Targeting Lipid Peroxidation for Cancer Treatment

Cancer is one of the highest prevalent diseases in humans. The chances of surviving cancer and its prognosis are very dependent on the affected tissue, body location, and stage at which the disease is diagnosed. Researchers and pharmaceutical companies worldwide are pursuing many attempts to look for compounds to treat this malignancy. Most of the current strategies to fight cancer implicate the use of compounds acting on DNA damage checkpoints, non-receptor tyrosine kinases activities, regulators of the hedgehog signaling pathways, and metabolic adaptations placed in cancer. In the last decade, the finding of a lipid peroxidation increase linked to 15-lipoxygenases isoform 1 (15-LOX-1) activity stimulation has been found in specific successful treatments against cancer. This discovery contrasts with the production of other lipid oxidation signatures generated by stimulation of other lipoxygenases such as 5-LOX and 12-LOX, and cyclooxygenase (COX-2) activities, which have been suggested as cancer biomarkers and which inhibitors present anti-tumoral and antiproliferative activities. These findings support the previously proposed role of lipid hydroperoxides and their metabolites as cancer cell mediators. Depletion or promotion of lipid peroxidation is generally related to a specific production source associated with a cancer stage or tissue in which cancer originates. This review highlights the potential therapeutical use of chemical derivatives to stimulate or block specific cellular routes to generate lipid hydroperoxides to treat this disease.

ROS-dependent Syk and Pyk2-mediated STAT1 activation is required for 15(S)-hydroxyeicosatetraenoic acid-induced CD36 expression and foam cell formation

15(S)-Hydroxyeicosatetraenoic acid (15(S)-HETE), the major 15-lipoxygenase 1/2 (15-LO1/2) metabolite of arachidonic acid (AA), induces CD36 expression through xanthine oxidase and NADPH oxidase-dependent ROS production and Syk and Pyk2-dependent STAT1 activation. In line with these observations, 15(S)-HETE also induced foam cell formation involving ROS, Syk, Pyk2, and STAT1-mediated CD36 expression. In addition, peritoneal macrophages from Western diet-fed ApoE(-/-) mice exhibited elevated levels of xanthine oxidase and NADPH oxidase activities, ROS production, Syk, Pyk2, and STAT1 phosphorylation, and CD36 expression compared to those from ApoE(-/-):12/15-LO(-/-) mice and these events correlated with increased lipid deposits, macrophage content, and lesion progression in the aortic roots. Human atherosclerotic arteries also showed increased 15-LO1 expression, STAT1 phosphorylation, and CD36 levels as compared to normal arteries. Together, these findings suggest that 12/15-LO metabolites of AA, particularly 12/15(S)-HETE, might play a crucial role in atherogenesis by enhancing foam cell formation.

Structure based design and synthesis of peptide inhibitor of human LOX-12: in vitro and in vivo analysis of a novel therapeutic agent for breast cancer

Human breast cancer cell proliferation involves a complex interaction between growth factors, steroid hormones and peptide hormones. The interaction of growth factors, such as epidermal growth factor (EGF), with their receptors on breast cancer cells can lead to the hydrolysis of phospholipids and release of fatty acid such as arachidonic acid, which can be further metabolized by cyclooxygenase (COX) and lipoxygenase (LOX) pathways to produce prostaglandins. The high concentration of prostaglandins has been associated with chronic inflammatory diseases and several types of human cancers. This is due to the over expression COX, LOX and other inflammatory enzymes. Ten peptides were designed and synthesized by solid phase peptide synthesis and analyzed in vitro for enzyme inhibition. Out of these peptides, YWCS had shown significant inhibitory effects. The dissociation constant (K_D) was determined by surface plasmon resonance (SPR) analysis and was found to be 3.39×10⁻⁸ M and 8.6×10⁻⁸ M for YWCS and baicalein (positive control), respectively. The kinetic constant Ki was 72.45×10⁻⁷ M as determined by kinetic assay. The peptide significantly reduced the cell viability of estrogen positive MCF-7 and estrogen negative MDA-MB-231 cell line with the half maximal concentration (IC₅₀) of 75 µM and 400 µM, respectively. The peptide also induced 49.8% and 20.8% apoptosis in breast cancer cells MCF-7 and MDA-MB-231, respectively. The YWCS was also found to be least hemolytic at a concentration of 358 µM. In vivo studies had shown that the peptide significantly inhibits tumor growth in mice (p<0.017). This peptide can be used as a lead compound and complement for ongoing efforts to develop differentiation therapies for breast cancer.

Cloning, expression, purification, crystallization and preliminary X-ray diffraction studies of a 12R-LOX-chaperone complex

Lipoxygenases are a family of nonheme iron-containing dioxygenases. An Escherichia coli expression system producing the bacterial chaperones GroES and GroEL was engineered and successfully used to produce large quantities of recombinant human 12R-LOX (LOXR; MW 80.34 kDa; 701 amino-acid residues). The co-overproduction of the two chaperones with 12R-LOX resulted in increased solubility of 12R-LOX and allowed the purification of milligram amounts of active enzyme for structural studies by X-ray diffraction. The lipoxygenase protein was purified on an affinity column and a gel-filtration column with chaperone protein (MW 57.16 kDa). The LOXR-chaperone complex was crystallized with ligand by the hanging-drop vapor-diffusion method using 1.5 M ammonium hydrogen phosphate as precipitant. The crystals belonged to the monoclinic system, space group P2(1), with unit-cell parameters a = 138.97, b = 266.11, c = 152.26 Å, β = 101.07°. Based on the calculated Matthews coefficient (3.1 Å(3) Da(-1)), it is estimated that one molecule of LOXR complexed with two molecules of chaperone is present in the asymmetric unit of the crystal lattice. X-ray diffraction data were collected to 4 Å resolution using synchrotron radiation.

Cloning, purification and characterization of non-human primate 12/15-lipoxygenases

The enzyme 15-lipoxygenase-1 (15-LO-1) possesses mainly 15-LO activity and has so far only been described in human cells and rabbit reticulocytes. The animal ortholog, except rabbit reticulocytes, is an enzyme with predominantly a 12-lipoxygenase activity, commonly referred to as 12/15-LO. We describe herein the characterization of the 12/15-LOs in Macaca mulatta (rhesus monkey) and in Pongo pygmaeus (orang-utan). The rhesus and the orang-utan enzymes have mainly 12-lipoxygenase and 15-lipoxygenase activity, respectively, and they display 94% and 98% identity to the human 15-LO-1 protein. The rhesus enzyme was functionally different from the human enzyme with respect to substrate utilization in that anandamide was used differently and that the rhesus enzymes positional specificity could be affected by the substrate concentration. Furthermore, genomic data indicate that chimpanzees express an enzyme with mainly 15-lipoxygenase activity whereas marmosets express an enzyme with mainly 12-LO activity. Taken together, the switch during evolution from a 12-lipoxygenating enzyme in lower primates to a 15-lipoxygenating enzyme in higher primates and man might be of importance for the biological function of this enzyme.

