Kinetic studies on arachidonate 5-lipoxygenase from rat basophilic leukemia cells

Characterization of Arachidonate 5S-Lipoxygenase from Danio rerio with High Activity for the Production of 5S- and 7S-Hydroxy Polyunsaturated Fatty Acids

A recombinant putative lipoxygenase (LOX) from Danio rerio (zebrafish), ALOX3c protein with 6-histidine tag, was purified using affinity chromatography, with a specific activity of 17.2 U mg^-1 for arachidonic acid (AA). The molecular mass of the native ALOX3c was 156 kDa composed of a 78-kDa dimer by gel-filtration chromatography. The product obtained from the conversion of AA was identified as 5S-hydroxyeicosatetraenoic acid (5S-HETE) by HPLC and LC-MS/MS analyses. The specific activity and catalytic efficiency of the LOX from D. rerio for polyunsaturated fatty acids (PUFAs) followed the order AA (17.2 U mg^-1, 1.96 s^-1 μM^-1) > docosahexaenoic acid (DHA, 13.6 U mg^-1, 0.91 s^-1 μM^-1) > eicosapentaenoic acid (EPA, 10.5 U mg^-1, 0.65 s^-1 μM^-1) and these values for AA were the highest among the 5S-LOXs reported to date. Based on identified products and substrate specificity, the enzyme is an AA 5S-LOX. The enzyme exhibited the maximal activity at pH 8.0 and 20 °C with 0.1 mM Zn²⁺ in the presence of 10 mM cysteine. Under these reaction conditions, 6.88 U mL^-1 D. rerio 5S-LOX converted 1.0 mM of AA, EPA, and DHA to 0.91 mM 5S-HETE, 0.72 mM 5S-hydroxyeicosapentaenoic acid (5S-HEPE), and 0.68 mM 7S-hydroxydocosahexaenoic acid (7S-HDHA) in 25, 30, and 25 min, corresponding to molar conversion rates of 91, 72, and 68% and productivities of 2.18, 1.44, and 1.63 mM h^-1, respectively. To the best of our knowledge, this study is the first to describe the bioconversion into 5S-HETE, 5S-HEPE, and 7S-HDHA for the application of biotechnological production.

Looking Back at the Early Stages of Redox Biology

The beginnings of redox biology are recalled with special emphasis on formation, metabolism and function of reactive oxygen and nitrogen species in mammalian systems. The review covers the early history of heme peroxidases and the metabolism of hydrogen peroxide, the discovery of selenium as integral part of glutathione peroxidases, which expanded the scope of the field to other hydroperoxides including lipid hydroperoxides, the discovery of superoxide dismutases and superoxide radicals in biological systems and their role in host defense, tissue damage, metabolic regulation and signaling, the identification of the endothelial-derived relaxing factor as the nitrogen monoxide radical (more commonly named nitric oxide) and its physiological and pathological implications. The article highlights the perception of hydrogen peroxide and other hydroperoxides as signaling molecules, which marks the beginning of the flourishing fields of redox regulation and redox signaling. Final comments describe the development of the redox language. In the 18th and 19th century, it was highly individualized and hard to translate into modern terminology. In the 20th century, the redox language co-developed with the chemical terminology and became clearer. More recently, the introduction and inflationary use of poorly defined terms has unfortunately impaired the understanding of redox events in biological systems.

Regulatory Phenomena in the Glutathione Peroxidase Superfamily

Significance: The selenium-containing Glutathione peroxidases (GPxs)1-4 protect against oxidative challenge, inhibit inflammation and oxidant-induced regulated cell death. Recent Advances: GPx1 and GPx4 dampen phosphorylation cascades predominantly via prevention of inactivation of phosphatases by H₂O₂ or lipid hydroperoxides. GPx2 regulates the balance between regeneration and apoptotic cell shedding in the intestine. It inhibits inflammation-induced carcinogenesis in the gut but promotes growth of established cancers. GPx3 deficiency facilitates platelet aggregation likely via disinhibition of thromboxane biosynthesis. It is also considered a tumor suppressor. GPx4 is expressed in three different forms. The cytosolic form proved to inhibit interleukin-1-driven nuclear factor κB activation and leukotriene biosynthesis. Moreover, it is a key regulator of ferroptosis, because it reduces hydroperoxy groups of complex lipids and silences lipoxygenases. By alternate substrate use, the nuclear form contributes to chromatin compaction. Mitochondrial GPx4 forms the mitochondrial sheath of spermatozoa and, thus, guarantees male fertility. Out of the less characterized GPxs, the cysteine-containing GPx7 and GPx8 are unique in contributing to oxidative protein folding in the endoplasmic reticulum by reacting with protein isomerase as an alternate substrate. A yeast 2-Cysteine glutathione peroxidase equipped with CP and CR was reported to sense H₂O₂ for inducing an adaptive response. Critical Issues: Most of the findings compiled are derived from tissue culture and/or animal studies only. Their impact on human physiology is sometimes questionable. Future Directions: The expression of individual GPxs and GPx-dependent regulatory phenomena are to be further investigated, in particular in respect to human health.

