Stereoselective oxidation of regioisomeric octadecenoic acids by fatty acid dioxygenases

Regulation of inflammation in cancer by dietary eicosanoids

BACKGROUND

Cancer is a major burden of disease worldwide and increasing evidence shows that inflammation contributes to cancer development and progression. Eicosanoids are derived from dietary polyunsaturated fatty acids, such as arachidonic acid (AA), and are mainly produced by a series of enzymatic pathways that include cyclooxygenase (COX), lipoxygenase (LOX), and cytochrome P-450 epoxygenase (CYP). Eicosanoids consist of at least several hundred individual molecules and play important roles in the inflammatory response and inflammation-related cancers.

SCOPE AND APPROACH

Dietary sources of AA and biosynthesis of eicosanoids from AA through different metabolic pathways are summarized. The bioactivities of eicosanoids and their potential molecular mechanisms on inflammation and cancer are revealed. Additionally, current challenges and limitations in eicosanoid research on inflammation-related cancer are discussed.

KEY FINDINGS AND CONCLUSIONS

Dietary AA generates a large variety of eicosanoids, including prostaglandins, thromboxane A2, leukotrienes, cysteinyl leukotrienes, lipoxins, hydroxyeicosatetraenoic acids (HETEs), and epoxyeicosatrienoic acids (EETs). Eicosanoids exert different bioactivities and mechanisms involved in the inflammation and related cancer developments. A deeper understanding of eicosanoid biology may be advantageous in cancer treatment and help to define cellular targets for further therapeutic development.

Enantioselective metabolomics by liquid chromatography-mass spectrometry

Metabolomics strives to capture the entirety of the metabolites in a biological system by comprehensive analysis, often by liquid chromatography hyphenated to mass spectrometry. A particular challenge thereby is the differentiation of structural isomers. Common achiral targeted and untargeted assays do not distinguish between enantiomers. This may lead to information loss. An increasing number of publications demonstrate that the enantiomeric ratio of certain metabolites can be meaningful biomarkers of certain diseases emphasizing the importance of introducing enantioselective analytical procedures in metabolomics. In this work, the state-of-the-art in the field of LC-MS based metabolomics is summarized with focus on developments in the recent decade. Methodologies, tagging strategies, workflows and general concepts are outlined. Selected biological applications in which enantioselective metabolomics has documented its usefulness are briefly discussed. In general, targeted enantioselective metabolomics assays are often based on a direct approach using chiral stationary phases (CSP) with polysaccharide derivatives, macrocyclic antibiotics, chiral crown ethers, chiral ion exchangers, donor-acceptor phases as chiral selectors. Rarely, these targeted assays focus on more than 20 analytes and usually are restricted to a certain metabolite class. In a variety of cases, pre-column derivatization of metabolites has been performed, especially for amino acids, to improve separation and detection sensitivity. Triple quadrupole instruments are the detection methods of first choice in targeted assays. Here, issues like matrix effect, absence of blank matrix impair accuracy of results. In selected applications, multiple heart cutting 2D-LC (RP followed by chiral separation) has been pursued to overcome this problem and alleviate bias due to interferences. Non-targeted assays, on the other hand, are based on indirect approach involving tagging with a chiral derivatizing agent (CDA). Besides classical CDAs numerous innovative reagents and workflows have been proposed and are discussed. Thereby, a critical issue for the accuracy is often neglected, viz. the validation of the enantiomeric impurity in the CDA. The majority of applications focus on amino acids, hydroxy acids, oxidized fatty acids and oxylipins. Some potential clinical applications are highlighted.

