Nitric oxide is a gas. It is highly reactive; that is, it participates in many chemical reactions. (It is one of the nitrogen oxides (“NOx“) in automobile exhaust and plays a major role in the formation of photochemical smog [Link].)
But NO also has many physiological functions.
They share these features:
- NO is synthesized within cells by an enzyme NO synthase (NOS).
- The human (and mouse) genome contains 3 different genes encoding NO synthases.
- nNOS (or NOS-1): found in neurons (hence the “n”).
- eNOS (or NOS-3): found in the endothelial (hence the “e”) cells that line the lumen of blood vessels.
- iNOS (or NOS-2): found in macrophages. (the “i” stands for “inducible”). Whereas the levels of nNOS and eNOS are relatively steady, expression of iNOS genes awaits an appropriate stimulus (e.g., invasion by a pathogen).