Sequence determinants for the reaction specificity of murine (12R)-lipoxygenase: targeted substrate modification and site-directed mutagenesis

Design, synthesis and evaluation of 2-aryl quinoline derivatives against 12R-lipoxygenase (12R-LOX): Discovery of first inhibitor of 12R-LOX

The 12R-lipoxygenase (12R-LOX), a (non-heme) iron-containing metalloenzyme belonging to the lipoxygenase (LOX) family catalyzes the conversion of arachidonic acid (AA) to its key metabolites. Studies suggested that 12R-LOX plays a critical role in immune modulation for the maintenance of skin homeostasis and therefore can be considered as a potential drug target for psoriasis and other skin related inflammatory diseases. However, unlike 12-LOX (or 12S-LOX) the enzyme 12R-LOX did not receive much attention till date. In our effort, the 2-aryl quinoline derivatives were designed, synthesized and evaluated for the identification of potential inhibitors of 12R-hLOX. The merit of selection of 2-aryl quinolines was assessed by in silico docking studies of a representative compound (4a) using the homology model of 12R-LOX. Indeed, in addition to participating in H-bonding with THR628 and LEU635 the molecule formed a hydrophobic interaction with VAL631. The desired 2-aryl quinolines were synthesized either via the Claisen-Schmidt condensation followed by one-pot reduction-cyclization or via the AlCl3 induced heteroarylation or via the O-alkylation approach in good to high (82-95%) yield. When screened against human 12R-LOX (12R-hLOX) in vitro four compounds (e.g. 4a, 4d, 4e and 7b) showed encouraging (>45%) inhibition at 100 μM among which 7b and 4a emerged as the initial hits. Both the compounds showed selectivity towards 12R-hLOX over 12S-hLOX, 15-hLOX and 15-hLOXB and concentration dependent inhibition of 12R-hLOX with IC50 = 12.48 ± 2.06 and 28.25 ± 1.63 μM, respectively. The selectivity of 4a and 7b towards 12R-LOX over 12S-LOX was rationalized with the help of molecular dynamics simulations. The SAR (Structure-Activity Relationship) within the present series of compounds suggested the need of a o-hydroxyl group on the C-2 phenyl ring for the activity. The compound 4a and 7b (at 10 and 20 µM) reduced the hyper-proliferative state and colony forming potential of IMQ-induced psoriatic keratinocytes in a concentration dependent manner. Further, both compounds decreased the protein levels of Ki67 and the mRNA expression of IL-17A in the IMQ-induced psoriatic-like keratinocytes. Notably, 4a but not 7b inhibited the production of IL-6 and TNF-α in the keratinocyte cells. In the preliminary toxicity studies (i.e. teratogenicity, hepatotoxicity and heart rate assays) in zebrafish both the compounds showed low safety (<30 µM) margin. Overall, being the first identified inhibitors of 12R-LOX both 4a and 7b deserve further investigations.

Paralog- and ortholog-specificity of inhibitors of human and mouse lipoxygenase-isoforms

Lipoxygenases (ALOX-isoforms) are lipid peroxidizing enzymes, which have been implicated in cell differentiation and maturation but also in the biosynthesis of lipid mediators playing important roles in the pathogenesis of inflammatory, hyperproliferative and neurological diseases. In mammals these enzymes are widely distributed and the human genome involves six functional genes encoding for six distinct human ALOX paralogs. In mice, there is an orthologous enzyme for each human ALOX paralog but the catalytic properties of human and mouse ALOX orthologs show remarkable differences. ALOX inhibitors are frequently employed for deciphering the biological role of these enzymes in mouse models of human diseases but owing to the functional differences between mouse and human ALOX orthologs the uncritical use of such inhibitors is sometimes misleading. In this study we evaluated the paralog- and ortholog-specificity of 13 frequently employed ALOX-inhibitors against four recombinant human and mouse ALOX paralogs (ALOX15, ALOX15B, ALOX12, ALOX5) under different experimental conditions. Our results indicated that except for zileuton, which exhibits a remarkable paralog-specificity for mouse and human ALOX5, no other inhibitor was strictly paralog specific but some compounds exhibit an interesting ortholog-specificity. Because of the variable isoform specificities of the currently available ALOX inhibitors care must be taken when the biological effects of these compounds observed in complex in vitro and in vivo systems are interpreted.

