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The chemical compound nitric oxide is a gas with chemical formula NO. It is an important signaling molecule in the body of mammals including humans, one of the few gaseous signaling molecules known. It is also a toxic air pollutant produced by automobile engines and power plants.
Nitric oxide (NO) should not be confused with nitrous oxide (N2O) or with nitrogen dioxide (NO2).
The nitric oxide molecule is a free radical which makes it very reactive and unstable. In air, it quickly reacts with oxygen to form the poisonous nitrogen dioxide.
Production and environmental effects
At high temperatures molecular nitrogen and oxygen can combine to form nitric oxide. A major natural source is lightning. Human activity has drastically increased the production of nitric oxide in combustion chambers. One purpose of catalytic converters in cars is to partially reverse this reaction.
Nitric oxide in the air may later convert to nitric acid which has been implicated in acid rain. Furthermore, both NO and NO2 participate in the ozone layer depletion.
Technical applications
Nitric oxide has few industrial uses. It is an intermediate of the Ostwald process which converts ammonia into nitric acid.
Nitric oxide can be used for detecting surface radicals on polymers. Quenching of surface radicals with nitric oxide results in incorporation of nitrogen, which can be quantified by means of X-ray photoelectron spectroscopy.
Biological functions
See also: Endothelium-derived relaxing factor (EDRF) and signal transduction
In the body, nitric oxide is synthesized from arginine and oxygen by the enzyme nitric oxide synthase (NOS).
The endothelium (inner lining) of blood vessels use nitric oxide to signal the surrounding smooth muscle to relax, thus dilating the artery and increasing blood flow. This underlies the action of nitroglycerin, amyl nitrate and other nitrate derivatives in the treatment of heart disease: the compounds are converted to nitric oxide (by a process that is not completely understood) which in turn dilates the coronary artery (blood vessels around the heart), thereby increasing its blood supply. Nitric oxide also plays a role in erection of the penis, and explains the mechanism of sildenafil (Viagra®). The effects of the recreational drugs known as poppers are also thought to be due to nitric oxide.
Macrophages, certain cells of the immune system, produce nitric oxide in order to kill invading bacteria. Under certain conditions, this can backfire: fulminant infection (sepsis) causes excess production of nitric oxide by macrophages, leading to vasodilatation (widening of blood vessels) and probably being one of the main causes of hypotension (low blood pressure) in sepsis.
Nitric oxide also serves as a neurotransmitter between nerve cells. Unlike most other neurotransmitters that only transmit information from a presynaptic to a postsynaptic neuron, the small nitric oxide molecule can diffuse all over and can thereby act on several nearby neurons, even on those not connected by a synapse. It is conjectured that this process may be involved in memory through the maintenance of long-term potentiation.
Production of NO also plays a role in development and maintenance of erection by stimulating PDE5-related intracellular cGMP in the smooth muscle cells surrounding the blood vessels supplying the corpus cavernosum; through relaxation of these muscles, more blood can flow in.
The discovery of the biological functions of nitric oxide in the 1980s came as a complete surprise and caused quite a stir. Nitric oxide was named "Molecule of the Year" in 1992 by the journal Science, a Nitric Oxide Society was founded, and a scientific journal devoted entirely to nitric oxide was created. The Nobel Prize in Physiology or Medicine in 1998 was awarded to Ferid Murad, Robert F. Furchgott, and Louis Ignarro for the discovery of the signalling properties of nitric oxide. It is estimated that yearly about 3,000 scientific articles about the biological roles of nitric oxide are published.
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