<prompt> Nitric oxide is derived from which amino acid-HistidineLysineMethionineArginine  <response>: Ans. is d i.e., Arginine Nitric oxide NOo Nitric oxide, also called endothelium-derived-relaxing factor EDRF, is formed from amino acid arginine, by the action of the enzyme NO synthase, which is cytosolic.NO synthase Arginine ???????????- Citrulline NOo Three isoforms of NO synthase have been identified :- Neuronal NOS nNOS or AOS-1; ii Macrophage or inducible NOS iNOS or NOS-2; and iii Endothelial NOS eNOS or NOS-3. NOS-1 and NOS-3 are constitutive enzymes, whereas NOS2 is inducible in inflammatory conditions,o NOS-1 and NOS-3 are activated by Ca2.o NO synthase is a very complex enzyme, employing five redox cofactors : NADPH. FAD, FMN, heme, and tetrahydrobiopterin.o The most important physiological source of NO is endothelial ceils.o NO has a very short half-life approximately 3-4 seconds in tissues because it reacts with oxygen and superoxide. The product of reaction with superoxide is peroxynitirite ONOO, which decomposes to form the highly reactive OH radical. NO acts through group IIB hormonal mechanisms, i.e., through cGMPas the second messenger. 