Thromboxane synthase expression and thromboxane A2 production in the atherosclerotic lesion

Thromboxane A(2) (TXA(2)) is a potent prothrombotic and immune modulating lipid mediator, which is implicated in cardiovascular diseases, in particular, atherosclerotic lesion development and thrombogenicity. Here, we tested the hypothesis that thromboxane synthase (TXAS), the obligate enzyme required to synthesize TXA(2), is expressed within the human atherosclerotic lesion, thus potentially contributing to TXA(2) synthesis and disease development. In an animal study, different atherosclerosis-prone mouse strains were investigated and compared with control mice. In a patient study (n = 134), endarterectomies of carotid atherosclerotic lesions were compared with non-atherosclerotic arteries (n = 11). Expression of TXAS was evaluated by real-time quantitative reverse transcription PCR and immunohistochemistry. TXAS mRNA expression was increased within the vascular wall in mouse models of atherosclerosis with advanced lesions. In humans, TXAS was expressed in the atherosclerotic lesion, associated with increased inflammatory cells, in particular M2 polarized macrophages, and increased in atherosclerotic lesions of patients with recent symptoms of thrombotic events. Production of TXA(2) by plaque tissue, verified by gas chromatography-mass spectrometry, increased after addition of arachidonic acid or lipopolysaccharide, and was inhibited by the TXAS inhibitor furegrelate. The findings suggest that intraplaque TXA(2) generation may contribute to the development of atherosclerosis and its thrombotic complications in humans.

Biochemical characterization of the oxygenation of unsaturated fatty acids by the dioxygenase and hydroperoxide isomerase of Pseudomonas aeruginosa 42A2

We have studied oxygenation of fatty acids by cell extract of Pseudomonas aeruginosa 42A2. Oleic acid ((9Z)-18:1) was transformed to (10S)-hydroperoxy-(8E)-octadecenoic acid ((10S)-HPOME) and to (7S,10S)-dihydroxy-(8E)-octadecenoic acid (7,10-DiHOME). Experiments under oxygen-18 showed that 7,10-DiHOME contained oxygen from air and was formed sequentially from (10S)-HPOME by isomerization. (10R)-HPOME was not isomerized. The (10S)-dioxygenase and hydroperoxide isomerase activities co-eluted on ion exchange chromatography and on gel filtration with an apparent molecular size of approximately 50 kDa. 16:1n-7, 18:2n-6, and 20:1n-11 were also oxygenated to 7,10-dihydroxy fatty acids, and (8Z)-18:1 was oxygenated to 6,9-dihydroxy-(7E)-octadecenoic acid. A series of fatty acids with the double bond positioned closer to ((6Z)-18:1, (5Z,9Z)-18:2) or more distant from the carboxyl group ((11Z)-, (13Z)-, and (15Z)-18:1) were poor substrates. The oxygenation mechanism was studied with [7S-(2)H]18:1n-9, [7R-(2)H]18:2n-6, and [8R-(2)H]18:2n-6 as substrates. The pro-R hydrogen at C-8 was lost in the biosynthesis of (10S)-HPODE, whereas the pro-S hydrogen was lost and the pro-R hydrogen was retained at C-7 during biosynthesis of the 7,10-dihydroxy metabolites. Analysis of the fatty acid composition of P. aeruginosa revealed relatively large amounts of (9E/Z)-16:1 and (11E/Z)-18:1 and only traces of 18:1n-9. We found that (11Z)-18:1 (vaccenic acid) was transformed to (11S,14S)-dihydroxy-(12E)-octadecenoic acid and to a mixture of 11- and 12-HPOME, possibly due to reverse orientation of (11Z)-18:1 at the active site compared with oleic acid. The reaction mechanism of the hydroperoxide isomerase suggests catalytic similarities to cytochrome P450.

A bisallylic mini-lipoxygenase from cyanobacterium Cyanothece sp. that has an iron as cofactor

Lipoxygenases are enzymes that are found ubiquitously in higher animals and plants, but have only recently been identified in a number of bacteria. The genome of the diazotrophic unicellular cyanobacterium Cyanothece sp. harbors two genes with homology to lipoxygenases. Here we describe the isolation of one gene, formerly named csplox2. It was cloned, and the protein was expressed in Escherichia coli and purified. The purified enzyme belongs to the group of prokaryotic mini lipoxygenases, because it had a molecular mass of 65 kDa. Interestingly, it catalyzed the conversion of linoleic acid, the only endogenously found polyunsaturated fatty acid, primarily to the bisallylic hydroperoxide 11R-hydroperoxyoctadecadienoic acid. This product had previously only been described for the manganese lipoxygenase from the take all fungus, Gaeumannomyces graminis. By contrast, CspLOX2 was shown to be an iron lipoxygenase. In addition, CspLOX2 formed a mixture of typical conjugated lipoxygenase products, e.g. 9R- and 13S-hydroperoxide. The conversion of linoleic acid took place with a maximum reaction rate of 31 s(-1). Incubation of the enzyme with [(11S)-(2)H]linoleic acid led to the formation of hydroperoxides that had lost the deuterium label, thus suggesting that CspLOX2 catalyzes antarafacial oxygenation as opposed to the mechanism of manganese lipoxygenase. CspLOX2 could also oxidize diarachidonylglycerophosphatidylcholine with similar specificity as the free fatty acid, indicating that binding of the substrate takes place with a "tail-first" orientation. We conclude that CspLOX2 is a novel iron mini-lipoxygenase that catalyzes the formation of bisallylic hydroperoxide as the major product.

A tomato enzyme synthesizes (+)-7-iso-jasmonoyl-L-isoleucine in wounded leaves

Jasmonoyl-L-isoleucine (JA-Ile) is a key jasmonate signal that probably functions in all plant species. The JASMONATE RESISTANT 1 (JAR1) enzyme synthesizes JA-Ile in Arabidopsis [Arabidopsis thaliana (L.) Heynh.], but a similar enzyme from tomato [Solanum lycopersicum (L.)] was not previously described. Tomato SlJAR1 has 66% sequence identity with Arabidopsis JAR1 and the SlJAR1-GST fusion protein purified from Escherichia coli catalyzed the formation of JA-amino acid conjugates in vitro. Kinetic analysis showed the enzyme has a strong preference for Ile over Leu and Val and it was about 10-fold more active with (+)-7-iso-JA than with its epimer (-)-JA. Leaf wounding rapidly increased JA-Ile 50-fold to about 450 pmol g(-1) FW at 30 min after wounding, while conjugates with Leu, Phe, Val and Met were only marginally increased or not detected. Nearly all of the endogenous JA-Ile was the bioactive epimer (+)-7-iso-JA-Ile and there was no evidence for its conversion to (-)-JA-Ile up to 6 h after wounding. A transgenic RNAi approach was used to suppress SlJAR1 transcript that reduced JA-Ile accumulation by 50-75%, suggesting that other JA conjugating enzymes may be present. These results show that SlJAR1 synthesizes the bioactive conjugate (+)-7-iso-JA-Ile and this is the predominant isomer accumulated in wounded tomato leaves.

