Glucose repression and induction of enzyme synthesis in rat liver

Glucose has long been known as a key metabolite in intermediary metabolism. Its importance as a regulator of protein synthesis is emphasized by its ability to repress the induced synthesis of threonine dehydrase and ornithine transaminase in rat liver. Simultaneous with its repressive effect, glucose administration per os with casein induces glucose-6-phosphate dehydrogenase after a lag of 12 hr. Neither glucose alone nor casein alone were capable of inducing Zwischenferment. In contrast, glucokinase induction by glucose does not require the concomitant administration of protein. Glucokinase induction was inhibited by puromycin and actinomycin D. Insulin administration along with glucose resulted in some inhibition of induction while glucagon suppressed glucokinase induction more than 80 per cent. Fructose administration resulted in glucokinase induction to about two thirds that with glucose. Other carbohydrates tested gave little or no induction. A model to explain these results based on work with microorganisms is presented.