Mechanism of soluble epoxide hydrolase. Formation of an alpha-hydroxy ester-enzyme intermediate through Asp-333

Effects of age on lacrimal gland bioactive lipids

PURPOSE

Polyunsaturated fatty acids (PUFA) are a source of bioactive lipids regulating inflammation and its resolution.

METHODS

Changes in PUFA metabolism were compared between lacrimal glands (LGs) from young and aged C57BL/6 J mice using a targeted lipidomics assay, as was the gene expression of enzymes involved in the metabolism of these lipids.

RESULTS

Global reduction in PUFAs and their metabolites was observed in aged LGs compared to young controls, averaging between 25 and 66 % across all analytes. ꞷ-6 arachidonic acid (AA) metabolites were all reduced in aged LGs, where the changes in prostaglandin E2 (PGE2) and lipoxin A4 (LXA4) were statistically significant. Several other 5-lipoxygenase (5-LOX) mediated metabolites were significantly reduced in the aged LGs, including D-series resolvins (e.g., RvD4, RvD5, and RvD6). Along with the RvDs, several ꞷ-3 docosahexaenoic acid (DHA) metabolites such as 14-HDHA, neuroprotectin D1 (NPD1), Maresin 2 (MaR2), and MaR 1 metabolite (22-COOH-MaR1) were significantly reduced in aged LGs. Similarly, ꞷ-3 eicosapentaenoic acid (EPA) and its metabolites were significantly reduced in aged LGs, where the most significantly reduced was 18-HEPE. Using metabolite ratios (product:precursor) for specific metabolic conversions as surrogate enzymatic measures, reduced 12-LOX activity was identified in aged LGs.

CONCLUSION

In this study, global reduction of PUFAs and their metabolites was found in the LGs of aged female C57BL/6 J compared to young controls. A consistent reduction was observed across all detected lipid analytes except for ꞷ-3 docosapentaenoic acid (DPA) and its special pro-resolving mediator (SPM) metabolites in aged mice, suggesting an increased risk for LG inflammation.

Glimepiride, a novel soluble epoxide hydrolase inhibitor, protects against heart failure via increasing epoxyeicosatrienoic acids

BACKGROUND

Epoxyeicosatrienoic acids (EETs), which exert multiple endogenous protective effects, are hydrolyzed into less active dihydroxyeicosatrienoic acids (DHETs) by soluble epoxide hydrolase (sEH). However, commercial drugs related to EETs or sEH are not yet in clinical use.

METHODS

Firstly, the plasma concentration of EETs and DHETs of 316 patients with heart failure (HF) were detected and quantitated by liquid chromatography-tandem mass spectrometry. Then, transverse aortic constriction (TAC)-induced HF was introduced in cardiomyocyte-specific Ephx2-/- mice. Moreover, Western blot, real-time PCR, luciferase reporter, ChIP assays were employed to explore the underlying mechanism. Finally, multiple sEH inhibitors were designed, synthesized, and validated in vitro and in vivo.

RESULTS

The ratios of DHETs/EETs were increased in the plasma from patients with HF. Meanwhile, the expression of sEH was upregulated in the heart of patients and mice with HF, especially in cardiomyocytes. Cardiomyocyte-specific Ephx2-/- mice ameliorated cardiac dysfunction induced by TAC. Consistently, Ephx2 knockdown protected Angiotensin II (AngII)-treated cardiomyocytes via increasing EETs in vitro. Mechanistically, AngII could enhance the expression of transcript factor Krüppel-like factor 15 (KLF15), which in turn upregulated sEH. Importantly, glimepiride was identified as a novel sEH inhibitor, which benefited from the elevated EETs during HF.

CONCLUSIONS

Glimepiride attenuates HF in mice in part by increasing EETs.

CLINICAL TRIAL IDENTIFIER

NCT03461107 (https://clinicaltrials.gov).

Allosteric regulation of the soluble epoxide hydrolase by nitro fatty acids using a combined experimental and computational approach

The human soluble epoxide hydrolase (hsEH) is a key regulator of epoxy fatty acid (EpFA) metabolism. Inhibition of sEH can maintain endogenous levels of beneficial EpFAs and reduce the levels of their corresponding diol products, thus ameliorating a variety of pathological conditions including cardiovascular, central nervous system and metabolic diseases. The quest for orthosteric drugs that bind directly to the catalytic crevice of hsEH has been prolonged and sustained over the past decades, but the disappointing outcome of clinical trials to date warrants alternative pharmacological approaches. Previously, we have shown that hsEH can be allosterically inhibited by the endogenous electrophilic lipid 15-deoxy-Δ^12,14-Prostaglandin-J₂, via covalent adduction to two cysteines, C423 and C522. In this study, we explore the properties and behaviour of three electrophilic lipids belonging to the class of the nitro fatty acids, namely 9- and 10-nitrooleate and 10-nitrolinoleate. Biochemical and biophysical investigations revealed that, in addition to C423 and C522, nitro fatty acids can covalently bind to additional nucleophilic residues in hsEH C-terminal domain (CTD), two of which predicted in this study to be latent allosteric sites. Systematic mapping of the protein mutational space and evaluation of possible propagation pathways delineated selected residues, both in the allosteric patches and in other regions of the enzyme, envisaged to play a role on allosteric signalling. The responses elicited by the ligands on the covalent adduction sites supports future fragment-based design studies of new allosteric effectors for hsEH with increased efficacy and selectivity.

sEH promotes macrophage phagocytosis and lung clearance of Streptococcus pneumoniae

Epoxyeicosatrienoic acids (EETs) have potent antiinflammatory properties. Hydrolysis of EETs by soluble epoxide hydrolase/ epoxide hydrolase 2 (sEH/EPHX2) to less active diols attenuates their antiinflammatory effects. Macrophage activation is critical to many inflammatory responses; however, the role of EETs and sEH in regulating macrophage function remains unknown. Lung bacterial clearance of Streptococcus pneumoniae was impaired in Ephx2-deficient (Ephx2-/-) mice and in mice treated with an sEH inhibitor. The EET receptor antagonist EEZE restored lung clearance of S. pneumoniae in Ephx2-/- mice. Ephx2-/- mice had normal lung Il1b, Il6, and Tnfa expression levels and macrophage recruitment to the lungs during S. pneumoniae infection; however, Ephx2 disruption attenuated proinflammatory cytokine induction, Tlr2 and Pgylrp1 receptor upregulation, and Ras-related C3 botulinum toxin substrates 1 and 2 (Rac1/2) and cell division control protein 42 homolog (Cdc42) activation in PGN-stimulated macrophages. Consistent with these observations, Ephx2-/- macrophages displayed reduced phagocytosis of S. pneumoniae in vivo and in vitro. Heterologous overexpression of TLR2 and peptidoglycan recognition protein 1 (PGLYRP1) in Ephx2-/- macrophages restored macrophage activation and phagocytosis. Human macrophage function was similarly regulated by EETs. Together, these results demonstrate that EETs reduced macrophage activation and phagocytosis of S. pneumoniae through the downregulation of TLR2 and PGLYRP1 expression. Defining the role of EETs and sEH in macrophage function may lead to the development of new therapeutic approaches for bacterial diseases.

