Theoretical Characterization of the Step-by-Step Mechanism of Conversion of Leukotriene A4 to Leukotriene B4 Catalysed by the Enzyme Leukotriene A4 Hydrolase

LTA₄H is a bifunctional zinc metalloenzyme that converts leukotriene A₄ (LTA₄) into leukotriene B₄ (LTB₄), one of the most potent chemotactic agents involved in acute and chronic inflammatory diseases. In this reaction, LTA₄H acts as an epoxide hydrolase with a unique and fascinating mechanism, which includes the stereoselective attachment of one water molecule to the carbon backbone of LTA₄ several methylene units away from the epoxide moiety. By combining Molecular Dynamics simulations and Quantum Mechanics/Molecular Mechanics calculations, we obtained a very detailed molecular picture of the different consecutive steps of that mechanism. By means of a rather unusual 1,7-nucleophilic substitution through a clear S_N1 mechanism, the epoxide opens and the triene moiety of the substrate twists in such a way that the bond C₆-C₇ adopts its cis (Z) configuration, thus exposing the R face of C₁₂ to the addition of a water molecule hydrogen-bonded to ASP375. Thus, the two stereochemical features that are required for the bioactivity of LTB₄ appear to be closely related. The noncovalent π-π stacking interactions between the triene moiety and two tyrosines (TYR267 and, especially, TYR378) that wrap the triene system along the whole reaction explain the preference for the cis configuration inside LTA₄H.

Structural origins for the loss of catalytic activities of bifunctional human LTA4H revealed through molecular dynamics simulations

Human leukotriene A4 hydrolase (hLTA4H), which is the final and rate-limiting enzyme of arachidonic acid pathway, converts the unstable epoxide LTA4 to a proinflammatory lipid mediator LTB4 through its hydrolase function. The LTA4H is a bi-functional enzyme that also exhibits aminopeptidase activity with a preference over arginyl tripeptides. Various mutations including E271Q, R563A, and K565A have completely or partially abolished both the functions of this enzyme. The crystal structures with these mutations have not shown any structural changes to address the loss of functions. Molecular dynamics simulations of LTA4 and tripeptide complex structures with functional mutations were performed to investigate the structural and conformation changes that scripts the observed differences in catalytic functions. The observed protein-ligand hydrogen bonds and distances between the important catalytic components have correlated well with the experimental results. This study also confirms based on the structural observation that E271 is very important for both the functions as it holds the catalytic metal ion at its location for the catalysis and it also acts as N-terminal recognition residue during peptide binding. The comparison of binding modes of substrates revealed the structural changes explaining the importance of R563 and K565 residues and the required alignment of substrate at the active site. The results of this study provide valuable information to be utilized in designing potent hLTA4H inhibitors as anti-inflammatory agents.

32P-postlabeling analysis of DNA adducts formed by leukotriene A4 (LTA4)

Leukotriene A(4) (LTA(4)), a reactive electrophilic intermediate formed during the biosynthesis of inflammation-related lipid mediators, has been found to bind covalently to DNA. The major DNA adducts formed by LTA(4) in vitro and human cells have been identified by mass spectrometry on the nucleoside level. Here we investigated whether the thin-layer chromatography (TLC) (32)P-postlabeling method is suitable for the detection of LTA(4)-DNA adducts. The reaction of individual deoxynucleoside 3'-monophosphates with LTA(4) in aqueous basic solution yielded numerous adduct spots when analyzed by the two enrichment procedures of the (32)P-postlabeling method-nuclease P1 digestion and butanol extraction. Highest LTA(4)-adduct levels were found with deoxyguanosine 3'-phosphate (around one adduct per 10(4) normal nucleotides). Under similar reaction conditions LTA(4) (25-320 microM) was incubated with calf thymus DNA, then DNA adduct patterns and levels were determined with the TLC (32)P-postlabeling method using both enrichment versions. The same DNA adduct pattern consisting of up to seven spots was observed with both enrichment versions. DNA adduct formation by LTA(4) was concentration-dependent with major adducts being derived from deoxyguanosine. When a human monocytic cell line (Mono Mac 6) was stimulated with arachidonic acid and calcium ionophore LTA(4)-DNA adducts were detected by (32)P-postlabeling. However, the level of these endogenously formed DNA adducts was close to the detection limit (3 +/- 2 adducts per 10(8) normal nucleotides). In summary, the TLC (32)P-postlabeling method is suitable for studying DNA adduct formation by LTA(4) and can be used for further investigations on the link between inflammation and cancer.

