Nitric oxide or nitrogen monoxide is a chemical compound with chemical formula NO. This gas is an important signaling molecule in the body of mammals, including humans, and is an extremely important intermediate in the chemical industry. It is also a toxic air pollutant produced by automobile engines and power plants.

NO is an important messenger molecule involved in many physiological and pathological processes within the mammalian body both beneficial and detrimental. . Appropriate levels of NO production are important in protecting an organ such as the liver from ischemic damage. However sustained levels of NO production result in direct tissue toxicity and contribute to the vascular collapse associated with septic shock, whereas chronic expression of NO is associated with various carcinomas and inflammatory conditions including juvenile diabetes, multiple sclerosis, arthritis and ulcerative colitis.

Nitric oxide should not be confused with nitrous oxide (N₂O), a general anaesthetic and greenhouse gas, or with nitrogen dioxide (NO₂) which is another poisonous air pollutant. The nitric oxide molecule is a free radical, which is relevant to understanding its high reactivity. It reacts with the ozone in air to form nitrogen dioxide, signalled by the appearance of the reddish-brown color.

Despite being a startlingly simple molecule, NO is a fundamental player in the fields of neuroscience, physiology, and immunology, and was proclaimed “Molecule of the Year” in 1992

Production environmental effects

From a thermodynamic perspective, NO is unstable with respect to O₂ and N₂, although this conversion is very slow at ambient temperatures in the absence of a catalyst. Because the heat of formation of NO is endothermic, its synthesis from molecular nitrogen and oxygen requires elevated temperatures, >1000°C. A major natural source is lightning. The use of internal combustion engines has drastically increased the presence of nitric oxide in the environment. One purpose of catalytic converters in cars is to minimize NO emission by catalytic reversion to O₂ and N₂.

Nitric oxide in the air may convert to nitric acid, which has been implicated in acid rain. Furthermore, both NO and NO₂ participate in ozone layer depletion. Nitric oxide is a small highly diffusible gas and a ubiquitous bioactive molecule.

Mechanism of action

There are several mechanisms by which NO has been demonstrated to affect the biology of living cells. These include oxidation of iron containing proteins such as ribonucleotide reductase and aconitase, activation of the soluble guanylate cyclase, ADP ribosylation of proteins, protein sulphhydryl group nitrosylation, and iron regulatory factor activation. NO has been demonstrated to activate NF-κB in peripheral blood mononuclear cells, an important transcription factor in iNOS gene expression in response to inflammation. . It was found that NO acts through the stimulation of the soluble guanylate cyclase which is a heterodimeric enzyme with subsequent formation of cyclic GMP. Cyclic GMP activates protein kinases and leads ultimately to the dephosphorylation of the myosin light chain.