Enzyme-catalyzed and enzyme-triggered pathways in dioxygenation of 1-monolinoleoyl-rac-glycerol by potato tuber lipoxygenase

Dissecting lipid metabolism in meibomian glands of humans and mice: An integrative study reveals a network of metabolic reactions not duplicated in other tissues

Lipids comprise the bulk of the meibomian gland secretion (meibum) which is produced by meibocytes. Complex arrays of lipogenic reactions in meibomian glands, which we collectively call meibogenesis, have not been explored on a molecular level yet. Our goals were to elucidate the possible biosynthetic pathways that underlie the generation of meibum, reveal similarities in, and differences between, lipid metabolism in meibomian glands and other organs and tissues, and integrate meibomian gland studies into the field of general metabolomics. Specifically, we have conducted detailed analyses of human and mouse specimens using genomic, immunohistochemical, and lipidomic approaches. Among equally highly expressed genes found in meibomian glands of both species were those related to fatty acid elongation, branching, desaturation, esterification, reduction of fatty acids to alcohols, and cholesterol biosynthesis. Importantly, corresponding lipid products were detected in meibum of both species using lipidomic approaches. For the first time, a cohesive, unifying biosynthetic scheme that connects genomic, lipidomic, and immunohistochemical observations is outlined and discussed.

Lipoxygenases mediate the effect of essential fatty acid in skin barrier formation: a proposed role in releasing omega-hydroxyceramide for construction of the corneocyte lipid envelope

A barrier to water loss is vital to maintaining life on dry land. Formation of the mammalian skin barrier requires both the essential fatty acid linoleate and the two lipoxygenases 12R-lipoxygenase (12R-LOX) and epidermal lipoxygenase-3 (eLOX3), although their roles are poorly understood. Linoleate occurs in O-linoleoyl-ω-hydroxyceramide, which, after hydrolysis of the linoleate moiety, is covalently attached to protein via the free ω-hydroxyl of the ceramide, forming the corneocyte lipid envelope, a scaffold between lipid and protein that helps seal the barrier. Here we show using HPLC-UV, LC-MS, GC-MS, and (1)H NMR that O-linoleoyl-ω-hydroxyceramide is oxygenated in a regio- and stereospecific fashion by the consecutive actions of 12R-LOX and eLOX3 and that these products occur naturally in pig and mouse epidermis. 12R-LOX forms 9R-hydroperoxy-linoleoyl-ω-hydroxyceramide, further converted by eLOX3 to specific epoxyalcohol (9R,10R-trans-epoxy-11E-13R-hydroxy) and 9-keto-10E,12Z esters of the ceramide; an epoxy-ketone derivative (9R,10R-trans-epoxy-11E-13-keto) is the most prominent oxidized ceramide in mouse skin. These products are absent in 12R-LOX-deficient mice, which crucially display a near total absence of protein-bound ω-hydroxyceramides and of the corneocyte lipid envelope and die shortly after birth from transepidermal water loss. We conclude that oxygenation of O-linoleoyl-ω-hydroxyceramide is required to facilitate the ester hydrolysis and allow bonding of the ω-hydroxyceramide to protein, providing a coherent explanation for the roles of multiple components in epidermal barrier function. Our study uncovers a hitherto unknown biochemical pathway in which the enzymic oxygenation of ceramides is involved in building a crucial structure of the epidermal barrier.

Lipidomics of human Meibomian gland secretions: Chemistry, biophysics, and physiological role of Meibomian lipids

