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Biological functions of nitric oxide
The gas nitric oxide plays a wide variety of roles in biological organisms.
==Nitric oxide synthesis==
Nitric oxide synthases (NOSs) synthesize the metastable free radical nitric oxide (NO). There are three isoforms of the NOS enzyme: endothelial (eNOS), neuronal (nNOS), and inducible (iNOS) - each with separate functions. The neuronal enzyme (NOS-1) and the endothelial isoform (NOS-3) are calcium-dependent and produce low levels of this gas as a cell signaling molecule. The inducible isoform (NOS-2) is calcium-independent and produces large amounts of gas that can be cytotoxic. NOS oxidizes the guanidine group of L-arginine in a process that consumes five electrons and results in the formation of NO with stoichiometric formation of L-citrulline. The process involves the oxidation of NADPH and the reduction of molecular oxygen. The transformation occurs at a catalytic site adjacent to a specific binding site of L-arginine.〔Ignarro L.J. (2001): Nitric Oxide. A Novel Signal Transduction Mechanism For Transcellular Communication; 16: 477- 483.〕
NO is an important regulator and mediator of numerous processes in the nervous, immune, and cardiovascular systems. These include vascular smooth muscle relaxation, resulting in arterial vasodilation and increasing blood flow.〔Weller, Richard, (Could the sun be good for your heart? ) TedxGlasgow March 2012, posted January 2013〕 NO is also a neurotransmitter and has been associated with neuronal activity and various functions such as avoidance learning. NO also partially mediates macrophage cytotoxicity against microbes and tumor cells. Besides mediating normal functions, NO is implicated in pathophysiologic states as diverse as septic shock, hypertension, stroke, and neurodegenerative diseases.〔Davies, S.A., Stewart, E.J., Huesmaan, G.R and Skaer, N. J. (1997): Neuropeptide stimulation of the nitric oxide signalling pathway in Drosophila melanogaster Malpighian tubules. ''Am. J. Physiol.''.; 273, R823-827.〕
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