The soluble epoxide hydrolase encoded by EPXH2 is a bifunctional enzyme with novel lipid phosphate phosphatase activity

Cell-specific subcellular localization of soluble epoxide hydrolase in human tissues

Soluble epoxide hydrolase (sEH) is a phase-I xenobiotic metabolizing enzyme having both an N-terminal phosphatase activity and a C-terminal epoxide hydrolase activity. Endogenous hydrolase substrates include arachidonic acid epoxides, which have been involved in regulating blood pressure and inflammation. The subcellular localization of sEH has been controversial. Earlier studies using mouse and rat liver suggested that sEH may be cytosolic and/or peroxisomal. In this study we applied immunofluorescence and confocal microscopy using markers for different subcellular compartments to evaluate sEH colocalization in an array of human tissues. Results showed that sEH is both cytosolic and peroxisomal in human hepatocytes and renal proximal tubules and exclusively cytosolic in other sEH-containing tissues such as pancreatic islet cells, intestinal epithelium, anterior pituitary cells, adrenal gland, endometrium, lymphoid follicles, prostate ductal epithelium, alveolar wall, and blood vessels. sEH was not exclusively peroxisomal in any of the tissues evaluated. Our data suggest that human sEH subcellular localization is tissue dependent, and that sEH may have tissue- or cell-type-specific functionality. To our knowledge, this is the first report showing the subcellular localization of sEH in a wide array of human tissues.

The soluble epoxide hydrolase gene harbors sequence variation associated with susceptibility to and protection from incident ischemic stroke

Stroke is the leading cause of severe disability and the third leading cause of death, accounting for one of every 15 deaths in the USA. We investigated the association of polymorphisms in the soluble epoxide hydrolase gene (EPHX2) with incident ischemic stroke in African-Americans and Whites. Twelve single nucleotide polymorphisms (SNPs) spanning EPHX2 were genotyped in a case-cohort sample of 1336 participants from the Atherosclerosis Risk in Communities (ARIC) study. In each racial group, Cox proportional hazard models were constructed to assess the relationship between incident ischemic stroke and EPHX2 polymorphisms. A score test method was used to investigate the association of common haplotypes of the gene with risk of ischemic stroke. In African-Americans, two common EPHX2 haplotypes with significant and opposing relationships to ischemic stroke risk were identified. In Whites, two common haplotypes showed suggestive indication of an association with ischemic stroke risk but, as in African-Americans, these relationships were in opposite direction. These findings suggest that multiple variants exist within or near the EPHX2 gene, with greatly contrasting relationships to ischemic stroke incidence; some associated with a higher incidence and others with a lower incidence.

Mechanisms for incorporation of glycerol-derived precursors into polyketide metabolites

Several polyketide secondary metabolites are shown by feeding experiments to incorporate glycerol-derived 3-carbon starter units, 2-carbon extender units, or 3-carbon branches into their hydrocarbon chains. In recent years, genetic studies have begun to elucidate the mechanisms by which this occurs. In this article we review the incorporation of glycerol-derived precursors into polyketides and propose new mechanisms for the incorporation processes.

Computational Study of Phosphatase Activity in Soluble Epoxide Hydrolase: High Efficiency through a Water Bridge Mediated Proton Shuttle

Recently, a new branch of fatty acid metabolism has been opened by the novel phosphatase activity found in the N-terminal domain of the, hence bifunctional, soluble epoxide hydrolase (sEH). Importantly, this finding has also provided a new site for drug targeting in sEH's activity regulation. Classical MD and hybrid Car-Parrinello QM/MM calculations have been performed to investigate the reaction mechanism of the phosphoenzyme intermediate formation in the first step of the catalysis. The results support a concerted multi-event reaction mechanism: (1) a dissociative in-line nucleophilic substitution for the phosphoryl transfer reaction; (2) a double proton transfer involved in the formation of a good leaving group in the transition state. The presence of a water bridge in the substrate/enzyme complex allowed an efficient proton shuttle, showing its key role in speeding up the catalysis. The calculated free energy of the favored catalytic pathway is approximately 19 kcal/mol, in excellent agreement with experimental data.

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.