EPR spectroscopy and electrospray ionization mass spectrometry reveal distinctive features of the iron site in leukocyte 12-lipoxygenase

The procedure for the expression and purification of recombinant porcine leukocyte 12-lipoxygenase using Escherichia coli [K.M. Richards, L.J. Marnett, Biochemistry 36 (1997) 6692-6699] was updated to make it possible to produce enough protein for physical measurements. Electrospray ionization tandem mass spectrometry confirmed the amino acid sequence. The redox properties of the cofactor iron site were examined by EPR spectroscopy at 25K following treatment with a variety of fatty acid hydroperoxides. Combination of the enzyme in a stoichiometric ratio with the hydroperoxides led to a g4.3 signal in EPR spectra instead of the g6 signal characteristic of similarly treated soybean lipoxygenase-1. Native 12-lipoxygenase was also subjected to electrospray ionization mass spectrometry. There was evidence for loss of the mass of an iron atom from the protein as the pH was lowered from 5 to 4. Native ions in these samples indicated that iron was lost without the protein completely unfolding.

Inhibitory activity of salicylic acid on lipoxygenase-dependent lipid peroxidation

BACKGROUND

Since iron is essential for lipoxygenase activity and salicylic acid (SA) can interact with the metal, possible lipoxygenase inhibition by SA was investigated.

METHODS

Kinetic spectrophotometric evaluation of enzymatic lipid peroxidation catalyzed by soybean lipoxygenase (SLO), rabbit reticulocyte 15-lipoxygenase (RR15-LOX), porcine leukocyte 12-lipoxygenase (PL12-LOX) and human recombinant 5-lipoxygenase (HR5-LOX) with and without SA.

RESULTS

SA inhibited linoleic, arachidonic and docosahexaenoic acid or human lipoprotein peroxidation catalyzed by SLO with IC50 of, respectively, 107, 153, 47 and 108 microM. Using the same substrates, SA inhibited RR15-LOX with IC50 of, respectively, 49, 63, 27 and 51 microM. Further, arachidonic acid peroxidation catalyzed by PL12-LOX and HR5-LOX was inhibited by SA with IC50 of 101 and 168 microM, respectively. Enzymatic inhibition was complete, reversible and non-competitive. Conceivably due to its lower hydrophobicity, aspirin was less effective, indicating acetylation-independent enzyme inhibition. SA and aspirin were ineffective peroxyl radical scavengers but readily reduced Fe3+, i.e. FeCl3, to Fe2+, suggesting their capacity to reduce Fe3+ at the enzyme active site. Indeed, similar to the catecholic redox inhibitor nordihydroguaiaretic acid, SA inhibited with the same efficiency both ferric and the native ferrous SLO form, indicating that these compounds reduce the active ferric enzyme leading to its inactivation.

GENERAL SIGNIFICANCE

SA can inhibit lipoxygenase-catalyzed lipid peroxidation at therapeutic concentrations, suggesting its possible inhibitory activity against enzymatic lipid peroxidation in the clinical setting.

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.

Human platelet 12-lipoxygenase, new findings about its activity, membrane binding and low-resolution structure

Human platelet 12-lipoxygenase (hp-12LOX, 662 residues+iron nonheme cofactor) and its major metabolite 12S-hydroxyeicosatetraenoic acid have been implicated in cardiovascular and renal diseases, many types of cancer and inflammatory responses. However, drug development is slow due to a lack of structural information. The major hurdle in obtaining a high-resolution X-ray structure is growing crystals, a process that requires the preparation of highly homogenous, reproducible and stable protein samples. To understand the properties of hp-12LOX, we have expressed and studied the behavior, function and low-resolution structure of the hp-12LOX His-tagged recombinant enzyme and its mutants in solution. We have found that it is a dimer easily converted into bigger aggregates, which are soluble/covalent-noncovalent/reversible. The heavier oligomers show a higher activity at pH 8, in contrast to dimers with lower activity showing two maxima at pH 7 and pH 8, indicating the existence of two different conformers. In the seven-point C-->S mutant, aggregation is diminished, activity has one broad peak at pH 8 and there is no change in specificity. Truncation of the N(t)-beta-barrel domain (PLAT, residues 1-116) reduces activity to approximately 20% of that shown by the whole enzyme, does not affect regio- or stereospecificity and lowers membrane binding by a factor of approximately 2. "NoPLAT" mutants show strong aggregation into oligomers containing six or more catalytic domains regardless of the status of the seven cysteine residues tested. Time-of-flight mass spectrometry suggests two arachidonic acid molecules bound to one molecule of enzyme. Small angle X-ray scattering studies (16 A resolution, chi approximately 1) suggest that two hp-12LOX monomers are joined by the catalytic domains, with the PLAT domains floating on the flexible linkers away from the main body of the dimer.

Molecular dioxygen enters the active site of 12/15-lipoxygenase via dynamic oxygen access channels

Cells contain numerous enzymes that use molecular oxygen for their reactions. Often, their active sites are buried deeply inside the protein, which raises the question whether there are specific access channels guiding oxygen to the site of catalysis. Choosing 12/15-lipoxygenase as a typical example for such oxygen-dependent enzymes, we determined the oxygen distribution within the protein and defined potential routes for oxygen access. For this purpose, we have applied an integrated strategy of structural modeling, molecular dynamics simulations, site-directed mutagenesis, and kinetic measurements. First, we computed the 3D free-energy distribution for oxygen, which led to identification of four oxygen channels in the protein. All channels connect the protein surface with a region of high oxygen affinity at the active site. This region is localized opposite to the nonheme iron providing a structural explanation for the reaction specificity of this lipoxygenase isoform. The catalytically most relevant path can be obstructed by L367F exchange, which leads to a strongly increased Michaelis constant for oxygen. The blocking mechanism is explained in detail by reordering the hydrogen-bonding network of water molecules. Our results provide strong evidence that the main route for oxygen access to the active site of the enzyme follows a channel formed by transiently interconnected cavities whereby the opening and closure are governed by side chain dynamics.

