Characterization of Human Placental Cytosolic Adenosine 3′,5′-Monophosphate Phosphodiest erase by Inhibitors and Insulin Treatment*

To gain insight into the regulation of low Km cAMP phosphodiesterases (PDE) by insulin in human tissues, PDEs in human placenta were studied. Human placenta contained cAMP PDEs in particulate and cytosolic fractions. More than 99% of the total activity was localized in the cytosolic fraction. The cytosolic fraction exhibited at least four cyclic nucleotide PDEs when fractionated by DEAE-cellulose chromatography. The first form was a calmodulin-activated PDE which hydrolyzed both cGMP and cAMP. The second form was a high affinity cAMP PDE with a nonlinear kinetic characteristic, but was not inhibited by either cGMP or cilostamide (either compound is known to specifically inhibit rat insulin-sensitive cAMP PDE). The third form was a low Km cAMP PDE, but was only modestly sensitive to inhibition by cGMP or cilostamide. The fourth form was a cAMP PDE which showed high sensitivity to inhibition by cGMP or cilostamide. The IC50 values of the fourth form were comparable to those of rat adipose insulin-sensitive PDE. However, its Km for cAMP was 2 microM, which is about 10 times higher than that of the rat enzyme. Insulin treatment on placenta tissues stimulated at least two PDEs, the third and fourth forms. To our knowledge, this is the first report to describe insulin-sensitive cAMP PDEs in the cytosolic fraction of human placenta.