Epoxygenases are a set of membrane-bound,

heme Heme, or haem (pronounced / hi:m/ ), is a precursor to hemoglobin, which is necessary to bind oxygen in the bloodstream. Heme is biosynthesized in both the bone marrow and the liver. In biochemical terms, heme is a coordination complex "consis ...

-containing

cytochrome P450 Cytochromes P450 (CYPs) are a Protein superfamily, superfamily of enzymes containing heme as a cofactor (biochemistry), cofactor that functions as monooxygenases. In mammals, these proteins oxidize steroids, fatty acids, and xenobiotics, and are ...

(CYP P450 or just CYP) enzymes that metabolize

polyunsaturated fatty acids Polyunsaturated fatty acids (PUFAs) are fatty acids that contain more than one double bond in their backbone. This class includes many important compounds, such as essential fatty acids and those that give drying oils their characteristic proper ...

epoxide In organic chemistry, an epoxide is a cyclic ether () with a three-atom ring. This ring approximates an equilateral triangle, which makes it strained, and hence highly reactive, more so than other ethers. They are produced on a large scale for ...

products that have a range of biological activities. The most thoroughly studied substrate of the CYP epoxylgenases is arachidonic acid. This

polyunsaturated fatty acid Polyunsaturated fatty acids (PUFAs) are fatty acids that contain more than one double bond in their backbone. This class includes many important compounds, such as essential fatty acids and those that give drying oils their characteristic proper ...

is metabolized by

cyclooxygenase Cyclooxygenase (COX), officially known as prostaglandin-endoperoxide synthase (PTGS), is an enzyme (specifically, a family of isozymes, ) that is responsible for formation of prostanoids, including thromboxane and prostaglandins such as pr ...

s to various

prostaglandin 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 derive ...

thromboxane 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 is named for its ...

, and

prostacyclin 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. When used as a drug, it is also known as epoprosteno ...

metabolites in what has been termed the first pathway 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 ...

production; it is also metabolized by various lipoxygenases to hydroxyeicosatetraenoic acids (e.g. 5-Hydroxyeicosatetraenoic acid, 12-Hydroxyeicosatetraenoic acid, 15-hydroxyicosatetraenoic acid) and

leukotrienes Leukotrienes are a family of eicosanoid inflammatory mediators produced in leukocytes by the oxidation of arachidonic acid (AA) and the essential fatty acid eicosapentaenoic acid (EPA) by the enzyme arachidonate 5-lipoxygenase. Leukotri ...

(e.g.

leukotriene B4 Leukotriene B4 (LTB4) is a leukotriene involved in inflammation. It has been shown to promote insulin resistance in obese mice. Biochemistry Leukotriene B4 (LTB4) is a leukotriene involved in inflammation. It is produced from leukocytes in r ...

leukotriene C4 Leukotriene C4 (LTC4) is a leukotriene. LTC4 has been extensively studied in the context of allergy and asthma. In cells of myeloid origin such as mast cells, its biosynthesis is orchestrated by translocation to the nuclear envelope along with c ...

) 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 (the epoxygenases) metabolize arachidonic acid to a set of products (the eicosatrienoic acid

s, abbreviated as EETs, which are classified as

nonclassic eicosanoid Nonclassic eicosanoids are biologically active signaling molecules made by oxygenation of twenty-carbon fatty acids other than the classic eicosanoids. Terminology : "Eicosanoid" is the collective term for oxygenated derivatives of three diffe ...

s) 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 Receptor may refer to: * Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a ...

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.

CYP epoxygenases

The cytochrome P450 (CYP) superfamily of membrane-bound (typically endoplasmic reticulum-bound) enzymes contain a

cofactor and therefore are

hemoprotein A hemeprotein (or haemprotein; also hemoprotein or haemoprotein), or heme protein, is a protein that contains a heme prosthetic group. They are a very large class of metalloproteins. The heme group confers functionality, which can include oxygen ...

s. The superfamily comprises more than 11,000 genes categorized into 1,000 families that are distributed broadly throughout bacteria,

archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...

, fungi, plants, animals, and even viruses (see

Cytochrome P450 Cytochromes P450 (CYPs) are a Protein superfamily, superfamily of enzymes containing heme as a cofactor (biochemistry), cofactor that functions as monooxygenases. In mammals, these proteins oxidize steroids, fatty acids, and xenobiotics, and are ...

). The CYP enzymes metabolize an enormously large variety of small and large molecules including foreign chemical substances, i.e.

xenobiotic A xenobiotic is a chemical substance found within an organism that is not naturally produced or expected to be present within the organism. It can also cover substances that are present in much higher concentrations than are usual. Natural compo ...

s and pharmaceuticals, as well as a diversity of endogenously formed substances such as various steroids,

vitamin D Vitamin D is a group of fat-soluble secosteroids responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, and many other biological effects. In humans, the most important compounds in this group are vitamin D3 (c ...

bilirubin Bilirubin (BR) (Latin for "red bile") is a red-orange compound that occurs in the normal catabolic pathway that breaks down heme in vertebrates. This catabolism is a necessary process in the body's clearance of waste products that arise from the ...

cholesterol Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell mem ...

