Biochemistry and molecular biology of nitric oxide synthases

Many mammalian cells synthesize nitric oxide (NO). Three different isoforms of NO synthase have been characterized, purified and cloned. The identity of the three isoforms at the amino acid level is 50-60%, in cases where the same isoform has been cloned from more than one species > 90% identity is found between species. Isozyme I is present in neuronal cells of the brain (where NO may mediate synaptic plasticity), in peripheral neurons (where NO acts as an atypical neurotransmitter relaxing vascular and non-vascular smooth muscle), in specialized epithelial cells, and in human skeletal muscle. Macrophages are induced with bacterial endotoxin and/or cytokines to express isozyme II. The high concentrations of NO produced by this inform have cytostatic effects on parasitic microorganisms and tumor cells. Very similar isozymes can be induced in various other cells such as smooth muscle cells, mesangial cells, endothelial cells, fibroblasts, hepatocytes, etc. The induced enzyme from rat hepatocytes has been cloned and shows 94% sequence identity with the mouse macrophage enzyme. Endothelial cells contain isoform III of NO synthase which seems to be unique for this cell type. Endothelium-derived NO is a physiologically significant vasodilator and inhibitor of platelet aggregation and adhesion. NO can also prevent leukocyte adhesion to the endothelium, and has also been shown to inhibit the proliferation of vascular smooth muscle cells. Thus at least three distinct isoforms of NO synthase are responsible for NO formation in mammals.