Cyclic guanosine monophosphate-specific phosphodiesterase type 5 is an enzyme () from the phosphodiesterase class. It is found in various tissues, most prominently the corpus cavernosum and the retina. It has also been recently discovered to play a vital role in the cardiovascular system. The phosphodiesterase (PDE) isozymes, found in several tissues including the rod and cone photoreceptor cells of the retina, belong to a large family of cyclic nucleotide PDEs that catalyze cAMP and cGMP hydrolysis. The interest in PDEs as molecular targets of drug action has grown with the development of isozyme-selective PDE inhibitors that offer potent inhibition of selected isozymes without the side-effects attributed to nonselective inhibitors such as theophylline. Sildenafil, vardenafil, tadalafil, and avanafil are PDE5 inhibitors that are significantly more potent and selective than zaprinast and other early PDE5 inhibitors. Action of PDE5 PDE5 is an enzyme that accepts cGMP and breaks it down. Sildenafil, vardenafil and tadalafil are inhibitors of this enzyme, which bind to the catalytic site of PDE5. Both inhibitors bind with high affinity and specificity, and cGMP-binding to the allosteric sites stimulates binding of PDE5 inhibitors at the catalytic site. The kinetics of inhibitor binding and inhibition of catalysis imply the existence of two PDE5 conformers, and results of native gel electrophoresis reveal that PDE5 exists in two apparently distinct conformations, i.e., an extended conformer and a more compact conformer. PDE5 activity is modulated by a rapidly reversible redox switch. Chemical reduction of PDE5 relieves autoinhibition of enzyme functions; allosteric cGMP-binding activity is increased 10-fold, and catalytic activity is increased ~3-fold. The redox effect on allosteric cGMP-binding occurs in the isolated regulatory domain. A change in the state of reduction of PDE5 or the isolated regulatory domain is associated with an apparent conformational change similar to that caused by phosphorylation. Tissue distribution of PDE5 PDE5 is expressed in human colonic cells and in intestinal tissue and its activity is regulated by intracellular cGMP levels in these cells that increase on GCC activation. This presumably occurs through binding of cGMP to the GAF domains in the N-terminus of PDE5, resulting in allosteric activation of the enzyme. The mechanism of action of E4021 on both the nonactivated and activated forms of rod PDE6 because both states are relevant to understanding how PDE5-selective inhibitors may alter signal transduction pathways in photoreceptor cells. PDE5-selective inhibitors may show good discrimination of PDE5 from most other PDE isoforms.