3',5'-cyclic nucleotide phosphodiesterase from human brain

3':5'-Cyclic nucleotide phosphodiesterase was isolated from human brain and characterized. After the first stage of purification on phenyl-Sepharose, the enzyme activity was stimulated by Ca2+ and micromolar concentrations of cGMP. High pressure liquid chromatography on a DEAE-TSK-3SW column permitted to identify three ranges of enzymatic activity designated as PDE I, PDE II and PDE III. Neither of the three enzymes possessed a high selectivity for cAMP and cGMP substrates. The catalytic activity of PDE I and PDE II increased in the presence of Ca2+-calmodulin (up to 6-fold); the degradation of cAMP was decreased by cGMP. The Ca2+-calmodulin stimulated PDE I and PDE II activity was decreased by W-7. PDE I and PDE II can thus be classified as Ca2+-calmodulin-dependent phosphodiesterases. With cAMP as substrate, the PDE III activity increased in the presence of micromolar concentrations of cGMP (up to 10-fold), Ca2+ and endogenous calmodulin (up to 2-3-fold). No additivity in the effects of saturating concentrations of these compounds on PDE III was observed. Ca2+ did not influence the rate of cGMP hydrolysis catalyzed by PDE III. In comparison with PDE I and PDE II, the inhibition of PDE III was observed at higher concentrations of W-7 and was not limited by the basal level of the enzyme. These results do not provide any evidence in favour of the existence of several forms of the enzyme in the PDE III fraction. The double regulation of PDE III creates some difficulties for its classification.