Mutagenesis studies on the amino acid residues involved in the iron-binding and the activity of human 5-lipoxygenase

Diminished arachidonate 5-lipoxygenase perturbs phase separation and transcriptional response of Runx2 to reverse pathological ventricular remodeling

BACKGROUND

Arachidonate 5-lipoxygenase (Alox5) belongs to a class of nonheme iron-containing dioxygenases involved in the catalysis of leukotriene biosynthesis. However, the effects of Alox5 itself on pathological cardiac remodeling and heart failure remain elusive.

METHODS

The role of Alox5 in pathological cardiac remodeling was investigated by Alox5 genetic depletion, AAV9-mediated overexpression in cardiomyocytes, and a bone marrow (BM) transplantation approach. Neonatal rat cardiomyocytes were used to explore the effects of Alox5 in vitro. Molecular and signaling pathways were revealed by CUT &Tag, IP-MS, RNA sequencing and bioinformatic analyses.

FINDINGS

Untargeted metabolomics showed that serum 5-HETE (a primary product of Alox5) levels were little changed in patients with cardiac hypertrophy, while Alox5 expression was significantly upregulated in murine hypertensive cardiac samples and human cardiac samples of hypertrophy, which prompted us to test whether high Alox5 levels under hypertensive stimuli were directly associated with pathologic myocardium in an enzymatic activity-independent manner. Herein, we revealed that Alox5 deficiency significantly ameliorated transverse aortic constriction (TAC)-induced hypertrophy. Cardiomyocyte-specific Alox5 depletion attenuated hypertensive ventricular remodeling. Conversely, cardiac-specifical Alox5 overexpression showed a pro-hypertrophic cardiac phenotype. Ablation of Alox5 in bone marrow-derived cells did not affect pathological cardiac remodeling and heart failure. Mechanically, Runx2 was identified as a target of Alox5. In this regard, Alox5 PEST domain could directly bind to Runx2 PTS domain, promoting nuclear localization of Runx2 in an enzymatic activity-independent manner, simultaneously contributed to liquid-liquid phase separation (LLPS) of Runx2 at specific domain in the nucleus and increased transcription of EGFR in cardiomyocytes. Runx2 depletion alleviated hypertrophy in Ang II-pretreated Alox5-overexpressing cardiomyocytes.

INTERPRETATION

Overall, our study demonstrated that targeting Alox5 exerted a protective effect against cardiac remodeling and heart failure under hypertensive stimuli by disturbing LLPS of Runx2 and substantial reduction of EGFR transcription activation in cardiomyocytes. Our findings suggest that negative modulation of Alox5-Runx2 may provide a therapeutic approach against pathological cardiac remodeling and heart failure.

FUNDING

National Natural Science Foundation of China.

Human 5-lipoxygenase regulates transcription by association to euchromatin

Human 5-lipoxygenase (5-LO) is the key enzyme of leukotriene biosynthesis, mostly expressed in leukocytes and thus a crucial component of the innate immune system. In this study, we show that 5-LO, besides its canonical function as an arachidonic acid metabolizing enzyme, is a regulator of gene expression associated with euchromatin. By Crispr-Cas9-mediated 5-LO knockout (KO) in MonoMac6 (MM6) cells and subsequent RNA-Seq analysis, we identified 5-LO regulated genes which could be clustered to immune/defense response, cell adhesion, transcription and growth/developmental processes. Analysis of differentially expressed genes (DEG) identified cyclooxygenase-2 (COX2, PTGS2) and kynureninase (KYNU) as strongly regulated 5-LO target genes. 5-LO knockout affected MM6 cell adhesion and tryptophan metabolism via inhibition of the degradation of the immunoregulator kynurenine. By subsequent FAIRE-Seq and 5-LO ChIP-Seq analyses, we found an association of 5-LO with euchromatin, with prominent 5-LO binding to promoter regions in actively transcribed genes. By enrichment analysis of the ChIP-Seq results, we identified potential 5-LO interaction partners. Furthermore, 5-LO ChIP-Seq peaks resemble patterns of H3K27ac histone marks, suggesting that 5-LO recruitment mainly takes place at acetylated histones. In summary, we demonstrate a noncanonical function of 5-LO as transcriptional regulator in monocytic cells.