Selenium and selenoproteins in prostanoid metabolism and immunity

Selenium (Se) is an essential trace element that functions in the form of the 21st amino acid, selenocysteine (Sec) in a defined set of proteins. Se deficiency is associated with pathological conditions in humans and animals, where incorporation of Sec into selenoproteins is reduced along with their expression and catalytic activity. Supplementation of Se-deficient population with Se has shown health benefits suggesting the importance of Se in physiology. An interesting paradigm to explain, in part, the health benefits of Se stems from the observations that selenoprotein-dependent modulation of inflammation and efficient resolution of inflammation relies on mechanisms involving a group of bioactive lipid mediators, prostanoids, which orchestrate a concerted action toward maintenance and restoration of homeostatic immune responses. Such an effect involves the interaction of various immune cells with these lipid mediators where cellular redox gatekeeper functions of selenoproteins further aid in not only dampening inflammation, but also initiating an effective and active resolution process. Here we have summarized the current literature on the multifaceted roles of Se/selenoproteins in the regulation of these bioactive lipid mediators and their immunomodulatory effects.

Selenium and redox signaling

Selenium compounds that contain selenol functions or can be metabolized to selenols are toxic via superoxide and H₂O₂ generation, when ingested at dosages beyond requirement. At supra-nutritional dosages various forms of programmed cell death are observed. At physiological intakes, selenium exerts its function as constituent of selenoproteins, which overwhelmingly are oxidoreductases. Out of those, the glutathione peroxidases counteract hydroperoxide-stimulated signaling cascades comprising inflammation triggered by cytokines or lipid mediators, insulin signaling and different forms of programmed cell death. Similar events are exerted by peroxiredoxins, which functionally depend on the selenoproteins of the thioredoxin reductase family. The thiol peroxidases of both families can, however, also act as sensors for hydroperoxides, thereby initiating signaling cascades. Although the interaction of selenoproteins with signaling events has been established by genetic techniques, the in vivo relevance of these findings is still hard to delineate for several reasons: The biosynthesis of individual selenoproteins responds differently to variations of selenium intakes; selenium is preferentially delivered to privileged tissues via inter-organ trafficking and receptor-mediated uptake, and only half of the selenoproteins known by sequence have been functionally characterized. The fragmentary insights do not allow any uncritical use of selenium for optimizing human health.

Catalytic mechanisms and specificities of glutathione peroxidases: variations of a basic scheme

Kinetics and molecular mechanisms of GPx-type enzymes are reviewed with emphasis on structural features relevant to efficiency and specificity. In Sec-GPxs the reaction takes place at a single redox centre with selenocysteine as redox-active residue (peroxidatic Sec, U(P)). In contrast, most of the non-vertebrate GPx have the U(P) replaced by a cysteine (peroxidatic Cys, C(P)) and work with a second redox centre that contains a resolving cysteine (C(R)). While the former type of enzymes is more or less specific for GSH, the latter are reduced by "redoxins". The common denominator of the GPx family is the first redox centre comprising the (seleno)cysteine, tryptophan, asparagine and glutamine. In this architectural context the rate of hydroperoxide reduction by U(P) or C(P), respectively, is enhanced by several orders of magnitude compared to that of free selenolate or thiolate. Mammalian GPx-1 dominates H(2)O(2) metabolism, whereas the domain of GPx-4 is the reduction of lipid hydroperoxides with important consequences such as counteracting 12/15-lipoxygenase-induced apoptosis and regulation of inflammatory responses. Beyond, the degenerate GSH specificity of GPx-4 allows selenylation and oxidation to disulfides of protein thiols. Heterodimer formation of yeast GPx with a transcription factor is discussed as paradigm of a redox sensing that might also be valid in vertebrates.

Seleno-independent glutathione peroxidases

Glutathione peroxidases (GPXs, EC 1.11.1.9) were first discovered in mammals as key enzymes involved in scavenging of activated oxygen species (AOS). Their efficient antioxidant activity depends on the presence of the rare amino-acid residue selenocysteine (SeCys) at the catalytic site. Nonselenium GPX-like proteins (NS-GPXs) with a Cys residue instead of SeCys have also been found in most organisms. As SeCys is important for GPX activity, the function of the NS-GPX can be questioned. Here, we highlight the evolutionary link between NS-GPX and seleno-GPX, particularly the evolution of the SeCys incorporation system. We then discuss what is known about the enzymatic activity and physiological functions of NS-GPX. Biochemical studies have shown that NS-GPXs are not true GPXs; notably they reduce AOS using reducing substrates other than glutathione, such as thioredoxin. We provide evidence that, in addition to their inefficient scavenging action, NS-GPXs act as AOS sensors in various signal-transduction pathways.

Glutathione peroxidases and redox-regulated transcription factors

Analysis of the selenoproteome identified five glutathione peroxidases (GPxs) in mammals: cytosolic GPx (cGPx, GPx1), phospholipid hydroperoxide GPx (PHGPX, GPx4), plasma GPx (pGPX, GPx3), gastrointestinal GPx (GI-GPx, GPx2) and, in humans, GPx6, which is restricted to the olfactory system. GPxs reduce hydroperoxides to the corresponding alcohols by means of glutathione (GSH). They have long been considered to only act as antioxidant enzymes. Increasing evidence, however, suggests that nature has not created redundant GPxs just to detoxify hydroperoxides. cGPx clearly acts as an antioxidant, as convincingly demonstrated in GPx1-knockout mice. PHGPx specifically interferes with NF-kappaB activation by interleukin-1, reduces leukotriene and prostanoid biosynthesis, prevents COX-2 expression, and is indispensable for sperm maturation and embryogenesis. GI-GPx, which is not exclusively expressed in the gastrointestinal system, is upregulated in colon and skin cancers and in certain cultured cancer cells. GI-GPx is a target for Nrf2, and thus is part of the adaptive response by itself, while PHGPx might prevent cancer by interfering with inflammatory pathways. In conclusion, cGPx, PHGPx and GI-GPx have distinct roles, particularly in cellular defence mechanisms. Redox sensing and redox regulation of metabolic events have become attractive paradigms to unravel the specific and in part still enigmatic roles of GPxs.

