Thromboxane A2

Last updated
Thromboxane A2
Thromboxane A2 acsv.svg
ThromboxaneA2 spacefill.png
3D model (JSmol)
MeSH Thromboxane+A2
PubChem CID
  • InChI=1S/C20H32O5/c1-2-3-6-9-15(21)12-13-17-16(18-14-20(24-17)25-18)10-7-4-5-8-11-19(22)23/h4,7,12-13,15-18,20-21H,2-3,5-6,8-11,14H2,1H3,(H,22,23)/b7-4-,13-12+/t15-,16+,17+,18-,20+/m0/s1 X mark.svgN
  • CCCCC[C@@H](/C=C/[C@@H]1[C@H]([C@@H]2C[C@@H](O2)O1)C/C=C\CCCC(=O)O)O
Molar mass 352.471 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)
Infobox references

Thromboxane A2 (TXA2) is a type of thromboxane that is produced by activated platelets during hemostasis and has prothrombotic properties: it stimulates activation of new platelets as well as increases platelet aggregation. This is achieved by activating the thromboxane receptor, which results in platelet-shape change, inside-out activation of integrins, and degranulation. [1] Circulating fibrinogen binds these receptors on adjacent platelets, further strengthening the clot. Thromboxane A2 is also a known vasoconstrictor [2] [3] [4] [5] and is especially important during tissue injury and inflammation. It is also regarded as responsible for Prinzmetal's angina.

Receptors that mediate TXA2 actions are thromboxane A2 receptors. The human TXA2 receptor (TP) is a typical G protein-coupled receptor (GPCR) with seven transmembrane segments. In humans, two TP receptor splice variants – TPα and TPβ – have so far been cloned.

Synthesis and breakdown

TXA2 is generated from prostaglandin H2 by thromboxane-A synthase in a metabolic reaction which generates approximately equal amounts of 12-Hydroxyheptadecatrienoic acid (12-HHT). Aspirin irreversibly inhibits platelet cyclooxygenase 1 preventing the formation of prostaglandin H2, and therefore thromboxane A2.

TXA2 is very unstable in aqueous solution, since it is hydrated within about 30 seconds to the biologically inactive thromboxane B2. 12-HHT, while once thought to be an inactive byproduct of TXA2 synthesis, has recently been shown to have a range of potentially important actions, some of which relate to the actions of TXA2 (see 12-Hydroxyheptadecatrienoic acid). [6] Due to its very short half life, TXA2 primarily functions as an autocrine or paracrine mediator in the nearby tissues surrounding its site of production. Most work in the field of TXA2 is done instead with synthetic analogs such as U46619 and I-BOP. [7] In human studies, 11-dehydrothromboxane B2 levels are used to indirectly measure TXA2 production. [8] [9]

Eicosanoid synthesis. Eicosanoid synthesis.svg
Eicosanoid synthesis.

Related Research Articles

Prostaglandin Group of physiologically active lipid compounds

The prostaglandins (PG) are a group of physiologically active lipid compounds called eicosanoids having diverse hormone-like effects in animals. Prostaglandins have been found in almost every tissue in humans and other animals. They are derived enzymatically from the fatty acid arachidonic acid. Every prostaglandin contains 20 carbon atoms, including a 5-carbon ring. They are a subclass of eicosanoids and of the prostanoid class of fatty acid derivatives.

An antiplatelet drug (antiaggregant), also known as a platelet agglutination inhibitor or platelet aggregation inhibitor, is a member of a class of pharmaceuticals that decrease platelet aggregation and inhibit thrombus formation. They are effective in the arterial circulation where anticoagulants have little effect.


Prostacyclin (also called prostaglandin I2 or PGI2) is a prostaglandin member of the eicosanoid family of lipid molecules. It inhibits platelet activation and is also an effective vasodilator.


Thromboxane is a member of the family of lipids known as eicosanoids. The two major thromboxanes are thromboxane A2 and thromboxane B2. The distinguishing feature of thromboxanes is a 6-membered ether-containing ring.

Thromboxane-A synthase

Thromboxane A synthase 1 , also known as TBXAS1, is a cytochrome P450 enzyme that, in humans, is encoded by the TBXAS1 gene.

Thromboxane receptor

The thromboxane receptor (TP) also known as the prostanoid TP receptor is a protein that in humans is encoded by the TBXA2R gene, The thromboxane receptor is one among the five classes of prostanoid receptors and was the first eicosanoid receptor cloned. The TP receptor derives its name from its preferred endogenous ligand thromboxane A2.

Most of the eicosanoid receptors are integral membrane protein G protein-coupled receptors (GPCRs) that bind and respond to eicosanoid signaling molecules. Eicosanoids are rapidly metabolized to inactive products and therefore are short-lived. Accordingly, the eicosanoid-receptor interaction is typically limited to a local interaction: cells, upon stimulation, metabolize arachidonic acid to an eicosanoid which then binds cognate receptors on either its parent cell or on nearby cells to trigger functional responses within a restricted tissue area, e.g. an inflammatory response to an invading pathogen. In some cases, however, the synthesized eicosanoid travels through the blood to trigger systemic or coordinated tissue responses, e.g. prostaglandin (PG) E2 released locally travels to the hypothalamus to trigger a febrile reaction. An example of a non-GPCR receptor that binds many eicosanoids is the PPAR-γ nuclear receptor.

