Glutamine is the most abundant amino acid in plasma and is non-essential because it is synthesized by a ubiquitously expressed enzyme, glutamine synthetase. It is important for inter-organ nitrogen transport and has a central role in nitrogen and ammonia metabolism. It is an amino group donor for glucose precursors, other amino acids, purines and pyrimidines, and is a regulator of acid-base balance. In addition, it is regarded as having a major role in the pathophysiology of hyperammonaemia due to its osmotic effect on astrocyte swelling. Figure 4 shows the localization of the urea cycle and glutamine synthetase in different compartments of the liver as well as the intercellular glutamine cycling in relation to ureagenesis.