Activation of hepatic acetyl-CoA carboxylase by glutamate and Mg²⁺ is mediated by protein phosphatase-2A

The activation of hepatic acetyl-CoA carboxylase by Na⁺-co-transported amino acids such as glutamine has been attributed mainly to the stimulation of its dephosphorylation by accumulating dicarboxylic acids, e.g. glutamate. We report here on a hepatic species of protein phosphatase-2A that activates acetyl-CoA carboxylase in the presence of physiological concentrations of glutamate or Mg²⁺ and, under these conditions, accounts for virtually all the hepatic acetyl-CoA carboxylase phosphatase activity. Glutamate also stimulated the dephosphorylation of a synthetic pentadecapeptide encompassing the Ser-79 phosphorylation site of rat acetyl-CoA carboxylase, but did not affect the dephosphorylation of other substrates such as phosphorylase. Conversely, protamine, which stimulated the dephosphorylation of phosphorylase, inhibited the activation of acetyl-CoA carboxylase. A comparison with various species of muscle protein phosphatase-2A showed that the stimulatory effects of glutamate and Mg²⁺ on the acetyl-CoA carboxylase phosphatase activity are largely mediated by the regulatory A subunit. Glutamate and Mg²⁺ emerge from our study as novel regulators of protein phosphatase-2A when acting on acetyl-CoA carboxylase.