Prostaglandin receptors or prostanoid receptors represent a sub-class of cell surface membrane receptors that are regarded as the primary receptors for one or more of the classical, naturally occurring prostanoids viz., prostaglandin D2,, PGE2, PGF2alpha, prostacyclin (PGI2), thromboxane A2 (TXA2), and PGH2. They are named based on the prostanoid to which they preferentially bind and respond, e.g. the receptor responsive to PGI2 at lower concentrations than any other prostanoid is named the Prostacyclin receptor (IP). One exception to this rule is the receptor for thromboxane A2 (TP) which binds and responds to PGH2 and TXA2 equally well.

Prostaglandin H<sub>2</sub> Chemical compound

Prostaglandin H2 is a type of prostaglandin and a precursor for many other biologically significant molecules. It is synthesized from arachidonic acid in a reaction catalyzed by a cyclooxygenase enzyme. The conversion from Arachidonic acid to Prostaglandin H2 is a two step process. First, COX-1 catalyzes the addition of two free oxygens to form the 1,2-Dioxane bridge and a peroxide functional group to form Prostaglandin G2. Second, COX-2 reduces the peroxide functional group to a Secondary alcohol, forming Prostaglandin H2. Other peroxidases like Hydroquinone have been observed to reduce PGG2 to PGH2. PGH2 is unstable at room temperature, with a half life of 90-100 seconds, so it is often converted into a different prostaglandin.


Cyclooxygenase 1 (COX-1), also known as prostaglandin G/H synthase 1, prostaglandin-endoperoxide synthase 1 or prostaglandin H2 synthase 1, is an enzyme that in humans is encoded by the PTGS1 gene. In humans it is one of two cyclooxygenases.

GPR31 protein in humans

G-protein coupled receptor 31 also known as 12-(S)-HETE receptor is a protein that in humans is encoded by the GPR31 gene. The human gene is located on chromosome 6q27 and encodes a G-protein coupled receptor protein composed of 319 amino acids.


Protease-activated receptor 4 (PAR-4), also known as coagulation factor II (thrombin) receptor-like 3, is a protein that in humans is encoded by the F2RL3 gene.

Leukotriene B<sub>4</sub> receptor 2

Leukotriene B4 receptor 2, also known as BLT2, BLT2 receptor, and BLTR2, is an Integral membrane protein that is encoded by the LTB4R2 gene in humans and the Ltbr2 gene in mice.


Guanine nucleotide-binding protein subunit alpha-11 is a protein that in humans is encoded by the GNA11 gene. Together with GNAQ, it functions as a Gq alpha subunit.

Mechanism of action of aspirin

Aspirin causes several different effects in the body, mainly the reduction of inflammation, analgesia, the prevention of clotting, and the reduction of fever. Much of this is believed to be due to decreased production of prostaglandins and TXA2. Aspirin's ability to suppress the production of prostaglandins and thromboxanes is due to its irreversible inactivation of the cyclooxygenase (COX) enzyme. Cyclooxygenase is required for prostaglandin and thromboxane synthesis. Aspirin acts as an acetylating agent where an acetyl group is covalently attached to a serine residue in the active site of the COX enzyme. This makes aspirin different from other NSAIDs, which are reversible inhibitors. However, other effects of aspirin, such as uncoupling oxidative phosphorylation in mitochondria, and the modulation of signaling through NF-κB, are also being investigated. Some of its effects are like those of salicylic acid, which is not an acetylating agent.

11-Dehydrothromboxane B2 Chemical compound

11-Dehydrothromboxane B2 is produced from the breakdown of thromboxane A2. It is released by activated platelets and urine levels of 11-dehydro-TXB2 can be used to monitor the response to aspirin therapy when used to prevent heart disease and in diseases where platelet activation is prominent.

U46619 Chemical compound

U46619 is a stable synthetic analog of the endoperoxide prostaglandin PGH2 first prepared in 1975, and acts as a thromboxane A2 (TP) receptor agonist. It potently stimulates TP receptor-mediated, but not other prostaglandin receptor-mediated responses in various in vitro preparations. and exhibits many properties similar to thromboxane A2, including shape change and aggregation of platelets and smooth muscle contraction. U46619 is a vasoconstrictor that mimics the hydroosmotic effect of vasopressin.

