Lipoxygenase

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Lipoxygenase
2p0m.png
Structure of rabbit reticulocyte 15S-lipoxygenase. [1]
Identifiers
SymbolLipoxygenase
Pfam PF00305
InterPro IPR013819
PROSITE PDOC00077
SCOP2 2sbl / SCOPe / SUPFAM
OPM superfamily 80
OPM protein 2p0m
Available protein structures:
Pfam   structures / ECOD  
PDB RCSB PDB; PDBe; PDBj
PDBsum structure summary

Lipoxygenases (EC 1.13.11.-) (LOX) are a family of (non-heme) iron-containing enzymes, more specifically oxidative 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 autocrine signals that regulate the function of their parent cells, paracrine signals that regulate the function of nearby cells, and endocrine signals that regulate the function of distant cells.

Contents

The lipoxygenases are related to each other based upon their similar genetic structure and dioxygenation activity. However, one lipoxygenase, ALOXE3, while having a lipoxygenase genetic structure, possesses relatively little dioxygenation activity; rather its primary activity appears to be as an isomerase that catalyzes the conversion of hydroperoxy unsaturated fatty acids to their 1,5-epoxide, hydroxyl derivatives.

Lipoxygenases are found in eukaryotes (plants, fungi, animals, protists); while the third domain of terrestrial life, the archaea, possesses proteins with a slight (~20%) amino acid sequence similarity to lipoxygenases, these proteins lack iron-binding residues and therefore are not projected to possess lipoxygenase activity. [2]

Biochemistry

Based on detailed analyses of 15-lipoxygenase 1 and stabilized 5-lipoxygenase, lipoxygenase structures consist of a 15 kilodalton N-terminal beta barrel domain, a small (e.g. ~0.6 kilodalton) linker inter-domain (see Protein domain § Domains and protein flexibility), and a relatively large C-terminal catalytic domain which contains the non-heme iron critical for the enzymes' catalytic activity. [3] Most of the lipoxygenases (exception, ALOXE3) catalyze the reaction Polyunsaturated fatty acid + O2 → fatty acid hydroperoxide in four steps:

The (—OO−) residue may then be protonated to form a hydroperoxide group (—OOH) and further metabolized by the lipoxygenase to e.g. leukotrienes, hepoxilins, and various specialized pro-resolving mediators, or reduced by ubiquitous cellular glutathione peroxidases to a hydroxy group thereby forming hydroxylated (—OH) polyunsaturated fatty acids such as the hydroxyeicosatetraenoic acids and HODEs (i.e. hydroxyoctadecaenoic acids). [3]

Polyunsaturated fatty acids that serve as substrates for one or more of the lipoxygenases include the omega 6 fatty acids, arachidonic acid, linoleic acid, dihomo-γ-linolenic acid, and adrenic acid; the omega-3 fatty acids, eicosapentaenoic acid, docosahexaenoic acid, and alpha-linolenic acid; and the omega-9 fatty acid, mead acid. [4] Certain types of the lipoxygenases, e.g. human and murine 15-lipoxygenase 1, 12-lipoxygenase B, and ALOXE3, are capable of metabolizing fatty acid substrates that are constituents of phospholipids, cholesterol esters, or complex lipids of the skin. [3] Most lipoxygenases catalyze the formation of initially formed hydroperoxy products that have S chirality. Exceptions to this rule include the 12R-lipoxygenases of humans and other mammals (see below). [3] [4] [5]

Lipoxygenases depend on the availability of their polyunsaturated fatty acid substrates which, particularly in mammalian cells, is normally maintained at extremely low levels. In general, various phospholipase A2s and diacylglycerol lipases are activated during cell stimulation, proceed to release these fatty acids from their storage sites, and thereby are key regulators in the formation of lipoxygenase-dependent metabolites. [3] In addition, cells, when so activated, may transfer their released polyunsaturated fatty acids to adjacent or nearby cells which then metabolize them through their lipoxygenase pathways in a process termed transcellular metabolism or transcellular biosynthesis. [6]

Biological function and classification

These enzymes are most common in plants where they may be involved in a number of diverse aspects of plant physiology including growth and development, pest resistance, and senescence or responses to wounding. [7] In mammals a number of lipoxygenases isozymes are involved in the metabolism of eicosanoids (such as prostaglandins, leukotrienes and nonclassic eicosanoids). [8] Sequence data is available for the following lipoxygenases:

Plant lipoxygenases

Plants express a variety of cytosolic lipoxygenases (EC 1.13.11.12; InterPro :  IPR001246 ) as well as what seems to be a chloroplast isozyme. [9] Plant lipoxygenase in conjunction with hydroperoxide lyases are responsible for many fragrances and other signalling compounds. One example is cis-3-hexenal, the odor of freshly cut grass.