Development of a fluorescence-based enzyme assay of human 5-lipoxygenase

Leukotrienes are important mediators in a number of inflammatory diseases and therefore are a target of several therapeutic approaches. The first committed step in the synthesis of leukotrienes is the conversion of arachidonic acid to leukotriene A(4) (LTA(4)) in two successive reactions catalyzed by 5-lipoxygenase (5-LOX). Assays to measure 5-LOX activity typically have been low throughput and time consuming. In this article, we describe a fluorescence assay that is amenable to high-throughput screening in a 384-well microplate format. The fluorescent signal is measured during oxidation of 2',7'-dichlorodihydrofluorescein diacetate (H2DCFDA) by human 5-LOX. The assay has been found to reliably identify small molecule inhibitors of human 5-LOX. The IC(50) values of several 5-LOX inhibitors in this new assay are comparable to those determined in a standard spectrophotometric assay that measures the formation of the 5(S)-hydroperoxyeicosatetraenoic acid (5-HpETE) product. In addition, we demonstrate the use of the assay in a high-throughput screen of the Pfizer compound collection to identify inhibitors of 5-LOX.

A conserved tyrosine residue of Saccharomyces cerevisiae leukotriene A4 hydrolase stabilizes the transition state of the peptidase activity

Saccharomyces cerevisiae leukotriene A4 hydrolase (LTA4H) is a bifunctional aminopeptidase/epoxide hydrolase and a member of the M1 family of metallopeptidases. In order to obtain a more thorough understanding of the aminopeptidase activity of the enzyme, two conserved tyrosine residues, Tyr244 and Tyr456, were altered to phenylalanine and the mutant proteins characterized by determining KM and kcat for various amino acid beta-naphthylamide substrates. While mutation of Tyr456 exhibited minimal effect on catalysis, mutation of Tyr244 caused an overall 25-100-fold reduction in catalytic activity for all substrates tested. Furthermore, LTA4H Y244F exhibited a 40-fold decrease in affinity for RB-3014, a transition state analog inhibitor, implicating Tyr244 in transition state stabilization.

Synthesis of 8,9-leukotriene A4 by murine 8-lipoxygenase

Arachidonate 8-lipoxygenase was identified in phorbol ester induced mouse skin. We expressed the enzyme in an Escherichia coli system using pET-15b carrying an N-terminal histidine-tag sequence. The enzyme, purified by nickel-nitrilotriacetate affinity chromatography, showed specific activity of about 0.1 micromol/min/mg of protein with arachidonic acid as a substrate. When metabolites of arachidonic acid were reduced and analyzed by reverse-phase HPLC, 8-hydroxy derivative was a major product as measured by absorbance at 235 nm. In addition, three polar compounds (I, II, and III) were detected by measuring absorbance at 270 nm. These compounds were also produced when the enzyme was incubated with 8-hydroperoxyeicosa-5,9,11,14-tetraenoic acid. Neither heat-inactivated enzyme nor mutated enzyme produced these compounds, suggesting that they are enzymatically generated. Ultraviolet spectra of these compounds showed typical triplet peaks around 270 nm, indicating that they have a triene structure. Molecular weight of these compounds was determined to be 336 by liquid chromatography-mass spectrometry, indicating that they carry two hydroxyl groups. Compounds I and III were generated even under anaerobic condition, indicating that oxygenation reaction was not required for their generation from 8-hydroperoxyeicosa-5,9,11,14-tetraenoic acid. By analogy to the reactions of 5-lipoxygenase pathway where leukotriene A4 is generated, it is suggested that 8-hydroperoxyeicosa-5,9,11,14-tetraenoic acid is converted by the 8-lipoxygenase to 8,9-epoxyeicosa-5,10,12,14-tetraenoic acid which degrades to compounds I and III by non-enzymatic reaction. In contrast, compound II was not generated under anaerobic condition, indicating that it was produced by oxygenation reaction. Taken together, 8-lipoxygenase catalyzes both dehydration reaction to yield 8,9-epoxy derivative and oxygenation reaction presumably at 15-position of 8-hydroperoxyeicosa-5,9,11,14-tetraenoic acid.

