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 an air pollutant produced by cigarette smoke, 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 air pollutant. The nitric oxide molecule is a free radical, which is relevant to understanding its high reactivity.
Despite being a simple molecule, NO is a fundamental player in the fields of neuroscience, physiology, and immunology, and was proclaimed “Molecule of the Year” in 1992.