An illustrative transformation involving a hydroperoxide lyase. Here cis-3-hexenal is generated from linolenic acid to the hydroperoxide by the action of a lipoxygenase followed by the lyase. O2aseLyaseFatty.png
An illustrative transformation involving a hydroperoxide lyase. Here cis-3-hexenal is generated from linolenic acid to the hydroperoxide by the action of a lipoxygenase followed by the lyase.

Human lipoxygenases

With the exception of the gene encoding 5-LOX (ALOX5), which is located on chromosome 10q11.2, all six human LOX genes are located on chromosome 17.p13 and code for a single chain protein of 75–81 kilodaltons that consists of 662–711 amino acids. Mammalian LOX genes contain 14 (ALOX5, ALOX12, ALOX15, ALOX15B) or 15 (ALOX12B, ALOXE3) exons with exon/intron boundaries at highly conserved positions. [11] [12] The 6 human lipoxygenases along with some of the major products that they make, as well as some of their associations with genetic diseases, are as follows: [11] [13] [14] [15] [16]

Two lipoxygenases may act in series to make di-hydroxy or tri-hydroxy products that have activities quite different than either lipoxyenases' products. This serial metabolism may occur in different cell types that express only one of the two lipoxygenases in a process termed transcellular metabolism. For example, ALOX5 and ALOX15 or, alternatively, ALOX5 and ALOX12 can act serially to metabolize arachidonic acid into lipoxins (see 15-Hydroxyeicosatetraenoic acid §§  Further metabolism ​ and Activities of 15(S)-HpETE, 15(S)-HETE, 15(R)-HpETE, 15(R)-HETE, and 15-oxo-ETE and Lipoxin § Synthesis) while ALOX15 and possibly ALOX15B can act with ALOX5 to metabolize eicosapentaenoic acid to resolvin D's (see Resolvin § Biochemistry and production).

Mouse lipoxygenases

The mouse is a common model to examine lipoxygenase function. However, there are some key differences between the lipoxygenases between mice and men that make extrapolations from mice studies to humans difficult. In contrast to the 6 functional lipoxygenases in humans, mice have 7 functional lipoxygenases and some of the latter have different metabolic activities than their human orthologs. [11] [19] [21] In particular, mouse Alox15, unlike human ALOX15, metabolizes arachidonic acid mainly to 12-HpETE and mouse Alox15b, in contrast to human ALOX15b, is primarily an 8-lipoxygenase, metabolizing arachdionic acid to 8-HpETE; there is no comparable 8-HpETE-forming lipoxygenase in humans. [22]

Rabbit 15-lipoxygenase (blue) with inhibitor (yellow) bound in the active site Lipoxygenase 1LOX.png
Rabbit 15-lipoxygenase (blue) with inhibitor (yellow) bound in the active site

3D structure

There are several lipoxygenase structures known including: soybean lipoxygenase L1 and L3, coral 8-lipoxygenase, human 5-lipoxygenase, rabbit 15-lipoxygenase and porcine leukocyte 12-lipoxygenase catalytic domain. The protein consists of a small N-terminal PLAT domain and a major C-terminal catalytic domain (see Pfam database), which contains the active site. In both plant and mammalian enzymes, the N-terminal domain contains an eight-stranded antiparallel β-barrel, but in the soybean lipoxygenases this domain is significantly larger than in the rabbit enzyme. The plant lipoxygenases can be enzymatically cleaved into two fragments which stay tightly associated while the enzyme remains active; separation of the two domains leads to loss of catalytic activity. The C-terminal (catalytic) domain consists of 18-22 helices and one (in rabbit enzyme) or two (in soybean enzymes) antiparallel β-sheets at the opposite end from the N-terminal β-barrel.