, and fatty acids. Humans have 57 putatively active CYP genes and 58 CYP

pseudogenes Pseudogenes are nonfunctional segments of DNA that resemble functional genes. Most arise as superfluous copies of functional genes, either directly by DNA duplication or indirectly by reverse transcription of an mRNA transcript. Pseudogenes are ...

of which only a few are

(PUFA) epoxygenases, i.e. enzymes with the capacity to attach atomic oxygen (see Allotropes of oxygen#Atomic oxygen) to the carbon-carbon double bonds of long chain PUFA to form their corresponding epoxides. These CYP epoxygenases represent a family of enzymes that consists of several members of the CYP1 and CYP2 subfamilies. The metabolism of the straight chain 20-carbon polyunsaturated fatty

eicosatetraenoic acid Eicosatetraenoic acid (ETA) designates any straight chain 20:4 fatty acid. Eicosatetraenoic acid belongs to the family of eicosanoids, molecules synthesized from oxidized polyunsaturated fatty acids (PUFAs) to mediate cell-cell communication. The ei ...

, arachidonic acid, by certain CYP epoxygenases is a good example of their action. Arachidonic acid has 4 cis-configured double bonds (see

Cis–trans isomerism ''Cis''–''trans'' isomerism, also known as geometric isomerism or configurational isomerism, is a term used in chemistry that concerns the spatial arrangement of atoms within molecules. The prefixes "''cis''" and "''trans''" are from Latin: " ...

) located between carbons 5-6, 8-9, 11-12, and 14-15

Double bond In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betwee ...

s. (The cis configuration is termed ''Z'' in the IUPAC

Chemical nomenclature A chemical nomenclature is a set of rules to generate systematic names for chemical compounds. The nomenclature used most frequently worldwide is the one created and developed by the International Union of Pure and Applied Chemistry (IUPAC). The ...

used here.). It is therefore 5''Z'',8''Z'',11''Z'',14''Z''-eicosatetraenoic acid. Certain CYP epoxygenases attack these double bounds to form their respective eicosatrienoic acid epoxide regioisomers (see Structural isomer, section on position isomerism egioisomerism. The products are therefore 5,6-EET (i.e. 5,6-epoxy-8''Z'',11''Z'',14''Z''-eicosatetrienoic acid), 8,9-EET (i.e. 5,6-epoxy-8''Z'',11''Z'',14''Z''-eicosatetrienoic acid), 11,12-EET (i.e. 11,12-epoxy-5''Z'',8''Z'',14''Z''-eicosatetrienoic acid), and/or 14,15-EET (i.e. 14,15-epoxy-5''Z'',8''Z'',11''Z''-eicosatetrainoic acid, the structure of which is illustrated in the attached figure). Note that the eicosatetraenoate substrate loses one double bound to become an eicosatrienoic acid with three double bonds and that the epoxygenases typically form a mixture of ''R''/''S'' enantiomers at the attacked double bound position. Thus, the CYP epoxygenases which attack arachidonic acid's double bound between carbon 14 and 15 form a mixture of 14''R'',15''S''-ETE and 14''S'',15''R''-ETE. However, each CYP epoxygenase often shows preferences in the position of the double bound on which they act, partial selectivity in the ''R''/''S'' enantiomer ratios that they make at each double bound position, and different double bond position preferences and ''R''/''S'' selectivity ratios with different PUFA substrates. Finally, the product epoxides are short-lived in cells, generally existing for only several seconds before being converted by a

Soluble epoxide hydrolase Soluble epoxide hydrolase (sEH) is a bifunctional enzyme that in humans is encoded by the EPHX2 gene. sEH is a member of the epoxide hydrolase family. This enzyme, found in both the cytosol and peroxisomes, binds to specific epoxides and convert ...

(also termed epoxide hydrolase 2 or sEH) to their corresponding dihydroxy-eicosatetraenoic acid (diHETE) products, e.g. 14,15-HETE rapidly becomes a mixture of 14(''S''),15(''R'')-diHETE and 14(''R''),15(''S'')-diHETE. Although there are exceptions, the diHETE products are generally far less active than their epoxide precursors; the sEH pathway is therefore regarded as an inactivating pathway which functions to limit epoxide activity. The catalytic activity of endoplasmic reticulum-bound cytochrome P450 enzymes, including the epoxygenases, depends upon

Cytochrome P450 reductase Cytochrome P450 reductase (; also known as NADPH:ferrihemoprotein oxidoreductase, NADPH:hemoprotein oxidoreductase, NADPH:P450 oxidoreductase, P450 reductase, POR, CPR, CYPOR) is a membrane-bound enzyme required for electron transfer from NADP ...

(POR); it transfers

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...

s to, and thereby regenerates the activity of, the CYPs. The human gene that expresses POR is highly polymorphic (see

Gene polymorphism A gene is said to be polymorphic if more than one allele occupies that gene's locus within a population. In addition to having more than one allele at a specific locus, each allele must also occur in the population at a rate of at least 1% to gen ...

); many of the polymorphic variant PORs cause significantly decreases or increases in the activity of the CYPs, including the epoxygenases. Scores of drugs have been shown to either inhibit or induce one or more of the CYP epoxygenases;

CYP epoxygenase substrates and products

The most studied substrate of the CYP epoxygenases is the

omega-6 fatty acid Omega-6 fatty acids (also referred to as ω-6 fatty acids or ''n''-6 fatty acids) are a family of polyunsaturated fatty acids that have in common a final carbon-carbon double bond in the ''n''-6 position, that is, the sixth bond, counting from ...