Synthesis of 13R,20-dihydroxy-docosahexaenoic acid by site-directed mutagenesis of lipoxygenase derived from Oscillatoria nigro-viridis PCC 7112

Lipoxygenases (LOXs) are implicated in the biosynthesis of pro- and anti-inflammatory lipid mediators involved in immune cell signaling, most of which catalyze peroxidation of polyunsaturated fatty acids by distinct regio- and stereoselectivity. Current reports suggested that conserved amino acid, Gly in R-LOXs and Ala in S-LOXs, in the catalytic domain play an important role in determining the position as well as the stereochemistry of the functional group. Recently, we have confirmed that the catalytic specificity of cyanobacterial lipoxygenase, named Osc-LOX, with alanine at 296 was 13S-type toward linoleic acid, and producing a 17S- hydroxy-docosahexaenoic acid from docosahexaenoic acid (DHA). Here, we aimed to change the catalytic property of LOX from13S-LOX to 9R-LOX by replacing Ala with Gly and to produce a lipid mediators different from the wild-type using DHA. Finally, we succeeded in generating human endogenous a 13R-hydroxy-docosahexaenoic acid and a 13R,20-dihydroxy-docosahexaenoic acid from DHA through an enzymatic reaction using the Osc-LOX-A296G. Our study could enable physiological studies and pharmaceutical research for the 13R,20-dihydroxy-docosahexaenoic acid.

The enzymology of human eicosanoid pathways: the lipoxygenase branches

Eicosanoids are short-lived derivatives of polyunsaturated fatty acids that serve as autocrine and paracrine signaling molecules. They are involved numerous biological processes of both the well state and disease states. A thorough understanding of the progression the disease state and homeostasis of the well state requires a complete evaluation of the systems involved. This review examines the enzymology for the enzymes involved in the production of eicosanoids along the lipoxygenase branches of the eicosanoid pathways with particular emphasis on those derived from arachidonic acid. The enzymatic parameters, protocols to measure them, and proposed catalytic mechanisms are presented in detail.

Eicosanoid biosynthesis in marine mammals

After 300 million years of evolution, the first land-living mammals reentered the marine environment some 50 million years ago. The driving forces for this dramatic lifestyle change are still a matter of discussion but the struggle for food resources and the opportunity to escape predators probably contributed. Reentering the oceans requires metabolic adaption putting evolutionary pressure on a number of genes. To explore whether eicosanoid signaling has been part of this adaptive response, we first explored whether the genomes of marine mammals involve functional genes encoding for key enzymes of eicosanoid biosynthesis. Cyclooxygenase (COX) and lipoxygenase (ALOX) genes are present in the genome of all marine mammals tested. Interestingly, ALOX12B, which has been implicated in skin development of land-living mammals, is lacking in whales and dolphins and genes encoding for its sister enzyme (ALOXE3) involve premature stop codons and/or frameshifting point mutations, which interrupt the open reading frames. ALOX15 orthologs have been detected in all marine mammals, and the recombinant enzymes exhibit similar catalytic properties as those of land-living species. All marine mammals express arachidonic acid 12-lipoxygenating ALOX15 orthologs, and these data are consistent with the Evolutionary Hypothesis of ALOX15 specificity. These enzymes exhibit membrane oxygenase activity and introduction of big amino acids at the triad positions altered the reaction specificity in favor of arachidonic acid 15-lipoxygenation. Thus, the ALOX15 orthologs of marine mammals follow the Triad concept explaining their catalytic specificity.

Bioactive lipids and pathological retinal angiogenesis

Angiogenesis, disruption of the retinal barrier, leukocyte-adhesion and oedema are cardinal signs of proliferative retinopathies that are associated with vision loss. Therefore, identifying factors that regulate these vascular dysfunctions is critical to target pathological angiogenesis. Given the conflicting role of bioactive lipids reported in the current literature, the goal of this review is to provide the reader a clear road map of what has been accomplished so far in the field with specific focus on the role of polyunsaturated fatty acids (PUFAs)-derived metabolites in proliferative retinopathies. This necessarily entails a description of the different retina cells, blood retina barriers and the role of (PUFAs)-derived metabolites in diabetic retinopathy, retinopathy of prematurity and age-related macular degeneration as the most common types of proliferative retinopathies.

