The stoichiometry of oxygen uptake and conjugated diene formation during the dioxygenation of linoleic acid by the pure reticulocyte lipoxygenase. Evidence for aerobic hydroperoxidase activity

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

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

A simple and efficient method for hemoglobin removal from mammalian tissue cytosol by zinc sulfate and its application to the study of lipoxygenase

A simple and efficient method is described to remove hemoglobin (Hb) from human term placental cytosol to study dioxygenase and co-oxidase activities of lipoxygenase. In the untreated samples, 70%-80% of the linoleic acid-dependent dioxygenase and co-oxidase activities were found to be associated with the pseudo-lipoxygenase activity of Hb. Zinc sulfate (0.5 mM) precipitated >97% of the Hb present in the cytosol. The dioxygenase activity of the ZnSO4 treated cytosol exhibited a Vmax value of 313 nmoles linoleic acid hydroperoxide formed/min/mg protein and a K(M) of 1.4 mM for linoleic acid. The ZnSO4 treated cytosol displayed co-oxidase activity toward benzidine, dimethoxybenzidine, guaiacol, pyrogallol, tetramethylbenzidine and tetramethyl-p-phenylenediamine. Nordihydroguaiaretic acid, 5,8,11-eicosatriynoic acid, butylated hydroxyanisole, butylated hydroxytoluene and gossypol caused concentration dependent inhibition of dioxygenase and co-oxidase activities. These results suggest ZnSO4 precipitation of Hb from cytosol does not alter the functional characteristics of the human term placental lipoxygenase.

Co-oxidation of NADH and NADPH by a mammalian 15-lipoxygenase: inhibition of lipoxygenase activity at near-physiological NADH concentrations

The purified 15-lipoxygenase from rabbit reticulocytes is capable of oxidizing NADH in the presence of linoleic acid and oxygen. This co-oxidation proceeds at a rate that amounts to approx. 7% of linoleic acid oxygenation rates. Although NADH inhibits the lipoxygenase reaction with linoleic acid as substrate (46% inhibition at 0.2 mM NADH), the reaction specificity of the enzyme was not altered since (13S)-hydroperoxy-(9Z,11E)-octadecadienoic acid was identified as the major reaction product. NADH oxidation was inhibited by NAD+ (uncompetitive with respect to linoleate and mixed/competitive with respect to NADH), and NADPH or NMNH could substitute for NADH with slightly different apparent Km values. NADH oxidation was enhanced at lower oxygen tension, but was completely prevented under anaerobic conditions. Computer-assisted modelling of 15-lipoxygenase/NADH interaction and sequence alignments of mammalian lipoxygenases with NADH-dependent enzymes suggested that there is no specific binding of the coenzyme at the putative fatty acid-binding site of lipoxygenases. These results suggest that NAD(P)H might be oxidized by a radical intermediate formed during the dioxygenase cycle of the lipoxygenase reaction but that NADH oxidation might not proceed at the active site of the enzyme. The mechanism and possible biological consequences of 15-lipoxygenase-catalysed NAD(P)H oxidation are discussed.

Inhibition of soybean lipoxygenase-1 by a diaryl-N-hydroxyurea by reduction of the ferric enzyme

It has been proposed that catechols and other antioxidants inhibit lipoxygenase activity by reducing the active Fe3+ form of the enzyme [Kemal et al. (1987) Biochemistry 26, 7064-7072]. In this model, reductively inactivated lipoxygenase can be reactivated by reaction with the hydroperoxide product in a pseudoperoxidase reaction. The contribution of enzyme reduction in the inhibition of the activity of soybean lipoxygenase-1 by the reducing inhibitor N-(4-chlorophenyl)-N-hydroxy-N'-(3-chlorophenyl)-urea (CPHU) has been evaluated quantitatively. The inhibition by CPHU of the oxygenation of linoleic acid to 13-hydroperoxy-9,11-octadecadienoic acid (13-HpODE) was accompanied by an initial lag phase which could be eliminated by the presence of exogenous 13-HpODE at the initiation of the reaction. In addition, both 13-HpODE and CPHU were found to be consumed during the lipoxygenase reaction, indicating occurrence of both oxygenase and pseudoperoxidase reactions. When analyzed individually, both the oxygenase reaction at different linoleic acid and O2 concentrations and the pseudoperoxidase reaction at different 13-HpODE and CPHU concentrations were found to follow ping-pong kinetics. A rate equation for the lipoxygenase-catalyzed reaction in the presence of reducing agent was derived considering that the inhibition of the oxygenase reaction is the combined result of 13-HpODE consumption and formation of inactive Fe2+ enzyme due to occurrence of the pseudoperoxidase reaction. By comparing the experimental data with those predicted by the rate equation, it is concluded that the inactivation of the enzyme by reduction can quantitatively account for the inhibition caused by CPHU.