Human Meibomian gland secretions (MGS) are a complex mixture of diverse lipids that are produced by Meibomian glands that are located in the upper and the lower eyelids. During blinking, MGS are excreted onto the ocular surface, spread and mix with aqueous tears that are produced by lachrymal glands, and form an outermost part of an ocular structure called "the tear film" (TF). The main physiological role of TF is to protect delicate ocular structures (such as cornea and conjunctiva) from desiccating. Lipids that are produced by Meibomian glands are believed to "seal" the aqueous portion of TF by creating a hydrophobic barrier and, thus, retard evaporation of water from the ocular surface, which enhances the protective properties of TF. As lipids of MGS are interacting with underlying aqueous sublayer of TF, the chemical composition of MGS is critical for maintaining the overall stability of TF. There is a consensus that a small, but important part of Meibomian lipids, namely polar, or amphiphilic lipids, is of especial importance as it forms an intermediate layer between the aqueous layer of TF and its upper (and much thicker) lipid layer formed mostly of very nonpolar lipids, such as wax esters and cholesteryl esters. The purpose of this review is to summarize the current knowledge on the lipidomics of human MGS, including the discussions of the most effective modern analytical techniques, chemical composition of MGS, biophysical properties of Meibomian lipid films, and their relevance for the physiology of TF. Previously published results obtained in numerous laboratories, as well as novel data generated in the author's laboratory, are discussed. It is concluded that despite a substantial progress in the area of Meibomian glands lipidomics, there are large areas of uncertainty that need to be addressed in future experiments.

Dioxygenase activity of epidermal lipoxygenase-3 unveiled: typical and atypical features of its catalytic activity with natural and synthetic polyunsaturated fatty acids

Epidermal lipoxygenase-3 (eLOX3) exhibits hydroperoxide isomerase activity implicated in epidermal barrier formation, but its potential dioxygenase activity has remained elusive. We identified herein a synthetic fatty acid, 9E,11Z,14Z-20:3ω6, that was oxygenated by eLOX3 specifically to the 9S-hydroperoxide. Reaction showed a pronounced lag phase, which suggested that eLOX3 is deficient in its activation step. Indeed, we found that high concentrations of hydroperoxide activator (e.g. 65 μM) overcame a prolonged lag phase (>1 h) and unveiled a dioxygenase activity with arachidonic acid; the main products were the 5-, 9-, and 7-hydroperoxyeicosatetraenoic acids (HPETEs). These were R/S mixtures (ranging from ∼50:50 to 73:27), and as the bis-allylic 7-HPETE can be formed only inside the enzyme active site, the results indicate there is oxygen availability along either face of the reacting fatty acid radical. That the active site oxygen supply is limited is implied from the need for continuous re-activation, as carbon radical leakage leaves the enzyme in the unactivated ferrous state. An Ala-to-Gly mutation, known to affect the positioning of O(2) in the active site of other lipoxygenase enzymes, led to more readily activated reaction and a significant increase in the 9R- over the 5-HPETE. Activation and cycling of the ferric enzyme are thus promoted using the 9E,11Z,14Z-20:3ω6 substrate, by continuous hydroperoxide activation, or by the Ala-to-Gly mutation. We suggest that eLOX3 represents one end of a spectrum among lipoxygenases where activation is inefficient, favoring hydroperoxide isomerase cycling, with the opposite end represented by readily activated enzymes in which dioxygenase activity is prominent.

Recombinant maize 9-lipoxygenase: expression, purification, and properties

Expression of maize 9-lipoxygenase was performed and optimized in Escherichia coli Rosetta(DE3)pLysS. The purity of recombinant protein obtained during Q-Sepharose and Octyl-Sepharose chromatographies in an LP system at 4 degrees C was >95%. Maximum activity of the lipoxygenase reaction was observed at pH 7.5. Enzyme stability was studied at pH 4.5 to 9.5 and in the presence of different compounds: phenylmethanesulfonyl fluoride, beta-mercaptoethanol, ammonium sulfate, and glycerol. HPLC and GC-MS analysis showed that enzyme produced 99% 9S-hydroperoxide from linoleic acid. 13-Hydroperoxide (less than 1%) consisted of S- and R-enantiomers in ratio 2 : 3.

