Masaki Nakane

Three isozymes of nitric oxide (NO) synthase (EC 1.14.13.39) have been identified and the cDNAs for these enzymes isolated. In humans, isozymes I (in neuronal and epithelial cells), II (in cytokine-induced cells), and III (in endothelial cells) are encoded for by three different genes located on chromosomes 12, 17, and 7, respectively. The deduced amino acid sequences of the human isozymes show less than 59% identity. Across species, amino acid sequences for each isoform are well conserved (> 90% for isoforms I and III, > 80% for isoform II). All isoforms use L-arginine and molecular oxygen as substrates and require the cofactors NADPH, 6(R)-5,6,7,8-tetrahydrobiopterin, flavin adenine dinucleotide, and flavin mononucleotide. They all bind calmodulin and contain heme. Isoform I is constitutively present in central and peripheral neuronal cells and certain epithelial cells. Its activity is regulated by Ca2+ and calmodulin. Its functions include long-term regulation of synaptic transmission in the central nervous system, central regulation of blood pressure, smooth muscle relaxation, and vasodilation via peripheral nitrergic nerves. It has also been implicated in neuronal death in cerebrovascular stroke. Expression of isoform II of NO synthase can be induced with lipopolysaccharide and cytokines in a multitude of different cells. Based on sequencing data there is no evidence for more than one inducible isozyme at this time. NO synthase II is not regulated by Ca2+; it produces large amounts of NO that has cytostatic effects on parasitic target cells by inhibiting iron-containing enzymes and causing DNA fragmentation. Induced NO synthase II is involved in the pathophysiology of autoimmune diseases and septic shock. Isoform III of NO synthase has been found mostly in endothelial cells. It is constitutively expressed, but expression can be enhanced, eg, by shear stress. Its activity is regulated by Ca2+ and calmodulin. NO from endothelial cells keeps blood vessels dilated, prevents the adhesion of platelets and white cells, and probably inhibits vascular smooth muscle proliferation.

The particulate enzyme responsible for the synthesis of endothelium-derived relaxing factor has been purified from cultured and native (noncultured) bovine aortic endothelial cells. Purification of the solubilized particulate enzyme preparation by affinity chromatography on adenosine 2',5'-bisphosphate coupled to Sepharose followed by Superose 6 gel filtration chromatography resulted in a single protein band after denaturing polyacrylamide gel electrophoresis that corresponded to approximately 135 kDa. The enzyme activity in the various fractions was assayed by its stimulatory effect on soluble guanylyl cyclase of rat fetal lung fibroblasts (RFL-6 cells), by the formation of L-citrulline from L-arginine, by measuring nitrite/nitrate formation, and by bioassay on endothelium-denuded vascular strips. Endothelium-derived relaxing factor synthase was purified 3419-fold from the crude particulate fraction of cultured bovine aortic endothelial cells with a 12% recovery (RFL-6 assay). Purified endothelium-derived relaxing factor synthase required L-arginine, NADPH, Ca2+, calmodulin, and 5,6,7,8-tetrahydrobiopterin for full activity.

Nitric oxide synthases (NOS Types I-III) generate nitric oxide (NO), which in turn activates soluble guanylyl cyclase (GC-S). The distribution of this NO-mediated (nitrinergic) signal transduction pathway in the body is unclear. A polyclonal monospecific antibody to rat cerebellum NOS-I and a monoclonal antibody to rat lung GC-S were employed to localize the protein components of this pathway in different rat organs and tissues. We confirmed the localization of NOS-I in neurons of the central and peripheral nervous system, where NO may regulate cerebral blood flow and mediate long-term potentiation. GC-S was located in NOS-negative neurons, indicating that NO acts as an intercellular signal molecule or neurotransmitter. However, NOS-I was not confined to neurons but was widely distributed over several non-neural cell types and tissues. These included glia cells, macula densa of kidney, epithelial cells of lung, uterus, and stomach, and islets of Langerhans. Our findings suggest that NOS-I is the most widely distributed isoform of NOS and, in addition to its neural functions, regulates secretion and non-vascular smooth muscle function. With the exception of bone tissue, NADPH-diaphorase (NADPH-d) activity was generally co-localized with NOS-I immunoreactivity in both neural and non-neural cells, and is a suitable histochemical marker for NOS-I but not a selective neuronal marker.

Complementary DNA clones corresponding to human brain nitric oxide (NO) synthase have been isolated. The deduced amino acid sequence revealed an overall identity with rat brain NO synthase of about 93% and contained all suggested consensus sites for binding of the co-factors. The cDNA transfected COS-1 cells showed significant NO synthase activity with the typical co-factor requirements. Unexpectedly, messenger RNA levels of this isoform of NO synthase was more abundant in human skeletal muscle than human brain. Moreover, we detected high NO synthase activity and the expressed protein in human skeletal muscle by Western blot analysis, indicating a possible novel function of NO in skeletal muscle.