Soluble epoxide hydrolase (sEH) is a bifunctional enzyme that in humans is encoded by the EPHX2 gene. [5] [6] [7] sEH is a member of the epoxide hydrolase family. This enzyme, found in both the cytosol and peroxisomes, binds to specific epoxides and converts them to the corresponding diols. A different region of this protein also has lipid-phosphate phosphatase activity. Mutations in the EPHX2 gene have been associated with familial hypercholesterolemia. [5]
While most highly expressed in the liver, sEH is also expressed in other tissues including vascular endothelium, leukocytes, red blood cells, smooth muscle cells, adipocytes and the kidney proximal tubule. [6] In the human brain, the enzyme is distributed widely, mostly in neuronal cell bodies, as well as in astrocytes and oligodendrocytes. [8]
The form of sEH in the intracellular environment is a homodimer with two distinct activities in two separate structural domains of each monomer: the C-terminal epoxide hydrolase activity (soluble epoxide hydrolase: EC 3.3.2.10) and the N-terminal phosphatase activity (lipid-phosphate phosphatase: EC 3.1.3.76). [6] sEH converts epoxides, or three membered cyclic ethers, to their corresponding diols through the addition of a molecule of water. [6] The resulting diols are more water-soluble than the parent epoxides, and so are more readily excreted by the organism. [6]
The C-term-EH catalyzes the addition of water to an epoxide to yield a vicinal diol (reaction 1). [6] The Nterm-phos hydrolyzes phosphate monoesters, such as lipid phosphates, to yield alcohols and phosphoric acid (reaction 2). [6] The C-term-EH hydrolyzes one important class of lipid signaling molecules that includes many epoxyeicosatrienoic acids (EETs) that have vasoactive, anti-inflammatory and analgesic properties. [9]
sEH also appears to be the hepoxilin hydrolase that is responsible for inactivating the epoxyalcohol metabolites of arachidonic acid, hepoxilin A3 and hepoxiin B3. [10] [11]
The sEH was first identified in the cytosolic fraction of mouse liver through its activity on epoxide containing substrates such as juvenile hormone and lipid epoxides such as epoxystearate. [12] The soluble EH activity was shown to be distinct from that of the microsomal epoxide hydrolase (mEH) previously discovered with a different substrate selectivity and cellular localization than the mEH. Studies using a lipid epoxide as a substrate detected this activity in the soluble fraction of multiple organs, though at a lesser amount than in liver and kidney. [13] The enzyme activity was detected in rabbits, mice and rats, and humans, and it is now believed to be ubiquitous in vertebrates. [14] The proposed enzyme was first named cytosolic epoxide hydrolase; however, after its discovery inside the peroxisomes of some organs, it was renamed soluble epoxide hydrolase or sEH. [14]
sEH has a restricted substrate selectivity, and has not been shown to hydrolyze any toxic or mutagenic xenobiotics. [6] Conversely, the sEH plays a major role in the in vivo metabolism of endogenous lipid epoxides, such as the EETs and squalene oxide, a key intermediate in the synthesis of cholesterol. [6] EETs are lipid signaling molecules that function in an autocrine and paracrine manner. [15] They are produced when arachidonic acid is metabolized by cytochrome p450s (CYPs). [15] These enzymes epoxidize the double bonds in arachidonic acid to form four regioisomers. [6] Arachidonic acid is also the precursor of the prostaglandins and the leukotrienes, which are produced by cyclooxygenases and lipoxygenases, respectively. [9] These lipids play a role in asthma, pain, and inflammation and are the targets of several pharmaceuticals. [16] The EET receptor or receptors have not been identified, but several tools for the study of EET biology have been developed, these include small molecule sEH inhibitors, EET mimics and sEH genetic models. Through the use of these tools, as well as the EETs themselves, the EETs have been found to have anti-inflammatory and vasoactive properties. [6] Several disease models have been used, including Ang-II induced hypertension and surgical models of brain and heart ischemia. In vitro models such as isolated coronary rings and platelet aggregation assays have also been employed. [6]
The proposed role of sEH in the regulation of hypertension can be used as a simple model of sEH function in the kidney. [17] Here the EETs are vasodilatory, and can be thought of as balancing other vasoconstrictive signals. sEH hydrolyzes the EETs to form the dihydroxyeicosatrienoic acids (DHETs). [17] These molecules are more water-soluble and are more easily metabolized by other enzymes, so the vasodilatory signal is removed from the site of action through excretion, tipping the balance of vasoconstrictive and vasodilatory signals towards vasoconstriction. This change in the lipid signaling increases vascular resistance to blood flow and blood pressure. [6] By reducing sEH epoxide hydrolase activity, and thereby shutting off the major route of metabolism of the EETs, the levels of these molecules can be stabilized or increased, increasing blood flow and reducing hypertension. [17] This reduction in sEH activity can be achieved in genetic models in which sEH has been knocked out, or through the use of small molecule sEH inhibitors. [18]
