Site-directed mutagenesis studies on the iron-binding domain and the determinant for the substrate oxygenation site of porcine leukocyte arachidonate 12-lipoxygenase

Hydroperoxidation of Docosahexaenoic Acid by Human ALOX12 and pigALOX15-mini-LOX

Human lipoxygenase 12 (hALOX12) catalyzes the conversion of docosahexaenoic acid (DHA) into mainly 14S-hydroperoxy-4Z,7Z,10Z,12E,16Z,19Z-docosahexaenoic acid (14S-H(p)DHA). This hydroperoxidation reaction is followed by an epoxidation and hydrolysis process that finally leads to maresin 1 (MaR1), a potent bioactive specialized pro-resolving mediator (SPM) in chronic inflammation resolution. By combining docking, molecular dynamics simulations, and quantum mechanics/molecular mechanics calculations, we have computed the potential energy profile of DHA hydroperoxidation in the active site of hALOX12. Our results describe the structural evolution of the molecular system at each step of this catalytic reaction pathway. Noteworthy, the required stereospecificity of the reaction leading to MaR1 is explained by the configurations adopted by DHA bound to hALOX12, along with the stereochemistry of the pentadienyl radical formed after the first step of the mechanism. In pig lipoxygenase 15 (pigALOX15-mini-LOX), our calculations suggest that 14S-H(p)DHA can be formed, but with a stereochemistry that is inadequate for MaR1 biosynthesis.

The Reaction Specificity of Mammalian ALOX15 Orthologs is Changed During Late Primate Evolution and These Alterations Might Offer Evolutionary Advantages for Hominidae

Arachidonic acid lipoxygenases (ALOXs) have been implicated in the immune response of mammals. The reaction specificity of these enzymes is decisive for their biological functions and ALOX classification is based on this enzyme property. Comparing the amino acid sequences and the functional properties of selected mammalian ALOX15 orthologs we previously hypothesized that the reaction specificity of these enzymes can be predicted based on their amino acid sequences (Triad Concept) and that mammals, which are ranked in evolution below gibbons, express arachidonic acid 12-lipoxygenating ALOX15 orthologs. In contrast, Hominidae involving the great apes and humans possess 15-lipoxygenating enzymes (Evolutionary Hypothesis). These two hypotheses were based on sequence data of some 60 mammalian ALOX15 orthologs and about half of them were functionally characterized. Here, we compared the ALOX15 sequences of 152 mammals representing all major mammalian subclades expressed 44 novel ALOX15 orthologs and performed extensive mutagenesis studies of their triad determinants. We found that ALOX15 genes are absent in extant Prototheria but that corresponding enzymes frequently occur in Metatheria and Eutheria. More than 90% of them catalyze arachidonic acid 12-lipoxygenation and the Triad Concept is applicable to all of them. Mammals ranked in evolution above gibbons express arachidonic acid 15-lipoxygenating ALOX15 orthologs but enzymes with similar specificity are only present in less than 5% of mammals ranked below gibbons. This data suggests that ALOX15 orthologs have been introduced during Prototheria-Metatheria transition and put the Triad Concept and the Evolutionary Hypothesis on a much broader and more reliable experimental basis.

Mutagenesis, Hydrogen-Deuterium Exchange, and Molecular Docking Investigations Establish the Dimeric Interface of Human Platelet-Type 12-Lipoxygenase

It was previously shown that human platelet 12S-lipoxygenase (h12-LOX) exists as a dimer; however, the specific structure is unknown. In this study, we create a model of the dimer through a combination of computational methods, experimental mutagenesis, and hydrogen-deuterium exchange (HDX) investigations. Initially, Leu183 and Leu187 were replaced by negatively charged glutamate residues and neighboring aromatic residues were replaced with alanine residues (F174A/W176A/L183E/L187E/Y191A). This quintuple mutant disrupted both the hydrophobic and π-π interactions, generating an h12-LOX monomer. To refine the determinants for dimer formation further, the L183E/L187E mutant was generated and the equilibrium shifted mostly toward the monomer. We then submitted the predicted monomeric structure to protein-protein docking to create a model of the dimeric complex. A total of nine of the top 10 most energetically favorable docking conformations predict a TOP-to-TOP dimeric arrangement of h12-LOX, with the α-helices containing a Leu-rich region (L172, L183, L187, and L194), corroborating our experimental results showing the importance of these hydrophobic interactions for dimerization. This model was supported by HDX investigations that demonstrated the stabilization of four, non-overlapping peptides within helix α2 of the TOP subdomain for wt-h12-LOX, consistent with the dimer interface. Most importantly, our data reveal that the dimer and monomer of h12-LOX have distinct biochemical properties, suggesting that the structural changes due to dimerization have allosteric effects on active site catalysis and inhibitor binding.

Role of Human 15-Lipoxygenase-2 in the Biosynthesis of the Lipoxin Intermediate, 5S,15S-diHpETE, Implicated with the Altered Positional Specificity of Human 15-Lipoxygenase-1

The oxylipins, 5S,12S-dihydroxy-6E,8Z,10E,14Z-eicosatetraenoic acid (5S,12S-diHETE) and 5S,15S-dihydroxy-6E,8Z,11Z,13E-eicosatetraenoic acid (5S,15S-diHETE), have been identified in cell exudates and have chemotactic activity toward eosinophils and neutrophils. Their biosynthesis has been proposed to occur by sequential oxidations of arachidonic acid (AA) by lipoxygenase enzymes, specifically through oxidation of AA by h5-LOX followed by h12-LOX, h15-LOX-1, or h15-LOX-2. In this work, h15-LOX-1 demonstrates altered positional specificity when reacting with 5S-HETE, producing 90% 5S,12S-diHETE, instead of 5S,15S-diHETE, with kinetics 5-fold greater than that of h12-LOX. This is consistent with previous work in which h15-LOX-1 reacts with 7S-HDHA, producing the noncanonical, DHA-derived, specialized pro-resolving mediator, 7S,14S-diHDHA. It is also determined that oxygenation of 5S-HETE by h15-LOX-2 produces 5S,15S-diHETE and its biosynthetic kcat/KM flux is 2-fold greater than that of h15-LOX-1, suggesting that h15-LOX-2 may have a greater role in lipoxin biosynthesis than previously thought. In addition, it is shown that oxygenation of 12S-HETE and 15S-HETE by h5-LOX is kinetically slow, suggesting that the first step in the in vitro biosynthesis of both 5S,12S-diHETE and 5S,15S-diHETE is the production of 5S-HETE.

Recombinant Lipoxygenases and Hydroperoxide Lyases for the Synthesis of Green Leaf Volatiles

Green leaf volatiles (GLVs) are mainly C₆- and in rare cases also C₉-aldehydes, -alcohols, and -esters, which are released by plants in response to biotic or abiotic stresses. These compounds are named for their characteristic smell reminiscent of freshly mowed grass. This review focuses on GLVs and the two major pathway enzymes responsible for their formation: lipoxygenases (LOXs) and fatty acid hydroperoxide lyases (HPLs). LOXs catalyze the peroxidation of unsaturated fatty acids, such as linoleic and α-linolenic acids. Hydroperoxy fatty acids are further converted by HPLs into aldehydes and oxo-acids. In many industrial applications, plant extracts have been used as LOX and HPL sources. However, these processes are limited by low enzyme concentration, stability, and specificity. Alternatively, recombinant enzymes can be used as biocatalysts for GLV synthesis. The increasing number of well-characterized enzymes efficiently expressed by microbial hosts will foster the development of innovative biocatalytic processes for GLV production.