Emerging role of 12/15-Lipoxygenase (ALOX15) in human pathologies

12/15-lipoxygenase (12/15-LOX) is an enzyme, which oxidizes polyunsaturated fatty acids, particularly omega-6 and -3 fatty acids, to generate a number of bioactive lipid metabolites. A large number of studies have revealed the importance of 12/15-LOX role in oxidative and inflammatory responses. The in vitro studies have demonstrated the ability of 12/15-LOX metabolites in the expression of various genes and production of cytokine related to inflammation and resolution of inflammation. The studies with the use of knockout and transgenic animals for 12/15-LOX have further shown its involvement in the pathogenesis of a variety of human diseases, including cardiovascular, renal, neurological and metabolic disorders. This review summarizes our current knowledge on the role of 12/15-LOX in inflammation and various human diseases.

Mammalian lipoxygenases and their biological relevance

Lipoxygenases (LOXs) form a heterogeneous class of lipid peroxidizing enzymes, which have been implicated not only in cell proliferation and differentiation but also in the pathogenesis of various diseases with major public health relevance. As other fatty acid dioxygenases LOXs oxidize polyunsaturated fatty acids to their corresponding hydroperoxy derivatives, which are further transformed to bioactive lipid mediators (eicosanoids and related substances). On the other hand, lipoxygenases are key players in the regulation of the cellular redox homeostasis, which is an important element in gene expression regulation. Although the first mammalian lipoxygenases were discovered 40 years ago and although the enzymes have been well characterized with respect to their structural and functional properties the biological roles of the different lipoxygenase isoforms are not completely understood. This review is aimed at summarizing the current knowledge on the physiological roles of different mammalian LOX-isoforms and their patho-physiological function in inflammatory, metabolic, hyperproliferative, neurodegenerative and infectious disorders. This article is part of a Special Issue entitled "Oxygenated metabolism of PUFA: analysis and biological relevance".

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

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

An insight into the regiospecificity of linoleic acid peroxidation catalyzed by mammalian 15-lipoxygenases

15-Lipoxygenases (15-LOs) catalyze the peroxidation reaction of linoleic acid (LA) in mammals producing almost exclusively 13-(S)-hydroperoxyoctadecadienoic acid (13-(S)-HPODE). Although several hypotheses have been formulated, the molecular basis of such enzymatic regiospecificity is unclear. We have here combined quantum mechanics/molecular mechanics (QM/MM) calculations with molecular dynamics simulations to analyze the peroxidation mechanism using a complete rabbit 15-LO-1/LA solvated model. C9 and C13 being equivalent for planarity and spin density, the QM/MM potential energy profiles of the O2 addition to those two atoms were calculated. The difference in the potential energy barrier heights is clear enough to justify that O2 selectively attacks C13 giving 13-(S)-HPODE. Oxygenation at C9 is hindered by two steric-shielding residues (Leu597 and Gln548). The calculated free energy profile at 300 K for the O2 addition to C13 confirms that the peroxidation on C13 is a reversible viable process in agreement with experiments. Thus, the subsequent reduction of the peroxyl radical to give the final hydroperoxidated product is expected to give the irreversibility character to the overall process.

Steric analysis of epoxyalcohol and trihydroxy derivatives of 9-hydroperoxy-linoleic acid from hematin and enzymatic synthesis

We characterize the allylic epoxyalcohols and their trihydroxy hydrolysis products generated from 9R- and 9S-hydroperoxy-octadecenoic acid (HPODE) under non-enzymatic conditions, reaction with hematin and subsequent acid hydrolysis, and enzymatic conditions, incubation with Beta vulgaris containing a hydroperoxide isomerase and epoxide hydrolase. The products were resolved by HPLC and the regio and stereo-chemistry of the transformations were determined through a combination of (1)H NMR and GC-MS analysis of dimethoxypropane derivatives. Four trihydroxy isomers were identified upon mild acid hydrolysis of 9S,10S-trans-epoxy-11E-13S-hydroxyoctadecenoate: 9S,10R,13S, 9S,12R,13S, 9S,10S,13S and 9S,12S,13S-trihydroxy-octadecenoic acids, in the ratio 40:26:22:12. We also identified a prominent δ-ketol rearrangement product from the hydrolysis as mainly the 9-hydroxy-10E-13-oxo isomer. Short incubation (5 min) of 9R- and 9S-HPODE with B. vulgaris extract yielded the 9R- and 9S-hydroxy-10E-12R,13S-cis-epoxy products respectively. Longer incubation (60 min) gave one specific hydrolysis product via epoxide hydrolase, the 9R/S,12S,13S-trihydroxyoctadecenoate. These studies provide a practical approach for the isolation and characterization of allylic epoxy alcohol and trihydroxy products using a combination of HPLC, GC-MS and (1)H NMR.