Active site

The iron atom in lipoxygenases is bound by four ligands, three of which are histidine residues. [23] Six histidines are conserved in all lipoxygenase sequences, five of them are found clustered in a stretch of 40 amino acids. This region contains two of the three zinc-ligands; the other histidines have been shown [24] to be important for the activity of lipoxygenases.

The two long central helices cross at the active site; both helices include internal stretches of π-helix that provide three histidine (His) ligands to the active site iron. Two cavities in the major domain of soybean lipoxygenase-1 (cavities I and II) extend from the surface to the active site. The funnel-shaped cavity I may function as a dioxygen channel; the long narrow cavity II is presumably a substrate pocket. The more compact mammalian enzyme contains only one boot-shaped cavity (cavity II). In soybean lipoxygenase-3 there is a third cavity which runs from the iron site to the interface of the β-barrel and catalytic domains. Cavity III, the iron site and cavity II form a continuous passage throughout the protein molecule.

The active site iron is coordinated by Nε of three conserved His residues and one oxygen of the C-terminal carboxyl group. In addition, in soybean enzymes the side chain oxygen of asparagine is weakly associated with the iron. In rabbit lipoxygenase, this Asn residue is replaced with His which coordinates the iron via Nδ atom. Thus, the coordination number of iron is either five or six, with a hydroxyl or water ligand to a hexacoordinate iron.

Details about the active site feature of lipoxygenase were revealed in the structure of porcine leukocyte 12-lipoxygenase catalytic domain complex [23] [25] In the 3D structure, the substrate analog inhibitor occupied a U-shaped channel open adjacent to the iron site. This channel could accommodate arachidonic acid without much computation, defining the substrate binding details for the lipoxygenase reaction. In addition, a plausible access channel, which intercepts the substrate binding channel and extended to the protein surface could be counted for the oxygen path.

Biochemical classification

EC 1.13.11.12 lipoxygenase(linoleate:oxygen 13-oxidoreductase)linoleate + O2 = (9Z,11E,13S)-13-hydroperoxyoctadeca-9,11-dienoate
EC 1.13.11.31 arachidonate 12-lipoxygenase (arachidonate:oxygen 12-oxidoreductase)arachidonate + O2 = (5Z,8Z,10E,12S,14Z)-12-hydroperoxyicosa-5,8,10,14-tetraenoate
EC 1.13.11.33 arachidonate 15-lipoxygenase (arachidonate:oxygen 15-oxidoreductase)arachidonate + O2 = (5Z,8Z,11Z,13E,15S)-15-hydroperoxyicosa-5,8,11,13-tetraenoate
EC 1.13.11.34 arachidonate 5-lipoxygenase (arachidonate:oxygen 5-oxidoreductase)arachidonate + O2 = leukotriene A4 + H2
EC 1.13.11.40 arachidonate 8-lipoxygenase (arachidonate:oxygen 8-oxidoreductase)arachidonate + O2 = (5Z,8R,9E,11Z,14Z)-8-hydroperoxyicosa-5,9,11,14-tetraenoate

Soybean Lipoxygenase 1 exhibits the largest H/D kinetic isotope effect (KIE) on kcat (kH/kD) (81 near room temperature) so far reported for a biological system. Recently, an extremely elevated KIE of 540 to 730 was found in a double mutant Soybean Lipoxygenase 1. [26] Because of the large magnitude of the KIE, Soybean Lipoxygenase 1 has served as the prototype for enzyme-catalyzed hydrogen-tunneling reactions.

Human proteins expressed from the lipoxygenase family include ALOX12, ALOX12B, ALOX15, ALOX15B, ALOX5, and ALOXE3. While humans also possess the ALOX12P2 gene, which is an ortholog of the well-expressed Alox12P gene in mice, the human gene is a pseudogene; consequently, ALOX12P2 protein is not detected in humans. [27]

Related Research Articles

<span class="mw-page-title-main">Arachidonic acid</span> Fatty acid used metabolically in many organisms

Arachidonic acid is a polyunsaturated omega-6 fatty acid 20:4(ω-6), or 20:4(5,8,11,14). It is structurally related to the saturated arachidic acid found in cupuaçu butter. Its name derives from the ancient Greek neologism arachis (peanut), but peanut oil does not contain any arachidonic acid.

