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08-SEPTEMBER-2008 09:13:22 - Leukotriene LTA4 Note the four double bonds, three of them conjugated. This is a common property of A4, B4, C4, D4, and E4. LTA4 Note the four double bonds, three of them conjugated. This is a common property of A4, B4, C4, D4, and E4. LTB4 LTB4 LTC4 is a cysteinyl leukotriene, as are D4 and E4. LTC4 is a cysteinyl leukotriene, as are D4 and E4. LTD4 LTD4 LTE4 LTE4 Leukotrienes are naturally produced eicosanoid lipid mediators, which may be responsible for the effects an inflammatory response. Leukotrienes use both autocrine signalling and paracrine signalling to regulate the body's response. Leukotrienes are produced in the body from arachidonic acid by the enzyme 5-lipoxygenase. Their production by the body is part of a complex response that usually includes the production of histamine. Contents 1 Types 2 History and name 3 Biochemistry 3.1 Synthesis 3.2 Function 4 Leukotrienes in asthma 4.1 Role of cysteinyl leukotrienes 4.2 Leukotriene modifiers 5 See also 6 Notes 7 References 8 External links Types Examples of leukotrienes are LTA4, LTB4, LTC4, LTD4, LTE4, and LTF4. LTC4, LTD4 and LTE4 are often called cysteinyl leukotrienes due to the presence of the amino acid in their structure. Collectively, the cysteinyl leukotrienes make up the slow reacting substance of anaphylaxis SRS-A. There has also been postulated the existence of LTG4, a metabolite of LTE4 in which the cysteinyl moiety has been oxidized to an alpha-keto-acid i.e., the cysteine has been replaced by a pyruvate. Very little is known about this putative leukotriene. History and name The name leukotriene, introduced by Swedish biochemist Bengt Samuelsson in 1979, comes from the words leukocyte and triene indicating the compound's three conjugated double bonds. What would be later named leukotriene C, slow reaction smooth muscle-stimulating substance SRS was originally described between 1938 and 1940 by Feldberg and Kellaway.1 2 3 The researchers isolated SRS from lung tissue after a prolonged period following exposure to snake venom and histamine. Leukotrienes are commercially available to the research community. Biochemistry Synthesis Eicosanoid synthesis. Leukotrienes at right. Eicosanoid synthesis. Leukotrienes at right. Leukotrienes are synthesized in the cell from arachidonic acid by 5-lipoxygenase. The catalytic mechanism involves the insertion of an oxygen moiety at a specific position in the arachidonic acid backbone. The lipoxygenase pathway is active in leukocytes, including mast cells, eosinophils, neutrophils, monocytes and basophils. When such cells are activated, arachidonic acid is liberated from cell membrane phospholipids by phospholipase A2, and donated by the 5-lipoxygenase activating protein FLAP to 5-lipoxygenase. 5-lipoxygenase 5-LO uses FLAP to convert arachidonic acid into 5-hydroperoxyeicosatetraenoic acid 5-HPETE, which spontaneously reduces to 5-hydroxyeicosatetraenoic acid 5-HETE. The enzyme 5-LO acts again on 5-HETE to convert it into leukotriene A4 LTA4, an unstable epoxide. In cells equipped with LTA4 hydrolase, such as neutrophils and monocytes, LTA4 is converted to the dihydroxy acid leukotriene LTB4, which is a powerful chemoattractant for neutrophils acting at BLT1 and BLT2 receptors on the plasma membrane of these cells. In cells that express LTC4 synthase, such as mast cells and eosinophils, LTA4 is conjugated with the tripeptide glutathione to form the first of the cysteinyl-leukotrienes, LTC4. Outside the cell, LTC4 can be converted by ubiquitous enzymes to form successively LTD4 and LTE4, which retain biological activity. The cysteinyl-leukotrienes act at their cell-surface receptors CysLT1 and CysLT2 on target cells to contract bronchial and vascular smooth muscle, to increase permeability of small blood vessels, to enhance secretion of mucus in the airway and gut, and to recruit leukocytes to sites of inflammation. Both LTB4 and the cysteinyl-leukotrienes LTC4, LTD4, LTE4 