Fatty acid dioxygenase-cytochrome P450 fusion enzymes of filamentous fungal pathogens

Oxylipins designate oxygenated unsaturated C₁₈ fatty acids. Many filamentous fungi pathogens contain dioxygenases (DOX) in oxylipin biosynthesis with homology to human cyclooxygenases. They contain a DOX domain, which is often fused to a functional cytochrome P450 at the C-terminal end. A Tyr radical in the DOX domain initiates dioxygenation of linoleic acid by hydrogen abstraction with formation of 8-, 9-, or 10-hydroperoxy metabolites. The P450 domains can catalyze heterolytic cleavage of 8- and 10-hydroperoxides with oxidation of the heme thiolate iron for hydroxylation at C-5, C-7, C-9, or C-11 and for epoxidation of the 12Z double bond; thus displaying linoleate diol synthase (LDS) and epoxy alcohol synthase (EAS) activities. LSD activities are present in the rice blast pathogen Magnaporthe oryzae, Botrytis cinerea causing grey mold and the black scurf pathogen Rhizoctonia solani. 10R-DOX-EAS has been found in M. oryzae and Fusarium oxysporum. The P450 domains may also catalyze homolytic cleavage of 8- and 9-hydroperoxy fatty acids and dehydration to produce epoxides with an adjacent double bond, i.e., allene oxides, thus displaying 8- and 9-DOX-allene oxide synthases (AOS). F. oxysporum, F. graminearum, and R. solani express 9S-DOX-AOS and Zymoseptoria tritici 8S-and 9R-DOX-AOS. Homologues are present in endemic human-pathogenic fungi with extensive studies in Aspergillus fumigatus, A. flavus (also a plant pathogen) as well as the genetic model A. nidulans. 8R-and 10R-DOX appear to bind fatty acids "headfirst" in the active site, whereas 9S-DOX binds them "tail first" in analogy with cyclooxygenases. The biological relevance of 8R-DOX-5,8-LDS (also designated PpoA) was first discovered in relation to sporulation of A. nidulans and recently for development and programmed hyphal branching of A. fumigatus. Gene deletion DOX-AOS homologues in F. verticillioides, A. flavus, and A. nidulans alters, inter alia, mycotoxin production, sporulation, and gene expression.

Enantioselectivity Effects in Clinical Metabolomics and Lipidomics

Metabolomics and lipidomics have demonstrated increasing importance in underlying biochemical mechanisms involved in the pathogenesis of diseases to identify novel drug targets and/or biomarkers for establishing therapeutic approaches for human health. Particularly, bioactive metabolites and lipids have biological activity and have been implicated in various biological processes in physiological conditions. Thus, comprehensive metabolites, and lipids profiling are required to obtain further advances in understanding pathophysiological changes that occur in cells and tissues. Chirality is one of the most important phenomena in living organisms and has attracted long-term interest in medical and natural science. Enantioselective separation plays a pivotal role in understanding the distribution and physiological function of a diversity of chiral bioactive molecules. In this context, it has been the goal of method development for targeted and untargeted metabolomics and lipidomic assays. Herein we will highlight the benefits and challenges involved in these stereoselective analyses for clinical samples.

WITHDRAWN: Fatty acid dioxygenase-cytochrome P450 fusion enzymes of the top 10 fungal pathogens in molecular plant pathology and human-pathogenic fungi

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Concerted proton-electron transfer reactions of manganese-hydroxo and manganese-oxo complexes

