Temperature and the regulation of enzyme activity in poikilotherms. Properties of lungfish fructose diphosphatase

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Abstract
1. 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 (Mn2+ and Mg2+) are sigmoidal and Hill plots of these results suggest about 2 interacting substrate and cofactor sites/molecule of enzyme. 3. Mn2+ is an efficient positive modulator of the enzyme and Ka for Mn2+ is about 20–30-fold lower than the Ka for Mg2+. 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 Mn2+ appear to be dependent on temperature, whereas affinity for Mg2+ is temperature-independent. 6. The pH optimum of the enzyme depends on the presence of the particular cofactor. As pH increases, the Ka 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.