Mass spectrometric quantitation of deoxyguanosine and leukotriene A4-deoxyguanosine adducts of DNA

An assay was developed using electrospray ionization negative ion tandem mass spectrometry (MS) to identify and quantitate the major product in the reaction of leukotriene A(4) (LTA(4)) with deoxyguanosine (dGuo). A second quantitative assay was established using the same separation and detection techniques to determine the amount of dGuo isolated from enzymatically processed DNA. The amount of LTA(4)-dGuo adduct could then be analytically determined in DNA samples and normalized to the amount of dGuo that had been simultaneously derived from the DNA sample. Stable isotope-labeled internal standards used for these quantitative assays were readily synthesized from isotopically labeled [(15)N(5)(13)C(10)]deoxyguanosine triphosphate and analyzed for isotopic purity using MS. A comparison of fragment ions formed from stable isotope analogs of dGuo revealed the loss of deoxyribose and secondarily the loss of a series of stable neutral small molecules in a fashion similar to patterns described previously for the collisional fragmentation of protonated guanine determined by positive ion fast atom bombardment/MS/MS. The combined quantitative assays were used for the determination of the amount of endogenously formed LTA(4)-dGuo adducts observed in DNA when isolated human neutrophils that had been incubated with arachidonic acid were stimulated with calcium ionophore to initiate leukotriene biosynthesis.

Fatty acid binding proteins stabilize leukotriene A4: competition with arachidonic acid but not other lipoxygenase products

Leukotriene A(4) (LTA(4)) is a chemically reactive conjugated triene epoxide product derived from 5-lipoxygenase oxygenation of arachidonic acid. At physiological pH, this reactive compound has a half-life of less than 3 s at 37 degrees C and approximately 40 s at 4 degrees C. Regardless of this aqueous instability, LTA(4) is an intermediate in the formation of biologically active leukotrienes, which can be formed through either intracellular or transcellular biosynthesis. Previously, epithelial fatty acid binding protein (E-FABP) present in RBL-1 cells was shown to increase the half-life of LTA(4) to approximately 20 min at 4 degrees C. Five FABPs (adipocyte FABP, intestinal FABP, E-FABP, heart/muscle FABP, and liver FABP) have now been examined and also found to increase the half-life of LTA(4) at 4 degrees C to approximately 20 min with protein present. Stabilization of LTA(4) was examined when arachidonic acid was present to compete with LTA(4) for the binding site on E-FABP. Arachidonate has an apparent higher affinity for E-FABP than LTA(4) and was able to completely block stabilization of the latter. When E-FABP is not saturated with arachidonate, FABP can still stabilize LTA(4). Several lipoxygenase products, including 5-hydroxyeicosatetraenoic acid, 5,6-dihydroxyeicosatetraenoic acid, and leukotriene B(4), were found to have no effect on the stability of LTA(4) induced by E-FABP even when present at concentrations 3-fold higher than LTA(4).

Stabilization of leukotriene A4 by epithelial fatty acid-binding protein in the rat basophilic leukemia cell

