Purification and characterization of a human platelet cyclic nucleotide phosphodiesterase

Abstract

A cyclic nucleotide phosphodiesterase was extensively purified from the 100,000 g supernatal fraction of human platelets. The purification was 2500- to 3000-fold with 30% recovery of activity. The enzyme was isolated by DEAE-cellulose chromatography followed by adsorption to blue dextran-Sepharose and elution with cAMP. The protein has a MW of 140,000 as determined by gel filtration. On NaDodSO4-containing polyacrylamide gels the major bands is at 61,000 daltons, suggesting that the enzyme may exist as a dimer in solution under nondenaturing conditions. The enzyme requires Mg2+ or Mn2+ for activity. The Ca binding protein calmodulin does not stimulate hydrolysis of cAMP by this enzyme. The purified enzyme hydrolyzes both cAMP and cGMP with normal Km values of 0.18 .mu.M and 0.02 .mu.M, respectively. The hydrolysis of cGMP is only 1/10 as rapid as the hydrolysis of cAMP. cGMP does not stimulate cAMP hydrolysis but instead is a potent competitive inhibitor of cAMP hydrolysis. The enzyme is also competitively inhibited by the phosphodiesterase inhibitors papaverine, 3-isobutyl-1-methylxanthine and dipyridamole. The enzyme did not cross-react with an antibody raised to a cAMP phosphodiesterase isolated from dog kidney, indicating that the enzymes are not immunologically related. The inhibition of cAMP hydrolysis by cGMP suggests a possible regulatory link between these two cyclic nucleotides. One of the roles of cGMP in platelets may be to potentiate increases in intracellular cAMP by inhibiting the hydrolysis of cAMP by this enzyme.