Epoxyeicosatrienoic acid

Chemical structure of 14,15-epoxyeicosatrienoic acid.

The epoxyeicosatrienoic acids or EETs are signaling molecules formed by certain Cytochrome P450 epoxygenases on the 20-carbon essential fatty acid,arachidonic acid (see epoxygenase). These nonclassic eicosanoids act as short-range hormones, (i.e. they are autocrine and paracrine mediators) of the cardiovascular system and kidney. They produce vasorelaxation as well as anti-inflammatory and pro-fibrinolytic effects. EETs are metabolized by the soluble epoxide hydrolase to the corresponding vicinal diol, or dihydroxyeicosatrienoic acids (DHETs), which are biologically less active.

Structure

EETS are epoxide eicosatrienoic acid metabolites of the straight chain Eicosatetraenoic acid, arachidonic acid. Arachidonic acid has 4 cis (see Cis–trans isomerism; the cis configureation is termed Z in the IUPAC Chemical nomenclature used here) Double bonds located between carbons 5-6, 8-9, 11-12, and 14-15. It is therefore 5Z,8Z,11Z,14Z-eicosatetraenoic acid. Cytochrome P450 epoxygenases attack these double bounds to form their respective eicosatrienoic acid epoxide regioisomers (see Structural isomer, section on position isomerism (regioisomerism)) viz., 5,6-EET (i.e. 5,6-epoxy-8Z,11Z,14Z-eicosatetrienoic acid), 8,9-EET (i.e. 5,6-epoxy-8Z,11Z,14Z-eicosatetrienoic acid), 11,12-EET (i.e. 11,12-epoxy-5Z,8Z,14Z-eicosatetrienoic acid), or, as drawn in the attached figure, 14,15-EET (i.e. 14,15-epoxy-5Z,8Z,11Z-eicosatetrainoic acid) . The enzymes generally form both R/S enantiomers at each former double bound position; for example, cytochrome P450 epoxidases metabolize arachidonic acid to a mixture of 14R,15S-ETE and 14S,15R-ETE.

Production

The cytochrome P450 (CYP) superfamily of enzymes is distributed broadly throughout bacteria, archaea, fungi, plants, animals, and even viruses (see Cytochrome P450). The superfamily comprises more than 11,000 genes categorized into 1,000 families. Humans have 57 putatively active CYP genes and 58 CYP pseudogenes of which only a relatively few are EET-forming epoxygenases with the capacity to attach atomic oxygen (see Allotropes of oxygen#Atomic oxygen) to the carbon-carbon double bonds of unsaturated long chain fatty acids such as arachidonic acid. The CYP epoxygenases fall into several subfamilies including CYP1A, CYP2B, CYP2C, CYP2E, and CYP2J; in humans, CYP2C8, CYP2C9, CYP2C19, 2CYP2J2, and CYP231 isoforms are the main producers of EETs. The CYP epoxygenases can epoxidize any of the double bounds in arachidonic acid but most of them are relatively selective in that they make appreciable amounts of only one or two EETs with 11,12-EET and 14,15-EET accounting for 67%-80% of the product made by the cited CYP epoxidases as well as the main EETs made by mammalian tissues. CYP2C9, CYP2JP , and possibly the more recently characterized CYP2S1 appear to be the main produces of the EETs in humans with CYPP2C9 being the main EET producer in vascular endothelial cells and CYP2JP being highly expressed (although less catalytically active than CYP2C) in heart muscle, kidneys, pancreas, lung, and brain. CYP2S1 is expressed in macrophages, liver, lung, intestine, and spleen and is abundant in human and mouse atherosclerosis (i.e. Atheroma) plaques as well as inflamed tonsils.

ETEs are commonly produced by the stimulation of specific cell types. The stimulation causes arachidonic acid to be released form the sn-2 position of cellular phospholipids through the action of Phospholipase A2-type enzymes and subsequent attack of the released arachidonic acid by a CYP epoxidase. In a typical example of this mechanism, bradykinin or acetylcholine acting through their respective Bradykinin receptor B2 and muscarinic acetylcholine receptor M1 or muscarinic acetylcholine receptor M3 stimulate vascular endothelial cells to make and release EETs.

EET metabolism

In cells, the EETs are rapidly metabolized by a cytosolic soluble epoxide hydrolase (sEH) to form their corresponding Vicinal (chemistry) diol dihydroxyeicosatrienoic acids (diHETrEs), i.e. sEH converts 14,15-ETE to 14,15-diHETrE, 11,12-ETE to 11,12-diHETrE, 8,9-ETE to 8,9-diHETrE, and 5,6-ETE to 5,6-diHETrE. The product diHETrEs, like their epoxy precursors, are enantiomer mixtures; for instance, sEH converts 14,15-ETE to a mixture of 14(S),15(R)-diHETrE and 14(R),15(S)-diHETrE. Since the diHETrE products are generally far less active than their epoxide precursors, the sEH pathway of EET metabolism is regarded as a critical EET-inactivating pathway.

In addition to the sEH pathway, EETs may be acylated into phospholipids in an Acylation-like reaction. This pathway may serve to limit the action of EETs or store them for future release.

Biological effects

Generally, EETs cause:

• Calcium release from intracellular stores
• Increased sodium-hydrogen antiporter activity
• Increased cell proliferation
• Decreased cyclooxygenase activity

Other effects are specific to certain cells or locations:

• EETs are cardioprotective after ischemic heart attack and reperfusion.
• They act in the corpus cavernosum to maintain penile erection.
• Specific epoxidation of EET sites produces endogenous PPARα agonists.
• Decreased release of somatostatin, insulin and glucagon from endocrine cells.
• Vasodilation and angiogenesis in blood vessels.
• Increased risk of tumor adhesion on endothelial cells
• Decreased platelet aggregation