2017 by Taylor & Francis Group, LLC. The metabolism and functions of glutamine in healthy intestine or in pathological situations cannot be dissociated from those of glutamate since these two nonessential amino acids possess a partially common metabolic fate (Blachier et al. 2009). In cells equipped with the enzymatic activity glutaminase (like enterocytes in which the catalytic activity is high), glutamine can be converted to glutamate and ammonia. Conversely, in cells equipped with glutamine synthetase (GS) activity (like colonocytes that are characterized by a high GS activity in comparison with enterocytes) (Andriamihaja et al. 2010), glutamate can be condensed with ammonia to form glutamine (EklouLawson et al. 2009). However, glutamine, but not glutamate, can be used for the synthesis of several compounds, including purine and pyrimidine (Newsholme and Carrie 1994), N-acetylglucosamine and N-acetylgalactosamine; these latter two being involved in intestinal mucin synthesis (Reeds and Burrin 2001). If one considers the circulating concentrations of the 20 amino acids used for the synthesis of body protein measured in the plasma of healthy subjects, there is a striking difference between glutamate that is present at a very low concentration and glutamine that is present at the highest concentration among all amino acids (Cynober 2002). In food products like milk, the sum of glutamine and glutamate represents the highest protein-bound amino acids (Wu and Knabe 1994), and more generally glutamate by itself is one of the most abundant amino acid in alimentary protein (Beyreuther et al. 2007).