Human and mouse eLOX3 have distinct substrate specificities: implications for their linkage with lipoxygenases in skin

Genetic and biochemical evidence suggests a functional link between human 12R-lipoxygenase (12R-LOX) and epidermal lipoxygenase-3 (eLOX3) in normal differentiation of the epidermis; LOX-derived fatty acid hydroperoxide is isomerized by the atypical eLOX3 into a specific epoxyalcohol that is a potential mediator in the pathway. Mouse epidermis expresses a different complement of LOX enzymes, and therefore this metabolic linkage could differ. To test this concept, we compared the substrate specificities of recombinant mouse and human eLOX3 toward sixteen hydroperoxy stereoisomers of arachidonic and linoleic acids. Both enzymes metabolized R-hydroperoxides 2-3 times faster than the corresponding S enantiomers. Whereas 12R-hydroperoxyeicosatetraenoic acid (12R-HPETE) is the best substrate for human eLOX3 (2.4 s(-1); at 30 microM substrate), mouse eLOX3 shows the highest turnover with 8R-HPETE (2.9 s(-1)) followed by 8S-HPETE (1.3 s(-1)). Novel product structures were characterized from reactions of mouse eLOX3 with 5S-, 8R-, and 8S-HPETEs. 8S-HPETE is converted specifically to a single epoxyalcohol, identified as 10R-hydroxy-8S,9S-epoxyeicosa-5Z,11Z,14Z-trienoic acid. The substrate preference of mouse eLOX3 and the unique occurrence of an 8S-LOX enzyme in mouse skin point to a potential LOX pathway for the production of epoxyalcohol in murine epidermal differentiation.

Synthetic curcuminoids modulate the arachidonic acid metabolism of human platelet 12-lipoxygenase and reduce sprout formation of human endothelial cells

Platelet 12-lipoxygenase (P-12-LOX) is overexpressed in different types of cancers, including prostate cancer, and the level of expression is correlated with the grade of this cancer. Arachidonic acid is metabolized by 12-LOX to 12(S)-hydroxyeicosatetraenoic acid [12(S)-HETE], and this biologically active metabolite is involved in prostate cancer progression by modulating cell proliferation in multiple cancer-related pathways inducing angiogenesis and metastasis. Thus, inhibition of P-12-LOX can reduce these two processes. Several lipoxygenase inhibitors are known, including plant and mammalian lipoxygenases, but only a few of them are known inhibitors of P-12-LOX. Curcumin is one of these lipoxygenase inhibitors. Using a homology model of the three-dimensional structure of human P-12-LOX, we did computational docking of synthetic curcuminoids (curcumin derivatives) to identify inhibitors superior to curcumin. Docking of the known inhibitors curcumin and NDGA to P-12-LOX was used to optimize the docking protocol for the system in study. Over 75% of the compounds of interest were successfully docked into the active site of P-12-LOX, many of them sharing similar binding modes. Curcuminoids that did not dock into the active site did not inhibit P-12-LOX. From a set of the curcuminoids that were successfully docked and selected for testing, two were found to inhibit human lipoxygenase better than curcumin. False-positive curcuminoids showed high LogP (theoretical) values, indicating poor water solubility, a possible reason for lack of inhibitory activity or/and nonrealistic binding. Additionally, the curcuminoids inhibiting P-12-LOX were tested for their ability to reduce sprout formation of endothelial cells (in vitro model of angiogenesis). We found that only curcuminoids inhibiting human P-12-LOX and the known inhibitor NDGA reduced sprout formation. Only limited inhibition of sprout formation at approximately IC(50) concentrations has been seen. At IC(50), a substantial amount of 12-HETE can be produced by lipoxygenase, providing a stimulus for angiogenic sprouting of endothelial cells. Increasing the concentration of lipoxygenase inhibitors above IC(50), thus decreasing the concentration of 12(S)-HETE produced, greatly reduced sprout formation for all inhibitors tested. This universal event for all tested lipoxygenase inhibitors suggests that the inhibition of sprout formation was most likely due to the inhibition of human P-12-LOX but not other cancer-related pathways.

Novel human lipoxygenase inhibitors discovered using virtual screening with homology models

We report the discovery of new, low micromolar, small molecule inhibitors of human platelet-type 12- and reticulocyte 15-lipoxygenase-1 (12-hLO and 15-hLO) using structure-based methods. Specifically, we created homology models of 12-hLO and 15-hLO, based on the structure of rabbit 15-lipoxygenase, for in silico screening of a large compound library followed by in vitro screening of 20 top scoring molecules. Eight of these compounds inhibited either 12- or 15-human lipoxygenase with lower than 100 microM affinity. Of these, we obtained IC50 values for the three best inhibitors, all of which displayed low micromolar inhibition. One compound showed specificity for 15-hLO versus 12-hLO; however, a selective inhibitor for 12-hLO was not identified. As a control we screened 20 randomly selected compounds, of which none showed low micromolar inhibition. The new low-micromolar inhibitors appear to be suitable as leads for further inhibitor development efforts against 12-hLO and 15-hLO, based on the fact their size and chemical properties are appropriate to classify them as drug-like compounds. The models of these protein-inhibitor complexes suggest strategies for future development of selective lipoxygenase inhibitors.

Affinity Labeling of the Rabbit 12/15-Lipoxygenase Using Azido Derivatives of Arachidonic Acid

Lipoxygenases are lipid-peroxidizing enzymes, which have been implicated in the pathogenesis of important diseases. They consist of a single polypeptide chain, which is folded into a two-domain structure. The large catalytic domain contains the putative substrate-binding pocket and the catalytic non-heme iron. To identify structural elements of the rabbit 12/15-lipoxygenase that are involved in enzyme/substrate and/or enzyme/product interaction, we synthesized a set of radioactively labeled lipoxygenase substrates carrying a photoreactive azido group (17-azido-ETE, 18-azido-ETE, 19-azido-ETE) and used these compounds as affinity probes. After photoaffinity labeling, the enzyme was digested proteolytically and modified tryptic cleavage peptides were identified by a combination of radio-HPLC and mass spectral analysis. Following this strategy, we observed covalent linkage of a cleavage peptide that contained Ile593, which has previously been identified as the sequence determinant for the positional specificity. These data are consistent with the previous suggestion that this peptide lines the substrate-binding pocket. Surprisingly, we also observed strong labeling of cleavage peptides originating from the N-terminal beta-barrel domain, and our mass spectral data suggested covalent linkage of oxidized affinity probes. Taken together, these results confirm the previous conclusion that Ile593 and surrounding amino acids are constituents of the active site, but they also implicate the N-terminal beta-barrel in enzyme/substrate and/or enzyme/product interaction.

The role of lipoxygenase-isoforms in atherogenesis

Lipoxygenases (LOXs) form a heterogeneous family of lipid-peroxidizing enzymes, and several LOX-isoforms (12/15-LOX, 5-LOX) have been implicated in atherogenesis. However, the precise role of these enzymes is still a matter of discussion. 12/15-LOXs are capable of oxidizing lipoproteins (low-density lipoprotein (LDL), high-density lipoprotein (HDL)) to atherogenic forms, and functional inactivation of this enzyme in murine atherosclerosis models slows down lesion formation. In contrast, rabbits that overexpress this enzyme were protected from lesion formation when fed a lipid-rich diet. To contribute to this discussion, we recently investigated the impact of 12/15-LOX overexpression on in vitro foam cell formation. When 12/15-LOX-transfected J774 cells were incubated in culture with modified LDL, we found that intracellular lipid deposition was reduced in the transfected cells when compared with the corresponding control transfectants. This paper briefly summarizes the current status of knowledge on the biological activity of different LOX-isoforms in atherogenesis and will also provide novel experimental data characterizing the role of 12/15-LOX in cellular LDL modification and for in vitro foam cell formation.