Regulation of 5-lipoxygenase enzyme activity

In this article, regulation of human 5-lipoxygenase enzyme activity is reviewed. First, structural properties and enzyme activities are described. This is followed by the activating factors: Ca2+, membranes, ATP, and lipid hydroperoxide. Also, studies on phosphorylation of 5-lipoxygenase and nuclear localization sequences are reviewed.

Effects of gpx4 haploid insufficiency on GPx4 activity, selenium concentration, and paraquat-induced protein oxidation in murine tissues

Selenium-dependent glutathione peroxidase-4 (GPx4) catalyzes the reduction of phospholipid hydroperoxides. Because a full gpx4 knockout is embryonic lethal, we examined the effect of deletion of one copy of gpx4 on the activities of three selenoperoxidases (GPx1, GPx3, and GPx4), selenium concentrations, and pro-oxidant-induced protein oxidation in various tissues of mice. A total of 32 gpx4 hemizygous (GPx4+/-) and wild-type (WT) mice (8- to 10-weeks old; 16 males and 16 females) were fed a selenium-adequate diet and given an intraperitoneal injection of paraquat (PQ; 24 mg/kg body wt) or phosphate-buffered saline (PBS). All mice were euthanized 4 hrs after injection to collect tissues for analyses. In PBS-treated mice, GPx4 activities in lung, liver, kidney, and testes of GPx4+/- mice were 24-39% lower (P < 0.05) than in WT mice. Among PQ-treated mice, only testis GPx4 activity in GPx4+/- mice was significantly lower (54% P < 0.05) than WT mice. Selenium concentration in testes, but not in other tissues, was reduced (34% P < 0.05) in GPx4+/- mice compared with WT mice, irrespective of treatment. Tissue GPx1 activities and plasma GPx3 and alanine aminotransferase (ALT) activities were unaffected by PQ treatment or gpx4 hemizygosity. Total protein carbonyl was elevated (73% P < 0.05) by PQ only in lung, and this effect of PQ was independent of genotypes. In conclusion, gpx4 haploid insufficiency reduced GPx4 activities and/or selenium concentrations, but had no effect on pro-oxidant-induced protein oxidation in various tissues of mice.

The GI-GPx gene is a target for Nrf2

The gastrointestinal glutathione peroxidase (GI-GPx, GPx2) is a selenoprotein that was suggested to act as barrier against hydroperoxide absorption but has also been implicated in the control of inflammation and malignant growth. In CaCo-2 cells, GI-GPx was induced by t-butyl hydroquinone (tBHQ) and sulforaphane (SFN), i.e., "antioxidants" known to activate the "antioxidant response element" (ARE) via electrophilic thiol modification of Keap1 in the Nrf2/Keap1 system. The functional significance of a putative ARE in the GI-GPx promoter was validated by transcriptional activation of reporter gene constructs upon exposure to electrophiles (tBHQ, SFN, and curcumin) or overexpression of Nrf2 and by reversal of these effects by mutation of the ARE in the promoter and by overexpressed Keap1. Binding of Nrf2 to the ARE sequence in authentic gpx2 was corroborated by chromatin immunoprecipitation. Thus, the presumed natural antioxidants sulforaphane and curcumin may exert their anti-inflammatory and anticarcinogenic effects not only by induction of phase 2 enzymes but also by the up-regulation of the selenoprotein GI-GPx.

Redox events in interleukin-1 signaling

There is increasing evidence that reactive oxygen species (ROS) are mediators in growth factor and cytokine signaling pathways. Mechanisms by which ROS can interfere with signaling cascades may include regulation of protein activities by the modification of essential cysteines. Modification can be performed chemically or enzyme-catalyzed. Enzymes catalyzing a reversible thiol modification within proteins are to be able to react with both, ROS and protein thiols. If hydroperoxides are involved, promising candidates are peroxiredoxins and glutathione peroxidases (GPx), especially the phospholipid hydroperoxide GPx. Interleukin-1, one of the key players in inflammatory response, stimulates the production of ROS itself, but its signaling cascade can also be influenced by ROS and by thiol modifying agents. Targets are located in early, intermediate, and late events in the signaling cascade. We here summarize what is known about the effects of thiol modifying agents, selenium and glutathione peroxidases, on the assembly of the IL-1 receptor signaling complex as an early event, on the activation of NF-kappa B as an intermediate event, and on the expression of cell adhesion molecules as a late event in IL-1 signaling.

