Mechanisms and stereochemistry in fatty acid metabolism

Hydroxy fatty acids in the surface Earth system

Lipids are ubiquitous and highly abundant in a wide range of organisms and have been found in various types of environmental media. These molecules play a crucial role as organic tracers by providing a chemical perspective on viewing the material world, as well as offering a wealth of information on metabolic activities. Among the diverse lipid compounds, hydroxy fatty acids (HFAs) with one to multiple hydroxyl groups attached to the carbon chain stand out as important biomarkers for different sources of organic matter. HFAs are widespread in nature and are involved in biotransformation and oxidation processes in living organisms. The unique chemical and physical properties attributed to the hydroxyl group make HFAs ideal biomarkers in biomedicine and environmental toxicology, as well as organic geochemistry. The molecular distribution patterns of HFAs can be unique and diagnostic for a given class of organisms, including animals, plants, and microorganisms. Thus, HFAs can act as a valuable proxy for understanding the ecological relationships between different organisms and their environment. Furthermore, HFAs have numerous industrial applications due to their higher reactivity, viscosity, and solvent miscibility. This review paper integrates the latest research on the sources and chemical analyses of HFAs, as well as their applications in industrial/medicinal production and as biomarkers in environmental studies. This review article also provides insights into the biogeochemical cycles of HFAs in the surface Earth system, highlighting the importance of these compounds in understanding the complex interactions between living organisms and the environment.

Beyond Autoxidation and Lipoxygenases: Fatty Acid Oxidation Products in Plant Oils

For decades, research on oxidation of linoleic acid (LA, C18:2 n6) and α-linolenic acid (ALA, C18:3 n3) in plant oils has focused on autoxidatively formed and lipoxygenase-derived 9-hydro(pero)xy- and 13-hydro(pero)xy-LA and -ALA. Here, using a non-targeted approach, we show that other hydroxy fatty acids are more abundant in plant oils. Liquid chromatography-mass spectrometry and gas chromatography-mass spectrometry analyses unveiled highly abundant peaks in flaxseed and rapeseed oils. Using authentic reference standards, seven of the peaks were identified as 9-, 10-, 12-, 13-, and 15-HODE as well as 9- and 13-HOTrE. Additionally, six peaks were characterized based on the retention time, the exact mass of the [M-H]- ion, and its fragment ions as 16-OH-C18:3, 18-OH-C18:3, three isomers of 12-OH-C18:2, and one of 15-OH-C18:2. 16-OH-C18:3 and 18-OH-C18:3 were tentatively identified as 16-OH-ALA and 18-OH-ALA, respectively, based on autoxidation and terminal hydroxylation of ALA using CYP4F2. Investigation of formation pathways suggests that fatty acid desaturase 3 is involved in the formation of the 12-OH-C18:2 isomers, 15-HODE, and its isomer. The dominantly occurring 12-OH-C18:2 isomer was identified as 12R,S-OH-9Z,15Z-octadecadienoic acid (densipolic acid) based on a synthetic standard. The characterized oxylipins occurred in cold-pressed flaxseed and rapeseed oils at concentrations of up to 0.1 g/100 g and thus about sixfold higher than the well-known 9-hydro(pero)xy- and 13-hydro(pero)xy-LA and -ALA. Concentrations in sunflower oil were lower but increased when oil was pressed from preheated seeds. Overall, this study provides fundamental new information about the occurrence of oxidized fatty acids in plant oils, having the potential to characterize their quality and authenticity.

Understanding desaturation/hydroxylation activity of castor stearoyl Δ9-Desaturase through rational mutagenesis

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Rationally designed mutations in the Δ⁹ desaturase promoted hydroxylation activity.

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Proton and electron transfer to the active site is crucial for the Δ⁹D to desaturate

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Detailed analysis of all enzymatic products of the Δ⁹D was carried out

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Insight into the chemo-, and stereoselectivity of non-heme diiron enzymes was obtained

A recently proposed reaction mechanism of soluble Δ⁹ desaturase (Δ⁹D) allowed us to identify auxiliary residues His203, Asp101, Thr206 and Cys222 localized near the di-iron active site that are supposedly involved in the proton transfer (PT) to and from the active site. The PT, along with the electron transfer (ET), seems to be crucial for efficient desaturation. Thus, perturbing the major PT chains is expected to impair the native reaction and (potentially) amplify minor reaction channels, such as the substrate hydroxylation. To verify this hypothesis, we mutated the four residues mentioned above into their counterparts present in a soluble methane monooxygenase (sMMO), and determined the reaction products of mutants. We found that the mutations significantly promote residual monohydroxylation activities on stearoyl-CoA, often at the expense of native desaturation activity. The favored hydroxylation positions are C₉, followed by C₁₀ and C₁₁. Reactions with unsaturated substrate, oleoyl-CoA, yield erythro-9,10-diol, cis-9,10-epoxide and a mixture of allylic alcohols. Additionally, using 9- and 11-hydroxystearoyl-CoA, we showed that the desaturation reaction can proceed only with the hydroxyl group at position C₁₁, whereas the hydroxylation reaction is possible in both cases, i.e. with hydroxyl at position C₉ or C₁₁. Despite the fact that the overall outcome of hydroxylation is rather modest and that it is mostly the desaturation/hydroxylation ratio that is affected, our results broaden understanding of the origin of chemo- and stereoselectivity of the Δ⁹D and provide further insight into the catalytic action of the NHFe₂ enzymes.

