Skeletal muscle glycogen phosphorylase a kinetics: effects of adenine nucleotides and caffeine

J Appl Physiol (1985). 2001 Nov;91(5):2071-8. doi: 10.1152/jappl.2001.91.5.2071.


This study aimed to determine physiologically relevant kinetic and allosteric effects of P(i), 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 P(i) at 30 degrees 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 microM 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 P(i) levels. Experiments simulating a range of contraction (exercise) conditions in which P(i), AMP, and ADP were used at appropriate physiological concentrations demonstrated that each agent singly and in combination influences Phos a activity. Caffeine (50-100 microM) inhibited Phos a (Km approximately 8-14 mM, approximately 40-50% reduction in activity at 2-10 mM P(i)). The present in vitro data support a possible contribution of substrate (P(i)) 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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adenine Nucleotides / pharmacology*
  • Adenosine Diphosphate / pharmacology
  • Adenosine Monophosphate / pharmacology
  • Caffeine / pharmacology*
  • Enzyme Activators / pharmacology
  • Kinetics
  • Muscle, Skeletal / drug effects
  • Muscle, Skeletal / enzymology*
  • Phosphates / metabolism
  • Phosphodiesterase Inhibitors / pharmacology*
  • Phosphorylase a / isolation & purification
  • Phosphorylase a / metabolism*


  • Adenine Nucleotides
  • Enzyme Activators
  • Phosphates
  • Phosphodiesterase Inhibitors
  • Caffeine
  • Adenosine Monophosphate
  • Adenosine Diphosphate
  • Phosphorylase a