Biosynthesis of vicinal dihydroxy fatty acids in the red alga Gracilariopsis lemaneiformis: identification of a sodium-dependent 12-lipoxygenase and a hydroperoxide isomerase

Mammalian-Like Inflammatory and Pro-Resolving Oxylipins in Marine Algae

Oxylipins constitute a family of oxidized fatty acids, that are well known as tissue hormones in mammals. They contribute to inflammation and its resolution. The major classes of these lipid mediators are inflammatory prostaglandins (PGs) and leukotrienes (LTs) as well as pro-resolving resolvins (Rvs). Understanding their biosynthetic pathways and modes of action is important for anti-inflammatory interventions. Besides mammals, marine algae also biosynthesize mammalian-like oxylipins and thus offer new opportunities for oxylipin research. They provide prolific sources for these compounds and offer unique opportunities to study alternative biosynthetic pathways to the well-known lipid mediators. Herein, we discuss recent findings on the biosynthesis of oxylipins in mammals and algae including an alternative pathway to prostaglandin E2 , a novel pathway to a precursor of leukotriene B4 , and the production of resolvins in algae. We evaluate the pharmacological potential of the algal metabolites with implications in health and disease.

Stereopermutation on the Putative Structure of the Marine Natural Product Mucosin

A stereodivergent total synthesis has been executed based on the plausibly misassigned structure of the unusual marine hydrindane mucosin (1). The topological connectivity of the four contiguous all-carbon stereocenters has been examined by selective permutation on the highlighted core. Thus, capitalizing on an unprecedented stereofacial preference of the cis-fused bicycle[4.3.0]non-3-ene system when a Michael acceptor motif is incorporated, copper-mediated conjugate addition furnished a single diastereomer. Cued by the relative relationship reported for the appendices in the natural product, the resulting anti-adduct was elaborated into a probative target structure 1*.

Total synthesis based on the originally claimed structure of mucosin

The first total synthesis aimed at the naturally occurring eicosanoid bicycle mucosin is reported. A practical route has been devised allowing the issues relating to the previous assignment of stereochemistry to be examined. X-ray crystallography was performed on a late stage intermediate to pinpoint the topological relationship displayed by the featured bicyclo[4.3.0]non-3-ene scaffold.

Biologically Active Oxylipins from Enzymatic and Nonenzymatic Routes in Macroalgae

Marine algae are rich and heterogeneous sources of great chemical diversity, among which oxylipins are a well-recognized class of natural products. Algal oxylipins comprise an assortment of oxygenated, halogenated, and unsaturated functional groups and also several carbocycles, varying in ring size and position in lipid chain. Besides the discovery of structurally diverse oxylipins in macroalgae, research has recently deciphered the role of some of these metabolites in the defense and innate immunity of photosynthetic marine organisms. This review is an attempt to comprehensively cover the available literature on the chemistry, biosynthesis, ecology, and potential bioactivity of oxylipins from marine macroalgae. For a better understanding, enzymatic and nonenzymatic routes were separated; however, both processes often occur concomitantly and may influence each other, even producing structurally related molecules.

Methyl Jasmonate-Induced Lipidomic and Biochemical Alterations in the Intertidal Macroalga Gracilaria dura (Gracilariaceae, Rhodophyta)

The role of exogenously added methyl jasmonate (MeJA), a lipid-derived signaling compound, in inducing oxidative stress in the marine red macroalga Gracilaria dura was investigated. MeJA at a concentration of 1-100 µM was a strong stimulant of reactive oxygen species (H(2)O(2), HO· and O(2) (·-)) (P < 0.05) causing considerable oxidative stress in G. dura. This further led to lipid peroxidation and degradation of the pigments Chl a and phycocyanin, with a concomitant increase in phycoerythrin. The MeJA-induced oxidative burst also led to the induction of a fatty acid oxidation cascade, resulting in the synthesis of hydroxy-oxylipins and the up-regulation of the 13-lipoxygenase pathway. Electrospray ionization-mass spectrometry-based shotgun lipidomic analysis revealed that monogalactosyldiacylglycerol (a chloroplastic glycerolipid) and phosphatidylcholine (extrachloroplastidic phopholipid) were the most affected lipid classes. The degradation of 18:3-fatty acid-containing monogalactosyldiacylglycerol inferred that it provided fatty acyl chains for the biosynthesis of 13-hydroperoxylinolenic acid, which was further directed towards either the jasmonate pathway or other alternative pathways of the fatty acid oxidation cascade, analogous to higher plants. Also, G. dura modulated the lipid acyl chains in such a way that no significant change was observed in the fatty acid profile of the treated thalli as compared with those of the control, except for C16:0, C16:1 (n-9), C20:3 (n-6) and C20:4 (n-6) (P < 0.05). Furthermore, MeJA caused the accumulation of phenolic compounds and the up-regulation of enzymes involved in secondary metabolism such as polyphenol oxidase, shikimate dehydrogenase and phenylalanine ammonia-lyase, indicating a shift towards secondary metabolism as a defense strategy to combat the induced oxidative stress.