<span class="mw-page-title-main">Eicosanoid</span> Class of compounds

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 sub-category of oxylipins, i.e. oxidized fatty acids of diverse carbon units in length, and are distinguished from other oxylipins by their overwhelming importance as cell signaling molecules. Eicosanoids function in diverse physiological systems and pathological processes such as: mounting or inhibiting inflammation, allergy, fever and other immune responses; regulating the abortion of pregnancy and normal childbirth; contributing to the perception of pain; regulating cell growth; controlling blood pressure; and modulating the regional flow of blood to tissues. In performing these roles, eicosanoids most often act as autocrine signaling agents to impact their cells of origin or as paracrine signaling agents to impact cells in the proximity of their cells of origin. Eicosanoids may also act as endocrine agents to control the function of distant cells.

<span class="mw-page-title-main">Lipoxin</span> Acronym for lipoxygenase interaction product

A lipoxin (LX or Lx), an acronym for lipoxygenase interaction product, is a bioactive autacoid metabolite of arachidonic acid made by various cell types. They are categorized as nonclassic eicosanoids and members of the specialized pro-resolving mediators (SPMs) family of polyunsaturated fatty acid (PUFA) metabolites. Like other SPMs, LXs form during, and then act to resolve, inflammatory responses. Initially, two lipoxins were identified, lipoxin A4 (LXA4) and LXB4, but more recent studies have identified epimers of these two LXs: the epi-lipoxins, 15-epi-LXA4 and 15-epi-LXB4 respectively.

<span class="mw-page-title-main">Resolvin</span> Class of chemical compounds

Resolvins are specialized pro-resolving mediators (SPMs) derived from omega-3 fatty acids, primarily eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), as well as from two isomers of docosapentaenoic acid (DPA), one omega-3 and one omega-6 fatty acid. As autacoids similar to hormones acting on local tissues, resolvins are under preliminary research for their involvement in promoting restoration of normal cellular function following the inflammation that occurs after tissue injury. Resolvins belong to a class of polyunsaturated fatty acid (PUFA) metabolites termed specialized proresolving mediators (SPMs).

<span class="mw-page-title-main">Hepoxilin</span> Chemical compound

Hepoxilins (Hx) are a set of epoxyalcohol metabolites of polyunsaturated fatty acids (PUFA), i.e. they possess both an epoxide and an alcohol residue. HxA3, HxB3, and their non-enzymatically formed isomers are nonclassic eicosanoid derived from acid the (PUFA), arachidonic acid. A second group of less well studied hepoxilins, HxA4, HxB4, and their non-enzymatically formed isomers are nonclassical eicosanoids derived from the PUFA, eicosapentaenoic acid. Recently, 14,15-HxA3 and 14,15-HxB3 have been defined as arachidonic acid derivatives that are produced by a different metabolic pathway than HxA3, HxB3, HxA4, or HxB4 and differ from the aforementioned hepoxilins in the positions of their hydroxyl and epoxide residues. Finally, hepoxilin-like products of two other PUFAs, docosahexaenoic acid and linoleic acid, have been described. All of these epoxyalcohol metabolites are at least somewhat unstable and are readily enzymatically or non-enzymatically to their corresponding trihydroxy counterparts, the trioxilins (TrX). HxA3 and HxB3, in particular, are being rapidly metabolized to TrXA3, TrXB3, and TrXC3. Hepoxilins have various biological activities in animal models and/or cultured mammalian tissues and cells. The TrX metabolites of HxA3 and HxB3 have less or no activity in most of the systems studied but in some systems retain the activity of their precursor hepoxilins. Based on these studies, it has been proposed that the hepoxilins and trioxilins function in human physiology and pathology by, for example, promoting inflammation responses and dilating arteries to regulate regional blood flow and blood pressure.