The lipoxygenase pathway of Tupaia belangeri representing Scandentia. Genomic multiplicity and functional characterization of the ALOX15 orthologs in the tree shrew

The tree shrew (Tupaia belangeri) is a rat-sized mammal, which is more closely related to humans than mice and rats. However, the use of tree shrew to explore the patho-mechanisms of human inflammatory disorders has been limited since nothing is known about eicosanoid metabolism in this mammalian species. Eicosanoids are important lipid mediators exhibiting pro- and anti-inflammatory activities, which are biosynthesized via lipoxygenase and cyclooxygenase pathways. When we searched the tree shrew genome for the presence of cyclooxygenase and lipoxygenase isoforms we found copies of functional COX1, COX2 and LOX genes. Interestingly, we identified four copies of ALOX15 genes, which encode for four structurally distinct ALOX15 orthologs (tupALOX15a-d). To explore the catalytic properties of these enzymes we expressed tupALOX15a and tupALOX15c as catalytically active proteins and characterized their enzymatic properties. As predicted by the Evolutionary Hypothesis of ALOX15 specificity we found that the two enzymes converted arachidonic acid predominantly to 12S-HETE and they also exhibited membrane oxygenase activities. However, their reaction kinetic properties (K_M for arachidonic acid and oxygen, T- and pH-dependence) and their substrate specificities were remarkably different. In contrast to mice and humans, tree shrew ALOX15 isoforms are highly expressed in the brain suggesting a role of these enzymes in cerebral function. The genomic multiplicity and the tissue expression patterns of tree shrew ALOX15 isoforms need to be considered when the results of in vivo inflammation studies obtained in this animal are translated into the human situation.

Probing the Electrostatic and Steric Requirements for Substrate Binding in Human Platelet-Type 12-Lipoxygenase

Human platelet ALOX12 (hALOX12 or h12-LOX) has been implicated in a variety of human diseases. The present study investigates the active site of hALOX12 to more thoroughly understand how it positions the substrate and achieves nearly perfect regio- and stereospecificities (i.e., 100 ± 5% of the 12(S)-hydroperoxide product), utilizing site-directed mutagenesis. Specifically, we have determined that Arg402 is not as important in substrate binding as previously seen for hALOX15 but that His596 may play a role in anchoring the carboxy terminal of the arachidonic acid during catalysis. In addition, Phe414 creates a π-stacking interaction with a double bond of arachidonic acid (Δ¹¹), and Ala417/Val418 define the bottom of the cavity. However, the influence of Ala417/Val418 on the profile is markedly less for hALOX12 than that seen in hALOX15. Mutating these two residues to larger amino acids (Ala417Ile/Val418Met) only increased the generation of 15-HpETE by 24 ± 2%, but conversely, smaller residues at these positions converted hALOX15 to almost 100% hALOX12 reactivity [Gan et al. (1996) J. Biol. Chem. 271, 25412-25418]. However, we were able to increase 15-HpETE to 46 ± 3% by restricting the width of the active site with the Ala417Ile/Val418Met/Ser594Thr mutation, indicating both depth and width of the active site are important. Finally, residue Leu407 is shown to play a critical role in positioning the substrate correctly, as seen by the increase of 15-HpETE to 21 ± 1% for the single Leu407Gly mutant. These results outline critical differences between the active site requirements of hALOX12 relative to hALOX15 and explain both their product specificity and inhibitory differences.

Recent development of lipoxygenase inhibitors as anti-inflammatory agents

Inflammation is favorable in most cases, because it is a kind of body defensive response to external stimuli; sometimes, inflammation is also harmful, such as attacks on the body's own tissues. It could be that inflammation is a unified process of injury and resistance to injury. Inflammation brings extreme pain to patients, showing symptoms of rubor, swelling, fever, pain and dysfunction. As the specific mechanism is not clear yet, the current anti-inflammatory agents are given priority for relieving suffering of patients. Thus it is emergent to find new anti-inflammatory agents with rapid effect. Lipoxygenase (LOX) is a kind of rate-limiting enzyme in the process of arachidonic acid metabolism into leukotriene (LT) which mediates the occurrence of inflammation. The inhibition of LOX can reduce LT, thereby producing an anti-inflammatory effect. In this review, the LOX inhibitors reported in recent years are summarized, and, in particular, their activities, structure-activity relationships and molecular docking studies are emphasized, which will provide new ideas to design novel LOX inhibitors.

Mutagenesis of Sequence Determinants of Truncated Porcine ALOX15 Induces Changes in the Reaction Specificity by Altering the Catalytic Mechanism of Initial Hydrogen Abstraction

The reaction specificity of lipoxygenases is of physiological relevance since the various oxygenation products exhibit different biological activities. Among mammalian ALOX15 orthologs there are arachidonic acid 12- and 15-lipoxygenating enzymes and recent studies suggested an evolutionary switch in that reaction specificity during late primate development. Previous reports showed that 12-lipoxygenating ALOX15 orthologs can be converted to 15-lipoxygenating enzymes by site-directed mutagenesis of some sequence determinants. Unfortunately, the molecular basis for those alterations are not well understood. Here, the arachidonic acid 12-lipoxygenating N-terminal truncation variant of pig ALOX15, for which a crystal structure is available, was used to explore the catalytic mechanism of the specificity switch induced by mutagenesis of Val418 and Val419 sequence determinants. We found that Val418Ile+Val419Met double mutant is dominantly 15-lipoxygenating. Docking and MD simulations, and quantum mechanics/molecular mechanics calculations indicated that the wildtype energy barrier for arachidonic acid 15-lipoxygenation is 3.4 kcal mol^-1 higher than for 12-lipoxygenation. In contrast, for the Val418Ile+Val419Met double mutant the energy barrier for 12-lipoxygenation is 6.0 kcal mol^-1 higher than for 15-lipoxygenation. Our data suggest that enzyme-substrate complex geometries determine the value of these energy barriers and, as a consequence, the reaction specificity of ALOX15 orthologs.

Structural and functional basis of phospholipid oxygenase activity of bacterial lipoxygenase from Pseudomonas aeruginosa

Pseudomonas aeruginosa expresses a secreted LOX-isoform (PA-LOX, LoxA) capable of oxidizing polyenoic fatty acids to hydroperoxy derivatives. Here we report high-level expression of this enzyme in E. coli and its structural and functional characterization. Recombinant PA-LOX oxygenates polyenoic fatty acids including eicosapentaenoic acid and docosahexaenoic acid to the corresponding (n-6)S-hydroperoxy derivatives. This reaction involves abstraction of the proS-hydrogen from the n-8 bisallylic methylene. PA-LOX lacks major leukotriene synthase activity but converts 5S-HETE and 5S,6R/S-DiHETE to anti-inflammatory and pro-resolving lipoxins. It also exhibits phospholipid oxygenase activity as indicated by the formation of a specific pattern of oxygenation products from different phospholipid subspecies. Multiple mutagenesis studies revealed that PA-LOX does not follow classical concepts explaining the reaction specificity of mammalian LOXs. The crystal structure of PA-LOX was solved with resolutions of up to 1.48Å and its polypeptide chain is folded as single domain. The substrate-binding pocket consists of two fatty acid binding subcavities and lobby. Subcavity-1 contains the catalytic non-heme iron. A phosphatidylethanolamine molecule occupies the substrate-binding pocket and its sn1 fatty acid is located close to the catalytic non-heme iron. His377, His382, His555, Asn559 and the C-terminal Ile685 function as direct iron ligands and a water molecule (hydroxyl) completes the octahedral ligand sphere. Although the biological relevance of PA-LOX is still unknown its functional characteristics (lipoxin synthase activity) implicate this enzyme in a bacterial evasion strategy aimed at downregulating the hosts' immune system.

