Oscillatory synthesis of glucose 1,6-bisphosphate and frequency modulation of glycolytic oscillations in skeletal muscle extracts.

Oscillatory behavior of glycolysis in cell-free extracts of rat skeletal muscle involves bursts of phosphofructokinase activity, due to autocatalytic activation by fructose-1,6-P2. Glucose-1,6-P2 similarly might activate phosphofructokinase in an autocatalytic manner, because it is produced in a side reaction of phosphofructokinase and in a side reaction of phosphoglucomutase using fructose-1,6-P2. When muscle extracts were provided with 1 mM ATP and 10 mM glucose, glucose-1,6-P2 accumulated in a stepwise, but monotonic, manner to 0.7 microM in 1 h. The stepwise increases occurred during the phases when fructose-1,6-P2 was available, consistent with glucose-1,6-P2 synthesis in the phosphoglucomutase side reaction. Addition of 5-20 microM glucose-1,6-P2 increased the frequency of the oscillations in a dose-dependent manner and progressively shortened the time interval before the first burst of phosphofructokinase activity. Addition of 30 microM glucose-1,6-P2 blocked the oscillations. The peak values of the [ATP]/[ADP] ratio were then eliminated, and the average [ATP]/[ADP] ratio was reduced by half. In the presence of higher, near physiological concentrations of ATP and citrate (which reduce the activation of phosphofructokinase by glucose-1,6-P2), high physiological concentrations of glucose-1,6-P2 (50-100 microM) increased the frequency of the oscillations and did not block them. We conclude that autocatalytic activation of phosphofructokinase by fructose-1,6-P2, but not by glucose-1,6-P2, is the mechanism generating the oscillations in muscle extracts. Glucose-1,6-P2 may nevertheless play a role in facilitating the initiation of the oscillations and in modulating their frequency.