Comparison of Properties of the Cellular and Extracellular Phosphodiesterases Induced by Cyclic Adenosine 3′,5′-Monophosphate in Dictyostelium discoideum

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Abstract
The kinetic properties and susceptibilities to various agents of intracellular (particulate and soluble) and extracellular phosphodiesterases [EC 3.1.4.17] of Dictyostelium discoideum induced by cyclic adenosine 3′,5′-monophosphate (cyclic AMP) were studied and compared. Intracellular particulate phosphodiesterase was obtained by solubilization of the light mitochondrial fraction with Emulgen. The Michaelis constants of this enzyme were 4.5±0.7 and 10±0.7 μM, while those of the intracellular soluble phosphodiesterase were 4.6±0.3 and 13±2.8 μM. However, the Michaelis constant of the extracellular phosphodiesterase was 6.8±0.9 μM, differing from the values of the two intracellular enzymes. Susceptibilities of the enzyme activity to various agents (theophylline, caffeine, dithiothreitol, glutathione, etc .) were essentially the same among these three phosphodiesterases. In the presence of 10 mM ethylenediaminetetraacetate, the activities of the particulate and the soluble enzymes were both decreased to about 60±, while that of the extracellular enzyme remained at 90±. The inhibition constants of cyclic inosine monophosphate for the cellular enzymes (35 and 100 μM for the particulate enzyme, and 37 and 90 μM for the soluble one) were considerably different from the value for the extracellular enzyme (48 μM). These results suggest that the characteristics of these three phosphodiesterases are substantially similar, but that the affinity of the intracellular (particulate and soluble) enzymes for the substrate is somewhat different from that of the extracellular enzyme.

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