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Vol. 280, Issue 3, 1312-1318, 1997
Section of Endocrinology, Metabolism and Nutrition, Minneapolis
Veterans Affairs Medical Center and the Department of Medicine,
University of Minnesota, Minneapolis, Minnesota
Liver phosphorylase a is stimulated by adenosine
monophosphate. It is inhibited by adenosine diphosphate, adenosine
triphosphate and glucose. Using these effectors as well as other
potential in vivo effectors at concentrations
approximating those present in hepatocytes, we previously reported that
the net effect was nil, i.e., at estimated in
vivo concentration, the inhibitors neutralized the stimulatory
effect of adenosine monophosphate in a phosphorylase a
preparation. In addition, a concentration dependent inhibition by
glucose was not present. Therefore, we were interested in determining
if addition of caffeine, an inhibitor that synergizes with glucose,
would result in a reduction in activity in the presence of the other
effectors and restore regulation by physiological concentrations of
glucose. The effect of xanthine and xanthine analogs also were studied.
Purified liver phosphorylase a was used. Activity was
measured in the direction of glycogenolysis at 37°, pH 7.0 and under
initial rate conditions. Caffeine (1 mM) was added to individual and
various combinations of other effectors. The interactions among the
potential in vivo effectors when caffeine was present
were complex. However, when caffeine was present glucose again
regulated activity. This most likely was due to a synergistically
facilitated reduction in binding affinity for AMP by caffeine and
glucose. Theophylline and adenosine did not inhibit activity but
reduced AMP stimulation and facilitated glucose inhibition. Xanthine
and the other xanthine derivatives all strongly inhibited activity and
the inhibition was independent of other effectors.
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