Structure and ligand based drug design strategies in the development of novel 5- LOX inhibitors

Lipoxygenases (LOXs) are non-heme iron containing dioxygenases involved in the oxygenation of polyunsaturated fatty acids (PUFAs) such as arachidonic acid (AA). Depending on the position of insertion of oxygen, LOXs are classified into 5-, 8-, 9-, 12- and 15-LOX. Among these, 5-LOX is the most predominant isoform associated with the formation of 5-hydroperoxyeicosatetraenoic acid (5-HpETE), the precursor of non-peptido (LTB₄) and peptido (LTC₄, LTD₄, and LTE₄) leukotrienes. LTs are involved in inflammatory and allergic diseases like asthma, ulcerative colitis, rhinitis and also in cancer. Consequently 5-LOX has become target for the development of therapeutic molecules for treatment of various inflammatory disorders. Zileuton is one such inhibitor of 5-LOX approved for the treatment of asthma.

In the recent times, computer aided drug design (CADD) strategies have been applied successfully in drug development processes. A comprehensive review on structure based drug design strategies in the development of novel 5-LOX inhibitors is presented in this article. Since the crystal structure of 5-LOX has been recently solved, efforts to develop 5-LOX inhibitors have mostly relied on ligand based rational approaches. The present review provides a comprehensive survey on these strategies in the development of 5-LOX inhibitors.

Catalytic convergence of manganese and iron lipoxygenases by replacement of a single amino acid

Lipoxygenases (LOXs) contain a hydrophobic substrate channel with the conserved Gly/Ala determinant of regio- and stereospecificity and a conserved Leu residue near the catalytic non-heme iron. Our goal was to study the importance of this region (Gly(332), Leu(336), and Phe(337)) of a lipoxygenase with catalytic manganese (13R-MnLOX). Recombinant 13R-MnLOX oxidizes 18:2n-6 and 18:3n-3 to 13R-, 11(S or R)-, and 9S-hydroperoxy metabolites (∼80-85, 15-20, and 2-3%, respectively) by suprafacial hydrogen abstraction and oxygenation. Replacement of Phe(337) with Ile changed the stereochemistry of the 13-hydroperoxy metabolites of 18:2n-6 and 18:3n-3 (from ∼100% R to 69-74% S) with little effect on regiospecificity. The abstraction of the pro-S hydrogen of 18:2n-6 was retained, suggesting antarafacial hydrogen abstraction and oxygenation. Replacement of Leu(336) with smaller hydrophobic residues (Val, Ala, and Gly) shifted the oxygenation from C-13 toward C-9 with formation of 9S- and 9R-hydroperoxy metabolites of 18:2n-6 and 18:3n-3. Replacement of Gly(332) and Leu(336) with larger hydrophobic residues (G332A and L336F) selectively augmented dehydration of 13R-hydroperoxyoctadeca-9Z,11E,15Z-trienoic acid and increased the oxidation at C-13 of 18:1n-6. We conclude that hydrophobic replacements of Leu(336) can modify the hydroperoxide configurations at C-9 with little effect on the R configuration at C-13 of the 18:2n-6 and 18:3n-3 metabolites. Replacement of Phe(337) with Ile changed the stereospecific oxidation of 18:2n-6 and 18:3n-3 with formation of 13S-hydroperoxides by hydrogen abstraction and oxygenation in analogy with soybean LOX-1.

Stereocontrol of arachidonic acid oxygenation by vertebrate lipoxygenases: newly cloned zebrafish lipoxygenase 1 does not follow the Ala-versus-Gly concept

Animal lipoxygenases (LOXs) are classified according to their specificity of arachidonic acid oxygenation, and previous sequence alignments suggested that S-LOXs contain a conserved Ala at a critical position at the active site but R-LOXs carry a Gly instead. Here we cloned, expressed, and characterized a novel LOX isoform from the model vertebrate Danio rerio (zebrafish) that carries a Gly at this critical position, classifying this enzyme as putative arachidonic acid R-LOX. Surprisingly, the almost exclusive arachidonic acid oxygenation product was 12S-H(p)ETE (hydro(pero)xyeicosatetraenoic acid), and extensive mutation around Gly-410 failed to induce R-lipoxygenation. This finding prompted us to explore the importance of the corresponding amino acids in other vertebrate S-LOXs. We found that Ala-to-Gly exchange in human 15-LOX2 and human platelet 12-LOX induced major alterations in the reaction specificity with an increase of specific R-oxygenation products. For mouse 5-LOX and 12/15-LOX from rabbits, men, rhesus monkeys, orangutans, and mice, only minor alterations in the reaction specificity were observed. For these enzymes, S-HETE (hydroxyeicosatetraenoic acid) isomers remained the major oxygenation products, whereas chiral R-HETEs contributed only 10-30% to the total product mixture. Taken together these data indicate that the Ala-versus-Gly concept may not always predict the reaction specificity of vertebrate LOX isoforms.