Quantification of selected endogenous hydroxy-oxylipins from tropical marine macroalgae

The present study investigated the contents of hydroxy-oxylipins hydroxyoctadecadienoic acids (HODEs), hydroxyoctadecatrienoic acids (HOTrEs), and hydroxyeicosatetraenoic acids (HETEs) in 40 macroalgae belonging to the Chlorophyceae, Rhodophyceae and, Phaeophyceae. The hydroxy-oxylipin content was low and ranged from 0.14 ± 0.012 ng/g (Codium dwarkense) to 8,161.9 ± 253 ng/g (Chaetomorpha linum) among the Chlorophyceae, 345.4 ± 56.8 ng/g (Scytosiphon lomentaria) to 2,574.5 ± 155.5 ng/g (Stoechospermum marginatum) among the Phaeophyceae, and 19.4 ± 2.2 ng/g (Laurencia cruciata) to 1,753.1 ± 268.2 ng/g in Gracilaria corticata v. folifera) among the Rhodophyceae on fresh weight basis (p ≤ 0.01). The concentrations of C18-oxylipins were greater than C20-oxylipins in all the investigated macroalgae, except forUlva linza, Codium sursum, Dictyopteris deliculata, S. marginatum, Sargassum tenerrimum, Gracilaria spp. (except G. textorii), Rhodymenia sonderi, and Odonthalia veravalensis.The macroalgal species rich in HODEs, HOTrEs, and HETEs were segregated using principal component analysis. The red macroalgae showed the highest contents of HETEs, followed by brown and green macroalgae in consistent with their PUFA profiles. The relative contents of isomeric forms of oxylipins displayed the species-specific positional selectivity of lipoxygenase (LOX) enzyme in macroalgae. All the species exhibited 13-LOX specificity for linoleic acid analogous of higher plants, while 21 out of 40 species showed 9-LOX selectivity for the oxygenation of α-linolenic acid. No trend was observed for the oxygenation of arachidonic acid in macroalgae, except for in the Halymeniales, Ceramiales (except L. cruciata), and Corallinales. This study infers that LOX products, octadecanoids and eicosanoids, described in macroalgal taxa were similar to those of higher plants and mammals, respectively, and thus can be utilized as an alternative source of chemically synthesized oxylipin analogues in therapeutics, cosmetics, and nutritional oil supplements.

Production of hydroxy fatty acids by microbial fatty acid-hydroxylation enzymes

Hydroxy fatty acids are widely used in chemical, food, and cosmetic industries as starting materials for the synthesis of polymers and as additives for the manufacture of lubricants, emulsifiers, and stabilizers. They have antibiotic, anti-inflammatory, and anticancer activities and therefore can be applied for medicinal uses. Microbial fatty acid-hydroxylation enzymes, including P450, lipoxygenase, hydratase, 12-hydroxylase, and diol synthase, synthesize regio-specific hydroxy fatty acids. In this article, microbial fatty acid-hydroxylation enzymes, with a focus on region-specificity and diversity, are summarized and the production of mono-, di-, and tri-hydroxy fatty acids is introduced. Finally, the production methods of regio-specific and diverse hydroxy fatty acids, such as gene screening, protein engineering, metabolic engineering, and combinatory biosynthesis, are suggested.

Allergic asthmatics show divergent lipid mediator profiles from healthy controls both at baseline and following birch pollen provocation

BACKGROUND

Asthma is a respiratory tract disorder characterized by airway hyper-reactivity and chronic inflammation. Allergic asthma is associated with the production of allergen-specific IgE and expansion of allergen-specific T-cell populations. Progression of allergic inflammation is driven by T-helper type 2 (Th2) mediators and is associated with alterations in the levels of lipid mediators.

OBJECTIVES

Responses of the respiratory system to birch allergen provocation in allergic asthmatics were investigated. Eicosanoids and other oxylipins were quantified in the bronchoalveolar lumen to provide a measure of shifts in lipid mediators associated with allergen challenge in allergic asthmatics.

