Gronert K, Devchand P, Moussignac R-L, Serhan, CN. Novel docosatrienes and 17S-resolvins generated from docosahexaenoic acid in murine brain, human blood, and glial cells. J Biol Chem 2003;278:14677-87]. Trout brain cells biosynthesize a range of recently identified di- and tri-hydroxy-containing bioactive products from endogenous sources of DHA when challenged in vitro. We identified neuroprotectin D1, resolvin D5, resolvin D1 and resolvin D2 from trout brain cells. Each compound was identified on the basis of its characteristic physical chemical properties that included MS, MS-MS, UV spectra and chromatographic behavior. The monohydroxy products from DHA, signatures of DHA conversion by lipoxygenases, were also identified. These included both 14S-hydroxy-docosahexaenoic acid and 17S-hydroxy-docosahexaenoic acid. The biosynthesis of these novel bioactive lipid mediators, namely resolvins and protectins, by fish cells provides the first evidence for the conservation of these structures from fish to humans as chemical signals in diverse biological systems.

At what stage in metazoan evolution did leukotriene generation first appear?--key insights from cartilaginous fish

Leukotriene (LT) B4 is a key player in inflammatory responses in mammals. During the generation of this derivative of arachidonic acid, the unstable product of 5-lipoxygenase, termed LTA4, is converted to LTB4 by LTA4 hydrolase. Invertebrates do not generate LTs yet all vertebrates from bony fish onwards synthesize this compound. As cartilaginous fish are the most primitive living jawed vertebrates, we investigated if the leukocytes from such a fish, the Thornback ray (Raja clavata) could generate LTB4. Supernatants from ionophore-challenged leukocytes generated the 5-lipoxygenase products, 6-trans-LTB4 and 6-trans-12-epi-LTB4 but were unable to synthesize LTB4. To determine if these cells contained an active LTA4 hydrolase, LTA4 was incubated with lysates from ray leukocytes. Such preparations did not contain any demonstrable LTA4 hydrolase activity. Our findings imply at the stage of cartilaginous fish evolution over 350 million years ago that the evolution of an active LTA4 hydrolase had yet to occur.

The eicosanoid generating capacity of isolated cell populations from the gills of the rainbow trout, Oncorhynchus mykiss

Rainbow trout gill filaments generated a wide range of eicosanoid products following calcium ionophore challenge. The putative lipoxygenase products were separated by reverse phase high performance liquid chromatography (RP-HPLC), while prostanoids were quantified by enzyme immunoassay. Three main monohydroxy compounds containing conjugated dienes were observed after RP-HPLC namely 12-(S) hydroxyeicosatetraenoic acid (12-HETE), 12-(S) hydroxyeicosapentaenoic acid (12-HEPE) and 14-(S) hydroxydocosahexaenoic acid (14-HDHE), derived from endogenous arachidonic, eicosapentaenoic and docosahexaenoic acids, respectively. Their identification was confirmed by mass spectrometry. A further five compounds containing conjugated trienes were also observed but in lesser amounts. One of these products was identified as 8,15-dihydroxyeicosatetraenoic acid (8,15-DiHETE) based on its UV spectrum, co-elution with authentic standard on RP-HPLC and mass spectrometry. Overall, the generation of these products suggests the presence of 12- and possibly 15-lipoxygenase activities in trout gill acting on endogenous sources of fatty acid. To determine if the various cell types in trout gill had differing eicosanoid generating potential, gills were disrupted and the resultant cell suspensions separated by density gradient centrifugation. Following this three bands were formed on the gradients and the cell populations from these were characterised using periodic acid Schiff's (PAS) reactivity for mucosubstances, haematoxylin and eosin staining, and immunoreactivity with both monoclonal and polyclonal antibodies. The first band consisted of polygonal cells and other more minor cell types, the second cell band contained mainly polygonal and PAS-positive goblet epithelial cells, while the third band consisted of mainly erythrocytes. There were significant differences in the eicosanoid generating potential of the isolated cells, with cells from the second band generating significantly more 12-HETE and 8,15-DiHETE than those from both the first band and unfractionated populations. The eicosanoid generating activity of the trout gill epithelial cell line, RTG-W1, was also elucidated. It proved to be a modest generator of eicosanoids in that only low levels of thromboxane B2 and prostaglandin E2 were detected while no lipoxygenase products were observed.

