Complement membrane attack complex
The membrane attack complex or terminal complement complex is a structure formed on the surface of pathogen cell membranes as a result of the activation of the host's complement system, as such is one of the effector proteins of the immune system. The membrane-attack complex forms transmembrane channels; these channels disrupt the cell membrane of target cells, leading to death. Active MAC is composed of the subunits C5b, C7, C8 and several C9 molecules. A number of proteins participate in the assembly of the MAC. Freshly activated C5b binds to C6 to form a C5b-6 complex to C7 forming the C5b-6-7 complex; the C5b-6-7 complex binds to C8, composed of three chains, thus forming the C5b-6-7-8 complex. C5b-6-7-8 subsequently binds to C9 and acts as a catalyst in the polymerization of C9. MAC is composed of a complex of four complement proteins that bind to the outer surface of the plasma membrane, many copies of a fifth protein that hook up to one another, forming a ring in the membrane. C6-C9 all contain a common MACPF domain.
This region is homologous to cholesterol-dependent cytolysins from Gram-positive bacteria. The ring structure formed by C9 is a pore in the membrane that allows free diffusion of molecules in and out of the cell. If enough pores form, the cell is no longer able to survive. If the pre-MAC complexes of C5b-7, C5b-8 or C5b-9 do not insert into a membrane, they can form inactive complexes with Protein S; these fluid phase complexes do not bind to cell membranes and are scavenged by clusterin and vitronectin, two regulators of complement. The membrane attack complex is initiated when the complement protein C5 convertase cleaves C5 into C5a and C5b. All three pathways of the complement system initiate the formation of MAC. Another complement protein, C6, binds to C5b; the C5bC6 complex is bound by C7. This junction alters the configuration of the protein molecules exposing a hydrophobic site on C7 that allows the C7 to insert into the phospholipid bilayer of the pathogen. Similar hydrophobic sites on C8 and C9 molecules are exposed when they bind to the complex, so they can insert into the bilayer.
C8 is a complex made of C8 alpha-gamma. C8 alpha-gamma has the hydrophobic area. C8 alpha-gamma induces the polymerization of 10-16 molecules of C9 into a pore-forming structure known as the membrane attack complex. MAC has a hydrophobic external face allowing it to associate with the lipid bilayer. MAC has a hydrophilic internal face to allow the passage of water. Multiple molecules of C9 can join spontaneously in concentrated solution to form polymers of C9; these polymers can form a tube-like structure. CD59 acts to inhibit the complex; this exists on body cells to protect them from MAC. A rare condition, paroxysmal nocturnal haemoglobinuria, results in red blood cells that lack CD59; these cells can, therefore, be lysed by MAC. Deficiencies of C5 to C9 components does not lead to generic infections, but only to increased susceptibility to Neisseria spp. since these bacteria have a thick cell wall and glycocalix. Terminal complement pathway deficiency Perforin Pore-forming toxin Media related to Complement membrane attack complex at Wikimedia Commons Complement+Membrane+Attack+Complex at the US National Library of Medicine Medical Subject Headings
Inflammation is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, is a protective response involving immune cells, blood vessels, molecular mediators. The function of inflammation is to eliminate the initial cause of cell injury, clear out necrotic cells and tissues damaged from the original insult and the inflammatory process, initiate tissue repair; the five classical signs of inflammation are heat, redness and loss of function. Inflammation is a generic response, therefore it is considered as a mechanism of innate immunity, as compared to adaptive immunity, specific for each pathogen. Too little inflammation could lead to progressive tissue destruction by the harmful stimulus and compromise the survival of the organism. In contrast, chronic inflammation may lead to a host of diseases, such as hay fever, atherosclerosis, rheumatoid arthritis, cancer. Inflammation is therefore closely regulated by the body. Inflammation can be classified as either chronic.
Acute inflammation is the initial response of the body to harmful stimuli and is achieved by the increased movement of plasma and leukocytes from the blood into the injured tissues. A series of biochemical events propagates and matures the inflammatory response, involving the local vascular system, the immune system, various cells within the injured tissue. Prolonged inflammation, known as chronic inflammation, leads to a progressive shift in the type of cells present at the site of inflammation, such as mononuclear cells, is characterized by simultaneous destruction and healing of the tissue from the inflammatory process. Inflammation is not a synonym for infection. Infection describes the interaction between the action of microbial invasion and the reaction of the body's inflammatory response—the two components are considered together when discussing an infection, the word is used to imply a microbial invasive cause for the observed inflammatory reaction. Inflammation on the other hand describes purely the body's immunovascular response, whatever the cause may be.