Proton-Electron Transfer to the Active Site Is Essential for the Reaction Mechanism of Soluble Δ9-Desaturase

A full understanding of the catalytic action of non-heme iron (NHFe) and non-heme diiron (NHFe₂) enzymes is still beyond the grasp of contemporary computational and experimental techniques. Many of these enzymes exhibit fascinating chemo-, regio-, and stereoselectivity, in spite of employing highly reactive intermediates which are necessary for activations of most stable chemical bonds. Herein, we study in detail one intriguing representative of the NHFe₂ family of enzymes: soluble Δ⁹ desaturase (Δ⁹D), which desaturates rather than performing the thermodynamically favorable hydroxylation of substrate. Its catalytic mechanism has been explored in great detail by using QM(DFT)/MM and multireference wave function methods. Starting from the spectroscopically observed 1,2-μ-peroxo diferric P intermediate, the proton-electron uptake by P is the favored mechanism for catalytic activation, since it allows a significant reduction of the barrier of the initial (and rate-determining) H-atom abstraction from the stearoyl substrate as compared to the "proton-only activated" pathway. Also, we ruled out that a Q-like intermediate (high-valent diamond-core bis-μ-oxo-[Fe^IV]₂ unit) is involved in the reaction mechanism. Our mechanistic picture is consistent with the experimental data available for Δ⁹D and satisfies fairly stringent conditions required by Nature: the chemo-, stereo-, and regioselectivity of the desaturation of stearic acid. Finally, the mechanisms evaluated are placed into a broader context of NHFe₂ chemistry, provided by an amino acid sequence analysis through the families of the NHFe₂ enzymes. Our study thus represents an important contribution toward understanding the catalytic action of the NHFe₂ enzymes and may inspire further work in NHFe₍₂₎ biomimetic chemistry.

2: Final Report on the Safety Assessment of Glyceryl Ricinoleate

Glyceryl Ricinoleate is the monoester of glycerol and ricinoleic acid. Castor oil contains 87–90% Glycerol Ricinoleate. Ricinoleic acid is metabolized by both β-oxidation and α-oxidation.

Acute oral toxicity tests in mice indicated that Glyceryl Ricinoleate has an LD50greater than 25.0 ml/kg and is, at most, mildly irritating to unrinsed rabbit eyes. This ingredient was not a primary skin irritant.

Castor oil was nonmutagenic by the Ames test. Ricinoleic acid was not a carcinogen when tested in mice.

In human single-insult occlusive patch tests, no indication of skin irritation potential was observed in the two products containing 5.6% Glyceryl Ricinoleate.

The available data on Glyceryl Ricinoleate were insufficient to determine whether this ingredient, under each relevant condition of use, was either safe or not safe. The types of data required before a decision can be made include: (1) 28 day chronic dermal toxicity in guinea pigs, and (2) clinical sensitization and photosensitization studies (or an appropriate ultraviolet spectrum instead of the photosensitization data).

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

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

Metabolism of conjugated linoleic acids and 18 : 1 fatty acids by ruminal bacteria: products and mechanisms

Cultures of ruminal bacteria known to metabolize unsaturated fatty acids were grown in medium containing 50 microg ml(-1) of geometric and positional isomers of conjugated linoleic acid (CLA) or 18 : 1 fatty acids and 37.4 % deuterium oxide to investigate the mechanisms responsible for fatty acid metabolism. Butyrivibrio fibrisolvens JW11 converted cis-9,trans-11-18 : 2 and trans-9,trans-11-18 : 2 to trans-11-18 : 1 as the main product, labelled at C-9, and metabolized trans-10,cis-12-18 : 2 to trans-10-18 : 1, labelled at C-13, and smaller amounts of trans-12-18 : 1 and cis-12-18 : 1. Butyrivibrio proteoclasticus P-18 did not grow in the presence of cis-9,trans-11-18 : 2 or trans-10,cis-12-18 : 2, but grew in medium containing trans-9,trans-11-18 : 2, forming 18 : 0. Propionibacterium acnes, a ruminal species that isomerizes linoleic acid to trans-10,cis-12-18 : 2, did not metabolize CLA isomers further. B. fibrisolvens metabolized small amounts of trans-10-18 : 1, trans-11-18 : 1 and cis-9-18 : 1, but the products formed were not detected. B. proteoclasticus, on the other hand, carried out substantial conversion of 18 : 1 substrates to 18 : 0. P. acnes hydrated cis-9-18 : 1 and trans-11-18 : 1 to 10-OH-18 : 0, which was further oxidized to yield 10-O-18 : 0. The deuterium enrichment in the intermediates formed during incubations with 9,11 geometric isomers of CLA was about half that of the products from trans-10,cis-12 CLA and 18 : 1 isomers, suggesting that the reduction of 9,11 geometric isomers CLA by ruminal bacteria occurs via different mechanisms compared with the metabolism of other unsaturated fatty acids.

