NADPH oxidase
Encyclopedia
The NADPH oxidase is a membrane-bound enzyme
complex. It can be found in the plasma membrane as well as in the membrane of phagosome
.
Under normal circumstances, the complex is latent in neutrophils and is activated to assemble in the membranes during respiratory burst
.
It generates superoxide
by transferring electrons from NADPH inside the cell across the membrane
and coupling these to molecular oxygen
to produce the superoxide, which is a reactive free-radical. Superoxide can be produced in phagosome
s, which contain ingested bacteria
and fungi
, or it can be produced outside of the cell. In a phagosome, superoxide can spontaneously form hydrogen peroxide that will undergo further reactions to generate reactive oxygen species
(ROS).
Superoxide is capable of killing bacteria
and fungi by mechanisms that are not yet fully understood (see article on superoxide
), but may inactivate critical metabolic enzymes, initiate lipid peroxidation
, and liberate redox-active iron, which allows the generation of indiscriminate oxidants such as the hydroxyl radical. It is presumed that superoxide kills bacteria directly, as the virulence of many pathogens is dramatically attenuated when their superoxide dismutase
(SOD) genes are deleted. However, downstream products of superoxide also include hydrogen peroxide
and hypochlorous acid
, the reactive agent in bleach
.
Mutations in the NADPH oxidase subunit genes cause several Chronic Granulomatous Diseases
(CGD), such as
In these diseases, cells have a low capacity for phagocytosis, and persistent bacterial infections occur. Areas of infected cells are common, granulomas.
A similar disorder called neutrophil immunodeficiency syndrome
is linked to a mutation in the RAC2, also a part of the complex.
One study suggests a role for NADPH oxidase in ketamine
-induced loss of neuronal parvalbumin
and GAD67 expression. Similar loss is observed in schizophrenia
, and the results may point at the NADPH oxidase as a possible player in the pathophysiology of the disease. Nitro blue tetrazolium
is used in a diagnostic test, in particular, for chronic granulomatous disease, a disease in which there is a defect in NADPH oxidase; therefore, the phagocyte is unable to make the reactive oxygen species or radicals required for bacterial killing, resulting in bacteria thriving within the phagocyte. The higher the blue score the better the cell is at producing reactive oxygen species.
and DPI (diphenylene iodonium). Apocynin prevents the assembly of the NADPH oxidase subunits. Using apocynin to inhibit NADPH oxidase may have clinical benefits in the treatment of influenza
as it lessens influenza-induced lung inflammation in mice in vivo
.
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
complex. It can be found in the plasma membrane as well as in the membrane of phagosome
Phagosome
In cell biology, a phagosome is a vacuole formed around a particle absorbed by phagocytosis. The vacuole is formed by the fusion of the cell membrane around the particle. A phagosome is a cellular compartment in which pathogenic microorganisms can be killed and digested...
.
Subunits
It is made up of six subunits. These subunits are:- a Rho guanosine triphosphatase (GTPaseGTPaseGTPases are a large family of hydrolase enzymes that can bind and hydrolyze guanosine triphosphate . The GTP binding and hydrolysis takes place in the highly conserved G domain common to all GTPases.-Functions:...
), usually Rac1 or Rac2 (Rac stands for Rho-related C3 botulinum toxinBotulinum toxinBotulinum toxin is a protein produced by the bacterium Clostridium botulinum, and is considered the most powerful neurotoxin ever discovered. Botulinum toxin causes Botulism poisoning, a serious and life-threatening illness in humans and animals...
substrate) - Five "phox" units. (Phox stands for phagocytic oxidaseOxidaseAn oxidase is any enzyme that catalyzes an oxidation-reduction reaction involving molecular oxygen as the electron acceptor. In these reactions, oxygen is reduced to water or hydrogen peroxide ....
.)- gp91-PHOX (contains hemeHemeA heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as their prosthetic group; these are...
)(Nox2) - p22phox
- p40phox
- p47phox
- p67phox
- gp91-PHOX (contains heme
Function
Respiratory burst
Respiratory burst is the rapid release of reactive oxygen species from different types of cells....
.
It generates superoxide
Superoxide
A superoxide, also known by the obsolete name hyperoxide, is a compound that possesses the superoxide anion with the chemical formula O2−. The systematic name of the anion is dioxide. It is important as the product of the one-electron reduction of dioxygen O2, which occurs widely in nature...
by transferring electrons from NADPH inside the cell across the membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
and coupling these to molecular oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
to produce the superoxide, which is a reactive free-radical. Superoxide can be produced in phagosome
Phagosome
In cell biology, a phagosome is a vacuole formed around a particle absorbed by phagocytosis. The vacuole is formed by the fusion of the cell membrane around the particle. A phagosome is a cellular compartment in which pathogenic microorganisms can be killed and digested...
s, which contain ingested bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
and fungi
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
, or it can be produced outside of the cell. In a phagosome, superoxide can spontaneously form hydrogen peroxide that will undergo further reactions to generate reactive oxygen species
Reactive oxygen species
Reactive oxygen species are chemically reactive molecules containing oxygen. Examples include oxygen ions and peroxides. Reactive oxygen species are highly reactive due to the presence of unpaired valence shell electrons....
(ROS).
Superoxide is capable of killing bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
and fungi by mechanisms that are not yet fully understood (see article on superoxide
Superoxide
A superoxide, also known by the obsolete name hyperoxide, is a compound that possesses the superoxide anion with the chemical formula O2−. The systematic name of the anion is dioxide. It is important as the product of the one-electron reduction of dioxygen O2, which occurs widely in nature...