But because of how the two are correlated, words ending in the suffix -itis are sometimes informally described as referring to infection. For example, the word urethritis means only "urethral inflammation", but clinical health care providers discuss urethritis as a urethral infection because urethral microbial invasion is the most common cause of urethritis, it is useful to differentiate inflammation and infection because there are typical situations in pathology and medical diagnosis where inflammation is not driven by microbial invasion – for example, trauma and autoimmune diseases including type III hypersensitivity. Conversely, there is pathology where microbial invasion does not cause the classic inflammatory response – for example, parasitosis or eosinophilia. Acute inflammation is a short-term process appearing within a few minutes or hours and begins to cease upon the removal of the injurious stimulus, it involves a coordinated and systemic mobilization response locally of various immune and neurological mediators of acute inflammation.
In a normal healthy response, it becomes activated, clears the pathogen and begins a repair process and ceases. It is characterized by five cardinal signs:An acronym that may be used to remember the key symptoms is "PRISH", for pain, immobility and heat; the traditional names for signs of inflammation come from Latin: Dolor Calor Rubor Tumor Functio laesa The first four were described by Celsus, while loss of function was added by Galen. However, the addition of this fifth sign has been ascribed to Thomas Sydenham and Virchow. Redness and heat are due to increased blood flow at body core temperature to the inflamed site. Loss of function has multiple causes. Acute inflammation of the lung does not cause pain unless the inflammation involves the parietal pleura, which does have pain-sensitive nerve endings; the process of acute inflammation is initiated by resident immune cells present in the involved tissue resident macrophages, dendritic cells, Kupffer cells and mast cells. These cells possess surface receptors known as pattern recognition receptors, which recognize two subclasses of molecules: pathogen-associated molecular patterns and damage-associated molecular patterns.
PAMPs are compounds that are associated with various pathogens, but which are distinguishable from host molecules. DAMPs are compounds that are associated with host-related cell damage. At the onset of an infection, burn, or other injuries, these cells undergo activation and release inflammatory mediators responsible for the clinical signs of inflammation. Vasodilation and its resulting increased blood flow causes increased heat. Increased permeability of the blood vessels results in an exudation of plasma proteins and fluid into the tissue, which manifests itself as swelling; some of the released mediators such as bradykinin increase the sensitivity to pain. The mediator molecules alter the blood vessels to
Macrophages are a type of white blood cell, of the immune system, that engulfs and digests cellular debris, foreign substances, cancer cells, anything else that does not have the type of proteins specific to healthy body cells on its surface in a process called phagocytosis. These large phagocytes are found in all tissues, where they patrol for potential pathogens by amoeboid movement, they take various forms throughout the body. Besides phagocytosis, they play a critical role in nonspecific defense and help initiate specific defense mechanisms by recruiting other immune cells such as lymphocytes. For example, they are important as antigen presenters to T cells. In humans, dysfunctional macrophages cause severe diseases such as chronic granulomatous disease that result in frequent infections. Beyond increasing inflammation and stimulating the immune system, macrophages play an important anti-inflammatory role and can decrease immune reactions through the release of cytokines. Macrophages that encourage inflammation are called M1 macrophages, whereas those that decrease inflammation and encourage tissue repair are called M2 macrophages.
This difference is reflected in their metabolism. However, this dichotomy has been questioned as further complexity has been discovered. Human macrophages are about 21 micrometres in diameter and are produced by the differentiation of monocytes in tissues, they can be identified using flow cytometry or immunohistochemical staining by their specific expression of proteins such as CD14, CD40, CD11b, CD64, F4/80 /EMR1, lysozyme M, MAC-1/MAC-3 and CD68. Macrophages were first discovered by Élie Metchnikoff, a Russian zoologist, in 1884. A majority of macrophages are stationed at strategic points where microbial invasion or accumulation of foreign particles is to occur; these cells together as a group are known as the mononuclear phagocyte system and were known as the reticuloendothelial system. Each type of macrophage, determined by its location, has a specific name: Investigations concerning Kupffer cells are hampered because in humans, Kupffer cells are only accessible for immunohistochemical analysis from biopsies or autopsies.