The C2-like beta-barrel domain mediates the Ca2+-dependent resistance of 5-lipoxygenase activity against inhibition by glutathione peroxidase-1

Recently, we reported that in crude enzyme preparations, a monocyte-derived soluble protein (M-DSP) renders 5-lipoxygenase (5-LO) activity Ca2+-dependent. Here we provide evidence that this M-DSP is glutathione peroxidase (GPx)-1. Thus, the inhibitory effect of the M-DSP on 5-LO could be overcome by the GPx-1 inhibitor mercaptosuccinate and by the broad spectrum GPx inhibitor iodoacetate, as well as by addition of 13(S)-hydroperoxy-9Z,11E-octadecadienoic acid (13(S)-HPODE). Also, the chromatographic characteristics and the estimated molecular mass (80-100 kDa) of the M-DSP fit to GPx-1 (87 kDa), and GPx-1, isolated from bovine erythrocytes, mimicked the effects of the M-DSP. Intriguingly, only a trace amount of thiol (10 micro M GSH) was required for reduction of 5-LO activity by GPx-1 or the M-DSP. Moreover, the requirement of Ca2+ allowing 5-LO product synthesis in various leukocytes correlated with the respective GPx-1 activities. Mutation of the Ca2+ binding sites within the C2-like domain of 5-LO resulted in strong reduction of 5-LO activity by M-DSP and GPx-1, also in the presence of Ca2+. In summary, our data suggest that interaction of Ca2+ at the C2-like domain of 5-LO protects the enzyme against the effect of GPx-1. Apparently, in the presence of Ca2+, a low lipid hydroperoxide level is sufficient for 5-LO activation.

Selenium-dependent enzymes in endothelial cell function

Glutathione peroxidases and thioredoxin reductases are the main selenoproteins expressed by endothelial cells. These enzymes reduce hydroperoxides, their role in endothelial cell physiology, however, by far exceeds prevention of oxidative damage. Reactive oxygen and nitrogen species, especially superoxide, hydroperoxides, and nitric oxide, are crucial signaling molecules in endothelial cells. Their production is regulated by vascular NAD(P)H oxidases and the endothelial nitric oxide synthase. Their metabolism and physiological functions are coordinated by glutathione peroxidases and the thioredoxin/thioredoxin reductase system. Endothelial selenoproteins are involved in the regulation of the vascular tone by maintaining the superoxide anion/nitric oxide balance, of cell adhesion by controlling cell adhesion molecule expression, of apoptosis via inhibition/activation of apoptosis signal-regulating kinase-1, and of eicosanoid production by controlling the activity of cyclooxygenases and lipoxygenases. Accordingly, they regulate inflammatory processes and atherogenesis. The underlying mechanisms are various and differ between individual selenoproteins. Scavenging of hydroperoxides not only prevents oxidative damage, but also interferes with signaling cascades and enzymes involved. Modulation of proteins by hydroperoxide-driven thiol/disulfide exchange is a novel mechanism that needs to be further investigated. A better understanding of the complex interplay of selenoproteins in regulating endothelial cell functions will help to develop a rationale for an improvement of health by an optimum selenium supply.

Biological significance of phospholipid hydroperoxide glutathione peroxidase (PHGPx, GPx4) in mammalian cells

Reactive oxygen species (ROS) are known mediators of intracellular signal cascades. Excessive production of ROS may lead to oxidative stress, loss of cell function, and cell death by apoptosis or necrosis. Lipid hydroperoxides are one type of ROS whose biological function has not yet been clarified. Phospholipid hydroperoxide glutathione peroxidase (PHGPx, GPx4) is a unique antioxidant enzyme that can directly reduce phospholipid hydroperoxide in mammalian cells. This contrasts with most antioxidant enzymes, which cannot reduce intracellular phospholipid hydroperoxides directly. In this review, we focus on the structure and biological functions of PHGPx in mammalian cells. Recently, molecular techniques have allowed overexpression of PHGPx in mammalian cell lines, from which it has become clear that lipid hydroperoxides also have an important function as activators of lipoxygenase and cyclooxygenase, participate in inflammation, and act as signal molecules for apoptotic cell death and receptor-mediated signal transduction at the cellular level.

Arachidonate 5-lipoxygenase

In this article, it has been attempted to review data primarily on the activation of human 5-lipoxygenase, in vitro and in the cell. First, structural properties and enzyme activities are described. This is followed by the activating factors: Ca2+, membranes, ATP, and lipid hydroperoxide. Also, studies on phosphorylation of 5-lipoxygenase, interaction with other proteins, and the intracellullar mobility of 5-lipoxygenase, are reviewed.

Overexpression of PHGPx inhibits hydroperoxide-induced oxidation, NFkappaB activation and apoptosis and affects oxLDL-mediated proliferation of rabbit aortic smooth muscle cells

Rabbit abdominal aortic smooth muscle cells (SMC) were stably transfected with the cDNA of porcine phospholipid hydroperoxide glutathione peroxidase (PHGPx) by means of a retroviral gene transfer technique, to create a model for studying cellular processes relevant to atherogenesis. The transfected cells (SMC/PHGPx) had approximately 4-fold higher PHGPx activity when cultured in the presence of selenite whereas the parental cells did not show any significant increase in PHGPx or total GPx activity upon selenium supplementation. In situ functionality of PHGPx was validated by inhibition of linoleic acid hydroperoxide-induced toxicity, dihydrorhodamine oxidation, NFkappaB activation and apoptosis. SMC grown in 1% FCS responded to oxidized LDL (oxLDL) with a marked proliferation, as measured by [3H]thymidine incorporation, irrespective of selenium supplementation. In SMC/PHGPx grown with or without selenite under control conditions or exposed to native LDL, thymidine incorporation was generally depressed. Also, oxLDL-induced proliferation was lower in SMC/PHGPx compared to untransfected SMC up to 24 h of incubation. After 40 h, however, selenite supplementation restored maximum proliferation response to oxLDL in SMC/PHGPx. The results suggest a proliferative effect of endogenous hydroperoxides in SMC. They further reveal that hydroperoxy lipids of oxLDL contribute to the induction of proliferation, but also suggest involvement of hydroxy lipids in the response to oxLDL.

