Splanchnic Metabolism of Dietary Glutamine
Glutamine normally constitutes 8%-12% of total amino acid content of dietary proteins. In the gut, glutamine is the major source of energy for proliferating enterocytes, and in health, the gastrointestinal tract is the major organ of glutamine utilization (Souba et al. 1990). Stable isotope studies have demonstrated that approximately 40%-75% of enterally administered glutamine undergoes first- pass extraction within the splanchnic bed in healthy humans (Matthews et al. 1993; Hankard et al. 1995; Thibault et al. 2008; Kao et al. 2013), where its major fate is oxidation (Haisch et al. 2000). Along with its use by the gut for energy production and protein synthesis, another metabolic fate of glutamine in the gut is its conversion to citrulline in enterocytes (Curis et al. 2005). Citrulline, in turn, is the only precursor for de novo synthesis of arginine in the kidney.
Glutamine as a Nontoxic Nitrogen Carrier
Glutamine plays an important role in the storage and transport of ammonia in the body. Normally, the concentration of free ammonia in blood and tissues is low. Although the majority of nitrogen is excreted as urea, only hepatic cells contain all the enzymes of the urea cycle in significant quantities. Therefore, organs other than the liver must rely upon the glutamine synthetic pathway for ammonia detoxification and maintenance of nontoxic ammonia levels (Meister 1956). In the kidney, regulation of excretion of the ammonia formed from glutamine via glutaminase is also essential for maintaining the body’s acid-base status (Meister 1956; Biolo et al. 1995).