From rats and mice, they are difficult to isolate, after purification, only 5 million cells can be obtained from one mouse. Macrophages can express paracrine functions within organs that are specific to the function of that organ. In the testis for example, macrophages have been shown to be able to interact with Leydig cells by secreting 25-hydroxycholesterol, an oxysterol that can be converted to testosterone by neighbouring Leydig cells. Testicular macrophages may participate in creating an immune privileged environment in the testis, in mediating infertility during inflammation of the testis. Cardiac resident macrophages participate in electrical conduction via gap junction communication with cardiac myocytes. Macrophages can be classified on basis of the fundamental activation. According to this grouping there are classically activated macrophages, wound-healing macrophages and regulatory macrophages. Macrophages that reside in adult healthy tissues either derive from circulating monocytes or are established before birth and maintained during adult life independently of monocytes.
By contrast, most of the macrophages that accumulate at diseased sites derive from circulating monocytes. When a monocyte enters damaged tissue through the endothelium of a blood vessel, a process known as leukocyte extravasation, it undergoes a series of changes to become a macrophage. Monocytes are attracted to a damaged site by chemical substances through chemotaxis, triggered by a range of stimuli including damaged cells and cytokines released by macrophages at the site. At some sites such as the testis, macrophages have been shown to populate the organ through proliferation. Unlike short-lived neutrophils, macrophages survive longer in the body, up to several months. Macrophages are professional phagocytes and are specialized in removal of dying or dead cells and cellular debris; this role is important in chronic inflammation, as the early stages of inflammation are dominated by neutrophils, which are ingested by macrophages if they come of age. The neutrophils are at first attracted to a site, where they proliferate, before they are phagocytized by the macrophages.
When at the site, the first wave of neutrophils, after the process of aging and after the first 48 hours, stimulate the appearance of the macrophages whereby these macrophages will ingest the aged neutrophils. The removal of dying cells is, to a greater extent, handled by fixed macrophages, which will stay at strategic locations such as the lungs, neural tissue, bone and connective tissue, ingesting foreign materials such as pathogens and recruiting additional macrophages if needed; when a macrophage ingests a pathogen, the pathogen becomes trapped in a phagosome, which fuses with a lysosome. Within the phagolysosome and toxic peroxides digest the pathogen. However, some bacteria, such as Mycobacterium tuberculosis, have become resistant to these methods of digestion. Typhoidal Salmonellae induce their own phagocytos
PD-1 and PD-L1 inhibitors
PD-1 inhibitors and PD-L1 inhibitors are a group of checkpoint inhibitors being developed for the treatment of cancer. PD-1 and PD-L1 are both proteins present on the surface of cells. Immune checkpoint inhibitors such as these are emerging as a front-line treatment for several types of cancer. PD-1 and PD-L1 inhibitors act to inhibit the association of the programmed death-ligand 1 with its receptor, programmed cell death protein 1; the interaction of these cell surface proteins is involved in the suppression of the immune system and occurs following infection to limit the killing of bystander host cells and prevent autoimmune disease. This immune checkpoint is active in pregnancy, following tissue allografts, in different types of cancer; the concept of blocking PD-1 and PD-L1 for the treatment of cancer was first published in 2001. Pharmaceutical companies began attempting to develop drugs to block these molecules, the first clinical trial was launched in 2006, evaluating nivolumab; as of 2017, more than 500 clinical trials involving PD-1 and PD-L1 inhibitors have been conducted in more than 20,000 patients.
By the end of 2017, PD-1/PD-L1 inhibitors had been approved for the treatment of nine forms of cancer. In the cancer disease state, the interaction of PD-L1 on the tumor cells with PD-1 on a T-cell reduces T-cell function signals to prevent the immune system from attacking the tumor cells. Use of an inhibitor that blocks the interaction of PD-L1 with the PD-1 receptor can prevent the cancer from evading the immune system in this way. Several PD-1 and PD-L1 inhibitors are being trialled within the clinic for use in advanced melanoma, non-small cell lung cancer, renal cell carcinoma, bladder cancer and Hodgkin lymphoma, amongst other cancer types. Immunotherapy with these immune checkpoint inhibitors appears to shrink tumours in a higher number of patients across a wider range of tumour types and is associated with lower toxicity levels than other immunotherapies, with durable responses. However, de-novo and acquired resistance is still seen in a large proportion of patients. Hence PD-L1 inhibitors are considered to be the most promising drug category for many different cancers.
