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Leukotrienes are a family of
eicosanoid Eicosanoids are signaling molecules made by the enzymatic or non-enzymatic oxidation of arachidonic acid or other polyunsaturated fatty acids (PUFAs) that are, similar to arachidonic acid, around 20 carbon units in length. Eicosanoids are a s ...
inflammatory mediators Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, and is a protective response involving immune cells, blood vessels, and molecu ...
produced in leukocytes by the
oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
of arachidonic acid (AA) and the
essential fatty acid Essential fatty acids, or EFAs, are fatty acids that humans and other animals must ingest because the body requires them for good health but cannot synthesize them. Only two fatty acids are known to be essential for humans: alpha-linolenic ac ...
eicosapentaenoic acid Eicosapentaenoic acid (EPA; also icosapentaenoic acid) is an omega-3 fatty acid. In physiological literature, it is given the name 20:5(n-3). It also has the trivial name timnodonic acid. In chemical structure, EPA is a carboxylic acid with a 20-c ...
(EPA) by the
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
arachidonate 5-lipoxygenase Arachidonate 5-lipoxygenase, also known as ALOX5, 5-lipoxygenase, 5-LOX, or 5-LO, is a non-heme iron-containing enzyme (EC 1.13.11.34) that in humans is encoded by the ''ALOX5'' gene. Arachidonate 5-lipoxygenase is a member of the lipoxygenase fa ...
. Leukotrienes use lipid signaling to convey information to either the cell producing them (
autocrine signaling Autocrine signaling is a form of cell signaling in which a cell secretes a hormone or chemical messenger (called the autocrine agent) that binds to autocrine receptors on that same cell, leading to changes in the cell. This can be contrasted with pa ...
) or neighboring cells (
paracrine signaling Paracrine signaling is a form of cell signaling, a type of cellular communication in which a cell produces a signal to induce changes in nearby cells, altering the behaviour of those cells. Signaling molecules known as paracrine factors diffuse over ...
) in order to regulate immune responses. The production of leukotrienes is usually accompanied by the production of histamine and prostaglandins, which also act as inflammatory mediators. One of their roles (specifically, leukotriene D4) is to trigger contractions in the smooth muscles lining the bronchioles; their overproduction is a major cause of inflammation in
asthma Asthma is a long-term inflammatory disease of the airways of the lungs. It is characterized by variable and recurring symptoms, reversible airflow obstruction, and easily triggered bronchospasms. Symptoms include episodes of wheezing, co ...
and allergic
rhinitis Rhinitis, also known as coryza, is irritation and inflammation of the mucous membrane inside the nose. Common symptoms are a stuffy nose, runny nose, sneezing, and post-nasal drip. The inflammation is caused by viruses, bacteria, irrita ...
.
Leukotriene antagonist An antileukotriene, also known as leukotriene modifier and leukotriene receptor antagonist, is a medication which functions as a leukotriene-related enzyme inhibitor (arachidonate 5-lipoxygenase) or leukotriene receptor antagonist (cysteinyl leukot ...
s are used to treat these disorders by inhibiting the production or activity of leukotrienes.


History and name

The name ''leukotriene'', introduced by Swedish biochemist
Bengt Samuelsson Bengt Ingemar Samuelsson (born 21 May 1934) is a Swedish biochemist. He shared with Sune K. Bergström and John R. Vane the 1982 Nobel Prize for Physiology or Medicine for discoveries concerning prostaglandins and related substances. Education a ...
in 1979, comes from the words ''leukocyte'' and ''
triene In organic chemistry, polyenes are poly- unsaturated, organic compounds that contain at least three alternating double () and single () carbon–carbon bonds. These carbon–carbon double bonds interact in a process known as conjugation, result ...
'' (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. The researchers isolated SRS from lung tissue after a prolonged period following exposure to snake
venom Venom or zootoxin is a type of toxin produced by an animal that is actively delivered through a wound by means of a bite, sting, or similar action. The toxin is delivered through a specially evolved ''venom apparatus'', such as fangs or a st ...
and histamine. Leukotrienes are commercially available to the research community.


Types


Cysteinyl leukotrienes

LTC4, LTD4, LTE4 and LTF4 are often called cysteinyl leukotrienes due to the presence of the amino acid cysteine in their structure. The cysteinyl leukotrienes make up the
slow-reacting substance of anaphylaxis The slow-reacting substance of anaphylaxis or SRS-A is a mixture of the leukotrienes LTC4, LTD4 and LTE4. Mast cells secrete it during the anaphylactic reaction, inducing inflammation. It can be found in basophils. It induces prolonged, slow ...
(SRS-A). LTF4, like LTD4, is a metabolite of LTC4, but, unlike LTD4, which lacks the glutamic residue of
glutathione Glutathione (GSH, ) is an antioxidant in plants, animals, fungi, and some bacteria and archaea. Glutathione is capable of preventing damage to important cellular components caused by sources such as reactive oxygen species, free radicals, pe ...
, LTF4 lacks the
glycine Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid ( carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinog ...
residue of glutathione.