Gene deletion of 7,8-linoleate diol synthase of the rice blast fungus: studies on pathogenicity, stereochemistry, and oxygenation mechanisms

Linoleate diol synthases (LDS) are heme enzymes, which oxygenate 18:2n-6 sequentially to (8R)-hydroperoxylinoleic acid ((8R)-HPODE) and to (5S,8R)-dihydroxy-, (7S,8S)-dihydroxy-, or (8R,11S)-dihydroxylinoleic acids (DiHODE). The genome of the rice blast fungus, Magnaporthe oryzae, contains two genes with homology to LDS. M. oryzae oxidized 18:2n-6 to (8R)-HPODE and to (7S,8S)-DiHODE, (6S,8R)-DiHODE, and (8R,11S)-HODE. Small amounts of 10-hydroxy-(8E,12Z)-octadecadienoic acid and traces of 5,8-DiHODE were also detected by liquid chromatography-mass spectrometry. The contribution of the 7,8-LDS gene to M. oryzae pathogenicity was evaluated by replacement of the catalytic domain with hygromycin and green fluorescent protein variant (SGFP) cassettes. This genetically modified strain Delta7,8-LDS infected rice leaves and roots and formed appressoria and conidia as the native fungus. The Delta7,8-LDS mutant had lost the capacity to biosynthesize all the metabolites except small amounts of 8-hydroxylinoleic acid. Studies with stereospecifically deuterated linoleic acids showed that (8R)-HPODE was formed by abstraction of the pro-S hydrogen at C-8 and antarafacial oxygenation, whereas (7S,8S)-DiHODE and (8R,11S)-DiHODE were formed from (8R)-HPODE by suprafacial hydrogen abstraction and oxygenation at C-7 and C-11, respectively. A mac1 suppressor mutant (Delta mac1 sum1-99) of M. oryzae, which shows cAMP-independent protein kinase A activity, oxygenated 18:2n-6 to increased amounts of (10R)-HPODE and (5S,8R)-DiHODE. Expression of the 7,8-LDS gene but not of the second homologue was detected in the suppressor mutant. This suggests that PKA-mediated signaling pathway regulates the dioxygenase and hydroperoxide isomerase activities of M. oryzae.

Functional analysis of alpha-DOX2, an active alpha-dioxygenase critical for normal development in tomato plants

Plant alpha-dioxygenases initiate the synthesis of oxylipins by catalyzing the incorporation of molecular oxygen at the alpha-methylene carbon atom of fatty acids. Previously, alpha-DOX1 has been shown to display alpha-dioxygenase activity and to be implicated in plant defense. In this study, we investigated the function of a second alpha-dioxygenase isoform, alpha-DOX2, in tomato (Solanum lycopersicum) and Arabidopsis (Arabidopsis thaliana). Recombinant Slalpha-DOX2 and Atalpha-DOX2 proteins catalyzed the conversion of a wide range of fatty acids into 2(R)-hydroperoxy derivatives. Expression of Slalpha-DOX2 and Atalpha-DOX2 was found in seedlings and increased during senescence induced by detachment of leaves. In contrast, microbial infection, earlier known to increase the expression of alpha-DOX1, did not alter the expression of Slalpha-DOX2 or Atalpha-DOX2. The tomato mutant divaricata, characterized by early dwarfing and anthocyanin accumulation, carries a mutation at the Slalpha-DOX2 locus and was chosen for functional studies of alpha-DOX2. Transcriptional changes in such mutants showed the up-regulation of genes playing roles in lipid and phenylpropanoid metabolism, the latter being in consonance with the anthocyanin accumulation. Transgenic expression of Atalpha-DOX2 and Slalpha-DOX2 in divaricata partially complemented the compromised phenotype in mature plants and fully complemented it in seedlings, thus indicating the functional exchangeability between alpha-DOX2 from tomato and Arabidopsis. However, deletion of Atalpha-DOX2 in Arabidopsis plants did not provoke any visible phenotypic alteration indicating that the relative importance of alpha-DOX2 in plant physiology is species specific.

The crystal structure of oxylipin-conjugated barley LTP1 highlights the unique plasticity of the hydrophobic cavity of these plant lipid-binding proteins

The barley lipid transfer protein (LTP1) adducted by an alpha-ketol, (9-hydroxy-10-oxo-12(Z)-octadecenoic acid) exhibits an unexpected high lipid transfer activity. The crystal structure of this oxylipin-adducted LTP1, (LTP1b) was determined at 1.8A resolution. The covalently bound oxylipin was partly exposed at the surface of the protein and partly buried within the hydrophobic cavity. The structure of the oxylipidated LTP1 emphasizes the unique plasticity of the hydrophobic cavity of these plant lipid-binding proteins when compared to the other members of the family. The plasticity of the hydrophobic cavity and increase of its surface hydrophobicity induced by the oxylipin account for the improvement of the lipid transfer activity of LTP1b. These observations open new perspectives to explore the different biological functions of LTPs, including their allergenic properties.

Specificity of oxidation of linoleic acid homologs by plant lipoxygenases

The lipoxygenase-catalyzed oxidation of linoleic acid homologs was studied. While the linoleic acid oxidation by maize 9-lipoxygenase (9-LO) specifically produced (9S)-hydroperoxide, the dioxygenation of (11Z,14Z)-eicosadienoic (20:2) and (13Z,16Z)-docosadienoic (22:2) acids by the same enzyme lacked regio- and stereospecificity. The oxidation of 20:2 and 22:2 by 9-LO afforded low yields of racemic 11-, 12-, 14-, and 15-hydroperoxides or 13- and 17-hydroperoxides, respectively. Soybean 13-lipoxygenase-1 (13-LO) specifically oxidized 20:2, 22:2, and linoleate into (omega6S)-hydroperoxides. Dioxygenation of (9Z,12Z)-hexadecadienoic acid (16:2) by both 9-LO and 13-LO occurred specifically, affording (9S)- and (13S)-hydroperoxides, respectively. The data are consistent with the "pocket theory of lipoxygenase catalysis" (i.e. with the penetration of a substrate into the active center with the methyl end first). Our findings also demonstrate that the distance between carboxyl group and double bonds substantially determines the positioning of substrates within the active site.

Functional expression of five Arabidopsis fatty acyl-CoA reductase genes in Escherichia coli

Very long chain primary alcohols are significant components in cuticle waxes of plants. Fatty acyl-CoA reductases (FARs) catalyze the formation of a fatty alcohol from an acyl-CoA. The Arabidopsis (Arabidopsis thaliana) genome contains eight genes homologous to FAR genes from jojoba (Simmondsia chinensis), silk moth, wheat and mouse. Expression of six Arabidopsis FAR homologs in Escherichia coli resulted in production of alcohols from endogenous E. coli fatty acids by five of these genes, confirming that they encode for FAR enzymes. Only a truncated splicing version of the sixth gene was found, and this gene yielded a protein with no FAR activity. The five functional FAR enzymes yielded distinctly different compositions of fatty alcohols when expressed in E. coli, indicating that the different enzymes may be involved in the production of different types of alcohols in plant cells.

(+)-7-iso-Jasmonoyl-L-isoleucine is the endogenous bioactive jasmonate

Hormone-triggered activation of the jasmonate signaling pathway in Arabidopsis thaliana requires SCF(COI1)-mediated proteasome degradation of JAZ repressors. (-)-JA-L-Ile is the proposed bioactive hormone, and SCF(COI1) is its likely receptor. We found that the biological activity of (-)-JA-L-Ile is unexpectedly low compared to coronatine and the synthetic isomer (+)-JA-L-Ile, which suggests that the stereochemical orientation of the cyclopentanone-ring side chains greatly affects receptor binding. Detailed GC-MS and HPLC analyses showed that the (-)-JA-L-Ile preparations currently used in ligand binding studies contain small amounts of the C7 epimer (+)-7-iso-JA-L-Ile. Purification of each of these molecules demonstrated that pure (-)-JA-L-Ile is inactive and that the active hormone is (+)-7-iso-JA-L-Ile, which is also structurally more similar to coronatine. In addition, we show that pH changes promote conversion of (+)-7-iso-JA-L-Ile to the inactive (-)-JA-L-Ile form, thus providing a simple mechanism that can regulate hormone activity through epimerization.

tasselseed1 is a lipoxygenase affecting jasmonic acid signaling in sex determination of maize

Sex determination in maize is controlled by a developmental cascade leading to the formation of unisexual florets derived from an initially bisexual floral meristem. Abortion of pistil primordia in staminate florets is controlled by a tasselseed-mediated cell death process. We positionally cloned and characterized the function of the sex determination gene tasselseed1 (ts1). The TS1 protein encodes a plastid-targeted lipoxygenase with predicted 13-lipoxygenase specificity, which suggests that TS1 may be involved in the biosynthesis of the plant hormone jasmonic acid. In the absence of a functional ts1 gene, lipoxygenase activity was missing and endogenous jasmonic acid concentrations were reduced in developing inflorescences. Application of jasmonic acid to developing inflorescences rescued stamen development in mutant ts1 and ts2 inflorescences, revealing a role for jasmonic acid in male flower development in maize.