Mutation of an atypical oxirane oxyanion hole improves regioselectivity of the α/β-fold epoxide hydrolase Alp1U

Epoxide hydrolases (EHs) have been characterized and engineered as biocatalysts that convert epoxides to valuable chiral vicinal diol precursors of drugs and bioactive compounds. Nonetheless, the regioselectivity control of the epoxide ring opening by EHs remains challenging. Alp1U is an α/β-fold EH that exhibits poor regioselectivity in the epoxide hydrolysis of fluostatin C (compound 1) and produces a pair of stereoisomers. Herein, we established the absolute configuration of the two stereoisomeric products and determined the crystal structure of Alp1U. A Trp-186/Trp-187/Tyr-247 oxirane oxygen hole was identified in Alp1U that replaced the canonical Tyr/Tyr pair in α/β-EHs. Mutation of residues in the atypical oxirane oxygen hole of Alp1U improved the regioselectivity for epoxide hydrolysis on 1. The single site Y247F mutation led to highly regioselective (98%) attack at C-3 of 1, whereas the double mutation W187F/Y247F resulted in regioselective (94%) nucleophilic attack at C-2. Furthermore, single-crystal X-ray structures of the two regioselective Alp1U variants in complex with 1 were determined. These findings allowed insights into the reaction details of Alp1U and provided a new approach for engineering regioselective epoxide hydrolases.

Small Molecule Soluble Epoxide Hydrolase Inhibitors in Multitarget and Combination Therapies for Inflammation and Cancer

The enzyme soluble epoxide hydrolase (sEH) plays a central role in metabolism of bioactive lipid signaling molecules. The substrate-specific hydrolase activity of sEH converts epoxyeicosatrienoic acids (EETs) to less bioactive dihydroxyeicosatrienoic acids. EETs exhibit anti-inflammatory, analgesic, antihypertensive, cardio-protective and organ-protective properties. Accordingly, sEH inhibition is a promising therapeutic strategy for addressing a variety of diseases. In this review, we describe small molecule architectures that have been commonly deployed as sEH inhibitors with respect to angiogenesis, inflammation and cancer. We juxtapose commonly used synthetic scaffolds and natural products within the paradigm of a multitarget approach for addressing inflammation and inflammation induced carcinogenesis. Structural insights from the inhibitor complexes and novel strategies for development of sEH-based multitarget inhibitors are also presented. While sEH inhibition is likely to suppress inflammation-induced carcinogenesis, it can also lead to enhanced angiogenesis via increased EET concentrations. In this regard, sEH inhibitors in combination chemotherapy are described. Urea and amide-based architectures feature prominently across multitarget inhibition and combination chemotherapy applications of sEH inhibitors.

2-Oxaadamant-1-yl Ureas as Soluble Epoxide Hydrolase Inhibitors: In Vivo Evaluation in a Murine Model of Acute Pancreatitis

In vivo pharmacological inhibition of soluble epoxide hydrolase (sEH) reduces inflammatory diseases, including acute pancreatitis (AP). Adamantyl ureas are very potent sEH inhibitors, but the lipophilicity and metabolism of the adamantane group compromise their overall usefulness. Herein, we report that the replacement of a methylene unit of the adamantane group by an oxygen atom increases the solubility, permeability, and stability of three series of urea-based sEH inhibitors. Most of these oxa-analogues are nanomolar inhibitors of both the human and murine sEH. Molecular dynamics simulations rationalize the molecular basis for their activity and suggest that the presence of the oxygen atom on the adamantane scaffold results in active site rearrangements to establish a weak hydrogen bond. The 2-oxaadamantane 22, which has a good solubility, microsomal stability, and selectivity for sEH, was selected for further in vitro and in vivo studies in models of cerulein-induced AP. Both in prophylactic and treatment studies, 22 diminished the overexpression of inflammatory and endoplasmic reticulum stress markers induced by cerulein and reduced the pancreatic damage.

Isothermal Titration Calorimetry Enables Rapid Characterization of Enzyme Kinetics and Inhibition for the Human Soluble Epoxide Hydrolase

Isothermal titration calorimetry (ITC) is conventionally used to acquire thermodynamic data for biological interactions. In recent years, ITC has emerged as a powerful tool to characterize enzyme kinetics. In this study, we have adapted a single-injection method (SIM) to study the kinetics of human soluble epoxide hydrolase (hsEH), an enzyme involved in cardiovascular homeostasis, hypertension, nociception, and insulin sensitivity through the metabolism of epoxy-fatty acids (EpFAs). In the SIM method, the rate of reaction is determined by monitoring the thermal power, while the substrate is being depleted, overcoming the need for synthetic substrates and reducing postreaction processing. Our results show that ITC enables the detailed, rapid, and reproducible characterization of the hsEH-mediated hydrolysis of several natural EpFA substrates. Furthermore, we have applied a variant of the single-injection ITC method for the detailed description of enzyme inhibition, proving the power of this approach in the rapid screening and discovery of new hsEH inhibitors using the enzyme’s physiological substrates. The methods described herein will enable further studies on EpFAs’ metabolism and biology, as well as drug discovery investigations to identify and characterize hsEH inhibitors. This also promises to provide a general approach for the characterization of lipid catalysis, given the challenges that lipid metabolism studies pose to traditional spectroscopic techniques.

15-deoxy-Δ12,14-Prostaglandin J2 inhibits human soluble epoxide hydrolase by a dual orthosteric and allosteric mechanism

Human soluble epoxide hydrolase (hsEH) is an enzyme responsible for the inactivation of bioactive epoxy fatty acids, and its inhibition is emerging as a promising therapeutical strategy to target hypertension, cardiovascular disease, pain and insulin sensitivity. Here, we uncover the molecular bases of hsEH inhibition mediated by the endogenous 15-deoxy-Δ12,14-Prostaglandin J2 (15d-PGJ2). Our data reveal a dual inhibitory mechanism, whereby hsEH can be inhibited by reversible docking of 15d-PGJ2 in the catalytic pocket, as well as by covalent locking of the same compound onto cysteine residues C423 and C522, remote to the active site. Biophysical characterisations allied with in silico investigations indicate that the covalent modification of the reactive cysteines may be part of a hitherto undiscovered allosteric regulatory mechanism of the enzyme. This study provides insights into the molecular modes of inhibition of hsEH epoxy-hydrolytic activity and paves the way for the development of new allosteric inhibitors.