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.

On the role of molecular oxygen in lipoxygenase activation: comparison and contrast of epidermal lipoxygenase-3 with soybean lipoxygenase-1

The oxygenation of polyunsaturated fatty acids by lipoxygenases (LOX) is associated with a lag phase during which the resting ferrous enzyme is converted to the active ferric form by reaction with fatty acid hydroperoxide. Epidermal lipoxygenase-3 (eLOX3) is atypical in displaying hydroperoxide isomerase activity with fatty acid hydroperoxides through cycling of the ferrous enzyme. Yet eLOX3 is capable of dioxygenase activity, albeit with a long lag phase and need for high concentrations of hydroperoxide activator. Here, we show that higher O(2) concentration shortens the lag phase in eLOX3, although it reduces the rate of hydroperoxide consumption, effects also associated with an A451G mutation known to affect the disposition of molecular oxygen in the LOX active site. These observations are consistent with a role of O(2) in interrupting hydroperoxide isomerase cycling. Activation of eLOX3, A451G eLOX3, and soybean LOX-1 with 13-hydroperoxy-linoleic acid forms oxygenated end products, which we identified as 9R- and 9S-hydroperoxy-12S,13S-trans-epoxyoctadec-10E-enoic acids. We deduce that activation partly depends on reaction of O(2) with the intermediate of hydroperoxide cleavage, the epoxyallylic radical, giving an epoxyallylic peroxyl radical that does not further react with Fe(III)-OH; instead, it dissociates and leaves the enzyme in the activated free ferric state. eLOX3 differs from soybean LOX-1 in more tightly binding the epoxyallylic radical and having limited access to O(2) within the active site, leading to a deficiency in activation and a dominant hydroperoxide isomerase activity.

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.

Molecular enzymology of lipoxygenases

Lipoxygenases (LOXs) are lipid peroxidizing enzymes, implicated in the pathogenesis of inflammatory and hyperproliferative diseases, which represent potential targets for pharmacological intervention. Although soybean LOX1 was discovered more than 60years ago, the structural biology of these enzymes was not studied until the mid 1990s. In 1993 the first crystal structure for a plant LOX was solved and following this protein biochemistry and molecular enzymology became major fields in LOX research. This review focuses on recent developments in molecular enzymology of LOXs and summarizes our current understanding of the structural basis of LOX catalysis. Various hypotheses explaining the reaction specificity of different isoforms are critically reviewed and their pros and cons briefly discussed. Moreover, we summarize the current knowledge of LOX evolution by profiling the existence of LOX-related genomic sequences in the three kingdoms of life. Such sequences are found in eukaryotes and bacteria but not in archaea. Although the biological role of LOXs in lower organisms is far from clear, sequence data suggests that this enzyme family might have evolved shortly after the appearance of atmospheric oxygen on earth.

Eicosanoids in inflammation and cancer: the role of COX-2

Eicosanoids, a family of oxygenated metabolites of eicosapolyenoic fatty acids, such as arachidonic acid, formed via the lipoxygenase, cyclooxygenase (COX) and epoxygenase pathways, play an important role in the regulation of various pathophysiological processes, including inflammation and cancer. COX-2, the inducible isoform of COX, has emerged as the key enzyme regulating inflammation, and promises to play a considerable role in cancer. Although NSAIDs have been in use for centuries, the COX-2 selective inhibitors - coxibs - have emerged as potent anti-inflammatory drugs with fewer gastric side effects. As COX-2 plays a major role in neoplastic transformation and cancer growth, by downregulating apoptosis and promoting angiogenesis, invasion and metastasis, coxibs have a potential role in the prevention and treatment of cancer. Recent studies indicate their possible application in overcoming drug resistance by downregulating the expression of MDR-1. However, the cardiac side effects of some of the coxibs have limited their application in treating various inflammatory disorders and warrant the development of COX-2 inhibitors without side effects. This review will focus on the role of COX-2 in inflammation and cancer, with an emphasis on novel approaches to the development of COX-2 inhibitors without side effects.