, arachidonic acid. However, the CYP epoxygenases also metabolize other omega-6 fatty acids such as

linoleic acid Linoleic acid (LA) is an organic compound with the formula COOH(CH2)7CH=CHCH2CH=CH(CH2)4CH3. Both alkene groups are cis-trans isomerism, ''cis''. It is a fatty acid sometimes denoted 18:2 (n-6) or 18:2 ''cis''-9,12. A linoleate is a salt (chem ...

and the

omega-3 fatty acid Omega−3 fatty acids, also called Omega-3 oils, ω−3 fatty acids or ''n''−3 fatty acids, are polyunsaturated fatty acids (PUFAs) characterized by the presence of a double bond, three atoms away from the terminal methyl group in their chem ...

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 ...

and

docosahexaenoic acid Docosahexaenoic acid (DHA) is an omega-3 fatty acid that is a primary structural component of the human brain, cerebral cortex, skin, and retina. In physiological literature, it is given the name 22:6(n-3). It can be synthesized from alpha-lino ...

. The distinction between the omega-6 and omega-3 fatty acid substrates is important because omega-3 fatty acids metabolites can have lesser or different activities than omega-6 fatty acid metabolites; furthermore, they compete with the omega-6 fatty acids for the CYP epoxygenases thereby reducing the production of omega-6 fatty acid metabolites. The human CYP P450 enzymes identified to have epoxygenase activity on one or more PUFA include

CYP1A1 Cytochrome P450, family 1, subfamily A, polypeptide 1 is a protein that in humans is encoded by the ''CYP1A1'' gene. The protein is a member of the cytochrome P450 superfamily of enzymes. Function Metabolism of xenobiotics and drugs CYP1A1 ...

CYP1A2 Cytochrome P450 1A2 (abbreviated CYP1A2), a member of the cytochrome P450 mixed-function oxidase system, is involved in the metabolism of xenobiotics in the human body. In humans, the CYP1A2 enzyme is encoded by the ''CYP1A2'' gene. Function ...

CYP2C8 Cytochrome P4502C8 (abbreviated CYP2C8), a member of the cytochrome P450 mixed-function oxidase system, is involved in the metabolism of xenobiotics in the body. Cytochrome P4502C8 also possesses epoxygenase activity, i.e. it metabolizes long-cha ...

CYP2C9 Cytochrome P450 family 2 subfamily C member 9 (abbreviated CYP2C9) is an enzyme protein. The enzyme is involved in metabolism, by oxidation, of both xenobiotics, including drugs, and endogenous compounds, including fatty acids. In humans, the prote ...

CYP2C18 Cytochrome P450 2C18 is a protein that in humans is encoded by the ''CYP2C18'' gene. Function This gene encodes a member of the cytochrome P450 superfamily of enzymes. The cytochrome P450 proteins are monooxygenases which catalyze many reactio ...

CYP2C19 Cytochrome P450 2C19 (abbreviated CYP2C19) is an enzyme protein. It is a member of the CYP2C subfamily of the cytochrome P450 mixed-function oxidase system. This subfamily includes enzymes that catalyze metabolism of xenobiotics, including some p ...

CYP2E1 Cytochrome P450 2E1 (abbreviated CYP2E1, ) is a member of the cytochrome P450 mixed-function oxidase system, which is involved in the metabolism of xenobiotics in the body. This class of enzymes is divided up into a number of subcategories, includ ...

CYP2J2 Cytochrome P450 2J2 (CYP2J2) is a protein that in humans is encoded by the ''CYP2J2'' gene. CYP2J2 is a member of the cytochrome P450 superfamily of enzymes. The enzymes are oxygenases which catalyze many reactions involved in the metabolism of d ...

CYP2S1 Cytochrome P450 2S1 is a protein that in humans is encoded by the ''CYP2S1'' gene. The gene is located in chromosome 19q13.2 within a cluster including other CYP2 family members such as CYP2A6, CYP2A13, CYP2B6, and CYP2F1. Expression CYP2S1 is ...

CYP3A4 Cytochrome P450 3A4 (abbreviated CYP3A4) () is an important enzyme in the body, mainly found in the liver and in the intestine. It oxidizes small foreign organic molecules (xenobiotics), such as toxins or drugs, so that they can be removed from t ...

CYP4F2 Leukotriene-B(4) omega-hydroxylase 1 is an enzyme protein involved in the metabolism of various endogenous substrates (mainly the fatty acids) and xenobiotics (including pharmaceutical drugs). The most notable substrate of the enzyme is leukotrie ...

CYP4F3 Leukotriene-B(4) omega-hydroxylase 2 is an enzyme that in humans is encoded by the ''CYP4F3'' gene. CYP4F3 encodes two distinct enzymes, CYP4F3A and CYP4F3B, which originate from the alternative splicing of a single pre-mRNA precursor molecule; ...

CYP4A11 Cytochrome P450 4A11 is a protein that in humans is codified by the ''CYP4A11'' gene. Function This gene encodes a member of the cytochrome P450 superfamily of enzymes. The cytochrome P450 proteins are monooxygenases which catalyze many reacti ...

CYP4F8 Cytochrome P450 4F8 is a protein that in humans is encoded by the ''CYP4F8'' gene. Function This gene, CYP4F8, encodes a member of the cytochrome P450 superfamily of enzymes. The cytochrome P450 proteins are monooxygenases which catalyze many r ...

