Glutamate Dehydrogenase Is Important for Ammonia Fixation and Amino Acid Homeostasis in Brain During Hyperammonemia

Abstract

Impaired liver function may lead to hyperammonemia and risk for hepatic encephalopathy. In brain, detoxification of ammonia is mediated mainly by glutamine synthetase (GS) in astrocytes. This requires a continuous de novo synthesis of glutamate, likely involving the action of both pyruvate carboxylase (PC) and glutamate dehydrogenase (GDH). An increased PC activity upon ammonia exposure and the importance of PC activity for glutamine synthesis has previously been demonstrated while the importance of GDH for generation of glutamate as precursor for glutamine synthesis has received little attention. We therefore investigated the functional importance of GDH for brain metabolism during hyperammonemia. To this end, brain slices were acutely isolated from transgenic CNS-specific GDH null or litter mate control mice and incubated in aCSF containing [U-13C]glucose in the absence or presence of 1 or 5 mM ammonia. In another set of experiments, brain slices were incubated in aCSF containing 1 or 5 mM 15N-labeled NH4Cl and 5 mM unlabeled glucose. Tissue extracts were analyzed for isotopic labeling in metabolites and for total amounts of amino acids. As a novel finding, we reveal a central importance of GDH function for cerebral ammonia fixation and as a prerequisite for de novo synthesis of glutamate and glutamine during hyperammonemia. Moreover, we demonstrated an important role of the concerted action of GDH and alanine aminotransferase in hyperammonemia; the products alanine and α-ketoglutarate serve as an ammonia sink and as a substrate for ammonia fixation via GDH, respectively. The role of this mechanism in human hyperammonemic states remains to be studied.

Evolutionary origin and functional diversification of aminotransferases

“…Certain amino acid dehydrogenases release or incorporate free ammonium ion from or to the Cα of amino acids through NAD(P) + /NAD(P)H-dependent oxidative deamination or reductive amination, respectively ( 110 , 111 ). For instance, glutamate dehydrogenases (EC 1.4.1.2) play critical roles at the interface of carbon and nitrogen metabolism, such as by releasing ammonium ion from glutamate at the entry of the urea cycle ( 112 ) or by assimilating ammonium ion to generate glutamate from α-ketoglutarate ( 111 ) ( Fig. 2 ).…”

The Relationship between Plasma Alpha-1-Antitrypsin Polymers and Lung or Liver Function in ZZ Alpha-1-Antitrypsin-Deficient Patients

“…GLDH is a mitochondrial enzyme that catalyzes the conversion of glutamate to 2-oxoglutarate [29] and reflects leakage from damaged or necrotic hepatocytes. Some studies suggest that GLDH activity is crucial for survival during hyperammonemia [30].…”

Metabolic trade-offs associated with homeostatic adjustments in pelagic and benthic cephalopods: Comparative evaluations of NH4+/H+ transport machinery in gills

“…Consequently, further studies are needed to explore those nitrogen pathways in terms of ammonium regulation in cephalopods with different lifestyles. Among these metabolic processes, ammonia-related amino acid synthesis is one of the most important biochemical processes for maintaining organismic NH 4 + homeostasis (Levitt and Levitt, 2018;Voss et al, 2021). Under the Glu-Gln cycle, glutamate (Glu) is catabolized to glutamine (Gln), which coordinates with NH 4 + .…”