Dual oxidase 2, also known as DUOX2 or ThOX2 (for thyroid oxidase), is an enzyme that in humans is encoded by the DUOX2 gene. [5] [6] Dual oxidase is an enzyme that was first identified in the mammalian thyroid gland. In humans, two isoforms are found; hDUOX1 and hDUOX2 (this enzyme). The protein location is not exclusive to thyroid tissue; hDUOX1 is prominent in airway epithelial cells [7] and hDUOX2 in the salivary glands and gastrointestinal tract. [8] [9] [10]
Investigations into reactive oxygen species (ROS) in biological systems have, until recently, focused on characterization of phagocytic cell processes. It is now well accepted that production of such species is not restricted to phagocytic cells and can occur in eukaryotic non-phagocytic cell types via NADPH oxidase (NOX) or dual oxidase (DUOX). This new family of proteins, termed the NOX/DUOX family or NOX family of NADPH oxidases, consists of homologs to the catalytic moiety of phagocytic NADPH-oxidase, gp91phox. Members of the NOX/DUOX family have been found throughout eukaryotic species, including invertebrates, insects, nematodes, fungi, amoeba, algae, and plants (not found in prokaryotes). These enzymes clearly demonstrate regulated production of ROS as their sole function. Genetic analyses have implicated NOX/DUOX derived ROS in biological roles and pathological conditions including hypertension (NOX1), innate immunity (NOX2/DUOX), otoconia formation in the inner ear (NOX3) and thyroid hormone biosynthesis (DUOX1/2).DUOX2 is the isoform that generates H2O2 utilized by thyroid peroxidase (TPO) for the biosynthesis of thyroid hormones, [11] supported by the discovery of congenital hypothyroidism resultant from an inactivating mutation in the DUOX2 gene. [5] [12]
The family currently has seven members including NOX1, NOX2 (formerly known as gp91phox), NOX3, NOX4, NOX5, DUOX1 and DUOX2.
This protein is known as a dual oxidase because it has both a peroxidase homology domain and a gp91phox domain. [13]
Duox are also implicated in lung defence system [14] and especially in cystic fibrosis. [15] [16] [17]
Schema of duox implication in human lung defence system
NADPH oxidase is a membrane-bound enzyme complex that faces the extracellular space. It can be found in the plasma membrane as well as in the membranes of phagosomes used by neutrophil white blood cells to engulf microorganisms. Human isoforms of the catalytic component of the complex include NOX1, NOX2, NOX3, NOX4, NOX5, DUOX1, and DUOX2.
NADPH oxidase 2 (Nox2), also known as cytochrome b(558) subunit beta or Cytochrome b-245 heavy chain, is a protein that in humans is encoded by the NOX2 gene. The protein is a superoxide generating enzyme which forms reactive oxygen species (ROS).
Retinoid X receptor alpha (RXR-alpha), also known as NR2B1 is a nuclear receptor that in humans is encoded by the RXRA gene.
Neutrophil cytosol factor 1, also known as p47phox, is a protein that in humans is encoded by the NCF1 gene.
NADPH oxidase 1 is an enzyme that in humans is encoded by the NOX1 gene.
NADPH oxidase 4 is an enzyme that in humans is encoded by the NOX4 gene, and is a member of the NOX family of NADPH oxidases.
Thyroid hormone receptor beta (TR-beta) also known as nuclear receptor subfamily 1, group A, member 2 (NR1A2), is a nuclear receptor protein that in humans is encoded by the THRB gene.
Cytochrome b-245 light chain is a protein that in humans is encoded by the CYBA gene involved in superoxide production and phagocytosis.
Rac2 is a small signaling G protein, and is a member of the Rac subfamily of the family Rho family of GTPases. It is encoded by the gene RAC2.
Neutrophil cytosol factor 4 is a protein that in humans is encoded by the NCF4 gene.
Rho GDP-dissociation inhibitor 1 is a protein that in humans is encoded by the ARHGDIA gene.
Dual oxidase 1, also known as DUOX1 or ThOX1, is an enzyme which in humans is encoded by the DUOX1 gene. DUOX1 was first identified in the mammalian thyroid gland. In humans, two isoforms are found; hDUOX1 and hDUOX2. Human DUOX protein localization is not exclusive to thyroid tissue; hDUOX1 is prominent in airway epithelial cells and hDUOX2 in the salivary glands and gastrointestinal tract.
NADPH oxidase, EF-hand calcium binding domain 5, also known as NOX5, is a protein which in humans is encoded by the NOX5 gene.
NADPH oxidase organizer 1 is an enzyme that in humans is encoded by the NOXO1 gene.
NADPH oxidase 3 is an enzyme that in humans is encoded by the NOX3 gene.
NADPH oxidase activator 1 is an enzyme that in humans is encoded by the NOXA1 gene.
NADPH-dependent diflavin oxidoreductase 1 is an enzyme that in humans is encoded by the NDOR1 gene.
Iodotyrosine deiodinase, also known as iodotyrosine dehalogenase 1, is a type of deiodinase enzyme that scavenges iodide by removing it from iodinated tyrosine residues in the thyroid gland. These iodinated tyrosines are produced during thyroid hormone biosynthesis. The iodide that is scavenged by iodotyrosine deiodinase is necessary to again synthesize the thyroid hormones. After synthesis, the thyroid hormones circulate through the body to regulate metabolic rate, protein expression, and body temperature. Iodotyrosine deiodinase is thus necessary to keep levels of both iodide and thyroid hormones in balance.
The respiratory tract antimicrobial defense system is a layered defense mechanism which relies on components of both the innate and adaptive immune systems to protect the lungs and the rest of the respiratory tract against inhaled microorganisms.
Edgar Pick is an Israeli immunologist who is Professor Emeritus of Immunology in the Department of Clinical Microbiology and Immunology at the Faculty of Medicine at Tel Aviv University, Israel.
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