Selenium in biology: facts and medical perspectives

Several decades after the discovery of selenium as an essential trace element in vertebrates approximately 20 eukaryotic and more than 15 prokaryotic selenoproteins containing the 21st proteinogenic amino acid, selenocysteine, have been identified, partially characterized or cloned from several species. Many of these proteins are involved in redox reactions with selenocysteine acting as an essential component of the catalytic cycle. Enzyme activities have been assigned to the glutathione peroxidase family, to the thioredoxin reductases, which were recently identified as selenoproteins, to the iodothyronine deiodinases, which metabolize thyroid hormones, and to the selenophosphate synthetase 2, which is involved in selenoprotein biosynthesis. Prokaryotic selenoproteins catalyze redox reactions and formation of selenoethers in (stress-induced) metabolism and energy production of E. coli, of the clostridial cluster XI and of other prokaryotes. Apart from the specific and complex biosynthesis of selenocysteine, selenium also reversibly binds to proteins, is incorporated into selenomethionine in bacteria, yeast and higher plants, or posttranslationally modifies a catalytically essential cysteine residue of CO dehydrogenase. Expression of individual eukaryotic selenoproteins exhibits high tissue specificity, depends on selenium availability, in some cases is regulated by hormones, and if impaired contributes to several pathological conditions. Disturbance of selenoprotein expression or function is associated with deficiency syndromes (Keshan and Kashin-Beck disease), might contribute to tumorigenesis and atherosclerosis, is altered in several bacterial and viral infections, and leads to infertility in male rodents.

Tissue-specific functions of individual glutathione peroxidases

The family of glutathione peroxidases comprises four distinct mammalian selenoproteins. The classical enzyme (cGPx) is ubiquitously distributed. According to animal, cell culture and inverse genetic studies, its primary function is to counteract oxidative attack. It is dispensible in unstressed animals, and accordingly ranks low in the hierarchy of glutathione peroxidases. The gastrointestinal isoenzyme (GI-GPx) is most related to cGPx and is exclusively expressed in the gastrointestinal tract. It might provide a barrier against hydroperoxides derived from the diet or from metabolism of ingested xenobiotics. The extreme stability in selenium deficiency ranks this glutathione peroxidase highest in the hierarchy of selenoproteins and points to a more vital function than that of cGPx. Plasma GPx (pGPx) behaves similar to cGPx in selenium deficiency. It is directed to extracellular compartments and is expressed in various tissues in contact with body fluids, e.g., kidney, ciliary body, and maternal/fetal interfaces. It has to be rated as an efficient extracellular antioxidant device, though with low capacity because of the limited extracellular content of potential thiol substrates. Phospholipid hydroperoxide glutathione peroxidase (PHGPx), originally presumed to be a universal antioxidant enzyme protecting membrane lipids, appears to have adopted a variety of specific roles like silencing lipoxygenases and becoming an enzymatically inactive structural component of the mitochondrial capsule during sperm maturation. Thus, all individual isoenzymes are efficient peroxidases in principle, but beyond their mere antioxidant potential may exert cell- and tissue-specific roles in metabolic regulation, as is evident for PHGPx and may be expected for others.

Intracellular oxidation/reduction status in the regulation of transcription factors NF-kappaB and AP-1

The eukaryotic cell contains a multitude of pathways coupling environmental stimuli to the specific regulation of gene expression. Two early response transcriptional complexes, NF-kappaB and AP-1, appear to respond to environmental stressors by inducing the expression of response specific downstream genes. Both are well-characterized transcriptional regulatory factors that are induced by a wide variety of seemingly unrelated exogenous and endogenous agents and serve important roles in cell growth and differentiation, immunity, inflammation, and other preprogrammed cellular genetic processes. The activities of NF-kappaB and AP-1 are also affected following exposure to chemicals, drugs, or other agents that appear to alter the cellular oxidation/reduction (redox) status. From these observations, it has been suggested that changes in cellular oxidation/reduction status, communicated via a series of cellular redox-sensitive signaling circuitry employing metal- and thiol-containing proteins, serve as common mechanisms linking environmental stressors to adaptive cellular responses. As such, these transcription factors are ideal paradigms to study the mechanism and possible physiological significance of early response genes in the cellular response to changes in cellular redox status. In this article we summarize the evidence suggesting that cellular redox regulates these transcription factors.

