Nitric oxide, also known as nitrogen monoxide, is a molecule with chemical formula NO. It is a free radical and is an importantintermediate in the chemical industry. Nitric oxide is a by-product of combustion of substances in the air, as in automobile engines, fossil fuel power plants, and is produced naturally during the electrical discharges of lightning in thunderstorms.
In mammals including humans, NO is an important cellular signaling molecule involved in many physiological and pathological processes. It is a powerful vasodilator with a short half-life of a few seconds in the blood. Long-known pharmaceuticals likenitroglycerine and amyl nitrite were discovered, more than century after their first use in medicine, to be active through the mechanism of being precursors to nitric oxide.
Low levels of nitric oxide production are important in protecting organs such as the liver from ischemic damage. Chronic expression of NO is associated with various carcinomas and inflammatory conditions including Type-1 diabetes, multiple sclerosis, arthritis and ulcerative colitis.
Nitric oxide should not be confused with nitrous oxide (N2O), an anaesthetic and greenhouse gas, or with nitrogen dioxide (NO2), a brown toxic gas and a major air pollutant. However, nitric oxide is rapidly oxidised in air to nitrogen dioxide. Humphrey Davy discovered this to his discomfort, when he inhaled the gas early in his career.
Despite being a simple molecule, NO is an important biological regulator and is a fundamental component in the fields of neuroscience,physiology, and immunology, with discovery of its key roles leading to Nobel Prize winning research in these areas. It was proclaimed “Molecule of the Year” in 1992
Measurement of nitric oxide concentration
Nitric oxide concentration can be determined using a simple 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. This reaction also produces light (chemiluminescence), which can be measured with a photodetector. The amount of light produced is proportional to the amount of nitric oxide in the sample.
- NO + O3 → NO2 + O2 + light
Other methods of testing include electroanalysis (amperometric approach), where NO reacts with an electrode to induce a current or voltage change. The detection of NO radicals in biological tissues is particularly 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 (EPR).]