Arachidonate 5-lipoxygenase, also known as ALOX5, 5-lipoxygenase, 5-LOX, or 5-LO, is a non-heme iron-containing enzyme that in humans is encoded by the ALOX5 gene. Arachidonate 5-lipoxygenase is a member of the lipoxygenase family of enzymes. It transforms essential fatty acids (EFA) substrates into leukotrienes as well as a wide range of other biologically active products. ALOX5 is a current target for pharmaceutical intervention in a number of diseases.

<span class="mw-page-title-main">Mead acid</span> Chemical compound

Mead acid is an omega-9 fatty acid, first characterized by James F. Mead. As with some other omega-9 polyunsaturated fatty acids, animals can make Mead acid de novo. Its elevated presence in the blood is an indication of essential fatty acid deficiency. Mead acid is found in large quantities in cartilage.

<span class="mw-page-title-main">ALOX15</span> Lipoxygenase found in humans

ALOX15 is, like other lipoxygenases, a seminal enzyme in the metabolism of polyunsaturated fatty acids to a wide range of physiologically and pathologically important products. ▼ Gene Function

<span class="mw-page-title-main">ALOX12</span> Protein-coding gene in the species Homo sapiens

ALOX12, also known as arachidonate 12-lipoxygenase, 12-lipoxygenase, 12S-Lipoxygenase, 12-LOX, and 12S-LOX is a lipoxygenase-type enzyme that in humans is encoded by the ALOX12 gene which is located along with other lipoyxgenases on chromosome 17p13.3. ALOX12 is 75 kilodalton protein composed of 663 amino acids.

<span class="mw-page-title-main">5-Hydroxyeicosatetraenoic acid</span> Chemical compound

5-Hydroxyeicosatetraenoic acid (5-HETE, 5(S)-HETE, or 5S-HETE) is an eicosanoid, i.e. a metabolite of arachidonic acid. It is produced by diverse cell types in humans and other animal species. These cells may then metabolize the formed 5(S)-HETE to 5-oxo-eicosatetraenoic acid (5-oxo-ETE), 5(S),15(S)-dihydroxyeicosatetraenoic acid (5(S),15(S)-diHETE), or 5-oxo-15-hydroxyeicosatetraenoic acid (5-oxo-15(S)-HETE).

<span class="mw-page-title-main">ALOX15B</span> Protein-coding gene in humans

Arachidonate 15-lipoxygenase type II is an enzyme that in humans is encoded by the ALOX15B gene. ALOX15B, also known as 15-lipoxygenase-2, is distinguished from its related oxygenase, ALOX15 or 15-lipoxygenase-1.

<span class="mw-page-title-main">ALOX12B</span> Protein-coding gene in the species Homo sapiens

Arachidonate 12-lipoxygenase, 12R type, also known as ALOX12B, 12R-LOX, and arachidonate lipoxygenase 3, is a lipoxygenase-type enzyme composed of 701 amino acids and encoded by the ALOX12B gene. The gene is located on chromosome 17 at position 13.1 where it forms a cluster with two other lipoxygenases, ALOXE3 and ALOX15B. Among the human lipoxygenases, ALOX12B is most closely related in amino acid sequence to ALOXE3

<span class="mw-page-title-main">ALOXE3</span> Protein-coding gene in the species Homo sapiens

Epidermis-type lipoxygenase 3 is a member of the lipoxygenase family of enzymes; in humans, it is encoded by the ALOXE3 gene. This gene is located on chromosome 17 at position 13.1 where it forms a cluster with two other lipoxygenases, ALOX12B and ALOX15B. Among the human lipoxygenases, ALOXE3 is most closely related in amino acid sequence to ALOX12B. ALOXE3, ALOX12B, and ALOX15B are often classified as epidermal lipoxygenases, in distinction to the other three human lipoxygenases, because they were initially defined as being highly or even exclusively expressed and functioning in skin. The epidermis-type lipoxygenases are now regarded as a distinct subclass within the multigene family of mammalian lipoxygenases with mouse Aloxe3 being the ortholog to human ALOXE3, mouse Alox12b being the ortholog to human ALOX12B, and mouse Alox8 being the ortholog to human ALOX15B [supplied by OMIM]. ALOX12B and ALOXE3 in humans, Alox12b and Aloxe3 in mice, and comparable orthologs in other in other species are proposed to act sequentially in a multistep metabolic pathway that forms products that are structurally critical for creating and maintaining the skin's water barrier function.