Hsp90α Recruited by Sp1 Is Important for Transcription of 12(S)-Lipoxygenase in A431 Cells

Sp1 is a basic transcriptional factor that binds to the GC-rich region in the promoter of the target gene. It is involved in transcription of numerous genes by recruiting transcriptional factors to the promoters of target genes. In this study, we found in vivo and in vitro that Hsp90alpha was recruited to the GC-rich region of the 12(S)-lipoxygenase promoter through interaction with Sp1 in A431 cells by employing DNA affinity immunoprecipitation assay and chromatin immunoprecipitation assay. When Hsp90alpha was inhibited by geldanamycin (GA, a specific inhibitor of the Hsp90 family) or by siRNA of Hsp90alpha (to block its activity or to knockdown protein levels), respectively, luciferase activity (driven by the 12(S)-lipoxygenase promoter) and both mRNA and protein levels of 12(S)-lipoxygenase were reduced significantly in cells. In addition, the effect of GA was abolished when the Sp1 binding sites of 12(S)-lipoxygenase were mutated in A431 cells. Interestingly, binding of Sp1 to the 12(S)-lipoxygenase promoter was also decreased upon GA treatment in cells. In conclusion, our results indicate that Sp1 interacts with Hsp90alpha to recruit it to the promoter of 12(S)-lipoxygenase and then to regulate gene transcription by modulating the binding ability of Sp1 to promoters.

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.

A multifunctional lipoxygenase with fatty acid hydroperoxide cleaving activity from the moss Physcomitrella patens

A complex mixture of fatty acid-derived aldehydes, ketones, and alcohols is released upon wounding of the moss Physcomitrella patens. To investigate the formation of these oxylipins at the molecular level we isolated a lipoxygenase from P. patens, which was identified in an EST library by sequence homology to lipoxygenases from plants. Sequence analysis of the cDNA showed that it exhibits a domain structure similar to that of type2 lipoxygenases from plants, harboring an N-terminal import signal for chloroplasts. The recombinant protein was identified as arachidonate 12-lipoxygenase and linoleate 13-lipoxygenase with a preference for arachidonic acid and eicosapentaenoic acid. In contrast to any other lipoxygenase cloned so far, this enzyme exhibited in addition an unusual high hydroperoxidase and also a fatty acid chain-cleaving lyase activity. Because of these unique features the pronounced formation of (2Z)-octen-1-ol, 1-octen-3-ol, the dienal (5Z,8Z,10E)-12-oxo-dodecatrienoic acid and 12-keto eicosatetraenoic acid was observed when arachidonic acid was administered as substrate. 12-Hydroperoxy eicosatetraenoic acid was found to be only a minor product. Moreover, the P. patens LOX has a relaxed substrate tolerance accepting C(18)-C(22) fatty acids giving rise to even more LOX-derived products. In contrast to other lipoxygenases a highly diverse product spectrum is formed by a single enzyme accounting for most of the observed oxylipins produced by the moss. This single enzyme might, in a fast and effective way, be involved in the formation of signal and/or defense molecules thus contributing to the broad resistance of mosses against pathogens.

Probing the activity differences of simple and complex brominated aryl compounds against 15-soybean, 15-human, and 12-human lipoxygenase

Lipoxygenases (LO) have been implicated in asthma, immune disorders, and various cancers. As a consequence of these broad biological implications, there is great interest in understanding the effects of naturally occurring and environmental contaminants against its activity. On the basis of our earlier studies indicating that polybrominated diphenol ethers are potent inhibitors to mammalian 15-LO, we expanded our structure-activity study to include marine-derived brominated phenol ethers (including a newly discovered tribrominated diphenyl ether), dioxins, and bastadins, as well as the synthetic brominated fire retardants, brominated bisphenol A (BBPA), and polybrominated diphenyl ethers (PBDEs). We report herein the effects of 21 simple and complex organobromine compounds against human platelet 12-LO, human reticulocyte 15-LO, and soybean 15-LO-1.

Kinetic investigations of the rate-limiting step in human 12- and 15-lipoxygenase

Mammalian lipoxygenases have been implicated in several inflammatory disorders; however, the details of the kinetic mechanism are still not well understood. In this paper, human platelet 12-lipoxygenase (12-hLO) and human reticulocyte 15-lipoxygenase-1 (15-hLO) were tested with arachidonic acid (AA) and linoleic acid (LA), respectively, under a variety of changing experimental conditions, such as temperature, dissolved oxygen concentration, and viscosity. The data that are presented show that 12-hLO and 15-hLO have slower rates of product release (k(cat)) than soybean lipoxygenase-1 (sLO-1), but similar or better rates of substrate capture for the fatty acid (k(cat)/K(M)) or molecular oxygen [k(cat)/K(M(O)2)]. The primary, kinetic isotope effect (KIE) for 15-hLO with LA was determined to be temperature-independent and large ((D)k(cat) = 40 +/- 8), over the range of 10-35 degrees C, indicating that C-H bond cleavage is the sole rate-limiting step and proceeds through a tunneling mechanism. The (D)k(cat)/K(M) for 15-hLO, however, was temperature-dependent, consistent with our previous results [Lewis, E. R., Johansen, E., and Holman, T. R. (1999) J. Am. Chem. Soc. 121, 1395-1396], indicating multiple rate-limiting steps. This was confirmed by a temperature-dependent, k(cat)/K(M) solvent isotope effect (SIE), which indicated a hydrogen bond rearrangement step at low temperatures, similar to that of sLO-1 [Glickman, M. H., and Klinman, J. P. (1995) Biochemistry 34, 14077-14092]. The KIE could not be determined for 12-hLO due to its inability to efficiently catalyze LA, but the k(cat)/K(M) SIE was temperature-independent, indicating distinct rate-limiting steps from both 15-hLO and sLO-1.