Inhibition of arachidonic acid metabolism and its implication on cell proliferation and tumour-angiogenesis

Arachidonic acid (AA) and its metabolites have recently generated a heightened interest due to growing evidence of their significant role in cancer biology. Thus, inhibitors of the AA cascade, first and foremost COX inhibitors, which have originally been of interest in the treatment of inflammatory conditions and certain types of cardiovascular disease, are now attracting attention as an arsenal against cancer. An increasing number of investigations support their role in cancer chemoprevention, although the precise molecular mechanisms that link levels of AA, and its metabolites, with cancer progression have still to be elucidated. This article provides an overview of the AA cascade and focuses on the roles of its inhibitors and their implication in cancer treatment. In particular, emphasis is placed on the inhibition of cell proliferation and neo-angiogenesis through inhibition of the enzymes COX-2, 5-LOX and CYP450. Downstream effects of inhibition of AA metabolites are analysed and the molecular mechanisms of action of a selected number of inhibitors of catalytic pathways reviewed. Lastly, the benefits of dietary omega-3 fatty acids and their mechanisms of action leading to reduced cancer risk and impeded cancer cell growth are mentioned. Finally, a proposal is put forward, suggesting a novel and integrated approach in viewing the molecular mechanisms and complex interactions responsible for the involvement of AA metabolites in carcinogenesis and the protective effects of omega-3 fatty acids in inflammation and tumour prevention.

Mutation analysis of the human 5-lipoxygenase C-terminus: support for a stabilizing C-terminal loop

Lipoxygenases contain prosthetic iron, in human 5-lipoxygenase (5LO) the C-terminal isoleucine carboxylate constitutes one of five identified ligands. ATP is one of several factors determining 5LO activity. We compared properties of a series of 5LO C-terminal deletion mutants (one to six amino acid residues deleted). All mutants were enzymatically inactive (expected due to loss of iron), but expression yield (in E. coli) and affinity to ATP-agarose was markedly different. Deletion of up to four C-terminal residues was compatible with good expression and retained affinity to the ATP-column, as for wild-type 5LO. However when also the fifth residue was deleted (Asn-669) expression yield decreased and the affinity to ATP was markedly diminished. This was interpreted as a result of deranged structure and stability, due to loss of a hydrogen bond between Asn-669 and His-399. Mutagenesis of these residues supported this conclusion. In the structure of soybean lipoxygenase-1, a C-terminal loop was pointed out as important for correct orientation of the C-terminus. Accordingly, a hydrogen bond appears to stabilize such a C-terminal loop also in 5LO.

Characterization of calcium and magnesium binding domains of human 5-lipoxygenase

Two calcium binding sites, separated by about 9.3A, present in the loops that connect the beta-sheets of N-terminal domain contain the ligating residues F14, A15, G16, D79, and D18, D19, L76, respectively. Magnesium is found to bind in regions, which are marginally different owing to the disparity in the ionic radii of Ca2+ and Mg2+. The entropy analysis on the loops of 5-lipoxygenase, implementing the wormlike chain model, explains that the N-terminal beta-barrel is well suited to accommodate calcium binding sites. The large buried side chain area of W102 (compared to W13 and W75) and comparatively smaller fraction of side chain exposed to polar atoms corroborate the calcium induced higher affinity to phosphatidylcholine (PC). However, W80 lying in close proximity of the calcium binding sites is expected to have considerable PC affinity but negligible calcium induced effect on PC binding.

