ABSTRACT
Glutamine is the most abundant free amino acid in the human body. With an arterial plasma concentration of 0.5–0.8 mM, the circulating pool of glutamine constitutes 20%–25% of the plasma amino acids. This chapter provides a brief overview of the interorgan metabolism of glutamine followed by a more detailed discussion of the biosynthesis, kinetic and biophysical properties, and the regulation of the various glutaminase isoforms. It discusses the role of glutaminase in the tissue-specific metabolism of glutamine and presents some insight for future research. Most of the ingested glutamine is rapidly catabolized within the epithelial cells of the small intestine. The exogenous glutamine that is absorbed intact is catabolized primarily in the liver. Therefore, the abundant pool of glutamine within the body is maintained largely through de novo synthesis, which is catalyzed by glutamine synthetase. In mammals, glutamine synthetase is a cytosolic enzyme.
