Binding of inhibitor alters kinetic and physical properties of extracellular cyclic AMP phosphodiesterase from Dictyostelium discoideum.

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Abstract
The extracellular cyclic(c)AMP phosphodiesterase (3'',5''-cyclic-nucleotide 5''-nucleotidohydrolase, EC 3.1.4.17) produced by D. discoideum has 2 kinetic forms. The free enzyme has a Km of approximately 10 .mu.M. The 2nd form is the result of a complex formed with a heat-stable inhibitor and has a Km in the millimolar range. Treating the enzyme-inhibitor complex with dithiothreitol stimulated enzyme activity 20- to 100-fold and changed the Km from millimolar to micromolar. Dithiothreitol inactivated the inhibitor. Reconstituting purified enzyme with excess inhibitor returned the Km to the millimolar range. Under conditions known to inhibit the production of extracellular inhibitor or in mutants that lack it, extracellular phosphodiesterase activity was already high and could not be increased by dithiothreitol. The phosphodiesterase and inhibitor sedimented at 6 S and 3 S, respectively; the enzyme-inhibitor complex sedimented at 6.7 S.