Leukotriene A(4) (LTA(4)) is a chemically unstable triene epoxide product of 5-lipoxygenase metabolism of arachidonic acid. Despite this chemical reactivity and its synthesis at the perinuclear membrane, LTA(4) is enzymatically converted into the cysteinyl leukotrienes and leukotriene B(4). Furthermore, LTA(4) participates in transcellular biosynthesis and is thus transferred between cells as an intact molecule. A cytosolic fatty acid-binding protein present in the rat basophilic leukemia cells was identified using mass spectrometry. This protein was determined to be the stabilizing factor present in the cell cytosol responsible for increasing the effective chemical half-life of LTA(4). Rat epithelial fatty acid-binding protein (E-FABP) was isolated using partial protein purification and immunoprecipitation. In-gel digestion with trypsin followed by peptide fingerprint analysis using matrix-assisted laser desorption ionization mass spectrometry and sequencing the major tryptic peptide obtained from liquid chromatography/mass spectrometry/mass spectrometry analysis identified E-FABP in the active fraction. Semi-quantitative Western blot analysis indicated that E-FABP in the cytosolic fraction of RBL-1 cells was present at approximately 1-3 pmol/10(6) cells. E-FABP (9 microm) was tested for its ability to stabilize LTA(4), and at 37 degrees C E-FABP was able to increase the half-life of LTA(4) from the previously reported half-life less than 3 s to a half-life of approximately 7 min. These results present a novel function for the well studied fatty acid-binding protein as a participant in leukotriene biosynthesis that permits LTA(4) to be available for further enzymatic processing in various cellular regions.

Covalent binding of leukotriene A4 to DNA and RNA

Leukotriene A(4) (LTA(4)) is a highly reactive electrophilic intermediate formed during the biosynthesis of the lipid mediators leukotriene B(4) and leukotriene C(4). Deoxynucleosides were found to react as nucleophiles with LTA(4) in aqueous solutions as assessed by UV spectroscopy and electrospray ionization mass spectrometry. Aqueous solutions of native DNA and RNA were also found to react with LTA(4) as assessed by mass spectrometric analysis of the constituent nucleosides derived from enzymatic hydrolysis of the nucleic acids. The most abundant adducts were observed for guanine- and adenine-containing deoxynucleosides and nucleosides. At neutral pH, these reactions led to an overall modification of deoxyguanosine/guanosine residues in DNA and RNA at 15 +/- 1 adducts/10(7) bases and 230 +/- 20 adducts/10(7) bases, respectively, determined by quantitative assay using stable isotope-labeled LTA(4)-nucleoside adduct. An estimation of the relative reactivity of LTA(4) with each of the purine and pyrimidine bases in DNA and RNA was carried out by comparisons of the mass spectral ion abundance of the different adducts (LTA(4)-dAdo, LTA(4)-dCyd, LTA(4)-Thd, LTA(4)-Ado, LTA(4)-Cyd, and LTA(4)-Urd) to the ion signal of known amounts of LTA(4)-dGuo and LTA(4)-Guo standards. The data were corrected for different mass spectrometric response factors that were experimentally determined for each adduct product. The structures of the two most abundant LTA(4)-Guo products were determined by NMR, UV spectroscopy, and mass spectrometry to be 5-hydroxy,12-[Guo-N(2)-yl]-6,8,11,14-eicosatetraenoic acid. Stimulation of human neutrophils with calcium ionophore led to the covalent modification of DNA within the cell as determined by mass spectrometric analysis of lipophilic nucleosides obtained after hydrolysis of extracted DNA. These observations, combined with the intracellular site of 5-lipoxygenase translocation and LTA(4) biosynthesis at the nuclear envelope, suggest that LTA(4) may have access to DNA and RNA within cells and furthermore modify nucleic acids in situ following the activation of 5-lipoxygenase and initiation of LTA(4) biosynthesis.

Saccharomyces cerevisiae leukotriene A4 hydrolase: formation of leukotriene B4 and identification of catalytic residues