The Meibomian puzzle: combining pieces together

The purpose of this review was to summarize the available information on lipidomic analysis of human meibum and tear film, and critically evaluate the pertinent past and present analytical procedures and results obtained in various laboratories. Human meibum was shown to be a very complex mixture of lipids of various classes. For decades, their exact structures have remained elusive. Because of the limitations of the then-current techniques, most of the complex lipids that constitute meibum could not be analyzed as whole molecules and required prior hydrolysis and/or transesterification of the entire lipid pool. These procedures effectively made it very difficult, and often impossible, to reconstruct the complete structures of the original intact compounds, which prompted us to call this The Meibomian Puzzle. Modern techniques such as high-performance liquid chromatography in combination with mass spectrometry help in solving this puzzle by allowing a researcher to detect and analyze intact molecules of complex lipid compounds, even if present in extremely low concentrations. This current de-facto standard procedure in lipidomic analysis of natural lipids and their mixtures is compared with other experimental techniques such as nuclear magnetic resonance spectroscopy, infrared spectroscopy, gas chromatography, and thin layer chromatography, among the others. The results obtained by older techniques, and their limitations and deficiencies are discussed. It appears that some of the earlier findings did not withstand a scrupulous re-evaluation and need to be modified and/or corrected. The most intriguing development is the virtual absence in meibum of typical phospholipids - an important group of amphiphilic compounds whose role in the human tear film was thought to be to stabilize the entire tear film structure. Instead, another group of previously unidentified compounds, very long chain (O-acyl)-omega-hydroxy fatty acids, appears to be a stabilizing factor which might be related to tear film stability and deterioration. Thus, these compounds may become an important target in biochemistry and (patho)physiology of ocular surface and dry eye research.

The 1.85 A structure of an 8R-lipoxygenase suggests a general model for lipoxygenase product specificity

Lipoxygenases (LOX) play pivotal roles in the biosynthesis of leukotrienes and other biologically active eicosanoids derived from arachidonic acid. A mechanistic understanding of substrate recognition, when lipoxygenases that recognize the same substrate generate different products, can be used to help guide the design of enzyme-specific inhibitors. We report here the 1.85 A resolution structure of an 8R-lipoxygenase from Plexaura homomalla, an enzyme with a sequence approximately 40% identical to that of human 5-LOX. The structure reveals a U-shaped channel, defined by invariant amino acids, that would allow substrate access to the catalytic iron. We demonstrate that mutations within the channel significantly impact enzyme activity and propose a novel model for substrate binding potentially applicable to other members of this enzyme family.

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.

Human tear film and meibum. Very long chain wax esters and (O-acyl)-omega-hydroxy fatty acids of meibum

Human meibum was targetly analyzed for the presence of intact wax esters (WEs) and related compounds by means of reverse-phase HPLC in combination with ion trap mass spectrometry. The major detected WEs were based on C(18:n) (n = 1-4) unsaturated FAs ranking in the following order of abundance: C(18:1)>C(18:2)>C(18:3)>C(18:4). The major fatty alcohols (FAls) found in WE were of saturated nature and varied from C(18:0) to C(28:0). The three most abundant species were C(18:1)-FA esters of C(24:0), C(25:0), and C(26:0)-FAl. Typically, a major compound based on C(18:1)-FA and a saturated FAl was accompanied by a few related compounds based on a C(18:2), C(18:3), and C(18:4)-FA. Contrary to previous reports, no epoxy-WEs or epoxy-FAs were detected in fresh and 1-year-old meibum samples. More than 20 (O-acyl)-omega-hydroxy-FAs (OAHFAs) were observed. The main detected OAHFAs were based on very long-chain omega-hydroxy-FA (C(30:1), C(32:1), and C(34:1)) acylated through their omega-hydroxyls by a C(18:1)-FA. Due to their amphiphilic anionogenic nature, OAHFAs may be responsible for stabilization of the tear film lipid layer by creating an interface between the vast pool of strictly nonpolar lipids of meibum (WEs, cholesteryl esters, etc.) and the aqueous subphase beneath it, a role previously attributed to phospholipids.

Investigation of substrate binding and product stereochemistry issues in two linoleate 9-lipoxygenases

