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1 Department of Kinesiology, University of Waterloo, Waterloo, Ontario N2L 3G1; and 2 Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario, Canada N1G 2W1
This study aimed to determine physiologically relevant kinetic and allosteric effects of Pi, AMP, ADP, and caffeine on isolated skeletal muscle glycogen phosphorylase a (Phos a). In the absence of effectors, Phos a had Vmax = 221 ± 2 U/mg and Km = 5.6 ± 0.3 mM Pi at 30°C. AMP and ADP each increased Phos a Vmax and decreased Km in a dose-dependent manner. AMP was more effective than ADP (e.g., 1 µM AMP vs. ADP: Vmax = 354 ± 2 vs. 209 ± 8 U/mg, and Km = 2.3 ± 0.1 vs. 4.1 ± 0.3 mM). Both nucleotides were relatively more effective at lower Pi levels. Experiments simulating a range of contraction (exercise) conditions in which Pi, AMP, and ADP were used at appropriate physiological concentrations demonstrated that each agent singly and in combination influences Phos a activity. Caffeine (50-100 µM) inhibited Phos a (Km ~8-14 mM, ~40-50% reduction in activity at 2-10 mM Pi). The present in vitro data support a possible contribution of substrate (Pi) and allosteric effects to Phos a regulation in many physiological states, independent of covalent modulation of the percentage of total Phos in the Phos a form and suggest that caffeine inhibition of Phos a activity may contribute to the glycogen-sparing effect of caffeine.
glycogenolysis; enzyme kinetics; allosteric control; adenosine 5'-phosphate; adenosine 5'-diphosphate; physiological biochemistry; inorganic phosphate
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