PD-1 and PD-L1 inhibitors are related to CTLA4 inhibitors, such as ipilimumab. PD-1 and CTLA-4 are both at different phases of immune response. Current clinical trials are evaluating anti-PD-1 and PD-L1 drugs in combination with other immunotherapy drugs blocking LAG3, B7-H3, KIR, OX40, PARP, CD27, ICOS. Pembrolizumab was developed by Merck and first approved by the Food and Drug Administration in 2014 for the treatment of melanoma, it was approved for metastatic non-small cell lung cancer and head and neck squamous cell carcinoma. In 2017, it became the first immunotherapy drug approved for use based on the genetic mutations of the tumor rather than the site of the tumor, it was shown, that patients with higher non-synonymous mutation burden in their tumors respond better to the treatment. Both their objective response rate and progression-free survival was shown to be higher than in patients with low non-synonymous mutation burden. Nivolumab was developed by Bristol-Myers Squibb and first approved by the FDA in 2014 for the treatment of melanoma.
It was approved for squamous cell lung cancer, renal cell carcinoma, Hodgkin's lymphoma. Cemiplimab was developed by Regeneron and first approved by the FDA in 2018 for the treatment of cutaneous squamous cell carcinoma or locally advanced CSCC who are not candidates for curative surgery or curative radiation; as of 2017, at least five PD-1 inhibitors were under development. Pidilizumab, by Cure Tech AMP-224, by GlaxoSmithKline AMP-514, by GlaxoSmithKline PDR001, by Novartis Atezolizumab is a humanised IgG1 (immunoglobulin 1 antibody developed by Roche Genentech. In 2016, the FDA approved atezolizumab for non-small cell lung cancer. Avelumab is a human IgG1 antibody developed by Merck Serono and Pfizer. Avelumab is FDA approved for the treatment of metastatic merkel-cell carcinoma, it failed phase III clinical trials for gastric cancer. Durvalumab is a human IgG1 antibody developed by AstraZeneca. Durvalumab is FDA approved for the treatment of urothelial carcinoma and unresectable non-small cell lung cancer after chemoradiation.
At least two PD-L1 inhibitors are in the experimental phase of development. BMS-936559, by Bristol-Myers Squibb CK-301, by Checkpoint Therapeutics Immunotherapies as a group have off-target effects and toxicities common to them; some of these include interstitial pneumonitis, skin reactions, immune thrombocytopenia, encephalopathy, neuromuscular adverse events including myositis, Guillain-Barré syndrome, myasthenia gravis. Cancer immunotherapy#Immune checkpoints
Plasmin is an important enzyme present in blood that degrades many blood plasma proteins, including fibrin clots. The degradation of fibrin is termed fibrinolysis. In humans, the plasmin protein is encoded by the PLG gene. Plasmin is a serine protease. Apart from fibrinolysis, plasmin proteolyses proteins in various other systems: It activates collagenases, some mediators of the complement system, weakens the wall of the Graafian follicle, leading to ovulation, it cleaves fibrin, thrombospondin and von Willebrand factor. Plasmin, like trypsin, belongs to the family of serine proteases. Plasmin is released. Two major glycoforms of plasminogen are present in humans - type I plasminogen contains two glycosylation moieties, whereas type II plasminogen contains only a single O-linked sugar. Type II plasminogen is preferentially recruited to the cell surface over the type I glycoform. Conversely, type I plasminogen appears more recruited to blood clots. In circulation, plasminogen adopts a activation resistant conformation.
Upon binding to clots, or to the cell surface, plasminogen adopts an open form that can be converted into active plasmin by a variety of enzymes, including tissue plasminogen activator, urokinase plasminogen activator and factor XII. Fibrin is a cofactor for plasminogen activation by tissue plasminogen activator. Urokinase plasminogen activator receptor is a cofactor for plasminogen activation by urokinase plasminogen activator; the conversion of plasminogen to plasmin involves the cleavage of the peptide bond between Arg-561 and Val-562. Plasmin cleavage produces angiostatin. Full length plasminogen comprises seven domains. In addition to a C-terminal chymotrypsin-like serine protease domain, plasminogen contains an N-terminal Pan Apple domain together with five Kringle domains; the Pan-Apple domain contains important determinants for maintaining plasminogen in the closed form, the kringle domains are responsible for binding to lysine residues present in receptors and substrates. The X-ray crystal structure of closed plasminogen reveals that the PAp and SP domains maintain the closed conformation through interactions made throughout the kringle array.