Site-Directed Mutagenesis Studies on a Putative Fifth Iron Ligand of Mouse 8S-Lipoxygenase: Retention of Catalytic Activity on Mutation of Serine-558 to Asparagine, Histidine, or Alanine

The reported crystal structures of plant and animal lipoxygenases (LOX) show that the nonheme iron in the catalytic domain is ligated by three histidines, the C-terminal isoleucine, and in certain structures also by a fifth iron ligand, an asparagine or histidine residue. Mouse 8-LOX and its homologues (e.g., human 15-LOX-2) are unique in having a serine in place of the usual Asn or His in this fifth position. To investigate the importance of the residue in mouse 8-LOX structure-function, the serine-558 was replaced by asparagine, histidine, or alanine using oligonucleotide-directed mutagenesis. Wild-type mouse 8-LOX and the mutant cDNAs were expressed in HeLa cells infected with vaccinia virus encoding T7 RNA polymerase and their relative lipoxygenase activities assessed by incubation with [14C]arachidonic acid or [14C]linoleic acid followed by HPLC analysis of the products. The Ser558Asn and Ser558His mutants had equivalent or greater activity than wild-type 8-LOX. They also exhibited some 15-LOX activity, indicating that small structural perturbations (in this case to a residue identical in mouse 8-LOX and its 15-LOX-2 homologues) can interchange the positional specificity of these closely related enzymes. Remarkably, the Ser558Ala mutant exhibited significant 8-LOX activity, indicating that this position is not an essential iron ligand in the enzyme. We conclude that mouse 8-LOX is catalytically competent with only four amino acid iron ligands, and that Ser-558 of the wild-type enzyme does not play an essential role in catalysis.

Purification and molecular cloning of an 8R-lipoxygenase from the coral Plexaura homomalla reveal the related primary structures of R- and S-lipoxygenases

Lipoxygenases that form S configuration fatty acid hydroperoxides have been purified or cloned from plant and mammalian sources. Our objectives were to characterize one of the lipoxygenases with R stereospecificity, many of which are described in marine and freshwater invertebrates. Characterization of the primary structure of an R-specific enzyme should help provide a new perspective to consider the enzyme-substrate interactions that are the basis of the specificity of all lipoxygenases. We purified an 8R-lipoxygenase of the prostaglandin-containing coral Plexaura homomalla by cation and anion exchange chromatography. This yielded a colorless enzyme preparation, a band of approximately 100 kDa on SDS-polyacrylamide gel electrophoresis, and turnover numbers of 4000 min-1 of 8R-lipoxygenase activity in peak chromatographic fractions. The full-length cDNA was cloned by PCR using peptide sequence from the purified protein and by 5'- and 3'-rapid amplification of cDNA ends. The cDNA encodes a polypeptide of 715 amino acids, including over 70 amino acids identified by peptide microsequencing. A peptide presequence of 52 amino acids is cleaved to give the mature protein of 76 kDa; the difference from the estimated size by SDS-PAGE implies a post-translational modification of the P. homomalla enzyme. All of the iron-binding histidines of S-lipoxygenases are conserved in the 8R-lipoxygenase. However, the C-terminal amino acid is a threonine, as opposed to the isoleucine that provides the carboxylate ligand to the iron in all known S-lipoxygenases. These results establish that the 8R-lipoxygenase is related in primary structure to the S-lipoxygenases. A model of the basis of R and S stereospecificity is described.

DIOXYGENASES: Molecular Structure and Role in Plant Metabolism

▪ Abstract  Dioxygenases are nonheme iron-containing enzymes important in the biosynthesis of plant signaling compounds such as abscisic acid, gibberellins, and ethylene and also of secondary metabolites, notably flavonoids and alkaloids. Plant dioxygenases fall into two classes: lipoxygenases and 2-oxoacid-dependent dioxygenases. The latter catalyze hydroxylation, epoxidation, and desaturation reactions; some enzymes catalyze more than one type of reaction in successive steps in a biosynthetic pathway. This review highlights recent discoveries on both enzyme groups, particularly in relation to gibberellin biosynthesis, in vivo activity of 1-aminocyclopropane-1-carboxylate oxidase, and molecular structure/function relationships. Similarities between the roles of monooxygenases and dioxygenases are also discussed.

