THE CATALYTIC SUBUNIT OF PROTEIN KINASE-A TRIGGERS ACTIVATION OF THE TYPE-V CYCLIC GMP-SPECIFIC PHOSPHODIESTERASE FROM GUINEA-PIG LUNG

The type V cyclic GMP phosphodiesterase was partially purified from the high-speed supernatant of guinea-pig lung. The isoenzyme displayed linear kinetics for cyclic GMP hydrolysis, with K(m) = 2.2 +/- 0.2-mu-M and V(max.) = 1.3 +/- 0.08 nmol/min per mg. The selective type V phosphodiesterase inhibitor Zaprinast inhibited cyclic GMP hydrolysis with IC50 (concon. giving 50% inhibition) = 0.45 +/- 0.08-mu-M. Isobutylmethylxanthine promoted a 3-fold increase in the binding of cyclic GMP to the isoenzyme. The addition of the catalytic subunit of protein kinase A to an activation cocktail containing the partially purified type V phosphodiesterase resulted in a marked increase in V(max.) for cyclic GMP hydrolysis (approximately 10-fold at 40 units of protein kinase A). We have suggested that protein kinase A triggers phosphorylation of the phosphodiesterase, which results in activation of phosphodiesterase activity. In addition, the sensitivity to inhibition by Zaprinast is severely decreased (the IC50 for inhibition is 7.5 +/- 1.1-mu-M), suggesting that the potency of phosphodiesterase inhibitors is effected by phosphorylation of the enzyme.