Arachidonate 12-lipoxygenase, 12R type, also known as ALOX12B, 12''R''-LOX, and arachidonate lipoxygenase 3, is a
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 ...
-type enzyme composed of 701 amino acids and encoded by the ''ALOX12B''
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
.
The gene is located on chromosome 17 at position 13.1 where it forms a cluster with two other lipoxygenases,
ALOXE3
Epidermis-type lipoxygenase 3 (ALOXE3 or eLOX3) 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 lipo ...
and
ALOX15B
Arachidonate 15-lipoxygenase type II is an enzyme that in humans is encoded by the ''ALOX15B'' gene. ALOX15B, also known as 15-lipoxygenase-2 (15-LO-2 or 15-LOX-2), is distinguished from its related oxygenase, ALOX15 or 15-lipoxygenase-1.
Fun ...
.
Among the human lipoxygenases, ALOX12B is most closely (54% identity) related in amino acid sequence to
ALOXE3
Epidermis-type lipoxygenase 3 (ALOXE3 or eLOX3) 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 lipo ...
Activity
ALOX12B oxygenates
arachidonic acid by adding molecular oxygen (O
2) in the form of a
hydroperoxyl
The hydroperoxyl radical, also known as the hydrogen superoxide, is the protonated form of superoxide with the chemical formula HO2. This species plays an important role in the atmosphere and as a reactive oxygen species in cell biology.
Struc ...
(HO
2) residue to its 12th carbon thereby forming 12(''R'')-hydroperoxy-5''Z'',8''Z'',10''E'',14''Z''-icosatetraenoic acid (also termed 12(''R'')-HpETE or 12''R''-HpETE).
When formed in cells, 12''R''-HpETE may be quickly reduced to its
hydroxyl
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
analog (OH), 12(''R'')-hydroxy-5Z'',8''Z'',10''E'',14''Z''-eicosatetraenoic acid (also termed 12(''R'')-HETE or 12''R''-HETE), by ubiquitous
peroxidase
Peroxidases or peroxide reductases ( EC numberbr>1.11.1.x are a large group of enzymes which play a role in various biological processes. They are named after the fact that they commonly break up peroxides.
Functionality
Peroxidases typically ca ...
-type enzymes. These sequential metabolic reactions are:
ALOX12B is also capable of metabolizing free
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 ...
to 9(''R'')-hydroperoxy-10(E),12(Z)-octadecadienoic acid (9''R''-HpODE) which is also rapidly converted to its hydroxyl derivative, 9-Hydroxyoctadecadienoic acid (9''R''-HODE).
The ''S''
stereoisomer
In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in ...
of 9''R''-HODE, 9''S''-HODE, has a range of biological activities related to
oxidative stress
Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species and a biological system's ability to readily Detoxification, detoxify the reactive intermediates or to repair the resulting damage. Disturbances ...
and pain perception (see
9-Hydroxyoctadecadienoic acid
9-Hydroxyoctadecadienoic acid (or 9-HODE) has been used in the literature to designate either or both of two stereoisomer metabolites of the essential fatty acid, linoleic acid: 9(''S'')-hydroxy-10(''E''),12(''Z'')-octadecadienoic acid (9(''S'')- ...
. It is known or likely that 9''R''-HODE possesses at least some of these activities. For example, 9''R''-HODE, similar to 9''S''-HODE, mediates the perception of acute and chronic pain induced by heat, UV light, and inflammation in the skin of rodents (see
9-Hydroxyoctadecadienoic acid#9-HODEs as mediators of pain perception). However, production of these LA metabolites does not appear to be the primary function of ALOX12B; ALOX12B's primary function appears to be to metabolize linoleic acid that is not free but rather esterified to certain
Proposed principal activity of ALOX12B
ALOX12B targets
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 ...
(LA). LA is the most abundant fatty acid in the skin
epidermis
The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water rele ...
, being present mainly
esterified
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ar ...
to the omega-
hydroxyl
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
residue of
amide
In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it is ...
-linked omega-hydroxylated
very long chain fatty acid A very-long-chain fatty acid (VLCFA) is a fatty acid with 22 or more carbons. Their biosynthesis occurs in the endoplasmic reticulum. VLCFA's can represent up to a few percent of the total fatty acid content of a cell.