The enzymes manganese superoxide dismutase and manganese lipoxygenase use MnIII-hydroxo centres to mediate proton-coupled electron transfer (PCET) reactions with substrate. As manganese is earth-abundant and inexpensive, manganese catalysts are of interest for synthetic applications. Recent years have seen exciting reports of enantioselective C-H bond oxidation by Mn catalysts supported by aminopyridyl ligands. Such catalysts offer economic and environmentally-friendly alternatives to conventional reagents and catalysts. Mechanistic studies of synthetic catalysts highlight the role of Mn-oxo motifs in attacking substrate C-H bonds, presumably by a concerted proton-electron transfer (CPET) step. (CPET is a sub-class of PCET, where the proton and electron are transferred in the same step.) Knowledge of geometric and electronic influences for CPET reactions of Mn-hydroxo and Mn-oxo adducts enhances our understanding of biological and synthetic manganese centers and informs the design of new catalysts. In this Feature article, we describe kinetic, spectroscopic, and computational studies of MnIII-hydroxo and MnIV-oxo complexes that provide insight into the basis for the CPET reactivity of these species. Systematic perturbations of the ligand environment around MnIII-hydroxo and MnIV-oxo motifs permit elucidation of structure-activity relationships. For MnIII-hydroxo centers, electron-deficient ligands enhance oxidative reactivity. However, ligand perturbations have competing consequences, as changes in the MnIII/II potential, which represents the electron-transfer component for CPET, is offset by compensating changes in the pKa of the MnII-aqua product, which represents the proton-transfer component for CPET. For MnIV-oxo systems, a multi-state reactivity model inspired the development of significantly more reactive complexes. Weakened equatorial donation to the MnIV-oxo unit results in large rate enhancements for C-H bond oxidation and oxygen-atom transfer reactions. These results demonstrate that the local coordination environment can be rationally changed to enhance reactivity of MnIII-hydroxo and MnIV-oxo adducts.

Enantioselective high-performance liquid chromatography analysis of oxygenated polyunsaturated fatty acids

Oxygenated polyunsaturated fatty acids (PUFAs)play an outstanding role in the physiological and pathological regulation of several biological processes. These oxygenated metabolites can be produced both enzimatically, yielding almost pure enantiomers, and non-enzymatically. The free radical-mediated non-enzymatic oxidation commonly produces racemic mixtures which are used as biomarkers of oxidative stress and tissue damage. The biological activity of oxygenated PUFAs is often associated with only one enantiomer, making it necessary of availing of lipidomics platforms allowing to disclose the role of single enantiomers in health and disease. Polysaccharide-based chiral stationary phases (CSPs) play a dominating part in this setting. As for the cellulose backbone, 4-methylbenzoate derivatives exhibit very high chiral recognition ability towards this class of compounds. Concerning the phenylcarbamate derivatives of cellulose and amylose, the tris(3,5-dimethylphenylcarbamate) variants show the best enantioresolving ability for a variety of oxygenated PUFAs. Moreover, also the amylose tris(5-chloro-2-methylphenylcarbamate)-based selector produces relevant chromatographic performances. The extreme versatility of those CSPs mostly depends on their compatibility with the most relevant elution modes: normal- and reversed-phase, as well as polar organic/ionic-mode. In this review article, a selection of enantioseparation studies of different oxygenated PUFAs is reported, with both tris(benzoates) and tris(phenylcarbamates) of cellulose and amylose.

Biosynthesis of Oxylipins by Rhizoctonia solani with Allene Oxide and Oleate 8S,9S-Diol Synthase Activities

Oxylipin biosynthesis by fungi is catalyzed by both the lipoxygenase (LOX) family and the linoleate diol synthase (LDS) family of the peroxidase-cyclooxygenase superfamily. Rhizoctonia solani, a pathogenic fungus, infects staple crops such as potato and rice. The genome predicts three genes with 9-13 introns, which code for tentative dioxygenase (DOX)-cytochrome P450 fusion enzymes of the LDS family, and one gene, which might code for a 13-LOX. The objective was to determine whether mycelia or nitrogen powder of mycelia oxidized unsaturated C₁₈ fatty acids to LDS- or LOX-related metabolites. Mycelia converted 18:2n-6 to 8R-hydroxy-9Z,12Z-octadecadienoic acid and to an α-ketol, 9S-hydroxy-10-oxo-12Z-octadecenoic acid. In addition to these metabolites, nitrogen powder of mycelia oxidized 18:2n-6 to 9S-hydroperoxy-10E, 12Z-octadecadienoic, and 13S-hydroperoxy-9Z,11E-octadecadienoic acids; the latter was likely formed by the predicted 13-LOX. 18:1n-9 was transformed into 8S-hydroperoxy-9Z-octadecenoic and into 8S,9S-dihydroxy-10E-octadecenoic acids, indicating the expression of 8,9-diol synthase. The allene oxide, 9S(10)epoxy-10,12Z-octadecadienoic acid, is unstable and decomposes rapidly to the α-ketol above, indicating biosynthesis by 9S-DOX-allene oxide synthase. This allene oxide and α-ketol are also formed by potato stolons, which illustrates catalytic similarities between the plant host and fungal pathogen.

