The role of nitric oxide in cellular signaling in the past two decades has become one of the most rapidly growing areas in biology.Nitric oxide is a gas a free radical with an unshared electron that can regulate an ever-growing list of biological processes.Nitric oxide is formed from L-arginine by a family of enzymes called nitric oxide synthases.These enzymes have a complex requirement for a number of cofactors and regulators including NADPH,tetrahydrobioterin,flavins,calmodulin and heme.The enzymes are present in most cells and tissues.In many instances,nitric oxide mediates its biological effects by activating the soluble isoform of guanylyl cyclase and increasing cyclic GMP synthesis from GTP.Cyclic GMP,in turn,can activate cyclic GMP-dependent protein kinase (PKG) and can cause smooth muscles and blood vessels to relax,decrease platelet aggregation,alter neuron function,etc.