Unlike most fatty acids, V ...
s (VLCFAs) in a unique class of
ceramide
Ceramides are a family of waxy lipid molecules. A ceramide is composed of N-acetylsphingosine and a fatty acid. Ceramides are found in high concentrations within the cell membrane of eukaryotic cells, since they are component lipids that make up ...
s termed esterified omega-hydroxyacyl-
sphingosine
Sphingosine (2-amino-4-trans-octadecene-1,3-diol) is an 18-carbon amino alcohol with an unsaturated hydrocarbon chain, which forms a primary part of sphingolipids, a class of cell membrane lipids that include sphingomyelin, an important phospholip ...
(EOS). EOS is an intermediate component in a proposed multi-step metabolic pathway which delivers VLCFAs to the cornified lipid envelop in the skin's
Stratum corneum
The stratum corneum (Latin for 'horny layer') is the outermost layer of the epidermis. The human stratum corneum comprises several levels of flattened corneocytes that are divided into two layers: the ''stratum disjunctum'' and ''stratum compact ...
; the presence of these
wax
Waxes are a diverse class of organic compounds that are lipophilic, malleable solids near ambient temperatures. They include higher alkanes and lipids, typically with melting points above about 40 °C (104 °F), melting to giv ...
-like, hydrophobic VLCFAs is needed to maintain the skin's integrity and functionality as a water barrier (see
Lung microbiome#Role of the epithelial barrier).
ALOX12B metabolizes the LA in EOS to its 9-hydroperoxy derivative; ALOXE3 then converts this derivative to three products: a) 9''R'',10''R''-trans-
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 ...
,13''R''-hydroxy-10''E''-octadecenoic acid, b) 9-keto-10''E'',12''Z''-octadecadienoic acid, and c) 9''R'',10''R''-trans-epoxy-13-keto-11''E''-octadecenoic acid.
These ALOX12B-oxidized products signal for the
hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
(i.e. removal) of the oxidized products from EOS; this allows the multi-step metabolic pathway to proceed in delivering the VLCFAs to the cornified lipid envelop in the skin's Stratum corneum.
Tissue distribution
ALOX12B protein has been detected in humans that in the same tissues the express ALOXE3 and ALOX15B viz., upper layers of the human skin and tongue and in tonsils.
mRNA for it has been detected in additional tissues such as the lung, testis, adrenal gland, ovary, prostate, and skin with lower abundance levels detected in salivary and thyroid glands, pancreas, brain, and plasma blood leukocytes.
Clinical significance
Congenital ichthyosiform erythrodema
Deletions of ''Alox12b'' or ''AloxE2'' genes in mice cause a congenital scaly skin disease which is characterized by a greatly reduced skin water barrier function and is similar in other ways to the
autosomal recessive
In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...
nonbullous
Congenital ichthyosiform erythroderma Congenital ichthyosiform erythroderma (CIE), also known as nonbullous congenital ichthyosiform erythroderma, is a rare type of the ichthyosis family of skin diseases which occurs in 1 in 200,000 to 300,000 births. CIE comes under the umbrella term a ...
(ARCI) disease of humans. Mutations in many of the genes that encode proteins, including ALOX12B and ALOXE3, which conduct the steps that bring and then bind VLCFA to the stratums corneum are associated with ARCI. ARCI refers to nonsyndromic (i.e. not associated with other signs or symptoms)
congenital
A birth defect, also known as a congenital disorder, is an abnormal condition that is present at birth regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities can ...
Ichthyosis
Ichthyosis is a family of genetic skin disorders characterized by dry, thickened, scaly skin. The more than 20 types of ichthyosis range in severity of symptoms, outward appearance, underlying genetic cause and mode of inheritance (e.g., dominan ...
including
Harlequin-type ichthyosis
Harlequin-type ichthyosis is a genetic disorder that results in thickened skin over nearly the entire body at birth. The skin forms large, diamond/trapezoid/rectangle-shaped plates that are separated by deep cracks. These affect the shape of th ...