A new class of fatty acid allene oxide formed by the DOX-P450 fusion proteins of human and plant pathogenic fungi, C. immitis and Z. tritici

Linoleate dioxygenase-cytochrome P450 (DOX-CYP) fusion enzymes are common in pathogenic fungi. The DOX domains form hydroperoxy metabolites of 18:2n-6, which can be transformed by the CYP domains to 1,2- or 1,4-diols, epoxy alcohols, or to allene oxides. We have characterized two novel allene oxide synthases (AOSs), namely, recombinant 8R-DOX-AOS of Coccidioides immitis (causing valley fever) and 8S-DOX-AOS of Zymoseptoria tritici (causing septoria tritici blotch of wheat). The 8R-DOX-AOS oxidized 18:2n-6 sequentially to 8R-hydroperoxy-9Z,12Z-octadecadienoic acid (8R-HPODE) and to an allene oxide, 8R(9)-epoxy-9,12Z-octadecadienoic acid, as judged from the accumulation of the α-ketol, 8S-hydroxy-9-oxo-12Z-octadecenoic acid. The 8S-DOX-AOS of Z. tritici transformed 18:2n-6 sequentially to 8S-HPODE and to an α-ketol, 8R-hydroxy-9-oxo-12Z-octadecenoic acid, likely formed by hydrolysis of 8S(9)-epoxy-9,12Z-octadecadienoic acid. The 8S-DOX-AOS oxidized [8R-(2)H]18:2n-6 to 8S-HPODE with retention of the (2)H-label, suggesting suprafacial hydrogen abstraction and oxygenation in contrast to 8R-DOX-AOS. Both enzymes oxidized 18:1n-9 and 18:3n-3 to α-ketols, but the catalysis of the 8R- and 8S-AOS domains differed. 8R-DOX-AOS transformed 9R-HPODE to epoxy alcohols, but 8S-DOX-AOS converted 9S-HPODE to an α-ketol (9-hydroxy-10-oxo-12Z-octadecenoic acid) and epoxy alcohols in a ratio of ∼1:2. Whereas all fatty acid allene oxides described so far have a conjugated diene impinging on the epoxide, the allene oxides formed by 8-DOX-AOS are unconjugated.

In Silico/In Vivo Insights into the Functional and Evolutionary Pathway of Pseudomonas aeruginosa Oleate-Diol Synthase. Discovery of a New Bacterial Di-Heme Cytochrome C Peroxidase Subfamily

As previously reported, P. aeruginosa genes PA2077 and PA2078 code for 10S-DOX (10S-Dioxygenase) and 7,10-DS (7,10-Diol Synthase) enzymes involved in long-chain fatty acid oxygenation through the recently described oleate-diol synthase pathway. Analysis of the amino acid sequence of both enzymes revealed the presence of two heme-binding motifs (CXXCH) on each protein. Phylogenetic analysis showed the relation of both proteins to bacterial di-heme cytochrome c peroxidases (Ccps), similar to Xanthomonas sp. 35Y rubber oxidase RoxA. Structural homology modelling of PA2077 and PA2078 was achieved using RoxA (pdb 4b2n) as a template. From the 3D model obtained, presence of significant amino acid variations in the predicted heme-environment was found. Moreover, the presence of palindromic repeats located in enzyme-coding regions, acting as protein evolution elements, is reported here for the first time in P. aeruginosa genome. These observations and the constructed phylogenetic tree of the two proteins, allow the proposal of an evolutionary pathway for P. aeruginosa oleate-diol synthase operon. Taking together the in silico and in vivo results obtained we conclude that enzymes PA2077 and PA2078 are the first described members of a new subfamily of bacterial peroxidases, designated as Fatty acid-di-heme Cytochrome cperoxidases (FadCcp).

