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Abstract  The amino acid L-glutamine is an important substrate in intermediary metabolism as carrier of nitrogen and carbon moieties. Its homeostasis is controlled primarily by two enzymes, glutaminase (glutamine amidohydrolase) and glutamine synthetase (glutamate-ammonia ligase). This chapter provides an overview of the molecular and protein heterogeneity of these two enzymes and their physiological roles. Murine glutaminase deficiency and glutamine synthetase deficiency, both in Drosophila and in two patients, also are summarized. In humans, glutamine synthetase deficiency (MIM 138290, 610015) is associated with severe brain malformations, including underdeveloped gyri and intracranial cysts, as well as with respiratory failure and early death. Both patients were of Turkish descent and manifested a substitution of cysteine for arginine in exon 6 of the glutamine synthetase gene (locus 1q31). In addition, potential pathomechanisms of secondary glutamine synthetase deficiencies (e.g., ammonia-induced neurotoxicity and hepatic encephalopathy) also are discussed.

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