LTB4

LTB4 is synthesized ''in vivo'' from LTA4 by the enzyme LTA4 hydrolase. Its primary function is to recruit neutrophils to areas of tissue damage, though it also helps promote the production of inflammatory cytokines by various immune cells. Drugs that block the actions of LTB4 have shown some efficacy in slowing the progression of neutrophil-mediated diseases.


LTG4

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.


LTB5

Leukotrienes originating from the omega-3 class eicosapentanoic acid (EPA) have diminished inflammatory effects. LTB5 induces aggregation of rat neutrophils, chemokinesis of human polymorphonuclear neutrophils (PMN), lysosomal enzyme release from human PMN and potentiation of bradykinin-induced plasma exudation, although compared to LTB4, it has at least 30 times less potency.


Biochemistry


Synthesis

Leukotrienes are synthesized in the cell from arachidonic acid by
arachidonate 5-lipoxygenase Arachidonate 5-lipoxygenase, also known as ALOX5, 5-lipoxygenase, 5-LOX, or 5-LO, is a non-heme iron-containing enzyme (EC 1.13.11.34) that in humans is encoded by the ''ALOX5'' gene. Arachidonate 5-lipoxygenase is a member of the lipoxygenase fa ...
. 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 and other immunocompetent cells, including
mast cell A mast cell (also known as a mastocyte or a labrocyte) is a resident cell of connective tissue that contains many granules rich in histamine and heparin. Specifically, it is a type of granulocyte derived from the myeloid stem cell that is a par ...
s,
eosinophil Eosinophils, sometimes called eosinophiles or, less commonly, acidophils, are a variety of white blood cells (WBCs) and one of the immune system components responsible for combating multicellular parasites and certain infections in vertebrates. A ...
s,
neutrophil Neutrophils (also known as neutrocytes or heterophils) are the most abundant type of granulocytes and make up 40% to 70% of all white blood cells in humans. They form an essential part of the innate immune system, with their functions varying ...
s,
monocyte Monocytes are a type of leukocyte or white blood cell. They are the largest type of leukocyte in blood and can differentiate into macrophages and conventional dendritic cells. As a part of the vertebrate innate immune system monocytes also ...
s, and
basophil Basophils are a type of white blood cell. Basophils are the least common type of granulocyte, representing about 0.5% to 1% of circulating white blood cells. However, they are the largest type of granulocyte. They are responsible for inflammator ...
s. When such cells are activated, arachidonic acid is liberated from cell membrane phospholipids by phospholipase A2, and donated by the
5-lipoxygenase-activating protein Arachidonate 5-lipoxygenase-activating protein also known as 5-lipoxygenase activating protein, or FLAP, is a protein that in humans is encoded by the ''ALOX5AP'' gene. Function FLAP is necessary for the activation of 5-lipoxygenase and therefo ...
(FLAP) to 5-lipoxygenase. 5-
Lipoxygenase Lipoxygenases () are a family of (non-heme) iron-containing enzymes most of which catalyze the dioxygenation of polyunsaturated fatty acids in lipids containing a cis,cis-1,4- pentadiene into cell signaling agents that serve diverse roles as aut ...
(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. 5-HETE can be further metabolized to 5-oxo-ETE and 5-oxo-15-hydroxy-ETE, all of which have pro-inflammatory actions similar but not identical to those of LTB4 and mediated not by LTB4 receptors but rather by the OXE receptor (see 5-Hydroxyicosatetraenoic acid and 5-oxo-eicosatetraenoic acid). In cells equipped with LTA 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 Glutathione (GSH, ) is an antioxidant in plants, animals, fungi, and some bacteria and archaea. Glutathione is capable of preventing damage to important cellular components caused by sources such as reactive oxygen species, free radicals, pe ...
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 Cysteinyl leukotriene receptor 1, also termed CYSLTR1, is a receptor for cysteinyl leukotrienes (LT) (see leukotrienes#Cysteinyl leukotrienes). CYSLTR1, by binding these cysteinyl LTs (CysLTs; viz, LTC4, LTD4, and to a much lesser extent, LTE4) co ...
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 receptor G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large group of evolutionarily-related p ...
s. They may also act upon
peroxisome proliferator-activated receptor In the field of molecular biology, the peroxisome proliferator-activated receptors (PPARs) are a group of nuclear receptor proteins that function as transcription factors regulating the expression of genes. PPARs play essential roles in the regu ...
s. Leukotrienes are involved in asthmatic and allergic reactions and act to sustain inflammatory reactions. Several leukotriene receptor antagonists such as montelukast and
zafirlukast Zafirlukast is an orally administered leukotriene receptor antagonist (LTRA) used for the chronic treatment of asthma. While zafirlukast is generally well tolerated, headache and stomach upset often occur. Some rare side effects can occur, which ...
are used to treat
asthma Asthma is a long-term inflammatory disease of the airways of the lungs. It is characterized by variable and recurring symptoms, reversible airflow obstruction, and easily triggered bronchospasms. Symptoms include episodes of wheezing, co ...
. Recent research points to a role of 5-lipoxygenase in cardiovascular and neuropsychiatric illnesses. Leukotrienes are very important agents in the inflammatory response. Some such as LTB4 have a
chemotactic Chemotaxis (from '' chemo-'' + '' taxis'') is the movement of an organism or entity in response to a chemical stimulus. Somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemica ...
effect on migrating neutrophils, and as such help to bring the necessary cells to the tissue. Leukotrienes also have a powerful effect in
bronchoconstriction Bronchoconstriction is the constriction of the airways in the lungs due to the tightening of surrounding smooth muscle, with consequent coughing, wheezing, and shortness of breath. Causes The condition has a number of causes, the most common be ...
and increase vascular permeability.


