Nitric oxide synthase NOS oxidizes a guanidine nitrogen of arginine releasing nitric oxide in the form of a free radical and citrulline. Nitric oxide thus generated acts as a messenger in diverse functions including vasodilation neurotransmission, anti tumor and anti pathogenic activities. NOS is classified under three types: neuronal NOS (nNOS) or brain NOS (bNOS); inducible NOS (iNOS) or macrophage NOS (mNOS); and endothelial NOS (eNOS).

eNOS is a calcium/calmodulin dependent enzyme which undergoes several post translational modifications, including acylation with myristate and palmitate, and phosphorylation on numerous residues. As with the other members of the NOS family, eNOS derives the diffusible multifunctional second messenger NO from L arginine through a series of reactions in which L citrulline is an intermediate. eNOS plays an important role in controlling vascular tone, platelet aggregation, and cardiac myocyte function.