Investigation of fatty acid elongation and desaturation steps in Fusarium lateritium by quantitative two-dimensional deuterium NMR spectroscopy in chiral oriented media

The origin of hydrogen atoms during fatty acid biosynthesis in Fusarium lateritium has been quantified by isotope tracking close to natural abundance. Methyl linoleate was isolated from F. lateritium grown in natural abundance medium or in medium slightly enriched with labeled water, glucose, or acetate, and the (2)H incorporation was determined by quantitative (2)H-{(1)H} NMR in isotropic and chiral oriented solvents. Thus, the individual ((2)H/(1)H)(i) ratio at each pro-R and pro-S hydrogen position of the CH(2) groups along the chain can be analyzed. These values allow the isotope redistribution coefficients (a(ij)) that characterize the specific source of each hydrogen atom to be related to the nonexchangeable hydrogen atoms in glucose and to the medium water. In turn, these can be related to the stereoselectivity that operates during the introduction or removal of hydrogens along the fatty acid chain. First, at even CH(2) the pro-S hydrogen comes only from water by protonation, whereas the pro-R hydrogen is introduced partly via acetate but principally from water. Second, the nonexchangeable hydrogens of glucose (positions H-6,6 and H-1) are shown to be introduced to the odd CH(2) via the NAD(P)H pool used by both reductases involved in the elongation steps of the fatty acid chain. Third, it is proved that hydrogens removed at sites 9,10 and 12,13 during desaturation by Delta(9)- and Delta(12)-desaturases are pro-R, and that during these desaturation steps alpha-secondary kinetic isotope effects occur at the 9 and 12 positions and not at the 10 and 13 positions.

Regiospecific enzymatic oxygenation of cis-vaccenic acid in the marine phototrophic bacterium Erythrobacter sp. strain MG3

The fatty acid composition of the marine phototrophic bacterium Erythrobacter sp. strain MG3 was analysed. The involvement of an unusual enzymatic peroxidation of the allylic carbon 10 of cis-vaccenic acid in this strain was confirmed. This process, which seems to be a characteristic of some aerobic and anaerobic phototrophic bacteria, appeared to also act on the allylic carbon 10 of octadeca-5,11-dienoic acid. Enzymatic degradation of 10-hydroperoxyoctadec-11(cis)-enoic acid resulting from the peroxidation of cis-vaccenic acid mainly involves reduction to the corresponding hydroxy acid (probably catalysed by peroxygenases) and cleavage to the corresponding oxoacid, which is then biohydrogenated. Abiotic degradation of this hydroperoxide involves allylic rearrangement to 10-hydroperoxyoctadec-11(trans)-enoic and 12-hydroperoxyoctadec-10(trans)-enoic acids and cyclisation to the very unusual 7,10-epoxyoctadec-11(cis)-enoic acid. Several tests carried out at different periods of growth and under different growth conditions allowed to show that the induction of this enzymatic peroxidation process strongly depends on the physiological state of the cells and is enhanced during C-limitation and at low temperatures.

An oleate hydroxylase from the fungus Claviceps purpurea: cloning, functional analysis, and expression in Arabidopsis

Claviceps purpurea, a fungal pathogen responsible for ergot diseases in many agriculturally important cereal crops, produces high levels of ricinoleic acid (12-hydroxyoctadec-cis-9-enoic acid) in its sclerotia. It has been believed for many years that the biosynthesis of this fatty acid in C. purpurea involves a hydration process with linoleic acid as the substrate. Using degenerate polymerase chain reaction, we cloned a gene from the sclerotia encoding an enzyme (CpFAH) that has high sequence similarity to the C. purpurea oleate desaturase, but only low similarity to plant oleate hydroxylases. Functional analysis of CpFAH in yeast (Saccharomyces cerevisiae) indicated it acted predominantly as a hydroxylase, introducing hydroxyl groups at the 12-position of oleic acid and palmitoleic acid. As well, it showed Delta(12) desaturase activities on 16C and 18C monounsaturated fatty acids and, to a much lesser extent, omega(3) desaturase activities on ricinoleic acid. Heterologous expression of CpFAH under the guidance of a seed-specific promoter in Arabidopsis (Arabidopsis thaliana) wild-type and mutant (fad2/fae1) plants resulted in the accumulation of relatively higher levels of hydroxyl fatty acids in seeds. These data indicate that the biosynthesis of ricinoleic acid in C. purpurea is catalyzed by the fungal desaturase-like hydroxylase, and CpFAH, the first Delta(12) oleate hydroxylase of nonplant origin, is a good candidate for the transgenic production of hydroxyl fatty acids in oilseed crops.

