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Theophylline effect on the cyclic AMP degrading multienzyme sequence.

Biochemical Pharmacology

|November 1, 1982

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Theophylline competitively inhibits cyclic nucleotide phosphodiesterase and noncompetitively inhibits nucleotidase within a beef adrenal cortex enzyme cluster. This suggests adenosine, derived from cyclic AMP, may be physiologically active.

Area of Science:

Biochemistry
Enzymology
Molecular Biology

Background:

Membrane-bound 3'.5'-cyclic nucleotide phosphodiesterase (EC 3.1.4.17) is physically associated with nucleotidase and deaminase.
This enzyme cluster in beef adrenal cortex microsomes degrades cyclic AMP to inosine via AMP and adenosine.
Theophylline is a known inhibitor of phosphodiesterase, but its effects on the entire enzyme cluster were not fully understood.

Purpose of the Study:

To investigate the effect of theophylline on the membrane-bound multienzyme sequence.
To elucidate the mechanism of theophylline's inhibition on phosphodiesterase, nucleotidase, and deaminase activities.
To propose a model for theophylline's action on the enzyme cluster and discuss the physiological implications for adenosine production.

Main Methods:

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Enzyme kinetics studies were performed to analyze the inhibition patterns.
Competitive and noncompetitive inhibition kinetics were determined for phosphodiesterase and nucleotidase, respectively.
Deaminase activity was assessed to confirm the absence of inhibition.

Main Results:

Theophylline competitively inhibited phosphodiesterase, with mM concentrations affecting the high-affinity enzyme.
Phosphodiesterase activity was attributed to two enzymes with distinct Km values (0.3 microM and 60 microM).
Theophylline exhibited hyperbolic noncompetitive inhibition of nucleotidase (Ki = 0.25 mM) but did not inhibit deaminase.

Conclusions:

Theophylline modulates the enzyme cluster's behavior by acting on a single allosteric site.
A model is proposed where theophylline affects the kinetic behavior of the entire enzyme cluster.
The findings support the possibility that physiologically active adenosine is derived from cyclic AMP degradation by this enzyme cluster.