Herein we characterize the Arabidopsis thaliana AtLOX1 and tomato (Solanum lycopersicum) LOXA proteins as linoleate 9S-lipoxygenases (9-LOX), and use the enzymes to test a model that predicts a relationship between substrate binding orientation and product stereochemistry. The cDNAs were heterologously expressed in E. coli and the proteins partially purified by nickel affinity chromatography using a N-terminal (His)(6)-tag. Both enzymes oxygenated linoleic acid almost exclusively to the 9S-hydroperoxide with turnover numbers of 300-400/s. AtLOX1 showed a broad range of activity over the range pH 5-9 (optimal at pH 6); tomato LOXA also showed optimal activity around pH 5-7 dropping off more sharply at pH 9. Site-directed mutagenesis of a conserved active site Ala (Ala562 in AtLOX1, Ala 564 in tomato LOXA, and typically conserved as Ala in S-LOX and Gly in R-LOX), revealed that substitution with Gly led to the production of a mixture of 9S- and 13R-hydroperoxyoctadecadienoic acids from linoleic acid. To follow up on earlier reports of 9-LOX metabolism of anandamide (van Zadelhoff et al. Biochem. Biophys. Res. Commun. 248:33-38, 1998), we also tested this substrate with the mutants, which produced predictable shifts in product profile, including a shift from the prominent 11S-hydroperoxy derivative of wild-type to include the 15R-hydroperoxide. These results conform to a model that predicts a head-first substrate binding orientation for 9S-LOX. We also found that linoleoyl-phosphatidylcholine is not a 9S-LOX substrate, which is consistent with this conclusion.

Regulation of lipoxygenase activity by polyunsaturated lysophosphatidylcholines or their oxygenation derivatives

Lysophosphatidylcholines (lysoPCs) have been known to play a role as lipid mediators in various cellular responses. In this study, we examined whether lysoPC containing linoleoyl, arachidonoyl, or docosahexaenoyl groups or their peroxy derivatives affect lipoxygenase (LOX)-catalyzed oxygenation of native substrates. First, arachidonoyl lysoPC and docosahexaenoyl lysoPC were found to inhibit potato 5-LOX-catalyzed oxygenation of linoleic acid (LA) in a noncompetitive type with Ki values of 0.38 and 1.90 microM, respectively. Likewise, arachidonoyl lysoPC and docosahexaenoyl lysoPC also inhibited 5-LOX activity from rat basophilic leukemia cells-2H3 (RBL-2H3) with IC50 values (50% inhibitory concentration) of 18.5 +/- 3.06 and 30.6 +/- 1.06 microM, respectively. Additionally, arachidonoyl lysoPC and docosahexaenoyl lysoPC also inhibited 15-LOX activity from RBL-2H3 with IC50 values of 16.6 +/- 1.3 and 24.1 +/- 2.4 microM, respectively. In a separate experiment, where lysoPC peroxides were tested for the effect on soybean LOX-1, 15(S)-hydroperoxy-5,8,11,13-eicosatetraenoyl lysoPC and 17(S)-hydroperoxy-4,7,10,13,15,19-docosahexaenoyl lysoPC potently inhibited soybean LOX-1 activity with Ki values of 6.8 and of 1.54 microM, respectively. In contrast, 13(S)-hydroperoxy-9,11-octadecadienoyl lysoPC was observed to stimulate soybean LOX-1-catalyzed oxygenation of LA markedly with AC50 value (50% activatory concentration) of 1.5 microM. Taken together, it is proposed that lysoPCs containing polyunsaturated acyl groups or their peroxy derivatives may participate in the regulation of LOX activity in biological systems.

Understanding and analyzing meibomian lipids--a review

PURPOSE

This review is intended to bring to the informed reader the current state of knowledge about meibomian lipids and the art for analyzing them.

METHODS

At the forefront of any endeavor, there are controversies, and these, along with future directions in the field, are brought to the reader's attention.

RESULTS

Function and anatomy of meibomian glands are briefly covered, giving insight into possible mechanisms for secretory controls. Anatomically, some anomalies in meibomian gland distribution of different species, such as whales versus dolphins, are presented, and, for the first time, the structure of the meibomian glands in a selection of marsupials is presented. In attempting to make the literature more accessible, lipid structure and nomenclature are described, and these structures are related to their possible effects on the physicochemical properties of meibomian lipids. The advantages and disadvantages of various collection and storage techniques are described, as well as how gas chromatography and combined HPLC and mass spectrometry coupled with fragmentation are currently enabling us to determine the nature of the lipids in very small samples.

CONCLUSIONS

This review extends to discussing the lipids in tears (as opposed to meibomian gland lipids) and briefly highlights new thoughts about the interactions between proteins of the tear film and meibomian lipids. A model that includes proteins in the outer layer of the tear film is also presented. This model is currently being critically analyzed by the ocular community. It concludes briefly by highlighting possible further areas of research in this area.