Comparative fatty acid profiling ofMucor rouxiiunder different stress conditions

To understand the relationship between fatty acid metabolism and the growth morphology of Mucor rouxii, fatty acid profiling was studied comparatively in cells grown under conditions which included different atmospheric conditions or the addition of phenethyl alcohol (PEA). The significant difference in fatty acid profiles from M. rouxii grown under aerobic or anaerobic conditions was not found to be directly related to morphological growth. Oxygen limitation, which induced the formation of pure multipolar budding yeasts, led to a decrease in long-chain fatty acids-- particularly unsaturated fatty acids-- and an increase in medium-chain saturated fatty acids, a finding which contrasted with the aerobic cultures, including mycelia and PEA-induced bipolar budding cells. High levels of C18 : 1Delta(9) were found in aerobic yeast cultures with additional PEA when compared to that in the aerobically grown mycelia. The identification of unusual fatty acids in Mucor in response to alcoholic and hypoxic stresses - including odd-numbered fatty acids and 7-hydroxy dodecanoic acid (7-OH C12 : 0) in addition to the more common fatty acids - implied that an important role existed for these unusual fatty acids.

Quantitative deuterium isotopic profiling at natural abundance indicates mechanistic differences for delta 12-epoxidase and delta 12-desaturase in Vernonia galamensis

Quantitative (2)H NMR spectroscopy can determine the natural abundance ((2)H/(1)H) ratio at each site of a molecule. In natural products, variation in these values is related to the reaction mechanisms in the pertinent biosynthetic pathway. For the first time, this novel approach has been exploited to probe for mechanistic differences in the introduction of different functionalities into a long-chain fatty acid. Vernolic acid, a major component of the seed oil of Vernonia galamensis, contains both an epoxide and a desaturation. The site-specific isotopic distribution ((2)H/(1)H)(i) has been determined for both vernolic acid and linoleic acid isolated from the same V. galamensis oil. It is found that the ((2)H/(1)H) ratio of vernolic acid shows a pattern along the entire length of the chain, consistent with linoleic acid being its immediate precursor. Notably, the C13 relates to the C13 of linoleic acid but not to the C13 of oleic acid. Furthermore, the C12 and C13 positions in vernolic acid are less depleted, consistent with a change in hybridization state from sp(2) to sp(3). However, the C11 position shows a marked relative enrichment in the vernolic acid, implying that it plays a role in the epoxidase but not the desaturase mechanism. Thus, although it can be concluded that the catalytic mechanisms for the epoxidase and desaturase activities are similar, marked differences in the residual ((2)H/(1)H) patterns indicate that the reaction mechanisms are not identical.

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.

CLA isomers in milk fat from cows fed diets with high levels of unsaturated fatty acids

The concentrations of CLA isomers were determined by Ag+ -HPLC in the milk fat of cows fed a control diet consisting of hay ad libitum and 15 kg of fodder beets or this diet supplemented with oilseeds containing either high levels of oleic acid (rapeseed), linoleic acid (sunflower seed), or alphalinolenic acid (linseed). Highly significant (P < or = 0.001) correlations were found between the daily intakes of oleic acid and the concentration of the CLA isomer trans-7,cis-9 in milk fat; of linoleic acid and the CLA isomers trans-10,trans-12, trans-9,trans-11, trans-8,trans-10, trans-7,trans-9, trans-10,cis-12, cis-9,trans-11, trans-8,cis-10, and trans-7,cis-9; and of alpha-linolenic acid and the CLA isomers trans-12,trans-14, trans-11 ,trans-13, cis,trans/trans,cis-12,14, trans-11 ,cis-13, and cis-11 ,trans-13. CLA concentrations were also determined in the milk fat of cows grazing in the lowlands (600-650 m), the mountains (900-1210 m), and the highlands (1275-2120 m). The concentrations of many isomers were highest in milk fat from the highlands, but only three CLA isomers (cis-9,trans-11, trans-11 ,cis-13, and trans-8,cis-10) showed a nearly linear increase with elevation. Therefore, these three CLA isomers, and particularly the CLA isomer trans- 11,cis-13, the second-most important CLA in milk fat from cows grazing at the three altitudes, could be useful indicators of milk products of Alpine origin.