Anti-inflammatory activity of Chinese medicinal vine plants

Anti-inflammatory activities of ethanol extracts from nine vine plants used in traditional Chinese medicine to treat inflammatory conditions were evaluated against a panel of key enzymes relating to inflammation. The enzymes included cyclooxygenase-1 (COX-1), cyclooxygenase-2 (COX-2), phospholipase A(2) (PLA(2)), 5-lipoxygenase (5-LO) and 12-lipoxygenase (12-LO). The vine plants studied were: the stem of Spatholobus suberectus Dunn, the stem of Trachelospermum jasminoides Lem., the root from Tripterygium wilfordii Hook. f., the stem of Sinomenium acutum Rehder and Wilson, the stem of Piper kadsura (Choisy) Ohwi, the stem of Polygonum multiflorum Thunb., the root and stem from Tinospora sagittata Gagnep., the root of Tinospora sinensis (Lour.) Merrill, and the stem of Clematis chinensis Osbeck. All of the plant extracts showed inhibitory activities against at least one of the enzymes in various percentages depending upon the concentrations. The extract from S. suberectus was found to be active against all enzymes except COX-2. Its IC(50) values were 158, 54, 31 and 35 microg/ml in COX-1, PLA(2), 5-LO and 12-LO assays, respectively. T. jasminoides showed potent inhibitory activities against both COX-1 (IC(50) 35 microg/ml) and PLA(2) (IC(50) 33 microg/ml). The most potent COX-1, COX-2 and 5-LO inhibition was observed in the extract of T. wilfordii with the IC(50) values of 27, 125 and 22 microg/ml, respectively. The findings of this study may partly explain the use of these vine plants in traditional Chinese medicine for the treatment of inflammatory conditions.

Kinetics of inhibition of leukocyte 12-lipoxygenase by the isoform-specific inhibitor 4-(2-oxapentadeca-4-yne)phenylpropanoic acid

Lipoxygenases (LOXs) are a ubiquitous family of enzymes that catalyze the dioxygenation of polyunsaturated fatty acids. Their role in a diverse range of biological processes has prompted the development of a large number of lipoxygenase inhibitors of possible therapeutic and probative value. The isoform-selective inhibitor 4-(2-oxapentadeca-4-yne)phenylpropanoic acid (OPP) was previously shown to inhibit leukocyte-type 12-LOX by a novel mechanism in which it binds to both the ferrous and ferric forms of the enzyme. The current study provides a detailed kinetic model of this inhibition. Nonlinear regression analysis of OPP's inhibition of arachidonic acid dioxygenation indicated mixed inhibition toward the ferric form of 12-LOX with apparent K(I) values in the low micromolar range: 2.0 +/- 0.2 microM for the free enzyme and 4.5 +/- 0.7 microM for the substrate-bound form of the enzyme. Rapid kinetic techniques allowed OPP's inhibition of the activation of the enzyme from the ferrous to the ferric form to be investigated. Titration of ferrous 12-LOX with OPP indicated that it bound to the ferrous form with an apparent K(I) value of 70 +/- 20 nM, suggesting a significantly higher affinity for the ferrous form than for the ferric form of the enzyme. Investigation of the LOX inhibitors nordihydroguaiaretic acid, N-(4-chlorophenyl)-N-hydroxy-N'-(3-chlorophenyl)urea, BWA137C, and eicosatetraynoic acid revealed that eicosatetraynoic acid also inhibited the activation of 12-LOX. These results demonstrate that LOX inhibitors are capable of binding to multiple forms of LOXs with high affinity and suggest that inhibition of enzyme activation may be an unrecognized mechanism of inhibition of additional LOX inhibitors.

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.

Selective oxygenation of the endocannabinoid 2-arachidonylglycerol by leukocyte-type 12-lipoxygenase

The endogenous cannabinoid system appears to serve vascular, neurological, immunological, and reproductive functions. The identification of 2-arachidonylglycerol (2-AG) as an endogenous ligand for the central (CB1) and peripheral (CB2) cannabinoid receptors has prompted interest in enzymes capable of modifying or inactivating this endocannabinoid. Porcine leukocyte 12-liopoxygenase (12-LOX) oxygenated 2-AG to the 2-glyceryl ester of 12(S)-hydroperoxyeicosa-5,8,10,14-tetraenoic acid (12-HPETE-G). The k(cat)/K(M) for oxygenation of 2-AG was 40% of the value for arachidonic acid. In contrast to the results with leukocyte 12-LOX, 2-AG oxygenation was not detected with platelet-type 12-LOX. Among a series of structurally related arachidonyl esters, 2-AG served as the preferential substrate for leukocyte 12-LOX. 12(S)-Hydroxyeicosa-5,8,10,14-tetraenoic acid glyceryl ester (12-HETE-G) was produced following addition of 2-AG to COS-7 cells transiently transfected with leukocyte 12-LOX. These results demonstrate that leukocyte-type 12-LOX efficiently oxidizes 2-AG in vitro and in intact cells, suggesting a role for this oxygenase in the endogenous cannabinoid system.

Characterization of the murine epidermal 12/15-lipoxygenase

The murine lipoxygenase (LO) family consists of at least seven members classified according to the HETE (hydroxyeicosatetraenoic acid) metabolite generated during arachidonic acid metabolism and the site of tissue expression. At present there are four 12-lipoxygenases that are functionally distinct, vary in cell and tissue distribution, catalytic activity and each are products of separate, linked genes. They are "platelet-type" 12-LO (P-12LO), "leukocyte-type" 12-LO (L-12LO), "epidermal-type" 12-LO (e-12LO) and the most recently discovered 12(R)-LO. In this report we characterize e-12LO, which was overexpressed in the baculovirus/insect cell expression system. The enzyme functions as a dual specificity 12/15-lipoxygenase with a 12-HETE/15-HETE product ratio of approximately 6:1 with arachidonic acid as substrate. Several other polyunsaturated fatty acids served as substrates for e-12LO such as gamma-linolenic, dihomo-gamma-linolenic and eicosapentaenoic acids. A green fluorescent protein/e-12LO fusion protein was localized to the cytosol of transfected HEK 293 cells. The e-12LO gene was expressed in mouse oocytes and early embryos. Western blot analysis revealed high level expression in postnatal day 3 mouse epidermal lysates. Together these data suggest that e-12LO plays a role in normal epidermal function and as yet an undiscovered role in early development.

The N-terminal domain of 5-lipoxygenase binds calcium and mediates calcium stimulation of enzyme activity

Human 5-lipoxygenase (5-LO) is a key enzyme in the conversion of arachidonic acid into leukotrienes and lipoxins, mediators and modulators of inflammation. In this study, we localized a stimulatory Ca(2+)-binding site to the N-terminal region of the enzyme. Thus, in a (45)Ca(2+) overlay assay, the N-terminal 128 amino acids of recombinant human 5-LO (fused to glutathione S-transferase) bound radioactive calcium to about the same extent as intact 5-LO. The glutathione S-transferase fusion protein of the C-terminal part of 5-LO (amino acids 120-673) showed much weaker binding. A model of a putative 5-LO N-terminal domain was calculated based on the structure of rabbit reticulocyte 15-LO. This model resembles beta-sandwich C2 domains of other Ca(2+)-binding proteins. Comparison of our model with the C2 domain of cytosolic phospholipase A(2) suggested a number of amino acids, located in the loops that connect the beta-strands, as potential Ca(2+) ligands. Indeed, mutations particularly in loop 2 (N43A, D44A, and E46A) led to decreased Ca(2+) binding and a requirement for higher Ca(2+) concentrations to stimulate enzyme activity. Our data indicate that an N-terminal beta-sandwich of 5-LO functions as a C2 domain in the calcium regulation of enzyme activity.