Applicability of the triad concept for the positional specificity of mammalian lipoxygenases

The nomenclature of lipoxygenases (LOXs) is partly based on the positional specificity of arachidonic acid oxygenation, but there is no unifying concept explaining the mechanistic basis of this enzyme property. According to the triad model, Phe-353, Ile-418, and Ile-593 of the rabbit 12/15-LOX form the bottom of the substrate-binding pocket, and introduction of less space-filling residues at either of these positions favors arachidonic acid 12-lipoxygenation. The present study was aimed at exploring the validity of the triad concept for two novel primate 12/15-LOX (Macaca mulatta and Pongo pygmaeus) and for five known members of the mammalian LOX family (human 12/15-LOX, mouse 12/15-LOX, human 15-LOX2, human platelet type 12-LOX, and mouse (12R)-LOX). The enzymes were expressed as N-terminal His tag fusion proteins in E. coli, the potential sequence determinants were mutated, and the specificity of arachidonic acid oxygenation was quantified. Taken together, our data indicate that the triad concept explains the positional specificity of all 12/15-LOXs tested (rabbit, human, M. mulatta, P. pygmaeus, and mouse). For the new enzymes of M. mulatta and P. pygmaeus, the concept had predictive value because the positional specificity predicted on the basis of the amino acid sequence was confirmed experimentally. The specificity of the platelet 12-LOX was partly explained by the triad hypothesis, but the concept was not applicable for 15-LOX2 and (12R)-LOX.

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.

Thematic Review Series: Proteomics. An integrated omics analysis of eicosanoid biology

Eicosanoids have been implicated in a vast number of devastating inflammatory conditions, including arthritis, atherosclerosis, pain, and cancer. Currently, over a hundred different eicosanoids have been identified, with many having potent bioactive signaling capacity. These lipid metabolites are synthesized de novo by at least 50 unique enzymes, many of which have been cloned and characterized. Due to the extensive characterization of eicosanoid biosynthetic pathways, this field provides a unique framework for integrating genomics, proteomics, and metabolomics toward the investigation of disease pathology. To facilitate a concerted systems biology approach, this review outlines the proteins implicated in eicosanoid biosynthesis and signaling in human, mouse, and rat. Applications of the extensive genomic and lipidomic research to date illustrate the questions in eicosanoid signaling that could be uniquely addressed by a thorough analysis of the entire eicosanoid proteome.

Computational analysis of R and S isoforms of 12-lipoxygenases: homology modeling and docking studies

The present study is aimed at predicting human 12R-LOX structure by constructing a homology model. Based upon Blast results, rabbit reticulocyte 15-lipoxygenase 1LOX (protein data bank) was considered as a template for homology modeling. The 3D model was generated with Modeler in InsightII and further refined using AMBER. Further to understand the relationship of protein structure with stereo specificity, a comparative analysis of 12R-LOX model was done with that of 12S-LOX homology model to identify differences in the binding site topology and interacting residues. The large insertion of 31-aa seen in 12R-LOX is located beyond the N-terminal barrel and is accommodated on the outside of the protein without disruption of the overall tertiary structure. The 31-aa region includes SH3 domain binding PXXP motif, seven prolines and five arginines. The docking of the substrate, arachidonic acid was also performed. Our results show that the Gly441 and substrate orientation within the active site play an important role in stereo specificity of 12R-LOX.