12-Hydroxyeicosatetraenoic acid Chemical compound

12-Hydroxyeicosatetraenoic acid (12-HETE) is a derivative of the 20 carbon polyunsaturated fatty acid, arachidonic acid, containing a hydroxyl residue at carbon 12 and a 5Z,8Z,10E,14Z Cis–trans isomerism configuration in its four double bonds. It was first found as a product of arachidonic acid metabolism made by human and bovine platelets through their 12S-lipoxygenase enzyme(s). However, the term 12-HETE is ambiguous in that it has been used to indicate not only the initially detected "S" stereoisomer, 12S-hydroxy-5Z,8Z,10E,14Z-eicosatetraenoic acid, made by platelets, but also the later detected "R" stereoisomer, 12(R)-hydroxy-5Z,8Z,10E,14Z-eicosatetraenoic acid made by other tissues through their 12R-lipoxygenase enzyme, ALOX12B. The two isomers, either directly or after being further metabolized, have been suggested to be involved in a variety of human physiological and pathological reactions. Unlike hormones which are secreted by cells, travel in the circulation to alter the behavior of distant cells, and thereby act as Endocrine signalling agents, these arachidonic acid metabolites act locally as Autocrine signalling and/or Paracrine signaling agents to regulate the behavior of their cells of origin or of nearby cells, respectively. In these roles, they may amplify or dampen, expand or contract cellular and tissue responses to disturbances.

Thromboregulation is the series of mechanisms in how a primary clot is regulated. These mechanisms include, competitive inhibition or negative feedback. It includes primary hemostasis, which is the process of how blood platelets adhere to the endothelium of an injured blood vessel. Platelet aggregation is fundamental to repair vascular damage and the initiation of the blood thrombus formation. The elimination of clots is also part of thromboregulation. Failure in platelet clot regulation may cause hemorrhage or thrombosis. Substances called thromboregulators control every part of these events.

12-Hydroxyheptadecatrienoic acid Chemical compound

12-Hydroxyheptadecatrenoic acid is a 17 carbon metabolite of the 20 carbon polyunsaturated fatty acid, arachidonic acid. It was first detected and structurally defined by P. Wlodawer, Bengt I. Samuelsson, and M. Hamberg as a product of arachidonic acid metabolism made by microsomes isolated from sheep seminal vesicle glands and by intact human platelets. 12-HHT is less ambiguously termed 12-(S)-hydroxy-5Z,8E,10E-heptadecatrienoic acid to indicate the S stereoisomerism of its 12-hydroxyl residue and the Z, E, and E cis-trans isomerism of its three double bonds. The metabolite was for many years thought to be merely a biologically inactive byproduct of prostaglandin synthesis. More recent studies, however, have attached potentially important activity to it.


  1. Offermanns, Stefan (2006-12-08). "Activation of Platelet Function Through G Protein–Coupled Receptors". Circulation Research. 99 (12): 1293–1304. doi: 10.1161/01.res.0000251742.71301.16 . ISSN   0009-7330. PMID   17158345.
  2. Ding, Xueqin; Murray, Paul A. (November 2005). "Cellular mechanisms of thromboxane A2-mediated contraction in pulmonary veins". American Journal of Physiology. Lung Cellular and Molecular Physiology. 289 (5): L825–833. doi:10.1152/ajplung.00177.2005. ISSN   1040-0605. PMID   15964897.
  3. Yamamoto, K.; Ebina, S.; Nakanishi, H.; Nakahata, N. (November 1995). "Thromboxane A2 receptor-mediated signal transduction in rabbit aortic smooth muscle cells". General Pharmacology. 26 (7): 1489–1498. doi:10.1016/0306-3623(95)00025-9. ISSN   0306-3623. PMID   8690235.
  4. Smyth, Emer M (2010-04-01). "Thromboxane and the thromboxane receptor in cardiovascular disease". Clinical Lipidology. 5 (2): 209–219. doi:10.2217/CLP.10.11. ISSN   1758-4299. PMC   2882156 . PMID   20543887.
  5. Winn, R; Harlan, J; Nadir, B; Harker, L; Hildebrandt, J (September 1983). "Thromboxane A2 mediates lung vasoconstriction but not permeability after endotoxin". Journal of Clinical Investigation. 72 (3): 911–918. doi:10.1172/jci111062. ISSN   0021-9738. PMC   1129256 . PMID   6886010.
  6. Yokomizo, T. (2014). "Two distinct leukotriene B4 receptors, BLT1 and BLT2". Journal of Biochemistry. 157 (2): 65–71. doi: 10.1093/jb/mvu078 . PMID   25480980.
  7. Michael P. Walsh; et al. "Thromboxane A2-induced contraction of rat caudal arterial smooth muscle involves activation of Ca2+ entry and Ca2+sensitization: Rho-associated kinase-mediated phosphorylation of MYPT1 at Thr-855 but not Thr-697" (PDF). Archived from the original (PDF) on 2011-07-13.
  8. Catella F, Healy D, Lawson JA, FitzGerald GA (1986). "11-Dehydrothromboxane B2: a quantitative index of thromboxane A2 formation in the human circulation". PNAS. 83 (16): 5861–5865. Bibcode:1986PNAS...83.5861C. doi: 10.1073/pnas.83.16.5861 . PMC   386396 . PMID   3461463.
  9. Lordkipanidzé M, Pharand C, Schampaert E, Turgeon J, Palisaitis DA, Diodati JG (2007). "A comparison of six major platelet function tests to determine the prevalence of aspirin resistance in patients with stable coronary artery disease". Eur Heart J. 28 (14): 1702–1708. doi: 10.1093/eurheartj/ehm226 . PMID   17569678.