Phosphofructokinase and fructosebisphosphatase from muscle can interact at physiological concentrations with mutual effects on their kinetic behavior.

Phosphofructokinase (PFK) and fructosebisphosphatase (FBPase) from muscle studied at physiological concentrations have been found to influence the kinetic behavior of each other. Under these conditions PFK can be activated up to ca. 4-fold by FBPase, while the latter can be inhibited up to ca. 3-fold by the former. Diluted enzymes did not interact with each other; nevertheless, they did so in the presence of polyethylene glycol. Equimolar amounts of either glucosephosphate isomerase or aldolase had no effect on concentrated PFK. The kinetic interactions between PFK and FBPase should be taken into account for fuller understanding of their regulatory behavior in vivo.