Enzymatic oxidation of phenothiazines by lipoxygenase/H2O2 system

Lipoxygenase (LOX) (EC 1.13.11.12) oxidized a wide range of phenothiazine (Pt) tranquillizers to their corresponding radical cations in the presence of H2O2 by means of an enzymatic chemical second-order mechanism with substrate regeneration similar to that of horseradish peroxidase. The optimum pH of LOX for this hydroperoxidase activity was in the acid range (pH 3.0-4.0), as has been shown for other Pt oxidizing systems, such as peroxidase/H2O2 and haemoglobin. LOX showed Michaelis constants for Pt ranging from 1.4 to 8.5 mM and which, in some cases, e.g. trifluoperazine, displayed substrate inhibition. By contrast, it had a high affinity for H2O2 in the microM to mM range. A new, previously undescribed plot, which relates the enzymatic affinity and the apparent second-order decay of the cation radical, was developed to study the influence of the 2- and 10-substituents in the Pt ring. The implications of this new plot and the LOX-mediated Pt oxidation are also discussed.

Superoxide and peroxyl radical generation from the reduction of polyunsaturated fatty acid hydroperoxides by soybean lipoxygenase

Soybean lipoxygenase is shown to catalyze the breakdown of polyunsaturated fatty acid hydroperoxides to produce superoxide radical anion as detected by spin trapping with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). In addition to the DMPO/superoxide radical adduct, the adducts of peroxyl, acyl, carbon-centered, and hydroxyl radicals were identified in incubations containing linoleic acid and lipoxygenase. These DMPO radical adducts were observed just prior to the system becoming anaerobic. Only a carbon-centered radical adduct was observed under anaerobic conditions. The superoxide radical production required the presence of fatty acid substrates, fatty acid hydroperoxides, active lipoxygenase, and molecular oxygen. Superoxide radical production was inhibited when nordihydroguaiaretic acid, butylated hydroxytoluene, or butylated hydroxyanisole was added to the incubation mixtures. We propose that polyunsaturated fatty acid hydroperoxides are reduced to form alkoxyl radicals and that after an intramolecular rearrangement, the resulting hydroxyalkyl radical reacts with oxygen, forming a peroxyl radical which subsequently eliminates superoxide radical anion.

Reaction of xanthine oxidase-derived oxidants with lipid and protein of human plasma

Xanthine oxidase and purines have recently been detected in the circulation during acute viral infection and following hepatotoxicity and shock. Reactions of xanthine oxidase-generated oxidants with human plasma or bovine serum albumin (BSA) and egg phosphatidylcholine (PC) liposomes have been studied by measuring protein sulfhydryl oxidation and two markers of free radical-mediated lipid peroxidation, thiobarbituric acid reactive substances (TBARS) and conjugated dienes. Plasma incubated with 5 mU/ml xanthine oxidase (XO) and 0.5 mM hypoxanthine (Hx) for 2 h at 37 degrees C had 25-53% oxidation of sulfhydryl groups, with greater than 80% of the oxidation occurring during the first 20 min of the reaction. Concentrations of BSA similar to those present in serum, when exposed to XO/Hx-mediated oxidative stress, showed an even greater decrease in sulfhydryl concentration than that of plasma. No significant increase in plasma TBARS and conjugated dienes was observed during the 2-h incubation period in the presence of XO. Egg PC liposomes, suspended to a plasma phospholipid-equivalent concentration, showed a minor increase in TBARS and conjugated dienes under similar XO/Hx incubation conditions. In the presence of 0.23 mM BSA, lipid peroxidation was completely inhibited. A similar inhibition of lipid peroxidation was induced by cysteine but not by uric acid. Electrophoretic and arsenite-mediated sulfur reduction analysis revealed that BSA was oxidized beyond the disulfide form, with sulfenic acid formed during the initial period of oxidation. Protein sulfhydryls served as sacrificial antioxidants, preventing plasma lipid peroxidation, as well as being targets for oxidative damage. Plasma protein thiol oxidation was determined to be a more sensitive and specific indication of oxidant stress to the vascular compartment than assessment of lipid oxidation byproducts.