Evolutionary alteration of ALOX15 specificity optimizes the biosynthesis of antiinflammatory and proresolving lipoxins

ALOX15 (12/15-lipoxygenase) orthologs have been implicated in maturational degradation of intracellular organelles and in the biosynthesis of antiinflammatory and proresolving eicosanoids. Here we hypothesized that lower mammals (mice, rats, pigs) express 12-lipoxygenating ALOX15 orthologs. In contrast, 15-lipoxygenating isoforms are found in higher primates (orangutans, men), and these results suggest an evolution of ALOX15 specificity. To test this hypothesis we first cloned and characterized ALOX15 orthologs of selected Catarrhini representing different stages of late primate evolution and found that higher primates (men, chimpanzees) express 15-lipoxygenating orthologs. In contrast, lower primates (baboons, rhesus monkeys) express 12-lipoxygenating enzymes. Gibbons, which are flanked in evolution by rhesus monkeys (12-lipoxygenating ALOX15) and orangutans (15-lipoxygenating ALOX15), express an ALOX15 ortholog with pronounced dual specificity. To explore the driving force for this evolutionary alterations, we quantified the lipoxin synthase activity of 12-lipoxygenating (rhesus monkey, mouse, rat, pig, humIle418Ala) and 15-lipoxygenating (man, chimpanzee, orangutan, rabbit, ratLeu353Phe) ALOX15 variants and found that, when normalized to their arachidonic acid oxygenase activities, the lipoxin synthase activities of 15-lipoxygenating ALOX15 variants were more than fivefold higher (P < 0.01) [corrected]. Comparative molecular dynamics simulations and quantum mechanics/molecular mechanics calculations indicated that, for the 15-lipoxygenating rabbit ALOX15, the energy barrier for C13-hydrogen abstraction (15-lipoxygenation) was 17 kJ/mol lower than for arachidonic acid 12-lipoxygenation. In contrast, for the 12-lipoxygenating Ile418Ala mutant, the energy barrier for 15-lipoxygenation was 10 kJ/mol higher than for 12-lipoxygenation. Taken together, our data suggest an evolution of ALOX15 specificity, which is aimed at optimizing the biosynthetic capacity for antiinflammatory and proresolving lipoxins.

Structural and functional biology of arachidonic acid 15-lipoxygenase-1 (ALOX15)

Lipoxygenases (LOX) form a family of lipid peroxidizing enzymes, which have been implicated in a number of physiological processes and in the pathogenesis of inflammatory, hyperproliferative and neurodegenerative diseases. They occur in two of the three domains of terrestrial life (bacteria, eucarya) and the human genome involves six functional LOX genes, which encode for six different LOX isoforms. One of these isoforms is ALOX15, which has first been described in rabbits in 1974 as enzyme capable of oxidizing membrane phospholipids during the maturational breakdown of mitochondria in immature red blood cells. During the following decades ALOX15 has extensively been characterized and its biological functions have been studied in a number of cellular in vitro systems as well as in various whole animal disease models. This review is aimed at summarizing the current knowledge on the protein-chemical, molecular biological and enzymatic properties of ALOX15 in various species (human, mouse, rabbit, rat) as well as its implication in cellular physiology and in the pathogenesis of various diseases.

Cell signalling by reactive lipid species: new concepts and molecular mechanisms

The process of lipid peroxidation is widespread in biology and is mediated through both enzymatic and non-enzymatic pathways. A significant proportion of the oxidized lipid products are electrophilic in nature, the RLS (reactive lipid species), and react with cellular nucleophiles such as the amino acids cysteine, lysine and histidine. Cell signalling by electrophiles appears to be limited to the modification of cysteine residues in proteins, whereas non-specific toxic effects involve modification of other nucleophiles. RLS have been found to participate in several physiological pathways including resolution of inflammation, cell death and induction of cellular antioxidants through the modification of specific signalling proteins. The covalent modification of proteins endows some unique features to this signalling mechanism which we have termed the ‘covalent advantage’. For example, covalent modification of signalling proteins allows for the accumulation of a signal over time. The activation of cell signalling pathways by electrophiles is hierarchical and depends on a complex interaction of factors such as the intrinsic chemical reactivity of the electrophile, the intracellular domain to which it is exposed and steric factors. This introduces the concept of electrophilic signalling domains in which the production of the lipid electrophile is in close proximity to the thiol-containing signalling protein. In addition, we propose that the role of glutathione and associated enzymes is to insulate the signalling domain from uncontrolled electrophilic stress. The persistence of the signal is in turn regulated by the proteasomal pathway which may itself be subject to redox regulation by RLS. Cell death mediated by RLS is associated with bioenergetic dysfunction, and the damaged proteins are probably removed by the lysosome-autophagy pathway.

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.

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.

Structure, biochemistry and biology of hepoxilins

Hepoxilins are biologically relevant epoxy-hydroxy eicosanoids synthesized through the 12S-lipoxygenase (12S-LOX) pathway of the arachidonic acid (AA) metabolism. The pathway is bifurcated at the level of 12S-hydroperoxy-eicosatetraenoic acid (12S-HpETE), which can either be reduced to 12S-hydro-eicosatetraenoic acid (12S-HETE) or converted to hepoxilins. The present review gives an update on the biochemistry, biology and clinical aspects of hepoxilin-based drug development. The isolation, cloning and characterization of a rat leukocyte-type 12S-LOX from rat insulinoma RINm5F cells revealed a 12S-LOX possessing an intrinsic 8S/R-hydroxy-11,12-epoxyeicosa-5Z,9E,14Z-trienoic acid (HXA(3)) synthase activity. Site-directed mutagenesis studies on rat 12S-LOX showed that the HXA(3) synthase activity was impaired when the positional specificity of AA was altered. Interestingly, amino acid Leu353, and not conventional sequence determinants Met419 and Ile418, was found to be a crucial sequence determinant for AA oxygenation. The regulation of HXA(3) formation is dependent on the cellular overall peroxide tone. Cellular glutathione peroxidases (cGPxs) compete with HXA(3) synthase for 12S-HpETE as substrate either to reduce to 12S-HETE or to convert to HXA(3), respectively. Therefore, RINm5F cells, which are devoid of GPxs, are capable of converting AA or 12S-HpETE to HXA(3) under basal conditions, whereas cells overexpressing cGPx are unable to do so. HXA(3) exhibits a myriad of biological effects, most of which are associated with the stimulation of intracellular calcium or the transport of calcium across the membrane. The activation of HXA(3)-G-protein-coupled receptors explains many of the extracellular effects of HXA(3), including AA- and diacylglycerol (DAG) release in human neutrophils, insulin secretion in rat pancreatic beta-cells or islets, and synaptic actions in the brain. The availability of stable analogs of HXA(3), termed 10-hydroxy-11,12-cyclopropyl-eicosa-5Z,8Z,14Z-trienoic acid derivatives (PBTs), recently made several animal studies possible and explored the role of HXA(3) as a therapeutic in treatment of diseases. Thus, PBT-3 induced apoptosis in K562 tumour cells and inhibited growth of K562 CML solid tumours in nude mice. HXA(3) inhibited bleomycin-evoked lung fibrosis and inflammation in mice and the raised insulin level in the circulation of rats. At low glucose concentrations (0-3 mm), HXA(3) also stimulated insulin secretion in RINm5F cells through the activation of IRE1alpha, an endoplasmic reticulum-resident kinase. The latter regulates the protein folding for insulin biosynthesis. In conclusion, HXA(3)-mediated signaling may be involved in normal physiological functions, and hepoxilin-based drugs may serve as therapeutics in diseases such as type II diabetes and idiopathic lung fibrosis.