Expression, purification, and characterisation of human soluble Epoxide Hydrolase (hsEH) and of its functional C-terminal domain

The human soluble Epoxide Hydrolase (hsEH) is an enzyme involved in the hydrolysis of endogenous anti-inflammatory and cardio-protective signalling mediators known as epoxyeicosatrienoic acids (EETs). EETs’ conversion into the corresponding diols by hsEH generates non-bioactive molecules, thereby the enzyme inhibition would be expected to enhance the EETs bioavailability, and their beneficial properties. Numerous inhibitors have been developed to target the enzyme, some of which are showing promising antihypertensive and anti-inflammatory properties in vivo. Thus far, the preparation of the recombinant enzyme for enzymatic and structural in vitro studies has been performed mainly using a baculovirus expression system. More recently, it was reported that the enzyme could be exogenously expressed and isolated from E. coli, although limited amounts of active protein were obtained. We herein describe two novel methods to yield pure recombinant enzyme. The first describes the expression and purification of the full-length enzyme from eukaryotic cells HEK293-F, whilst the second concerns the C-terminal domain of hsEH obtained from the cost-effective and rapid E. coli prokaryotic system. The two methods successfully generated satisfactory amounts of functional enzyme, with virtually identical enzymatic activity. Overall, the protocols described in this paper can be employed for the recombinant expression and purification of active hsEH, to be used in future biomedical investigations and for high-throughput screening of inhibitors for potential use in the treatment of cardiovascular disease.

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hsEH is a key regulator of cardiovascular homeostasis.

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A HEK293-F mammalian expression system for hsEH full-length (FL) was developed.

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An E. coli expression system for the hsEH C-terminal Domain (CTD) was established.

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Both proteins exhibited the same enzymatic specific activity in vitro.

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The CTD preparation provides benefits of easy operation, and high yield and purity.

Chemical Proteomics Reveals Soluble Epoxide Hydrolase as a Therapeutic Target for Ocular Neovascularization

The standard-of-care therapeutics for the treatment of ocular neovascular diseases like wet age-related macular degeneration (AMD) are biologics targeting vascular endothelial growth factor signaling. There are currently no FDA approved small molecules for treating these blinding eye diseases. Therefore, therapeutic agents with novel mechanisms are critical to complement or combine with existing approaches. Here, we identified soluble epoxide hydrolase (sEH), a key enzyme for epoxy fatty acid metabolism, as a target of an antiangiogenic homoisoflavonoid, SH-11037. SH-11037 inhibits sEH in vitro and in vivo and docks to the substrate binding cleft in the sEH hydrolase domain. sEH levels and activity are up-regulated in the eyes of a choroidal neovascularization (CNV) mouse model. sEH is overexpressed in human wet AMD eyes, suggesting that sEH is relevant to neovascularization. Known sEH inhibitors delivered intraocularly suppressed CNV. Thus, by dissecting a bioactive compound's mechanism, we identified a new chemotype for sEH inhibition and characterized sEH as a target for blocking the CNV that underlies wet AMD.

Soluble Epoxide Hydrolase Inhibitory Activity of Components Isolated from Apios americana Medik

A new compound 1, 5-methoxy-2,5,7,4′-tetrahydroxy-coumaronochromone, along with seven known compounds (2–8), were isolated from Apios americana using open column chromatography. Their structures were established based on an analysis of 1D and 2D NMR, and MS spectra. Among these, two compounds 1 and 2 showed inhibitory activity on soluble epoxide hydrolase (sEH) at a concentration below 50 μM. The respective competitive (1) and mixed (2) inhibitors were revealed to have K_i values of 21.0 ± 0.8 and 14.5 ± 1.5 μM, based on the Dixon plot. The potential inhibitor (2) was visually presented in a predicted binding pose in the receptor by molecular docking. Additionally, molecular dynamics were performed for a detailed understanding of their complex by Gromacs 4.6.5 package.

Investigation of the binding mode of 1, 3, 4-oxadiazole derivatives as amide-based inhibitors for soluble epoxide hydrolase (sEH) by molecular docking and MM-GBSA

The soluble epoxide hydrolase (sEH) enzyme plays an important role in the metabolism of endogenous chemical mediators involved in the regulation of blood pressure and inflammation. Inhibition of sEH provides a new approach to the treatment of inflammation, hypertension and atherosclerosis. In this study, the binding modes and inhibition mechanisms of the new oxadiazole-based amide inhibitors of the human soluble epoxide hydrolase were investigated by molecular docking and molecular dynamics (MD) simulation followed by the MM-GBSA method to calculate the binding free energy of each inhibitor to sEH. The results obtained from the binding free energy (ΔG _binding) calculation and normal mode analysis indicate that the major favorable contributors are the van der Waals and electrostatic terms, whereas the polar solvation term opposes binding. In addition, a good agreement between the calculated ΔG _binding and the experimental IC₅₀ was obtained [correlation coefficient, r ² = 0.89 (with) and 0.87 (without) entropy]. Besides, comparison of the enthalpy changes (ΔG _MM-GBSA) with entropy changes (-TΔS) indicates that binding process of all inhibitors to sEH is enthalpy-driven. Based on the ΔG _binding on per residue decomposition, Asp335 and Tyr383 residues from the active site and Trp336, Leu499 and His524 residues from hydrophobic pockets contribute the most to ΔG _binding. Moreover, hydrogen bond analysis reveals that Tyr383, Tyr466 and Asp335 residues have an important role in the binding to inhibitors by forming hydrogen bonds with high occupancies. Our obtained results are useful for the understanding of the sEH-inhibitor interactions and may have great importance in the design of future sEH inhibitors.

Is Promiscuous CALB a Good Scaffold for Designing New Epoxidases?

Candida Antarctica lipase B (CALB) is a well-known enzyme, especially because of its promiscuous activity. Due to its properties, CALB was widely used as a benchmark for designing new catalysts for important organic reactions. The active site of CALB is very similar to that of soluble epoxide hydrolase (sEH) formed by a nucleophile-histidine-acid catalytic triad and an oxyanion hole typical for molecular structures derived from processes of α/β hydrolases. In this work we are exploring these similarities and proposing a Ser105Asp variant of CALB as a new catalyst for epoxide hydrolysis. In particular, the hydrolysis of the trans-diphenylpropene oxide (t-DPPO) is studied by means of quantum cluster models mimicking the active site of both enzymes. Our results, based on semi-empirical and DFT calculations, suggest that mutant Ser105Asp CALB is a good protein scaffold to be used for the bio-synthesis of chiral compounds.