Leukotrienes in asthma

Leukotrienes contribute to the pathophysiology of
asthma Asthma is a long-term inflammatory disease of the airways of the lungs. It is characterized by variable and recurring symptoms, reversible airflow obstruction, and easily triggered bronchospasms. Symptoms include episodes of wheezing, co ...
, especially in patients with aspirin-exacerbated respiratory disease ( AERD), and cause or potentiate 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
CYSLTR1 Cysteinyl leukotriene receptor 1, also termed CYSLTR1, is a receptor for cysteinyl leukotrienes (LT) (see leukotrienes#Cysteinyl leukotrienes). CYSLTR1, by binding these cysteinyl LTs (CysLTs; viz, LTC4, LTD4, and to a much lesser extent, LTE4 ...
and
CYSLTR2 Cysteinyl leukotriene receptor 2, also termed CYSLTR2, is a receptor for cysteinyl leukotrienes (LT) (see leukotrienes#Cysteinyl leukotrienes). CYSLTR2, by binding these cysteinyl LTs (CysLTs; viz, LTC4, LTD4, and to a much lesser extent, LT ...
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. The levels of cysteinyl leukotrienes, along with 8-isoprostane, have been reported to be increased in the EBC of patients with
asthma Asthma is a long-term inflammatory disease of the airways of the lungs. It is characterized by variable and recurring symptoms, reversible airflow obstruction, and easily triggered bronchospasms. Symptoms include episodes of wheezing, co ...
, correlating with disease severity. Cysteinyl leukotrienes may also play a role in adverse drug reactions in general and in contrast medium induced adverse reactions in particular. In excess, the cysteinyl leukotrienes can induce anaphylactic shock.


Leukotrienes in dementia

Leukotrienes are found to play an important role in the later stages of Alzheimer's disease and related
dementias Dementia is a disorder which manifests as a set of related symptoms, which usually surfaces when the brain is damaged by injury or disease. The symptoms involve progressive impairments in memory, thinking, and behavior, which negatively affec ...
in studies with animals. In tau transgenic mice, which develop tau pathology, " zileuton, a drug that inhibits leukotriene formation by blocking the 5-lipoxygenase enzyme" was found to reverse
memory loss Amnesia is a deficit in memory caused by brain damage or disease,Gazzaniga, M., Ivry, R., & Mangun, G. (2009) Cognitive Neuroscience: The biology of the mind. New York: W.W. Norton & Company. but it can also be caused temporarily by the use o ...
.


See also

* A chemical synthesis of Leukotriene A methyl ester *
Eoxin Eoxins are proposed to be a family of proinflammatory eicosanoids (signaling compounds that regulate inflammatory and immune responses). They are produced by human eosinophils (a class of white blood cells), mast cells, the L1236 Reed–Sternberg ...
s (14,15-leukotrienes)


References


Further reading

* Bailey, J. Martyn (1985) ''Prostaglandins, leukotrienes, and lipoxins: biochemistry, mechanism of action, and clinical applications'' Plenum Press, New York, * Lipkowitz, Myron A. and Navarra, Tova (2001) ''The Encyclopedia of Allergies'' (2nd ed.) Facts on File, New York, p. 167, * 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,


External links

* {{Authority control Eicosanoids Immunology Polyenes