13-HPODE and 13-HODE modulate cytokine-induced expression of endothelial cell adhesion molecules differently

Expression of cellular adhesion molecules (CAMs) at endothelial surfaces represents a physiological response to vascular damage and mediates the initiation of inflammation and possibly of atherogenesis. The cytokines TNF alpha and IL-1 are potent inducers of CAMs in endothelial cells. Reactive oxygen species comprising lipid oxidation products have been implicated in the signaling pathways of both TNF alpha and IL-1 and accordingly could modulate atherogenic events. We, therefore, investigated the potential role of the lipoxygenase product, 13-hydroperoxyoctadecadienoic acid (13-HPODE), which has also been identified in oxidized low density lipoproteins on CAM expression in HUVEC. 13-HPODE induced the expression of ICAM-1 in a concentration dependent manner up to 75 microM. Higher concentrations were toxic. Similar effects were observed with H2O2 and phosphatidylcholine hydroperoxide. VCAM-1 and E-selectin were not induced by 13-HPODE. 13-HPODE administered simultaneously with IL-1 or TNF alpha induced ICAM-1 additively, suggesting that hydroperoxides and cytokines act on the same signaling pathways. In contrast, pretreatment of cells with 50 microM 13-HPODE for 1 hour rather inhibited subsequent cytokine-induced ICAM-1 and E-selectin expression. Surprisingly, the reduction product of 13-HPODE, 13-hydroxyoctadecadienoic acid (13-HODE) proved to be an even better inducer of ICAM-1 than 13-HPODE. Pretreatment with 13-HODE did not show any inhibitory effect on ICAM-1 expression. Our data show that lipoxygenase products differentially affect CAM expression. 13-HPODE is stimulatory by itself and can positively or negatively affect cytokine signaling depending on time of exposure. 13-HODE induces CAM expression by itself but does not inhibit cytokine signaling. Thus, the interplay of lipoxygenase products with proinflammatory cytokines can not simply be explained by an oxidant-mediated facilitation of cytokine signaling.

Alpha- and beta- alkyl-substituted eicosapentaenoic acids: incorporation into phospholipids and effects on prostaglandin H synthase and 5-lipoxygenase

Alpha-ethyl-, alpha-methyl- and beta-methyl eicosapentaenoic acid (EPA) were prepared and their incorporation into cell lipids and effects on eicosanoid synthesis compared with EPA and docosahexaenoic acid (DHA). alpha- and beta-methyl EPA were incorporated into hepatocyte triacylglycerols as efficiently as EPA, whereas lesser amounts were found in phospholipids. alpha-ethyl EPA was not incorporated into phospholipids but small amounts were detected in triacylglycerol. All derivatives inhibited the synthesis of arachidonic acid, although less efficiently than EPA and DHA. The derivatives were poor substrates of prostaglandin H (PGH) synthase and 5-lipoxygenase, and they all inactivated PGH synthase. In isolated platelets, alpha-methyl EPA was a stronger inhibitor of TxB2 production than EPA, alpha-ethyl- and beta-methyl EPA. All derivatives were stronger inducers of peroxisomal beta-oxidation than EPA and DHA. This increased induction probably is a consequence of the blocked mitochondrial beta-oxidation of the derivatives.

Heneicosapentaenoate (21:5n-3): its incorporation into lipids and its effects on arachidonic acid and eicosanoid synthesis

6,9,12,15,18-Heneicosapentaenoic acid (21:5n-3) (HPA), present in small amounts in fish oils, has been prepared by chemical elongation of eicosapentaenoic acid (EPA) and its biological properties compared with EPA and docosahexaenoic acid (DHA). All the double bonds of HPA are displaced one carbon away from the carboxyl group when compared to EPA. HPA is incorporated into phospholipids and into triacylglycerol in cell culture to a similar extent as EPA and DHA. HPA is a stronger inhibitor of the conversion of alpha-linoleic acid and dihomo-gamma-linolenic acid to arachidonic acid (AA) in hepatoma cells than are EPA, DHA, and AA. HPA is a poor substrate for prostaglandin H synthase and for 5-lipoxygenase, but it inactivates prostaglandin H synthase as rapidly as do AA, EPA, and DHA. HPA inhibits thromboxane synthesis in isolated platelets as efficiently as EPA. EPA, HPA, and DHA are all weak inducers of acyl-CoA oxidase in hepatoma cells. Therefore, since fish oils contain only small amounts of HPA, it is unlikely that this fatty acid is of particular significance for the biological effects of these oils, possibly with the exception that it is a strong inhibitor of AA synthesis.

Use of a continuous assay of oxygen consumption to evaluate the pharmacology of 5-lipoxygenase inhibitors

A variety of assay systems have been utilized to evaluate the inhibition of the key enzyme in leukotriene (LT) biosynthesis, 5-lipoxygenase (5-LO). We have developed an assay utilizing a cytosolic preparation of 5-LO from rat basophilic leukemia (RBL-1) cells. Enzyme activity was monitored by continuous measurement of oxygen consumption. High performance liquid chromatography (HPLC) analysis of products showed exclusive generation of 5-LO products. The assay proved useful for the evaluation of a variety of chemical classes of lipoxygenase inhibitors and clearly differentiated those compounds which extended the lag phase (e.g. A-64077) as opposed to the propagation phase of the enzyme activity (e.g. SK & F 105561). The data generated were in reasonable agreement with results from the assay of isolated human monocyte 5-LO and, with the exception of compounds which appear to have a significant effect on 5-LO translocation (e.g. MK-886 and Wy-49 232), inhibition of LT production by intact monocytes. This assay system proved to be a convenient and informative method to analyze inhibition of 5-LO activity.

