an enzyme that catalyzes the reaction of l-arginine with 2O2 and 1.5NADPH to form NO, l-citrulline, 1.5NADP+, and 2H2O; there are an inducible and two constitutive forms of this enzyme: the constitutive forms play significant roles in regulating vascular tone, tissue blood flow, renal function, and others; in bone, brain, endothelium, granulocytes, pancreatic Z-cells, and peripheral nerves, the constitutive forms depend on calcium-calmodulin; in brain, the enzyme is cytosolic; in endothelium, it is membrane bound; the inducible form of the enzyme (e.g., by endotoxin) in hepatocytes, Kupffer cells, macrophages, and smooth muscle is not calmodulin dependent.