On the mechanism of a polyunsaturated fatty acid double bond isomerase from Propionibacterium acnes

The catalytic mechanism of Propionibacterium acnes polyunsaturated fatty acid isomerase (PAI) is explored by kinetic, spectroscopic, and thermodynamic studies. The PAI-catalyzed double bond isomerization takes place by selective removal of the pro-R hydrogen from C-11 followed by suprafacial transfer of this hydrogen to C-9 as shown by conversion of C-9-deuterated substrate isotopologs. Data on the midpoint potential, photoreduction, and cofactor replacement suggest PAI to operate via an ionic mechanism with the formation of FADH(2) and linoleic acid carbocation as intermediates. In line with this proposal, no radical intermediates were detected neither by stopped flow absorption nor by EPR spectroscopy. The substrate preference toward free fatty acids is determined by the interaction between Arg-88 and Phe-193, and the reaction rate is strongly affected by replacement of these amino acids, indicating that the efficiency of the hydrogen transfer relies on a fixed distance between the free carboxyl group and the N-5 atom of FAD. Combining data obtained for PAI from the structural studies and experiments described here suggests that at least two different prototypical active site geometries exist among polyunsaturated fatty acid double bond isomerases.

Tomato CYP74C3 is a multifunctional enzyme not only synthesizing allene oxide but also catalyzing its hydrolysis and cyclization

The mechanism of the recombinant tomato allene oxide synthase (LeAOS3, CYP74C3) was studied. Incubations of linoleic acid (9S)-hydroperoxide with dilute suspensions of LeAOS3 (10-20 s, 0 degrees C) yield mostly the expected allene oxide (12Z)-9,10-epoxy-10,12-octadecadienoic acid (9,10-EOD), which was detected as its methanol-trapping product. In contrast, the relative yield of 9,10-EOD progressively decreased when the incubations were performed with fourfold, tenfold, or 80-fold larger amounts of LeAOS3, while alpha-ketol and the cyclopentenone rac-cis-10-oxo-11-phytoenoic acid (10-oxo-PEA) became the predominant products. Both the alpha-ketol and 10-oxo-PEA were also produced when LeAOS3 was exposed to preformed 9,10-EOD, which was generated by maize allene oxide synthase (CYP74A). LeAOS3 also converted linoleic acid (13S)-hydroperoxide into the corresponding allene oxide, but with about tenfold lower yield of cyclopentenone. The results indicate that in contrast to the ordinary allene oxide synthases (CYP74A subfamily), LeAOS3 (CYP74C subfamily) is a multifunctional enzyme, catalyzing not only the synthesis, but also the hydrolysis and cyclization of allene oxide.

Mutation of a critical arginine in microsomal prostaglandin E synthase-1 shifts the isomerase activity to a reductase activity that converts prostaglandin H2 into prostaglandin F2alpha

Microsomal prostaglandin E synthase type 1 (mPGES-1) converts prostaglandin endoperoxides, generated from arachidonic acid by cyclooxygenases, into prostaglandin E2. This enzyme belongs to the membrane-associated proteins in eicosanoid and glutathione metabolism (MAPEG) family of integral membrane proteins, and because of its link to inflammatory conditions and preferential coupling to cyclooxygenase 2, it has received considerable attention as a drug target. Based on the high resolution crystal structure of human leukotriene C4 synthase, a model of mPGES-1 has been constructed in which the tripeptide co-substrate glutathione is bound in a horseshoe-shaped conformation with its thiol group positioned in close proximity to Arg-126. Mutation of Arg-126 into an Ala or Gln strongly reduces the enzyme's prostaglandin E synthase activity (85-95%), whereas mutation of a neighboring Arg-122 does not have any significant effect. Interestingly, R126A and R126Q mPGES-1 exhibit a novel, glutathione-dependent, reductase activity, which allows conversion of prostaglandin H2 into prostaglandin F2alpha. Our data show that Arg-126 is a catalytic residue in mPGES-1 and suggest that MAPEG enzymes share significant structural components of their active sites.

Diversity of the enzymatic activity in the lipoxygenase gene family of Arabidopsis thaliana

Lipoxygenases (LOX) catalyze the oxygenation of polyunsaturated fatty acids, the first step in the biosynthesis of a large group of biologically active fatty acid metabolites collectively named oxylipins. In the present study we report the characterization of the enzymatic activity of the six lipoxygenases found in the genome of the model plant Arabidopsis thaliana. Recombinant expressed AtLOX-1 and AtLOX-5 had comparable oxygenase activity with either linoleic acid or linolenic acid. AtLOX-2, AtLOX-3, AtLOX-4 and AtLOX-6 displayed a selective oxygenation of linolenic acid. Analyses by high-performance liquid chromatography and gas chromatography-mass spectrometry demonstrated that AtLOX-1 and AtLOX-5 are 9S-lipoxygenases, and AtLOX-2, AtLOX-3, AtLOX-4 and AtLOX-6 are 13S-lipoxygenases. None of the enzymes had dual positional specificity. The determined activities correlated with that predicted by their phylogenetic relationship to other biochemically-characterized plant lipoxygenases.

Analysis of oil composition in cultivars and wild species of oat (Avena sp.)

Oil quality and content were analyzed in 33 accessions from 13 wild species and 10 accessions of cultivated oat. Wild oat species tended to have higher oil and 18:1 fatty acid (FA) contents and lower amounts of 18:2 and 18:3 FAs as compared to cultivated oats. In addition to common FAs, minor amounts of several hydroxy and epoxy FAs were also present in the oat oil and mainly confined to specific lipid classes. These unusual FAs included the previously reported 15-hydroxy 18:2 (Delta9,12) (avenoleic acid) mostly found among polar lipids and a novel 7-hydroxyhexadecanoic acid located to 1,2-diacylglycerol. The present study highlights the potential of making use of the existing germplasm, consisting of wild oat species, in breeding programs for achieving new oat varieties that produce a range of oils with different FA compositions as well as having high oil contents. However, in one matter, oats apparently lack genetic diversity and that is for oil qualities that are highly enriched in the omega 3 (omega-3) FA 18:3. Consequently, developing oat cultivars with highly unsaturated oils will need involvement of other techniques such as biotechnology.

A lipoxygenase-divinyl ether synthase pathway in flax (Linum usitatissimum L.) leaves

Incubation of linoleic acid with an enzyme preparation from leaves of flax (Linum usitatissimum L.) led to the formation of a divinyl ether fatty acid, i.e. (9Z,11E,1'Z)-12-(1'-hexenyloxy)-9,11-dodecadienoic [(omega5Z)-etheroleic] acid, as well as smaller amounts of 13-hydroxy-9(Z),11(E)-octadecadienoic acid. The 13-hydroperoxide of linoleic acid afforded the same set of products, whereas incubations of alpha-linolenic acid and its 13-hydroperoxide afforded the divinyl ether (9Z,11E,1'Z,3'Z)-12-(1',3'-hexadienyloxy)-9,11-dodecadienoic [(omega5Z)-etherolenic] as the main product. Identification of both divinyl ethers was substantiated by their UV, mass-, (1)H NMR and COSY spectral data. In addition to the 13-lipoxygenase and divinyl ether synthase activities demonstrated by these results, flax leaves also contained allene oxide synthase activity as judged by the presence of endogenously formed (15Z)-cis-12-oxo-10,15-phytodienoic acid in all incubations.

