Evidence for the coupling of the regulations of tyrosine aminotransferase and phosphoenolpyruvate carboxykinase.

An experimental design which simulated a light–darkness schedule reproduced the daily rhythm of tyrosine aminotransferase in rat liver. A similar rhythm was identified for phosphoenolpyruvate carboxykinase. Both enzyme rhythms were profoundly and similarly modified by the amount of dietary protein fed to the animals. The liver glycogen content fluctuated in a manner that produced plots which were virtually the mirror images of the enzyme rhythm plots. The oscillations of phosphoenolpyruvate carboxykinase were less pronounced than those of tyrosine aminotransferase and the same quantitative relationship was found in response to infused nonpharmacological amounts of Cortisol.A relationship in the regulations of the two enzymes suggested by the present and other experiments is envisaged and discussed.