Epoxide hydrolase 2

Soluble epoxide hydrolase (sEH) is a bifunctional is_associated_with::enzyme that in humans is encoded by the EPHX2 is_associated_with::gene. sEH is a member of the is_associated_with::epoxide hydrolase family. This enzyme, found in both the is_associated_with::cytosol and is_associated_with::peroxisomes, binds to specific is_associated_with::epoxides and converts them to the corresponding is_associated_with::diols. A different region of this protein also has is_associated_with::lipid-phosphate phosphatase activity. Mutations in the EPHX2 gene have been associated with is_associated_with::familial hypercholesterolemia.

Tissue distribution
While most highly expressed in the liver, sEH is also expressed in other tissues including vascular is_associated_with::endothelium, leukocytes, red blood cells, smooth muscle cells, is_associated_with::adipocytes and the kidney proximal tubule.

Catalyzed reactions
The form of sEH in the intracellular environment is a is_associated_with::homodimer with two distinct activities in two separate structural domains of each monomer: the is_associated_with::C-terminal epoxide hydrolase activity (is_associated_with::soluble epoxide hydrolase: EC 3.3.2.10) and the is_associated_with::N-terminal phosphatase activity (is_associated_with::lipid-phosphate phosphatase: EC 3.1.3.76). sEH converts epoxides, or three membered cyclic ethers, to their corresponding diols through the addition of a molecule of water. The resulting diols are more water soluble than the parent epoxides, and so are more readily excreted by the organism.

The C-term-EH catalyzes the addition of water to an epoxide to yield a vicinal diol (reaction 1). The Nterm-phos hydrolyzes phosphate monoesters, such as lipid phosphates, to yield alcohols and phosphoric acid (reaction 2). The C-term-EH hydrolyzes one important class of lipid signaling molecules that includes many is_associated_with::epoxyeicosatrienoic acids (EETs) that have vasoactive, anti-inflammatory and analgesic properties.

Discovery
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. 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. The enzyme activity was detected in rabbits, mice and rats, and humans, and it is now believed to be ubiquitous in vertebrates. 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.

Function
sEH has a restricted substrate selectivity, and has not been shown to hydrolyze any toxic or mutagenic is_associated_with::xenobiotics. Conversely, the sEH plays a major role in the in vivo metabolism of endogenous lipid epoxides, such as the EETs and is_associated_with::squalene oxide, a key intermediate in the synthesis of cholesterol. EETs are lipid signaling molecules that function in an is_associated_with::autocrine and is_associated_with::paracrine manner. They are produced when arachidonic acid is metabolized by cytochrome p450s (CYPs). These enzymes epoxidize the double bonds in is_associated_with::arachidonic acid to form four regioisomers. Arachidonic acid is also the precursor of the prostaglandins and the leukotrienes, which are produced by cyclooxygenases and lipoxygenases, respectively. These lipids play a role in asthma, pain, and inflammation and are the targets of several pharmaceuticals. 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. Several disease models have been used, including Ang-II induced is_associated_with::hypertension and surgical models of brain and heart ischemia. In vitro models such as isolated coronary rings and is_associated_with::platelet aggregation assays have also been employed.

The proposed role of sEH in the regulation of hypertension can be used as a simple model of sEH function in the kidney. Here the EETs are is_associated_with::vasodilatory, and can be thought of as balancing other is_associated_with::vasoconstrictive signals. sEH hydrolyzes the EETs to form the dihydroxyeicosatrienoic acids (DHETs). 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. 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. 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.

This simplified model is complicated by a number of factors in vivo. The EETs display different properties in different vascular beds. 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. There are epoxides of other lipids besides arachidonic acid such as the omega three docosahexaenoic acid (DHA) and is_associated_with::eicosapentaenoic acid (EPA) epoxides. These lipid epoxides have been shown to have biological effects in vitro in which they inhibit platelet aggregation. In fact, in some assays they are more potent than the EETs. Other epoxidized lipids include the 18-carbon leukotoxin and isoleukotoxin. The diepoxide of linoleic acid can form tetrahydrofuran diols,

The phosphatase activity of sEH has been shown to hydrolyze in vitro lipid phosphates such as is_associated_with::terpene pyrophosphates or is_associated_with::lysophosphatidic acids. However, its biological role is still unknown.

Clinical significance
Through metabolism of EETs and other lipid mediators, sEH plays a role in several diseases, including is_associated_with::hypertension, is_associated_with::cardiac hypertrophy, is_associated_with::arteriosclerosis, brain and heart is_associated_with::ischemia/is_associated_with::reperfusion injury, cancer and pain. 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.

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. One such inhibitor, UC1153 (AR9281), was taken to a phase IIA clinal trial for treatment of hypertension by Arête Therapeutics. However, UC1153 failed the clinical trial, due in large part because of its poor pharmacokinetic properties. Since this trial, a different sEH inhibitor, GSK2256294, developed for is_associated_with::chronic obstructive pulmonary disease by is_associated_with::GlaxoSmithKline  has entered the pre-recruiting phase of a phase I clinical trial for obese male smokers. Thus, interest continues in sEH as a therapeutic target.

One indication of the possible therapeutic value of sEH inhibition comes from studies examining physiologically relevant is_associated_with::single nucleotide polymorphisms (SNPs) of sEH in human populations. 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. In these studies, two nonsynonymous SNPs were identified, R287Q and K55R. R287Q changes the is_associated_with::arginine in position 287 in the most frequent is_associated_with::allele to is_associated_with::glutamine, while K55R changes the is_associated_with::lysine in position 55 to an arginine. R287Q was associated with coronary artery calcification in African American population participating in the CARDIA study. 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 is_associated_with::homozygotes.