, and

CYP4F12 Cytochrome P450 4F12 is a protein that in humans is encoded by the ''CYP4F12'' gene. This gene encodes a member of the cytochrome P450 superfamily of enzymes and is part of a cluster of cytochrome P450 genes on chromosome 19. The cytochrome P450 ...

. CYP2C8 and CYP2C9 form particularly large amounts of

superoxide In chemistry, a superoxide is a compound that contains the superoxide ion, which has the chemical formula . The systematic name of the anion is dioxide(1−). The reactive oxygen ion superoxide is particularly important as the product of t ...

anion (chemical formula ) during their metabolism of polyunsaturated fatty acids; this

reactive oxygen species In chemistry, reactive oxygen species (ROS) are highly reactive chemicals formed from diatomic oxygen (). Examples of ROS include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen. The reduction of molecular oxygen () p ...

is toxic to cells and may be responsible for some of the activities ascribed to the epoxides made by the two CYPs.

Omega-6 fatty acids

Arachidonic acid

In humans, CYP1A1, CYP1A2,

, CYP2C18, CYP2C19, CYP2E1,

, and CYP2S1

isoforms A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene or gene family and are the result of genetic differences. While many perform the same or similar biological roles, some iso ...

metabolize arachidonic acid to Epoxyeicosatrienoic acids (i.e., EETs) as defined using recombinant CYPs in an ''

In vitro ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in biology an ...

microsome In cell biology, microsomes are heterogeneous vesicle-like artifacts (~20-200 nm diameter) re-formed from pieces of the endoplasmic reticulum (ER) when eukaryotic cells are broken-up in the laboratory; microsomes are not present in healthy, liv ...

assay. CYP2C9, and CYP2J2 appear to be the main producers of the EETs in humans with CYPP2C9 being the main unsaturated fatty acid epoxide producer in vascular endothelial cells and CYP2J2 being highly expressed (although less catalytically active than CYP2C9) particularly in heart muscle but also in kidneys, pancreas, lung, and brain; CYP2C8, CYP2C19, CYP2J2 are also implicated in converting arachidonic acid to epoxides in humans. Most of these CYPs preferentially form 14,15-ETE, somewhat lower levels of 11,12-EET, and far lower, trace, or undetectable levels of 8,9-ETE and 4,5-ETE. There are exceptions to this rule with, for example, CYPE1 forming 14,15-EET almost exclusively, CYP2C19 forming 8,9-EET at slightly higher levels than 14,15-EET, and CYP3A4 forming 11,12-EET at slightly higher levels than 14,15-ETE. 14,15-EET and 11,12-EET are the major EETs produced by mammalian, including human, 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 Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests pathogens, such as cancer ce ...

, liver, lung, intestine, and spleen and is abundant in human and mouse

atherosclerosis Atherosclerosis is a pattern of the disease arteriosclerosis in which the wall of the artery develops abnormalities, called lesions. These lesions may lead to narrowing due to the buildup of atheroma, atheromatous plaque. At onset there are usu ...

(i.e.

Atheroma An atheroma, or atheromatous plaque, is an abnormal and reversible accumulation of material in the inner layer of an arterial wall. The material consists of mostly macrophage cells, or debris, containing lipids, calcium and a variable amount o ...

) plaques as well as inflamed tonsils. CYP2S1 is expressed in

, liver, lung, intestine, and spleen; is abundant in human and mouse

(i.e.

) plaques as well as inflamed tonsils; and, in addition to forming epoxides of arachidonic acid (and other polyunsaturated fatty acids), CYP2S1 metabolizes

prostaglandin G2 Prostaglandin G2 is an organic peroxide belonging to the family of prostaglandins. The compound has been isolated as a solid, although it is usually used in vivo. It quickly converts into prostaglandin H2, a process catalyzed by the enzyme COX. ...

and

Prostaglandin H2 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 Pro ...

to 12-Hydroxyheptadecatrienoic acid. Possibly because of metabolizing and thereby inactivating the prostaglandins and/or because forming the bioactive metabolite, 12-hyddroxyheptadecatrienoic acid, rather than EETs, CYP2S1 may act to inhibit the function of monocytes and thereby limit

inflammation Inflammation (from la, wikt:en:inflammatio#Latin, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or Irritation, irritants, and is a protective response involving im ...

as well as other

immune response An immune response is a reaction which occurs within an organism for the purpose of defending against foreign invaders. These invaders include a wide variety of different microorganisms including viruses, bacteria, parasites, and fungi which could ...

s. The activities and clinical significance of the EETs are given on the epoxyeicosatrienoic acid page.

Linoleic acid

CYP2C9 and CYP2S1 are known to, and many or all of the other CYPs that act on arachidonic acid are thought to, metabolize the 18 carbon

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 ...

, 9(''Z''),12(''Z'')-octadecadienoic acid, i.e.

, at is 12,13 carbon-carbon double bout to form (+) and (-) epoxy optical isomers viz., the 12''S'',13''R''-epoxy-9(''Z'')-octadecaenoic and 12''R'',13''S''-epoxy-9(''Z'')-octadecaenoic acids; this set of optical isomers is also termed

vernolic acid Vernolic acid (leukotoxin) is a long chain fatty acid that is monounsaturated and contains an epoxide. It is the R,R-''cis'' epoxide derived from the C12–C13 alkene of linoleic acid. Vernolic acid was first definitively characterized in 1954. I ...