Crystal structure of an iron-dependent group III dehydrogenase that interconverts L-lactaldehyde and L-1,2-propanediol in Escherichia coli

The FucO protein, a member of the group III "iron-activated" dehydrogenases, catalyzes the interconversion between L-lactaldehyde and L-1,2-propanediol in Escherichia coli. The three-dimensional structure of FucO in a complex with NAD(+) was solved, and the presence of iron in the crystals was confirmed by X-ray fluorescence. The FucO structure presented here is the first structure for a member of the group III bacterial dehydrogenases shown experimentally to contain iron. FucO forms a dimer, in which each monomer folds into an alpha/beta dinucleotide-binding N-terminal domain and an all-alpha-helix C-terminal domain that are separated by a deep cleft. The dimer is formed by the swapping (between monomers) of the first chain of the beta-sheet. The binding site for Fe(2+) is located at the face of the cleft formed by the C-terminal domain, where the metal ion is tetrahedrally coordinated by three histidine residues (His200, His263, and His277) and an aspartate residue (Asp196). The glycine-rich turn formed by residues 96 to 98 and the following alpha-helix is part of the NAD(+) recognition locus common in dehydrogenases. Site-directed mutagenesis and enzyme kinetic assays were performed to assess the role of different residues in metal, cofactor, and substrate binding. In contrast to previous assumptions, the essential His267 residue does not interact with the metal ion. Asp39 appears to be the key residue for discriminating against NADP(+). Modeling L-1,2-propanediol in the active center resulted in a close approach of the C-1 hydroxyl of the substrate to C-4 of the nicotinamide ring, implying that there is a typical metal-dependent dehydrogenation catalytic mechanism.

Hepoxilin A3 synthase

Hepoxilins constitute a group of 12S-hydroperoxyeicosatetraenoic acid (12S-HpETE)-derived epoxy-hydroxy fatty acids that have been detected in various cell types and tissues. Although hepoxilin A3 (HXA3) exhibits a myriad of biological activities, its biosynthetic mechanism was not investigated in detail. Here we review the isolation, cloning, and characterization of a leukocyte-type 12S-lipoxygenase (12S-LOX) from rat insulinoma cells RINm5F, which exhibits an intrinsic hepoxilin A3 synthase activity. Confirmation for this observation was achieved by coimmunoprecipitation of HXA3 synthase activity with an anti-leukocyte 12S-LOX antibody, preparation of recombinant rat 12S-LOX enzyme from RINm5F cells, and assay of HXA3 synthase activity therein. Site-directed mutagenesis studies performed on rat 12S-LOX showed that 12-lipoxygenating enzyme species exhibit a strong HXA3 synthase activity that is impaired when the positional specificity of arachidonic acid is altered in favor of 15-lipoxygenation. Inasmuch as cellular glutathione peroxidases (cGPx and PHGPx) and HXA3 synthase compete for the same substrate 12S-HpETE, it can be proposed that the overall activity of glutathione peroxidases, representing the overall peroxide tone, finely tunes the rate of HXA3 formation.

Arachidonate 12-lipoxygenases with reference to their selective inhibitors

Lipoxygenase is a dioxygenase recognizing a 1-cis,4-cis-pentadiene of polyunsaturated fatty acids. The enzyme oxygenates various carbon atoms of arachidonic acid as a substrate and produces 5-, 8-, 12- or 15-hydroperoxyeicosatetraenoic acid with a conjugated diene chromophore. The enzyme is referred to as 5-, 8-, 12- or 15-lipoxygenase, respectively. Earlier we found two isoforms of 12-lipoxygenase, leukocyte- and platelet-type enzymes, which were distinguished by substrate specificity, catalytic activity, primary structure, gene intron size, and antigenicity. Recently, the epidermis-type enzyme was found as the third isoform. Attempts have been made to find isozyme-specific inhibitors of 12-lipoxygenase, and earlier we found hinokitiol, a tropolone, as a potent inhibitor selective for the platelet-type 12-lipoxygenase. More recently, we tested various catechins of tea leaves and found that (-)-gallocatechin gallate was a potent and selective inhibitor of human platelet 12-lipoxygenase with an IC50 of 0.14 microM. The compound was much less active with 12-lipoxygenase of leukocyte-type, 15-, 8-, and 5-lipoxygenases, and cyclooxygenases-1 and -2.

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

Mammalian lipoxygenases (LOXs) are categorized with respect to their positional specificity of arachidonic acid oxygenation. Site-directed mutagenesis identified sequence determinants for the positional specificity of these enzymes, and a critical amino acid for the stereoselectivity was recently discovered. To search for sequence determinants of murine (12R)-LOX, we carried out multiple amino acid sequence alignments and found that Phe(390), Gly(441), Ala(455), and Val(631) align with previously identified positional determinants of S-LOX isoforms. Multiple site-directed mutagenesis studies on Phe(390) and Ala(455) did not induce specific alterations in the reaction specificity, but yielded enzyme species with reduced specific activities and stereo random product patterns. Mutation of Gly(441) to Ala, which caused drastic alterations in the reaction specificity of other LOX isoforms, failed to induce major alterations in the positional specificity of mouse (12R)-LOX, but markedly modified the enantioselectivity of the enzyme. When Val(631), which aligns with the positional determinant Ile(593) of rabbit 15-LOX, was mutated to a less space-filling residue (Ala or Gly), we obtained an enzyme species with augmented catalytic activity and specifically altered reaction characteristics (major formation of chiral (11R)-hydroxyeicosatetraenoic acid methyl ester). The importance of Val(631) for the stereo control of murine (12R)-LOX was confirmed with other substrates such as methyl linoleate and 20-hydroxyeicosatetraenoic acid methyl ester. These data identify Val(631) as the major sequence determinant for the specificity of murine (12R)-LOX. Furthermore, we conclude that substrate fatty acids may adopt different catalytically productive arrangements at the active site of murine (12R)-LOX and that each of these arrangements may lead to the formation of chiral oxygenation products.

