Bacterial lipoxygenases, a new subfamily of enzymes? A phylogenetic approach

Lipoxygenases (EC. 1.13.11.12) are a non-heme iron enzymes consisting of one polypeptide chain folded into two domains, the N-terminal domain and the catalytic moiety β-barrel domain. They catalyze the dioxygenation of 1Z,4Z-pentadiene moieties of polyunsaturated fatty acids obtaining hydroperoxy fatty acids. For years, the presence of lipoxygenases was considered a eukaryotic feature, present in mammals, plants, small marine invertebrates, and fungi, but now, some lipoxygenase sequences have been detected on prokaryotic organisms, changing the idea that lipoxygenases are exclusively a eukaryotic affair. Lipoxygenases are involved in different types of reactions on eukaryote organisms where the biological role and the structural characteristics of these enzymes are well studied. However, these aspects of the bacterial lipoxygenases have not yet been elucidated and are unknown. This revision discusses biochemical aspects, biological applications, and some characteristics of these enzymes and tries to determine the existence of a subfamily of bacterial lipoxygenases in the context of the phylogeny of prokaryotic lipoxygenases, supporting the results of phylogenetic analyzes with the comparison and discussion of structural information of the first prokaryotic lipoxygenase crystallized and other eukaryotic lipoxygenases structures.     

Evolutionary aspects of lipoxygenases and genetic diversity of human leukotriene signaling

Leukotrienes are pro-inflammatory lipid mediators, which are biosynthesized via the lipoxygenase pathway of the arachidonic acid cascade. Lipoxygenases form a family of lipid peroxidizing enzymes and human lipoxygenase isoforms have been implicated in the pathogenesis of inflammatory, hyperproliferative (cancer) and neurodegenerative diseases. Lipoxygenases are not restricted to humans but also occur in a large number of pro- and eucaryotic organisms. Lipoxygenase-like sequences have been identified in the three domains of life (bacteria, archaea, eucarya) but because of lacking functional data the occurrence of catalytically active lipoxygenases in archaea still remains an open question. Although the physiological and/or pathophysiological functions of various lipoxygenase isoforms have been studied throughout the last three decades there is no unifying concept for the biological importance of these enzymes. In this review we are summarizing the current knowledge on the distribution of lipoxygenases in living single and multicellular organisms with particular emphasis to higher vertebrates and will also focus on the genetic diversity of enzymes and receptors involved in human leukotriene signaling.     

15-lipoxygenase-1: a prooxidant enzyme

Human and rabbit reticulocyte 15-lipoxygenase (15-lipoxygenase-1) and the leukocyte-type 12-lipoxygenases (12/15-lipoxygenases) of pig, beef, mouse and rat constitute a particular subfamily of mammalian lipoxygenases (reticulocyte-type lipoxygenases) with unique properties and functions. They catalyze enzymatic lipid peroxidation in complex biological structures via direct dioxygenation of phospholipids and cholesterol esters of biomembranes and plasma lipoproteins. Moreover, they are a source of free radicals initiating non-enzymatic lipid peroxidation and other oxidative processes. Expression and activity of reticulocyte-type lipoxygenases are highly regulated. Moreover, the susceptibility of intracellular membranes toward these lipoxygenases is controlled and may be increased together with lipoxygenase activity under conditions of oxidative stress. Thus, oxidative stress may favor a concerted package of lipoxygenase-mediated enzymatic and non-enzymatic lipid peroxidation and co-oxidative processes. Reaction of reticulocyte-type lipoxygenases with low-density lipoprotein renders the latter atherogenic and appears to be involved in the formation of atherosclerotic lesions.     

Enzymatic lipid peroxidation

Biosynthesis of certain biologically active substances (prostaglandins, thromboxanes, prostacyclins and leukotrienes) in animal tissues occurs with participation of cyclooxygenases and lipoxygenases, enzymic systems of lipid peroxidation. In normal physiological and pathological processes the enzymic lipid peroxidation by microsomal dioxygenases is considerably more active than the nonenzymic one in the same membrane structures. The molecular structure of the products of the enzymic and nonenzymic peroxidation of lipids also differs essentially. An assumption is advanced that cytosol lipoxygenase may be an easily dissociating component of the cyclooxygenase multienzymic complex and its transition from the biomembrane to the cell cytoplasm is accompanied by changes in the enzyme conformation and chemical nature of the products resulted from polyenic lipids oxidation catalyzed by the enzyme.     