Dissociative recombination of OPCl+ and OPCl2+: pushing the upper mass limit at CRYRING

The dissociative recombination of OPCl+ and OPCl2+ has been studied at the storage ring CRYRING. The rate constants as a function of electron temperature have been derived to be 7.63 x 10(-7)(Te/300)(-0.89) and >1.2 x 10(-6)(Te/300)(-1.22) cm3s(-1), respectively. The lower limit quoted for the latter rate constant reflects the experimental inability to detect all of the reaction products. The branching fractions from the reaction have been measured for OPCl+ at approximately 0 eV interaction energy and are determined to be N(O+P+Cl)=(16+/-7)%, N(O+PCl)=(16+/-3)% and N(OP+Cl)=(68+/-5)%. These values have been obtained assuming that the rearrangement channel forming P+ClO is negligible, and ab initio calculations using GAUSSIAN03 are presented for the ion structures and energetics to support such an assumption. Finally, the limitations to using heavy ion storage rings such as CRYRING for studies into the dissociative recombination of large singly charged molecular ions are discussed.

Enzymatic, but not non-enzymatic, 1O2-mediated peroxidation of polyunsaturated fatty acids forms part of the EXECUTER1-dependent stress response program in the flu mutant of Arabidopsis thaliana

The conditional flu mutant of Arabidopsis accumulates excess amounts of protochlorophyllide within plastid membranes in the dark and generates singlet oxygen upon light exposure. By varying the length of the dark period, the level of the photosensitizer protochlorophyllide may be modulated, and conditions have been established that either endorse the cytotoxicity of (1)O(2) or reveal its signaling role. Two criteria have been used to distinguish between these two modes of activity of (1)O(2): the impact of the EXECUTER1 mutation and the prevalence of either non-enzymatic or enzymatic lipid peroxidation. During illumination of etiolated flu seedlings, toxic effects of (1)O(2) prevail and non-enzymatic lipid peroxidation proceeds rapidly. In contrast, in light-grown flu plants that were subjected to an 8 h dark/light shift, lipid peroxidation occurred almost exclusively enzymatically. The resulting oxidation product, 13-hydroperoxy octadecatrienoic acid (13-HPOT), serves as a substrate for synthesis of 12-oxo phytodienoic acid (OPDA) and jasmonic acid (JA), both of which are known to control various metabolic and developmental processes in plants. Inactivation of the EXECUTER1 protein abrogates not only (1)O(2)-mediated cell death and growth inhibition of flu plants, but also enzymatic lipid peroxidation. However, inactivation of jasmonate biosynthesis in the aos/flu double mutant does not affect (1)O(2)-mediated growth inhibition and cell death. Hence, JA and OPDA do not act as second messengers during (1)O(2) signaling, but form an integral part of a stress-related signaling cascade activated by (1)O(2) that encompasses several signaling pathways known to be activated by abiotic and biotic stressors.

Oxo-phytodienoic acid-containing galactolipids in Arabidopsis: jasmonate signaling dependence

The jasmonate family of phytohormones, as represented by 12-oxo-phytodienoic acid (OPDA), dinor-phytodienoic acid (dn-OPDA), and jasmonic acid in Arabidopsis (Arabidopsis thaliana), has been implicated in a vast array of different developmental processes and stress responses. Recent reports indicate that OPDA and dn-OPDA occur not only as free acids in Arabidopsis, but also as esters with complex lipids, so-called arabidopsides. Recently, we showed that recognition of the two bacterial effector proteins AvrRpm1 and AvrRpt2 induced high levels of a molecule consisting of two OPDAs and one dn-OPDA esterified to a monogalactosyl diacylglycerol moiety, named arabidopside E. In this study, we demonstrate that the synthesis of arabidopsides is mainly independent of the prokaryotic lipid biosynthesis pathway in the chloroplast, and, in addition to what previously has been reported, arabidopside E as well as an all-OPDA analog, arabidopside G, described here accumulated during the hypersensitive response and in response to wounding. We also show that different signaling pathways lead to the formation of arabidopsides during the hypersensitive response and the wounding response, respectively. However, the formation of arabidopsides during both responses is dependent on an intact jasmonate signaling pathway. Additionally, we report inhibition of growth of the fungal necrotrophic pathogen Botrytis cinerea and in planta release of free jasmonates in a time frame that overlaps with the observed reduction of arabidopside levels. Thus, arabidopsides may have a dual function: as antipathogenic substances and as storage compounds that allow the slow release of free jasmonates.

Identification of dioxygenases required for Aspergillus development. Studies of products, stereochemistry, and the reaction mechanism

Aspergillus sp. contain ppoA, ppoB, and ppoC genes, which code for fatty acid oxygenases with homology to fungal linoleate 7,8-diol synthases (7,8-LDS) and cyclooxygenases. Our objective was to identify these enzymes, as ppo gene replacements show critical developmental aberrancies in sporulation and pathogenicity in the human pathogen Aspergillus fumigatus and the genetic model Aspergillus nidulans. The PpoAs of A. fumigatus and A. nidulans were identified as (8R)-dioxygenases with hydroperoxide isomerase activity, designated 5,8-LDS. 5,8-LDS transformed 18:2n-6 to (8R)-hydroperoxyoctadecadienoic acid ((8R)-HPODE) and (5S,8R)-dihydroxy-9Z,12Z-octadecadienoic acid ((5S,8R)-DiHODE). We also detected 8,11-LDS in A. fumigatus and (10R)-dioxygenases in both Aspergilli. The diol synthases oxidized [(8R)-(2)H]18:2n-6 to (8R)-HPODE with retention of the deuterium label, suggesting antarafacial hydrogen abstraction and insertion of molecular oxygen. Experiments with stereospecifically deuterated 18:2n-6 showed that (8R)-HPODE was isomerized by 5,8- and 8,11-LDS to (5S,8R)-DiHODE and to (8R,11S)-dihydroxy-9Z,12Z-octadecadienoic acid, respectively, by suprafacial hydrogen abstraction and oxygen insertion at C-5 and C-11. PpoCs were identified as (10R)-dioxygenases, which catalyzed abstraction of the pro-S hydrogen at C-8 of 18:2n-6, double bond migration, and antafacial insertion of molecular oxygen with formation of (10R)-hydroxy-8E,12Z-hydroperoxyoctadecadienoic acid ((10R)-HPODE). Deletion of ppoA led to prominent reduction of (8R)-H(P)ODE and complete loss of (5S,8R)-DiHODE biosynthesis, whereas biosynthesis of (10R)-HPODE was unaffected. Deletion of ppoC caused biosynthesis of traces of racemic 10-HODE but did not affect the biosynthesis of other oxylipins. We conclude that ppoA of Aspergillus sp. may code for 5,8-LDS with catalytic similarities to 7,8-LDS and ppoC for linoleate (10R)-dioxygenases. Identification of these oxygenases and their products will provide tools for analyzing the biological impact of oxylipin biosynthesis in Aspergilli.

Dissociative recombination study of N3+: Cross section and branching fraction measurements

We report an investigation into the dissociative recombination of the azide radical cation, N(3) (+). The reaction rate constant has been measured to be 6.47 x 10(-7) cm(3) s(-1) at room temperature. This value is smaller than those reported earlier for the ion-electron neutralization of N(3) (+) at nitrogen atmospheric pressure. A strong propensity to dissociate through the N(2)+N channel has been observed.

Rotational state effects in the dissociative recombination of H2+

We have studied the dissociative recombination (DR) of molecular hydrogen ions with slow electrons over a range of collision energies from 0 to 400 meV. By employing a pulsed expansion source for rotational cooling and by exploiting super elastic collisions with near-0-eV electrons in a heavy ion storage ring for vibrational cooling, we observe a highly structured DR cross section, comparable to that reported for HD+. Using para-hydrogen-enriched ion beams, we identify for the first time features in the DR cross sections attributed to nu=0, J=even molecules (para-H2) and nu=0, J=odd (ortho-H2) molecules, separately. Indications are given that para levels have different DR rate coefficients from ortho levels for the first four vibrational levels at near-0-eV collisions.