, linoleic acid 12:13-oxide, and leukotoxin. CYPC2C9 is known and the other arachidonic acid-metabolizing CYPs are thought to likewise attack linoleic acid at its 9,10 carbon-carbon double bound to form 9''S'',10''R''-epoxy-12(''Z'')-octadecaenoic and 9''R'',10''S''-epoxy-12(''Z'')-octadecaenoic acid optical isomers; this set of optical isomers is also termed

coronaric acid Coronaric acid (isoleukotoxin) is a mono-unsaturated, epoxide derivative of the di-saturated fatty acid, linoleic acid (i.e. 9(''Z''),12(''Z'') octadecadienoic acid. It is a mixture of the two optically active isomers of 12(''Z'') 9,10-epoxy-octa ...

, linoleic acid 9,10-oxide, and isoleukotoxin These linoleic acid-derived leukotoxin and isoleukotoxin sets of optical isomers possess activities similar to that of other leukotoxins such as the pore-forming leukotoxin family of

RTX toxin The RTX toxin superfamily is a group of cytolysins and cytotoxins produced by bacteria. There are over 1000 known members with a variety of functions. The RTX family is defined by two common features: characteristic repeats in the toxin protein se ...

virulence factor proteins secreted by

gram-negative bacteria Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall ...

, e.g.

Aggregatibacter actinomycetemcomitans ''Aggregatibacter actinomycetemcomitans'' is a Gram-negative, facultative anaerobe, nonmotile bacterium that is often found in association with localized aggressive periodontitis, a severe infection of the periodontium. It is also suspected to b ...

and E. coli. That is, they are toxic to leukocytes as well as many other cell types and when injected into rodents produce multiple organ failure and respiratory distress. These effects appear due to the conversion of leukotoxin to its dihydroxy counterparts, 9''S'',10''R''- and 9''R'',10''S''-dihydroxy-12(''Z'')-octadecaenoic acids, and isoleukotoxin to its 12''R'',13''S''- and 12''S'',13''R''-dihydroxy-9(''Z'')-octadecenoic acid counterparts by

soluble epoxide hydrolase Soluble epoxide hydrolase (sEH) is a bifunctional enzyme that in humans is encoded by the EPHX2 gene. sEH is a member of the epoxide hydrolase family. This enzyme, found in both the cytosol and peroxisomes, binds to specific epoxides and convert ...

. Some studies suggest but have not proven that leukotoxin and isoleukotoxin, acting primarily if not exclusively through their respective dihydroxy counterparts, are responsible for or contribute to multiple organ failure, respiratory distress, and certain other cataclysmic diseases in humans.

Adrenic acid

Adrenic acid Docosatetraenoic acid designates any straight chain 22:4 fatty acid. (''See'' essential fatty acid for nomenclature.) One isomer is of particular interest: * all-''cis''-7,10,13,16-docosatetraenoic acid is an ω-6 fatty acid with the common nam ...

or 7(''Z''),10(''Z''),13(''Z''),16(''Z'')-docosatetraenoic acid, an abundant fatty acid in the adrenal gland, kidney, vasculature, and early human brain, is metabolized primarily to 7(''Z''),10(''Z''),13(''Z'')-16,17-epoxy-docosatrienoic acid and smaller amounts of its 7,8-, 10,11-, and 13,14-epoxide-docosatrienoic acids by bovine coronary arteries and adrenal zona glomerulosa cells through the apparent action of an unidentified CYP epoxygenase(s); the eSH-dependent metabolism of these eoxide, 7,8-, 10,11-, and 13,14-dihydroxy-docosatrienoic acids relaxes pre-contracted coronary and adrenal gland arteries suggesting that the dihydroxy metabolites may act as vascular endothelium-derived

Endothelium-derived relaxing factor Endothelium-derived relaxing factor (EDRF) is a name for a substance that Robert F. Furchgott discovered had the eponymous properties. Today, it is firmly established this substance is nitric oxide (NO). Endothelium produces NO which then diffu ...

Omega-3 fatty acids

Eicosapentaenoic acid

5(''Z''),8(''Z''),11(''Z''),14(''Z''),17(''Z'')-

(EPA) is metabolized by the same CYP epoxygenases that metabolize arachidonic acid primarily to 17,18-epoxy-5(''Z''),8(''Z''),11(''Z''),14(''Z'')-eicosatetranoic acid and usually far smaller or undetectable amounts of EPA's 5,6-, 8,9-, 11,12-, or 14,15-epoxides; however, CYP2C9 metabolizes EPA primarily to 14,15-epoxy-5(''Z''),8(''Z''),11(''Z''),17(''Z'')-eicosatetranoic acid, CYP2C11 forms appreciable amounts of this 14,15-epoxide in addition to the 17,18-epoxide, and CYP2C18 forms appreciable amounts of the 11,12 epoxide (11,12-epoxy-5(''Z''),8(''Z''),14(''Z''),17(''Z'')-eicosatetranoic acid) in addition to the 17,18-epoxide. Furthermore, CYP4A11, CYP4F8, and CYP4F12, which are CYP monooxygenase rather than CYP epoxygeanse in that they metabolize arachidonic acid to monohydroxy eicosatetraenoic acid products (see 20-Hydroxyeicosatetraenoic acid), i.e. 19-hydroxy- and/or 18-hydroxy-eicosatetranoic acids, takes on epoxygenase activity in converting EPA primarily to its 17,18-epoxy metabolite (see

epoxyeicosatetraenoic acid Epoxyeicosatetraenoic acids (EEQs or EpETEs) are a set of biologically active epoxides that various cell types make by metabolizing the omega 3 fatty acid, eicosapentaenoic acid (EPA), with certain cytochrome P450 epoxygenases. These epoxygenases ...

