Temperature and the regulation of enzyme activity in poikilotherms. Fructose diphosphatase from migrating salmon.

1. The calculated energy charge of the liver cell from migrating salmon is very low (0.464), in keeping with the extended starvation and high rates of muscular and biosynthetic activity of these organisms. 2. Affinity of fructose 1,6-diphosphatase for substrate increases with a decrease in temperature. 3. Arrhenius plots of the saturation kinetics are complex and suggest an interconversion of one or more forms of the enzyme; this interconversion is dependent on the identity of the cofactor. 4. Affinity of salmon fructose 1,6-diphosphatase for its allosteric inhibitor (AMP) is lower than in other fructose 1,6-diphosphatases and this enzyme-AMP interaction is largely insensitive to temperature. The functional significance of diminished AMP-sensitivity is that it allows normal or high fructose 1,6-diphosphatase activity during a low energy charge. 5. These findings suggest mechanisms for the maintenance of high rates of gluconeogenesis in salmon during spawning migration.