Determination of the metal ion dependence and substrate specificity of a hydratase involved in the degradation pathway of biphenyl/chlorobiphenyl

BphH is a divalent metal ion-dependent hydratase that catalyzes the formation of 2-keto-4-hydroxypentanoate from 2-hydroxypent-2,4-dienoate (HPDA). This reaction lies on the catabolic pathway of numerous aromatics, including the significant environmental pollutant, polychlorinated biphenyls (PCBs). BphH from the PCB degrading bacterium, Burkholderia xenoverans LB400, was overexpressed and purified to homogeneity. Atomic absorption spectroscopy and Scatchard analysis reveal that only one divalent metal ion is bound to each enzyme subunit. The enzyme exhibits the highest activity when Mg2+ was used as cofactor. Other divalent cations activate the enzyme in the following order of effectiveness: Mg2+ > Mn2+ > Co2+ > Zn2+ > Ca2+. This differs from the metal activation profile of the homologous hydratase, MhpD. UV-visible spectroscopy of the Co2+-BphH complex indicates that the divalent metal ion is hexa-coordinated in the enzyme. The nature of the metal ion affected only the kcat and not the Km values in the BphH hydration of HPDA, suggesting that cation has a catalytic rather than just a substrate binding role. BphH is able to transform alternative substrates substituted with methyl- and chlorine groups at the 5-position of HPDA. The specificity constants (kcat/Km) for 5-methyl and 5-chloro substrates are, however, lowered by eight- and 67-fold compared with the unsubstituted substrate. Significantly, kcat for the chloro-substituted substrate is eightfold lower compared with the methyl-substituted substrate, showing that electron withdrawing substituent at the 5-position of the substrate has a negative influence on enzyme catalysis.

Epoxide hydrolases: their roles and interactions with lipid metabolism

The epoxide hydrolases (EHs) are enzymes present in all living organisms, which transform epoxide containing lipids by the addition of water. In plants and animals, many of these lipid substrates have potent biologically activities, such as host defenses, control of development, regulation of inflammation and blood pressure. Thus the EHs have important and diverse biological roles with profound effects on the physiological state of the host organisms. Currently, seven distinct epoxide hydrolase sub-types are recognized in higher organisms. These include the plant soluble EHs, the mammalian soluble epoxide hydrolase, the hepoxilin hydrolase, leukotriene A4 hydrolase, the microsomal epoxide hydrolase, and the insect juvenile hormone epoxide hydrolase. While our understanding of these enzymes has progressed at different rates, here we discuss the current state of knowledge for each of these enzymes, along with a distillation of our current understanding of their endogenous roles. By reviewing the entire enzyme class together, both commonalities and discrepancies in our understanding are highlighted and important directions for future research pertaining to these enzymes are indicated.

Epoxide Hydrolases: Structure, Function, Mechanism, and Assay

Epoxide hydrolases are a class of enzymes important in the detoxification of genotoxic compounds, as well as in the control of physiological signaling molecules. This chapter gives an overview on the function, structure, and enzymatic mechanism of structurally characterized epoxide hydrolases and describes selected assays for the quantification of epoxide hydrolase activity.

Sequence and structure of epoxide hydrolases: a systematic analysis

Epoxide hydrolases (EC 3.3.2.3) are ubiquitous enzymes that catalyze the hydrolysis of epoxides to the corresponding vicinal diols. More than 100 epoxide hydrolases (EH) have been identified or predicted, and 3 structures are available. Although they catalyze the same chemical reaction, sequence similarity is low. To identify conserved regions, all EHs were aligned. Phylogenetic analysis identified 12 homologous families, which were grouped into 2 major superfamilies: the microsomal EH superfamily, which includes the homologous families of Mammalian, Insect, Fungal, and Bacterial EHs, and the cytosolic EH superfamily, which includes Mammalian, Plant, and Bacterial EHs. Bacterial EHs show a high sequence diversity. Based on structure comparison of three known structures from Agrobacterium radiobacter AD1 (cytosolic EH), Aspergillus niger (microsomal EH), Mus musculus (cytosolic EH), and multisequence alignment and phylogenetic analysis of 95 EHs, the modular architecture of this enzyme family was analyzed. Although core and cap domain are highly conserved, the structural differences between the EHs are restricted to only two loops: the NC-loop connecting the core and the cap and the cap-loop, which is inserted into the cap domain. EHs were assigned to either of three clusters based on loop length. By using this classification, core and cap region of all EHs, NC-loops and cap-loops of 78% and 89% of all EHs, respectively, could be modeled. Representative models are available from the Lipase Engineering Database, http://www.led.uni-stuttgart.de.