Homology modeling of 5-lipoxygenase and hints for better inhibitor design

Lipoxygenases (LOXs) are a group of enzymes involved in the oxygenation of polyunsaturated fatty acids. Among these 5-lipoxygenase (5-LOX) is the key enzyme leading to the formation of pharmacologically important leukotrienes and lipoxins, the mediators of inflammatory and allergic disorders. In view of close functional similarity to mammalian lipoxygenase, potato 5-LOX is used extensively. In this study, the homology modeling technique has been used to construct the structure of potato 5-LOX. The amino acid sequence identity between the target protein and sequence of template protein 1NO3 (soybean LOX-3) searched from NCBI protein BLAST was 63%. Based on the template structure, the protein model was constructed by using the Homology program in InsightII. The protein model was briefly refined by energy minimization steps and validated using Profile-3D, ERRAT and PROCHECK. The results showed that 99.3% of the amino acids were in allowed regions of Ramachandran plot, suggesting that the model is accurate and its stereochemical quality good. Like all LOXs, 5-LOX also has a two-domain structure, the small N-terminal beta-barrel domain and a larger catalytic domain containing a single atom of non-heme iron coordinating with His525, His530, His716 and Ile864. Asn720 is present in the fifth coordination position of iron. The sixth coordination position faces the open cavity occupied here by the ligands which are docked. Our model of the enzyme is further validated by examining the interactions of earlier reported inhibitors and by energy minimization studies which were carried out using molecular mechanics calculations. Four ligands, nordihydroguaiaretic acid (NDGA) having IC(50) of 1.5 microM and analogs of benzyl propargyl ethers having IC(50) values of 760 microM, 45 microM, and no inhibition respectively were selected for our docking and energy minimization studies. Our results correlated well with the experimental data reported earlier, which proved the quality of the model. This model generated can be further used for the design and development of more potent 5-LOX inhibitors.

Properties of a mini 9R-lipoxygenase from Nostoc sp. PCC 7120 and its mutant forms

Lipoxygenases (LOXs) consist of a class of enzymes that catalyze the regio- and stereospecific dioxygenation of polyunsaturated fatty acids. Current reports propose that a conserved glycine residue in the active site of R-lipoxygenases and an alanine residue at the corresponding position in S-lipoxygenases play a crucial role in determining the stereochemistry of the product. Recently, a bifunctional lipoxygenase with a linoleate diol synthase activity from Nostoc sp. PCC7120 with R stereospecificity and the so far unique feature of carrying an alanine instead of the conserved glycine in the position of the sequence determinant for chiral specificity was identified. The recombinant carboxy-terminal domain was purified after expression in Escherichia coli. The ability of the enzyme to use linoleic acid esterified to a bulky phosphatidylcholine molecule as a substrate suggested a tail-fist binding orientation of the substrate. Site directed mutagenesis of the alanine to glycine did not cause alterations in the stereospecificity of the products, while mutation of the alanine to valine or isoleucine modified both regio- and enantioselectivity of the enzyme. Kinetic measurements revealed that substitution of Ala by Gly or Val did not significantly influence the reaction characteristics, while the A162I mutant showed a reduced vmax. Based on the mutagenesis data obtained, we suggest that the existing model for stereocontrol of the lipoxygenase reaction may be expanded to include enzymes that seem to have in general a smaller amino acid in R and a bulkier one in S lipoxygenases at the position that controls stereospecificity.

Factors influencing the rearrangement of bis-allylic hydroperoxides by manganese lipoxygenase

Manganese lipoxygenase (Mn-LOX) catalyzes the rearrangement of bis-allylic S-hydroperoxides to allylic R-hydroperoxides, but little is known about the reaction mechanism. 1-Linoleoyl-lysoglycerophosphatidylcholine was oxidized in analogy with 18:2n-6 at the bis-allylic carbon with rearrangement to C-13 at the end of lipoxygenation, suggesting a "tail-first" model. The rearrangement of bis-allylic hydroperoxides was influenced by double bond configuration and the chain length of fatty acids. The Gly316Ala mutant changed the position of lipoxygenation toward the carboxyl group of 20:2n-6 and 20:3n-3 and prevented the bis-allylic hydroperoxide of 20:3n-3 but not 20:2n-6 to interact with the catalytic metal. The oxidized form, Mn(III)-LOX, likely accepts an electron from the bis-allylic hydroperoxide anion with the formation of the peroxyl radical, but rearrangement of 11-hydroperoxyoctadecatrienoic acid by Mn-LOX was not reduced in D(2)O (pD 7.5), and aqueous Fe(3+) did not transfer 11S-hydroperoxy-9Z,12Z,15Z-octadecatrienoic acid to allylic hydroperoxides. Mutants in the vicinity of the catalytic metal, Asn466Leu and Ser469Ala, had little influence on bis-allylic hydroperoxide rearrangement. In conclusion, Mn-LOX transforms bis-allylic hydroperoxides to allylic by a reaction likely based on the positioning of the hydroperoxide close to Mn(3+) and electron transfer to the metal, with the formation of a bis-allylic peroxyl radical, beta-fragmentation, and oxygenation under steric control by the protein.