Leukotriene A(4) hydrolase in mammals is a bifunctional zinc metalloenzyme that catalyzes the hydrolysis of leukotriene A(4) into the proinflammatory mediator leukotriene B(4), and also possesses an aminopeptidase activity. Recently we cloned and characterized an leukotriene A(4) hydrolase from Saccharomyces cerevisiae as a leucyl aminopeptidase with an epoxide hydrolase activity. Here we show that S. cerevisiae leukotriene A(4) hydrolase is a metalloenzyme containing one zinc atom complexed to His-340, His-344, and Glu-363. Mutagenetic analysis indicates that the aminopeptidase activity follows a general base mechanism with Glu-341 and Tyr-429 as the base and proton donor, respectively. Furthermore, the yeast enzyme hydrolyzes leukotriene A(4) into three compounds, viz., 5S,6S-dihydroxy-7,9-trans-11,14-cis-eicosatetraenoic acid, leukotriene B(4), and Delta(6)-trans-Delta(8)-cis-leukotriene B(4), with a relative formation of 1:0.2:0.1. In addition, exposure of S. cerevisiae leukotriene A(4) hydrolase to leukotriene A(4) selectively inactivates the epoxide hydrolase activity with a simultaneous stimulation of the aminopeptidase activity. Moreover, kinetic analyses of wild-type and mutated S. cerevisiae leukotriene A(4) hydrolase suggest that leukotriene A(4) binds in one catalytic mode and one tight-binding, regulatory mode. Exchange of a Phe-424 in S. cerevisiae leukotriene A(4) hydrolase for a Tyr, the corresponding residue in human leukotriene A(4) hydrolase, results in a protein that converts leukotriene A(4) into leukotriene B(4) with an improved efficiency and specificity. Hence, by a single point mutation, we could make the active site better suited to bind and turn over the substrate leukotriene A(4), thus mimicking a distinct step in the molecular evolution of S. cerevisiae leukotriene A(4) hydrolase toward its mammalian counterparts.

Covalent binding of LTA(4) to nucleosides and nucleotides

Leukotriene A(4) (LTA(4)) is a chemically reactive conjugated triene epoxide that is formed by 5-lipoxygenase and is an intermediate in the formation of the biologically active eicosanoids leukotriene B(4) and leukotriene C(4). The present study was undertaken to determine whether or not LTA(4) could serve as an electrophilic species that nucleosides and nucleotides could attack, ultimately resulting in a covalent adduct. Electrospray ionization mass spectrometry and tandem mass spectrometry were used to study the covalent binding of LTA(4) with uridine, cytidine, adenosine, and guanosine. The reaction with guanosine was found to yield five major and at least six minor adduct species. Reversed phase HPLC and mass spectrometric data suggested that the guanosine attacked LTA(4) either at carbon-12 or carbon-6 with opening the epoxide at carbon-5 to yield a series of adducts characterized by the molecular anion [M-H](-) at m/z 600.3. Reactions of LTA(4) with mixtures of nucleosides and nucleotides revealed that guanine-containing nucleosides were the most reactive toward LTA(4). The facility of the reaction of guanine with LTA(4) raises the possibility that this intermediate of leukotriene biosynthesis formed on or near the cellular nuclear envelope may react with nucleosides and nucleotides present in RNA or DNA.

LTA(4)-derived 5-oxo-eicosatetraenoic acid: pH-dependent formation and interaction with the LTB(4) receptor of human polymorphonuclear leukocytes

5-oxo-(7E,9E,11Z,14Z)-eicosatetraenoic acid (5-oxo-ETE) has been identified as a non-enzymatic hydrolysis product of leukotriene A(4) (LTA(4)) in addition to 5,12-dihydroxy-(6E,8E,10E, 14Z)-eicosatetraenoic acids (5,12-diHETEs) and 5,6-dihydroxy-(7E,9E, 11Z,14Z)-eicosatetraenoic acids (5,6-diHETEs). The amount of 5-oxo-ETE detected in the mixture of the hydrolysis products of LTA(4) was found to be pH-dependent. After incubation of LTA(4) in aqueous medium, the ratio of 5-oxo-ETE to 5,12-diHETE was 1:6 at pH 7.5, and 1:1 at pH 9.5. 5-Oxo-ETE was isolated from the alkaline hydrolysis products of LTA(4) in order to evaluate its effects on human polymorphonuclear (PMN) leukocytes. 5-Oxo-ETE induced a rapid and dose-dependent mobilization of calcium in PMN leukocytes with an EC(50) of 250 nM, as compared to values of 3.5 nM for leukotriene B(4) (LTB(4)500 nM for 5(S)-hydroxy-(6E,8Z,11Z,14Z)-eicosatetraenoic acid (5-HETE). Pretreatment of the cells with LTB(4) totally abolished the calcium response induced by 5-oxo-ETE. In contrast, the preincubation with 5-oxo-ETE did not affect the calcium mobilization induced by LTB(4). The calcium response induced by 5-oxo-ETE was totally inhibited by the specific LTB(4) receptor antagonist LY223982. These data demonstrate that 5-oxo-ETE can induce calcium mobilization in PMN leukocyte via the LTB(4) receptor in contrast to the closely related analog 5-oxo-(6E,8Z,11Z, 14Z)-eicosatetraenoic acid which is known to activate human neutrophils by a mechanism independent of the receptor for LTB(4).

