Glutamate, at the Interface between Amino Acid and Carbohydrate Metabolism

Abstract

The liver is the major site of gluconeogenesis, the major organ of amino acid catabolism and the only organ with a complete urea cycle. These metabolic capabilities are related, and these relationships are best exemplified by an examination of the disposal of the daily protein load. Adults, ingesting a typical Western diet, will consume approximately 100 g protein/d; the great bulk of this is metabolized by the liver. Although textbooks suggest that these amino acids are oxidized in the liver, total oxidation cannot occur within the confines of hepatic oxygen uptake and ATP homeostasis. Rather, most amino acids are oxidized only partially in the liver, with the bulk of their carbon skeleton being converted to glucose. The nitrogen is converted to urea and, to a lesser extent, to glutamine. The integration of the urea cycle with gluconeogenesis ensures that the bulk of the reducing power (NADH) required in the cytosol for gluconeogenesis can be provided by ancillary reactions of the urea cycle. Glutamate is at the center of these metabolic events for three reasons. First, through the well-described transdeamination system involving aminotransferases and glutamate dehydrogenase, glutamate plays a key catalytic role in the removal of alpha-amino nitrogen from amino acids. Second, the "glutamate family" of amino acids (arginine, ornithine, proline, histidine and glutamine) require the conversion of these amino acids to glutamate for their metabolic disposal. Third, glutamate serves as substrate for the synthesis of N-acetylglutamate, an essential allosteric activator of carbamyl phosphate synthetase I, a key regulatory enzyme in the urea cycle.