Soluble epoxide hydrolase inhibitors and heart failure

Cardiovascular disease remains one of the leading causes of death in the Western societies. Heart failure (HF) is due primarily to progressive myocardial dysfunction accompanied by myocardial remodeling. Once HF develops, the condition is, in most cases, irreversible and is associated with a very high mortality rate. Soluble epoxide hydrolase (sEH) is an enzyme that catalyzes the hydrolysis of epoxyeicosatrienoic acids (EETs), which are lipid mediators derived from arachidonic acid through the cytochrome P450 epoxygenase pathway. EETs have been shown to have vasodilatory, antiinflammatory, and cardioprotective effects. When EETs are hydrolyzed by sEH to corresponding dihydroxyeicosatrienoic acids, their cardioprotective activities become less pronounced. In line with the recent genetic study that has identified sEH as a susceptibility gene for HF, the sEH enzyme has received considerable attention as an attractive therapeutic target for cardiovascular diseases. Indeed, sEH inhibition has been demonstrated to have antihypertensive and antiinflammatory actions, presumably due to the increased bioavailability of endogenous EETs and other epoxylipids, and several potent sEH inhibitors have been developed and tested in animal models of cardiovascular disease including hypertension, cardiac hypertrophy, and ischemia/reperfusion injury. sEH inhibitor treatment has been shown to effectively prevent pressure overload- and angiotensin II-induced cardiac hypertrophy and reverse the pre-established cardiac hypertrophy caused by chronic pressure overload. Application of sEH inhibitors in several cardiac ischemia/reperfusion injury models reduced infarct size and prevented the progressive cardiac remodeling. Moreover, the use of sEH inhibitors prevented the development of electrical remodeling and ventricular arrhythmias associated with cardiac hypertrophy and ischemia/reperfusion injury. The data published to date support the notion that sEH inhibitors may represent a promising therapeutic approach for combating detrimental cardiac remodeling and HF.

The role of epoxide hydrolases in health and disease

Epoxide hydrolases (EH) are ubiquitously expressed in all living organisms and in almost all organs and tissues. They are mainly subdivided into microsomal and soluble EH and catalyze the hydration of epoxides, three-membered-cyclic ethers, to their corresponding dihydrodiols. Owning to the high chemical reactivity of xenobiotic epoxides, microsomal EH is considered protective enzyme against mutagenic and carcinogenic initiation. Nevertheless, several endogenously produced epoxides of fatty acids function as important regulatory mediators. By mediating the formation of cytotoxic dihydrodiol fatty acids on the expense of cytoprotective epoxides of fatty acids, soluble EH is considered to have cytotoxic activity. Indeed, the attenuation of microsomal EH, achieved by chemical inhibitors or preexists due to specific genetic polymorphisms, is linked to the aggravation of the toxicity of xenobiotics, as well as the risk of cancer and inflammatory diseases, whereas soluble EH inhibition has been emerged as a promising intervention against several diseases, most importantly cardiovascular, lung and metabolic diseases. However, there is reportedly a significant overlap in substrate selectivity between microsomal and soluble EH. In addition, microsomal and soluble EH were found to have the same catalytic triad and identical molecular mechanism. Consequently, the physiological functions of microsomal and soluble EH are also overlapped. Thus, studying the biological effects of microsomal or soluble EH alterations needs to include the effects on both the metabolism of reactive metabolites, as well as epoxides of fatty acids. This review focuses on the multifaceted role of EH in the metabolism of xenobiotic and endogenous epoxides and the impact of EH modulations.

Isolation and characterization of Xenopus soluble epoxide hydrolase

Soluble epoxide hydrolase (sEH) contributes to cell growth, but the contribution of sEH to embryonic development is not well understood. In this study, Xenopus sEH cDNA was isolated from embryos of Xenopus laevis. The Xenopus sEH was expressed in Escherichia coli and was purified. The epoxide hydrolase and phosphatase activities of purified sEH were investigated. The Xenopus sEH did not show phosphatase activity toward 4-methylumbelliferyl phosphate or several lysophosphatidic acids although it had EH activity. The amino acid sequence of Xenopus sEH was compared with that reported previously. We found amino acid substitutions of the 29th Thr to Asn and the 146th Arg to His and prepared a sEH mutant (N29T/H146R), designed as mutant 1. Neither wild-type sEH nor mutant 1 had phosphatase activity. Additional substitution of the 11th Gly with Asp was found by comparison with human sEH which has phosphatase activity, but the Xenopus sEH mutant G11D prepared as mutant 2 did not have phosphatase activity. The epoxide hydrolase activity of sEH seemed to be similar to that of human sEH, while Xenopus sEH did not have phosphatase activity toward several substrates that human sEH metabolizes.

Substituted phenyl groups improve the pharmacokinetic profile and anti-inflammatory effect of urea-based soluble epoxide hydrolase inhibitors in murine models

Soluble epoxide hydrolase inhibitors (sEHIs) are anti-inflammatory, analgesic, anti-hypertensive, cardio- and renal-protective in multiple animal models. However, the earlier adamantyl-containing urea-based inhibitors are rapidly metabolized. Therefore, new potent inhibitors with the adamantyl group replaced by a substituted phenyl group were synthesized to presumptively offer better pharmacokinetic (PK) properties. Here we describe the improved PK profile of these inhibitors and the anti-inflammatory effect of the most promising one in a murine model. The PK profiles of inhibitors were determined following p.o. administration and serial bleeding in mice. The anti-inflammatory effect of 1-trifluoromethoxyphenyl-3-(1-propionylpiperidin-4-yl)urea (TPPU), the most promising inhibitor among the five sEHIs tested, was investigated in a lipopolysaccharide (LPS)-challenged murine model. The earlier broadly-used adamantyl-containing sEHI, trans-4-[4-(3-adamantan-1-yl-ureido)-cyclohexyloxy]-benzoic acid (t-AUCB), was used for comparison. Compared with the earlier adamantyl-containing urea-based inhibitors, substituted phenyl-containing urea-based inhibitors afford more favorable PK properties, such as higher Cmaxs, larger AUCs and longer t1/2s, which, as expected, show more stable metabolic stability. Moreover, oral administration of TPPU dramatically reversed the shifts caused by LPS-challenge in plasma levels of inflammatory cytokines, epoxides and corresponding diols, which is more potent than t-AUCB. The substituted phenyl-containing sEHIs are more metabolically stable than those with adamantyl group, resulting in more potent efficacy in vivo. This indicates a new strategy for development of sEHIs for further study toward clinical trials.

Obtaining optical purity for product diols in enzyme-catalyzed epoxide hydrolysis: contributions from changes in both enantio- and regioselectivity

Enzyme variants of the plant epoxide hydrolase StEH1 displaying improved stereoselectivities in the catalyzed hydrolysis of (2,3-epoxypropyl)benzene were generated by directed evolution. The evolution was driven by iterative saturation mutagenesis in combination with enzyme activity screenings where product chirality was the decisive selection criterion. Analysis of the underlying causes of the increased diol product ratios revealed two major contributing factors: increased enantioselectivity for the corresponding epoxide enantiomer(s) and, in some cases, a concomitant change in regioselectivity in the catalyzed epoxide ring-opening half-reaction. Thus, variant enzymes that catalyzed the hydrolysis of racemic (2,3-epoxypropyl)benzene into the R-diol product in an enantioconvergent manner were isolated.