Epoxygenases are a set of membrane-bound, heme-containing cytochrome P450 enzymes that metabolize polyunsaturated fatty acids to epoxide products that have a range of biological activities. The most thoroughly studied substrate of the CYP epoxylgenases is arachidonic acid. This polyunsaturated fatty acid is metabolized by cyclooxygenases to various prostaglandin, thromboxane, and prostacyclin metabolites in what has been termed the first pathway of eicosanoid production; it is also metabolized by various lipoxygenases to hydroxyeicosatetraenoic acids and leukotrienes in what has been termed the second pathway of eicosanoid production. The metabolism of arachidonic acid to epoxyeicosatrienoic acids by the CYP epoxygenases has been termed the third pathway of eicosanoid metabolism. Like the first two pathways of eicosanoid production, this third pathway acts as a signaling pathway wherein a set of enzymes metabolize arachidonic acid to a set of products that act as secondary signals to work in activating their parent or nearby cells and thereby orchestrate functional responses. However, none of these three pathways is limited to metabolizing arachidonic acid to eicosanoids. Rather, they also metabolize other polyunsaturated fatty acids to products that are structurally analogous to the eicosanoids but often have different bioactivity profiles. This is particularly true for the CYP epoxygenases which in general act on a broader range of polyunsaturated fatty acids to form a broader range of metabolites than the first and second pathways of eicosanoid production. Furthermore, the latter pathways form metabolites many of which act on cells by binding with and thereby activating specific and well-characterized receptor proteins; no such receptors have been fully characterized for the epoxide metabolites. Finally, there are relatively few metabolite-forming lipoxygenases and cyclooxygenases in the first and second pathways and these oxygenase enzymes share similarity between humans and other mammalian animal models. The third pathway consists of a large number of metabolite-forming CYP epoxygenases and the human epoxygenases have important differences from those of animal models. Partly because of these differences, it has been difficult to define clear roles for the epoxygenase-epoxide pathways in human physiology and pathology.

<span class="mw-page-title-main">12-Hydroxyeicosatetraenoic acid</span> 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 (Z=cis, E=trans) 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 (i.e. ALOX12) 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 (12(S)-HETE or 12S-HETE), made by platelets, but also the later detected "R" stereoisomer, 12(R)-hydroxy-5Z,8Z,10E,14Z-eicosatetraenoic acid (also termed 12(R)-HETE or 12R-HETE) 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.

<span class="mw-page-title-main">Maresin</span> Chemical compound

Maresin 1 (MaR1 or 7R,14S-dihydroxy-4Z,8E,10E,12Z,16Z,19Z-docosahexaenoic acid) is a macrophage-derived mediator of inflammation resolution coined from macrophage mediator in resolving inflammation. Maresin 1, and more recently defined maresins, are 12-lipoxygenase-derived metabolites of the omega-3 fatty acid, docosahexaenoic acid (DHA), that possess potent anti-inflammatory, pro-resolving, protective, and pro-healing properties similar to a variety of other members of the specialized proresolving mediators (SPM) class of polyunsaturated fatty acid (PUFA) metabolites. SPM are dihydroxy, trihydroxy, and epoxy-hydroxy metabolites of long chain PUFA made by certain dioxygenase enzymes viz., cyclooxygenases and lipoxygenases. In addition to the maresins, this class of mediators includes: the 15-lipoxygenase (i.e. ALOX15 and/or possibly ALOX15B)-derived lipoxin A4 and B4 metabolites of the omega 6 fatty acid, arachidonic acid; the cyclooxygenase 2-derived resolvin E series metabolites of the omega 3 fatty acid, eicosapentaenoic acid; certain 15-lipoxygenase-derived resolvin D series metabolites of DHA; certain other 15-lipoxygenase-derived protectin D1 and related metabolites of DHA; and the more recently defined and therefore less fully studied 15-lipoxygenase-derived resolvin Dn-3DPA metabolites of the omega-3 fatty acid n-3 docosapentaenoic acid (n-3 DPA or clupanodonic acid), the cyclooxygenase 2-derived resolvin T metabolites of this clupanodonic acid, and the 15-lipoxygenase-derived products of the N-acetylated fatty acid amide of the DHA metabolite, docosahexaenoyl ethanolamide.

