Effects of monovalent cations on the catalytic activity of pig liver phosphofructokinase

The monovalent cations NH 4 +, K +, and Rb + activate pig liver phosphofructokinase by increasing the maximal velocity. In the presence of these cations the enzyme retains sigmoid kinetics with respect to fructose-6-phosphate. However, these cations bring about a decrease in the [S] 0.5 for fructose-6-phosphate to an extent directly proportional to their ionic volumes. The apparent dissociation constants of NH 4 +, K +, and Rb + for the enzyme at 0.5 mM ATP and 4 mM Fru6P are 0.2 mM, 8 mM, and 15 mM, respectively. The maximal velocity of the enzyme in the presence of saturating concentrations of Rb + is about 70% of that seen with NH 4 + or K +. The monovalent cations Li +, Na +, and Cs + inhibit the enzyme at high concentrations (> 50 mM) by decreasing the maximal velocity. Although the efficiency of inhibition by these cations qualitatively increases with decreasing size, there is no obvious quantitative relationship between efficiency of inhibition and any parameter of ionic size.