Determinants of reactivity and selectivity in soluble epoxide hydrolase from quantum mechanics/molecular mechanics modeling

Soluble epoxide hydrolase (sEH) is an enzyme involved in drug metabolism that catalyzes the hydrolysis of epoxides to form their corresponding diols. sEH has a broad substrate range and shows high regio- and enantioselectivity for nucleophilic ring opening by Asp333. Epoxide hydrolases therefore have potential synthetic applications. We have used combined quantum mechanics/molecular mechanics (QM/MM) umbrella sampling molecular dynamics (MD) simulations (at the AM1/CHARMM22 level) and high-level ab initio (SCS-MP2) QM/MM calculations to analyze the reactions, and determinants of selectivity, for two substrates: trans-stilbene oxide (t-SO) and trans-diphenylpropene oxide (t-DPPO). The calculated free energy barriers from the QM/MM (AM1/CHARMM22) umbrella sampling MD simulations show a lower barrier for phenyl attack in t-DPPO, compared with that for benzylic attack, in agreement with experiment. Activation barriers in agreement with experimental rate constants are obtained only with the highest level of QM theory (SCS-MP2) used. Our results show that the selectivity of the ring-opening reaction is influenced by several factors, including proximity to the nucleophile, electronic stabilization of the transition state, and hydrogen bonding to two active site tyrosine residues. The protonation state of His523 during nucleophilic attack has also been investigated, and our results show that the protonated form is most consistent with experimental findings. The work presented here illustrates how determinants of selectivity can be identified from QM/MM simulations. These insights may also provide useful information for the design of novel catalysts for use in the synthesis of enantiopure compounds.

Soluble epoxide hydrolase inhibition reveals novel biological functions of epoxyeicosatrienoic acids (EETs)

Early on, intriguing biological activities were found associated with the EETs using in vitro systems. Although the EETs other than the 5,6-isomer, are quite stable chemically, they are quickly degraded enzymatically with the sEH accounting in many cases for much of the metabolism. This rapid degradation often made it difficult to associate biological effects with the administration of EETs and other lipid epoxides particularly in vivo. Thus, it is the power to inhibit the sEH that has facilitated the demonstration of many physiological processes associated with EETs and possibly other epoxy fatty acids. In the last few years it has become clear that major roles of the EETs include modulation of blood pressure and modulation of inflammatory cascades. There are a number of other physiological functions now associated with the EETs including angiogenesis, neurohormone release, cell proliferation, G protein signaling, modulation of ion channel activity, and a variety of effects associated with modulation of NFkappaB. More recently we observed a role of the EETs as modulated by sEHI in reducing non-neuropathic pain. The array of biological effects observed with sEHI illustrates the power of modulating the degradation of chemical mediators in addition to the modulation of their biosynthesis, receptor binding and signal transduction. Many of these biological effects can be modulated by sEHIs but also by the natural eicosanoids and their mimics all of which offer therapeutic potential.

Theoretical Study of the Full Reaction Mechanism of Human Soluble Epoxide Hydrolase

The complete reaction mechanism of soluble epoxide hydrolase (sEH) has been investigated by using the B3LYP density functional theory method. Epoxide hydrolases catalyze the conversion of epoxides to their corresponding vicinal diols. In our theoretical study, the sEH active site is represented by quantum-chemical models that are based on the X-ray crystal structure of human soluble epoxide hydrolase. The trans-substituted epoxide (1S,2S)-beta-methylstyrene oxide has been used as a substrate in the theoretical investigation of the sEH reaction mechanism. Both the alkylation and the hydrolytic half-reactions have been studied in detail. We present the energetics of the reaction mechanism as well as the optimized intermediates and transition-state structures. Full potential energy curves for the reactions involving nucleophilic attack at either the benzylic or the homo-benzylic carbon atom of (1S,2S)-beta-methylstyrene oxide have been computed. The regioselectivity of epoxide opening has been addressed for the two substrates (1S,2S)-beta-methylstyrene oxide and (S)-styrene oxide.

Catalytic mechanism of limonene epoxide hydrolase, a theoretical study

The catalytic mechanism of limonene epoxide hydrolase (LEH) was investigated theoretically using the density functional theory method B3LYP. LEH is part of a novel limonene degradation pathway found in Rhodococcus erythropolis DCL14, where it catalyzes the hydrolysis of limonene-1,2-epoxide to give limonene-1,2-diol. The recent crystal structure of LEH was used to build a model of the LEH active site composed of five amino acids and a crystallographically observed water molecule. With this model, hydrolysis of different substrates was investigated. It is concluded that LEH employs a concerted general acid/general base-catalyzed reaction mechanism involving protonation of the substrate by Asp101, nucleophilic attack by water on the epoxide, and abstraction of a proton from water by Asp132. Furthermore, we provide an explanation for the experimentally observed regioselective hydrolysis of the four stereoisomers of limonene-1,2-epoxide.

Catalysis of potato epoxide hydrolase, StEH1

The kinetic mechanism of epoxide hydrolase (EC 3.3.2.3) from potato, StEH1 (Solanum tuberosum epoxide hydrolase 1), was studied by presteady-state and steady-state kinetics as well as by pH dependence of activity. The specific activities towards the different enantiomers of TSO (trans-stilbene oxide) as substrate were 43 and 3 micromol x min(-1) x mg(-1) with the R,R- or S,S-isomers respectively. The enzyme was, however, enantioselective in favour of the S,S enantiomer due to a lower K(m) value. The pH dependences of kcat with R,R or S,S-TSO were also distinct and supposedly reflecting the pH dependences of the individual kinetic rates during substrate conversion. The rate-limiting step for TSO and cis- and trans-epoxystearate was shown by rapid kinetic measurements to be the hydrolysis of the alkylenzyme intermediate. Functional characterization of point mutants verified residues Asp105, Tyr154, Tyr235 and His300 as crucial for catalytic activity. All mutants displayed drastically decreased enzymatic activities during steady state. Presteady-state measurements revealed the base-deficient H300N (His300-->Asn) mutant to possess greatly reduced efficiencies in catalysis of both chemical steps (alkylation and hydrolysis).