This simplified model is complicated by a number of factors in vivo. The EETs display different properties in different vascular beds. [15] The DHETs are more readily excreted, but they have yet to be fully characterized, and may possess biological properties themselves, complicating the balance of signals described in the simplified model. [6] There are epoxides of other lipids besides arachidonic acid such as the omega three docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) epoxides. [19] These lipid epoxides have been shown to have biological effects in vitro in which they inhibit platelet aggregation. [20] In fact, in some assays they are more potent than the EETs. [21] Other epoxidized lipids include the 18-carbon leukotoxin and isoleukotoxin. [22] The diepoxide of linoleic acid can form tetrahydrofuran diols, [23]
sEH metabolizes the biologically active epoxyalcohol metabolites of arachidnoic acid, hepoxilin A3 (8-hydroxy-11S,12Sepoxy-(5Z,8Z,14Z)-eicosatrienoic acid) to trioxilin A3 (8,11,12-trihydroxy-(5Z,9E,14Z)-eicosatrienoic acid) and hepoxilin B3 (10-hydroxy-11S,12Sepoxy-(5Z,9E,14Z)-eicosatrienoic acid) to trioxlin B3 (10,11,12-trihydroxy-(5Z,9E,14Z)-eicosatrienoic acid. [24] These trihydroxy products are generally considered to be inactive and the sEH pathway is generally considered to limit the actions of the hepoxilins. [11] [24]
The phosphatase activity of sEH has been shown to hydrolyze in vitro lipid phosphates such as terpene pyrophosphates or lysophosphatidic acids. [6] Studies suggest a potential role of sEH in regulating cholesterol biosynthesis and metabolism in the brain. If the N-terminal domain of sEH is regulating cholesterol metabolism, it emplies that higher levels of its phosphatase activity could potentially increase brain cholesterol concentrations. [25] However, its biological role is still unknown.
Through metabolism of EETs and other lipid mediators, sEH plays a role in several diseases, including hypertension, cardiac hypertrophy, arteriosclerosis, brain and heart ischemia/reperfusion injury, cancer and pain. [15] Because of its possible role in cardiovascular and other diseases, sEH is being pursued as a pharmacological target, and potent small molecule inhibitors are available. [18]
Because of the implications to human health, sEH has been pursued as a pharmaceutical target and several sEH inhibitors have been developed in the private and public sectors. [18] One such inhibitor, UC1153 (AR9281), was taken to a phase IIA clinal trial for treatment of hypertension by Arête Therapeutics. [26] However, UC1153 failed the clinical trial, due in large part because of its poor pharmacokinetic properties. [18] Since this trial, a different sEH inhibitor, GSK2256294, developed for chronic obstructive pulmonary disease by GlaxoSmithKline has entered the pre-recruiting phase of a phase I clinical trial for obese male smokers. [27] EicOsis designs and applies sEH inhibitors towards treating chronic pain in humans, companion animals and horses. The inhibitor EC1728 has been shown to successfully treat equine laminitis and alleviate inflammatory pain in dogs and cats and is currently undergoing clinical trials in horses. The sEH inhibitor EC5026 has been selected as the therapeutic for diabetic neuropathy and recently entered Phase 1 clinical trials. [28] Thus, interest continues in sEH as a therapeutic target. Another drug described as a small-molecule thrombolytic with multiple mechanisms of action, SMTP-7, has been found to act as a sEH inhibitor, but is still at early experimental stages. [29] [30]
One indication of the possible therapeutic value of sEH inhibition comes from studies examining physiologically relevant single nucleotide polymorphisms (SNPs) of sEH in human populations. [31] The Coronary Artery Risk Development in Young Adults (CARDIA) and the Atherosclerosis Risk in Communities (ARIC) studies both associated SNPs in the sEH coding region with coronary heart disease. [32] [33] In these studies, two nonsynonymous SNPs were identified, R287Q and K55R. R287Q changes the arginine in position 287 in the most frequent allele to glutamine, while K55R changes the lysine in position 55 to an arginine. R287Q was associated with coronary artery calcification in African American population participating in the CARDIA study. [32] [34] The K55R allele is associated with the risk of developing coronary heart disease in Caucasians participating in the ARIC study, where it was also associated with a higher risk of hypertension and ischemic stroke in male homozygotes. [33]
 | The 2013 version of this article was updated by an external expert under a dual publication model. The corresponding academic peer reviewed article was published in Gene and can be cited as: Todd R Harris, Bruce D. Hammock (10 September 2013). "Soluble epoxide hydrolase: gene structure, expression and deletion". Gene . Gene Wiki Review Series. 526 (2): 61–74. doi:10.1016/J.GENE.2013.05.008. ISSN 0378-1119. PMC 3733540 . PMID 23701967. Wikidata Q28291292. |
Cytochrome P450 2E1 is a member of the cytochrome P450 mixed-function oxidase system, which is involved in the metabolism of xenobiotics in the body. This class of enzymes is divided up into a number of subcategories, including CYP1, CYP2, and CYP3, which as a group are largely responsible for the breakdown of foreign compounds in mammals.