Oxylipins produced by the 9-lipoxygenase pathway in Arabidopsis regulate lateral root development and defense responses through a specific signaling cascade

Arabidopsis thaliana seedling growth with pure oxylipins resulted in root waving, loss of root apical dominance, and decreased root elongation. 9-Hydroxyoctadecatrienoic acid (9-HOT) was a potent inducer of root waving. Studies with noxy2 (for nonresponding to oxylipins2), a new 9-HOT-insensitive mutant, and coronatine insensitive1-1 (jasmonate-insensitive) revealed at least three signaling cascades mediating the oxylipin actions. Treatment with 9-HOT resulted in a reduction in lateral roots and an increase in stage V primordia. Roots showed strong 9-lipoxygenase (9-LOX) activity, and root primordia expressed 9-LOX genes. These results, along with findings that noxy2 and mutants with defective 9-LOX activity showed increased numbers of lateral roots, suggest that 9-HOT, or a closely related 9-LOX product, is an endogenous modulator of lateral root formation. Histochemical and molecular analyses revealed that 9-HOT activated events common to development and defense responses. A subset of 9-HOT-responding root genes was also induced in leaves after 9-HOT treatment or pathogen inoculation. The results that noxy2 displayed altered root development, enhanced susceptibility to Pseudomonas, and reduced the activation of 9-HOT-responding genes are consistent with mechanistic links among these processes. The nature of the changes detected suggests that oxylipins from the 9-LOX pathway function in cell wall modifications required for lateral root development and pathogen arrest.

Liquid chromatography-mass spectrometry utilizing multi-stage fragmentation for the identification of oxysterols

In humans, the brain accounts for about 20% of the body's free cholesterol, most of which is synthesized de novo in brain. To maintain cholesterol balance throughout life, cholesterol becomes metabolized to 24S-hydroxycholesterol, principally in neurons. In mouse, rat, and probably human, metabolism to 24S-hydroxycholesterol accounts for about 50% of cholesterol turnover; however, the route by which the remainder is turned over has yet to be elucidated. Here, we describe a novel liquid chromatography (LC) multi-stage fragmentation mass spectrometry (MS(n)) methodology for the identification, with high sensitivity (low pg), of cholesterol metabolites in rat brain. The methodology includes derivatization to enhance ionization, exact mass analysis at high resolution to identify potential metabolites, and LC-MS(n) (n=3) to allow their characterization. 24S-hydroxycholesterol was confirmed as a major oxysterol in rat brain, and other oxysterols identified for the first time in brain included 24,25-, 24,27-, 25,27-, 6,24,- 7alpha,25-, and 7alpha,27-dihydroxycholesterols. In addition, 3beta-hydroxy-5-oxo-5,6-secocholestan-6-al and its aldol, two molecules linked to amyloidogenesis of proteins, were characterized in rat brain.

Specific Adduction of Plant Lipid Transfer Protein by an Allene Oxide Generated by 9-Lipoxygenase and Allene Oxide Synthase

Lipid transfer proteins (LTPs) are ubiquitous plant lipid-binding proteins that have been associated with multiple developmental and stress responses. Although LTPs typically bind fatty acids and fatty acid derivatives in a non-covalent way, studies on the LTPs of barley seeds have identified an abundantly occurring covalently modified form, LTP1b, the lipid ligand of which has resisted clarification. In the present study, this adduct was identified as the alpha-ketol 9-hydroxy-10-oxo-12(Z)-octadecenoic acid. Further studies on the formation of LTP1b demonstrated that the ligand was introduced by nucleophilic attack of the free carboxylate group of the Asp-7 residue of the protein at carbon-9 of the allene oxide fatty acid 9(S),10-epoxy-10,12(Z)-octadecadienoic acid. This reactive oxylipin was produced in barley seeds by oxygenation of linoleic acid by 9-lipoxygenase followed by dehydration of the resulting hydroperoxide by allene oxide synthase. The generation of protein-oxylipin adducts represents a new function for plant allene oxide synthases, enzymes that have earlier been implicated mainly in the biosynthesis of the jasmonate family of plant hormones. Additionally, the LTP-allene oxide synthase interaction opens new perspectives regarding the roles of LTPs in the signaling of plant defense and development.

Hydroperoxide lyases (CYP74C and CYP74B) catalyze the homolytic isomerization of fatty acid hydroperoxides into hemiacetals

The conversion of linoleic acid 9-hydroperoxide (9-HPOD) by recombinant melon (Cucumis melo L.) hydroperoxide lyase (HPL, CYP74C subfamily) was studied. Short (5 s-1 min) incubations at 0 degrees C followed by rapid extraction and trimethylsilylation made it possible to trap a new unstable (t(1/2) <30 s) product, i.e. the hemiacetal (1'E,3'Z)-9-hydroxy-9-(1',3'-nonadienyloxy)-nonanoic acid. Identification was performed by GC-MS analysis and substantiated by the formation of trimethylsilyl 9-trimethylsilyloxy-9-nonyloxy-nonanoate upon catalytic hydrogenation and by (2)H-labelling experiments. Both (18)O atoms of [(18)O(2)-hydroperoxy]9-HPOD were incorporated into the hemiacetal. Along with the hemiacetal, three chain-cleavage products, i.e. the enol (1E,3Z)-nonadienol and the hydrates of 3(Z)-nonenal and 9-oxononanoic acid, were trapped as their trimethylsilyl derivatives. The kinetics of (18)O incorporation from [(18)O(2)]9-HPOD provided strong evidence that the cleavage products originated in the hemiacetal. Linolenic and linoleic acid 13-hydroperoxides served as substrates for recombinant HPLs of melon, alfalfa (Medicago sativa) and guava (Psidium guajava), and in each case hemiacetals and enols were detectable by the trapping technique. The data obtained demonstrated that CYP74C and CYP74B HPLs act as isomerases performing a homolytic rearrangement of fatty acid hydroperoxides into short-lived hemiacetals which upon decomposition produce 3(Z)-nonenal, 3(Z)-hexenal and other short chain aldehydes.

Oxylipin profiling of the hypersensitive response in Arabidopsis thaliana. Formation of a novel oxo-phytodienoic acid-containing galactolipid, arabidopside E

Oxidation products of unsaturated fatty acids, collectively known as oxylipins, function as signaling molecules in plants during development, wounding, and insect and pathogen attack. Certain oxylipins are also known to have direct cytotoxic effects on pathogens. We used inducible expression of bacterial avirulence proteins in planta to study the involvement of oxylipins in race-specific defense against bacterial pathogens. We demonstrate that recognition of the Pseudomonas syringae avirulence protein AvrRpm1 induces 9- and 13-lipoxygenase-dependent oxylipin synthesis in Arabidopsis thaliana. The major oxylipins accumulated were jasmonic acid, 12-oxo-phytodienoic acid, and dinor-oxo-phytodienoic acid. The majority of the newly formed oxylipins (>90%) was found to be esterified to glycerolipids, whereby 12-oxo-phytodienoic acid and dinor-oxo-phytodienoic acid were found to be esterified to a novel galactolipid. The structure of the substance was determined as a monogalactosyldiacylglycerol containing two 12-oxo-phytodienoic acids and one dinor-oxo-phytodienoic acid acyl chain and was given the trivial name arabidopside E. This substance accumulated to surprisingly high levels, 7-8% of total lipid content, and was shown to inhibit growth of a bacterial pathogen in vitro. Arabidopside E was formed also after recognition of the avirulence protein AvrRpt2, suggesting that this could be a conserved feature of defense reactions against bacterial pathogens. In conclusion, the data presented suggest a role of enzymatically formed oxylipins, especially the octadecanoids and arabidopside E in race-specific resistance against bacterial pathogens.