Evaluation of 5- and 6-fluoro derivatives of arachidonic acid and 5,8,14-eicosatrienoic acid as substrates and inhibitors of 5-lipoxygenase

The 5- and 6-fluoro derivatives of arachidonic acid (5F-ETE and 6F-ETE) were evaluated as substrates of rat basophilic leukaemia cell (RBL-1) 5-lipoxygenase. 5F-ETE was found to be a poor substrate and was converted into a single product, 5-oxoeicosa-6,8,11,14-tetraenoic acid (5-oxo-ETE). 6F-ETE was a good substrate and was mainly converted into 5-hydroperoxy-6-fluoroeicosa-6,8,11,14-tetraenoic acid (5-OOH-6F-ETE) with concomitant formation of a small amount of 5-oxo-6-fluoroeicosa-6,8,11,14-tetraenoic acid (5-oxo-6F-ETE). However the formation of 5,12-dihydroxy-6-fluoroeicosa-6,8,10,14-tetraenoic acids, epimeric at C-12, was not observed. Eicosa-5(Z),8(Z),14(Z)-trienoic acid (ET), previously described as a good substrate of 5-lipoxygenase, is oxidized mainly to 5-hydroperoxyeicosa-6,8,14-trienoic acid (5-OOH-ET), which does not serve as a substrate for the leukotriene A4 (LTA4) synthase activity of 5-lipoxygenase [Navé, Dulery, Gaget & Ducep (1988) Prostaglandins 36, 385-398]. To allow a better estimation of the effect of fluorine substitution on the rate of oxidation of the 5,8-cis,cis-diene moiety by 5-lipoxygenase, the 5- and 6-fluoro derivatives of ET were studied as substrates. Qualitatively, the metabolism of 5F-ET and 6F-ET was found to be similar to that observed for 5F-ETE and 6F-ETE. Quantitatively, 6F-ET proved to be a somewhat better substrate than ET, whereas 5F-ET was poorly metabolized. The relative ability of arachidonic acid, ET and the corresponding 5- and 6-fluoro derivatives to inhibit the 5-lipoxygenase-catalysed oxidation of eicosa-5(Z),8(Z)-dienoic acid (ED) was also investigated. 6F-ETE and 5F-ETE were found to be effective and about equipotent inhibitors of 5-lipoxygenase in the micromolar range. In view of their close structural similarity to arachidonic acid, these two inhibitors are expected to be important tools in the study of the 5-lipoxygenase pathway in vivo.

Improved purification of 12-lipoxygenase from rat basophilic leukemia cells and conditions for optimal enzyme activity

12-Lipoxygenase from rat basophilic leukemia cells was purified about 300-fold by protein-HPLC in a single run. Maximal 12-lipoxygenase activity was observed at pH 7.5, while the enzyme became almost inactive at pH 6 and 9. Although Ca2+ was not essential for 12-lipoxygenase activity, the partially purified enzyme was stimulated approx. 2-fold in the presence of 0.1-5.0 mM Ca2+. Contrary to 5-lipoxygenase from RBL-1 cells, 12-lipoxygenase was not inactivated by preincubation with Ca2+ for 1-10 min, nor was it stimulated by 0.1-10 mM ATP.

12-Lipoxygenase from rat basophilic leukemia cells: separation from 5-lipoxygenase and temperature-dependent inactivation by hydroperoxy fatty acid

12-Lipoxygenase and 5-lipoxygenase from rat basophilic leukemia cells were separated by protein-HPLC in a single step. Upon incubation in the presence of Ca2+, 12-lipoxygenase converted arachidonic acid into 12(S)-hydroxyeicosatetraenoic acid and linoleic acid into 13(S)-hydro(pero)xyoctadecadienoic acid. The reaction products were analyzed by reversed-phase and chiral straight-phase HPLC with ultraviolet-detection. Using the cytosolic fraction of rat basophilic leukemia cells, optimal 12-lipoxygenase activity was observed at 10 degrees C. At 37 degrees C 12-lipoxygenase was very rapidly inactivated by its own product, hydroperoxy fatty acid, at low concentrations (10-100 nM).

Metabolism of unsaturated fatty acids by RBL-1 5-lipoxygenase: influence of substrate solubility and product inactivation

Several alternative fatty acid substrates have been employed to characterise the kinetics of rat basophilic leukaemia cell (RBL-1) 5-lipoxygenase. Using arachidonic acid (AA) as substrate, enzymes rates declined at high substrate concentrations (greater than 25 microM) and were associated with pronounced lag phases. The concentrations of AA at which apparent substrate inhibition and lag phases were observed were comparable with those at which AA induced emulsion formation in aqueous media. No evidence for substrate inhibition or lag phases was observed using eicosapentaenoic acid (EPA), a more soluble substrate which did not induce emulsion formation at concentrations up to 100 microM. Reactions catalysed by RBL-1 5-lipoxygenase terminated before exhaustion of substrate. AA and EPA induced time-dependent enzyme inactivation at concentrations 100-fold lower than their apparent Km values for the enzyme. The ability of several fatty acids to induce time-dependent inactivation was directly proportional to their substrate potency. We conclude that apparent substrate inhibition is a consequence of a change from monomeric to micellar substrate which has a lower affinity for the enzyme and that premature termination of the enzyme reactions is a consequence of product-induced enzyme inactivation.