Human soluble epoxide hydrolase: structural basis of inhibition by 4-(3-cyclohexylureido)-carboxylic acids

X-ray crystal structures of human soluble epoxide hydrolase (sEH) complexed with four different dialkylurea inhibitors bearing pendant carboxylate "tails" of varying length have been determined at 2.3-3.0 A resolution. Similarities among inhibitor binding modes reinforce the proposed roles of Y381 and/or Y465 as general acids that protonate the epoxide ring of the substrate in concert with nucleophilic attack of D333 at the electrophilic epoxide carbon. Additionally, the binding of these inhibitors allows us to model the binding mode of the endogenous substrate 14,15-epoxyeicosatrienoic acid. Contrasts among inhibitor binding modes include opposite orientations of inhibitor binding in the active-site hydrophobic tunnel. Alternative binding orientations observed for this series of inhibitors to human sEH, as well as the binding of certain dialkylurea inhibitors to human sEH and murine sEH, complicate the structure-based design of human sEH inhibitors with potential pharmaceutical applications in the treatment of hypertension. Thus, with regard to the optimization of inhibitor designs targeting human sEH, it is critical that human sEH and not murine sEH be utilized for inhibitor screening, and it is critical that structures of human sEH-inhibitor complexes be determined to verify inhibitor binding orientations that correlate with measured affinities.

Epoxide hydrolases: mechanisms, inhibitor designs, and biological roles

Organisms are exposed to epoxide-containing compounds from both exogenous and endogenous sources. In mammals, the hydration of these compounds by various epoxide hydrolases (EHs) can not only regulate their genotoxicity but also, for lipid-derived epoxides, their endogenous roles as chemical mediators. Recent findings suggest that the EHs as a family represent novel drug discovery targets for regulation of blood pressure, inflammation, cancer progression, and the onset of several other diseases. Knowledge of the EH mechanism provides a solid foundation for the rational design of inhibitors, and this review summarizes the current understanding of the catalytic mechanism of the EHs. Although the overall EH mechanism is now known, the molecular basis of substrate selectivity, possible allosteric regulation, and many fine details of the catalytic mechanism remain to be solved. Finally, recent development in the design of EH inhibitors and the EH biological role are discussed.

Identification and Characterization of an Ovary-Selective Isoform of Epoxide Hydrolase1

A novel ovary-selective gene was identified by suppression subtractive hybridization (SSH) that is expressed only during the mouse periovulatory phase of a stimulated estrous cycle. Analysis of the protein encoded by the full-length cDNA revealed that the majority of it, with the exception of the first 44 amino acids, matched soluble epoxide hydrolase (Ephx2, referred to as Ephx2A). By comparing the cDNA sequence of this newly identified variant of soluble epoxide hydrolase (referred to as Ephx2B) with the mouse genome database, an exon was identified that corresponds to its unique 5' cDNA sequence. Through the use of an Ephx2A-specific probe, Northern blot analysis revealed that this mRNA was also expressed in the ovary, with the highest level of expression occurring during the luteal phase of a stimulated estrous cycle. In situ hybridization revealed that Ephx2B mRNA expression was restricted to granulosa cells of preovulatory follicles. Ephx2A mRNA expression, however, was detectable in follicles at different stages of development, as well as in the corpus luteum. Total ovarian epoxide hydrolase activity increased following the induction of follicular development, and remained elevated through the periovulatory and postovulatory stages of a stimulated estrous cycle. The change in enzyme activity paralleled the combined mRNA expression profiles for both Ephx2A and Ephx2B, thus supporting a role for epoxide metabolism in ovarian function.

Soybean epoxide hydrolase: identification of the catalytic residues and probing of the reaction mechanism with secondary kinetic isotope effects

Soybean epoxide hydrolase catalyzes the oxirane ring opening of 9,10-epoxystearate via a two-step mechanism involving the formation of an alkylenzyme intermediate, which, in contrast to most epoxide hydrolases studied so far, was found to be the rate-limiting step. We have probed residues potentially involved in catalysis by site-directed mutagenesis. Mutation of His(320), a residue predicted from sequence analysis to belong to the catalytic triad of the enzyme, considerably slowed down the second half-reaction. This kinetic manipulation provoked an accumulation of the reaction intermediate, which could be trapped and characterized by electrospray ionization mass spectrometry. As expected, mutation of Asp(126) totally abolished the activity of the enzyme from its crucial function as nucleophile involved in the formation of the alkylenzyme. In line with its role as the partner of His(320) in the "charge relay system," mutation of Asp(285) dramatically reduced the rate of catalysis. However, the mutant D285L still exhibited a very low residual activity, which, by structural analysis and mutagenesis, has been tentatively attributed to Glu(195), another acidic residue of the active site. Our studies have also confirmed the fundamental role of the conserved Tyr(175) and Tyr(255) residues, which are believed to activate the oxirane ring. Finally, we have determined the secondary tritium kinetic isotope effects on the epoxide opening step of 9,10-epoxystearate. The large observed values, i.e. (T)(V/K(m)) approximately 1.30, can be interpreted by the occurrence of a very late transition state in which the epoxide bond is broken before the nucleophilic attack by Asp(126) takes place.

QSAR and classification of murine and human soluble epoxide hydrolase inhibition by urea-like compounds

A data set of 348 urea-like compounds that inhibit the soluble epoxide hydrolase enzyme in mice and humans is examined. Compounds having IC(50) values ranging from 0.06 to >500 microM (murine) and 0.10 to >500 microM (human) are categorized as active or inactive for classification, while quantitation is performed on smaller compound subsets ranging from 0.07 to 431 microM (murine) and 0.11 to 490 microM (human). Each compound is represented by calculated structural descriptors that encode topological, geometrical, electronic, and polar surface features. Multiple linear regression (MLR) and computational neural networks (CNNs) are employed for quantitative models. Three classification algorithms, k-nearest neighbor (kNN), linear discriminant analysis (LDA), and radial basis function neural networks (RBFNN), are used to categorize compounds as active or inactive based on selected data split points. Quantitative modeling of human enzyme inhibition results in a nonlinear, five-descriptor model with root-mean-square errors (log units of IC(50) [microM]) of 0.616 (r(2) = 0.66), 0.674 (r(2) = 0.61), and 0.914 (r(2) = 0.33) for training, cross-validation, and prediction sets, respectively. The best classification results for human and murine enzyme inhibition are found using kNN. Human classification rates using a seven-descriptor model for training and prediction sets are 89.1% and 91.4%, respectively. Murine classification rates using a five-descriptor model for training and prediction sets are 91.5% and 88.6%, respectively.

The N-terminal domain of mammalian soluble epoxide hydrolase is a phosphatase

The mammalian soluble epoxide hydrolase (sEH) is an enzyme with multiple functions, being implicated in detoxification of xenobiotic epoxides as well as in regulation of physiological processes such as blood pressure. The enzyme is a homodimer, in which each subunit is composed of two domains. The 35-kDa C-terminal domain has an alpha/beta hydrolase fold and harbors the catalytic center for the EH activity. The 25-kDa N-terminal domain has a different alpha/beta fold and belongs to the haloacid dehalogenase superfamily of enzymes. The catalytic properties of the enzyme reported so far can all be explained by the action of the C-terminal domain alone. The function of the N-terminal domain, other than in structural stabilization of the dimer, has therefore remained unclear. By structural comparison of this domain to other haloacid dehalogenase family members, we identified a putative active site containing all necessary components for phosphatase activity. Subsequently, we found rat sEH hydrolyzed 4-nitrophenyl phosphate with a rate constant of 0.8 s(-1) and a K(m) of 0.24 mM. Recombinant human sEH lacking the C-terminal domain also displayed phosphatase activity. Presence of a phosphatase substrate did not affect epoxide turnover nor did epoxides affect dephosphorylation by the intact enzyme, indicating both catalytic sites act independently. The enzyme was unable to hydrolyze 4-nitrophenyl sulfate, suggesting its role in xenobiotic metabolism does not extend beyond phosphates. Thus, we propose this domain participates instead in the regulation of the physiological functions associated with sEH.

