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Temperature and the regulation of enzyme activity in poikilotherms : regulatory properties of fish fructose-1, 6-diphosphatase Behrisch, Hans Werner

Abstract

The regulatory properties of fructose 1,6-diphosphatase (FDPase) from liver of rainbow trout (Salmo gairdneri), South American lungfish (Lepidosiren paradoxa) and migrating pink salmon (Oncorhynchus gorbuscha) were examined over the physiological temperature ranges of the organisms. In rainbow trout saturation curves for substrate (fructose 1,6-diphosphate), and a cofactor (Mg²⁺ ) are sigmoidal, and the evidence suggests cooperative interaction between the binding sites for these ligands. The affinity of the trout enzyme is approximately 50-to 100-fold higher for Mn²⁺ than for Mg²⁺ and the Mn²⁺ saturation curve is hyperbolic. The enzyme is inhibited by Ca²⁺ and Zn²⁺ and this inhibition appears to be competitive with respect to cofactor. The trout FDPase has an alkaline pH optimum and high pH values enhance FDPase affinity for cofactor. Low concentrations of 5'AMP inhibit the rainbow trout FDPase and the enzyme-AMP interaction is sensitive to temperature; thus, Ki for AMP at 25° is approximately 30-fold higher than at 0°. Furthermore, increasing concentrations of the cofactors tend to reduce and/or reverse the AMP inhibition and it is suggested that these ions may play a role in regulating the efficiency of AMP as an inhibitor of FDPase activity. Similar to the trout FDPase, substrate and cofactor saturation curves for the lungifsh FDPase are sigmoid and Hill plots of the data suggest homotropic interaction between their respective binding sites. Affinity of the lungfish enzyme for substrate increases markedly at low temperature, with the result that at physiological concentrations of substrate, the velocity of the reaction is essentially independent of temperature. In addition, increasing pH values stimulate enzyme activity and increase affinity of the lungfish FDPase for cofactor. The lungfish enzyme is inhibited by AMP, as is the trout FDPase, but affinity for the allosteric inhibitor is reduced and this FDPase-AMP interaction is independent of temperature. It is suggested that the decreased sensitivity to AMP is adaptive in view of the aestivating habit of the South American lungfish, under which condition the animal would be starved for long periods of time and a low energy charge (high concentrations of AMP) would probably exist in the liver cell. The regulatory properties of FDPase from migrating salmon were investigated. The calculated energy charge of the liver cell from these fish is very low (0.464) in keeping with the extended starvation and high rates of muscular and biosynthetic activity in these organisms. As in the trout and lungfish enzymes, affinity of salmon FDPase for substrate increases with a decrease in temperature. As a result at physiological concentrations of substrate enzyme activity is independent of temperature. Arrhenius plots of the saturation kinetics are complex and suggest an inter-conversion of one or more forms of the enzyme. Subsequent examination of the enzyme by sucrose density gradient centrifugation at different temperatures indicates that such thermally-dependent changes in conformation of the protein do occur, a finding which also offers a basis for the observed temperature-dependent changes in Km for substrate. The affinity of salmon FDPase for its allosteric inhibitor, AMP, is lower than in other FDPases and this enzyme-AMP interaction is largely insensitive to temperature. The functional significance of this reduced sensitivity to AMP is that it allows normal, indeed high, FDPase activity during conditions of low energy charge. These findings suggest mechanisms for the maintenance of high rates of gluconeogenesis in salmon during spawning migration.

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