Effects of sparsely and densely ionizing radiation on plants

One of the main purposes leading botanists to investigate the effects of ionizing radiations is to understand plant behaviour in space, where vegetal systems play an important role for nourishment, psychological support and functioning of life support systems. Ground-based experiments have been performed with particles of different charge and energy. Samples exposed to X- or γ-rays are often used as reference to derive the biological efficiency of different radiation qualities. Studies where biological samples are exposed directly to the space radiation environment have also been performed. The comparison of different studies has clarified how the effects observed after exposure are deeply influenced by several factors, some related to plant characteristics (e.g. species, cultivar, stage of development, tissue architecture and genome organization) and some related to radiation features (e.g. quality, dose, duration of exposure). In this review, we report main results from studies on the effect of ionizing radiations, including cosmic rays, on plants, focusing on genetic alterations, modifications of growth and reproduction and changes in biochemical pathways especially photosynthetic behaviour. Most of the data confirm what is known from animal studies: densely ionizing radiations are more efficient in inducing damages at several different levels, in comparison with sparsely ionizing radiation.     

Biosynthesis, catabolism, and biological properties of HPETEs, hydroperoxide derivatives of arachidonic acid

The oxygenation of arachidonic acid by lipoxygenases results in the formation of HPETEs (hydroperoxyeicosatetraenoic acids), the first products of the LOX pathway. These compounds are short lived and are catabolised into various families of more stable compounds of which the HETEs, hepoxilins, lipoxins and leukotrienes have been identified so far. The development of new techniques have helped to identify and understand the structures of various HPETEs and only recently the biological effects of HPETEs and their various catabolites are being unraveled. Although lipoxygenases are ubiquitous, not all tissues possess the same spectrum of lipoxygenase enzymes. Hence different HPETEs can be formed in different tissues. Recent studies have revealed that HPETEs or products derived from them possess a diversity of important biological properties including the regulation of electrolyte flux and eicosanoid and corticosterone syntheses, release of histamine, regulation of oocyte maturation and release of various reproductive hormones. HPETEs appear to be involved in some pathological conditions viz, skin psoriasis, Clarkson's disease, nerve injury and spinal cord ischemia. These novel eicosanoids are associated with the release of insulin as well as renin. Recently HPETEs have been suggested to act as second messengers in the Aplysia sensory neurons and its catabolite, hepoxilin, has been demonstrated to have effects on mammalian hippocampal neurons. The purpose of this review is to provide a brief summary of the formation of the HPETEs and the various families of compounds derived from them as well as the various types of biological activities for these products described so far.     

The differential effect of disulfiram on lipoxygenases from Glycine max

A kinetic analysis of the effect of disulfiram on the isoenzymes of lipoxygenase from soybean has been carried out. The compound is an effective inhibitor of type-2 isoenzymes but has no effect on the type-1 isoenzyme under the conditions employed in this study. The inhibitory effect is reversible and therefore does not result from covalent modification of the enzyme. The inhibition is manifest as a prolongation of the lag phase commonly seen in progress curves for lipoxygenases rather than as a reduction of the catalysed rate. A variety of structurally related and unrelated compounds have been investigated to identify the nature of the inhibitory effect. The antioxidant properties of disulfiram account for its ability to inhibit type-2 lipoxygenases. The inhibitory effect of antioxidants on type-2 lipoxygenase is only partly reversed when product hydroperoxide is included in the incubation mixture. These observations support the conclusion that free radical intermediates are integral to the catalytic mechanism of type-2 lipoxygenases.     

Activity of soybean lipoxygenase isoforms against esterified fatty acids indicates functional specificity

In soybean (Glycine max L.) vegetative tissue at least five lipoxygenase isozymes are present. Four of these proteins have been localized to the paraveinal mesophyll, a layer of cells that is thought to function in assimilate partitioning. In order to determine the role of the lipoxygenase isozymes within the soybean plant, the leaf lipoxygenases were cloned into bacterial expression vectors and expressed in Escherichia coil. The recombinant lipoxygenases were then characterized as to substrate preference, pH profiles for the most common plant lipoxygenase substrates, linoleic acid, and alpha-linolenic acid, and the reaction products with the substrates linoleic acid, alpha-linolenic acid, arachidonic acid, gamma-linolenic acid, and the triacylglycerol trilinolein. All five enzymes were shown to be (13S)-lipoxygenases against linoleic acid. The results of these assays also indicate that two of these isozymes are highly active against esterified fatty acid groups, such as those found in triacylglycerols. Lipid analysis of leaves from plants subjected to sink limitation conditions indicates that the soybean leaf lipoxygenases are active in vivo against both free fatty acids and esterified lipids, and that the quantities of lipoxygenase products found in leaf tissue show a positive correlation with the level of lipoxygenase in the leaf. Implications for the putative role of these enzymes in the paraveinal mesophyll are discussed.     

