Skeletal muscle glycogen phosphorylase alpha kinetics: Effects of adenine nucleotides and caffeine

This study aimed to determine physiologically relevant kinetic and alloster ic effects of Pi, AMP, ADP, and caffeine on isolated skeletal muscle glycog en phosphorylase a (Phos a). In the absence of effectors, Phos a had V-max = 221 +/- 2 U/mg and K-m = 5.6 +/- 0.3 mM P-i at 30 degreesC. AMP and ADP e ach increased Phos a V-max and decreased K-m in a dose-dependent manner. AM P was more effective than ADP (e.g., 1 muM AMP vs. ADP: V-max = 354 +/- 2 v s. 209 +/- 8 U/mg, and K-m = 2.3 +/- 0.1 vs. 4.1 +/- 0.3 mM). Both nucleoti des were relatively more effective at lower P-i levels. Experiments simulat ing a range of contraction (exercise) conditions in which P-i, AMP, and ADP were used at appropriate physiological concentrations demonstrated that ea ch agent singly and in combination influences Phos a activity. Caffeine (50 -100 muM) inhibited Phos a (K-m similar to8-14 mM, similar to 40-50% reduct ion in activity at 2-10 mM P-i). The present in vitro data support a possib le contribution of substrate (P-i) and allosteric effects to Phos a regulat ion in many physiological states, independent of covalent modulation of the percentage of total Phos in the Phos a form and suggest that caffeine inhi bition of Phos a activity may contribute to the glycogen-sparing effect of caffeine.