<span class="mw-page-title-main">15-Hydroxyeicosatetraenoic acid</span> Chemical compound

15-Hydroxyeicosatetraenoic acid (also termed 15-HETE, 15(S)-HETE, and 15S-HETE) is an eicosanoid, i.e. a metabolite of arachidonic acid. Various cell types metabolize arachidonic acid to 15(S)-hydroperoxyeicosatetraenoic acid (15(S)-HpETE). This initial hydroperoxide product is extremely short-lived in cells: if not otherwise metabolized, it is rapidly reduced to 15(S)-HETE. Both of these metabolites, depending on the cell type which forms them, can be further metabolized to 15-oxo-eicosatetraenoic acid (15-oxo-ETE), 5(S),15(S)-dihydroxy-eicosatetraenoic acid (5(S),15(S)-diHETE), 5-oxo-15(S)-hydroxyeicosatetraenoic acid (5-oxo-15(S)-HETE), a subset of specialized pro-resolving mediators viz., the lipoxins, a class of pro-inflammatory mediators, the eoxins, and other products that have less well-defined activities and functions. Thus, 15(S)-HETE and 15(S)-HpETE, in addition to having intrinsic biological activities, are key precursors to numerous biologically active derivatives.

Eoxins are proposed to be a family of proinflammatory eicosanoids. They are produced by human eosinophils, mast cells, the L1236 Reed–Sternberg cell line derived from Hodgkin's lymphoma, and certain other tissues. These cells produce the eoxins by initially metabolizing arachidonic acid, an omega-6 (ω-6) fatty acid, via any enzyme possessing 15-lipoxygenase activity. The product of this initial metabolic step, 15(S)-hydroperoxyeicosatetraenoic acid, is then converted to a series of eoxins by the same enzymes that metabolize the 5-lipoxygenase product of arachidonic acid metabolism, i.e. 5-Hydroperoxy-eicosatetraenoic acid to a series of leukotrienes. That is, the eoxins are 14,15-disubstituted analogs of the 5,6-disubstituted leukotrienes.

<span class="mw-page-title-main">13-Hydroxyoctadecadienoic acid</span> Chemical compound

13-Hydroxyoctadecadienoic acid (13-HODE) is the commonly used term for 13(S)-hydroxy-9Z,11E-octadecadienoic acid (13(S)-HODE). The production of 13(S)-HODE is often accompanied by the production of its stereoisomer, 13(R)-hydroxy-9Z,11E-octadecadienoic acid (13(R)-HODE). The adjacent figure gives the structure for the (S) stereoisomer of 13-HODE. Two other naturally occurring 13-HODEs that may accompany the production of 13(S)-HODE are its cis-trans (i.e., 9E,11E) isomers viz., 13(S)-hydroxy-9E,11E-octadecadienoic acid (13(S)-EE-HODE) and 13(R)-hydroxy-9E,11E-octadecadienoic acid (13(R)-EE-HODE). Studies credit 13(S)-HODE with a range of clinically relevant bioactivities; recent studies have assigned activities to 13(R)-HODE that differ from those of 13(S)-HODE; and other studies have proposed that one or more of these HODEs mediate physiological and pathological responses, are markers of various human diseases, and/or contribute to the progression of certain diseases in humans. Since, however, many studies on the identification, quantification, and actions of 13(S)-HODE in cells and tissues have employed methods that did not distinguish between these isomers, 13-HODE is used here when the actual isomer studied is unclear.

Specialized pro-resolving mediators are a large and growing class of cell signaling molecules formed in cells by the metabolism of polyunsaturated fatty acids (PUFA) by one or a combination of lipoxygenase, cyclooxygenase, and cytochrome P450 monooxygenase enzymes. Pre-clinical studies, primarily in animal models and human tissues, implicate SPM in orchestrating the resolution of inflammation. Prominent members include the resolvins and protectins.

References

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