Deuterium NMR used to indicate a common mechanism for the biosynthesis of ricinoleic acid by Ricinus communis and Claviceps purpurea

Previous studies have shown that ricinoleic acid from castor bean oil of Ricinus communis is synthesized by the direct hydroxyl substitution of oleate, while it has been proposed that ricinoleate is formed by hydration of linoleate in the ergot fungus Claviceps purpurea. The mechanism of the enzymes specific to ricinoleate synthesis has not yet been established, but hydroxylation and desaturation of fatty acids in plants apparently involve closely related mechanisms. As mechanistic differences in the enzymes involved in the biosynthesis of natural products can lead to different isotopic distributions in the product, we could expect ricinoleate isolated from castor or ergot oil to show distinct (2)H distribution patterns. To obtain information concerning the substrate and isotope effects that occur during the biosynthesis of ricinoleate, the site-specific natural deuterium distributions in methyl ricinoleate isolated from castor oil and in methyl ricinoleate and methyl linoleate isolated from ergot oils have been measured by quantitative (2)H NMR. First, the deuterium profiles for methyl ricinoleate from the plant and fungus are equivalent. Second, the deuterium profile for methyl linoleate from ergot is incompatible with this chemical species being the precursor of methyl ricinoleate. Hence, it is apparent that 12-hydroxylation in C. purpurea is consistent with the biosynthetic mechanisms proposed for R. communis and is compatible with the general fundamental mechanistic similarities between hydroxylation and desaturation previously proposed for plant fatty acid biosynthesis.

Rapid-mix and chemical quench studies of ferredoxin-reduced stearoyl-acyl carrier protein desaturase

Stearoyl-ACP Delta9 desaturase (Delta9D) catalyzes the NADPH- and O(2)-dependent insertion of a cis double bond between the C9 and C10 positions of stearoyl-ACP (18:0-ACP) to produce oleoyl-ACP (18:1-ACP). This work revealed the ability of reduced [2Fe-2S] ferredoxin (Fd) to act as a catalytically competent electron donor during the rapid conversion of 18:0-ACP into 18:1-ACP. Experiments on the order of addition for substrate and reduced Fd showed high conversion of 18:0-ACP to 18:1-ACP (approximately 95% per Delta9D active site in a single turnover) when 18:0-ACP was added prior to reduced Fd. Reactions of the prereduced enzyme-substrate complex with O(2) and the oxidized enzyme-substrate complex with reduced Fd were studied by rapid-mix and chemical quench methods. For reaction of the prereduced enzyme-substrate complex, an exponential burst phase (k(burst) = 95 s(-1)) of product formation accounted for approximately 90% of the turnover expected for one subunit in the dimeric protein. This rapid phase was followed by a slower phase (k(linear) = 4.0 s(-1)) of product formation corresponding to the turnover expected from the second subunit. For reaction of the oxidized enzyme-substrate complex with excess reduced Fd, a slower, linear rate (k(obsd) = 3.4 s(-1)) of product formation was observed over approximately 1.5 turnovers per Delta9D active site potentially corresponding to a third phase of reaction. An analysis of the deuterium isotope effect on the two rapid-mix reaction sequences revealed only a modest effect on k(burst) ((D)k(burst) approximately 1.5) and k(linear) (D)k(linear) approximately 1.4), indicating C-H bond cleavage does not contribute significantly to the rate-limiting steps of pre-steady-state catalysis. These results were used to assemble and evaluate a minimal kinetic model for Delta9D catalysis.

Characterization of 18:1 and 18:2 isomers produced during microbial biohydrogenation of unsaturated fatty acids from canola and soya bean oil in the rumen of lactating cows

Ruminal production of biohydrogenation intermediates in response to unsaturated oils was assessed using 24 Jersey cows fed a control diet or the control diet supplemented at 35 g/kg dry matter (DM) with canola, soya bean, or a mixture of equal amounts of canola plus soya bean oil for 4-weeks. Total fatty acid content averaged 63 or 35 g/kg DM for oil-supplemented diets or control. Oleic acid accounted for 6, 29, 21 or 12 g/kg DM in the control, canola, mixture, or soya bean oil diet, respectively. Linoleic acid averaged 17, 19, 26, or 33 g/kg DM and linolenic acid 5, 5, 6 or 8 g/kg DM for control, canola, mixture, or soya bean oil. Concentrations of cis12-, trans11-, trans13+14, and trans15-18:1 were 0.81, 2.99, 2.24, and 0.73 mg/g rumen fluid, respectively, in response to soya bean oil and were 126, 90, 45, and 38% greater compared with other diets. Trans11cis15-, cis9trans11- and cis9 cis11-18:2 also were greater when soya bean oil (0.30, 0.34 and 0.01 mg/g, respectively) was fed compared with other treatments (0.12, 0.21 and 0.004 mg/g, respectively). Feeding canola oil resulted in greater concentrations of trans4-, trans5-, trans6+7+8-, trans9- and trans10-18:1 (0.20, 0.25, 0.87, 0.39 and 0.70 mg/g, respectively) compared with other diets (0.09, 0.15, 0.36, 0.20 and 0.46 mg/g, respectively). Trans10cis12-18:2 concentration did not differ as a result of diet and averaged 0.002 mg/g rumen contents. The pattern of 18:1 and 18:2 isomers formed during ruminal biohydrogenation depends greatly on dietary profile of unsaturated fatty acids.