Kinetic investigation of the rate-limiting step of manganese- and iron-lipoxygenases

Lipoxygenases (LOX) oxidize polyunsaturated fatty acids to hydroperoxides, which are generated by proton coupled electron transfer to the metal center with FeIIIOH- or MnIIIOH-. Hydrogen abstraction by FeIIIOH- of soybean LOX-1 (sLOX-1) is associated with a large deuterium kinetic isotope effect (D-KIE). Our goal was to compare the D-KIE and other kinetic parameters at different temperatures of sLOX-1 with 13R-LOX with catalytic manganese (13R-MnLOX). The reaction rate and the D-KIE of sLOX-1 with unlabeled and [11-2H2]18:2n-6 were almost temperature independent with an apparent D-KIE of ∼56 at 30°C, which is in agreement with previous studies. In contrast, the reaction rate of 13R-MnLOX increased 7-fold with temperature (8-50°C), and the apparent D-KIE decreased linearly from ∼38 at 8°C to ∼20 at 50°C. The kinetic lag phase of 13R-MnLOX was consistently extended at low temperatures. The Phe337Ile mutant of 13R-MnLOX, which catalyzes antarafacial hydrogen abstraction and oxygenation in analogy with sLOX-1, retained the large D-KIE and its temperature-dependent reaction rate. The kinetic differences between 13R-MnLOX and sLOX-1 may be due to protein dynamics, hydrogen donor-acceptor distances, and to the metal ligands, which may not equalize the 0.7V-gap between the redox potentials of the free metals.

An ancient relative of cyclooxygenase in cyanobacteria is a linoleate 10S-dioxygenase that works in tandem with a catalase-related protein with specific 10S-hydroperoxide lyase activity

In the course of exploring the scope of catalase-related hemoprotein reactivity toward fatty acid hydroperoxides, we detected a novel candidate in the cyanobacterium Nostoc punctiforme PCC 73102. The immediate neighboring upstream gene, annotated as "cyclooxygenase-2," appeared to be a potential fatty acid heme dioxygenase. We cloned both genes and expressed the cDNAs in Escherichia coli, confirming their hemoprotein character. Oxygen electrode recordings demonstrated a rapid (>100 turnovers/s) reaction of the heme dioxygenase with oleic and linoleic acids. HPLC, including chiral column analysis, UV, and GC-MS of the oxygenated products, identified a novel 10S-dioxygenase activity. The catalase-related hemoprotein reacted rapidly and specifically with linoleate 10S-hydroperoxide (>2,500 turnovers/s) with a hydroperoxide lyase activity specific for the 10S-hydroperoxy enantiomer. The products were identified by NMR as (8E)10-oxo-decenoic acid and the C8 fragments, 1-octen-3-ol and 2Z-octen-1-ol, in ∼3:1 ratio. Chiral HPLC analysis established strict enzymatic control in formation of the 3R alcohol configuration (99% enantiomeric excess) and contrasted with racemic 1-octen-3-ol formed in reaction of linoleate 10S-hydroperoxide with hematin or ferrous ions. The Nostoc linoleate 10S-dioxygenase, the sequence of which contains the signature catalytic sequence of cyclooxygenases and fungal linoleate dioxygenases (YRWH), appears to be a heme dioxygenase ancestor. The novel activity of the lyase expands the known reactions of catalase-related proteins and functions in Nostoc in specific transformation of the 10S-hydroperoxylinoleate.