Chloride ions further bridge the PAp / KR4 and SP / KR2 interfaces, explaining the physiological role of serum chloride in stabilizing the closed conformer. The structural studies reveal that differences in glycosylation alter the position of KR3; these data help explain the functional differences between the type I and type II plasminogen glycoforms. In closed plasminogen, access to the activation bond targeted for cleavage by tPA and uPA is blocked through the position of the KR3/KR4 linker sequence and the O-linked sugar on T346; the position of KR3 may hinder access to the activation loop. The Inter-domain interactions block all kringle ligand-binding sites apart from that of KR-1, suggesting that the latter domain governs pro-enzyme recruitment to targets. Analysis of an intermediate plasminogen structure suggests that plasminogen conformational change to the open form is initiated through KR-5 transiently peeling away from the PAp domain; these movements expose the KR5 lysine-binding site to potential binding partners, suggest a requirement for spatially distinct lysine residues in eliciting plasminogen recruitment and conformational change respectively.
Plasmin is inactivated by proteins such as α2-antiplasmin. The mechanism of plasmin inactivation involves the cleavage of an α2-macroglobulin at the bait region by plasmin; this initiates a conformational change such that the α2-macroglobulin collapses about the plasmin. In the resulting α2-macroglobulin-plasmin complex, the active site of plasmin is sterically shielded, thus decreasing the plasmin's access to protein substrates. Two additional events occur as a consequence of bait region cleavage, namely a h-cysteinyl-g-glutamyl thiol ester of the α2-macroglobulin becomes reactive and a major conformational change exposes a conserved COOH-terminal receptor binding domain; the exposure of this receptor binding domain allows the α2-macroglobulin protease complex to bind to clearance receptors and be removed from circulation. Plasmin deficiency may lead to thrombosis. Plasminogen deficiency in mice leads to defective liver repair, defective wound healing, reproductive abnormalities. In humans, a rare disorder called plasminogen deficiency type I is caused by mutations of the PLG gene and is manifested by ligneous conjunctivitis.
Plasmin has been shown to interact with Thrombospondin 1, Alpha 2-antiplasmin and IGFBP3. Moreover, plasmin induces the generation of bradykinin in mice and humans through high molecular weight kininogen cleavage; the MEROPS online database for peptidases and their inhibitors: S01.233 Plasmin at the US National Library of Medicine Medical Subject Headings This article incorporates text from the United States National Library of Medicine, in the public domain
Immune checkpoints are regulators of the immune system. These pathways are crucial for self-tolerance, which prevents the immune system from attacking cells indiscriminately. Inhibitory checkpoint molecules are targets for cancer immunotherapy due to their potential for use in multiple types of cancers. Approved checkpoint inhibitors block CTLA4 and PD-1 and PD-L1. For the related basic science discoveries, James P. Allison and Tasuku Honjo won the Tang Prize in Biopharmaceutical Science and the Nobel Prize in Physiology or Medicine in 2018. Four stimulatory checkpoint molecules are members of the tumor necrosis factor receptor superfamily—CD27, CD40, OX40, GITR and CD137. Another two stimulatory checkpoint molecules belongs to the B7-CD28 superfamily—CD28 itself and ICOS. CD27: This molecule supports antigen-specific expansion of naïve T cells and is vital for the generation of T cell memory. CD27 is a memory marker of B cells. CD27's activity is governed by the transient availability of its ligand, CD70, on lymphocytes and dendritic cells.
CD27 costimulation is known to suppresses Th17 effector cell function. The American biotech company Celldex Therapeutics is working on CDX-1127, an agonistic anti-CD27 monoclonal antibody which in animal models has been shown to be effective in the context of T cell receptor stimulation. CD28: This molecule is constitutively expressed on all human CD4+ T cells and on around half of all CD8 T cells. Binding with its two ligands are CD80 and CD86, expressed on dendritic cells, prompts T cell expansion. CD28 was the target of the TGN1412'superagonist' which caused severe inflammatory reactions in the first-in-man study in London in March 2006. CD40: This molecule, found on a variety of immune system cells including antigen presenting cells has CD40L, otherwise known as CD154 and transiently expressed on the surface of activated CD4+ T cells, as its ligand. CD40 signaling is known to ‘license’ dendritic cells to mature and thereby trigger T-cell activation and differentiation. A now-defunct Seattle-based biotechnology company called VLST in-licensed an anti-CD40 agonist monoclonal antibody from Pfizer in 2012.