METHODS

Eighty-seven lipid mediators representing the cyclooxygenase (COX), lipoxygenase (LOX) and cytochrome P450 (CYP) metabolic pathways were screened via LC-MS/MS following off-line extraction of bronchoalveolar lavage fluid (BALF). Multivariate statistics using OPLS were employed to interrogate acquired oxylipin data in combination with immunological markers.

RESULTS

Thirty-two oxylipins were quantified, with baseline asthmatics possessing a different oxylipin profile relative to healthy individuals that became more distinct following allergen provocation. The most prominent differences included 15-LOX-derived ω-3 and ω-6 oxylipins. Shared-and-Unique-Structures (SUS)-plot modeling showed a correlation (R(2) = 0.7) between OPLS models for baseline asthmatics (R(2)Y[cum] = 0.87, Q(2)[cum] = 0.51) and allergen-provoked asthmatics (R(2)Y[cum] = 0.95, Q(2)[cum] = 0.73), with the majority of quantified lipid mediators and cytokines contributing equally to both groups. Unique structures for allergen provocation included leukotrienes (LTB(4) and 6-trans-LTB(4)), CYP-derivatives of linoleic acid (epoxides/diols), and IL-10.

CONCLUSIONS

Differences in asthmatic relative to healthy profiles suggest a role for 15-LOX products of both ω-6 and ω-3 origin in allergic inflammation. Prominent differences at baseline levels indicate that non-symptomatic asthmatics are subject to an underlying inflammatory condition not observed with other traditional mediators. Results suggest that oxylipin profiling may provide a sensitive means of characterizing low-level inflammation and that even individuals with mild disease display distinct phenotypic profiles, which may have clinical ramifications for disease.

Applications of stereospecifically-labeled Fatty acids in oxygenase and desaturase biochemistry

Oxygenation and desaturation reactions are inherently associated with the abstraction of a hydrogen from the fatty acid substrate. Since the first published application in 1965, stereospecific placement of a labeled hydrogen isotope (deuterium or tritium) at the reacting carbons has proven a highly effective strategy for investigating the chemical mechanisms catalyzed by lipoxygenases, hemoprotein fatty acid dioxygenases including cyclooxygenases, cytochromes P450, and also the desaturases and isomerases. This review presents a synopsis of all published studies through 2010 on the synthesis and use of stereospecifically labeled fatty acids (71 references), and highlights some of the mechanistic insights gained by application of stereospecifically labeled fatty acids.

A lipoxygenase with linoleate diol synthase activity from Nostoc sp. PCC 7120

The dioxygenation of PUFAs (polyunsaturated fatty acids) in plants is mainly catalysed by members of the LOX (lipoxygenase) enzyme family. LOX products may be further metabolized, and are known as signalling substances in plant development and in responses to wounding and pathogen attack. In contrast with the situation in eukaryotes, information on the relevance of lipid peroxide metabolism in prokaryotic organisms is scarce. Therefore, we aimed to analyse LOXs and oxylipin patterns of cyanobacterial origin. A search of the genomic sequence of the cyanobacterium Nostoc sp. PCC 7120 suggested an open reading frame encoding a putative LOX named NspLOX that harboured an N-terminal extension. Individual analysis of recombinant C-terminal domain revealed enzymatic activity as a linoleate (9R)-LOX. Analysis of the full-length NspLOX protein, however, revealed linoleate diol synthase activity, generating (10E,12E)-9,14-dihydroxy-10,12-octadecadienoic acid as the main product from LA (linoleic acid) and (10E,12E,14E)-9,16-dihydroxy-10,12,14-octadecatrienoic acid as the main product from ALA (α-LA) substrates respectively, with ALA as preferred substrate. The enzyme exhibited a broad pH optimum between pH 7 and pH 10. Soluble extracts of Nostoc sp. contain more 9-LOX-derived hydroperoxides in sonified than in non-sonified cells, but products of full-length NspLOX were not detectable under the conditions used. As no other LOX-like sequence was identified in the genome of Nostoc sp. PCC 7120, the results presented suggest that (9R)-LOX-derived oxylipins may represent the endogenous products of NspLOX. Based on the biochemical results of NspLOX, we suggest that this bifunctional enzyme may represent a more ancient way to control the intracellular amount of oxylipins in this cyanobacterium.

