Table 1

Some characteristics of human nitric oxide synthases (NOS) (enzyme classification EC 1.14.23)

IsoformAlternative namesMolecular weight (kD) as monomer1-150 Intracellular locationHuman cellular sourcesPresence Activation
nNOSNeuronal NOS ncNOS
NOS1
NOS type I		155Membrane associatedNeurons, adrenal medullary, renal macula densa, glia, astrocytes, ConstitutiveCa++increase leading to calmodulin binding
iNOSInducible NOS NOS2
NOS type II		125–135CytosolicMacrophages, monocytes, leukocytes, endothelium, smooth muscle, neutrophils, retinal pigmented epithelium, astrocytes, microglial, hepatocytes, Küpffer cells, fibroblasts, mesangiumInducibleTranscriptional induction
Ca++ independent, calmodulin always bound
eNOSEndothelial NOS ecNOS
NOS3
NOS type III		135Membrane (inactive) Cytosolic (active)Endothelium, platelets, smooth muscleConstitutiveCa++ increase leading to calmodulin binding

  • ↵1-150 All enzymes active as dimers.