Regiospecific enzymatic oxygenation of cis-vaccenic acid during aerobic senescence of the halophilic purple sulfur bacterium Thiohalocapsa halophila

A regiospecific oxygenation of the allylic carbon 10 of cis-vaccenic acid has been observed in senescent cells of the halophilic purple sulfur bacterium Thiohalocapsa halophila incubated under aerobic conditions in darkness. The results obtained strongly suggest that these enzymatic processes involve the initial dioxygenase-mediated formation of 1 O-hydroperoxyoctadec-cis-11-enoic acid, which is not accumulated in the cells of T. halophila owing to its high cytotoxic properties. Deuterium labeling and GC-MS analyses enabled us to demonstrate that subsequent enzymatic conversions of this allylic hydroperoxide involved reduction, cleavage, isomerization, and saturation reactions. Some of the specific oxidation products thus formed could constitute potential T. halophila biomarkers.

Stereospecificity of an enzymatic monoene 1,4-dehydrogenation reaction: conversion of (Z)-11-tetradecenoic acid into (E,E)-10,12-tetradecadienoic acid

In this article, we report the first stereochemical study of an enzymatic 1,4-dehydrogenation reaction, namely, the transformation of (Z)-11-tetradecenoic acid into (E,E)-10,12-tetradecadienoic acid, involved in the sex pheromone biosynthesis of the moth Spodoptera littoralis. The investigation was carried out using the labeled substrates (R)-[10-(2)H]- and (S)-[10-(2)H]-tridecanoic acids ((R)-2 and (S)-2, respectively) and (R)-[2,2,3,3,13-(2)H(5)]- and (S)-[2,2,3,3,13-(2)H(5)]-tetradecanoic acids ((R)-1 and (S)-1, respectively). Probes (R)-2 and (S)-2 were prepared as described in a previous article.(1) The synthesis of the pentadeuterated chiral substrates (R)-1 and (S)-1 was accomplished by kinetic resolution of the racemic 12-tridecyn-2-ol (6) with immobilized porcine pancreatic lipase. The enantiomerically pure alcohols (R)-6 and (S)-6 were transformed into the final acids (S)-1 and (R)-1, respectively, by a sequence of well-established reactions. The analyses of methanolyzed lipidic extracts from glands incubated separatedly with each individual probe showed that in the transformation of (Z)-11-tetradecenoic acid into (E,E)-10,12-tetradecadienoic acid, both pro-(R) hydrogen atoms at C-10 and C-13 are removed from the substrate. This is the first example reported of a desaturase with pro-(R)/pro-(R) stereospecificities that gives rise to (E)-double bonds. A mechanistic explanation for the stereochemical outcome of this reaction is advanced.

Stereospecificity of the (Z)-9 desaturase that converts (E)-11-tetradecenoic acid into (Z,E)-9,11-tetradecadienoic acid in the biosynthesis of Spodoptera littoralis sex pheromone

Moth pheromone glands contain desaturases that catalyze the formation of conjugated dienoic fatty acids. In this article we present the first stereochemical study on one of these enzymes, namely the Delta(9) desaturase of (E)-11-tetradecenoic acid, using the moth Spodoptera littoralis as a biological model and enantiopure deuterated probes derived from tridecanoic acid. Gas chromatography coupled to mass spectrometry analysis of methanolyzed lipidic extracts from glands incubated with each individual probe showed that in the transformation of (E)-11-tetradecenoic acid into (Z,E)-9,11-tetradecadienoic acid both pro-(R) hydrogen atoms at C9 and C10 are removed from the substrate.

Glyceryl-ether monooxygenase [EC 1.14.16.5]. A microsomal enzyme of ether lipid metabolism

The history, biological, and medical aspects of glyceryl ethers, as well as their chemical syntheses, biosynthesis, and their chemical and physical properties are briefly reviewed as background information for appreciating the importance of the enzyme glyceryl-ether monooxygenase, and for embarking on new studies of this enzyme. The occurrence, isolation and general properties of the microsomal, membrane-bound, glyceryl-ether monooxygenase from rat liver are described. Radiometric, nonradiometric, and coupled and direct spectrophotometric assays for this enzyme are detailed. The effects of detergents on the kinetics of this enzyme are described together with the stoichiometry and the effects of inhibitors. The structure-activity relationships of pterin cofactors and of ether lipid substrates, including their stereospecificities, have been summarized from enzyme kinetic data which are also tabulated. The mechanism of enzymic hydroxylation of glyceryl ethers and a model for the active site of glyceryl-ether monooxygenase are proposed from these apparent kinetic data. Notes on useful future studies of this monooxygenase have been made.