Mammalian arachidonate 15-lipoxygenases structure, function, and biological implications

Lipoxygenases (LOXs) constitute a heterogeneous family of lipid peroxidizing enzymes capable of oxygenating polyunsaturated fatty acids to their corresponding hydroperoxy derivatives. In mammals, LOXs are classified with respect to their positional specificity of arachidonic acid oxygenation into 5-, 8-, 12-, and 15-LOXs. Arachidonate 15-LOXs may be sub-classified into a reticulocyte-type (type-1) and an epidermis-type (type-2) enzyme. Since the leukocyte-type 12-LOXs are very similar to the reticulocyte-type 15-LOXs, these enzymes are designated 12/15-LOXs. Several LOX isoforms, in particular the reticulocyte-type 15-LOX and the human 5-LOX, are well characterized with respect to their structural and functional properties On the other hand, the biological role of most LOX-isozymes including the reticulocyte-type 15-LOC is far from clear. This review is intended to summarize the recent developments in 15-LOX research with particular emphasis to molecular enzymology and regulation of gene expression. In addition, the major hypotheses on the physiological and patho-physiological roles of 15-LOXs will be discussed briefly.

Arachidonate 12-lipoxygenases

Arachidonate 12-lipoxygenase introduces a molecular oxygen at carbon 12 of arachidonic acid to generate a 12-hydroperoxy derivative. The enzymes generate 12-hydroperoxy derivatives with either S- or R-configurations. There are three isoforms of 12S-lipoxygenases named after the cells where they were first identified; platelet, leukocyte and epidermis. The leukocyte-type enzyme is widely distributed among cells, but the tissue distribution varies substantially from species to species. The platelet and epidermal enzymes are present in only a relatively limited number of cell types. Although the structures and enzymatic properties of the three isoforms of 12S-lipoxygenases have been elucidated, the physiological roles of the 12S-lipoxygenases are not yet fully understood. There are important roles for the enzymes and their products in several biological systems including those involved in atherosclerosis and neurotransmission.

Mutagenesis and modelling of linoleate-binding to pea seed lipoxygenase

We have produced a model to define the linoleate-binding pocket of pea 9/13-lipoxygenase and have validated it by the construction and characterization of eight point mutants. Three of the mutations reduced, to varying degrees, the catalytic centre activity (kcat) of the enzyme with linoleate. In two of the mutants, reductions in turnover were associated with changes in iron-coordination. Multiple sequence alignments of recombinant plant and mammalian lipoxygenases of known positional specificity, and the results from numerous other mutagenesis and modelling studies, have been combined to discuss the possible role of the mutated residues in pea 9/13-lipoxygenase catalysis. A new nomenclature for recombinant plant lipoxygenases based on positional specificity has subsequently been proposed. The null-effect of mutating pea 9/13-lipoxygenase at the equivalent residue to that which controlled dual positional specificity in cucumber 13/9-lipoxygenase, strongly suggests that the mechanisms controlling dual positional specificity in pea 9/13-lipoxygenase and cucumber 13/9-lipoxygenase are different. This was supported from modelling of another isoform of pea lipoxygenase, pea 13/9-lipoxygenase. Dual positional specificity in pea lipoxygenases is more likely to be determined by the degree of penetration of the methyl terminus of linoleate and the volume of the linoleate-binding pocket rather than substrate orientation. A single model for positional specificity, that has proved to be inappropriate for arachidonate-binding to mammalian 5-, 12- and 15-lipoxygenases, would appear to be true also for linoleate-binding to plant 9- and 13-lipoxygenases.

Structural basis for lipoxygenase specificity. Conversion of the human leukocyte 5-lipoxygenase to a 15-lipoxygenating enzyme species by site-directed mutagenesis

Mammalian lipoxygenases constitute a heterogeneous family of lipid-peroxidizing enzymes, and the various isoforms are categorized with respect to their positional specificity of arachidonic acid oxygenation into 5-, 8-, 12-, and 15-lipoxygenases. Structural modeling suggested that the substrate binding pocket of the human 5-lipoxygenase is 20% bigger than that of the reticulocyte-type 15-lipoxygenase; thus, reduction of the active-site volume was suggested to convert a 5-lipoxygenase to a 15-lipoxygenating enzyme species. To test this "space-based" hypothesis of the positional specificity, the volume of the 5-lipoxygenase substrate binding pocket was reduced by introducing space-filling amino acids at critical positions, which have previously been identified as sequence determinants for the positional specificity of other lipoxygenase isoforms. We found that single point mutants of the recombinant human 5-lipoxygenase exhibited a similar specificity as the wild-type enzyme but double, triple, and quadruple mutations led to a gradual alteration of the positional specificity from 5S- via 8S- toward 15S-lipoxygenation. The quadruple mutant F359W/A424I/N425M/A603I exhibited a major 15S-lipoxygenase activity (85-95%), with (8S,5Z,9E,11Z,14Z)-8-hydroperoxyeicosa-5,9 ,11, 14-tetraenoic acid being a minor side product. These data indicate the principle possibility of interconverting 5- and 15-lipoxygenases by site-directed mutagenesis and appear to support the space-based hypothesis of positional specificity.

Hinokitiol, a selective inhibitor of the platelet-type isozyme of arachidonate 12-lipoxygenase

Hinokitiol (4-isopropyltropolone), a constituent of Japanese cypress, reversibly inhibited platelet-type 12-lipoxygenase with an IC(50) of 0.1 microM, and the enzyme activity was almost lost at 1 microM. The compound was much less active with other lipoxygenase enzymes with higher IC(50) values (leukocyte-type 12-lipoxygenase, 50 microM; soybean lipoxygenase, 17 microM; 15-lipoxygenase-1, >100 microM; 5-lipoxygenase, 17 microM). Hinokitiol up to 100 microM had almost no effect on cyclooxygenases-1 and -2. Their structure-activity relationship examined with various tropolone derivatives indicated the requirements of the 2-hydroxyl group and 4-alkyl group for the potent and selective inhibition of platelet-type 12-lipoxygenase.

Structural basis for the positional specificity of lipoxygenases

The positional specificity of arachidonic acid oxygenation is currently the decisive parameter for classification of mammalian lipoxygenases but, unfortunately, the structural reasons for lipoxygenase specificity are not well understood. Although there are no direct structural data on lipoxygenase/substrate interaction, experiments with modified fatty acid substrates and mutagenesis studies suggest that for 12- and 15-lipoxygenases, arachidonic acid slides into the substrate-binding pocket with its methyl end ahead. For arachidonate 5- and/or 8-lipoxygenation two alternative models for the enzyme/substrate interaction have been developed: 1) The orientation-determined model and 2) the space-determined model. This review explores the experimental data available on the mechanistic reasons for lipoxygenase specificity and concludes that each of the above-mentioned hypotheses may be valid for arachidonate 5-lipoxygenation under certain circumstances.

Shape and specificity in mammalian 15-lipoxygenase active site. The functional interplay of sequence determinants for the reaction specificity

Previous mutagenesis studies along with molecular modeling using the x-ray coordinates of the rabbit 15-lipoxygenase have led to the suggestion that the size of the substrate binding pocket may play an essential role in determining the oxygenation specificity of 5-, 12-, and 15-lipoxygenases. Based on the x-ray crystal structure of rabbit 15-lipoxygenase, Ile(593) appeared to be important in defining size and shape of the substrate-binding site in 15-lipoxygenases. We found that substitution of Ile(593) with alanine shifted the positional specificity of this enzyme toward 12-lipoxygenation. To compare the importance of position 593 with previously defined determinants for the oxygenation specificity, we introduced small (alanine-scan) or large amino acids (phenylalanine-scan) at critical positions surrounding the putative fatty acid-binding site, so that the volume of the pocket was either increased or decreased. Enlargement or alteration in packing density within the substrate binding pocket in the rabbit 15-lipoxygenase increased the share of 12-lipoxygenase products, whereas a smaller active site favored 15-lipoxygenation. Simultaneous substitution of both large and small residues in the context of either a 15- or 12-lipoxygenase indicated that there is a functional interplay of the sequence determinants for lipoxygenation specificity. If the 15-lipoxygenase active site is enlarged excessively, however, no lipoxygenation was observed anymore. Together these results indicate the importance of the overall size and shape of the arachidonic acid binding pocket in defining the specificity of lipoxygenase reaction.