The Swiss pharmaceutical company Roche acquired this project when VLST was shut down in 2013. CD122: This molecule, the Interleukin-2 receptor beta sub-unit, is known to increase proliferation of CD8+ effector T cells; the American biotechnology company Nektar Therapeutics is working on NKTR-214, a CD122-biased immune-stimulatory cytokine Phase I results announced in Nov 2016. CD137: When this molecule called 4-1BB, is bound by CD137 ligand, the result is T-cell proliferation. CD137-mediated signaling is known to protect T cells, in particular, CD8+ T cells from activation-induced cell death; the German biotech company Pieris Pharmaceuticals has developed an engineered lipocalin, bi-specific for CD137 and HER2. OX40: This molecule called CD134, has OX40L, or CD252, as its ligand. Like CD27, OX40 promotes the expansion of effector and memory T cells, however it is noted for its ability to suppress the differentiation and activity of T-regulatory cells, for its regulation of cytokine production.
OX40's value as a drug target lies in the fact that, being transiently expressed after T-cell receptor engagement, it is only upregulated on the most antigen-activated T cells within inflammatory lesions. Anti-OX40 monoclonal antibodies have been shown to have clinical utility in advanced cancer; the pharma company AstraZeneca has three drugs in development targeting OX40: MEDI0562 is a humanised OX40 agonist. The ligand for GITR is expressed on antigen presenting cells. Antibodies to GITR have been shown to promote an anti-tumor response through loss of Treg lineage stability; the biotech company TG Therapeutics is working on anti-GITR antibodiesICOS: This molecule, short for Inducible T-cell costimulator, called CD278, is expressed on activated T cells. Its ligand is ICOSL, expressed on B cells and dendritic cells; the molecule seems to be important in T cell effector function. The American biotechnology company Jounce Therapeutics is developing an ICOS agonist. A2AR: The Adenosine A2A receptor is regarded as an important checkpoint in cancer therapy because adenosine in the immune microenvironment, leading to the activation of the A2a receptor, is negative immune feedback loop and the tumor microenvironment has high concentrations of adenosine.
B7-H3: called CD276, was understood to be a co-stimulatory molecule but is now regarded as co-inhibitory. The American biotechnology company MacroGenics is working on MGA271 is an Fc-optimized monoclonal antibody that targets B7-H3. B7-H3’s receptors have not yet been identified. B7-H4: called VTCN1, is expressed by tumor cells and tumor-associated macrophages and plays a role in tumour escape. BTLA: This molecule, short for B and T Lymphocyte Attenuator and called CD272, has HVEM as its ligand. Surface expression of BTLA is downregulated during differentiation of human CD8+ T cells from the naive to effector cell phenotype, however tumor-specific human CD8+ T cells express high levels of BTLA. CTLA-4: short for Cytotoxic T-Lymphocyte-Associated protein 4 and called CD152, is the target of Bristol-Myers Squibb's melanoma drug Yervoy, which gained FDA approval in March 2011. Expression of CTLA-4 on Treg cells serves to control T cell proliferation. IDO: short for Indoleamine 2,3-dioxygenase, is a tryptophan catabolic enzyme with immune-inhibitory properties.
Another important molecule is TDO, tryptophan 2,3-dioxygenase. IDO is known to suppress T and NK
Nitric oxide is a colorless gas with the formula NO. It is one of the principal oxides of nitrogen. Nitric oxide is a free radical, i.e. it has an unpaired electron, sometimes denoted by a dot in its chemical formula, i.e. ·NO. Nitric oxide is a heteronuclear diatomic molecule, a historic class that drew researches which spawned early modern theories of chemical bonding. An important intermediate in chemical industry, nitric oxide forms in combustion systems and can be generated by lightning in thunderstorms. In mammals, including humans, nitric oxide is a signaling molecule in many physiological and pathological processes, it was proclaimed the "Molecule of the Year" in 1992. The 1998 Nobel Prize in Physiology or Medicine was awarded for discovering nitric oxide's role as a cardiovascular signalling molecule. Nitric oxide should not be confused with nitrous oxide, an anesthetic, or with nitrogen dioxide, a brown toxic gas and a major air pollutant. Upon condensing to a liquid, nitric oxide dimerizes to dinitrogen dioxide, but the association is weak and reversible.