Effect of ionizing radiation on the kinetic parameters of rat cerebral cortex Na, K-ATPase

A study was made of the effect of ionizing radiation of 10.3 and 180.6 mC/kg on kinetic parameters of the processes of activation of Na,K-ATPase of rat brain cortex by Mg-ATP-substrate and Na+ and K+ ions. The obtained results prompt an assumption that a conformational rearrangement occurs under the effect of ionizing radiation which is not identical after relatively small and lethal radiation doses.     

Conserved histidine residues in soybean lipoxygenase: functional consequences of their replacement.

Sequences of 13 lipoxygenases from various plant and mammalian species, thus far reported, display a motif of 38 amino acid residues which includes 5 conserved histidines and a 6th histidine about 160 residues downstream. These residues occur at positions 494, 499, 504, 522, 531, and 690 in soybean lipoxygenase isozyme L-1. Since the participation of iron in the lipoxygenase reaction has been established and existing evidence based on M?ssbauer and EXAFS spectroscopy suggests that histidines may be involved in iron binding, the effect of the above residues has been examined in soybean lipoxygenase L-1. Six singly mutated lipoxygenases have been produced in which each of the His residues has been replaced with glutamine. Two additional mutants have been constructed wherein the codons for His-494 and His-504 have been replaced by serine codons. All of the mutant lipoxygenases, which were obtained by expression in Escherichia coli, have mobilities identical to that of the wild-type enzyme on denaturing gel electrophoresis and respond to lipoxygenase antibodies. The mutated proteins H499Q, H504Q, H504S, and H690Q are virtually inactive, while H522Q has about 1% of the wild-type activity. H494Q, H494S, and H531Q are about 37%, 8%, and 20% as active as the wild type, respectively. His-517 is conserved in the several lipoxygenase isozymes but not in the animal isozymes. The mutant H517Q has about 33% of the wild-type activity. The inactive mutants, H499Q, H504Q, H504S, and H690Q, become insoluble when heated for 3 min at 65 degrees C, as does H522Q. The other mutants and the wild-type are stable under these conditions.(ABSTRACT TRUNCATED AT 250 WORDS)     

The role of regulatory networks of the cell response to damages in radiation effects

In view of modern knowledge and concepts about components, function and mechanisms of response of cell molecular structures to damaging effects, response which is generating specialized modules of reactions, it is shown that main components of the mechanism of maintenance of genome constancy at ionizing radiation exposure are checkpoints of cell cycle, DNA repair and apoptosis. They operate under the control of a genetic system at participation of Tp53 gene, corresponding protein and of regulatory networks formed by cascades of mitogen-activated protein kinases (MAPK). At ionizing radiation exposure the MAPK special modules participate in formation of radiation effect: ERK 1/2 (extracellular signal-regulated kinase 1 and 2), JNK/SAPK (c-Jun N-terminal kinase/stress activated protein kinase) and p38 MAPK. Executing physiological functions of maintenance of normal life activity of cells, they do not lose this capacity after exposure to ionizing radiation, participating in formation of radiation effect in a wide range of doses, and are inactivated only by exposure to very high doses. It is concluded that in light of the modern data the main problem is not a problem of mechanisms of biological effect of ionizing radiation but a problem of biological mechanisms of radiation exposure.     

Fatty acid-derived lipid mediators and blood platelet aggregation

Polyunsaturated fatty acids of nutritional value may affect cell functions after their release from cell lipid storage sites, especially phospholipids, and specific oxygenation by cyclooxygenases, lipoxygenases and cytochrome P₄₅₀. The end-products, namely prostanoids, leukotrienes, and mono-, di- and tri-hydroxy derivatives exhibit a variety of biological effects, especially on vascular cells, leukocytes and platelets. This paper reviews some results obtained with blood platelets as target cells, showing that various lipoxygenase end-products, mainly mono- and di-hydroxy derivatives, are inhibitors (IC₅₀ in μM range) of arachidonic acid-induced aggregation either at the cycloxygenase or thromboxane receptor site level.     

