PHYSIOLOGICAL REGULATION OF RAT LIVER PHOSPHOENOLPYRUVATE CARBOXYKINASE (GTP) BY INSULIN INSIGNIFICANCE OF A CYCLIC AMP-INDEPENDENT MECHANISM

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Abstract
The effect of re-feeding glucose, protein or fat and the effect of insulin injection on the activity of hepatic phosphoenolpyruvate [PEP] carboxykinase (GTP: oxaloacetate carboxy-lyase (transphosphorylating) EC 4.1.1.32), the concentration of hepatic cyclic[c]AMP and the level of serum insulin was investigated in starved rats. Under all conditions examined the concentration of serum insulin was elevated to a high degree. Only rats re-fed with glucose responded to the increase in serum insulin with a decrease in PEP carboxykinase activity, while the activity of the enzyme remained unchanged or was elevated after re-feeding protein or fat or after insulin injection, respectively. Since under all conditions there was a close correlation between cAMP concentration and PEP carboxykinase activity, but not between the insulin level and enzyme activity, it is concluded that the hormone physiologically regulates PEP carboxykinase activity by decreasing the intrahepatic cAMP concentration rather than by the postulated cAMP-independent inhibition of specific mRNA translation.

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