Docosahexaenoic acid

4(''Z''),7(''Z''),10(''Z''),13(''Z''),16(''Z''),19(''Z'')-

(DHA) is metabolized by the same CYP epoxygenases that metabolize arachidonic acid to form epoxide-containing

docosapentaenoic acid Docosapentaenoic acid (DPA) designates any straight chain 22:5 fatty acid, that is a straight chain open chain type of polyunsaturated fatty acid (PUFA) which contains 22 carbons and 5 double bonds. DPA is primarily used to designate two isomers, ...

products, particularly 19,20-epoxy-4(''Z''),7(''Z''),10(''Z''),13(''Z''),16(''Z'')-docosapentenoic acid. These docosapentaenoic acid epoxides or

Epoxydocosapentaenoic acid Epoxide docosapentaenoic acids (epoxydocosapentaenoic acids, EDPs, or EpDPEs) are metabolites of the 22-carbon straight-chain omega-3 fatty acid, docosahexaenoic acid (DHA). Cell types that express certain cytochrome P450 (CYP) epoxygenases metab ...

s (EDPs) have a somewhat different set of activities than, and thereby may serve in part as counterpoises to, the EET's; EDPs may also be responsible for some the beneficial effects attributed to

-rich foods such as

fish oil Fish oil is oil derived from the tissues of oily fish. Fish oils contain the omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), precursors of certain eicosanoids that are known to reduce inflammation in the b ...

(see

α-Linolenic acid

The 18 carbon

, 9

α-Linolenic acid ''alpha''-Linolenic acid (ALA), also known as α-Linolenic acid (from Greek language, Greek ''alpha'' meaning "first" and ''linon'' meaning flax), is an omega-3 fatty acid, ''n''−3, or omega-3, essential fatty acid. ALA is found in many seeds ...

or 9(''Z''),12(''Z''),15(''Z'')-octadecatrienoic acid, is metabolized primarily to 9(''Z''),12(''Z'')-15,16-epoxy-octadecadienoic acid but also to smaller amounts of its 8,10- and 12,13-epoxides in the serum, liver, lung, and spleen of mice treated with a drug that increases the expression of CYP1A1, CYP1A2 and/or CYP1B1. These epoxides are also found in the plasma of humans and their levels greatly increase in subjects given an α-linolenic acid-rich diet.

Genetic polymorphism A gene is said to be polymorphic if more than one allele occupies that gene's locus within a population. In addition to having more than one allele at a specific locus, each allele must also occur in the population at a rate of at least 1% to gen ...
in CYP epoxygenases

Human CYP epoxygenase genes come in many

single nucleotide polymorphism In genetics, a single-nucleotide polymorphism (SNP ; plural SNPs ) is a germline substitution of a single nucleotide at a specific position in the genome. Although certain definitions require the substitution to be present in a sufficiently larg ...

(SNP) variants some of which code for epoxygenase products with altered activity. Investigation into the impact of these variants on the bearers' health (i.e.

phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...

) is an invaluable area of research which offers the opportunity to define the function of the epoxygenases and their polyunsaturated fatty acid metabolites in humans. However, SNP variants that cause altered polyunsaturated fatty acid metabolism may also cause altered metabolism of their other substrates, i.e. diverse

(e.g.

NSAID Non-steroidal anti-inflammatory drugs (NSAID) are members of a therapeutic drug class which reduces pain, decreases inflammation, decreases fever, and prevents blood clots. Side effects depend on the specific drug, its dose and duration of ...

) and endotiotic (e.g. the primary female sex hormone,

estradiol Estradiol (E2), also spelled oestradiol, is an estrogen steroid hormone and the major female sex hormone. It is involved in the regulation of the estrous and menstrual female reproductive cycles. Estradiol is responsible for the development of f ...

) compounds: the latter effects may lead to clinical manifestations that overshadow any manifestations resulting from changes in polyunsaturated fatty acid metabolism. The most common SNP epoxygenase variants are as follows. 1) CYP2C8*3 (30411A>G, rs10509681, Lys399Arg) converts arachidonic acid to 11,12-EET and 14,15-EET with a turnover rate less than half that of wild type CYP2C8; in a single recent report, male but not female carriers of the CYP2C8*3 allele had an increased risk of essential hypertension. Bearers of this SNP may or may not show increased risk of developing acute gastrointestinal bleeding during the use of

non-steroidal anti-inflammatory drugs Non-steroidal anti-inflammatory drugs (NSAID) are members of a therapeutic drug class which reduces pain, decreases inflammation, decreases fever, and prevents blood clots. Side effects depend on the specific drug, its dose and duration of ...