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

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

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.

Mutations at the C-terminal isoleucine and other potential iron ligands of 5-lipoxygenase

The non-heme iron centre in human 5-lipoxygenase was studied. Recombinant enzyme was expressed in Escherichia coli, purified and assayed for iron content and enzyme activity. For non-mutated enzyme, the iron content was 1.01 +/- 0.19 mol/mol. Deletion of the C-terminal Ile673 resulted in an iron content of 0.03 +/- 0.07 mol/mol and undetectable lipoxygenase activity. Mutations at His367, Glu376 and Asn554 led to drastically decreased enzyme activity (< 2% of non-mutated control) but iron was still present. In addition to Glu376, eight other conserved acidic residues (Asp/Glu) in 5-lipoxygenase were replaced, none of which was crucial for enzyme activity. We conclude that Ile673 is an iron ligand in 5-lipoxygenase, while our results do not support that Glu376 or Asn554 have this function. The possible role of His367 as a replaceable iron ligand is discussed.

The structure and function of lipoxygenase

Lipoxygenases are non-heme iron enzymes that catalyze the dioxygenation of polyunsaturated fatty acids, yielding hydroperoxides. The first crystal structures have recently been published, revealing an unusual iron site. There have also been substantial developments in the analysis of the kinetics of the reaction, including the observation of uniquely large primary deuterium isotope effects. Exploitation of these results should enable substantial progress in understanding what appears to be a complicated and fascinating chemical mechanism.

Two genes for carbohydrate catabolism are divergently transcribed from a region of DNA containing the hexC locus in Pseudomonas aeruginosa PAO1

The hexC locus of Pseudomonas aeruginosa PAO1 was localized to a 247-bp segment of chromosomal DNA on the multicopy broad-host-range vector pRO1614. The presence of this plasmid (pPZ196) in strain PAO1 produced the so-called "hexC effect," a two- to ninefold increase in the activities of four carbohydrate catabolism enzymes, glucokinase, glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydratase, and 2-keto-3-deoxy-6-phosphogluconate aldolase. The extent of the hexC effect was restricted, since three independently regulated metabolic enzymes were not affected by the presence of the hexC plasmid. Furthermore, the hexC-containing plasmid did not suppress catabolite repression control. Nucleotide sequence analysis of the segment of DNA encompassing hexC revealed a 128-bp region rich in adenosine-plus-thymine (AT) content separating two divergent open reading frames (ORFs). Transcriptional start sites for these two genes were mapped to the intergenic region, demonstrating that this sequence contained overlapping divergent promoters. The intergenic region contained potential regulatory sequences such as dyad symmetry motifs, polydeoxyadenosine tracts, and a sequence matching the integration host factor recognition site in Escherichia coli. One of the ORFs encoded a 610-amino-acid protein with 55 to 60% identity to 6-phosphogluconate dehydratase from E. coli and Zymomonas mobilis. The second ORF coded for a protein of 335 amino acids that displayed 45 to 60% identity to the NAD-dependent glyceraldehyde-3-phosphate dehydrogenase (GAP) family of enzymes. The NAD-dependent GAP gene on the P. aeruginosa chromosome was previously unmapped. GAP was found to exhibit the hexC-dependent increase in its basal activity, establishing it as a fifth catabolic enzyme in the multioperonic hex regulon.