Regiochemical and Stereochemical Evidence for Enzyme-Initiated Catalysis in Dual Positional Specific Maize Lipoxygenase-1

Dual positional specific maize lipoxygenase-1 catalyzed the formation of racemic mixtures of four possible regioisomers and was strongly inhibited by the radical scavenger, 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinoxy radical. Molecular modeling studies indicated that the oxygen-binding cavity is segregated from the substrate-binding cavity. The data suggest that a bis-allylic radical reaction intermediate is generated enzymatically, released from the enzyme active site, and subsequently oxygenated outside of the enzyme active site by a nonenzymatic mechanism.

Biochemical characterization of the dual positional specific maize lipoxygenase and the dependence of lagging and initial burst phenomenon on pH, substrate, and detergent during pre-steady state kinetics

The wound-inducible lipoxygenase obtained from maize is one of the nontraditional lipoxygenases that possess dual positional specificity. In this paper, we provide our results on the determination and comparison of the kinetic constants of the maize lipoxygenase, with or without detergents in the steady state, and characterization of the dependence of the kinetic lag phase or initial burst, on pH, substrate, and detergent in the pre-steady state of the lipoxygenase reaction. The oxidation of linoleic acid showed a typical lag phase in the pre-steady state of the lipoxygenase reaction at pH 7.5 in the presence of 0.25% Tween-20 detergent. The reciprocal correlation between the induction period and the enzyme level indicated that this lag phenomenon was attributable to the slow oxidative activation of Fe (II) to Fe (III) at the active site of the enzyme as observed in other lipoxygenase reactions. Contrary to the lagging phenomenon observed at pH 7.5 in the presence of Tween-20, a unique initial burst was observed at pH 6.2 in the absence of detergents. To our knowledge, the initial burst in the oxidation of linoleic acid at pH 6.2 is the first observation in the lipoxygenase reaction. Kinetic constants (K(m) and k(cat) values) were largely dependent on the presence of detergent. An inverse correlation of the initial burst period with enzyme levels and interpretations on kinetic constants suggested that the observed initial burst in the oxidation of linoleic acid could be due to the availability of free fatty acids as substrates for binding with the lipoxygenase enzyme.

Pathophysiological roles of ASK1-MAP kinase signaling pathways

Apoptosis signal-regulating kinase 1 (ASK1) is a mitogenactivated protein kinase (MAPK) kinase kinase that activates JNK and p38 kinases. ASK1 is activated by various stresses, such as reactive oxygen species (ROS), endoplasmic reticulum (ER) stress, lipopolysaccharide (LPS) and calcium influx which are thought to be responsible for the pathogenesis or exacerbations of various human diseases. Recent studies revealed the involvement of ASK1 in ROS- or ER stressrelated diseases, suggesting that ASK1 may be a potential therapeutic target of various human diseases. In this review, we focus on the current findings for the relationship between pathogenesis and ASK1-MAPK pathways.

Dihydroxydocosahexaenoic acids of the neuroprotectin D family: synthesis, structure, and inhibition of human 5-lipoxygenase

