Enzymology. An enzyme that catalyses the conversion of arginine, NADPH, and oxygen to citrulline, nitric oxide, and NADP+. Three distinct isoforms of nitric oxide synthase (NOS) are found in mammals:
neuronal NOS (nNOS or nitric oxide synthase type I) which is a calcium-dependent form found primarily in nerve tissue;
eNOS and nNOS are constitutive isoforms which are always present whereas the expression of iNOS can be induced by cytokines (e.g., interferon-gamma, tumor necrosis factor, interleukin). The stimuli and conditions that determine iNOS expression are cell and species specific.