), but may inactivate critical metabolic enzymes, initiate lipid peroxidation
Lipid peroxidation
Lipid peroxidation refers to the oxidative degradation of lipids. It is the process in which free radicals "steal" electrons from the lipids in cell membranes, resulting in cell damage. This process proceeds by a free radical chain reaction mechanism...
, and liberate redox-active iron, which allows the generation of indiscriminate oxidants such as the hydroxyl radical. It is presumed that superoxide kills bacteria directly, as the virulence of many pathogens is dramatically attenuated when their superoxide dismutase
Superoxide dismutase
Superoxide dismutases are a class of enzymes that catalyze the dismutation of superoxide into oxygen and hydrogen peroxide. As such, they are an important antioxidant defense in nearly all cells exposed to oxygen...
(SOD) genes are deleted. However, downstream products of superoxide also include hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
and hypochlorous acid
Hypochlorous acid
Hypochlorous acid is a weak acid with the chemical formula HClO. It forms when chlorine dissolves in water. It cannot be isolated in pure form due to rapid equilibration with its precursor...
, the reactive agent in bleach
Bleach
Bleach refers to a number of chemicals that remove color, whiten, or disinfect, often via oxidation. Common chemical bleaches include household chlorine bleach , lye, oxygen bleach , and bleaching powder...
.
Role in pathology
NADPH oxidase is a major cause of atherosclerosis, and NADPH oxidase inhibitors may reverse atherosclerosis. Atherosclerosis is caused by the accumulation of macrophages containing cholesterol (foam cells) in artery walls (in the intima). NADPH oxidase produces ROSs. These ROSs activate an enzyme that makes the macrophages adhere to the artery wall (by polymerizing actin fibers). This process is counterbalanced by NADPH oxidase inhibitors, and by antioxidants. An imbalance in favor of ROS produces atherosclerosis. In vitro studies have found that the NADPH oxidase inhibitors apocynin and diphenyleneiodonium, along with the antioxidants N-acetyl-cystine and resveratrol, depolymerized the actin, broke the adhesions, and allowed foam cells to migrate out of the intima.Mutations in the NADPH oxidase subunit genes cause several Chronic Granulomatous Diseases
Chronic granulomatous disease
Chronic granulomatous disease is a diverse group of hereditary diseases in which certain cells of the immune system have difficulty forming the reactive oxygen compounds used to kill certain ingested pathogens...
(CGD), such as
- X-linked chronic granulomatous disease (CGD)
- autosomal recessive cytochrome b-negative CGD
- autosomal recessive cytochrome b-positive CGD type I
- autosomal recessive cytochrome b-positive CGD type II.
In these diseases, cells have a low capacity for phagocytosis, and persistent bacterial infections occur. Areas of infected cells are common, granulomas.
A similar disorder called neutrophil immunodeficiency syndrome
Neutrophil immunodeficiency syndrome
Neutrophil immunodeficiency syndrome is a condition caused by mutations in the Rac2 gene.- See also :* Immunodeficiency with hyper-IgM* List of cutaneous conditions* Chronic granulomatous disease- References :...
is linked to a mutation in the RAC2, also a part of the complex.
One study suggests a role for NADPH oxidase in ketamine
Ketamine
Ketamine is a drug used in human and veterinary medicine. Its hydrochloride salt is sold as Ketanest, Ketaset, and Ketalar. Pharmacologically, ketamine is classified as an NMDA receptor antagonist...
-induced loss of neuronal parvalbumin
Parvalbumin
Parvalbumin is a calcium-binding albumin protein with low molecular weight .It has three EF hand motifs and is structurally related to calmodulin and troponin C...
and GAD67 expression. Similar loss is observed in schizophrenia
Schizophrenia
Schizophrenia is a mental disorder characterized by a disintegration of thought processes and of emotional responsiveness. It most commonly manifests itself as auditory hallucinations, paranoid or bizarre delusions, or disorganized speech and thinking, and it is accompanied by significant social...
, and the results may point at the NADPH oxidase as a possible player in the pathophysiology of the disease. Nitro blue tetrazolium
Nitro blue tetrazolium chloride
Nitro blue tetrazolium is a chemical compound composed of two tetrazole moieties. It is used in immunology for sensitive detection of alkaline phosphatase . NBT serves as the oxidant and BCIP is the AP-substrate ....
is used in a diagnostic test, in particular, for chronic granulomatous disease, a disease in which there is a defect in NADPH oxidase; therefore, the phagocyte is unable to make the reactive oxygen species or radicals required for bacterial killing, resulting in bacteria thriving within the phagocyte. The higher the blue score the better the cell is at producing reactive oxygen species.
Chemical reaction
- NADPH + 2O2 ↔ NADP+ + 2O2°- + H+
Inhibition
NADPH oxidase can be inhibited by apocyninApocynin
Apocynin, also known as acetovanillone, is a natural organic compound structurally related to vanillin. It has been isolated from a variety of plant sources and is being studied for its variety of pharmacological properties.- History :...
and DPI (diphenylene iodonium). Apocynin prevents the assembly of the NADPH oxidase subunits. Using apocynin to inhibit NADPH oxidase may have clinical benefits in the treatment of influenza
Influenza
Influenza, commonly referred to as the flu, is an infectious disease caused by RNA viruses of the family Orthomyxoviridae , that affects birds and mammals...
as it lessens influenza-induced lung inflammation in mice in vivo
In vivo
In vivo is experimentation using a whole, living organism as opposed to a partial or dead organism, or an in vitro controlled environment. Animal testing and clinical trials are two forms of in vivo research...
.