Amphibolic role of the Krebs cycle in the insulin-stimulated protein synthesis.

It has been a generally held view that insulin does not significantly affect the incorporation of amino acids into liver protein. This interpretation was based on data obtained from studies using the branched chain amino acids, which are poorly metabolized by the hepatic tissue. The effect of insulin on 14CO2 formation and protein incorporation of several 1-14C-labeled or U-14C-labeled amino acids was studied in isolated rat hepatocytes and diaphragm pieces. It was shown that insulin enhanced 14CO2 formation and protein incorporation primarily of those carbons of amino acids which are metabolized through the mitochondrial Krebs cycle. Using aminooxyacetic acid (0.5 mM), a potent inhibitor of the transamination reaction, it was shown that there exists an "insulin-sensitive" pool of glutamate which is preferentially utilized for protein synthesis in the presence of insulin. The insulin effect on protein incorporation of 14C-labeled glutamate generated in the Krebs cycle was abolished in the presence of aminooxyacetic acid. We interpret these results to signify that mitochondrial transamination of alpha-ketoglutarate to glutamate is essential for insulin stimulation of 14C incorporation into hepatocyte protein.