|MoBio > Signaling via Hydrophobic Molecules|
Nitric oxide (NO) is produced from the following reaction:
where NOS represents NO synthase. After NO is generated in a cell, it diffuses to act on target molecules in neighboring cells. NO may stimulate soluble guanylyl cyclase to produce cGMP which regulates several enzymes and ion channels. In smooth muscle, an important action of cGMP is to induce muscle relaxation. Normally, cGMP will soon be converted into GMP by phosphodiesterase. The well known drug for impotence, Viagra (sildenafil citrate), inhibits phosphodiesterase.
Figure 6-B-2. Structures of cAMP and cGMP.
Arachidonic acid is generated from phospholipid hydrolysis catalyzed by phospholipase. Like NO, its target is usually not located in the cell where it was first generated. After diffusing to target cells, arachidonic acid may activate protein kinase, resulting in phosphorylation of target molecules. Many of its target molecules are involved in learning and other neuronal activities.
The major role of steroids is to regulate transcription, since many steroid receptors are transcription factors.