Effect of Dietary Fat and Sucrose on the Activities of Several Rat Hepatic Enzymes and Their Diurnal Response to a Meal

Abstract

Regulation of the cytoplasmic enzymes, pyruvate kinase (PK), glucokinase (GK), phosphoenolpy ruvate carboxykinase (PEPCK), fructose-1,6-diphosphatase (FDP), ATP citrate-lyase (ATP-CL), NAD-malate dehydrogenase (NAD-MD), NADP-malate dehydrogenase (NADP-MD), glutamic-pyruvic transaminase (GPT), glucose-6-phosphate dehydrogenase (G6PD), and 6-phosphogluconate dehydrogenase (6PGD), in rat liver by dietary fat (F diet) and dietary sucrose (S diet) was investigated. Mealfeeding the S diet to adult rats for 5 and 9 months resulted in a diurnal dietary response (i.e., food response) variation of FDP, GK, ATP-CL, 6PGD, and PK, while meal-feeding the S diet to young rats resulted in diurnal dietary response variation of ATP-CL, G6PD, NADP-MD, 6PGD, GPT, and PK. Meal-feeding the fat diet results in essentially no diurnal variation in enzyme activity. The overall effect of meal-feeding, as compared with ad libitum feeding, of the S diet was to increase the levels of G6PD, ATP-CL, and NADP-MD and to decrease the level of PEck in the meal-fed rats. Young rats meal-fed the two diets have higher enzyme activities than meal-fed adult rats for the observed enzymes (except for GPT and NAD-MD). In general, hepatic levels of the enzymes studied are low in the F diet-fed animals and markedly higher for the S diet-fed animals. These results suggest that dietary carbohydrate specifically induces those enzymes involved in carbohydrate metabolism, whereas dietary fat does not affect their levels. On the basis of prior evidence for an early requirement of RNA synthesis for sucrose induction of G6PD, this widespread induction of liver enzymes by carbohydrate must indicate either increased synthesis of ribosomal RNA with later regulation of synthesis specifically of these enzymes or increased synthesis of a rather large group of specific messenger RNAs i.e., coordinate genetic control of a number of these enzyme messenger RNAs.