Epoxide hydrolases (EHs), also known as epoxide hydratases, are enzymes that metabolize compounds that contain an epoxide residue; they convert this residue to two hydroxyl residues through an epoxide hydrolysis reaction to form diol products. Several enzymes possess EH activity. Microsomal epoxide hydrolase, soluble epoxide hydrolase, and the more recently discovered but not as yet well defined functionally, epoxide hydrolase 3 (EH3) and epoxide hydrolase 4 (EH4) are structurally closely related isozymes. Other enzymes with epoxide hydrolase activity include leukotriene A4 hydrolase, Cholesterol-5,6-oxide hydrolase, MEST (gene) (Peg1/MEST), and Hepoxilin-epoxide hydrolase. The hydrolases are distinguished from each other by their substrate preferences and, directly related to this, their functions.
The epoxyeicosatrienoic acids or EETs are signaling molecules formed within various types of cells by the metabolism of arachidonic acid by a specific subset of cytochrome P450 enzymes termed cytochrome P450 epoxygenases. These nonclassic eicosanoids are generally short-lived, being rapidly converted from epoxides to less active or inactive dihydroxy-eicosatrienoic acids (diHETrEs) by a widely distributed cellular enzyme, soluble epoxide hydrolase (sEH), also termed epoxide hydrolase 2. The EETs consequently function as transiently acting, short-range hormones; that is, they work locally to regulate the function of the cells that produce them or of nearby cells. The EETs have been most studied in animal models where they show the ability to lower blood pressure possibly by a) stimulating arterial vasorelaxation and b) inhibiting the kidney's retention of salts and water to decrease intravascular blood volume. In these models, EETs prevent arterial occlusive diseases such as heart attacks and brain strokes not only by their anti-hypertension action but possibly also by their anti-inflammatory effects on blood vessels, their inhibition of platelet activation and thereby blood clotting, and/or their promotion of pro-fibrinolytic removal of blood clots. With respect to their effects on the heart, the EETs are often termed cardio-protective. Beyond these cardiovascular actions that may prevent various cardiovascular diseases, studies have implicated the EETs in the pathological growth of certain types of cancer and in the physiological and possibly pathological perception of neuropathic pain. While studies to date imply that the EETs, EET-forming epoxygenases, and EET-inactivating sEH can be manipulated to control a wide range of human diseases, clinical studies have yet to prove this. Determination of the role of the EETS in human diseases is made particularly difficult because of the large number of EET-forming epoxygenases, large number of epoxygenase substrates other than arachidonic acid, and the large number of activities, some of which may be pathological or injurious, that the EETs possess.
Cytochrome P450, family 1, subfamily A, polypeptide 1 is a protein that in humans is encoded by the CYP1A1 gene. The protein is a member of the cytochrome P450 superfamily of enzymes.
Docosatetraenoic acid designates any straight chain 22:4 fatty acid.
