cAMP-mediated phosphorylation of the low-Km cAMP phosphodiesterase markedly stimulates its catalytic activity.

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Abstract
Treatment of intact human platelets with the adenylate cyclase agonist forskolin (100 .mu.M) resulted in an increase in cAMP phosphodiesterase activity in freeze-thaw lysates. When the low-Km (high affinity), cGMP-inhibited cAMP phosphodiesterase was isolated from such lysates by blue dextran-Separose chromatography, the specific activity of the enzyme was increased an average of 11-fold over similarly processed control platelets. The increase in the low-Km, cGMP-inhibited cAMP phosphodiesterase activity was inhibited when platelets were incubated with the protein kinase inhibitor H-8 prior to treatment with forskolin, suggesting that the stimulation of cAMP phosphodiesterase activity involved a cAMP-dependent phosphorylation. When intact platelets that had been prelabeled with 32Pi were treated with forskolin and the low-Km, cGMP-inhibited phosphodiesterase was isolated by blue dextran-Sepharose chromatography, a protein of 110,000 kDa was phosphorylated. By using a monospecific antiserum to the purified phosphodiesterase, this protein was shown to be the low-Km cGMP-inhibited cAMP phosphodiesterase by electrophoretic transfer blot (Western blot) analysis and by immunoprecipitation. The stable prostacyclin analog iloprost also stimulated the low-Km cAMP phosphodiesterase activity about 2-fold and caused phosphorylation of the enzyme. These results suggest that phosphorylation of the low-Km, cGMP-inhibited phosphodiesterase may be an important regulatory mechanism for this enzyme in platelets.