Eicosanoid biosynthesis in an advanced deuterostomate invertebrate, the sea squirt (Ciona intestinalis)

The eicosanoid generating potential of tunic, branchial basket, intestine, ovary and tadpole larvae from the sea squirt, Ciona intestinalis, was examined using a combination of reverse phase high performance liquid chromatography, gas chromatography-mass spectrometry and enzyme immunoassay. All organs examined synthesized the lipoxygenase products 12-hydroxyeicosapentaenoic acid (12-HEPE) and 8-HEPE implying that both 8- and 12-lipoxygenase activity are widely distributed in this species. In addition, tunic and branchial basket generated significant amounts of 8,15-diHEPE and smaller amounts of 8,15-dihydroxyeicosatetraenoic acid (8,15-diHETE), while tunic alone generated small amounts of conjugated tetraene-containing material with a UV chromophore and mass ion characteristic of a lipoxin-like compound. The broad range lipoxygenase inhibitors, esculetin and nordihydroguaiaretic acid, both caused a significant dose dependent inhibition of 12-HEPE and 8,15-diHEPE biosynthesis in tunic, while the specific 5-lipoxygenase inhibitor, REV-5901, and the specific 5-lipoxygenase activating protein inhibitor, MK-866, had no observable effect on the lipoxygenase profile of this tissue. Tunic, branchial basket, intestine and ovary all generated significant amounts of prostaglandin (PG) E and PGF immunoreactive material and smaller amounts of thromboxane B immunoreactive material as measured by enzyme immunoassay. The non-specific cyclooxygenase (COX) inhibitor, indomethacin, the selective COX-1 inhibitors, resveratrol and valerylsalicylate, and the specific COX-2 inhibitors, NS-398, etolodac and DFU (5,5-dimethyl-3-(3-fluorophenyl)-4-(4-methylsulphonyl) phenyl-2(5H)-furanone) all caused a significant dose dependent inhibition of the biosynthesis of PGE immunoreactive material. However, the specific COX-2 inhibitors were most effective, perhaps implying that a COX-2-like enzyme may be present in this species.

Gas-liquid chromatography-mass spectrometry of hydroxy fatty acids as their methyl esters tert.-butyldimethylsilyl ethers

tert.-Butyldimethylsilyl ethers of secondary hydroxy fatty acid methyl esters (tBDMS-O-FAMEs) produce stable derivatives amenable to gas-liquid chromatography (GLC) and mass spectrometry (MS). Derivatives produce prominent molecular mass minus 57 [M-57]+ fragment ions and unique marker fragment ions indicating the location of the secondary hydroxyl groups along the aliphatic chain from the omega-2 carbon to carbon numbers 5 from the carboxylic terminus, in addition to yielding information regarding carbon chain length, and degree of unsaturation. The tBDMS-derivatives of C-2, C-3 hydroxy fatty acids and the unique GLC-MS data of gamma- and delta-lactones are also presented. Though several tBDMS-O-FAMEs with centrally located hydroxyl groups were not chromatographically resolved, the combination of GLC retention times and monitoring of key diagnostic fragment ions of each tBDMS-derivative, when applied to mixtures containing all hydroxy isomers of palmitic through arachidic acid methyl esters, and to several monounsaturated, monohydroxylated fatty acid methyl esters, allowed for their unambiguous identification. Coupled with derivative stability, permitting their purification and concentration, this method was applied to the identification of trace lipids isolated from bovine skim milk which contained a complex mixture of hydroxy fatty acids of which 19 were newly identified.