Hepoxilins (Hx) are a set of epoxyalcohol metabolites of polyunsaturated fatty acids (PUFA), i.e. they possess both an epoxide and an alcohol residue. HxA3, HxB3, and their non-enzymatically formed isomers are nonclassic eicosanoid derived from acid the (PUFA), arachidonic acid. A second group of less well studied hepoxilins, HxA4, HxB4, and their non-enzymatically formed isomers are nonclassical eicosanoids derived from the PUFA, eicosapentaenoic acid. Recently, 14,15-HxA3 and 14,15-HxB3 have been defined as arachidonic acid derivatives that are produced by a different metabolic pathway than HxA3, HxB3, HxA4, or HxB4 and differ from the aforementioned hepoxilins in the positions of their hydroxyl and epoxide residues. Finally, hepoxilin-like products of two other PUFAs, docosahexaenoic acid and linoleic acid, have been described. All of these epoxyalcohol metabolites are at least somewhat unstable and are readily enzymatically or non-enzymatically to their corresponding trihydroxy counterparts, the trioxilins (TrX). HxA3 and HxB3, in particular, are being rapidly metabolized to TrXA3, TrXB3, and TrXC3. Hepoxilins have various biological activities in animal models and/or cultured mammalian tissues and cells. The TrX metabolites of HxA3 and HxB3 have less or no activity in most of the systems studied but in some systems retain the activity of their precursor hepoxilins. Based on these studies, it has been proposed that the hepoxilins and trioxilins function in human physiology and pathology by, for example, promoting inflammation responses and dilating arteries to regulate regional blood flow and blood pressure.
In enzymology, a hepoxilin-epoxide hydrolase is an enzyme that catalyzes the conversion of the epoxyalcohol metabolites arachidonic acid, hepoxilin A3 and hepoxilin B3 to their tri-hydroxyl products, trioxolin A3 and trioxilin B3, respectively. These reactions in general inactivate the two biologically active hepoxilins.
In enzymology, a microsomal epoxide hydrolase (mEH) is an enzyme that catalyzes the hydrolysis reaction between an epoxide and water to form a diol.
The enzyme lipid-phosphate phosphatase (EC 3.1.3.76) catalyzes the reaction
Cytochrome P450 2J2 (CYP2J2) is a protein that in humans is encoded by the CYP2J2 gene. CYP2J2 is a member of the cytochrome P450 superfamily of enzymes. The enzymes are oxygenases which catalyze many reactions involved in the metabolism of drugs and other xenobiotics) as well as in the synthesis of cholesterol, steroids and other lipids.
Cytochrome P450 4A11 is a protein that in humans is codified by the CYP4A11 gene.
Cytochrome P450 4F8 is a protein that in humans is encoded by the CYP4F8 gene.
Cytochrome P450 4F12 is a protein that in humans is encoded by the CYP4F12 gene.
Epoxygenases are a set of membrane-bound, heme-containing cytochrome P450 enzymes that metabolize polyunsaturated fatty acids to epoxide products that have a range of biological activities. The most thoroughly studied substrate of the CYP epoxylgenases is arachidonic acid. This polyunsaturated fatty acid is metabolized by cyclooxygenases to various prostaglandin, thromboxane, and prostacyclin metabolites in what has been termed the first pathway of eicosanoid production; it is also metabolized by various lipoxygenases to hydroxyeicosatetraenoic acids and leukotrienes in what has been termed the second pathway of eicosanoid production. The metabolism of arachidonic acid to epoxyeicosatrienoic acids by the CYP epoxygenases has been termed the third pathway of eicosanoid metabolism. Like the first two pathways of eicosanoid production, this third pathway acts as a signaling pathway wherein a set of enzymes metabolize arachidonic acid to a set of products that act as secondary signals to work in activating their parent or nearby cells and thereby orchestrate functional responses. However, none of these three pathways is limited to metabolizing arachidonic acid to eicosanoids. Rather, they also metabolize other polyunsaturated fatty acids to products that are structurally analogous to the eicosanoids but often have different bioactivity profiles. This is particularly true for the CYP epoxygenases which in general act on a broader range of polyunsaturated fatty acids to form a broader range of metabolites than the first and second pathways of eicosanoid production. Furthermore, the latter pathways form metabolites many of which act on cells by binding with and thereby activating specific and well-characterized receptor proteins; no such receptors have been fully characterized for the epoxide metabolites. Finally, there are relatively few metabolite-forming lipoxygenases and cyclooxygenases in the first and second pathways and these oxygenase enzymes share similarity between humans and other mammalian animal models. The third pathway consists of a large number of metabolite-forming CYP epoxygenases and the human epoxygenases have important differences from those of animal models. Partly because of these differences, it has been difficult to define clear roles for the epoxygenase-epoxide pathways in human physiology and pathology.
