Uric acid inhibits liver phosphorylase a activity under simulated in vivo conditions

We have reported that glycogen synthesis and degradation can occur in vivo without a significant change in the amount of phosphorylase a present. These data suggest the presence of a regulatable mechanism for inhibiting phosphorylase a activity in vivo. Several effectors have been described. AMP stimulates, whereas ADP, ATP, and glucose inhibit activity. Of these effectors, only the glucose concentration changes under normal conditions; thus it could regulate phosphorylase a activity in vivo. We previously have reported that, when all of these effectors were present at physiological concentrations, the net effect was no change in phosphorylase a activity. Addition of caffeine, an independent inhibitor of activity, to the above effectors not only resulted in inhibition but also restored a glucose concentration-dependent inhibition. Because uric acid is an endogenous xanthine derivative, we decided to determine whether it had an effect on phosphorylase a activity. Independently, uric acid did not affect activity; however, when added at a presumed physiological concentration in combination with AMP, ADP, ATP, and glucose, it inhibited activity. A modest but not statistically significant glucose concentration-dependent inhibition was also present. Thus uric acid may play an important role in regulating phosphorylase a activity in vivo.