Occurrence of 9- and 13-keto-octadecadienoic acid in biological membranes oxygenated by the reticulocyte lipoxygenase

Membranes of intact rabbit reticulocytes and rat liver mitochondrial membranes oxygenated by the pure reticulocyte lipoxygenase contain 13-keto-9Z,11E-octadecadienoic acid and 9-keto-10E,12Z-octadecadienoic acid. In mitochondrial membranes not treated with lipoxygenase and in rabbit erythrocyte membranes these products were not detected. The chemical structure of the compounds has been identified by cochromatography with authentic standards on various types of HPLC columns, by uv and ir spectroscopy and GC/MS. In the membranes of rabbit reticulocytes up to 2% of the linoleate residues are present as its 9- and 13-keto derivatives. Most of the keto compounds (up to 90%) are esterified in the membrane ester lipids, only about 10% were found in the free fatty acid fraction. It is proposed that the keto dienoic fatty acids are formed via decomposition of hydroperoxy polyenoic fatty acids originating from the oxygenation of the membrane lipids by the reticulocyte lipoxygenase.

Oxygenation of mitochondrial membranes by the reticulocyte lipoxygenase. Action on monoamine oxidase activities A and B

Incubation of isolated rat liver mitochondria with the pure rabbit reticulocyte lipoxygenase caused a time-dependent inactivation of the monoamine oxidase activities A and B. Furthermore, a conversion of the monoamine oxidase into a diamine oxidase was observed. The inactivation kinetics for both monoamine oxidase activities A and B showed a biphasic behaviour; a reversible short-term inhibition during the first 5 min of incubation was followed by an irreversible inactivation of the enzyme. The kinetic studies suggest that the slow irreversible inactivation of the monoamine oxidase activities is due to secondary reactions subsequent to the initial attack of the lipoxygenase on the mitochondrial outer membrane. During the interaction of the lipoxygenase with the mitochondria, only about 1.5% of the polyenoic fatty acids present in the mitochondrial membranes were oxygenated. The predominant products formed during the interaction of the lipoxygenase with the mitochondrial membranes are (13S)-hydro(pero)xy-9Z,11E-octadecadienoic acid and (15S)-hydro(pero)xy-5,8,11,13(Z,Z,Z,E)-eicosatetraenoic acid.

Lipid peroxidation of phosphatidylcholine liposomes depressed by the calcium channel blockers nifedipine and verapamil and by the antiarrhythmic-antihypoxic drug stobadine

Nifedipine, verapamil and stobadine were tested and compared with butylated hydroxytoluene (BHT) as possible free radical scavengers inhibiting lipid peroxidation in phosphatidylcholine liposomes. Liposomes were peroxidized by incubation in air at 50 degrees C. Verapamil less than nifedipine less than BHT less than stobadine depressed the lipid peroxidation as detected spectroscopically for conjugate diene and thiobarbituric acid product formation. Verapamil and stobadine were tested as OH radical scavengers in a Fenton-type reaction against spin trap 5,5'-dimethyl-1-pyrroline-N-oxide (DMPO), as detected by ESR spectroscopy. The tested drugs competed with DMPO in trapping OH radicals, with stobadine being more effective than verapamil. ESR spectra of nifedipine in the incubated liposomes revealed that nifedipine could be involved in free radical reactions in the liposomes leading to nifedipine-stable radical(s) which were immobilized in the membrane. The obtained results suggest that some of the beneficial effects of the studied drugs can be mediated in disease by their ability to scavenge free radicals and by their protective effect on lipid peroxidation.