Characterization of anacardic acids by micellar electrokinetic chromatography and mass spectrometry

A possibility of using capillary electrophoresis for separation of anacardic acids (6-alkylsalicylic acids) has been studied. Conventional micellar electrokinetic chromatography (MEKC) in non-coated fused silica capillaries and reversed-flow micellar electrokinetic chromatography (RF-MEKC) in capillaries coated with polydimethylacrylamide was applied for separation of anacardic acids extracted from cashew nuts. Influence of the composition of background electrolyte on the resolution of anacardic acid isomers was evaluated. Separations were performed using sodium dodecyl sulphate (SDS) micelles and mixed micelles of SDS and polyoxyethylene lauryl ether as a pseudostationary phase. To further improve the separation in RF-MEKC, beta-cyclodextrin and a dual cyclodextrin system of beta-cyclodextrin with heptakis-6-sulphato-beta-cyclodextrin was added to the working electrolyte. Best separation of anacardic acids were achieved in the polydimethylacrylamide-coated capillary using 10 mM phosphate background electrolyte pH 6.5 with addition of 1 M urea, 20% acetonitrile, 10 mM of beta-cyclodextrin and 1 mM of heptakis-6-sulfo-beta-cyclodextrin. Mass spectrometry was used for the identification of anacardic acids in the extract from cashew nuts in single and tandem mode using Q-TOF instrument. Nine anacardic acids were identified in the extract form the cashew nuts.

Synthesis of 3-oxalinolenic acid and β-oxidation-resistant 3-oxa-oxylipins

3-Oxalinolenic acid (3-oxa-9(Z),12(Z),15(Z)-octadecatrienoic acid or (6(Z),9(Z),12(Z)-pentadecatrienyloxy)acetic acid) was synthesized from 5(Z),8(Z),11(Z),14(Z),17(Z)-eicosapentaenoic acid by a sequence involving the C15 aldehyde 3(Z),6(Z),9(Z),12(Z)-pentadecatetraenal as a key intermediate. Conversion of the aldehyde by isomerization and two steps of reduction afforded 6(Z),9(Z),12(Z)-pentadecatrienol, which was coupled to bromoacetate to afford after purification by HPLC >99%-pure 3-oxalinolenic acid in 10-15% overall yield. 3-Oxalinolenic acid was efficiently oxygenated by soybean lipoxygenase-1 into 3-oxa-13(S)-hydroperoxy-9(Z),11(E),15(Z)-octadecatrienoic acid, and this hydroperoxide could be further converted chemically into 3-oxa-13(S)-hydroxy-9(Z),11 (E),15(Z)-octadecatrienoic acid and 3-oxa-13-oxo-9(Z),11 (E),15(Z)-octadecatrienoic acid. The 3-oxa-hydroperoxide also served as the substrate for the plant enzymes allene oxide synthase, divinyl ether synthase, and hydroperoxide lyase to produce 3-oxa-12-oxo-10,15(Z)-phytodienoic acid and other 3-oxa-oxylipins that were characterized by MS. 3-Oxalinolenic acid was not oxygenated by 9-lipoxygenase from tomato but was converted at a slow rate into 3-oxa-9(S)-hydroperoxy-10(E),12(Z),15(Z)-octadecatrienoic acid by recombinant maize 9-lipoxygenase. Recombinant alpha-dioxygenase-1 from Arabidopsis thaliana catalyzed the conversion of 3-oxalinolenic acid into a 2-hydroperoxide, which underwent spontaneous degradation into a mixture of 6,9,12-pentadecatrienol and 6,9,12-pentadecatrienyl formate. A novel alpha-dioxygenase from the moss Physcomitrella patens was cloned and expressed and was found to display the same activity with 3-oxalinolenic acid as Arabidopsis thaliana alpha-dioxygenase-1. Lipoxygenase-generated 3-oxa-oxylipins are resistant toward beta-oxidation and have the potential for displaying enhanced biological activity in situations where activity is limited by metabolic degradation.

Dissociative recombination of protonated methanol

The branching ratios of the different reaction pathways and the overall rate coefficients of the dissociative recombination reactions of CH3OH2+ and CD3OD2+ have been measured at the CRYRING storage ring located in Stockholm, Sweden. Analysis of the data yielded the result that formation of methanol or deuterated methanol accounted for only 3 and 6% of the total rate in CH3OH2+ and CD3OD2+, respectively. Dissociative recombination of both isotopomeres mainly involves fragmentation of the C-O bond, the major process being the three-body break-up forming CH3, OH and H (CD3, OD and D). The overall cross sections are best fitted by sigma = 1.2 +/- 0.1 x 10(-15) E(-1.15 +/- 0.02) cm2 and sigma = 9.6 +/- 0.9 x 10(-16) E(-1.20 +/- 0.02) cm2 for CH3OH2+ and CD3OD2+, respectively. From these values thermal reaction rate coefficients of k(T) = 8.9 +/- 0.9 x 10(-7) (T/300)(-0.59 +/- 0.02) cm3 s(-1) (CH3OH2+) and k(T) = 9.1 +/- 0.9 x 10(-7) (T/300)(-0.63 +/- 0.02) cm3 s(-1) (CD3OD2+) can be calculated. A non-negligible formation of interstellar methanol by the previously proposed mechanism via radiative association of CH3+ and H2O and subsequent dissociative recombination of the resulting CH3OH2+ ion to yield methanol and hydrogen atoms is therefore very unlikely.

Evaluation of the antimicrobial activities of plant oxylipins supports their involvement in defense against pathogens

Plant oxylipins are a large family of metabolites derived from polyunsaturated fatty acids. The characterization of mutants or transgenic plants affected in the biosynthesis or perception of oxylipins has recently emphasized the role of the so-called oxylipin pathway in plant defense against pests and pathogens. In this context, presumed functions of oxylipins include direct antimicrobial effect, stimulation of plant defense gene expression, and regulation of plant cell death. However, the precise contribution of individual oxylipins to plant defense remains essentially unknown. To get a better insight into the biological activities of oxylipins, in vitro growth inhibition assays were used to investigate the direct antimicrobial activities of 43 natural oxylipins against a set of 13 plant pathogenic microorganisms including bacteria, oomycetes, and fungi. This study showed unequivocally that most oxylipins are able to impair growth of some plant microbial pathogens, with only two out of 43 oxylipins being completely inactive against all the tested organisms, and 26 oxylipins showing inhibitory activity toward at least three different microbes. Six oxylipins strongly inhibited mycelial growth and spore germination of eukaryotic microbes, including compounds that had not previously been ascribed an antimicrobial activity, such as 13-keto-9(Z),11(E),15(Z)-octadecatrienoic acid and 12-oxo-10,15(Z)-phytodienoic acid. Interestingly, this first large-scale comparative assessment of the antimicrobial effects of oxylipins reveals that regulators of plant defense responses are also the most active oxylipins against eukaryotic microorganisms, suggesting that such oxylipins might contribute to plant defense through their effects both on the plant and on pathogens, possibly through related mechanisms.

Thermal conversions of trimethylsilyl peroxides of linoleic and linolenic acids

The trimethylsilyl (TMS) peroxides/esters of the fatty acid hydroperoxides (9S,10E,12Z)-9-hydroperoxy-10,12-octadecadienoic acid (9-HPOD) and (9Z,11E,13S,15Z)-13-hydroperoxy-9,11,15-octadecatrienoic acid (13-HPOT) were subjected to gas chromatography-mass spectrometry and products formed by thermal rearrangements were identified. The main products were decadienals and the TMS derivatives of 13-oxo-9,11-tridecadienoic acid, epoxyalcohols, hemiacetals, and ketodienes. Oxy radicals as well as epoxyallylic radicals served as intermediates in the formation of these compounds. The thermal TMS peroxide conversions documented provided biomimetic models for enzymatic conversions of fatty acid hydroperoxides and also offered a method to generate an array of oxylipin derivatives of value as reference compounds in GC-MS studies.

