[5] Best studied are mammals, where three distinct genes encode NOS isozymes: neuronal (nNOS or NOS-1), cytokine-inducible (iNOS or NOS-2) and endothelial (eNOS or NOS-3).

NO activates guanylate cyclase, which induces smooth muscle relaxation by: eNOS plays a critical role in embryonic heart development and morphogenesis of coronary arteries and cardiac valves.

[10] Recently, NOS activity has been demonstrated in several bacterial species, including the notorious pathogens Bacillus anthracis and Staphylococcus aureus.

[15] As opposed to the critical calcium-dependent regulation of constitutive NOS enzymes (nNOS and eNOS), iNOS has been described as calcium-insensitive, likely due to its tight non-covalent interaction with calmodulin (CaM) and Ca2+.

[15] While evidence for ‘baseline’ iNOS expression has been elusive, IRF1 and NF-κB-dependent activation of the inducible NOS promoter supports an inflammation mediated stimulation of this transcript.

[16] Induction of the high-output iNOS usually occurs in an oxidative environment, and thus high levels of NO have the opportunity to react with superoxide leading to peroxynitrite formation and cell toxicity.

These properties may define the roles of iNOS in host immunity, enabling its participation in anti-microbial and anti-tumor activities as part of the oxidative burst of macrophages.

[17] It has been suggested that pathologic generation of nitric oxide through increased iNOS production may decrease tubal ciliary beats and smooth muscle contractions and thus affect embryo transport, which may consequently result in ectopic pregnancy.

[18] Endothelial NOS (eNOS), also known as nitric oxide synthase 3 (NOS3), generates NO in blood vessels and is involved with regulating vascular function.

[21] Bacterial NOS (bNOS) has been shown to protect bacteria against oxidative stress, diverse antibiotics, and host immune response.

Bacteria late in the log phase who do not possess bNOS fail to upregulate SodA, which disables the defenses against harmful oxidative stress.

Initially, bNOS may have been present to prepare the cell for stressful conditions but now seems to help shield the bacteria against conventional antimicrobials.

Binding of calmodulin appears to act as a "molecular switch" to enable electron flow from flavin prosthetic groups in the reductase domain to heme.

Unlike other enzymes where BH4 is used as a source of reducing equivalents and is recycled by dihydrobiopterin reductase (EC 1.5.1.33), BH4 activates heme-bound O2 by donating a single electron, which is then recaptured to enable nitric oxide release.

In NOS1 and NOS3, physiological concentrations of Ca2+ in cells regulate the binding of calmodulin to the "latch domains", thereby initiating electron transfer from the flavins to the heme moieties.

Specifically, NO has been shown to play an important negative feedback regulatory role on NOS3, and therefore vascular endothelial cell function.

[25] This process, known formally as S-nitrosation (and referred to by many in the field as S-nitrosylation), has been shown to reversibly inhibit NOS3 activity in vascular endothelial cells.

This process may be important because it is regulated by cellular redox conditions and may thereby provide a mechanism for the association between "oxidative stress" and endothelial dysfunction.

In addition, both NOS1 and NOS2 have been shown to form ferrous-nitrosyl complexes in their heme prosthetic groups that may act partially to self-inactivate these enzymes under certain conditions[citation needed].