Four cyclic nucleotide phosphodiesterase (PDE) activities were separated from low-speed supernatants of homogenates of human cardiac ventricle by DEAE-Sepharose chromatography, and designated PDE I-PDE IV in order of elution with an increasing salt gradient. PDE I was a Ca2+/calmodulin-stimulated activity, and PDE II was an activity with a high Km for cyclic AMP which was stimulated by low concentrations of cyclic GMP. Human ventricle PDE III had Km values of 0.14 microM (cyclic AMP) and 4 microM (cyclic GMP), and showed simple Michaelis-Menten kinetics with both substrates. PDE IV is a previously unrecognized activity in cardiac muscle, the human enzyme having Km values of 2 microM (cyclic AMP) and 50 microM (cyclic GMP). PDE III and PDE IV were not activated by cyclic nucleotides or calmodulin. Four PDE activities were also isolated from guinea-pig ventricle, and had very similar kinetic properties. By gel filtration, the Mr of PDE III was 60,000, and that of PDE IV 45,000. The drug SK&F 94120 selectively and competitively inhibited PDE III with a Ki value of 0.8 microM (human), showing simple hyperbolic inhibition kinetics. Rolipram (Schering ZK 62711) and Ro 20-1724 (Roche), which have previously been reported to inhibit PDE III-like activities strongly, were shown to be weak inhibitors of human and guinea-pig PDE III enzymes (Ki values greater than 25 microM), but potent inhibitors of PDE IV [Ki values 2.4 microM (Rolipram) and 3.1 microM (Ro 20-1724) with human PDE IV]. The inhibition in all cases demonstrated simple hyperbolic competition. These observations suggest that the previously reported complex inhibition of PDE III-type activities from cardiac muscle was caused by incomplete separation of the PDE III from other enzymes, particularly PDE IV.

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