During aerobic oxidation of docosahexaenoic acid (DHA), soybean lipoxygenase (sLOX) has been shown to form 7,17(S)-dihydro(pero)xydocosahexaenoic acid [7,17(S)-diH(P)DHA] along with its previously described positional isomer, 10,17(S)-dihydro(pero)xydocosahexa-4Z,7Z,11E,13Z,15E,19Z-enoic acid. 7,17(S)-diH(P)DHA was also obtained via sLOX-catalyzed oxidation of either 17(S)-hydroperoxydocosahexaenoic acid [17(S)-HPDHA] or 17(S)-hydroxydocosahexaenoic acid [17(S)-HDHA]. The structures of the products were elucidated by normal-phase, reverse-phase, and chiral-phase HPLC analyses and by ultraviolet, NMR, and tandem mass spectroscopy and GC-MS. 7,17(S)-diH(P)DHA was shown to have 4Z,8E,10Z,13Z,15E,19Z geometry of the double bonds. In addition, a compound apparently identical to the sLOX-derived 7,17(S)-diH(P)DHA was produced by another enzyme, potato tuber LOX, in the reactions of oxygenation of either 17(S)-HPDHA or 17(S)-HDHA. All of the dihydroxydocosahexaenoic acids (diHDHAs) formed by either of the enzymes were clearly produced through double lipoxygenation of the corresponding substrate. 7,17(S)-diHDHA inhibited human recombinant 5-lipoxygenase in the reaction of arachidonic acid (AA) oxidation. In standard conditions with 100 microM AA as substrate, the IC(50) value for 7,17(S)-diHDHA was found to be 7 microM, whereas IC(50) for 10,17(S)-DiHDHA was 15 microM. Similar inhibition by the diHDHAs was observed with sLOX, a quintessential 15LOX, although the strongest inhibition was produced by 10,17(S)-diHDHA (IC(50) = 4 microM). Inhibition of sLOX by 7,17(S)-diHDHA was slightly less potent, with an IC(50) value of 9 microM. These findings suggest that 7,17(S)-diHDHA along with its 10,17(S) counterpart might have anti-inflammatory and anticancer activities, which could be exerted, at least in part, through direct inhibition of 5LOX and 15LOX.

A one-step method of 10,17-dihydro(pero)xydocosahexa-4Z,7Z,11E,13Z,15E,19Z-enoic acid synthesis by soybean lipoxygenase

A product of lipoxygenase (LOX) oxidation of docosahexaenoic acid (DHA), 10,17-dihydro(pero)xydocosahexa-4Z,7Z,11E,13Z,15E,19Z-enoic acid [10,17(S)-diH(P)DHA] was obtained through various reaction pathways that involved DHA, 17(S)-hydro(pero)xydocosahexa-4Z,7Z,11Z,13Z,15E,19Z-enoic acid [17(S)-H(P)DHA], soybean lipoxygenase (sLOX), and potato tuber lipoxygenase (ptLOX) in various combinations. The structure of the product was confirmed by HPLC, ultraviolet (UV) light spectrometry, GC-MS, tandem MS, and NMR spectroscopy. It has been found that 10,17(S)-diH(P)DHA formed by sLOX through direct oxidation of either DHA or 17(S)-H(P)DHA was apparently identical to the product of ptLOX oxidation of the latter. The sLOX- and ptLOX-derived samples of 10,17-diHDHAs coeluted under the conditions of normal, reverse, and chiral phase HPLC analyses, displayed identical UV absorption spectra with maxima at 260, 270, and 280 nm, and had similar one-dimensional and two-dimensional proton NMR spectra. Analysis of their NMR spectra led to the conclusion that 10,17-diHDHA formed by sLOX had solely 11E,13Z,15E configuration of the conjugated triene fragment, which was identical to the previously published structure of its ptLOX-derived counterpart. Based on the cis,trans geometry of the reaction products, the conclusion is made that in the tested conditions sLOX catalyzed direct double dioxygenation of DHA. Compared with the previously described two-enzyme method that involved sLOX and ptLOX, the current simplified one-enzyme procedure uses only sLOX as the catalyst of both dioxygenation steps.

On the relationships of substrate orientation, hydrogen abstraction, and product stereochemistry in single and double dioxygenations by soybean lipoxygenase-1 and its Ala542Gly mutant