Hydroperoxidase activity of lipoxygenase: hydrogen peroxide-dependent oxidation of xenobiotics

Since H2O2 is one of the major biologically available peroxides, its ability to support hydroperoxidase activity of highly purified soybean lipoxygenase was examined by monitoring co-oxidation of selected xenobiotics. All of the eight chemicals tested were found to be oxidized in the presence of H2O2. Tetramethylbenzidine oxidation was completely inhibited by the classical lipoxygenase inhibitor nordihydroguaiaretic acid. The reaction was enzymatic in nature and exhibited a acidic pH optimum. The data clearly indicate, for the first time, that H2O2 can efficiently replace fatty acid hydroperoxide in a xenobiotic oxidation reaction medicated by the hydroperoxidase activity of lipoxygenase.

A kinetic model for lipoxygenases based on experimental data with the lipoxygenase of reticulocytes

A comprehensive kinetic model for lipoxygenase catalysis is proposed which includes the simultaneous occurrence of dioxygenase and hydroperoxidase activities and is based on the assumption of a single binding site for substrate fatty acid and product. The aerobic reaction of purified lipoxygenase from rabbit reticulocytes with 9,12(Z,Z)-octadecadienoic acid (linoleic acid) as substrate was studied. The rate constants and the dissociation constants of this enzyme were calculated for the model from progress curves; the model describes correctly the experimental data. The following kinetic features of the reticulocyte enzyme are assumed to apply generally to lipoxygenases. (a) The enzyme shows autoactivation by its product. (b) The rate-limiting step is the hydrogen abstraction. (c) Both substrate fatty acid and its product are competitive inhibitors of the lipoxygenase. (d) Lowering the oxygen concentration enhances the degree of substrate inhibition, whereas product inhibition is not influenced. (e) If substrate is in excess the oxygen concentration determines the share of dioxygenase and hydroperoxidase activities of the enzyme. As predicted from the model it was found that at low concentrations of oxygen the regio- and stereo-specificities of the dioxygenation are diminished. During the autoactivation phase the steady-state approximation does not hold.

Applications of electron spin resonance spectroscopy to the identification of radicals produced during lipid peroxidation

Electron spin resonance (ESR) spectroscopy, which is the only commonly available method for directly detecting free radicals in biological systems, has now been quite extensively used to study radicals produced by metabolism of xenobiotic chemicals and the interaction of such species with lipid molecules. This review examines a variety of different xenobiotic systems and tissues and summarises the information obtained from these studies, with particular reference to the elucidation of the nature of the radicals involved in the initiation and propagation of lipid peroxidation.

Peroxidase, an alternate pathway to cytochrome P-450 for xenobiotic metabolism in skin: partial purification and properties of the enzyme from neonatal rat skin

Peroxidase activity was partially purified from neonatal (3 to 6 days old) rat skin. The membrane-bound peroxidase activity was extracted with 0.5 M calcium chloride and was monitored spectrophotometrically at 470 nm with 2-methoxyphenol (guaiacol) and hydrogen peroxide as substrates. Subcellular distribution studies indicated the activity to be highest and comparable in nuclei and mitochondria, lowest in microsomes, and absent in cytosol. The peroxidase activity was partially purified by affinity chromatography on concanavalin A-sepharose 4B and by gel filtration using Bio-Gel P-150. Purification factors from these two steps were about 25 and 4, respectively. Peroxidase extraction in the presence of 2 mM N-ethylmaleimide increased activity about twofold. The combination of 2 mM N-ethylmaleimide and 10% (w/v) glycerol was found to be optimal for preservation of activity. Peroxidase activity increased linearly with increases in protein concentration, time, and guaiacol concentration. Activity was inhibited approximately 75% by 0.1 mM potassium cyanide or 0.05 mM sodium azide. Pyrogallol, hydroquinone, p-cresol, catechol, benzidine, 3,3'-dimethoxybenzidine, tetramethylbenzidine and p-phenylenediamine also acted as substrates for the rat cutaneous peroxidase.