Leukotriene formation by human polymorphonuclear leukocytes from endogenous arachidonate. Physiological triggers and modulation by prostanoids

Human polymorphonuclear leukocytes (PMN) were isolated from freshly drawn venous blood by Dextran sedimentation and discontinuous Percoll gradient centrifugation. The effects of several putative triggers of the leukotriene formation such as C5a, PAF, FMLP, C3a, PMA, LTC4, LTD4, LTB4 or arachidonate were studied by RP-HPLC analysis. 280 nM C5a, 100 nM FMLP, 1 microM PAF or 20 microM arachidonate induced a marginal formation of 1.5-18 ng of LTB4 plus LTB4 metabolites/2 x 10(7) PMN. 560 nM C3a, 100 nM PMA, 1 microM LTC4, 1 microM LTD4 and 1 microM LTB4 each failed to induce any formation of 5-lipoxygenase products. Pretreatment of the cells with 40 microM ethylmercurithiosalicylate (merthiolate) enhanced the leukotriene formation by 100 nM FMLP about 40-fold, by 280 nM C3a about 120-fold and by 1 microM PAF about 14-fold. Merthiolate itself induced no leukotriene formation from human PMN and reduced the leukotriene formation by 20 microM arachidonate. The FMLP/merthiolate-induced activation of the PMN was concentration-dependent in respect to both FMLP and merthiolate. 1 microM LTC4, 1 microM LTD4 or 1 microM LTB4 also failed to trigger any LTB4 formation of merthiolate-treated PMN. 560 nM C3a or 100 nM PMA in combination with 40 microM merthiolate induced a slight formation of 28 ng and 10 ng of LTB4 plus LTB4 metabolites, respectively. The FMLP/merthiolate-induced leukotriene formation was modulated by prostanoids. PGE2, PGE1, PGD2 and 6-keto-PGE1 each evoked a concentration-dependent inhibition of the leukotriene formation with IC50 values of 0.07 microM, 0.18 microM, 0.27 microM and 6 microM respectively. In addition, significant inhibitory effects by PGI2, Iloprost (a carbacyclin analogue of prostacyclin), PGF2a or 6-keto-PGF1a were achieved; the corresponding IC50 values, however, amounted to 19-59 microM. Thus these compounds were about 500-fold less potent in comparison with PGE2 in inhibiting LTB4 formation by human PMN.

Involvement of glutathione peroxidase activity in the stimulation of 5-lipoxygenase activity by glutathione-depleting agents in human polymorphonuclear leukocytes

We recently demonstrated activation of 5-lipoxygenase activity in human polymorphonuclear leukocytes (PMN) on preincubation of the cells with glutathione-depleting agents, namely 1-chloro-2,4-dinitrobenzene (Dnp-C1) and azodicarboxylic acid bis[dimethylamide] (diamide). In this paper we show that Dnp-C1, but not diamide, impairs the reduction of added organic peroxides in whole PMN. Also, since co-incubation of fatty acid hydroperoxides with arachidonate caused activation of 5-lipoxygenase, we propose that Dnp-C1 increases the peroxide level in PMN which is required for the onset of lipoxygenase activity. This could be substantiated in PMN homogenates by a glutathione-dependent depression of arachidonate 5-lipoxygenation. At higher arachidonate concentrations and in the presence of Ca2+ the glutathione effect was not observed but additional glutathione peroxidase also blocked this maximally stimulated 5-lipoxygenase. Together with other experiments, it became obvious that the formation of leukotrienes, but also of 15-lipoxygenase products, requires a sharply defined threshold level of fatty acid hydroperoxides which are generated by the lipoxygenases and counteracted by glutathione-dependent peroxidase(s). Dnp-C1 influences this equilibrium by removing glutathione and thereby inhibiting glutathione-dependent peroxidase activity. From our data we conclude that it is the physiological function of the peroxidase activity in PMN to determine an efficiently regulated threshold level of hydroperoxide products, below which no activation of 5-lipoxygenase or 15-lipoxygenase can occur.

5-Hydroperoxyeicosatetraenoic acid (5-HPETE) enhances the synthesis of 1-O-alkyl-2-sn-acetyl-glycero-3-phosphocholine (PAF) in fMet-Leu-Phe-stimulated HL-60 cells: key role of diacylglycerol (DAG) in activation of protein kinase C (PKC)

We investigated the effect of 5-hydroperoxyeicosatetraenoic acid (5-HPETE) on the PAF formation in fMet-Leu-Phe-stimulated HL-60 cells. 5-HPETE was found to enhance the PAF synthesis in fmlp-stimulated cells without causing additional mobilization of intracellular calcium. However, a significant increase in diacylglycerol (DAG) levels due to 5-HPETE was observed, which in turn activated the protein kinase C (PKC). Obviously, PKC is responsible for the activation of phospholipase A2 and the release of lyso-PAF and AA from complex lipid stores. Further, the dose-dependent increase in DAG production in absence of simultaneous increase in total inositol phosphates is indicative of an additional source for DAG besides PIP2.