The N–N distance in crystalline NO is 218 pm, nearly twice the N–O distance. Since the heat of formation of ·NO is endothermic, NO can be decomposed to the elements. Catalytic converters in cars exploit this reaction: 2 NO → O2 + N2; when exposed to oxygen, nitric oxide converts into nitrogen dioxide: 2 NO + O2 → 2 NO2. This conversion has been speculated as occurring via the ONOONO intermediate. In water, nitric oxide reacts with water to form nitrous acid; the reaction is thought to proceed via the following stoichiometry: 4 NO + O2 + 2 H2O → 4 HNO2. Nitric oxide reacts with fluorine and bromine to form the nitrosyl halides, such as nitrosyl chloride: 2 NO + Cl2 → 2 NOCl. With NO2 a radical, NO combines to form the intensely blue dinitrogen trioxide: NO + NO2 ⇌ ON−NO2; the addition of a nitric oxide moiety to another molecule is referred to as nitrosylation. Nitric oxide reacts with acetone and an alkoxide to a diazeniumdiolate or nitrosohydroxylamine and methyl acetate: This reaction, discovered around 1898, remains of interest in nitric oxide prodrug research.
Nitric oxide can react directly with sodium methoxide, forming sodium formate and nitrous oxide. Nitric oxide reacts with transition metals to give complexes called metal nitrosyls; the most common bonding mode of nitric oxide is the terminal linear type. Alternatively, nitric oxide can serve as a one-electron pseudohalide. In such complexes, the M−N−O group is characterized by an angle between 120° and 140°; the NO group can bridge between metal centers through the nitrogen atom in a variety of geometries. In commercial settings, nitric oxide is produced by the oxidation of ammonia at 750–900 °C with platinum as catalyst: 4 NH3 + 5 O2 → 4 NO + 6 H2OThe uncatalyzed endothermic reaction of oxygen and nitrogen, effected at high temperature by lightning has not been developed into a practical commercial synthesis: N2 + O2 → 2 NO In the laboratory, nitric oxide is conveniently generated by reduction of dilute nitric acid with copper: 8 HNO3 + 3 Cu → 3 Cu2 + 4 H2O + 2 NOAn alternative route involves the reduction of nitrous acid in the form of sodium nitrite or potassium nitrite: 2 NaNO2 + 2 NaI + 2 H2SO4 → I2 + 4 NaHSO4 + 2 NO 2 NaNO2 + 2 FeSO4 + 3 H2SO4 → Fe23 + 2 NaHSO4 + 2 H2O + 2 NO 3 KNO2 + KNO3 + Cr2O3 → 2 K2CrO4 + 4 NOThe iron sulfate route is simple and has been used in undergraduate laboratory experiments.
So-called NONOate compounds are used for nitric oxide generation. Nitric oxide concentration can be determined using a chemiluminescent reaction involving ozone. A sample containing nitric oxide is mixed with a large quantity of ozone; the nitric oxide reacts with the ozone to produce oxygen and nitrogen dioxide, accompanied with emission of light: NO + O3 → NO2 + O2 + hνwhich can be measured with a photodetector. The amount of light produced is proportional to the amount of nitric oxide in the sample. Other methods of testing include electroanalysis, where ·NO reacts with an electrode to induce a current or voltage change; the detection of NO radicals in biological tissues is difficult due to the short lifetime and concentration of these radicals in tissues. One of the few practical methods is spin trapping of nitric oxide with iron-dithiocarbamate complexes and subsequent detection of the mono-nitrosyl-iron complex with electron paramagnetic resonance. A group of fluorescent dye indicators that are available in acetylated form for intracellular measurements exist.
The most common compound is 4,5-diaminofluorescein. Nitric oxide reacts with the hydroperoxy radical to form nitrogen dioxide, which can react with a hydroxyl radical to produce nitric acid: ·NO + HO2•→ •NO2 + •OH ·NO2 + •OH → HNO3Nitric acid, along with sulfuric acid, contribute acid rain deposition. Furthermore, ·NO participates in ozone layer depletion. In this process, nitric oxide reacts with stratospheric ozone to form O2 and nitrogen dioxide: ·NO + O3 → NO2 + O2As seen in the Concentration Measurement section, this reaction is utilized to measure concentrations of ·NO in control volumes; as seen in the Acid deposition section, nitric oxide can transform into nitrogen dioxide. Symptoms of short-term nitrogen dioxide exposure include nausea and headache. Long-term effects could include impaired respiratory function. NO is a gaseous signaling molecule, it is a key vertebrate biological messenger. It is