Novel oxylipins formed from docosahexaenoic acid by potato lipoxygenase--10(S)-hydroxydocosahexaenoic acid and 10,20-dihydroxydocosahexaenoic acid

Potato tuber lipoxygenase (ptLOX) has been shown to catalyze the aerobic formation of at least four major oxygenated derivatives of DHA. Two of the products--7,17(S)- and 10,17(S)-dihydro(pero)xy-DHA [7,17- and 10,17-diH(P)DHA]--were formed from soybean 15-LOX-derived 17(S)-hydro(pero)xy-DHA [17(S)-H(P)DHA], whereas two novel oxylipin compounds--10(S)-hydro(pero)xy-DHA and 10,20-dihydro(pero)xy-DHA [10(S)-H(P)DHA and 10,20-diH(P)DHA, respectively]--were the major direct products of DHA oxidation by ptLOX. The reactions proceeded relatively slowly but could be stimulated by catalytic amounts of SDS. Micromolar concentrations of 10(S)-HPDHA effectively abolished the kinetic lag period of ptLOX activation. Enzymatic activity with DHA or 17(S)-HPDHA as substrate was about 8% of that with linoleic acid--a standard natural ptLOX substrate--whereas 17(S)-HDHA was converted at a rate of approximately 1%. The enzyme was relatively unstable and quickly inactivated during the reaction with DHA on with 17(S)-HPDHA (first-order kinetic constant of inactivation kin = 1.5 +/- 0.3 min(-1)), but not with 17(S)-HDHA. Both 7,17- and 10,20-diH(P)DHA were clearly products of double oxygenation catalyzed by soybean 15-LOX and/or ptLOX. Our observation that ptLOX could convert 17-HDHA to 10,17-diH(P)DHA indicates that this dihydroxylated derivative of DHA also can be formed via a double lipoxygenation mechanism.

Conversion of linoleic acid into novel oxylipins by the mushroom Agaricus bisporus

Oxylipins are associated with important processes of the fungal life cycle, such as spore formation. Here, we report the formation of FA metabolites in Agaricus bisporus. Incubation of a crude extract of lamellae with linoleic acid (18:2) led to the extensive formation of two oxylipins. They were identified as 8(R)-hydroxy-9Z,12Z-octadecadienoic acid (8-HOD) and 8(R),11 (S)-dihydroxy-9Z,12Z-octadecadienoic acid (8,11-diHOD) by using RP-HPLC, GC-MS, IR, GC-MS analysis of diastereomeric derivatives, and 1H NMR and 13C NMR spectroscopy. Neither compound has been reported before in A. bisporus. Oleic (18:1), alpha-linolenic (18:3n-3), and gamma-linolenic (18:3n-6) acids were converted into their 8-hydroxy derivatives as well, and 18:3n-3 was further metabolized to its 8,11-diol derivative. Reactions with [U-13C]18:2 demonstrated that the compounds 8-HOD and 8,11-diHOD were formed from exogenously supplied 18:2. When [U-13C]8-HOD was supplied, it was not converted into 8,11-diHOD, indicating that it was not an intermediate in the formation of 8,11-diHOD. When a crude extract of A. bisporus was incubated under an atmosphere of 16O2/18O2, the two hydroxyl groups of 8,11-diHOD contained either two 180 atoms or two 60 atoms. Species that contained one of each isotope could not be detected. We propose that the formation of the 8,11-dihydroxy compounds occurs through either an 8,11-endoperoxy, an 8-peroxo free radical, or an 8-hydroperoxy intermediate. In the latter case, the reaction should be catalyzed by dioxygenase with novel specificity.

Alpha-dioxygenases

Alpha-dioxygenases constitute a family of fatty acid-metabolizing enzymes recently discovered in plants. The present paper gives a brief overview of the literature dealing with these enzymes and additionally reports the new finding of an alpha-dioxygenase in the moss, Physcomitrella patens, and some properties of this enzyme.

Biosynthesis and isomerization of 11-hydroperoxylinoleates by manganese- and iron-dependent lipoxygenases

Manganese lipoxygenase (Mn-LO) oxygenates linoleic acid (LA) to a mixture of the hydroperoxides--11 (S)-hydroperoxy-9Z,12Z-octadecadienoic acid [11(S)-HPODE] and 13(R)-hydroperoxy-9Z,11 E-octadecadienoic acid [13(R)-HPODE]-- and also catalyzes the conversion of 11 (S)-HPODE to 13(R)-HPODE via oxygen-centered (LOO-) and carbon-centered (L.) radicals [Hamberg, M., Su, C., and Oliw, E. (1998) Manganese Lipoxygenase. Discovery of a Bis-allylic Hydroperoxide as Product and Intermediate in a Lipoxygenase Reaction, J. Biol. Chem. 273, 13080-13088]. The aims of the present work were to investigate whether 11-HPODE can also be produced by iron-dependent lipoxygenases and to determine the enzymatic transformations of stereoisomers of 11-HPODE by lipoxygenases. Rice leaf pathogen-inducible lipoxygenase, but not soybean lipoxygenase-1 (sLO-1), generated a low level of 11-HPODE (0.4%) besides its main hydroperoxide, 13(S)-HPODE, on incubation with LA. Steric analysis revealed that 11-HPODE was enriched with respect to the R enantiomer [74% 11(R)]. In agreement with previous results, 11 (S)-HPODE incubated with Mn-LO provided 13(R)-HPODE, and the same conversion also took place with the methyl ester of 11(S)-HPODE. 11(R,S)-HPODE was metabolized biphasically in the presence of Mn-LO, i.e., by a rapid phase during which the 11(S)-enantiomer was converted into 13(R)-HPODE and a slow phase during which the 11(R)-enantiomer was converted into 9(R)-HPODE. sLO-1 catalyzed a slow conversion of 11 (S)-HPODE into a mixture of 13(R)-HPODE (75%), 9(S)-HPODE (10%), and 13(S)-HPODE (10%), whereas 11(R,S)-HPODE produced a mixture of nearly racemic 13-HPODE (approximately 70%) and 9-HPODE (approximately 30%). The results showed that 11HPODE can also be produced by an iron-dependent LO and suggested that the previously established mechanism of isomerization of 11(S)-HPODE involving suprafacial migration of O2 is valid also for the isomerizations of 11(R)-HPODE by Mn-LO and of 11(S)-HPODE by sLO-1.

Hidden stereospecificity in the biosynthesis of divinyl ether fatty acids

Incubations of [8(R)-2H]9(S)-hydroperoxy-10(E),12(Z)-octadecadienoic acid, [14(R)-2H]13(S)-hydroperoxy-9(Z),11(E)-octadecadienoic acid and [14(S)-2H]13(S)-hydroperoxy-9(Z),11(E)-octadecadienoic acid were performed with preparations of plant tissues containing divinyl ether synthases. In agreement with previous studies, generation of colneleic acid from the 8(R)-deuterated 9(S)-hydroperoxide was accompanied by loss of most of the deuterium label (retention, 8%), however, the opposite result (98% retention) was observed in the generation of 8(Z)-colneleic acid from the same hydroperoxide. Formation of etheroleic acid and 11(Z)-etheroleic acid from the 14(R)-deuterated 13(S)-hydroperoxide was accompanied by loss of most of the deuterium (retention, 7-8%), and, as expected, biosynthesis of these divinyl ethers from the corresponding 14(S)-deuterated hydroperoxide was accompanied by retention of deuterium (retention, 94-98%). Biosynthesis of omega5(Z)-etheroleic acid from the 14(R)- and 14(S)-deuterated 13(S)-hydroperoxides showed the opposite results, i.e. 98% retention and 4% retention, respectively. The experiments demonstrated that biosynthesis of divinyl ether fatty acids from linoleic acid 9- and 13-hydroperoxides takes place by a mechanism that involves stereospecific abstraction of one of the two hydrogen atoms alpha to the hydroperoxide carbon. Furthermore, a consistent relationship between the absolute configuration of the hydrogen atom eliminated (R or S) and the configuration of the introduced vinyl ether double bond (E or Z) emerged from these results. Thus, irrespective of which hydroperoxide regioisomer served as the substrate, divinyl ether synthases abstracting the pro-R hydrogen generated divinyl ethers having an E vinyl ether double bond, whereas enzymes abstracting the pro-S hydrogen produced divinyl ethers having a Z vinyl ether double bond.

