A model study of the fructose diphosphatase-phosphofructokinase substrate cycle.

A fructose diphosphatase-phosphofructokinase substrate cycle has been reconstructed in vitro to provide a system that recycles fructose 6-phosphate and hydrolyses ATP to ADP and P(i). The concerted actions of glucose phosphate isomerase, phosphofructokinase, aldolase and triose phosphate isomerase catalysed the loss of (3)H from [5-(3)H,U-(14)C]glucose 6-phosphate. This was used as the basis of a method for the estimation of the fructose diphosphatase-phosphofructokinase substrate cycle. For the reconstructed cycle, the rate of decrease of the (3)H/(14)C ratio in [5-(3)H,U-(14)C]hexose 6-phosphate was proportional to the rate of fructose 6-phosphate substrate cycling. A detailed theoretical treatment of this relationship is developed, which enables the rate of substrate cycling to be determined in vivo.