5-Lipoxygenase: enzyme expression and regulation of activity

5-Lipoxygenase catalyzes the transformation of arachidonic acid to leukotriene A4. This unstable intermediate can be converted to leukotriene B4 by LTA4-hydrolase or to leukotriene C4 by LTC4-synthase. Leukotrienes are involved in host defense reactions and play an important role in inflammatory diseases like asthma, inflammatory bowel disease and arthritis. The capability to release leukotrienes is restricted to a few cell types. Under pathophysiological conditions, leukotrienes are released from granulocytes, mast cells or macrophages. During hematopoiesis the competence of these cells for leukotriene biosynthesis is supposed to be upregulated. In mature cells, 5-lipoxygenase activity is tightly regulated and seems to be under the control of additional cellular components. One cellular component, a membrane-bound peptide termed FLAP, which is necessary for 5-LO activity in intact cells has been recently identified. Inhibitors of FLAP function prevent translocation of 5-lipoxygenase from cytosol to the membrane and inhibit 5-LO activation. Thus, the understanding of the regulatory mechanisms of cellular leukotriene biosynthesis provides new concepts for the development of antiinflammatory drugs. This review focuses on the regulation of gene expression and activity of 5-lipoxygenase.

Interfacial kinetic reaction of human 5-lipoxygenase

The kinetics of human 5-lipoxygenase were investigated in the presence of Tween 20 using a continuous spectrophotometric assay. Using the mixture at a constant molar ratio of arachidonate/Tween 20 at pH 8.0, the steady-state velocity on a varied arachidonate concentration did not follow simple Michaelis-Menten-type kinetics and double-reciprocal plot analysis gave hyperbolic curves. However, by introducing the concept of a local pH change, it was possible to analyze the kinetics as simple Michaelis-Menten type. The concept of a local pH change implies that when utilizing an acidic and amphiphilic substance as a substrate, such as arachidonate, the medium around the substrate is acidified with an increased concentration of substrate. This concept was explained rationally by two experiments. Consequently, the data were transformed according to a local pH change and analyzed according to a dual phospholipid model as has been proposed for phospholipase A2 [Hendrickson, H. S. and Dennis, E. A. (1984) Kinetic analysis of the dual phospholipid model for phosphalipase A2, J. Biol. Chem. 259, 5734-5739]. It is concluded that 5-lipoxygenase performs an interfacial reaction in the arachidonate/Tween 20 mixed micelles in the same manner as phospholipase A2. The values of Km were almost constant (about 0.07 molar fraction), even when arachidonate molar ratios were changed in the surface of the mixed micelles. The values for Ks (the association constant of the enzyme to the micelle interface) ranged over 0.21-0.48 microM. The Vmax was 25.76 mumol.min-1.mg-1. This concept of a local pH change could be used extensively with enzymes which utilize both amphiphilic and acidic substances as substrates.

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.

Characterization of the non-heme iron center of human 5-lipoxygenase by electron paramagnetic resonance, fluorescence, and ultraviolet-visible spectroscopy: redox cycling between ferrous and ferric states

Purified human 5-lipoxygenase, a non-heme iron containing enzyme, has been characterized by electron paramagnetic resonance, (EPR), ultraviolet (UV)-visible and fluorescence spectroscopy. As isolated, the enzyme is largely in the ferrous state and shows a weak X-band EPR signal extending from 0 to 700 G at 15 K, tentatively ascribed to integer spin Fe(II). Titration of the protein with 13-HPOD (13-hydroperoxyoctadecadienoic acid) generates a strong multicomponent EPR signal in the g' approximately 6 region, a yellow color associated with an increased absorption between 310 and 450 nm (epsilon 330nm = 2400 M-1 cm-1), and a 17% decrease in the intrinsic protein fluorescence. The multiple component nature of the g' approximately 6 signal indicates that the metal center in its oxidized state exists in more than one but related forms. The g' approximately 6 EPR signal and the yellow color reach a maximum when approximately 1 mol of 13-HPOD is added/mol of iron; the resultant EPR spectrum accounts quantitatively for all of the iron in the protein with a signal at g' = 4.3 representing less than 3% of the total iron in the majority of samples. Addition of a hydroxyurea reducing agent abolished the g' approximately 6 signal and yellow color of the protein and also reversed the decrease in fluorescence caused by the oxidant 13-HPOD. The results indicate that the g' approximately 6 EPR signal, the yellow color, and the decreased fluorescence are associated with the formation of the Fe(III) form of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Crystallographic determination of the active site iron and its ligands in soybean lipoxygenase L-1