Regorafenib, sold under the brand name Stivarga among others, is an oral multi-kinase inhibitor developed by Bayer which targets angiogenic, stromal and oncogenic receptor tyrosine kinase (RTK). Regorafenib shows anti-angiogenic activity due to its dual targeted VEGFR2-TIE2 tyrosine kinase inhibition. Since 2009 it was studied as a potential treatment option in multiple tumor types. By 2015 it had two US approvals for advanced cancers.
Coronaric acid (leukotoxin or leukotoxin A) is a mono-unsaturated, epoxide derivative of the di-saturated fatty acid, linoleic acid (i.e. 9(Z),12(Z) octadecadienoic acid). It is a mixture of the two optically active isomers of 12(Z) 9,10-epoxy-octadecenoic acid. This mixture is also termed 9,10-epoxy-12Z-octadecenoic acid or 9(10)-EpOME and when formed by or studied in mammalians, leukotoxin.
Epoxide docosapentaenoic acids are metabolites of the 22-carbon straight-chain omega-3 fatty acid, docosahexaenoic acid (DHA). Cell types that express certain cytochrome P450 (CYP) epoxygenases metabolize polyunsaturated fatty acids (PUFAs) by converting one of their double bonds to an epoxide. In the best known of these metabolic pathways, cellular CYP epoxygenases metabolize the 20-carbon straight-chain omega-6 fatty acid, arachidonic acid, to epoxyeicosatrienoic acids (EETs); another CYP epoxygenase pathway metabolizes the 20-carbon omega-3 fatty acid, eicosapentaenoic acid (EPA), to epoxyeicosatetraenoic acids (EEQs). CYP epoxygenases similarly convert various other PUFAs to epoxides. These epoxide metabolites have a variety of activities. However, essentially all of them are rapidly converted to their corresponding, but in general far less active, vicinal dihydroxy fatty acids by ubiquitous cellular soluble epoxide hydrolase. Consequently, these epoxides, including EDPs, operate as short-lived signaling agents that regulate the function of their parent or nearby cells. The particular feature of EDPs distinguishing them from EETs is that they derive from omega-3 fatty acids and are suggested to be responsible for some of the beneficial effects attributed to omega-3 fatty acids and omega-3-rich foods such as fish oil.
Epoxyeicosatetraenoic acids are a set of biologically active epoxides that various cell types make by metabolizing the omega 3 fatty acid, eicosapentaenoic acid (EPA), with certain cytochrome P450 epoxygenases. These epoxygenases can metabolize EPA to as many as 10 epoxides that differ in the site and/or stereoisomer of the epoxide formed; however, the formed EEQs, while differing in potency, often have similar bioactivities and are commonly considered together.
Epoxide hydrolase 3 is a protein that in humans is encoded by the EPHX3 gene. It is the third defined isozyme in a set of epoxide hydrolase isozymes, the epoxide hydrolases. This set includes the Microsomal epoxide hydrolase ; the epoxide hydrolase 2 ; and the far less well defined enzymatically, epoxide hydrolase 4. All four enzyme contain an Alpha/beta hydrolase fold suggesting that they have Hydrolysis activity. EH1, EH2, and EH3 have been shown to have such activity in that they add water to epoxides of unsaturated fatty acids to form vicinal cis products; the activity of EH4 has not been reported. The former three EH's differ in subcellular location, tissue expression patterns, substrate preferences, and thereby functions. These functions include limiting the biologically actions of certain fatty acid epoxides, increasing the toxicity of other fatty acid epoxides, and contributing to the metabolism of drugs and other xenobiotics.
Jorge H. Capdevila is an American biochemist and professor emeritus of Medicine at Vanderbilt University Medical School, Nashville, TN. He was named a Fellow of the American Heart Association in 2002 and received the 2004 American Heart Association's “Novartis Excellence Award for Hypertension Research” for his contributions to our understanding of the molecular basis of hypertension. Capdevila's pioneering identification of roles for the Cytochrome P450 (P450) enzymes in the metabolism of arachidonic acid (AA) and of the physiological and pathophysiological importance of these enzymes and products was recognized in a special section honoring him at the 14th International Winter Eicosanoid Conference, Baltimore, MD, March 11–14, 2012. Capdevila received an “Outstanding Achievement Award” from the Eicosanoid Research Foundation at their 15th International Bioactive Lipid Conference in Puerto Vallarta, Mexico, on October 22–25, 2017, and is listed as a noteworthy medical and biochemical educator in the 2008 62nd volume of Marquis “Who’s Who in America”.
