Control of substrate cycling at fructose phosphates in a reconstituted muscle glycolytic system

Using a glycolytic system reconstituted from purified muscle enzymes, it has been demonstrated that addition of fructose bisphosphatase results in a net ATP turnover, indicative of substrate cycling. This occurred in conditions closely simulating those of muscle in the resting state. The cycling depended on the presence of free Mg²⁺ ions; the concentration of free Mg²⁺ in muscle tissue was estimated to be 0.8 mm, and in these conditions the fructose bisphosphatase was approximately 50% active. The effective K_i of fructose bisphosphatase for AMP was estimated to be 1.0 μm, using the equilibrium constants of the creatine kinase and myokinase reactions to calculate AMP concentrations. Neither citrate nor cyclic AMP at physiological concentrations affected substrate cycling significantly.