Properties of two multifunctional plant fatty acid acetylenase/desaturase enzymes

The properties of the Delta6 desaturase/acetylenase from the moss Ceratodon purpureus and the Delta12 acetylenase from the dicot Crepis alpina were studied by expressing the encoding genes in Arabidopsis thaliana and Saccharomyces cerevisiae. The acetylenase from C. alpinaDelta12 desaturated both oleate and linoleate with about equal efficiency. The desaturation of oleate gave rise to 9(Z),12(E)- and 9(Z),12(Z)-octadecadienoates in a ratio of approximately 3 : 1. Experiments using stereospecifically deuterated oleates showed that the pro-R hydrogen atoms were removed from C-12 and C-13 in the introduction of the 12(Z) double bond, whereas the pro-R and pro-S hydrogen atoms were removed from these carbons during the formation of the 12(E) double bond. The results suggested that the Delta12 acetylenase could accommodate oleate having either a cisoid or transoid conformation of the C(12)-C(13) single bond, and that these conformers served as precursors of the 12(Z) and 12(E) double bonds, respectively. However, only the 9(Z),12(Z)-octadecadienoate isomer could be further desaturated to 9(Z)-octadecen-12-ynoate (crepenynate) by the enzyme. The evolutionarily closely related Delta12 epoxygenase from Crepis palaestina had only weak desaturase activity but could also produce 9(Z),12(E)-octadecadienoate from oleate. The Delta6 acetylenase/desaturase from C. purpureus, on the other hand, produced only the 6(Z) isomers using C16 and C18 acyl groups possessing a Delta9 double bond as substrates. The Delta6 double bond was efficiently further converted to an acetylenic bond by a second round of desaturation but only if the acyl substrate had a Delta12 double bond and that this was in the Z configuration.

The "heterolytic hydroperoxide lyase" is an isomerase producing a short-lived fatty acid hemiacetal

To elucidate the reaction mechanism of hydroperoxide lyase (HPL), the enzyme from guava (Psidium guajava) fruits, was incubated for 10-60 s at 0 degrees C with 13-HPOT. The products were rapidly extracted and derivatized by trimethylsilylation. Two trapping products, namely the trimethylsilyl ether/ester derivatives of the hemiacetal 12-(1'-hydroxy-3'-hexenyloxy)-9,11-dodecadienoic acid and the enol (9Z,11E)-12-hydroxy-9,11-dodecadienoic acid, were detected by gas chromatography-mass spectrometry (GC-MS) analyses. The structural assignments were supported by mass spectra recorded for (a) hydrogenated products; (b) products biosynthesized from [9,10,12,13,15,16] 13-HPOT or [(18)O(2)]13-HPOT; (c) chemically prepared reference compounds. Kinetic experiments showed that the hemiacetal and enol were both unstable and transiently appearing compounds (half-lives, ca. 20 s and 2 min, respectively). Hemiacetal and enol biosynthesized from [(18)O(2)]13-HPOT retained two and one (18)O atoms, respectively, whereas no (18)O was incorporated from [(18)O]water. The data demonstrated that: (1) the true enzymatic product formed from 13-HPOT in the presence of HPL is a short-lived hemiacetal; (2) the hemiacetal spontaneously dissociates into (3Z)-hexenal and the unstable enol form of (9Z)-12-oxo-9-dodecenoic acid; (3) the enzymatic isomerization of 13-HPOT into the hemiacetal occurs homolytically.

Isolation and structures of two divinyl ether fatty acids from Clematis vitalba

[1-14C]Linolenic acid was incubated with a homogenate of leaves of Clematis vitalba, a plant belonging to the Ranunculaceae family. Analysis of the reaction product by reversed-phase high-performance liquid radiochromatography demonstrated the presence of the following labeled oxylipins: 12-oxo-10, 15(Z)-phytodienoic acid, 9(S)-hydroxy-10(E), 12(Z), 15(Z)-octadecatrienoic acid, omega5(Z)-etherolenic acid, and 9-[1'(E), 3'(Z),6'(Z)-nonatrienyloxy]-8(Z)-nonenoic acid [8(Z)-colnelenic acid]. The last compound was a new divinyl ether FA, and an analogous compound, i.e., 9-[1'(E),3'(Z)-nonadienyloxy]-8(Z)-nonenoic acid [8(Z)-colneleic acid], was obtained following incubation of linoleic acid with the Clematis homogenate. Structures of the two divinyl ethers were assigned by spectral and chromatographic comparison with authentic compounds prepared synthetically using previously described methodology. Separate incubation of the 9- and 13-hydroperoxides of linolenic acid demonstrated that the first hydroperoxide served as the precursor of 8(Z)-colnelenic acid and indicated the presence in C. vitalba of a new divinyl ether synthase acting on 9-lipoxygenase-generated hydroperoxides. A close structural relationship between this enzyme and the well-studied divinyl ether synthase in the potato and tomato seems likely.

Activation of the fatty acid alpha-dioxygenase pathway during bacterial infection of tobacco leaves. Formation of oxylipins protecting against cell death

A pathogen-induced oxygenase showing homology to prostaglandin endoperoxide synthases-1 and -2 was recently characterized by in vitro experiments as a fatty acid alpha-dioxygenase catalyzing formation of unstable 2(R)-hydroperoxy fatty acids. To study the activity of this enzyme under in vivo conditions and to elucidate the fate of enzymatically produced 2-hydroperoxides, leaves of tobacco were analyzed for the presence of alpha-dioxygenase-generated compounds as well as for lipoxygenase (LOX) products and free fatty acids. Low basal levels of 2-hydroxylinolenic acid (0.4 nmol/g leaves fresh weight) and 8,11,14-heptadecatrienoic acid (0.1 nmol/g) could be demonstrated. These levels increased strongly upon infection with the bacterium Pseudomonas syringae pv syringae (548 and 47 nmol/g, respectively). Transgenic tobacco plants overexpressing alpha-dioxygenase were developed, and incompatible infection of such plants led to a dramatic elevation of 2-hydroxylinolenic acid (1778 nmol/g) and 8,11,14-heptadecatrienoic acid (86 nmol/g), whereas the levels of LOX products were strongly decreased. Further analysis of oxylipins in infected leaves revealed the presence of a number of 2-hydroxy fatty acids differing with respect to chain length and degree of unsaturation as well as two new doubly oxygenated oxylipins identified as 2(R),9(S)-dihydroxy-10(E),12(Z),15(Z)-octadecatrienoic acid and 2(R),9(S)-dihydroxy-10(E),12(Z)-octadecadienoic acid. alpha-Dioxygenase-generated 2-hydroxylinolenic acid, and to a lesser extent lipoxygenase-generated 9-hydroxyoctadecatrienoic acid, exerted a tissue-protective effect in bacterially infected tobacco leaves.

Detection of an enol intermediate in the hydroperoxide lyase chain cleavage reaction

Guava (Psidium guajava) hydroperoxide lyase (HPL) preparations were incubated with [1-(14)C](9Z,11E,13S,15Z)-13-hydroperoxy-9,11,15-octadecatrienoic acid for 1 min at 0 degrees C, followed by rapid extraction/trimethylsilylation. Analysis of the trimethylsilylated products by gas chromatography-mass spectrometry and radio-high-performance liquid chromatography revealed a single predominant (14)C-labelled compound, identified by its (1)H-nuclear magnetic resonance, ultraviolet and mass spectra as the trimethylsilyl ether/ester of (9Z,11E)-12-hydroxy-9,11-dodecadienoic acid. Longer time incubations afford smaller yield of this enol due to its partial tautomerization into (9Z)-12-oxo-9-dodecenoic acid. The data obtained demonstrate that formation of (9Z)-12-oxo-9-dodecenoic acid in the HPL reaction is preceded by unstable enol oxylipin, and further suggest that hemiacetals are the true products of HPL catalysis.