Mechanism of inhibition of porcine leukocyte 12-lipoxygenase by the isoform-specific inhibitor 4-(2-oxapentadeca-4-yne)phenylpropanoic acid

The mechanism of inhibition of porcine leukocyte 12-lipoxygenase by 4-(2-oxapentadeca-4-yne)phenylpropanoic acid (OPP) was investigated. This compound is selective for the leukocyte form of the 12-lipoxygenase and inhibits the purified recombinant enzyme with an IC(50) value of approximately 2 microM. OPP induced a concentration-dependent lag phase in the oxygenation of arachidonic acid and decreased the maximal rate of reaction. Addition of the fatty acid hydroperoxide 13(S)-hydroperoxyoctadecadienoic acid (13-HPODE) to the reaction greatly reduced the OPP-induced lag. Lineweaver-Burk analysis of the effect of OPP on 12-lipoxygenase kinetics with arachidonic acid indicated that it was a mixed-type inhibitor. OPP was not metabolized by 12-lipoxygenase as evidenced by its quantitative recovery from incubations with stoichiometric amounts of enzyme and 13-HPODE or arachidonic acid. OPP inhibited the pseudoperoxidase activity of the enzyme with 13-HPODE and the reducing agent, BWA137C. Lineweaver-Burk analysis of the effect of OPP on pseudoperoxidase kinetics suggested that OPP was competitive with 13-HPODE. Single-turnover experiments indicated that OPP inhibited the reduction of 13-HPODE by a stoichiometric amount of ferrous 12-lipoxygenase. Addition of 13-HPODE shortened the OPP-induced lag phase but did not affect the maximal rate of enzyme activity. In addition, OPP had no effect on total product formation in either the presence or the absence of 5 microM 13-HPODE when the reaction was allowed to go to completion. All of these observations are consistent with a model for inhibition of 12-lipoxygenase activity in which OPP slows the oxidation of the inactive ferrous enzyme to the active ferric enzyme and competes with arachidonic acid for the ferric enzyme.

cDNA cloning of 15-lipoxygenase type 2 and 12-lipoxygenases of bovine corneal epithelium

Bovine corneal epithelium contains arachidonate 12- and 15-lipoxygenase activity, while human corneal epithelium contains only 15-lipoxygenase activity. Our purpose was to identify the corneal 12- and 15-lipoxygenase isozymes. We used cDNA cloning to isolate the amino acid coding nucleotide sequences of two bovine lipoxygenases. The translated sequence of one lipoxygenase was 82% identical with human 15-lipoxygenase type 2 and 75% identical with mouse 8-lipoxygenase, whereas the other translated nucleotide sequence was 87% identical with human 12-lipoxygenase of the platelet type. Expression of 15-lipoxygenase type 2 and platelet type 12-lipoxygenase mRNAs were detected by Northern analysis. In addition to these two lipoxygenases, 12-lipoxygenase of leukocyte (tracheal) type was detected by polymerase chain reaction (PCR), sequencing, and Northern analysis. Finally, PCR and sequencing suggested that human corneal epithelium contains 15-lipoxygenase types 1 and 2.

cDNA cloning of a 8-lipoxygenase and a novel epidermis-type lipoxygenase from phorbol ester-treated mouse skin

Using a combination of PCR cloning and conventional screening procedures, we isolated from phorbol ester-treated mouse epidermis two full length cDNA clones encoding novel lipoxygenases. One of the cDNAs turned out to be identical to the recently cloned 8-lipoxygenase [Jisaka et al., J. Biol. Chem. 272 (1997) 24 410-24 416], the open reading frame of the second one corresponded to a protein of 701 amino acids with a calculated molecular mass of 80.6 kDa. The amino acid sequence showed 50.8% identity to human 15-lipoxygenase 2, approximately 40% to 5-lipoxygenase and 35% to 12- and 15-lipoxygenases. A unique structural feature is the insertion of 31 amino acid residues in the amino-terminal part of the molecule. Based on these data, we conclude that this epidermis-derived cDNA encodes a novel lipoxygenase isoform termed provisionally epidermis-type lipoxygenase 2 (e-LOX 2).

Leukocyte 12-lipoxygenase: expression, purification, and investigation of the role of methionine residues in turnover-dependent inactivation and 5,8,11,14-eicosatetraynoic acid inhibition

Porcine leukocyte 12-lipoxygenase cDNA was cloned into the expression vectors pSE280, pSE380, and pSE420. pSE380 yielded the highest level of 12-lipoxygenase activity when these vectors were tested for expression in Escherichia coli Top10 cells. Optimal expression of the protein from this vector occurred in cells cultured at 30 degrees C and harvested 18 h following induction of expression by 0.5 mM isopropyl thiogalactoside (IPTG). The enzyme was purified from the 100000 g supernatant to 98% homogeneity by a combination of ammonium sulfate precipitation, anion exchange chromatography, and chromatofocusing. Addition of dithiothreitol and catalase to buffers at various steps in the purification protocol enabled the isolation of enzyme having a specific activity of 12 micromol min(-1) mg(-1). The recovery of purified protein from this expression system was 56%, resulting in a 109-fold purification. On the basis of amino acid sequence comparisons between mammalian 15- and 12-lipoxygenases, three methionine residues in the porcine leukocyte 12-lipoxygenase (M338L, M367V, and M562L) were targeted for mutation to assess their potential role in turnover-dependent inactivation and inhibition by 5,8,11,14-eicosatetraynoic acid (ETYA). The mutants were expressed and purified by the same procedure used for the wild-type enzyme. These amino acid changes did not significantly alter enzyme catalysis as judged by the kinetic constants Km and k(cat)/Km, nor did they affect the rate of turnover-dependent inactivation or inhibition by ETYA. The results indicate that these methionine residues do not play a pivotal role in catalysis, autoinactivation, or sensitivity to inhibition by acetylenic compounds.

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.

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.