Discovery of a linoleate 9S-dioxygenase and an allene oxide synthase in a fusion protein of Fusarium oxysporum

Fusarium oxysporum is a devastating plant pathogen that oxidizes C₁₈ fatty acids sequentially to jasmonates. The genome codes for putative dioxygenase (DOX)-cytochrome P450 (CYP) fusion proteins homologous to linoleate diol synthases (LDSs) and the allene oxide synthase (AOS) of Aspergillus terreus, e.g., FOXB_01332. Recombinant FOXB_01332 oxidized 18:2n-6 to 9S-hydroperoxy-10(E),12(Z)-octadecadienoic acid by hydrogen abstraction and antarafacial insertion of molecular oxygen and sequentially to an allene oxide, 9S(10)-epoxy-10,12(Z)-octadecadienoic acid, as judged from nonenzymatic hydrolysis products (α- and γ-ketols). The enzyme was therefore designated 9S-DOX-AOS. The 9S-DOX activity oxidized C₁₈ and C₂₀ fatty acids of the n-6 and n-3 series to hydroperoxides at the n-9 and n-7 positions, and the n-9 hydroperoxides could be sequentially transformed to allene oxides with only a few exceptions. The AOS activity was stereospecific for 9- and 11-hydroperoxides with S configurations. FOXB_01332 has acidic and alcoholic residues, Glu⁹⁴⁶-Val-Leu-Ser⁹⁴⁹, at positions of crucial Asn and Gln residues (Asn-Xaa-Xaa-Gln) of the AOS and LDS. Site-directed mutagenesis studies revealed that FOXB_01332 and AOS of A. terreus differ in catalytically important residues suggesting that AOS of A. terreus and F. oxysporum belong to different subfamilies. FOXB_01332 is the first linoleate 9-DOX with homology to animal heme peroxidases and the first 9-DOX-AOS fusion protein.

Expression of fusion proteins of Aspergillus terreus reveals a novel allene oxide synthase

Aspergilli oxidize C18 unsaturated fatty acids by dioxygenase-cytochrome P450 fusion proteins to signal molecules involved in reproduction and host-pathogen interactions. Aspergillus terreus expresses linoleate 9R-dioxygenase (9R-DOX) and allene oxide synthase (AOS) activities in membrane fractions. The genome contains five genes (ATEG), which may code for a 9R-DOX-AOS fusion protein. The genes were cloned and expressed, but none of them oxidized 18:2n-6 to 9R-hydroperoxy-10(E),12(Z)-octadecadienoic acid (9R-HPODE). ATEG_02036 transformed 9R-HPODE to an unstable allene oxide, 9(R),10-epoxy-10,12(Z)-octadecadienoic acid. A substitution in the P450 domain (C1073S) abolished AOS activity. The N964V and N964D mutants both showed markedly reduced AOS activity, suggesting that Asn(964) may facilitate homolytic cleavage of the dioxygen bond of 9R-HPODE with formation of compound II in analogy with plant AOS (CYP74) and prostacyclin synthase (CYP8A1). ATEG_03992 was identified as 5,8-linoleate diol synthase (5,8-LDS). Replacement of Asn(878) in 5,8-LDS with leucine (N878L) mainly shifted ferryl oxygen insertion from C-5 toward C-6, but replacements of Gln(881) markedly affected catalysis. The Q881L mutant virtually abolished the diol synthase activity. Replacement of Gln(881) with Asn, Glu, Asp, or Lys residues augmented the homolytic cleavage of 8R-HPODE with formation of 10-hydroxy-8(9)-epoxy-12(Z)-octadecenoic acid (erythro/threo, 1-4:1) and/or shifted ferryl oxygen insertion from C-5 toward C-11. We conclude that homolysis and heterolysis of the dioxygen bond with formation of compound II in AOS and compound I in 5,8-LDS are influenced by Asn and Gln residues, respectively, of the I-helices. AOS of A. terreus appears to have evolved independently of CYP74 but with an analogous reaction mechanism.

