Allosteric sites of phosphodiesterase-5 (PDE5) - A potential role in negative feedback regulation of cGMP signaling in corpus cavernosum

To date, relative cellular levels of cGMP and cGMP-binding proteins have no t been considered important in the regulation of smooth muscle or any other tissue. In rabbit penile corpus cavernosum, intracellular cGMP was determi ned to be 18 +/- 4 nM, whereas the cGMP-binding sites of types I alpha and I beta cGMP-dependent protein kinase (PKG) and cGMP-binding cGMP-specific p hosphodiesterase (PDES) were 58 +/- 14 nM and 188 +/- 6 nM, respectively, a s estimated by two different methods for each protein. Thus, total cGMP-bin ding sites (246 nM) greatly exceed total cGMP. Given this excess of cGMP-bi nding sites and the high affinities of PKG and PDE5 for cGMP, it is Likely that a large portion of intracellular cGMP is associated with these protein s, which could provide a dynamic reservoir for cGMP. Phosphorylation of PDE 5 by PKG is known to increase the affinity of PDE5 allosteric sites for cGM P suggesting the potential for regulation of a reservoir of cGMP bound to t his protein. Enhanced binding of cGMP by phosphorylated PDE5 could reduce t he amount of cGMP available for activation of PKG, contributing to feedback inhibition of smooth muscle relaxation or other processes. This introduces a new concept for cyclic nucleotide signaling.