Purification and molecular cloning of an 8R-lipoxygenase from the coral Plexaura homomalla reveal the related primary structures of R- and S-lipoxygenases

Lipoxygenases that form S configuration fatty acid hydroperoxides have been purified or cloned from plant and mammalian sources. Our objectives were to characterize one of the lipoxygenases with R stereospecificity, many of which are described in marine and freshwater invertebrates. Characterization of the primary structure of an R-specific enzyme should help provide a new perspective to consider the enzyme-substrate interactions that are the basis of the specificity of all lipoxygenases. We purified an 8R-lipoxygenase of the prostaglandin-containing coral Plexaura homomalla by cation and anion exchange chromatography. This yielded a colorless enzyme preparation, a band of approximately 100 kDa on SDS-polyacrylamide gel electrophoresis, and turnover numbers of 4000 min-1 of 8R-lipoxygenase activity in peak chromatographic fractions. The full-length cDNA was cloned by PCR using peptide sequence from the purified protein and by 5'- and 3'-rapid amplification of cDNA ends. The cDNA encodes a polypeptide of 715 amino acids, including over 70 amino acids identified by peptide microsequencing. A peptide presequence of 52 amino acids is cleaved to give the mature protein of 76 kDa; the difference from the estimated size by SDS-PAGE implies a post-translational modification of the P. homomalla enzyme. All of the iron-binding histidines of S-lipoxygenases are conserved in the 8R-lipoxygenase. However, the C-terminal amino acid is a threonine, as opposed to the isoleucine that provides the carboxylate ligand to the iron in all known S-lipoxygenases. These results establish that the 8R-lipoxygenase is related in primary structure to the S-lipoxygenases. A model of the basis of R and S stereospecificity is described.

Fatty acid metabolism in freshwater fish with particular reference to polyunsaturated fatty acids

Fatty acids in fish can arise from two sources: synthesis de novo from non-lipid carbon sources within the animal, or directly from dietary lipid. Acetyl-CoA derived mainly from protein can be converted to saturated fatty acids via the combined action of acetyl-CoA carboxylase and fatty acid synthetase. The actual rate of fatty acid synthesis de novo is inversely related to the level of lipid in the diet. Freshwater fish can desaturate endogenously-synthesized fatty acids to monounsaturated fatty acids via a delta 9 desaturase but lack the necessary enzymes for complete de novo synthesis of polyunsaturated fatty acids which must therefore be obtained preformed from the diet. Most freshwater fish species can desaturate and elongate 18:2(n-6) and 18:3(n-3) to their C20 and C22 homologues but the pathways involved remain ill-defined. Cyclooxygenase and lipoxygenase enzymes can convert C20 polyunsaturated fatty acids to a variety of eicosanoid products. The dietary ratio of (n-3) to (n-6) polyunsaturated fatty acids influences the pattern of eicosanoids formed. The beta-oxidation of fatty acids can occur in both mitochondria and peroxisomes but mitochondrial beta-oxidation is quantitatively more important and can utilise a wide range of fatty acid substrates.

Eicosanoid generating capacities of different tissues from the rainbow trout, Oncorhynchus mykiss

The eicosanoid generating potential of the brain, gills, skin, ovary, muscle, eye, liver, spleen, heart, and alimentary canal in the rainbow trout, Oncorhynchus mykiss, was examined. All the organs/tissues examined synthesized the 12-lipoxygenase products, 12-hydroxyeicosatetraenoic acid (12-HETE), and 12-hydroxyeicosapentaenoic acid (12-HEPE), implying the widespread nature of this enzyme in trout. Both prostaglandin E and LTC were also found in variable amounts in the organs, with the greatest amount of PGE found in the gill. Leukotriene (LT) B4 and LTB5 were found in supernatants from calcium ionophore-challenged brain, skin, ovary, liver, spleen, and heart, but the lipoxins A4 and A5 were only present in brain, ovary, and spleen in relatively small amounts. As lipoxins have previously been shown to be synthesized by macrophages in rainbow trout [Pettitt et al., J. Biol. Chem. 266, 8720-8726 (1991)], and related cells (microglial cells) are found in the brain of mammals, the localization of macrophage-like cells in trout brain was investigated immunocytochemically. Monoclonal antibodies specific for trout leucocytes failed to identify any microglial-like cells in sections of the brain, although microvessels containing immuno-positive reaction products were observed. A number of distinct lipoxygenase products were found in supernatants of ionophore-challenged gill, including 14-hydroxydocosahexaenoic acid, 12-HETE, and 12-HEPE, and a large number of dihydroxy fatty acid derivatives with conjugated triene chromophores. One of these products was tentatively identified as 8(R),15(S)-dihydroxyeicosatetraenoic acid, a dual 12- and 15-lipoxygenase product, but apparently no LTB4 was generated by this tissue.