Ineffective hepatic clearance of excess
ammonia in the form of
urea, as occurs in
urea cycle enzymopathies (UCDs) and in
liver failure, leads to increases in circulating and tissue concentrations of
glutamine and a positive correlation between brain
glutamine and the severity of neurological symptoms. Studies using 1H/13C Nuclear Magnetic Resonance (NMR) spectroscopy reveal increased de novo synthesis of
glutamine in the brain in
acute liver failure (ALF) but increases of synthesis rates per se do not correlate with either the severity of
encephalopathy or
brain edema. Skeletal muscle becomes primarily responsible for removal of excess
ammonia in
liver failure and in UCDs, an adaptation that results from a post-translational induction of the
glutamine synthetase (GS) gene. The importance of muscle in
ammonia removal in
hyperammonemia accounts for the resurgence of interest in maintaining adequate
dietary protein and the use of agents aimed at the stimulation of muscle GS. Alternative or additional metabolic and regulatory pathways that impact on brain
glutamine homeostasis in
hyperammonemia include (i)
glutamine deamination by the two
isoforms of
glutaminase, (ii)
glutamine transamination leading to the production of the putative
neurotoxin alpha-ketoglutaramate and (iii) alterations of high affinity astrocytic
glutamine transporters (SNATs). Findings of reduced expression of the
glutamine transporter SNAT-5 (responsible for
glutamine clearance from the astrocyte) in ALF raise the possibility of "
glutamine trapping" within these cells. Such a trapping mechanism could contribute to
cytotoxic brain edema and to the imbalance between excitatory and inhibitory neurotransmission in this disorder.