The N-terminal β-barrel domain of mammalian lipoxygenases including mouse 5-lipoxygenase is not essential for catalytic activity and membrane binding but exhibits regulatory functions

Mammalian lipoxygenases (LOXs) have been implicated in cell differentiation and in the pathogenesis of inflammatory and hyperproliferative diseases. The available structural information indicated that lipoxygenases constitute single polypeptide chain enzymes consisting of a small N-terminal β-barrel domain and a larger C-terminal subunit that harbors the catalytic non-heme iron. Because of its structural similarity to C2-domains of lipases the N-terminal β-barrel domain of lipoxygenases, which comprises about 110 amino acids, has been implicated in membrane binding and activity regulation. To explore the functional relevance of the C2-domain in more detail and to develop a more comprehensive hypothesis on the biological role of this structural subunit we performed gene technical truncation on various mammalian LOX isoforms (12/15-LOXs of various species, human 15-LOX2, mouse 5-LOX) and quantified catalytic activity and membrane binding properties of the truncated recombinant enzyme species. We found that the C2-domain is not essential for catalytic activity and does hardly impact reaction specificity. Truncated enzyme species exhibit impaired membrane binding properties and altered reaction kinetics. Taken together, our data suggests a regulatory importance of the N-terminal β-barrel domain for mammalian lipoxygenase isoforms.     

Post-translational modifications of protein in response to ionizing radiation

Based on central dogma of genetics, protein is the embodiment and executor of genetic function, post-translational modifications (PTMs) of protein are particularly important and involved in almost all aspects of cell biology and pathogenesis. Studies have shown that ionizing radiation (IR) alters gene expression much more profoundly and a broad variety of cell-process pathways, lots of proteins are modified and activated. Our understanding of the protein in response to ionizing radiation is steadily increasing. Among the various biological processes known to induce radioresistance, PTMs have attracted marked attention in recent years. The present review summarizes the latest knowledge about how PTMs response to ionizing radiation and pathway analysis were conducted. The data provided insights into biological effects of IR and contributing to the development of novel IR-based strategies.     

Radiation induced peroxidative damage: mechanism and significance

An interest has been generated in free radicals after the discovery of superoxide dismutase. These free radicals cause a number of diseases and are involved in the detrimental effect of ionizing radiation. Efforts have been made to understand their role in damage and death of the cell using lipid peroxidation process. Lipid peroxidation is an important effect of radiation on membranes, which apart from DNA, are critical targets of radiation action. This paper addresses the basic mechanism of radiation induced lipid peroxidation. Various factors, which determine the mode and magnitude of lipid peroxidation, are also discussed. Lipid peroxidation is shown to have importance in understanding the modifications of radiation effects. Efforts are made to show similarities between radiolytic and non-radiolytic lipid peroxidation. Recent findings related to the close link between radiation-induced lipid peroxidation and apoptosis are likely to open new avenues for future research and to develop new approaches for radiomodification of biological effects.     

Lipoxygenases and their involvement in programmed cell death

Lipoxygenases are a family of enzymes which dioxygenate unsaturated fatty acids, thus initiating lipoperoxidation of membranes and the synthesis of signaling molecules. Consequently, they induce structural and metabolic changes in the cell in a number of pathophysiological conditions. Recently, a pro-apoptotic effect of lipoxygenase, and of the hydroperoxides produced thereof, has been reported in different cells and tissues, leading to cell death. Anti-apoptotic effects of lipoxygenases have also been reported; however, this has often been based on the use of enzyme inhibitors. Here we review the characteristics of the lipoxygenase family and its involvement in the initiation of oxidative stress-induced apoptosis. Finally, we discuss the role of lipoxygenase activation in apoptosis of animal and plant cells, suggesting a common signal transduction pathway in cell death conserved through evolution of both kingdoms.     