Inhibitory effect of phospholipid hydroperoxide glutathione peroxidase on the activity of lipoxygenases and cyclooxygenases

The partially purified phospholipid hydroperoxide glutathione peroxidase (PHGPx) from A431 cells was used to systematically compare the inhibitory effect on the enzyme activity of various lipoxygenases and cyclooxygenases. Under the standard assay system, platelet 12-lipoxygenase, 15-lipoxygenase, and cyclooxygenase-2 were the most sensitive to the inhibition by PHGPx. 5-Lipoxygenase and cyclooxygenase-1 were less sensitive to the inhibition by PHGPx than platelet 12-lipoxygenase and cyclooxygenase-2, respectively, and the difference was approximately 10-fold. Reduction of 12(S)-hydroperoxyeicosatetraenoic acid to 12(S)-hydroxyeicosatetraenoic acid by PHGPx was observed in the presence of glutathione (GSH), and the inhibitory effect of PHGPx on 12-lipoxygenase-catalyzed arachidonate metabolism was reversed by the addition of exogenous lipid hydroperoxide. The results indicate that PHGPx directly reduced lipid hydroperoxides and then down-regulated the activity of arachidonate oxygenases. Moreover, a high-level expression of PHGPx mRNA and its 12-lipoxygenase-inhibitory activity was observed in cancer cells and endothelial cells, and these results suggest that PHGPx may play a significant role in the regulation of reactive oxygen species formation in these cells.

Conversion of cucumber linoleate 13-lipoxygenase to a 9-lipoxygenating species by site-directed mutagenesis

Multiple lipoxygenase sequence alignments and structural modeling of the enzyme/substrate interaction of the cucumber lipid body lipoxygenase suggested histidine 608 as the primary determinant of positional specificity. Replacement of this amino acid by a less-space-filling valine altered the positional specificity of this linoleate 13-lipoxygenase in favor of 9-lipoxygenation. These alterations may be explained by the fact that H608V mutation may demask the positively charged guanidino group of R758, which, in turn, may force an inverse head-to-tail orientation of the fatty acid substrate. The R758L+H608V double mutant exhibited a strongly reduced reaction rate and a random positional specificity. Trilinolein, which lacks free carboxylic groups, was oxygenated to the corresponding (13S)-hydro(pero)xy derivatives by both the wild-type enzyme and the linoleate 9-lipoxygenating H608V mutant. These data indicate the complete conversion of a linoleate 13-lipoxygenase to a 9-lipoxygenating species by a single point mutation. It is hypothesized that H608V exchange may alter the orientation of the substrate at the active site and/or its steric configuration in such a way that a stereospecific dioxygen insertion at C-9 may exclusively take place.

Expression of leukocyte-type 12-lipoxygenase and reticulocyte-type 15-lipoxygenase in rabbits

From a rabbit reticulocyte library a full length cDNA was isolated which predicted a novel lipoxygenase (LOX) sharing 99% identical amino acids with the rabbit 15-lipoxygenase. HPLC product analysis of the bacterially expressed protein identified it as a leukocyte-type 12-lipoxygenase (1.12-LOX). This proves the co-expression of a 15-lipoxygenase and a 1.12-lipoxygenase in one mammalian species. Among the six amino acids that are different to rabbit 15-lipoxygenase, leucine 353 is shown to be the primary determinant for 12-positional specificity. In the 3'-untranslated region of the 12-LOX-mRNA a CU-rich, 20-fold repetitive element has been found, closely related to the differentiation control element (DICE) of the rabbit 15-LOX-mRNA which is organized by ten repeats of 19 bases. By genomic PCR the 3'-terminal part of the gene for the novel 12-lipoxygenase containing the introns 10-13 has been amplified and sequenced. The introns were very similar in length to the corresponding 15-lipoxygenase introns with 89% to 95% identical nucleotide sequences. By screening a rabbit reticulocyte library an alternative 15-lipoxygenase transcript of 3.6 kb has been detected containing a 1019 nucleotides longer 3'-untranslated region (UTR2) than the main 2.6 kb mRNA. The determination of the tissue distribution by Northern blotting showed that the 3.6 kb mRNA2 was only expressed in non-erythroid tissues, whereas the 2.6 kb mRNA1 was exclusively expressed in reticulocytes. The only cell type which has been found to express the 1.12-lipoxygenase abundantly are monocytes. The results indicate that the expression of 1.12-lipoxygenase and 15-lipoxygenase is highly regulated. The UTR2 of the 15-LOX-mRNA2 contained a novel eight-fold repetitive CU-rich motif of 23 bases length which is related but not identical to the DICE of 19 bases in the UTR1. The analysis of a genomic recombinant of the complete 9.0 kb Alox15 gene confirmed that UTR1 and UTR2 are not interrupted by an additional intron.

Affinities of various mammalian arachidonate lipoxygenases and cyclooxygenases for molecular oxygen as substrate

In an attempt to study affinities for molecular oxygen of mammalian arachidonate oxygenases, which remain unclarified at present, we determined activities of platelet-type 12-lipoxygenase, leukocyte-type 12-lipoxygenase, 5-lipoxygenase, 15-lipoxygenase, cyclooxygenase-1 and cyclooxygenase-2 at various oxygen concentrations. Activities of all the tested enzymes were assessed by oxygenation of radioactive arachidonic acid under hypoxic conditions, and part of the enzymes were also assayed by monitoring oxygen consumption. Their Km values for oxygen ranged between 10 and 26 microM. These results should be considered in investigations of arachidonic acid metabolism in pathophysiological processes associated with hypoxia.

Intra- and extracellular expression of rabbit reticulocyte 15-lipoxygenase in the Baculovirus/insect cell system

Rabbit reticulocyte 15-lipoxygenase was expressed as intracellular enzyme and as export protein in High Five cells. While intracellular expression in the baculovirus/insect cell system was already reported for various mammalian lipoxygenases, we have developed a strategy for secretion of this cytosolic enzyme using the signal sequence of human interleukin-2. Expression levels of 10 mg/liter (intracellular strategy) and 18 mg/liter (extracellular strategy) were obtained. The recombinant enzyme expressed as intracellular protein was purified to apparent homogeneity by anion-exchange chromatography on a Mono-Q column with an overall recovery of 80% enzyme activity. For the final enzyme preparation, a specific linoleic acid oxygenase activity of 16.7 micromol 13-hydroperoxyoctadeca-9,11-dieic acid formation mg-1 min-1 was determined, which corresponds to a molecular turnover number of 21 s-1. Similar turnover rates have been reported for the native rabbit 15-lipoxygenase. Extracellularly expressed recombinant 15-lipoxygenase exhibited a heterogeneity in anion-exchange chromatography. Three major peaks of 15-lipoxygenase activity were eluted from a Mono-Q column and the relative amounts of these isoforms varied from batch to batch of enzyme expression. One of the major isoenzymes which cochromatographed with the native 15-lipoxygenase was purified to homogeneity from the cell-free culture supernatant and exhibited a specific activity of 5.1 micromol 13-hydroperoxyoctadeca-9,11-dienoic acid formation mg-1 min-1 (turnover rate of 6.1 s-1). The recombinant enzyme species were characterized with respect to their protein-chemical and enzymatic properties and no differences to the native rabbit 15-lipoxygenase were detected.