,
Lamellar ichthyosis
Lamellar ichthyosis, also known as ichthyosis lamellaris and nonbullous congenital ichthyosis, is a rare inherited skin disorder, affecting around 1 in 600,000 people.
Presentation
Affected babies are born in a collodion membrane, a shiny, waxy- ...
, and
Congenital ichthyosiform erythroderma Congenital ichthyosiform erythroderma (CIE), also known as nonbullous congenital ichthyosiform erythroderma, is a rare type of the ichthyosis family of skin diseases which occurs in 1 in 200,000 to 300,000 births. CIE comes under the umbrella term a ...
.
ARCI has an incidence of about 1/200,000 in European and North American populations; 40 different mutations in ''ALOX12B'' and 13 different mutations in ''ALOXE3'' genes account for a total of about 10% of ARCI case; these mutations uniformly cause a total loss of ALOX12B or ALOXE3 function (see
mutations
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...
).
Proliferative skin diseases
In
psoriasis
Psoriasis is a long-lasting, noncontagious autoimmune disease characterized by raised areas of abnormal skin. These areas are red, pink, or purple, dry, itchy, and scaly. Psoriasis varies in severity from small, localized patches to complete ...
and other proliferative skin diseases such as the
erythroderma
Erythroderma is an inflammatory skin disease with redness and scaling that affects nearly the entire cutaneous surface.Freedberg, et al. (2003). ''Fitzpatrick's Dermatology in General Medicine''. (6th ed.). McGraw-Hill. p. 436. . This term applie ...
s underlying lung cancer,
cutaneous T cell lymphoma
Cutaneous T-cell lymphoma (CTCL) is a class of non-Hodgkin lymphoma, which is a type of cancer of the immune system. Unlike most non-Hodgkin lymphomas (which are generally B-cell-related), CTCL is caused by a mutation of T cells. The cancerous ...
, and drug reactions, and in
discoid lupus
Discoid lupus erythematosus is the most common type of chronic cutaneous lupus (CCLE), an Autoimmune disease, autoimmune skin condition on the lupus erythematosus spectrum of illnesses. It presents with red, painful, inflamed and coin-shaped patch ...
,
seborrheic dermatitis
Seborrhoeic dermatitis, sometimes inaccurately referred to as seborrhoea, is a long-term skin disorder. Symptoms include red, scaly, greasy, itchy, and inflamed skin. Areas of the skin rich in oil-producing glands are often affected including the ...
, subacute
cutaneous lupus erythematosus
Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.
Other animal coverings, such as the arthropod exoskeleton, have different de ...
, and
pemphigus foliaceus
Pemphigus foliaceus is an autoimmune blistering disease ( bullous disorder) of the skin. Pemphigus foliaceus causes a characteristic inflammatory attack at the subcorneal layer of epidermis, which results in skin lesions that are scaly or cruste ...
, cutaneous levels of ALOX12B
mRNA
In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein.
mRNA is ...
and 12''R''-HETE are greatly increased.
It is not clear if these increases contribute to the disease by, for example, 12''R''-HETE induction of inflammation, or are primarily a consequence of skin proliferation.
Embryogenesis
The expression of Alox12b and Aloxe3
mRNA
In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein.
mRNA is ...
in mice parallels, and is proposed to be instrumental for, skin development in mice
embryogenesis
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...
; the human
ortholog
Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments of DNA can have shared ancestry because of three phenomena: either a spec ...
s of these genes, i.e. ALOX12B and ALOXE3, may have a similar role in humans.
Essential fatty acid deficiency
Severe dietary deficiency of polyunsaturated
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 ...
s leads to the
essential fatty acid deficiency
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 ...
syndrome that is characterized by scaly skin and excessive water loss; in humans and animal models the syndrome is fully reversed by dietary omega 6 fatty acids, particularly linoleic acid.
It is proposed that this deficiency disease resembles and has a similar basis to Congenital ichthyosiform erythrodema; that is, it is at least in part due to a deficiency of linoleic acid and thereby in the EOS-based delivery of VLCFA to the stratum corneum.
References
Further reading
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External links
* {{UCSC gene info, ALOX12B
Cell biology
Fatty acids