Structural characterization of the covalent attachment of leukotriene A3 to leukotriene A4 hydrolase

Leukotriene A4 (LTA4) hydrolase catalyzes the conversion of the unstable epoxide LTA4 [5(S)-trans-5,6-oxido-11,14-cis-eicosatetraenoic acid] into proinflammatory LTB4. During the process of catalyzing this reaction, the enzyme is suicide inactivated by its substrate. In addition, LTA3, and analogue of LTA4 that lacks the C14-C15 double bond, is a potent suicide inhibitor of LTA4 hydrolase. We have synthesized [3H]LTA3 and used this ligand to demonstrate that LTA3 can covalently label LTA4 hydrolase and that this labeling is specifically competed for by bestatin and LTA4. Incubation of recombinant human LTA4 hydrolase with LTA3 followed by proteolysis (endoproteinase Lys-C) resulted in a peptide map with a single modified peptide defining the location of the LTA3 covalent attachment region. This modified 21-amino-acid peptide had a UV absorption spectrum corresponding to a conjugated triene chromophore which established conservation of this structural unit after covalent interaction of LTA3 with LTA4 hydrolase. MALDI-TOF mass spectrometric analysis of the 21-amino-acid peptide adduct revealed an abundant MH+ at m/z 2658, consistent with the predicted nominal mass of the sequenced peptide with the addition of a single LTA3 moiety. Proteolysis of LTA4 hydrolase modified with LTA3 was performed sequentially with endo-Asp-N and endo-Lys-C. The resulting peptide isolated by reverse-phase high-performance liquid chromatography was analyzed by mass spectroscopy revealing two related peptides, D371-K385 (m/z 2018.0) and D375-K385 (m/z 1577.8), both of which retained the elements of LTA3. Postsource decay of m/z 1577.8 resulted in an abundant ion at m/z 536 and an ion of lesser abundance at m/z 856 consistent with cleavage between V381 and P382 that supported assignment of the modified tyrosine residue at Y383. These results suggest nucleophilic attack of a tyrosine residue (Y383) at the conjugated triene epoxide of LTA3 resulting in a triene ether carbinol covalent adduct.

Identification of 5-keto-(7E,9E,11Z,14Z)-eicosatetraenoic acid as a novel nonenzymatic rearrangement product of leukotriene A4

Leukotriene A4 (LTA4), the reaction product of 5-lipoxygenase in human polymorphonuclear (PMN) leukocytes, is transformed both to LTB4 and a mixture of 5,6- and 5,12-dihydroxy-eicosatetraenoic acids (diHETE) via nonenzymatic hydrolysis. Evidence has been obtained that LTA4 is also converted to 5-keto-(7E,9E,11Z,14Z)-eicosatetraenoic acid (5-oxo-ETE). The compound was isolated from the products of the 5-lipoxygenase reaction and its structure elucidated by UV spectroscopy, LC-MS, two-dimensional [1H]NMR spectroscopy and chemical reduction to the corresponding alcohol. The 5-oxo-ETE represented about 14% of the LTA4 hydrolysis products as compared to 72 and 14% for the 5,12-diHETE and 5,6-diHETE, respectively. A similar profile of hydrolysis products was obtained after incubation of synthetic LTA4 in aqueous buffer. Human PMN leukocytes produced 5-oxo-ETE in an arachidonic acid-dependent and MK-886-inhibitable manner. The 5-oxo-ETE caused 50% inhibition of 5-lipoxygenase activity at 1 microM. These results demonstrate that the nonenzymatic conversion of LTA4, in addition to the previously described hydrolysis products, yields 5-oxo-ETE during both the 5-lipoxygenase reaction and arachidonic acid oxidation by human PMN leukocytes. They indicate that allylic epoxides can rearrange in aqueous media at physiological pH to spontaneously form beta,gamma-unsaturated ketones.