Recent findings associate the control of stereochemistry in lipoxygenase (LOX) catalysis with a conserved active site alanine for S configuration hydroperoxide products, or a corresponding glycine for R stereoconfiguration. To further elucidate the mechanistic basis for this stereocontrol we compared the stereoselectivity of the initiating hydrogen abstraction in soybean LOX-1 and an Ala542Gly mutant that converts linoleic acid to both 13S and 9R configuration hydroperoxide products. Using 11R-(3)H- and 11S-(3)H-labeled linoleic acid substrates to examine the initial hydrogen abstraction, we found that all the primary hydroperoxide products were formed with an identical and highly stereoselective pro-S hydrogen abstraction from C-11 of the substrate (97-99% pro-S-selective). This strongly suggests that 9R and 13S oxygenations occur with the same binding orientation of substrate in the active site, and as the equivalent 9R and 13S products were formed from a bulky ester derivative (1-palmitoyl-2-linoleoylphosphatidylcholine), one can infer that the orientation is tail-first. Both the EPR spectrum and the reaction kinetics were altered by the R product-inducing Ala-Gly mutation, indicating a substantial influence of this Ala-Gly substitution extending to the environment of the active site iron. To examine also the reversed orientation of substrate binding, we studied oxygenation of the 15S-hydroperoxide of arachidonic acid by the Ala542Gly mutant soybean LOX-1. In addition to the usual 5S, 15S- and 8S, 15S-dihydroperoxides, a new product was formed and identified by high-performance liquid chromatography, UV, gas chromatography-mass spectrometry, and NMR as 9R, 15S-dihydroperoxyeicosa-5Z,7E,11Z,13E-tetraenoic acid, the R configuration "partner" of the normal 5S,15S product. This provides evidence that both tail-first and carboxylate end-first binding of substrate can be associated with S or R partnerships in product formation in the same active site.

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.

On the structure and synthesis of neuroprotectin D1, a novel anti-inflammatory compound of the docosahexaenoic acid family

Potato tuber lipoxygenase was shown to convert 17(S)-hydro(pero)xydocasahexaenoic acid in 10,17(S)-dihydro(pero)xydocosahexa-4Z,7Z,11E,13Z,15E,19Z-enoic acid [10,17(S)-diHDHA] which was formed apparently through a double lipoxygenation mechanism. No traces of 10,17(S)-dihydro(pero)xydocosahexa-4Z,7Z,11E,13E,15Z,19Z-enoic acid were found among the reaction products. It is very likely that a described earlier "neuroprotectin D1" [or "10,17(S)docosatriene"], a novel and potent anti-inflammatory compound derived from docosahexaenoic acid, was, in fact, 10,17(S)-dihydroxydocosahexa-4Z,7Z,11E,13Z,15E,19Z-enoic acid formed through a double lipoxygenation mechanism instead of a previously thought epoxidation/isomerization mechanism.

Inhibition of potato lipoxygenase by linoleyl hydroxamic acid: kinetic and EPR spectral evidence for a two-step reaction

The reaction mechanism of an electrophoretically pure potato tuber lipoxygenase (ptLOX) was studied by EPR spectroscopy. An EPR spectrum of the 'native' ptLOX recorded at 4.5+/-0.5 K showed signals of a high-spin (pseudo) axial Fe(3+) with a g-value of approx. 6.3+/-0.1 with a shoulder at g=5.9+/-0.1, and a rhombic Fe(3+) signal at g=4.35+/-0.05. When the enzyme was treated with a 2-fold molar excess of 13(S)-hydroperoxyoctadecadienoic acid [13(S)-HPODE], a 3-fold increase in the integral intensity of the g=6.3 signal was observed, indicating that 25% of the native ptLOX iron was in ferrous state. The positional isomer 9(S)-HPODE caused similar spectral changes. Therefore the catalytic centre of ptLOX appears to accommodate both positional isomers of linoleic acid hydroperoxides in a manner that ensures proper alignment of their hydroperoxy groups with the iron centre of the enzyme. Treatment of the Fe(3+)-ptLOX form with a 3-fold molar excess of linoleyl hydroxamic acid (LHA) completely quenched the g=6.3 signal. Concurrently, a dramatic increase in the signal at g=4.35 was detected, which was attributed to a newly formed LHA-Fe(3+)-ptLOX complex. The spectral characteristics of the complex are similar to those of a 4-nitrocatechol-Fe(3+)-ptLOX complex. From these observations, we conclude that LHA did not reduce Fe(3+) to Fe(2+), but rather formed a LHA-Fe(3+)-ptLOX complex. Formation of such a complex may be responsible for the inhibitory activity of LHA, at least in the initial stages of enzyme inhibition. A prolonged 15 min incubation of the complex at 23+/-1 degrees C led to the partial quenching of the g=4.35 signal. The quenching is attributed to the reduction of Fe(3+)-ptLOX by LHA, with concomitant formation of its oxidation product(s). A kinetic scheme for the inhibition is proposed.