Oleate 6-hydroxylase activity in the membrane fraction from an oleaginous fungus, Mortierella ramanniana var. angulispora

Conversion of [1-14C]oleoyl-CoA (OA-CoA) in subcellular fractions of an oleaginous fungus, Mortierella ramanniana var. angulispora (IFO 8187) was investigated. The membrane fraction actively catalyzed the transformation of OA-CoA into one metabolite, which was identified as 6-hydroxy oleic acid (6-HOOA) by GC-MS analysis. The enzyme activity to form 6-HOOA was dependent on OA-CoA concentrations and increased linearly with incubation time and protein concentrations under appropriate experimental conditions. Since the enzyme activity required molecular oxygen and reduced pyridine nucleotides (NADH and NADPH), it was most probable that the formation of 6-HOOA was ascribed to direct hydroxylation to the C6 position, oleate 6-hydroxylase. 6-HOOA was formed from free oleic acid (OA) and OA-NH4 salt as substrate as well as OA-CoA, but not from OA covalently bound to phosphatidylcholine or triacylglycerol. Since CoA stimulated the hydroxylation of free OA and OA-NH4 salt but not that of OA-CoA, OA may be converted to OA-CoA and then hydroxylated. The enzyme activity was distinctly reduced by addition of lysophosphatidic acid, phosphatidic acid and dithiothreitol. Since hydroxy fatty acids are very minor components in this fungus, it may raise the possibility that the 6-hydroxylase activity works for other functions such as comprising one step for desaturation as well as forming the hydroxy fatty acids.

Resonance Raman evidence for an Fe-O-Fe center in stearoyl-ACP desaturase. Primary sequence identity with other diiron-oxo proteins

The stearoyl-ACP delta 9 desaturase from plants is a new example of a growing number of proteins that contain oxo- or hydroxo-bridged diiron clusters. On the basis of differences in primary sequence motifs providing the cluster ligands and upon structural differences elucidated by X-ray crystallography, we now propose that the presently known, soluble diiron-oxo proteins can be grouped into two classes, I and II. Class I contains hemerythrin, myohemerythrin, and, possibly, purple acid phosphatase. Class II contains ribonucleotide reductases, bacterial hydrocarbon hydroxylases (methane monooxygenase, toluene-4-monooxygenase, and phenol hydroxylase), rubrerythrin, and stearoyl-ACP desaturases. Through the use of resonance Raman spectroscopy, we have detected symmetric (vs = 519 cm-1) and asymmetric (vas = 747 cm-1) vibrational modes in the castor stearoyl-ACP delta 9 desaturase, which are typical of oxo-bridged diiron clusters. These frequencies shift by -18 and -34 cm-1, respectively, in H218O, proving that the bridging ligand is readily exchangeable with solvent (t1/2 = 7 min). Calculation of an approximately 123 degrees Fe-O-Fe angle from the position of vs and vas and from the 18O-dependent shift in these frequencies suggests that the diiron-oxo cluster in the desaturase is triply bridged in the diferric state. In the diferrous state, the two iron sites of the cluster are structurally inequivalent, as shown by differential temperature dependence of the Mössbauer quadrupole splittings. For the class II diiron-oxo proteins, primary sequence alignments reveal conserved amino acid residues which act as iron cluster ligands, participate in a hydrogen-bonding network, and are potentially involved in O2 binding and activation. Based on this conservation, a structural model for the stearoyl-ACP delta 9 desaturase active site is proposed that has strong similarity to both ribonucleotide reductase and methane monooxygenase. However, after single turnover of the diferous state with 18O2, 18O is not detected in the oxo bridge of the castor desaturase. This is in contrast to the outcome observed for ribonucleotide reductase, suggesting the desaturase and ribonucleotide reductase differ in certain aspects of their respective O2-activation reactions.

Eight histidine residues are catalytically essential in a membrane-associated iron enzyme, stearoyl-CoA desaturase, and are conserved in alkane hydroxylase and xylene monooxygenase

The eukaryotic fatty acid desaturases are iron-containing enzymes that catalyze the NAD-(P)H- and O2-dependent introduction of double bonds into methylene-interrupted fatty acyl chains. Examination of deduced amino acid sequences for the membrane desaturases from mammals, fungi, insects, higher plants, and cyanobacteria has revealed three regions of conserved primary sequence containing HX(3 or 4)H,HX(2 or 3)HH, and HX(2 or 3)HH. This motif is also present in the bacterial membrane enzymes alkane hydroxylase (omega-hydroxylase) and xylene monooxygenase. Hydropathy analyses indicate that these enzymes contain up to three long hydrophobic domains which would be long enough to span the membrane bilayer twice. The conserved His-containing regions have a consistent positioning with respect to these potential membrane spanning domains. Taken together, these observations suggest that the membrane fatty acid desaturases and hydrocarbon hydroxylases have a related protein fold, possibly arising from a common ancestral origin. In order to examine the functional role of these conserved His residues, we have made use of the ability of the rat delta 9 desaturase gene to complement a yeast strain deficient in the delta 9 desaturase gene function (ole1). By site-directed mutagenesis, eight conserved His residues in the rat delta 9 desaturase were individually converted to Ala. Each His-->Ala mutation failed to complement the yeast ole1 mutant. In contrast, mutation of three nonconserved flanking His residues or a partially conserved Arg residue within the conserved motif to Ala allowed for complementation of the ole1 phenotype.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of linoleic acid on desaturation and uptake of deuterium-labeled palmitic and stearic acids in humans