Immunological comparison of lipoxygenase isozymes-1 and -2 with soybean seedling lipoxygenases

An affinity-purified polyclonal antibody against soybean seed lipoxygenase-2 was prepared and used to characterize the immunological relatedness of lipoxygenase isozymes 1 and 2 and lipoxygenases from soybean seedling roots, hypocotyls, leaves, and cotyledons. All soybean lipoxygenases tested cross-reacted with the anti-lipoxygenase-2. Cross-reactivity of seed-derived lipoxygenases was evidenced by formation of a line of identity in double-diffusion tests, by positive results on an immunoblot, and by antibody precipitation of enzyme activity. Levels of anti-lipoxygenase-2, which inhibited lipoxygenase-2 activity, had no effect on lipoxygenase-1 activity. Root, hypocotyl, and leaf lipoxygenases did not form precipitation lines in double-diffusion tests but the anti-lipoxygenase-2 did inhibit and precipitate lipoxygenase activity from these sources as well as cross-react on immunoblots. All the cross-reactive lipoxygenases examined were found to have the same apparent molecular weight. Lipoxygenase activity found in soybean seedling roots, hypocotyls, leaves, and cotyledons is associated with proteins which are all immunologically related to the seed-derived enzymes.     

The linear no-threshold model does not hold for low-dose ionizing radiation

Almost all of the data on the biological effects of ionizing radiation come from studies of high doses. However, the human population is unlikely to be exposed to such doses. Regulatory limits for radiation exposure are based on the linear no-threshold model, which predicts that the relationship between biological effects and radiation dose is linear, and that any dose has some effect. Chromosomal changes are an important effect of ionizing radiation because of their role in carcinogenesis. Here we exposed pKZ1 mice to single whole-body X-radiation doses as low as 1 microGy. We observed three different phases of response: (1) an induction of inversions at ultra-low doses, (2) a reduction below endogenous inversion frequency at low doses, and (3) an induction of inversions again at higher doses. These results do not fit a linear no-threshold model, and they may have implications for the way in which regulatory standards are presently set and for understanding radiation effects.     

Lipoxygenase inhibitors, eicosapolynoic acids, attenuate the short-term plasticity of the cholinoreceptors of snail neurons]

On identified Helix neurones RPa3 and LPa3 with the use of the double-electrode voltage clamp technique on the membrane the influence was studied of three polyacetilenic analogues of natural polyenoic acids which were the inhibitors of their lipoxygenase oxidation on the dynamics of inward current extinction, caused by the repeated iontophoretic applications of acetylcholine to soma. It was found that 5, 8, 11, 14-eicosatetraynoic acid (30-60 microM) and 5, 8, 11, 14, 17-eicosapentaynoic acid (4-5 microM) decreased the amplitude of inward current caused by acetylcholine leading and weakened its extinction at the repeated applications. The third analogue -8, 11, 14-eicosatrynoic acid had no modulating influence on the value of current and on its extinction. The supposition was made that lipoxygenase metabolites of polyenoic acids regulated plasticity of Helix neurones cholinoreceptors. Considering different inhibiting by the used compounds of various lipoxygenases the most probable was participation in regulation of cholinoreceptors plasticity of those eicosanoids which were formed from arachidonic acid under the influence of 5-lipoxygenase. Regulating role of eicosanoids formed at the action of other lipoxygenases was not excluded.     

Structural basis for pH-dependent alterations of reaction specificity of vertebrate lipoxygenase isoforms

Lipoxygenases have been classified according to their specificity of fatty acid oxygenation and for several plant enzymes pH-dependent alterations in the product patterns have been reported. Assuming that the biological role of mammalian lipoxygenases is based on the formation of specific reaction products, pH-dependent alterations would impact enzymes' functionality. In this study we systematically investigated the pH-dependence of vertebrate lipoxygenases and observed a remarkable stability of the product pattern in the near physiological range for the wild-type enzyme species. Site-directed mutagenesis of selected amino acids and alterations in the substrate concentrations induced a more pronounced pH-dependence of the reaction specificity. For instance, for the V603H mutant of the human 15-lipoxygenase-2 8-lipoxygenation was dominant at acidic pH (65%) whereas 15-H(p)ETE was the major oxygenation product at pH 8. Similarly, the product pattern of the wild-type mouse 8-lipoxygenase was hardly altered in the near physiological pH range but H604F exchange induced strong pH-dependent alterations in the positional specificity. Taken together, our data suggest that the reaction specificities of wild-type vertebrate lipoxygenase isoforms are largely resistant towards pH alterations. However, we found that changes in the assay conditions (low substrate concentration) and introduction/removal of a critical histidine at the active site impact the pH-dependence of reaction specificity for some lipoxygenase isoforms.