Inhibition of fructose 1,6-bisphosphatase by excess substrate and its reversal by monovalent cations

Purified rabbit liver fructose 1,6-bisphosphatase is maximally active with 2 μM fructose 1,6-bisphosphate. Above this concentration the substrate becomes inhibotory. Inhibition is reversed by $\text{NH}{}_4{}^{\mathrm{+}}$ or by physiological concentrations of K⁺. Substrate inhibition and its modification by monovalent cations may play a role in the regulation of gluconeogenesis at the step catalyzed by fructose 1,6-bisphosphatase.