(NSAIDs) that are its substrates such as aceclofenac,

celecoxib Celecoxib, sold under the brand name Celebrex among others, is a COX-2 inhibitor and nonsteroidal anti-inflammatory drug (NSAID). It is used to treat the pain and inflammation in osteoarthritis, acute pain in adults, rheumatoid arthritis, ankyl ...

diclofenac Diclofenac, sold under the brand name Voltaren, among others, is a nonsteroidal anti-inflammatory drug (NSAID) used to treat pain and inflammatory diseases such as gout. It is taken by mouth or rectally in a suppository, used by injection, or ...

, ibuprofen,

indomethacin Indometacin, also known as indomethacin, is a nonsteroidal anti-inflammatory drug (NSAID) commonly used as a prescription medication to reduce fever, pain, stiffness, and swelling from inflammation. It works by inhibiting the production of ...

lornoxicam Lornoxicam, also known as chlortenoxicam, is a nonsteroidal anti-inflammatory drug (NSAID) of the oxicam class with analgesic (pain relieving), anti-inflammatory and antipyretic (fever reducing) properties. It is available in oral administration, ...

meloxicam Meloxicam, sold under the brand name Mobic among others, is a nonsteroidal anti-inflammatory medication (NSAID) used to treat pain and inflammation in rheumatic diseases and osteoarthritis. It is used by mouth or by intravenous, injection into a ...

naproxen Naproxen is a nonsteroidal anti-inflammatory drug (NSAID) used to treat pain, menstrual cramps, inflammatory diseases such as rheumatoid arthritis, gout and fever. It is taken orally. It is available in immediate and delayed release formulat ...

piroxicam Piroxicam is a nonsteroidal anti-inflammatory drug (NSAID) of the oxicam class used to relieve the symptoms of painful inflammatory conditions like arthritis. Piroxicam works by preventing the production of endogenous prostaglandins which are in ...

tenoxicam Tenoxicam, sold under the brand name Mobiflex among others, is a nonsteroidal anti-inflammatory drug (NSAID). It is used to relieve inflammation, swelling, stiffness, and pain associated with rheumatoid arthritis, osteoarthritis, ankylosing spon ...

, and

valdecoxib Valdecoxib is a nonsteroidal anti-inflammatory drug (NSAID) used in the treatment of osteoarthritis, rheumatoid arthritis, and painful menstruation and menstrual symptoms. It is a selective cyclooxygenase-2 inhibitor. It was patented in 1995. V ...

. 2) CYP2J2*7 (−76G>T, rs890293, upstream

Promoter (genetics) In genetics, a promoter is a sequence of DNA to which proteins bind to initiate transcription of a single RNA transcript from the DNA downstream of the promoter. The RNA transcript may encode a protein (mRNA), or can have a function in and of ...

site) has decreased binding of the

Sp1 transcription factor Transcription factor Sp1, also known as specificity protein 1* is a protein that in humans is encoded by the SP1 gene. Function The protein encoded by this gene is a zinc finger transcription factor that binds to GC-rich motifs of many promote ...

resulting in its lowered expressionas and lowered levels of EETs in plasma. Carriers of this SNP among a Uyghur population in China had a higher risk of

coronary artery disease Coronary artery disease (CAD), also called coronary heart disease (CHD), ischemic heart disease (IHD), myocardial ischemia, or simply heart disease, involves the reduction of blood flow to the heart muscle due to build-up of atherosclerotic pla ...

. However, CYP2J2*7 carriers showed no association with hypertension, heart attack, or stroke in a study of 5,740 participants of the cardiovascular cohort of the Malmö Diet and Cancer study; since other studies have afforded contradictory results, this allele is currently regarded as not associated with cardiovascular diseases (see Epoxyeicosatrienoic acid#Clinical significance). Bearers of this SNP in a Chinese Population had a higher risk of younger onset of

type 2 diabetes Type 2 diabetes, formerly known as adult-onset diabetes, is a form of diabetes mellitus that is characterized by high blood sugar, insulin resistance, and relative lack of insulin. Common symptoms include increased thirst, frequent urinatio ...

and among a Chinese Han population had a higher risk of

Alzheimer's disease Alzheimer's disease (AD) is a neurodegeneration, neurodegenerative disease that usually starts slowly and progressively worsens. It is the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in short-term me ...

. 3) CYP2C8*2 (11054A>T, rs11572103, Ile269Phe) and CYP2C8*4 (11041C>, rs1058930, variants have reduced arachidonic acid-metabolizing activity but have not been clearly associated with cardiovascular or other diseases. 4) CYPC28*4 (3608C>T, rs1058930, Ile264Met) has reduced arachidonic acid metabolizing activity. It has not been associated with cardiovascular diseases but has a higher incidence in subjects with

type II diabetes Type 2 diabetes, formerly known as adult-onset diabetes, is a form of diabetes mellitus that is characterized by high blood sugar, insulin resistance, and relative lack of insulin. Common symptoms include increased thirst, frequent urinatio ...

in a small sample of Caucasians in Germany. 5) The CYP2C9*2 (3608C>T, rs1799853, Arg144Cys) variant has a 50% reduction in polyunsaturated fatty acid metabolizing activity compared to CYP2C9 wild type; carriers of it show no association with cardiovascular disease but exhibit poor metabolism of the anti-coagulating, ''blood-thinning'' agent,

warfarin Warfarin, sold under the brand name Coumadin among others, is a medication that is used as an anticoagulant (blood thinner). It is commonly used to prevent blood clots such as deep vein thrombosis and pulmonary embolism, and to prevent st ...