Reaction mechanism of soluble epoxide hydrolase: insights from molecular dynamics simulations

Molecular dynamics simulations have been performed to gain insights into the catalytic mechanism of the hydrolysis of epoxides to vicinal diols by soluble epoxide hydrolase (sEH). The binding of a substrate, 1S,2S-trans-methylstyrene oxide, was studied in two conformations in the active site of the enzyme. It was found that only one is likely to be found in the active enzyme. In the preferred conformation the phenyl group of the substrate is pi-sandwiched between two aromatic residues, Tyr381 and His523, whereas the other conformation is pi-stacked with only one aromatic residue, Trp334. Two simulations were carried out to 1 ns for each conformation to evaluate the protonation state of active site residue His523. It was found that a protonated histidine is essential for keeping the active site from being disrupted. Long time scale, 4 ns, molecular dynamics simulation was done for the structure with the most likely combination of binding conformation and protonation state of His523. Near Attack Conformers (NACs) are present 5.3% of the time and nucleophilic attack on either epoxide carbon atom, approximately 75% on C(1) and approximately 25% on C(2), is found. A maximum of one hydrogen bond between the epoxide oxygen and either of the active site tyrosines, Tyr465 and Tyr381, is present, in agreement with experimental mutagenesis results that reveal a slight loss in activity if one tyrosine is mutated and essential loss of all activity upon double mutation of the two tyrosines in question. It was found that a hydrogen bond from Tyr465 to the substrate oxygen is essential for controlling the regioselectivity of the reaction. Furthermore, a relationship between the presence of this hydrogen bond and the separation of reactants was found. Two groups of amino acid segments were identified each as moving collectively. Furthermore, an overall anti-correlation was found between the movements of these two individually collectively moving groups, made up by parts of the cap-region, including the two tyrosines, and the site of the catalytic triad, respectively. This overall anti-correlated collective domain motion is, perhaps, involved in the conversion of E.NAC to E.TS.

Overexpression of Arabidopsis thaliana soluble epoxide hydrolase 1 in Pichia pastoris and characterisation of the recombinant enzyme

Epoxide hydrolases are enzymes involved in metabolism and defense of plants. Genome scanning suggested the presence of several genes encoding epoxide hydrolase in Arabidopsis thaliana. To assure that the predicted genes are functional and the translated products have epoxide hydrolase activity analysis at the protein level is needed. We have started to clone the cDNAs and overexpress them for catalytic and physico-chemical analysis. We here report that Pichia pastoris serves as an efficient system for overexpression of soluble epoxide hydrolase 1 (AtsEH1) from A. thaliana. A tag containing six histidine residues was added to the N-terminus to enable efficient one-step purification on nickel-agarose. The enzyme was expressed at levels >18 mg.L(-1) of culture and a French Press was found to be effective to achieve cell lysis. The recombinant enzyme had a molecular mass of 37 or 38 kDa based on SDS-PAGE or MALDI-TOF analysis, respectively. The enzyme was highly active towards the substrate trans-stilbene oxide (TSO) and had a pH optimum at 7 and a temperature optimum at 54 degrees C. Using TSO as substrate the K(m) and V(max) values were determined to 5 micro M and 2 micromol min(-1) mg protein(-1), respectively. The activity was 50-fold lower towards cis-stilbene oxide. The stability over time was tested from 20 to 54 degrees C and the enzyme lost activity at varying degrees at the temperatures tested but was stable for several months at 4 degrees C.

Stereochemical features of the hydrolysis of 9,10-epoxystearic acid catalysed by plant and mammalian epoxide hydrolases

cis-9,10-epoxystearic acid was used as a tool to probe the active sites of epoxide hydrolases (EHs) of mammalian and plant origin. We have compared the stereochemical features of the hydrolysis of this substrate catalysed by soluble and membrane-bound rat liver EHs, by soluble EH (purified to apparent homogeneity) obtained from maize seedlings or celeriac roots, and by recombinant soybean EH expressed in yeast. Plant EHs were found to differ in their enantioselectivity, i.e. their ability to discriminate between the two enantiomers of 9,10-epoxystearic acid. For example, while the maize enzyme hydrated both enantiomers at the same rate, the EH from soybean exhibited very high enantioselectivity in favour of 9R,10S-epoxystearic acid. This latter enzyme also exhibited a strict stereoselectivity, i.e. it hydrolysed the racemic substrate with a very high enantioconvergence, yielding a single chiral diol product, threo-9R,10R-dihydroxystearic acid. Soybean EH shared these distinctive stereochemical features with the membrane-bound rat liver EH. The stereochemical outcome of these enzymes probably results from a stereoselective attack by the nucleophilic residue on the oxirane ring carbon having the (S)-configuration, leading to the presumed (in plant EH) covalent acyl-enzyme intermediate. In sharp contrast, the reactions catalysed by cytosolic rat liver EH exhibited a complete absence of enantioselectivity and enantioconvergence; this latter effect might be ascribed to a regioselective formation of the acyl-enzyme intermediate involving C-10 of 9,10-epoxystearic acid, independent of its configuration. Thus, compared with soybean EH, the active site of rat liver soluble EH displays a very distinct means of anchoring the oxirane ring of the fatty acid epoxides, and therefore appears to be a poor model for mapping the catalytic domain of plant EHs.