Secretion of two novel enzymes, manganese 9S-lipoxygenase and epoxy alcohol synthase, by the rice pathogen Magnaporthe salvinii

The mycelium of the rice stem pathogen, Magnaporthe salvinii, secreted linoleate 9S-lipoxygenase (9S-LOX) and epoxy alcohol synthase (EAS). The EAS rapidly transformed 9S-hydroperoxy-octadeca-10E,12Z-dienoic acid (9S-HPODE) to threo 10 (11)-epoxy-9S-hydroxy-12Z-octadecenoic acid, but other hydroperoxy FAs were poor substrates. 9S-LOX was expressed in Pichia pastoris. Recombinant 9S-LOX oxidized 18:2n-6 directly to 9S-HPODE, the end product, and also to two intermediates, 11S-hydroperoxy-9Z,12Z-octadecenoic acid (11S-HPODE; ∼5%) and 13R-hydroperoxy-9Z,11E-octadecadienoic acid (13R-HPODE; ∼1%). 11S- and 13R-HPODE were isomerized to 9S-HPODE, probably after oxidation to peroxyl radicals, β-fragmentation, and oxygen insertion at C-9. The 18:3n-3 was oxidized at C-9, C-11, and C-13, and to 9,16-dihydroxy-10E,12,14E-octadecatrienoic acid. 9S-LOX contained catalytic manganese (Mn:protein ∼0.2:1; Mn/Fe, 1:0.05), and its sequence could be aligned with 77% identity to 13R-LOX with catalytic manganese lipoxygenase (13R-MnLOX) of the Take-all fungus. The Leu350Met mutant of 9S-LOX shifted oxidation of 18:2n-6 from C-9 to C-13, and the Phe347Leu, Phe347Val, and Phe347Ala mutants of 13R-MnLOX from C-13 to C-9. In conclusion, M. salvinii secretes 9S-LOX with catalytic manganese along with a specific EAS. Alterations in the Sloane determinant of 9S-LOX and 13R-MnLOX with larger and smaller hydrophobic residues interconverted the regiospecific oxidation of 18:2n-6, presumably by altering the substrate position in relation to oxygen insertion.

Novel insights into cyclooxygenases, linoleate diol synthases, and lipoxygenases from deuterium kinetic isotope effects and oxidation of substrate analogs

Cyclooxygenases (COX) and 8R-dioxygenase (8R-DOX) activities of linoleate diol synthases (LDS) are homologous heme-dependent enzymes that oxygenate fatty acids by a tyrosyl radical-mediated hydrogen abstraction and antarafacial insertion of O(2). Soybean lipoxygenase-1 (sLOX-1) contains non-heme iron and oxidizes 18:2n-6 with a large deuterium kinetic isotope effect (D-KIE). The aim of the present work was to obtain further mechanistic insight into the action of these enzymes by using a series of n-6 and n-9 fatty acids and by analysis of D-KIE. COX-1 oxidized C(20) and C(18) fatty acids in the following order of rates: 20:2n-6>20:1n-6>20:3n-9>20:1n-9 and 18:3n-3≥18:2n-6>18:1n-6. 18:2n-6 and its geometrical isomer (9E,12Z)18:2 were both mainly oxygenated at C-9 by COX-1, but the 9Z,12E isomer was mostly oxygenated at C-13. A cis-configured double bond in the n-6 position therefore seems important for substrate positioning. 8R-DOX oxidized (9Z,12E)18:2 at C-8 in analogy with 18:2n-6, but the 9E,12Z isomer was mainly subject to hydrogen abstraction at C-11 and oxygen insertion at C-9 by 8R-DOX of 5,8-LDS. sLOX-1 and 13R-MnLOX oxidized [11S-(2)H]18:2n-6 with similar D-KIE (~53), which implies that the catalytic metals did not alter the D-KIE. Oxygenation of 18:2n-6 by COX-1 and COX-2 took place with a D-KIE of 3-5 as probed by incubations of [11,11-(2)H(2)]- and [11S-(2)H]18:2n-6. In contrast, the more energetically demanding hydrogen abstractions of the allylic carbons of 20:1n-6 by COX-1 and 18:1n-9 by 8R-DOX were both accompanied by large D-KIE (>20).