Stereochemistry of the Aplysia neuronal 12-lipoxygenase: specific potentiation of FMRFamide action by 12(S)-HPETE

Nervous tissue of the marine mollusc, Aplysia californica, generates arachidonic acid metabolites in response to neurotransmitters such as histamine or FMRFamide. In addition, identified neurons of Aplysia respond to the pharmacologic application of some of these products, particularly those of the 12-lipoxygenase pathway. We investigated the chirality of the initial Aplysia 12-lipoxygenase product, 12-HPETE, in preparation for more detailed metabolic studies and for the analysis of the physiological activity of the endogenous lipid. Neural homogenates and intact ganglia exclusively generate 12(S)-HPETE as do the better characterized mammalian lipoxygenases. The direct application of 12(S)-HPETE to cultured sensory neurons induced a hyperpolarization which averaged 2.6 mV. We did not find any difference between the response to the naturally-occurring 12(S)-HPETE and its diastereomer, 12(R)-HPETE which is not generated in Aplysia. Both isomers were significantly more effective than 15(S)-HPETE. In contrast, 12(S)-HPETE, but not 12(R)-HPETE, was a potent modulator of the action of the molluscan neuropeptide, FMRFamide. Prior application of 12(S)-HPETE to cultured sensory neurons increased the subsequent response to a submaximal dose of FMRFamide by 60%. On the other hand, 12(R)-HPETE reduced the subsequent response to the peptide by 30%. The lack of stereospecificity in the direct effect of the lipids differs markedly from their stereospecific effects as modulators of FMRFamide action. This suggests that there may be an important neurophysiologic role for these lipid modulators which is distinct from their direct effects, and also indicates that there are multiple sites and mechanisms by which lipid hydroperoxides act on neurons in Aplysia.

Conversion of human 15-lipoxygenase to an efficient 12-lipoxygenase: the side-chain geometry of amino acids 417 and 418 determine positional specificity

Positional specificity determinants of human 15-lipoxygenase were examined by site-directed mutagenesis and by kinetic analysis of the wild-type and variant enzymes. By comparing conserved differences among sequences of 12- and 15-lipoxygenases, a small region responsible for functional differences between 12- and 15-lipoxygenases has been identified. Furthermore, the replacement of only two amino acids in 15-lipoxygenase (at 417 and 418 in the primary sequence) by those found in certain 12-lipoxygenases results in an enzyme that has activity similar to 12-lipoxygenase. An examination of the activity of nine variants of lipoxygenase demonstrated that the amino acid side-chain bulk and geometry of residues 417 and 418 are the key components of the positional specificity determinant of 15-lipoxygenase. Overexpression of a variant (containing valines at positions 417 and 418) that performs predominantly 12-lipoxygenation was achieved in a baculo-virus-insect cell culture system. This variant was purified to > 90% homogeneity and its kinetics were compared with the wild-type 15-lipoxygenase. The variant enzyme has no change in its apparent KM for arachidonic acid and a minor (3-fold) change in its Vmax. For linoleic acid, the variant has no change in its KM and a 10-fold reduction in its Vmax, as expected for an enzyme performing predominantly 12-lipoxygenation. The results are consistent with a model in which two amino acids of 15-lipoxygenase (isoleucine 417 and methionine 418) constitute a structural element which contributes to the regiospecificity of the enzyme. Replacement of these amino acids with those found in certain 12-lipoxygenases results in an enzyme which can bind arachidonic acid in a catalytic register that prefers 12-lipoxygenation.

Arachidonate 12-lipoxygenase of rat pineal glands: catalytic properties and primary structure deduced from its cDNA

When a crude extract of rat pineal glands (the 1000 x g supernatant of a homogenate) was incubated with arachidonic acid, 12-hydroxy-5,8,10,14-eicosatetraenoic acid was found as a major product. The 12-lipoxygenase of rat pineal gland also reacted with linoleic and alpha-linolenic acids at 35% and 101% the rate of arachidonate 12-oxygenation, respectively. Upon Western blot analysis using polyclonal antibody against porcine leukocyte 12-lipoxygenase, the cytosol fraction of rat pineal gland showed a positive band with a molecular weight of approx. 74 kDa. A full-length cDNA for this enzyme was cloned from a cDNA library of rat pineal gland and the identity of the 12-lipoxygenase cDNA was confirmed by its expression in E. coli. The amino acid sequence of the enzyme was deduced from the nucleotide sequence of the cDNA, encoding 663 amino acids with a calculated molecular weight of 75,305. The enzyme showed 72% identity of amino acid sequence with porcine leukocyte 12-lipoxygenase and 73% with bovine tracheal 12-lipoxygenase, but only 59% with human platelet 12-lipoxygenase. Taken together, the high reactivity with C-18 fatty acids, the immunoreactivity and the amino acid homology data indicate that the rat pineal 12-lipoxygenase is more closely related to leukocyte 12-lipoxygenase than to platelet 12-lipoxygenase. Upon RNA blot analysis, by far the highest content of 12-lipoxygenase mRNA was observed in the pineal gland and negligible amounts of mRNA were detected in other parts of the brain. The predominant presence of 12-lipoxygenase mRNA in pineal gland was confirmed by in situ hybridization of rat brain. Significant amounts of 12-lipoxygenase mRNA were also detected in rat spleen, aorta, lung and leukocytes.

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

cDNA for arachidonate 12-lipoxygenase of porcine leukocytes was expressed in Escherichia coli. The recombinant 12-lipoxygenase was purified by immunoaffinity chromatography to near homogeneity with a specific activity of about 1.5 mumol/min per mg protein. Each of eight histidine residues, which were well-conserved among various mammalian lipoxygenases and presumed as ligands for non-heme iron, was substituted with leucine by site-directed mutagenesis. Each mutant enzyme was immunoaffinity-purified to near homogeneity. Mutations of His-361, -366 and -541 caused a total loss of enzyme activity, and the iron content was much lower (0.10, 0.06 and 0.06 g atom/mol protein) than that of the wild-type enzyme (0.53). Mutations of His-128 and -356 gave 159% and 162% specific activity of the wild-type enzyme, and the iron contents were 0.55 and 0.52 g atom/mol protein. Substitution of His-426 decreased the activity to 5%, but the iron content was 0.4 g atom/mol protein. The expression level of mutants at His-384 and -393 was too low to precisely determine the iron content. Taken together, His-361, -366 and -541 may play important roles for iron-binding in catalytically active 12-lipoxygenase. Since a high homology of amino acid sequence was known between porcine leukocyte 12-lipoxygenase and mammalian 15-lipoxygenases, we attempted to convert the 12-lipoxygenase to a 15-lipoxygenase. A double mutation of Val-418 and -419 to Ile and Met increased the ratio of 15- and 12-lipoxygenase activities from 0.1 to 5.7.