Phospholipases and Galactolipases Trigger Oxylipin-Mediated Wound-Activated Defence in the Red Alga Gracilaria chilensis against Epiphytes

We investigated the wound response of the commercially important red alga, Gracilaria chilensis, in order to obtain insight into its interaction with epiphytic pests. After wounding, the host releases free fatty acids as well as the hydroxylated eicosanoids, 8R-hydroxy eicosatetraenoic acid (8-HETE) and 7S,8R-dihydroxy eicosatetraenoic acid (7,8-di-HETE). While the release of free arachidonic acid and subsequent formation of 8-HETE is controlled by phospholipase A, 7,8-di-HETE production is independent of this lipase. This dihydroxylated fatty acid might be directly released from galactolipids. Physiologically relevant concentrations of oxylipins reduced spore settlement of Acrochaetium sp. (Rhodophyta, Acrochaetiaceae) and suppressed the development of hapteria in Ceramium rubrum (Rhodophyta, Ceramiaceae) when these model epiphytes were exposed to artificial surfaces that contained 8-HETE or 7,8-di-HETE. Thus, the immediate release of oxylipins can be seen as G. chilensis defence against epiphytes.

Expression profiling of Chondrus crispus (Rhodophyta) after exposure to methyl jasmonate

Methyl jasmonate (MeJA) is a plant hormone important for the mediation of signals for developmental processes and defence reactions in higher plants. The effects of MeJA and the signalling pathways on other photosynthetic organism groups are largely unknown, even though MeJA may have very important roles. Therefore the effects of MeJA in a red alga were studied. A medium-scale expression profiling approach to identify genes regulated by MeJA in the red seaweed Chondrus crispus is described here. The expression profiles were studied 0, 2, 4, 6, 12, and 24 h after the addition of MeJA to the seawater surrounding the algae. The changes in the transcriptome were monitored using cDNA microarrays with 1920 different cDNA representing 1295 unique genes. The responses of selected genes were verified with real-time PCR and the correlation between the two methods was generally satisfying. The study showed that 6% of genes studied showed a response to the addition of MeJA and the most dynamic response was seen after 6 h. Genes that showed up-regulation included several glutathione S-transferases, heat shock protein 20, a xenobiotic reductase, and phycocyanin lyase. Down-regulated transcripts included glucose kinase, phosphoglucose isomerase, and a ribosomal protein. A comparison between different functional groups showed an up-regulation of stress-related genes and a down-regulation of genes involved in energy conversion and general metabolism. It is concluded that MeJA, or a related compound, has a physiological role as a stress hormone in red algae. This study represents to our knowledge the first analysis of gene expression using cDNA microarrays in a red macroalga.

Characterization of a membrane-associated apoplastic lipoxygenase in Phaseolus vulgaris L

An extracytoplasmic 86.7 kDa protein was isolated from intercellular washing fluids (IWF) of Phaseolus vulgaris etiolated hypocotyls. Micro sequencing of tryptic peptides of the 86.7 kDa protein revealed 100% identity with a bean lipoxygenase (LOX) protein fragment. Purified P87-LOX exhibited LOX activity characterized by an optimal pH of 6.0 and linolenic acid as an optimal substrate, and was classified as a 13-LOX with respect to its positional specificity of linoleic acid oxygenation. A protein identical to P87-LOX, as determined by MALDI-TOF analysis and biochemical characterization, was purified from hypocotyl microsomes. Immunoblot analysis showed that P87-LOX is present in plasma membrane-enriched fractions, from which it was solubilized using high ionic strength buffers. These observations suggest that P87-LOX is a peripheral protein associated to the apoplastic face of the plasma membrane.

Identification of hydroxy fatty acids by liquid chromatography-atmospheric pressure chemical ionization mass spectroscopy in Euglena gracilis

Hydroxy fatty acids from Euglena gracilis were identified by reverse-phase high performance liquid chromatography coupled to a mass spectrometer run in atmospheric pressure chemical ionization positive ion mode. These metabolites were converted to methyl esters to improve stability and chromatographic properties. A detection limit of 20 pg/microl per injection was determined for 5-HETE methyl ester based on the signal to noise ratio of the m/z 317 ion which corresponds to the loss of a hydroxyl group (M-17) and the major fragment in all HETE methyl esters studied. This is the first report for these metabolites in E. gracilis.

The innate immunity of a marine red alga involves oxylipins from both the eicosanoid and octadecanoid pathways