Objectives of this study were to investigate the desaturation of stearic acid (18:0) and palmitic acid (16:0), to determine if differences in their metabolism provide a reasonable explantation for differences in their effect on serum cholesterol levels, and to investigate the affect of linoleic acid on delta 9-desaturase products in man. Deuterium-labeled 16:0 and 18:0 were used to follow the metabolism of these fatty acids in young adult male subjects that were pre-fed diets containing two different levels of linoleic acid. Results indicate that absorption of 16:0 and 18:0 was similar when all components of the mixture used to formulate the deuterated fat mixture were kept above the melting point of tristearin. The percent of 18:0 desaturated to 9c-18:1 was higher than the percent of 16:0 desaturated to 9c-16:1 (9.2% vs. 3.9%). The subject-to-subject variability suggests that differences in ability to desaturate saturated fatty acids may be related to the variability observed in response of serum cholesterol levels to dietary saturated fatty acids. Data for the distribution of 16:0 and 18:0 between triacylglycerol and phosphatidylcholine (PC) was markedly different. Based on PC data, phospholipid acyltransferase selectivity was about 2-fold higher for 18:0 than for 16:0. A 2-fold difference in the linoleic acid content of the pre-fed diets had little influence on desaturation or distribution of 16:0 and 18:0 between plasma lipid classes. A deuterium isotope effect was estimated to reduce delta 9-desaturase enzyme activity by 30-50%.

The hydrogenation of some cis- and trans-octadecenoic acids to stearic acid by a rumen Fusocillus sp

The hydrogenation of a range of double-bond positional and configurational octadecenoic acid isomers (cis (delta 2 and delta 4 to delta 13) and trans (delta 2 and delta 5 to delta 13] to stearic acid by a rumen Fusocillus sp. were examined. The cis and trans delta 5 to delta 13 isomers were all hydrogenated to some extent by late-log-phase cultures added to suspensions of individual isomers and incubated for a further 3 h. Of the cis-isomers, delta 5 to delta 11 (79-73% conversion to stearic acid) were the preferred substrates. delta 12-cis- (30%) and delta 13-cis-isomers (5%) were poorly hydrogenated. Of the trans-isomers, delta 8, delta 9 and delta 10 were 45% converted to stearic acid, the other isomers were poorly hydrogenated. These results are in agreement with less extensive studies using sheep rumen micro-organisms. When cultures were grown from small inocula in media containing individual isomers more extensive hydrogenation was found than with late-log-phase cultures. At 24 h, cis delta 2, delta 4 and delta 5 gave the highest conversions to stearic acid (90%) followed by the cis delta 6 to delta 12 and trans delta 8 to delta 10 isomers (approximately 75%), although at 6 and 12 h delta 9-trans gave higher yields of stearic acid than delta 9-cis, probably because the growth of the cis cultures showed a longer log-phase.

Lipid metabolism in fungi

So far, reviews that have appeared on fungal lipids present data mainly on the lipid composition of these organisms and the influence of lipids on their physiology. These reviews provide little information about the enzymes of lipid metabolism in these organisms and it is assumed, by most workers, that lipid synthesis in all fungi takes place as in Saccharomyces cervesiae, the only fungus in which the complete pathways of phospholipid biosynthesis have been worked out. During the last few years, literature has accumulated on lipid metabolic enzymes of other fungi, as investigators became increasingly interested in this area of research. The present review, after an introduction, will be divided into different sections and each section will deal, comparatively, with various aspects of fungal lipid metabolism and physiology. This review will, therefore, bring out the differences or similarities of lipid metabolism in diverse fungal species.

Fatty acid alpha-hydroxylation and its relation to myelination

alpha-Hydroxylation is an enzymatic reaction by which long-chain fatty acids are converted to their alpha-hydroxy derivatives. This reaction, in animals, can be detected only in developing brain and is the rate-determining step in the synthesis of hydroxycerebroside, which is an indispensable and abundant myelin lipid. In addition to a particulate fraction from brain, two cytoplasmic factors, one heat-stable and the other heat-labile, are required for alpha-hydroxylation. During the past eight years we have been investigating alpha-hydroxylation. Our progress is summarized and discussed here.