Kinetic Properties of the Phosphofructokinase from Erythrocytes of Rats and Rabbits

Abstract

The activity of the phosphofructokinase from erythrocytes of rats and rabbits, and presumably of the enzyme from other sources depend on the presence of either K + or NH 4 + ions. Their effects are not due to ionic strength. The apparent activation constant, K D , is about 1 mM for NH 4 + and 10 mM for K + . While K + ions appear to act only as activators of the catalytic site, NH 4 + ions have a double action. They act both as activators and as positive effectors which abolish the cooperativity, presumably by affecting the interaction between the subunits. They increase the affinity of the enzyme for fructose 6‐phosphate and ATP as substrates, while the affinity for ATP as inhibitor is weakened. These effects are probably secondary to the changed interaction between the subunits. It is concluded that only data on the phosphofructokinase in K + system are representative of normal intracellular conditions. In certain physiological and pathological states the effect of NH 4 + ions on the allosteric properties of the enzyme is probably important. P i is an activator of both the K + ‐ and the NH 4 + ‐activated enzyme, with an approximate K D of 1.6 mM. Its effect is therefore of importance under physiological and pathological conditions. The data on the purified K + ‐activated enzyme are in good agreement with changes of the metabolic pattern of the intact erythrocyte.