Simultaneous expression of leukocyte-type 12-lipoxygenase and reticulocyte-type 15-lipoxygenase in rabbits

In rabbit reticulocytes an arachidonic acid 15-lipoxygenase (15-LOX) is expressed at high yield. Rescreening a rabbit reticulocyte cDNA library for alternative 15-LOX transcripts, a full length cDNA which encodes a novel lipoxygenase was isolated. The predicted amino acid sequence of this enzyme shared a high degree (99%) of identity with the reticulocyte-type 15-lipoxygenase. Among the six amino acid residues different in both enzymes a Phe-Leu exchange was detected at position 353. Recently, site-directed mutagenesis studies have revealed that this amino acid exchange converts a 15-lipoxygenase to a 12-lipoxygenase. In fact, when the novel enzyme was expressed in Escherichia coli, mainly 12-lipoxygenation of arachidonic acid was observed. The recombinant enzyme exhibited a rather broad substrate specificity. Various C-18 and C-20 polyenoic fatty acids and even complex substrates such as biomembranes were effectively oxygenated. Thus, the novel enzyme may be classified as leukocyte-type 12-lipoxygenase. Genomic polymerase chain reaction of the 3' region of the leukocyte-type 12-lipoxygenase gene indicated that introns 10 to 13 differed to about 10% from the corresponding sequences of the 15-lipoxygenase gene although their size and the intron-exon organization were very similar. In the 3'-untranslated region of the novel mRNA a C+U-rich, 20-fold repetitive element was found which appears to be highly related to the differentiation control element of the 15-lipoxygenase mRNA. Activity assays with a variety of cells and tissues prepared from normal rabbits suggested that only peripheral monocytes abundantly express the enzyme, suggesting a tissue-specific regulation of gene expression. These data indicate for the first time the co-expression of two separate genes for a reticulocyte-type 15-lipoxygenase and for a leukocyte-type 12-lipoxygenase in one species. This is of importance for the implication of both enzymes in red blood cell development and atherogenesis.

The iron ligand sphere geometry of mammalian 15-lipoxygenases

We investigated the geometry of the iron ligand sphere of the native rabbit 15-lipoxygenase (15-LOX) by X-ray absorption spectroscopy using synchrotron radiation. The soybean LOX-1 was used as a reference compound because its iron ligand sphere is well characterized. For structural information the X-ray absorption spectra were evaluated using the Excurve Program (CCLRC Daresbury Laboratory, Warrington, U.K.). From the positions of the absorption edges and from the intensities of the 1s-3d pre-edge transition peaks a six-coordinate ferrous iron was concluded for the rabbit 15-LOX. Evaluation of the extended region of the absorption spectra suggested six nitrogen and/or oxygen atoms as direct iron ligands, and the following binding distances were determined (means+/-S.D.; estimated accuracy is +/-0.001nm for bond distances, on the basis of more than 22 X-ray absorption spectra): 0.213+/-0.001nm, 0.213+/-0. 001 nm, 0.236+/-0.001 nm, 0.293+/-0.001 nm, 0.189+/-0.001 nm and 0. 242+/-0.001. Lyophilization of the LOX altered the binding distances but did not destroy the octahedral iron ligand sphere. For construction of a structural model of the iron ligand sphere the binding distances extracted from the X-ray spectra were assigned to specific amino acids (His-360, -365, -540, -544 and the C-terminal Ile-662) by molecular modelling using the crystal coordinates of the soybean LOX-1 and of a rabbit 15-LOX-inhibitor complex.

A selective inhibitor of arachidonate 5-lipoxygenase scavenging peroxide activator

A novel compound termed YT-18 (2,3-dihydro-2,4,6,7-tetramethyl-2-[(4-phenyl-1-piperazinyl) methyl]-5-benzofuranamine) selectively inhibited 5-lipoxygenases of porcine leukocytes (IC50 value, 7.5 microM), human leukocytes (1.5 microM), and rat basophilic leukemia cells (14 microM), which are responsible for bioactive leukotriene synthesis. In contrast, the compound up to 1 mM had almost no effect on 12-lipoxygenases of leukocytes and platelets, 15-lipoxygenase, and cyclooxygenases-1 and -2. YT-18 also inhibited the leukotriene synthesis in intact rat basophilic leukemia cells. In the 5-lipoxygenase reaction, YT-18 caused a lag phase, thereby delaying the start of the reaction. The lag was abolished by the addition of 13-hydroperoxy-linoleic acid in a dose-dependent manner, and most (but not all) of the reduced 5-lipoxygenase activity was recovered.

Production of arachidonic acid metabolites by the colon adenocarcinoma cell line HT29 cl.19A and their effect on chloride secretion

Eicosanoids were found in large amounts in the colonic mucosa of patients suffering from inflammatory bowel diseases and colonic adenocarcinoma. The aim of this study was to evaluate the role of the intestinal epithelial cells in the arachidonic acid metabolism and their functional response to certain eicosanoids. We used the human adenocarcinoma epithelial cell line HT29 cl.19A cell, which is an in vitro model of colon carcinoma and ion transport. These cells were found to express 5- and 15-lipoxygenase, leukotriene A4 hydrolase and cyclooxygenase-1 and -2 mRNAs. We observed an arachidonic acid metabolism via 5-lipoxygenase pathway despite the lack of FLAP mRNA expression and that certain eicosanoids such as hydroperoxy- and hydroxyeicosatetraenoic acids stimulate chloride secretion.

Differential inactivation of alcohol dehydrogenase isoenzymes in Zymomonas mobilis by oxygen

Zymomonas mobilis is endowed with two isoenzymes of fermentative alcohol dehydrogenase, a zinc-containing enzyme (ADH I) and an iron-containing enzyme (ADH II). The activity of ADH I remains fully conserved, while ADH II activity decays when anaerobic cultures are shifted to aerobiosis. This differential response depends on the metal present on each isoenzyme, since pure preparations of ADH I are resistant to oxidative inactivation and preparations of zinc-containing ADH II, obtained by incubation of pure ADH II with ZnCl2, showed no modification of the target for oxidative damage (His277-containing peptide). It was consistently found that the activity of the zinc-containing ADH II, once submitted to oxidative treatment, was fully restored when iron was reintroduced into the enzyme structure. These results indicate that zinc bound to these proteins plays an important role in the protection of their active centers against oxidative damage and may have relevant biochemical and physiological consequences in this species.

Phenylalanine 353 is a primary determinant for the positional specificity of mammalian 15-lipoxygenases

Mammalian lipoxygenases are implicated in the biosynthesis of inflammatory mediators, in the pathogenesis of atherosclerosis and in the process of blood cell differentiation and maturation. With respect to their reaction specificity, three major types of mammalian lipoxygenases (15-lipoxygenases, 12-lipoxygenases and 5-lipoxygenases) may be classified. Although this nomenclature is commonly used, the mechanistic reasons for the positional specificity of lipoxygenases are not well understood. We investigated the structural reasons for lipoxygenase specificity by a combination of chimera formation and site-directed mutagenesis, and identified phenylalanine 353 as primary determinant for the positional specificity of rabbit reticulocyte 15-lipoxygenase. Modeling of the enzyme-substrate interaction suggested that the alignment of arachidonic acid at the active site appears to be influenced by this residue. According to the substrate orientation, the 15-lipoxygenase may be differentiated from two types of mammalian 12-lipoxygenases.