are partly degraded in local tissues, and ultimately become inactive metabolites in the liver. Function Leukotrienes act principally on a subfamily of G protein coupled receptors. They may also act upon peroxisome proliferator-activated receptors. Leukotrienes are involved in asthmatic and allergic reactions and act to sustain inflammatory reactions; several leukotriene receptor antagonists e.g. montelukast and zafirlukast are used to treat asthma. Recent research points to a role of 5-lipoxygenase in cardiovascular and neuropsychiatric illnesses.4 Leukotrienes are very important agents in the inflammatory response. Some such as LTB4 have a chemotactic effect on migrating neutrophils, and as such help to bring the necessary cells to the tissue. Leukotrienes also have a powerful effect in bronchoconstriction, they also increase vascular permeability. Leukotrienes in asthma Leukotrienes assist in the pathophysiology of asthma, causing or potentiating the following symptoms: airflow obstruction increased secretion of mucus mucosal accumulation bronchoconstriction infiltration of inflammatory cells in the airway wall Role of cysteinyl leukotrienes Cysteinyl leukotriene receptors CysLT1 and CysLT2 are present on mast cells, eosinophil and endothelial cells. During cysteinyl leukotriene interaction, they can stimulate proinflammatory activities such as endothelial cell adherence and chemokine production by mast cells. As well as mediating inflammation, they induce asthma and other inflammatory disorders, thereby reducing the airflow to the alveoli. In excess, the cysteinyl leukotrienes can induce anaphylactic shock.5 Zileuton blocks 5-lipoxygenase inhibiting the synthetic pathway of leukotriene metabolism. Zileuton affects the LTB4 pathway, montelukast doesn't. Leukotriene modifiers See leukotriene antagonist See also A chemical synthesis of Leukotriene A methyl ester Notes ^ Feldberg W, Kellaway CH. Liberation of histamine and formation of lyscithin-like substances by cobra venom. J Physiol 1938;94:187-226. ^ Feldberg W, Holden HF, Kellaway CH. The formation of lyscithin and of a muscle-stimulating substance by snake venoms. J Physiol 1938;94:232-248. ^ Kellaway CH, Trethewie ER. 1940. The liberation of a slow reacting smooth-muscle stimulating substance in anaphylaxis pdf. Q J Exp Physiol 30: 121-145. ^ Manev R, Manev H 2004. 5-Lipoxygenase as a putative link between cardiovascular and psychiatric disorders. Crit Rev Neurobiol 16 1-2: 181-6. doi:10.1615/CritRevNeurobiol.v16.i12.190. PMID 15581413. ^ Brocklehurst, W 1960, The release of histamine and formation of a slow-reacting substance SRS-A during anaphylactic shock, J Physiol 151: 416-35, http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrievedb=PubMedlist_uids=13804592dopt=Citation References Lipkowitz, Myron A. and Navarra, Tova 2001 The Encyclopedia of Allergies 2nd ed. Facts on File, New York, p. 167, ISBN 0-8160-4404-X Samuelsson, Bengt ed. 2001 Advances in prostaglandin and leukotriene research: basic science and new clinical applications: 11th International Conference on Advances in Prostaglandin and Leukotriene Research: Basic Science and New Clinical Applications, Florence, Italy, June 4-8, 2000 Kluwer Academic Publishers, Dordrecht, ISBN 1-4020-0146-0 Bailey, J. Martyn 1985 Prostaglandins, leukotrienes, and lipoxins: biochemistry, mechanism of action, and clinical applications Plenum Press, New York, ISBN 0-306-41980-7 External links MeSH Leukotrienes v d e Autacoids, unsaturated fatty acids: Eicosanoids Prostanoids Prostaglandins - Thromboxanes A2, B2 Leukotrienes A4 - B4 - Slow reacting substance of anaphylaxis C4, D4, E4 Other Arachidonic acids - Eicosapentaenoic acid - Lipoxins Retrieved from http://en..org/wiki/Leukotriene Categories: Eicosanoids Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages Deutsch Español Français Italiano 日本語 Nederlands Polski Português This page was last modified on 20 July 2008, at 20:43
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