The synthesis and degradation of rat liver and kidney fructose bisphosphatase in Vivo

Abstract

The activity of fructose bisphosphatase (EC 3.1.3.11) in rat liver and kidney under controlled conditions of diet, age, and animal management was shown to be constant during several hormonal and dietary regimens. The only exception found was in animals maintained on a high protein diet, in whom an increase in enzyme activity was observed in the liver. The activities of two adaptive enzymes, phosphoenolpyruvate carboxykinase (EC 4.1.1.32) and ATP citrate lyase (EC 4.1.3.8), were also measured in liver under the same regimens. Their activities varied in a manner predictable from previous investigations. Fructose bisphosphatase was purified from rat liver, and a specific antibody was obtained that was used to isolate the enzyme from liver and kidney tissue extracts. The relative rate of enzyme synthesis was measured by pulse-labeling rats with [4,5- 3H]leucine and isolating the enzyme protein by immunoprecipitation. The fractional rate of [ 3H]leucine incorporation into fructose bisphosphatase in relation to its incorporation into the soluble protein fraction of liver and kidney was determined. The fractional rate was not affected by the diet or by the hormone treatments utilized. The degradation rate in vivo of the liver and kidney enzyme was determined by giving injections to rats of NaH 14CO 3 and following the disappearance of 14C from the enzyme protein. The degradation rate constants for the liver and kidney enzyme in chow-fed animals were 0.016 and 0.013 h −1, respectively. These values correspond to half lives of about 45 and 55 h, respectively. The feeding of a 90% protein or an 80% carbohydrate diet did not alter these parameters. As both liver and kidney fructose bisphosphatase showed little variation in the level of enzyme activity or the rate of synthesis and degradation of fructose bisphosphatase under a variety of dietary and hormonal conditions in the intact rat, we propose that this enzyme is a constitutive protein in these organs.