Catalytic and kinetic properties of purified high-affinity cyclic AMP phosphodiesterase from dog kidney |
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Authors: | P M Epstein S J Strada K Sarada W J Thompson |
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Affiliation: | 1. Department of Pharmacology, University of Connecticut Health Center, Farmington, Connecticut 06032 U.S.A.;2. Department of Pharmacology, University of Texas Medical School, Houston, Texas 77030 U.S.A. |
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Abstract: | High-affinity cyclic AMP phosphodiesterase purified to homogeneity from dog kidney was studied with respect to its stability, its catalytic and kinetic properties, and its sensitivity to pharmacological agents. The enzyme was shown to rapidly lose activity upon dilution to low protein concentrations in aqueous media, but this activity loss was largely prevented by the presence of bovine serum albumin or ethylene glycol. Similarly, maximum activity required bovine serum albumin to be present during incubation for activity analysis. Enzyme activity required a divalent cation; Mg2+, Mn2+, and Co2+ each supported activity, but highest activity was obtained with Mg2. The temperature optimum ranged from 30 to 45 °C and depended on substrate concentration; the Ea = 10,600 cal/mol. The pH optimum of the enzyme was broad, with a maximum from pH 8.0 to 9.5. The enzyme exhibits linear Michaelis-Menton kinetics for hydrolysis of cyclic AMP at all substrate concentrations tested and for hydrolysis of cyclic GMP at > 20 μm. The Km for cyclic AMP hydrolysis was 2 μm, and that for cyclic GMP hydrolysis was 312 μm. The Ki values for the competitive inhibition of hydrolysis of each substrate by the other were similar to their Km values suggesting a single active site. Cyclic AMP hydrolysis was weakly inhibited by cyclic GMP, cyclic IMP, adenine, and adenosine, but was not inhibited by the mono-, di, or trinucleotides of adenosine, guanosine, or inosine. Activity was competitively inhibited with Ki values in the micromolar range by drugs representative of methylxanthines, isoquinolines, pyrazolopyridines, imidazolidinones, triazolopyrimidines, pyridylethylenediamines, phenothiazines, and calcium antagonists. The results are discussed with reference to the similarities and differences between high- and low-affinity phosphodiesterase forms. |
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