Nitric oxide synthases (NOS, E.C. 1.14.13.39) are a family of oxidoreductases that synthesize nitric oxide (NO.) via the NADPH and oxygen-dependent consumption of L-arginine with the resultant by-product, L-citrulline. There are 3 NOS isoforms and they are related by their capacity to produce NO, highly conserved organization of functional domains and significant homology at the amino acid level. NOS isoforms are functionally distinguished by the cell type where they are expressed, intracellular targeting and transcriptional and post-translation mechanisms regulating enzyme activity. The nomenclature suggested by NC-IUPHAR of NOS I, II and III [8] has not gained wide acceptance, and the 3 isoforms are more commonly referred to as neuronal NOS (nNOS), inducible NOS (iNOS) and endothelial NOS (eNOS) which reflect the location of expression (nNOS and eNOS) and inducible expression (iNOS). All are dimeric enzymes that shuttle electrons from NADPH, which binds to a C-terminal reductase domain, through the flavins FAD and FMN to the oxygenase domain of the other monomer to enable the BH4-dependent reduction of heme bound oxygen for insertion into the substrate, L-arginine. Electron flow from reductase to oxygenase domain is controlled by calmodulin binding to canonical calmodulin binding motif located between these domains. eNOS and nNOS isoforms are activated at concentrations of calcium greater than 100 nM, while iNOS shows higher affinity for Ca²⁺/calmodulin (CALM2, CALM3, CALM1, P62158) with great avidity and is essentially calcium-independent and constitutively active. Efficient stimulus-dependent coupling of nNOS and eNOS is achieved via subcellular targeting through respective N-terminal PDZ and fatty acid acylation domains whereas iNOS is largely cytosolic and function is independent of intracellular location. nNOS is primarily expressed in the brain and neuronal tissue, iNOS in immune cells such as macrophages and eNOS in the endothelial layer of the vasculature although exceptions in other cells have been documented. L-NAME and related modified arginine analogues are inhibitors of all three isoforms, with IC₅₀ values in the micromolar range.

The reductase domain of NOS catalyses the reduction of cytochrome c and other redox-active dyes [7]. NADPH:O₂ oxidoreductase catalyses the formation of superoxide anion/H₂O₂ in the absence of L-arginine and sapropterin.

* Key recommended reading is highlighted with an asterisk

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Subcommittee members: Andreas Papapetropoulos (Co-chairperson) Drug Discovery & Pharmacology Group Department of Pharmaceutical Chemistry Faculty of Pharmacy University of Athens Athens 15771, Greece Csaba Szabo, MD, PhD (Co-chairperson) Department of Anesthesiology The University of Texas Medical Branch at Galveston 601 Harborside Drive Galveston, TX 77555, USA Timothy R. Billiar Department of Surgery University of Pittsburgh Pittsburgh, PA, USA Giuseppe Cirino Department of Experimental Pharmacology University of Naples-Federico II Via Domenico Montesano 49 80131, Naples, Italy David Fulton Vascular Biology Center Georgia Regents University Sanders Building, CB 3940 Augusta, Georgia, USA Roberto Motterlini Director of Research (DR2) Equipe 12 INSERM U955, Faculty of Medicine University of Paris Est Creteil 94010 Creteil France