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1. The properties of fructose diphosphatase from the liver of rainbow trout (Salmo gairdnerii) were examined over the physiological temperature range of the organism. 2. Saturation curves for substrate (fructose 1,6-diphosphate) and a cofactor (Mg(2+)) are sigmoidal, and Hill plots of the results suggest a minimum of two interacting fructose 1,6-diphosphate sites and two interacting Mg(2+) sites per molecule of enzyme. 3. Mn(2+)-saturation curves are hyperbolic, and the K(a) for Mn(2+), which inhibits the enzyme at high concentrations, is 50-100-fold lower than the K(a) for Mg(2+). 4. Fructose diphosphatase is inhibited by low concentrations of AMP; this inhibition appears to be decreased and reversed by increasing the concentrations of Mg(2+) and Mn(2+). Higher concentrations of AMP are required to inhibit the trout fructose diphosphatase in the presence of Mn(2+). 5. The affinities of fructose diphosphatase for fructose diphosphate and Mn(2+) appear to be temperature-independent, whereas the affinities for Mg(2+) and AMP are highly temperature-dependent. 6. The pH optimum of the enzyme depends on the concentrations of Mg(2+) and Mn(2+). In addition, pH determines the K(a) for Mg(2+); at high pH, K(a) for Mg(2+) is lowered. 7. The enzyme is inhibited by Ca(2+) and Zn(2+), and the inhibition is competitive with respect to both cations. 8. The possible roles of these ions and AMP in the modulation of fructose diphosphatase and gluconeogenic activity are discussed in relation to temperature adaptation.

Temperature and the regulation of enzyme activity in poikilotherms. Properties of rainbow-trout fructose diphosphatase