The oxygenated derivatives of fatty acids, known as oxylipins, are pivotal signaling molecules in animals and terrestrial plants. In animal systems, eicosanoids regulate cell differentiation, immune responses, and homeostasis. In contrast, terrestrial plants use derivatives of C18 and C16 fatty acids as developmental or defense hormones. Marine algae have emerged early in the evolution of eukaryotes as several distinct phyla, independent from the animal and green-plant lineages. The occurrence of oxylipins of the eicosanoid family is well documented in marine red algae, but their biological roles remain an enigma. Here we address the hypothesis that they are involved with the defense mechanisms of the red alga Chondrus crispus. By investigating its association with a green algal endophyte Acrochaete operculata, which becomes invasive in the diploid generation of this red alga, we showed that (1) when challenged by pathogen extracts, the resistant haploid phase of C. crispus produced both C20 and C18 oxylipins, (2) elicitation with pathogen extracts or methyl jasmonate activated the metabolism of C20 and C18 polyunsaturated fatty acids to generate hydroperoxides and cyclopentenones such as prostaglandins and jasmonates, and (3) C20 and C18 hydroperoxides as well as methyl jasmonate did induce shikimate dehydrogenase and Phe ammonialyase activities in C. crispus and conferred an induced resistance to the diploid phase, while inhibitors of fatty acid oxidation reduced the natural resistance of the haploid generation. The dual nature of oxylipin metabolism in this alga suggests that early eukaryotes featured both animal- (eicosanoids) and plant-like (octadecanoids) oxylipins as essential components of innate immunity mechanisms.

Hydroperoxy-arachidonic acid mediated n-hexanal and (Z)-3- and (E)-2-nonenal formation in Laminaria angustata

In higher plants, C6 and C9 aldehydes are formed from C18 fatty acids, such as linoleic or linolenic acid, through formation of 13- and 9-hydroperoxides, followed by their stereospecific cleavage by fatty acid hydroperoxide lyases (HPL). Some marine algae can also form C6 and C9 aldehydes, but their precise biosynthetic pathway has not been elucidated fully. In this study, we show that Laminaria angustata, a brown alga, formed C6 and C9 aldehydes enzymatically. The alga forms C9 aldehydes exclusively from the C20 fatty acid, arachidonic acid, while C6 aldehydes are derived either from C18 or from C20 fatty acid. The intermediates in the biosynthetic pathway were trapped by using a glutathione/glutathione peroxidase system, and subjected to structural analyses. Formation of (S)-12-, and (S)-15-hydroperoxy arachidonic acids [12(S)HPETE and 15(S)HPETE] from arachidonic acid was confirmed by chiral HPLC analyses. These account respectively for C9 aldehyde and C6 aldehyde formation, respectively. The HPL that catalyzes formation of C9 aldehydes from 12(S)HPETE seems highly specific for hydroperoxides of C20 fatty acids.

A dual function alpha-dioxygenase-peroxidase and NAD(+) oxidoreductase active enzyme from germinating pea rationalizing alpha-oxidation of fatty acids in plants

An enzyme with fatty acid alpha-oxidation activity (49 nkat mg(-1); substrate: lauric acid) was purified from germinating pea (Pisum sativum) by a five-step procedure to apparent homogeneity. The purified protein was found to be a 230-kD oligomer with two dominant subunits, i.e. a 50-kD subunit with NAD(+) oxidoreductase activity and a 70-kD subunit, homolog to a pathogen-induced oxygenase, which in turn shows significant homology to animal cyclooxygenase. On-line liquid chromatography-electrospray ionization-tandem mass spectrometry revealed rapid alpha-oxidation of palmitic acid incubated at 0 degrees C with the purified alpha-oxidation enzyme, leading to (R)-2-hydroperoxypalmitic acid as the major product together with (R)-2-hydroxypalmitic acid, 1-pentadecanal, and pentadecanoic acid. Inherent peroxidase activity of the 70-kD fraction decreased the amount of the (R)-2-hydroperoxy product rapidly and increased the level of (R)-2-hydroxypalmitic acid. Incubations at room temperature accelerated the decline toward the chain-shortened aldehyde. With the identification of the dual function alpha-dioxygenase-peroxidase (70-kD unit) and the related NAD(+) oxidoreductase (50-kD unit) we provided novel data to rationalize all steps of the classical scheme of alpha-oxidation in plants.

5-Lipoxygenase-derived oxylipins from the red alga Rhodymenia pertusa

The lipid extract of the temperate red alga Rhodymenia pertusa has yielded four eicosanoid metabolites, three of which are new natural products. Using principally NMR and MS techniques, their structures were deduced as 5R,6S-dihydroxy-7(E),9(E),11(Z),14(Z)-eicosatetraenoic acid (5R,6S-diHETE), 5R*,6S*-dihydroxy-7(E),9(E),11(Z),14(Z),17(Z)-eicosapentaenoic acid (5R*,6S*-diHEPE), 5-hydroxy-6(E),8(Z),11(Z),14(Z)-eicosatetraenoic acid (5-HETE), 5-hydroxy-6(E),8(Z),11(Z),14(Z),17(Z)-eicosapentaenoic acid (5-HEPE). The co-occurrence of these metabolites strongly suggests that R. pertusa contains a unique 5R-lipoxygenase system acting on both arachidonic and eicosapentaenoic acids.

