The complex conversion of arginine to citrulline and nitric oxide is shown in Figure 21.3. The enzyme catalyzing the reaction is nitric oxide synthase.

Nitric oxide, is involved in many signaling processes in the body. For example, it is a signal-transducing agent in the vasodilation of endothelial vascular cells and underlying smooth muscle. It is also involved in signaling decreases in blood pressure, and inhibiting platelet aggregation. In the inflammatory and immune responses, an inducible form of nitric oxide synthase produces nitric oxide at levels sufficient to be toxic to pathogenic organisms. Finally, it can act in neurotransmission in the central nervous system and stimulate erection of the penis.

Nitric oxide is a gas so it can diffuse rapidly into neighboring cells and control their metabolism. It is also unstable, with a half-life of 1 to 5 seconds, so its effects are short-lived. In the cell, nitric oxide acts primarily by stimulating cyclic GMP synthesis. The drug, Viagra, acts by inhibiting cyclic GMP breakdown, thereby prolonging the effect of nitric oxide.