Intact cell and cell-free phosphorylation and concomitant activation of a low Km, cAMP phosphodiesterase found in human platelets.

A monoclonal antibody (CGI-5) directed against the cGMP-inhibited phosphodiesterase isolated from bovine heart was used to examine the phosphorylation of this isozyme in human platelets. PGE1 promoted the phosphorylation of this isozyme, identified as a 110 kDa peptide following SDS-gel electrophoresis. Phosphorylation resulted in approximately a 40% increase in the cGMP-inhibited phosphodiesterase activity. Cell-free experiments demonstrated that cAMP-dependent protein kinase phosphorylated the cGMP-inhibited phosphodiesterase, and that this could be blocked by the heat stable inhibitor peptide (PKI). Phosphorylation of the cGMP-inhibited phosphodiesterase increases the Vmax for cAMP hydrolysis approximately 50%, but does not affect the Km for cAMP (0.12 microM).