Purification and catalytic activities of the two domains of the allene oxide synthase-lipoxygenase fusion protein of the coral Plexaura homomalla

The conversion of fatty acid hydroperoxides to allene epoxides is catalyzed by a cytochrome P450 in plants and, in coral, by a 43-kDa catalase-related hemoprotein fused to the lipoxygenase that synthesizes the 8R-hydroperoxyeicosatetraenoic acid (8R-HPETE) substrate. We have expressed the separate lipoxygenase and allene oxide synthase (AOS) domains of the coral protein in Escherichia coli (BL21 cells) and purified the proteins; this system gives high expression (1.5 and 0.3 micromol/liter, respectively) of catalytically active enzymes. Both domains show fast reaction kinetics. Catalytic activity of the lipoxygenase domain is stimulated 5-fold by high concentrations of monovalent cations (500 mM Na(+), Li(+), or K(+)), and an additional 5-fold by 10 mM Ca(2+). The resulting rates of reaction are approximately 300 turnovers/s, 1-2 orders of magnitude faster than mammalian lipoxygenases. This makes the coral lipoxygenase well suited for partnership with the AOS domain, which shows maximum rates of approximately 1400 turnovers/s in the conversion of 8R-HPETE to the allene oxide. Some unusual catalytic activities of the two domains are described. The lipoxygenase domain converts 20.3omega6 partly to the bis-allylic hydroperoxide (10-hydroperoxyeicosa-8,11,14-trienoic acid). Metabolism of the preferred substrate of the AOS domain, 8R-HPETE, is inhibited by the enantiomer 8S-HPETE. Although the AOS domain has homology to catalase in primary structure, it is completely lacking in catalatic action on H(2)O(2); catalase itself, as expected from its preference for small hydroperoxides, is ineffective in allene oxide synthesis from 8R-HPETE.

alpha-oxidation of fatty acids in higher plants. Identification of a pathogen-inducible oxygenase (piox) as an alpha-dioxygenase and biosynthesis of 2-hydroperoxylinolenic acid

A pathogen-inducible oxygenase in tobacco leaves and a homologous enzyme from Arabidopsis were recently characterized (Sanz, A., Moreno, J. I., and Castresana, C. (1998) Plant Cell 10, 1523-1537). Linolenic acid incubated at 23 degrees C with preparations containing the recombinant enzymes underwent alpha-oxidation with the formation of a chain-shortened aldehyde, i.e., 8(Z),11(Z), 14(Z)-heptadecatrienal (83%), an alpha-hydroxy acid, 2(R)-hydroxy-9(Z),12(Z),15(Z)-octadecatrienoic acid (15%), and a chain-shortened fatty acid, 8(Z),11(Z),14(Z)-heptadecatrienoic acid (2%). When incubations were performed at 0 degrees C, 2(R)-hydroperoxy-9(Z),12(Z),15(Z)-octadecatrienoic acid was obtained as the main product. An intermediary role of 2(R)-hydroperoxy-9(Z), 12(Z),15(Z)-octadecatrienoic acid in alpha-oxidation was demonstrated by re-incubation experiments, in which the hydroperoxide was converted into the same alpha-oxidation products as those formed from linolenic acid. 2(R)-Hydroperoxy-9(Z),12(Z), 15(Z)-octadecatrienoic acid was chemically unstable and had a half-life time in buffer of about 30 min at 23 degrees C. Extracts of cells expressing the recombinant oxygenases accelerated breakdown of the hydroperoxide (half-life time, about 3 min at 23 degrees C), however, this was not attributable to the recombinant enzymes since the same rate of hydroperoxide degradation was observed in the presence of control cells not expressing the enzymes. No significant discrimination between enantiomers was observed in the degradation of 2(R,S)-hydroperoxy-9(Z)-octadecenoic acid in the presence of recombinant oxygenases. A previously studied system for alpha-oxidation in cucumber was re-examined using the newly developed techniques and was found to catalyze the same conversions as those observed with the recombinant enzymes, i.e. enzymatic alpha-dioxygenation of fatty acids into 2(R)-hydroperoxides and a first order, non-stereoselective degradation of hydroperoxides into alpha-oxidation products. It was concluded that the recombinant enzymes from tobacco and Arabidopsis were both alpha-dioxygenases, and that members of this new class of enzymes catalyze the first step of alpha-oxidation in plant tissue.