Biosynthesis of vicinal dihydroxy fatty acids in the red alga Gracilariopsis lemaneiformis: identification of a sodium-dependent 12-lipoxygenase and a hydroperoxide isomerase

Biosynthesis of the vicinal diol fatty acid (12R,13S)-dihydroxy-(5Z,8Z,10E,14Z)-eicosatetrae noic acid from arachidonic acid was studied in preparations of the red alga Gracilariopsis lemaneiformis. The transformation consisted of initial 12-lipoxygenase-catalyzed oxygenation of arachidonic acid into (12S)-hydroperoxy-(5Z,8Z,10E,14Z)-eicosatetraeno ic acid followed by hydroperoxide isomerase-catalyzed conversion of the hydroperoxide into (12R,13S)-dihydroxyeicosatetraenoic acid. Short time incubations and trapping experiments with glutathione peroxidase revealed that (12S)-hydroperoxyeicosatetraenoic acid existed as a free intermediate in the overall conversion. The 12-lipoxygenase was mainly present in the soluble fraction of homogenate of G. lemaneiformis. Further, gel filtration experiments showed that the soluble 12-lipoxygenase was a protein having a molecular weight of 84,000-89,000. The enzymatic activity of 12-lipoxygenase isolated by gel filtration was weak; however, addition of 0.8-1 M sodium chloride to such desalted enzyme increased the activity 20-fold. Experiments with different salts revealed that sodium ion was specifically responsible for the stimulatory effect. Hydroperoxide isomerase was about equally distributed between the high speed supernatant and particulate fractions. Gel filtration of hydroperoxide isomerase present in the soluble fraction showed two peaks of activity corresponding to proteins having molecular weights of 220,000 or greater, and 40,000-45,000. The stereochemical course of the biosynthesis of vicinal diol fatty acids was determined using stereospecifically deuterated 6,9,12-octadecatrienoic acids. The 12-lipoxygenase-catalyzed reaction consisted of antarafacial hydrogen removal and oxygen insertion, whereas the hydroperoxide isomerase catalyzed an intramolecular oxygenation which occurred with retention of the configuration of the carbon atom hydroxylated.

Structure-function properties of human platelet 12-lipoxygenase: chimeric enzyme and in vitro mutagenesis studies

Mutant and chimeric lipoxygenases were expressed in human embryonal kidney 293 cells to assess the importance of amino acids and domains for catalytic activity and positional specificity of molecular oxygen insertion. Histidines 360, 365, and 540, when changed to glutamine residues, completely abolished human platelet 12-lipoxygenase activity. Altered histidines at positions 355, 383, and 392 retained enzymatic activity. The former three histidines could possibly serve as ligands for the catalytically essential non-heme iron atom. Amino acids adjacent (residues 398-417) to the five centrally located histidines conserved among all plant and animal lipoxygenases controlled to a limited extent the positional specificity of oxygenation of 12-lipoxygenase. Variant A417I and the triple variant K416Q/A417I/V418M, designed to introduce 15-lipoxygenase substitutions, transformed the platelet 12-lipoxygenase that synthesizes exclusively 12-hydro(pero)xy-eicosatetraenoic acid (12-H(P)ETE) to an enzyme capable of 10-20% 15-lipoxygenation. When all amino acids between positions 398-429 of 12-lipoxygenase had the corresponding 15-lipoxygenase sequence, the enzyme made 66% 15-lipoxygenase products. The latter enzyme had markedly reduced enzyme activity, though, indicating an apparent shift in the optimal alignment of substrate at the active site for hydrogen atom abstraction. The platelet enzyme could not be altered to form 5-lipoxygenase products by similar manipulations of sequence within this region. Chimeric enzymes consisting of an NH2-terminal segment from one lipoxygenase and the COOH terminus from another lipoxygenase or large substitutions resulted in nonfunctional enzymes. NH2-terminal extensions, but not short deletions, could be tolerated functionally. These studies provide some new insights into lipoxygenase structure-function in the absence of an unresolved three-dimensional structure.

Human 15-lipoxygenase: induction by interleukin-4 and insights into positional specificity

Arachidonate 15-lipoxygenase (15-lipoxygenase) is a lipid-peroxidizing enzyme associated with specific inflammatory cells seen in asthma and atherosclerosis. In atherosclerosis, 15-lipoxygenase is induced in the macrophages of human and rabbit lesions and has been implicated in foam cell formation. In human lung, 15-lipoxygenase is preferentially expressed in airway epithelial cells and eosinophils. Our studies have focused both on the regulation of expression and on the structure-function relationships of the enzyme. To determine factors that could regulate expression, peripheral blood monocytes were purified and cultured with combinations of 18 factors. Only interleukin-4 (60 pM) induced 15-lipoxygenase mRNA, protein and enzymatic activity. Interferon-gamma (100 pM) inhibited the interleukin-4 dependent induction of 15-lipoxygenase. Results with cultured human airway cells were similar. These data suggest that expression of 15-lipoxygenase is regulated by interleukin-4, and that 15-lipoxygenase is a potential downstream effector molecule for this potent cytokine. In parallel studies, we have investigated determinants of positional specificity using site-directed mutagenesis and bacterial expression of human 15-lipoxygenase. Hypotheses for mutagenesis were derived from an analysis of conserved differences among multiple lipoxygenase sequences. Switching four amino acids in 15-lipoxygenase to their counterparts in 12-lipoxygenase resulted in a variant enzyme that produced equal 12- and 15-lipoxygenation. Further analysis has identified two amino acids that completely control the positional specificity of 15-lipoxygenase. These data have led to a preliminary model of the enzyme's active site region.

Molecular evolution of cyclooxygenase and lipoxygenase

Four oxygenases of the arachidonic acid cascade (cyclooxygenase, 5-lipoxygenase, 12-lipoxygenase and 15-lipoxygenase) were investigated by the method of computer-assisted sequence comparison. From the calculations, some aspects of evolution and function of these enzymes were revealed. (1) The evolutionary origin of cyclooxygenase was different from that of lipoxygenases. (2) Cyclooxygenase was a distantly related member of a peroxidase family. (3) Enzymes with 12-lipoxygenase activity were created independently twice by gene duplication.

Porcine leukocyte 5- and 12-lipoxygenases are iron enzymes

5- and 12-lipoxygenases isolated from porcine leukocytes were investigated by electron paramagnetic resonance at X-band and atomic absorption spectroscopy. For comparison potato 5-lipoxygenase was studied under identical experimental conditions. All three lipoxygenases contained between 0.7 and 0.9 Fe atoms/enzyme molecule. As isolated, both mammalian enzymes exhibited a characteristic EPR signal at low magnetic field with a maximum at g = 5.20 indicative of a high-spin ferric iron center. The signal was not affected by the oxidants 12-hydroperoxyeicosatetraenoic acid or arachidonic acid, nor was it affected by the reductant nordihydroguaiaretic acid. In the case of the potato enzyme an intense EPR signal with resonances at g = 7.50, 6.39 and 5.84 was only observed after addition of an oxidant, such as 9-hydroperoxyoctadecadienoic acid.

Analysis of non-heme iron in arachidonate 12-lipoxygenase of porcine leukocytes

Arachidonate 12-lipoxygenase of porcine leukocytes, which was purified to homogeneity by immunoaffinity chromatography, was analyzed for iron content by atomic absorption spectrophotometry. The enzyme contained 0.70 +/- 0.09 g atom of iron per mol of enzyme (mean +/- S.D., n = 4). Inorganic iron, which was added to the enzyme solution as an internal standard, was recovered in almost 100% yield. Among various iron chelators tested, only 2,2'-dipyridyl at 1 mM inactivated the enzyme by 87%, but the enzyme was not reactivated by the addition of excess ferrous or ferric iron.