Nitric oxide synthase

Nitric oxide (NO) is the only known gaseous biologically active signaling molecule. The ubiqitous nature of the NO molecule in physiological systems makes it and the family of enzymes that produced it, the nitric oxide sythases (NOSes), of great medical importance. It has been found to act in seemingly contradictory manners acting both in normal regulation and pathological conditions when over stimulated. Research of the NOSes has revealed it to be a unique enzyme family. Although NO and NOS activity is a field that has seen a prolific rate of research, especially over the last decade.

Isoforms
The nitric oxide synthase family of enzymes consists of three isoforms and their splice variants. The isoforms are produced by different genes, have different localizations and variable inhibitor sensitivities. The human isoforms have 51-57% homology between them, making them fairly similar1. Neuronal NOS was discovered first and predominates in neural tissues. It is referred to as nNOS or NOS-I. The second isoform to be isolated was iNOS, so called because it is inducible in a wide range of cell types. It is also referred to as NOS-II. In endothelial cells eNOS (NOS-III) is found to be prevalent. Splice variants of the three isoforms have also been identified and have slightly different characteristics than the three isoforms.2