Biogenesis and biological function of marine algal oxylipins

The biogenetic source of most marine algal oxylipins, which are many and of diverse structure, can logically be unified through a common lipoxygenase-derived hydroperoxide to epoxy allylic carbocation transformation. The biological role of oxylipins in algae remains an enigma, although numerous ideas have been put forth. Herein, we hypothesize and provide some evidence for an osmoregulatory role for these metabolites.

Manganese lipoxygenase. Discovery of a bis-allylic hydroperoxide as product and intermediate in a lipoxygenase reaction

Linoleic acid was incubated with manganese lipoxygenase (Mn-LO) from the fungus Gäumannomyces graminis. The product consisted of (13R)-hydroperoxy-(9Z,11E)-octadecadienoic acid ((13R)-HPOD) and a new hydroperoxide, (11S)-hydroperoxy-(9Z,12Z)-octadecadienoic acid ((11S)-HPOD). Incubation of (11R)-[2H]- and (11S)-[2H]linoleic acids with Mn-LO led to the formation of hydroperoxides that largely retained and lost, respectively, the deuterium label. Conversion of the (11S)-deuteriolinoleic acid was accompanied by a primary isotope effect, which manifested itself in a strongly reduced rate of formation of hydroperoxides and in a time-dependent accumulation of deuterium in the unconverted substrate. These experiments indicated that the initial step catalyzed by Mn-LO consisted of abstraction of the pro-S hydrogen of linoleic acid to produce a linoleoyl radical. (11S)-HPOD was converted into (13R)-HPOD upon incubation with Mn-LO. The mechanism of this enzyme-catalyzed hydroperoxide rearrangement was studied in experiments carried out with 18O2 gas or 18O2-labeled hydroperoxides. Incubation of [11-18O2](11S)-HPOD with Mn-LO led to the formation of (13R)-HPOD, which retained 39-44% of the 18O label, whereas (11S)-HPOD incubated with Mn-LO under 18O2 produced (13R)-HPOD, which had incorporated 57% of 18O. Furthermore, analysis of the isotope content of (11S)-HPOD remaining unconverted in such incubations demonstrated that [11-18O2](11S)-HPOD suffered a time-dependent loss of 18O when exposed to Mn-LO, whereas (11S)-HPOD incorporated 18O when incubated with Mn-LO under 18O2. On the basis of these experiments, it was proposed that the conversion of (11S)-HPOD into (13R)-HPOD occurred in a non-concerted way by deoxygenation into a linoleoyl radical. Subsequent reoxygenation of this intermediate by dioxygen attack at C-13 produced (13R)-HPOD, whereas attack at C-11 regenerated (11S)-HPOD. The hydroperoxide rearrangement occurred by oxygen rebound, although, as demonstrated by the 18O experiments, the oxygen molecule released from (11S)-HPOD exchanged with surrounding molecular oxygen prior to its reincorporation.

Stereochemistry of oxygenation of linoleic acid catalyzed by prostaglandin-endoperoxide H synthase-2

Linoleic acid was incubated with prostaglandin-endoperoxide H synthase-2 (PGHS-2) from ovine placenta. A product consisting of regio- and stereoisomeric hydroxyoctadecadienoic (HOD) acids was obtained. Analysis by straight-phase high-performance liquid chromatography followed by chiral-phase high-performance liquid chromatography demonstrated that linoleic acid was preferentially oxygenated at C-9 to produce the following mixture of HODs: 9(R)-HOD (52%), 9(S)-HOD (11%), 13(R)-HOD (2%), and 13(S)-HOD (35%). As a comparison, linoleic acid was incubated with microsomal prostaglandin-endoperoxide H synthase-1 (PGHS-1) from ovine vesicular gland. This resulted in a product having the following composition: 9(R)-HOD (73%), 9(S)-HOD (9%), 13(R)-HOD (1%), and 13(S)-HOD (17%). The stereochemistry of the hydrogen which was removed from C-11 during the conversion of linoleic acid into hydroxy acids in the presence of PGHS-1 or PGHS-2 was determined by incubation of [(11R)-2H]- and [(11S)-2H]linoleic acids followed by mass spectrometric analysis of the isotope contents of the individual hydroxy acid isomers. Both enzymes were found to catalyze oxygenations which involved stereospecific removal of the (11S) hydrogen and retention of the (11R) hydrogen. The major hydroxy acids, i.e., 9(R)-HOD and 13(S)-HOD, were formed from linoleic acid in reactions which involved antarafacial hydrogen abstraction and oxygen insertion. It is concluded that the initial steps of the PGHS-2- and PGHS-1-catalyzed oxygenations proceed with identical stereochemistry and involve stereospecific removal of the pro-S hydrogen from the omega 8-methylene group of the substrate.