The hepoxilin connection in the epidermis

The recent convergence of genetic and biochemical evidence on the activities of lipoxygenase (LOX) enzymes has implicated the production of hepoxilin derivatives (fatty acid epoxyalcohols) in the pathways leading to formation of the water-impermeable barrier of the outer epidermis. The enzymes 12R-LOX and eLOX3 are mutated in a rare form of congenital ichthyosis, and, in vitro, the two enzymes function together to convert arachidonic acid to a specific hepoxilin. Taken together, these lines of evidence suggest an involvement of these enzymes and their products in skin barrier function in all normal subjects. The natural occurrence of the specific hepoxilin products, and their biological role, whether structural or signaling, remain to be defined.

Effect of 15-lipoxygenase metabolites, 15-(S)-HPETE and 15-(S)-HETE on chronic myelogenous leukemia cell line K-562: reactive oxygen species (ROS) mediate caspase-dependent apoptosis

Growth inhibitory effects of 15-lipoxygenase-1 [13-(S)-HPODE and 13-(S)-HODE] and 15-lipoxygenase-2 [15-(S)-HPETE and 15-(S)-HETE] (15-LOX-1 and LOX-2) metabolites and the underlying mechanisms were studied on chronic myeloid leukemia cell line (K-562). The hydroperoxy metabolites, 15-(S)-HPETE and 13-(S)-HPODE rapidly inhibited the growth of K-562 cells by 3h with IC(50) values, 10 and 15microM, respectively. In contrast, the hydroxy metabolite of 15-LOX-2, 15-(S)-HETE, showed 50% inhibition only at 40microM by 6h and 13-(S)-HODE, hydroxy metabolite of 15-LOX-1, showed no significant effect up to 160microM. The cells exposed to 10microM of 15-(S)-HPETE and 40microM of 15-(S)-HETE showed typical apoptotic features like release of cytochrome c, caspase-3 activation and PARP-1 (poly(ADP) ribose polymerase-1) cleavage. A flow cytometry based DCFH-DA analysis and inhibitory studies with DPI, a pharmacological inhibitor of NADPH oxidase, NAC (N-acetyl cysteine) and GSH revealed that NADPH oxidase-mediated generation of ROS is responsible for caspase-3 activation and subsequent induction of apoptosis in the K-562 cell line.

A G316A Mutation of Manganese Lipoxygenase Augments Hydroperoxide Isomerase Activity

Lipoxygenases with R stereospecificity have a conserved Gly residue, whereas (S)-lipoxygenases have an Ala residue. Site-directed mutagenesis has shown that these residues control position and S/R stereospecificity of oxygenation. Recombinant Mn-LO was expressed in Pichia pastoris, and its conserved Gly-316 residue was mutated to Ala, Ser, Val, and Thr. The G316A mutant was catalytically active. We compared the catalytic properties of Mn-LO and the G316A mutant with 17:3n-3, 18:2n-6, 18:3n-3, and 19:3n-3 as substrates. Increasing the fatty acid chain length from C17 to C19 shifted the oxygenation by Mn-LO from the n-6 toward the n-8 carbon. The G316A mutant increased the oxygenation at the n-8 carbon of 17:3n-3 and at the n-10 carbon of the C17 and C18 fatty acids (from 1-2% to 7-11%). The most striking effect of the G316A mutant was a 2-, 7-, and 15-fold increase in transformation of the n-6 hydroperoxides of 19:3n-3, 18:3n-3, and 17:3n-3, respectively, to keto fatty acids and epoxyalcohols. The n-3 double bond was essential. An experiment under an oxygen-18 atmosphere showed that both oxygen atoms were retained in the epoxyalcohols. (R)-Hydroperoxides at n-6 of C17:3, 18:3, and 19:3 were transformed 5 times faster than S stereoisomers. The G316A mutant converted (13R)-hydroperoxylinolenic acid to 13-ketolinolenic acid (with an apparent K(m) of 0.01 mm) and to epoxyalcohols (viz. erythro- and threo-11-hydroxy-(12R,13R)-epoxy-(9Z,15Z)-octadecadienoic acids and one of the corresponding cis-epoxides as major products). A reducing lipoxygenase inhibitor stimulated the hydroperoxide isomerase activity, whereas a suicide-type lipoxygenase inhibitor reduced this activity. The n-3 double bond also appeared to influence the anaerobic formation of epoxyalcohols by Mn-LO, since 18:2n-6 and 18:3n-3 yielded different profiles of epoxyalcohols. Our results suggest that the G316A mutant augmented the hydroperoxide isomerase activity by positioning the hydroperoxy group at the n-6 carbon of n-3 fatty acids closer to the reduced catalytic metal.