The fatty acid 8,11-diol synthase of Aspergillus fumigatus is inhibited by imidazole derivatives and unrelated to PpoB

(8R)-Hydroperoxy-(9Z,12Z)-octadecadienoic acid (8-HPODE) is formed by aspergilli as an intermediate in biosynthesis of oxylipins with effects on sporulation. 8-HPODE is transformed by separate diol synthases to (5S,8R)-dihydroxy- and (8R,11S)-dihydroxy-(9Z,12Z)-octadecadienoic acids (5,8- and 8,11-DiHODE). The former is formed by the cytochrome P450 (P450) domain of 5,8-linoleate diol synthase (5,8-LDS or PpoA). Our aim was to characterize the 8,11-diol synthase of Aspergillus fumigatus, which is prominent in many strains. The 8,11-diol synthase was soluble and had a larger molecular size (>100 kDa) than most P450. Miconazole, ketoconazole, and 1-benzylimidazole, classical inhibitors of P450, reduced the biosynthesis of 8,11-DiHODE from 8-HPODE (apparent IC(50) values ~0.8, ~5, and ~0.6 μM, respectively), but did not inhibit the biosynthesis of 5,8-DiHODE. Analysis of hydroperoxides of regioisomeric C(18) and C(20) fatty acids showed that the 8,11-diol synthase was specific for certain hydroperoxides with R configuration. The suprafacial hydrogen abstraction and oxygen insertion at C-11 of 8-HPODE was associated with a small deuterium kinetic isotope effect ((H) k (cat)/(D) k (cat) ~1.5), consistent with P450-catalyzed oxidation. The genome of A. fumigatus contains over 70 P450 sequences. The reaction mechanism, size, and solubility of 8,11-diol synthase pointed to PpoB, a homologue of 5,8-LDS, as a possible candidate of this activity. Gene deletion of ppoB of A. fumigatus strains AF:∆ku80 and J272 did not inhibit biosynthesis of 8,11-DiHODE and recombinant PpoB appeared to lack diol synthase activity. We conclude that 8,11-DiHODE is formed from 8-HPODE by a soluble and substrate-specific 8,11-diol synthase with catalytic characteristics of class III P450.

Linolenate 9R-dioxygenase and allene oxide synthase activities of Lasiodiplodia theobromae

Jasmonic acid (JA) is synthesized from linolenic acid (18:3n-3) by sequential action of 13-lipoxygenase, allene oxide synthase (AOS), and allene oxide cyclase. The fungus Lasiodiplodia theobromae can produce large amounts of JA and was recently reported to form the JA precursor 12-oxophytodienoic acid. The objective of our study was to characterize the fatty acid dioxygenase activities of this fungus. Two strains of L. theobromae with low JA secretion (~0.2 mg/L medium) oxygenated 18:3n-3 to 5,8-dihydroxy-9Z,12Z,15Z-octadecatrienoic acid as well as 9R-hydroperoxy-10E,12Z,15Z-octadecatrienoic acid, which was metabolized by an AOS activity into 9-hydroxy-10-oxo-12Z,15Z-octadecadienoic acid. Analogous conversions were observed with linoleic acid (18:2n-6). Studies using [11S-(2)H]18:2n-6 revealed that the putative 9R-dioxygenase catalyzed stereospecific removal of the 11R hydrogen followed by suprafacial attack of dioxygen at C-9. Mycelia from these strains of L. theobromae contained 18:2n-6 as the major polyunsaturated acid but lacked 18:3n-3. A third strain with a high secretion of JA (~200 mg/L) contained 18:3n-3 as a major fatty acid and produced 5,8-dihydroxy-9Z,12Z,15Z-octadecatrienoic acid from added 18:3n-3. This strain also lacked the JA biosynthetic enzymes present in higher plants.