Defining the arachidonic acid binding site of human 15-lipoxygenase. Molecular modeling and mutagenesis

Mammalian lipoxygenases have been implicated in the pathogenesis of several inflammatory disorders and are, therefore, important targets for drug discovery. Both plant and mammalian lipoxygenases catalyze the dioxygenation of polyunsaturated fatty acids, which contain one or more 1,4-cis,cis-pentadiene units to yield hydroperoxide products. At the time this study was initiated, soybean lipoxygenase-1 was the only lipoxygenase for which an atomic resolution structure had been determined. No structure of lipoxygenase with substrate or inhibitor bound is currently available. A model of arachidonic acid docked into the proposed substrate binding site in the soybean structure is presented here. Analysis of this model suggested two residues, an aromatic residue and a positively charged residue, could be critical for substrate binding. Validation of this model is provided by site-directed mutagenesis of human 15-lipoxygenase, despite the low amino acid sequence identity between the soybean and mammalian enzymes. Both a positively charged amino acid residue (Arg402) and an aromatic amino acid residue (Phe414) are identified as critical for the binding of fatty acid substrates in human 15-lipoxygenase. Thus, binding determinants shown to be characteristic of non-enzymatic fatty acid-binding proteins are now implicated in the substrate binding pocket of lipoxygenases.

The environment of the lipoxygenase iron binding site explored with novel hydroxypyridinone iron chelators

The mechanisms of lipoxygenase inhibition by iron chelators have been investigated in human neutrophils and in isolated soybean lipoxygenase. Their Fe(III)-containing active sites have been targeted by synthesizing novel bidentate chelators from the hydroxypyridinone family sufficiently small to gain access through the hydrophobic channels of lipoxygenase. In stimulated human neutrophils, release of [3H]arachidonate-labeled eicosanoids is dependent on the lipid solubility of hydroxypyridinones, but larger hexadentate chelators have no effect on this or on total cellular leukotriene B4 production. Lipophilic hydroxypyridinones inhibit 5-lipoxygenase at equivalent concentrations to the established inhibitor, piriprost, and show additional but minor anti-phospholipase A2 activity. Soybean 15-lipoxygenase inhibition is also dependent on the lipid solubility and coordination structure of chelators. Inhibition is associated with the formation of chelate-iron complexes, which are removed by dialysis without restoration of enzyme activity. Only after adding back iron is activity restored. Electron paramagnetic resonance studies show the removal of the iron center signal (g = 6) is concomitant with formation of Fe(III)-chelator complexes, identical in spectral shape and g value to 3:1 hydroxypyridinone Fe(III) complexes. Removal of iron is not the only mechanism by which hydroxypyridinones can inhibit lipoxygenase in intact cells, however, as a lipophilic non-iron-binding hydroxypyridinone, which shows no inhibition of the soybean lipoxygenase activity, partially inhibits 5-lipoxygenase in intact neutrophils without inhibiting neutrophil phospholipase A2.

Suicide inactivation of porcine leukocyte 12-lipoxygenase associated with its incorporation of 15-hydroperoxy-5,8,11,13-eicosatetraenoic acid derivative

Two isozymes of arachidonate 12-lipoxygenase, platelet-type and leukocyte-type, which were distinguished by their substrate specificities and primary structures, were investigated with reference to 'suicide' inactivation. Upon reaction with arachidonic acid the leukocyte-type enzyme was inactivated rapidly during the catalysis, whereas the platelet-type enzyme did not show such a rapid inactivation. The two 12-lipoxygenase isozymes were incubated with various hydroperoxy and hydroxy products from arachidonic acid. (15S)-Hydroperoxy-5,8,11,13-eicosatetraenoic acid (15-HPETE) was found to be a unique substrate of the leukocyte-type 12-lipoxygenase as follows. (1) 15-HPETE was an active substrate for porcine leukocyte 12-lipoxygenase, and converted anaerobically to a 14,15-epoxy compound (14,15-leukotriene A4). (2) A rapid inactivation of the enzyme was observed within 2 min upon aerobic and anaerobic incubations with 15-HPETE. (3) 15-HPETE was rapidly incorporated into the enzyme in a nearly equimolar amount under both aerobic and anaerobic conditions. (4) Several findings suggested a covalent binding of 15-HPETE or its derivative to the enzyme. (5) Such a rapid and stoichiometric incorporation of 15-HPETE was not observed with the platelet-type 12-lipoxygenase. On the basis of these findings we presumed that 15-HPETE was transformed to 14,15-leukotriene A4, which was covalently bound to the leukocyte-type 12-lipoxygenase leading to the suicide inactivation of the enzyme.

Structure conservation in lipoxygenases: structural analysis of soybean lipoxygenase-1 and modeling of human lipoxygenases

Lipoxygenases are a class of non-heme iron dioxygenases which catalyze the hydroperoxidation of fatty acids for the biosynthesis of leukotrienes and lipoxins. The structure of the 839-residue soybean lipoxygenase-1 was used as a template to model human 5-, 12-, and 15-lipoxygenases. A distance-based algorithm for placing side chains in a low homology environment (only the four iron ligands were fixed during side chain placement) was devised. Twenty-six of the 56 conserved lipoxygenase residues were grouped in four distinct regions of the enzyme. These regions were analyzed to discern whether the side chain interactions could be duplicated in the models or whether alternate conformers should be considered. The effects of site directed mutagenesis variants were rationalized using the models of the human lipoxygenases. In particular, variants which shifted positional specificity between 12- and 15-lipoxygenase activity were analyzed. Analysis of active site residues produced a model which accounts for observed lipoxygenase positional specificity and stereospecificity.

Novel 10-substituted antipsoriatic anthrones as inhibitors of epidermal 12-lipoxygenase and lipid peroxidation in membranes

The ability of novel 10-substituted anthrones to inhibit 12-lipoxygenase (12-LO) in mouse epidermal homogenate and lipid peroxidation in both bovine brain phospholipid liposomes and erythrocyte ghosts was investigated, and compared with their ability to inhibit 5-lipoxygenase (5-LO) in bovine leukocytes. The compounds were fairly potent inhibitors of epidermal 12-LO, in addition to their strong inhibitory effects against leukocyte 5-LO. Although the antipsoriatic drug, anthralin, predominantly inhibited epidermal 12-LO, the novel derivatives were more selective 5-LO inhibitors. Compounds with free phenolic groups in the attached aromatic ring were also potent inhibitors of nonenzymatic lipid peroxidation in both sources of lipid substrate. This property was not correlated with their ability to inhibit the 5- and 12-LO pathways, suggesting that their mechanism of 5-/12-LO inhibition is not simply due to scavenging of peroxyl radicals generated at the active site of the enzymes. The compounds are dual-purpose inhibitors and may play a protective role against oxidative damage to psoriatic skin, in addition to their antiinflammatory 5-LO and 12-LO inhibitory properties.