<prompt> Nitric oxide is derived from which amino acid?HistidineLysineMethionineArginine  <response>: Ans:D.Arginine.NITRIC OXIDE NOENDOTHELIUM DERIVED RELAXING FACTOR EDRFNitric oxide, a vasodilator is formed by the action of nitric oxide synthase on Arginine. Synthesis:NO is formed by the action of Cytosolic enzyme NO synthase in endothelial neuronal cells.Substrate: Arginine, O 2 , NADPHCoenzyme: NADPH, FAD, FMN, Heme, tetrahydrobiopterinProduct: Citrulline, NOFeaturesReacts with oxygen superoxide ? peroxynitrite which decomposes to form highly reactive OH-radicalIt stimulates guanyl cyclase cGMPFunctionsRelaxes smooth muscles Prevents platelet aggregationFunctions as neurotransmitter in brainMediates tumoricidal, bactericidal actions of macrophagesInvolved in penile erection; sildenafil affects this process by inhibiting a cGMP dependent phosphodiesterase 