Polymorphisms in human soluble epoxide hydrolase: effects on enzyme activity, enzyme stability, and quaternary structure

Human soluble epoxide hydrolase (hsEH) has been shown to play a role in regulating blood pressure and inflammation. HsEH consists of an N-terminal phosphatase and a C-terminal epoxide hydrolase domain. In the present study, we examined the effects of polymorphisms in the hsEH gene on phosphatase activity, enzyme stability, and protein quaternary structure. The results showed that mutants Lys55Arg, Arg103Cys, Cys154Tyr, Arg287Gln, and the Arg103Cys/Arg287Gln (double mutant) have significantly lower phosphatase activity compared to the most frequent allele (MFA) of hsEH. In addition, the Lys55Arg, Arg103Cys, Cys154Tyr, Arg287Gln, and the double mutant have significantly lower kcat/Km values. The stabilities at 37 degrees C of purified Arg287Gln and Arg103Cys/Arg287Gln mutants were also significantly reduced compared to the MFA. HPLC size-exclusion studies showed that the MFA exists predominantly as a dimer. However, the Arg287Gln and Arg103Cys/Arg287Gln mutants show increased concentration of the monomer. We conclude that the Arg287Gln polymorphism disrupts putative intra- and inter-monomeric salt-bridges responsible for dimerization.

Polymorphism of the Soluble Epoxide Hydrolase Is Associated With Coronary Artery Calcification in African-American Subjects

Background— Modulation of endogenous epoxide levels by soluble epoxide hydrolase (sEH) in the endothelium represents an important mechanism in the regulation of cardiovascular function. We examined the relationship between a common, functional polymorphism of the human sEH gene and coronary artery calcification (CAC) in young, largely asymptomatic African-American and non-Hispanic white subjects.

Methods and Results— Multiple logistic regression and Tobit regression models were used to assess the relationship between the sEH Arg287Gln polymorphism and presence and quantity of CAC. Models adjusting for race (except in race-specific analyses), age, sex, smoking, body mass index, systolic blood pressure, LDL cholesterol, and HDL cholesterol were estimated. Allele and genotype frequency distributions were not significantly different between the 2 ethnic groups ( P =0.22; P =0.17, respectively). The Arg287Gln polymorphism of the sEH gene was a significant predictor of CAC status in African-American participants, either alone or after adjusting for other risk factors. African-American subjects with at least 1 copy of the Gln287 allele had a 2-fold greater risk of having CAC compared with those not carrying this allele (95% CI, 1.1 to 2.9; P =0.02). There was no relationship between Arg287Gln polymorphism and the probability of having CAC in white participants (OR, 0.8; 95% CI, 0.5 to 1.3; P =0.49). Inferences from multivariable Tobit regression were similar to those obtained in the logistic regression models, indicating that the Arg287Gln polymorphism was a significant independent predictor of both presence and quantity of CAC in African-American but not white subjects.

Conclusions— These data suggest an intriguing and possibly novel role for sEH in the pathogenesis of atherosclerosis, which deserves additional investigation.

The Telltale Structures of Epoxide Hydrolases

Traditionally, epoxide hydrolases (EH) have been regarded as xenobiotic-metabolizing enzymes implicated in the detoxification of foreign compounds. They are known to play a key role in the control of potentially genotoxic epoxides that arise during metabolism of many lipophilic compounds. Although this is apparently the main function for the mammalian microsomal epoxide hydrolase (mEH), evidence is now accumulating that the mammalian soluble epoxide hydrolase (sEH), despite its proven role in xenobiotic metabolism, also has a central role in the formation and breakdown of physiological signaling molecules. In addition, a certain class of microbial epoxide hydrolases has recently been identified that is an integral part of a catabolic pathway, allowing the use of specific terpens as sole carbon sources. The recently available x-ray structures of a number of EHs mirror their respective functions: the microbial terpen EH differs in its fold from the canonical alpha/beta hydrolase fold of the xenobiotic-metabolizing mammalian EHs. It appears that the latter fold is the perfect solution for the efficient detoxification of a large variety of structurally different epoxides by a single enzyme, whereas the smaller microbial EH, which has a particularly high turnover number with its prefered substrate, seems to be the better solution for the hydrolysis of one specific substrate. The structure of the sEH also includes an additional catalytic domain that has recently been shown to possess phosphatase activity. Although the physiological substrate for this second active site has not been identified so far, the majority of known phosphatases are involved in signaling processes, suggesting that the sEH phosphatase domain also has a role in the regulation of physiological functions.