A theoretical examination of the acid-catalyzed and noncatalyzed ring-opening reaction of an oxirane by nucleophilic addition of acetate. Implications to epoxide hydrolases

Ab initio and density functional calculations have been performed to gain a better understanding of the epoxide ring-opening reaction catalyzed by epoxide hydrolase. The S(N)2 reaction of acetate with 1S,2S-trans-2-methylstyrene oxide to provide the corresponding diol acetate ester was studied with and without general-acid catalysis. MP2 and DFT (B3LYP) calculations predict, for the noncatalyzed reaction, a central barrier of approximately 20-21 kcal/mol separating the reactants from products depending on which carbon center in the epoxide is undergoing attack. From these gas-phase reactions the immediate alkoxide products are not energetically far below their associated transition states such that the reaction is predicted to be endothermic. Inclusion of aqueous solvation effects via a polarizable continuum model predicts the activation barrier to increase by almost 10 kcal/mol due to the solvation of the acetate ion nucleophile. The activation barrier for the epoxide ring-opening reaction is reduced to approximately 10 kcal/mol when phenol, as the general-acid catalyst, is included in the gas-phase calculations. This is due to the immediate product being the neutral ester rather than the corresponding alkoxide. The transition state in the general-acid-catalyzed reaction is earlier than that for the noncatalyzed reaction and the reaction is highly exothermic. Molecular mechanics calculations of 1S,2S-trans-2-methylstyrene oxide in the active site of murine epoxide hydrolase show two possible binding conformations. Both conformers have the epoxide oxygen forming hydrogen bonds with the acidic hydrogens of the catalytic tyrosines (Tyr381 and Tyr465). These two conformations likely lead to different products since the nucleophile (Asp333-CO(2)(-)) is positioned to react with either carbon center in the epoxide.

Direct measurement of acylenzyme hydrolysis demonstrates rate-limiting deacylation in cleavage of physiological sequences by the processing protease Kex2

Saccharomyces cerevisiae Kex2 protease is the prototype for the family of eukaryotic proprotein convertases that includes furin, PC1/3, and PC2. These enzymes belong to the subtilase superfamily of serine proteases and are distinguished from degradative subtilisins by structural features and by their much more stringent substrate specificity. Pre-steady-state studies have shown that both Kex2 and furin exhibit an initial burst of 7-amino-4-methylcoumarin release in cleavage of peptidyl methylcoumarinamide substrates that are based on physiological cleavage sites. Thus, in cleavage of such substrates, formation of the acylenzyme intermediate is fast relative to some later step (deacylation or N-terminal product release). This behavior is significant, because Kex2 also exhibits burst kinetics in cleavage of peptide bonds. k(cat) for cleavage of a tetrapeptidyl methylcoumarinamide substrate based on the physiological yeast substrate pro-alpha-factor exhibits a weak solvent isotope effect, but neither this isotope effect nor temperature dependence studies with this substrate conclusively identify the rate-limiting step for Kex2 cleavage of this substrate. We therefore developed an assay to measure deacylation directly by pulse-chase incorporation of H(2)(18)O in a rapid-quenched-flow mixer followed by mass spectrometric quantitation. The results given by this assay rule out rate-limiting product release for cleavage of this substrate by Kex2. These experiments demonstrate that cleavage of the acylenzyme ester bond, as opposed to either the initial attack on the amide bond or product release, is rate-limiting for the action of Kex2 at physiological sequences. This work demonstrates a fundamental difference in the catalytic strategy of proprotein processing enzymes and degradative subtilisins.

Epoxide hydrolases: biochemistry and molecular biology

Epoxides are organic three-membered oxygen compounds that arise from oxidative metabolism of endogenous, as well as xenobiotic compounds via chemical and enzymatic oxidation processes, including the cytochrome P450 monooxygenase system. The resultant epoxides are typically unstable in aqueous environments and chemically reactive. In the case of xenobiotics and certain endogenous substances, epoxide intermediates have been implicated as ultimate mutagenic and carcinogenic initiators Adams et al. (Chem. Biol. Interact. 95 (1995) 57-77) Guengrich (Properties and Metabolic roles 4 (1982) 5-30) Sayer et al. (J. Biol. Chem. 260 (1985) 1630-1640). Therefore, it is of vital importance for the biological organism to regulate levels of these reactive species. The epoxide hydrolases (E.C. 3.3.2. 3) belong to a sub-category of a broad group of hydrolytic enzymes that include esterases, proteases, dehalogenases, and lipases Beetham et al. (DNA Cell Biol. 14 (1995) 61-71). In particular, the epoxide hydrolases are a class of proteins that catalyze the hydration of chemically reactive epoxides to their corresponding dihydrodiol products. Simple epoxides are hydrated to their corresponding vicinal dihydrodiols, and arene oxides to trans-dihydrodiols. In general, this hydration leads to more stable and less reactive intermediates, however exceptions do exist. In mammalian species, there are at least five epoxide hydrolase forms, microsomal cholesterol 5,6-oxide hydrolase, hepoxilin A(3) hydrolase, leukotriene A(4) hydrolase, soluble, and microsomal epoxide hydrolase. Each of these enzymes is distinct chemically and immunologically. Table 1 illustrates some general properties for each of these classes of hydrolases. Fig. 1 provides an overview of selected model substrates for each class of epoxide hydrolase.

Inhibition of insect juvenile hormone epoxide hydrolase: asymmetric synthesis and assay of glycidol-ester and epoxy-ester inhibitors of trichoplusia ni epoxide hydrolase

Juvenile hormone (JH) undergoes metabolic degradation by two major pathways involving JH esterase and JH epoxide hydrolase (EH). While considerable effort has been focussed on the study of JH esterase and the development of inhibitors for this enzyme, much less has been reported on the study of JH-EH. In this work, the asymmetric synthesis of two classes of inhibitors of recombinant JH-EH from Trichoplusia ni, a glycidol-ester series and an epoxy-ester series is reported. The most effective glycidol-ester inhibitor, compound 1, exhibited an I(50) of 1.2x10(-8) M, and the most effective epoxy-ester inhibitor, compound 11, exhibited an I(50) of 9.4x10(-8) M. The potency of the inhibitors was found to be dependent on the absolute configuration of the epoxide. In both series of inhibitors, the C-10 R-configuration was found to be significantly more potent that the corresponding C-10 S-configuration. A mechanism for epoxide hydration catalyzed by insect EH is also presented.

Biochemical evidence for the involvement of tyrosine in epoxide activation during the catalytic cycle of epoxide hydrolase

Epoxide hydrolases (EH) catalyze the hydrolysis of epoxides and arene oxides to their corresponding diols. The crystal structure of murine soluble EH suggests that Tyr(465) and Tyr(381) act as acid catalysts, activating the epoxide ring and facilitating the formation of a covalent intermediate between the epoxide and the enzyme. To explore the role of these two residues, mutant enzymes were produced and the mechanism of action was analyzed. Enzyme assays on a series of substrates confirm that both Tyr(465) and Tyr(381) are required for full catalytic activity. The kinetics of chalcone oxide hydrolysis show that mutation of Tyr(465) and Tyr(381) decreases the rate of binding and the formation of an intermediate, suggesting that both tyrosines polarize the epoxide moiety to facilitate ring opening. These two tyrosines are, however, not implicated in the hydrolysis of the covalent intermediate. Sequence comparisons showed that Tyr(465) is conserved in microsomal EHs. The substitution of analogous Tyr(374) with phenylalanine in the human microsomal EH dramatically decreases the rate of hydrolysis of cis-stilbene oxide. These results suggest that these tyrosines perform a significant mechanistic role in the substrate activation by EHs.