Five ligands of the active site iron atom in soybean lipoxygenase L-1 have been identified from the electron density map of the crystallized enzyme. The position of the iron atom can be readily and independently located from an anomalous difference electron density map. The ligands identified are His-499, His-504, His-690, Asn-694, and Ile-839, the carboxy-terminal residue. Our previous view that these three histidines are essential for activity and binding of iron, based on site-specific mutation studies, is confirmed. A sixth protein ligand is not present, and the sixth coordination site opens into a wide cleft. The structure of the soybean lipoxygenase was solved by multiple anomalous isomorphous replacements.

Investigation of the mechanism of non-turnover-dependent inactivation of purified human 5-lipoxygenase. Inactivation by H2O2 and inhibition by metal ions

Human 5-lipoxygenase is a non-heme iron protein which is reported to be highly unstable in the presence of oxygen. The results of this investigation demonstrate that H2O2 generated during air oxidation of thiols is the main factor in non-turnover-dependent inactivation of purified recombinant human 5-lipoxygenase for the following reasons: catalase protects against oxygen-dependent inactivation of the enzyme in the presence of dithiothreitol; the active, stable enzyme can be prepared under aerobic conditions with the exclusion of dithiothreitol and contaminating metal ions; 10 microM H2O2 causes the rapid inactivation of the enzyme. The native (ferrous) enzyme is approximately seven times more sensitive to inactivation by H2O2 than the ferric enzyme, suggesting that the mechanism of inactivation involves a Fenton-type reaction of the ferrous enzyme with H2O2, resulting in the formation of an activated oxygen species. Purification of 5-lipoxygenase under aerobic conditions (no dithiothreitol) results in an increase in both the specific activity of the purified protein [up to 70 mumol 5(S)-hydroperoxy-6-trans-8, 11, 14-cis-icosatetraenoic acid (5-HPETE)/mg protein] and in the ratio of specific activity to enzyme iron content compared to enzyme purified under anaerobic conditions in the presence of dithiothreitol. The reaction of the highly active 5-lipoxygenase enzyme shows a dependence on physiological intracellular calcium concentrations, half-maximal product formation being obtained at 0.9 microM free Ca2+. The maximal enzyme activity is also dependent on EDTA and dithiothreitol and low amounts of carrier protein, as well as the known activators PtdCho and ATP. Ca2+ can be substituted by Mn2+, Ba2+ and Sr2+, although lower levels of stimulation are obtained. 5-Lipoxygenase is strongly inhibited by low concentrations (< or = 10 microM) of Zn2+ and Cu2+. The inhibition by Cu2+ is apparently irreversible, whereas that by Zn2+ is slowly reversed (t1/2 = 2 min) in the presence of excess EDTA. These observations on the mechanism of non-turnover-dependent inactivation of 5-lipoxygenase, and the optimisation of assay conditions, have facilitated the purification of large quantities of relatively stable enzyme that will be useful for further kinetic and physical studies.

The characterization of 5 histidine-serine mutants of human 5-lipoxygenase

The physical and catalytic properties of 5 histidine-serine mutants of human 5-lipoxygenase (5-LO) have been characterized. The mutants HS363, HS391 and HS400 have activities, pH optima and stabilities similar to those of the wild type enzyme. The iron content of each of these mutants is 0.30-0.53 mol Fe/mol 5-LO which is within the range observed for the wild type enzyme. HS368 contains iron (0.15 and 0.43 mol Fe/mol 5-LO in 2 preparations) but has no detectable oxygenase, leukotriene A4 synthase or anaerobic arachidonate-dependent hydroperoxidase activities. HS368 does have significant reducing agent-dependent hydroperoxidase activity suggesting that His-368 may not be an iron ligand but rather may be involved in interactions with arachidonic acid or the formation of the arachidonyl radical intermediate. HS373 contains no iron and has no detectable activities.