Cytochrome P450 metabolites of arachidonic acid in human placenta

Little is known about the epoxygenase pathway of the arachidonic acid cascade in uterine tissues. In this paper, we describe the formation of epoxyeicosatrienoic acids (EETs) and dihydroxyeicosatrienoic acids (DHETs) in human term placenta after uncomplicated pregnancies. Metabolism of [3H]-arachidonic acid was analyzed in short term tissue cultures of placenta by reverse phase HPLC. Major metabolites coeluted with authentic EETs and DHETs. The formation of EETs was inhibited by carbon monoxide. In non-radioactive incubations with biopsies from seven different placentas, sufficient material for GC/MS analysis was obtained. The combined media were purified by solid phase extraction and reverse phase HPLC. The fraction coeluting with DHETs was derivatized with pentafluorobenzylbromide (PFB) and bis-(trimethylsilyl)-trifluoroacetylacetamide (BSTFA) and analyzed by GC/NICI/MS/MS. 11, 12-DHET and 14, 15-DHET were identified by their mass spectra displaying specific fragments at m/z 149 and m/z 189, respectively. Our results suggest that the epoxygenase pathway is active in human term placenta.

Epoxy allylic carbocations as conceptual intermediates in the biogenesis of diverse marine oxylipins

Marine organisms, especially marine algae, are extremely rich in a diversity of novel oxylipin structures. Many of these oxylipins contain functionalities and rings of a type and location unknown in mammalian systems. In this perspective reviewing marine oxylipins, a proposal is formulated for the central intermediacy of an epoxy allylic carbocation in the biogenesis of these diverse structures. This proposal is strengthened by the relatively large number of examples which are consistent with this type of mechanistic transformation.

Studies on linoleic acid 8R-dioxygenase and hydroperoxide isomerase of the fungus Gaeumannomyces graminis

Linoleic acid is sequentially converted to 7S,8S-dihydroxy-9Z,12Z-octadecadienoic acid by the 8R-dioxygenase and hydroperoxide isomerase of the fungus Gaeumannomyces graminis, which is a common pathogen of wheat. The objective of this study was to separate and characterize the two enzyme activities. The isomerase activity was found mainly in the microsomal fraction of the mycelia and the 8R-dioxygenase in the cytosol. The 8R-dioxygenase could be partially purified by ammonium sulfate precipitation, gel filtration, ion exchange chromatography or isoelectric focusing. The 8R-dioxygenase was unstable during purification, but it could be stabilized by glutathione, glutathione peroxidase and ethylenediaminetetraacetic acid. Several protease inhibitors reduced the enzyme activity. Gel filtration with Sephacryl S-300 showed that most 8R-dioxygenase activity was eluted with the front with little retention. Isoelectric focusing in the presence of ethylene glycol (20%) indicated an isoelectric point of pl 6.1-6.3. The enzyme was retained on strong anion exchange columns at pH 7.4 and could be eluted with 0.3-0.5 M NaCl. Incubation of the enzyme with 0.1 mM linoleic acid led to partial inactivation, which may indicate product inhibition. Paracetamol and the lipoxygenase inhibitor ICI 230,487 at 30 microM inhibited the 8R-dioxygenase by 44 and 58%, respectively. 8R-hydroperoxy-9Z,12Z-octadecadienoic acid was isolated from incubations of linoleic acid with the partially purified enzyme or with the cytosol in the presence of p-hydroxymercuribenzoate. The hydroperoxide was rapidly converted by the hydroperoxide isomerase in the microsomal fractions to 7S,8S-dihydroxy-9Z,12Z-octadecadienoic acid.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure and biosynthesis of marine algal oxylipins

Diverse marine life, including algae, sponges, molluscs, corals, tunicates, and bacteria, have been found to possess a variety of structurally unique oxylipins. The algae are the best characterized of these organisms for their oxylipins, which have now been described from more than 30 species representing the three major groups of macrophytic algae (Rhodophyta = reds, Chlorophyta = greens, and Phaeophyceae = browns). A number of recent studies have sought to understand the biosynthetic origin and mechanistic chemistry which leads to the formation of these unique marine substances. In general, the red algae metabolize C20 acids via 12-lipoxygenase-initiated pathways, green algae metabolize C18 acids at C-9 and C-13, and brown algae metabolize both C18 and C20 acids, principally by lipoxygenases with n-6 specificity. This review updates the records of new oxylipins from marine algae and describes thoughts on their biogenesis as well as specific experiments aimed at probing these hypotheses.