The properties of fructose diphosphatase from liver of South American lungfish (Lepidosiren paradoxa) were examined. 2. Saturation curves for substrate (fructose diphosphate) and both cofactors (Mn(2+) and Mg(2+)) are sigmoidal and Hill plots of these results suggest about 2 interacting substrate and cofactor sites/molecule of enzyme. 3. Mn(2+) is an efficient positive modulator of the enzyme and K(a) for Mn(2+) is about 20-30-fold lower than the K(a) for Mg(2+). 4. Lungfish fructose diphosphatase is inhibited by low concentrations of AMP, and the affinity of the enzyme for AMP is insensitive to temperature. 5. The affinities of fructose diphosphatase for fructose diphosphate and Mn(2+) appear to be dependent on temperature, whereas affinity for Mg(2+) is temperature-independent. 6. The pH optimum of the enzyme depends on the presence of the particular cofactor. As pH increases, the K(a) values of both cations are lowered, maximum velocities are increased and the saturation curves for cofactor become hyperbolic. 7. The possible roles of these ions, pH and substrate in the modulation of fructose diphosphatase and gluconeogenic activity in the lungfish are discussed in relation to aestivation and temperature adaptation.
