ABSTRACT

Glutamine, the most abundant free amino acid in the plasma, is classified as a nonessen­ tial amino acid in textbooks because mammalian tissues possess the ability to produce it from glutamate, ammonia and ATP via the enzyme glutamine synthetase. A dynamic balance exists in the body between glutamine utilization and release by individual tissues-processes which collectively maintain circulating glutamine concentrations at approximately 0.6 mM in humans. The simultaneous synthesis and catabolism of glutamine was first described by Hans Krebs in tissues from many different species in 1935.14 Since that time, this amino acid has received a lot of attention given its diverse and essential roles in cellular metabolism for normal and neoplastic tissues and cells. Among the glutamine-utilizing biosynthetic and metabolic path­ ways are: gluconeogenesis, glutathione synthesis, oxidative energy metabolism (Krebs Cycle, (CO2)), protein synthesis, ureagenesis, aminosugar biosynthesis, renal ammoniagenesis, NAD biosynthesis, de novo nucleotide biosynthesis, amino acid biosynthesis (transamination path­ ways), fatty acid/phospholipid biosynthesis and brain neurotransmitter biosynthesis. Figure 3.1 depicts the major pathways for glutamine transport and metabolism in mammalian cells. The disposition of glutamine to any one or a combination of these pathways is dependent on the role of specific cells within a tissue, and may change depending on the nutritional/hormonal status of the individual. For a detailed discussion of glutamine metabolism, the reader is directed to several comprehensive reviews.15'20

Nutritional Support in Cancer and Transplant Patients, edited by Rifat Latifi and Ronald C. Merrell. ©2001 Eurekah.com.