. These carriers are susceptible to the gastrointestinal bleeding side effects of warfarin and the NSAID cited above. 6 ) CYP2C9*3 (42624A>C, rs1057910, Iso359Leu) encodes an expoxygenase with reduced arachidonic acid metabolizing activity. This allele has not been directly associated with cardiovascular diseases but may be associated with the poor metabolism and therefore adverse reactions to warfarin, NSAID, sulfonylurea-containing oral

hypoglycemic agent Drugs used in diabetes treat diabetes mellitus by altering the glucose level in the blood. With the exceptions of insulin, most GLP receptor agonists ( liraglutide, exenatide, and others), and pramlintide, all are administered orally and are th ...

s, and the anti-(epilepsy) drug, phenytoin. 7) CYP2C19*2 (19154G>A, rs4244285, Il264Met) and CYP2C19*3 (17948G>A, rs4986893, His212X) are loss-of-function

null allele A null allele is a nonfunctional allele (a variant of a gene) caused by a genetic mutation. Such mutations can cause a complete lack of production of the associated gene product or a product that does not function properly; in either case, the alle ...

s; carriers of the CYP2C19*3 but not the CYP2C19*2 allele showed a ''reduced'' risk of developing essential hypertension in a large Korean population study. Bearers of null alleles would be expected to be poor metabolizers of several drugs which are CYP2C19*2 or CYP2C19*3 substrates. This is particularly the case with

Clopidogrel Clopidogrel — sold under the brand name Plavix, among others — is an antiplatelet medication used to reduce the risk of heart disease and stroke in those at high risk. It is also used together with aspirin in heart attacks and following ...

, a drug used to block platelet activation, blood clotting, and thereby heart attack, stroke, and peripheral artery occlusion in people at high risk of these events; CYP2C19 metabolizes clopidogrel to it active form. Consequently, patients with severe deficiencies in this CYP, i.e. bearers of CYP2C19*3 or CYP2C19*2 alleles, fail to gain protection from clopidogrel and have a higher risk of the cited cardiovascular events than clopidogrel-treated patients bearing wild type CYP2C19 alleles. 8) CYPC19*17 (-800C>T, rs12248560, site upstream

gene promoter In genetics, a promoter is a sequence of DNA to which proteins bind to initiate transcription of a single RNA transcript from the DNA downstream of the promoter. The RNA transcript may encode a protein (mRNA), or can have a function in and of ...

site) causes overproduction of its epoxygenase and thereby the ultra fast metabolism of arachidonic acid. Bearers of this allele have not been associated with cardiovascular diseases but clearly show a decreased risk of developing breast cancer and

endometriosis Endometriosis is a disease of the female reproductive system in which cells similar to those in the endometrium, the layer of tissue that normally covers the inside of the uterus, grow outside the uterus. Most often this is on the ovaries, f ...

possible because their rapid metabolism of estrogen leads to lower estrogen levels and thereby a lower risk of these estrogen-fueled diseases. These bearers also a higher rate of metabolism of, and therefore reduced responsiveness to, certain

proton pump inhibitor Proton-pump inhibitors (PPIs) are a class of medications that cause a profound and prolonged reduction of stomach acid production. They do so by irreversibly inhibiting the stomach's H+/K+ ATPase proton pump. They are the most potent inhibitors ...

and

antidepressant Antidepressants are a class of medication used to treat major depressive disorder, anxiety disorders, chronic pain conditions, and to help manage addictions. Common side-effects of antidepressants include dry mouth, weight gain, dizziness, hea ...

drugs.

Genetic polymorphism in Cytochrome P450 reductase

As indicated above, Cytochrome P450 reductase (POR) is responsible for regenerating the activity of CYPs including the epoxygenases. Several genetic variants of the human POR gene impact epoxygenase activity. For example, POR

Missense mutation In genetics, a missense mutation is a point mutation in which a single nucleotide change results in a codon that codes for a different amino acid. It is a type of nonsynonymous substitution. Substitution of protein from DNA mutations Missense m ...

s A287P and R457H lead to reductions in the activity of CYP2C19 and CYP2C9, respectively, whereas A503V and Q153R missense mutations lead to small increases in the activity of CYP2C9. While these and other POR genetic variants have not yet been associated with epoxygenase-related disease, they contribute to the marked variability in the activity of the epoxygenases between individuals.

References

{{reflist, 2 Enzymes Inflammations

CYP epoxygenases

CYP epoxygenase substrates and products

Omega-6 fatty acids

Arachidonic acid

Linoleic acid

Adrenic acid

Omega-3 fatty acids

Eicosapentaenoic acid

Docosahexaenoic acid

α-Linolenic acid

Genetic polymorphism A gene is said to be polymorphic if more than one allele occupies that gene's locus within a population. In addition to having more than one allele at a specific locus, each allele must also occur in the population at a rate of at least 1% to gen ... in CYP epoxygenases

Genetic polymorphism in Cytochrome P450 reductase

References

Genetic polymorphism A gene is said to be polymorphic if more than one allele occupies that gene's locus within a population. In addition to having more than one allele at a specific locus, each allele must also occur in the population at a rate of at least 1% to gen ...
in CYP epoxygenases