title: Pivotal role of glutamine synthetase in ammonia detoxification.
abstract: Glutamine synthetase (GS) catalyzes condensation of ammonia with glutamate to glutamine. Glutamine serves, with alanine, as a major nontoxic interorgan ammonia carrier. Elimination of hepatic GS expression in mice causes only mild hyperammonemia and hypoglutaminemia but a pronounced decrease in the whole-body muscle-to-fat ratio with increased myostatin expression in muscle. Using GS-knockout/liver and control mice and stepwise increments of enterally infused ammonia, we show that ∼35% of this ammonia is detoxified by hepatic GS and ∼35% by urea-cycle enzymes, while ∼30% is not cleared by the liver, independent of portal ammonia concentrations ≤2 mmol/L. Using both genetic (GS-knockout/liver and GS-knockout/muscle) and pharmacological (methionine sulfoximine and dexamethasone) approaches to modulate GS activity, we further show that detoxification of stepwise increments of intravenously (jugular vein) infused ammonia is almost totally dependent on GS activity. Maximal ammonia-detoxifying capacity through either the enteral or the intravenous route is ∼160 μmol/hour in control mice. Using stable isotopes, we show that disposal of glutamine-bound ammonia to urea (through mitochondrial glutaminase and carbamoylphosphate synthetase) depends on the rate of glutamine synthesis and increases from ∼7% in methionine sulfoximine-treated mice to ∼500% in dexamethasone-treated mice (control mice, 100%), without difference in total urea synthesis.
  
  
  
  Hepatic GS contributes to both enteral and systemic ammonia detoxification. Glutamine synthesis in the periphery (including that in pericentral hepatocytes) and glutamine catabolism in (periportal) hepatocytes represents the high-affinity ammonia-detoxifying system of the body. The dependence of glutamine-bound ammonia disposal to urea on the rate of glutamine synthesis suggests that enhancing peripheral glutamine synthesis is a promising strategy to treat hyperammonemia. Because total urea synthesis does not depend on glutamine synthesis, we hypothesize that glutamate dehydrogenase complements mitochondrial ammonia production. (Hepatology 2017;65:281-293).
authors.affiliation: Tytgat Institute for Liver and Intestinal Research, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.
authors.forename: Theodorus B M
authors.initials: TB
authors.lastname: Hakvoort
authors.affiliation: Tytgat Institute for Liver and Intestinal Research, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.
authors.forename: Youji
authors.initials: Y
authors.lastname: He
authors.affiliation: Department of Genetic Metabolic Disease, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.
authors.forename: Wim
authors.initials: W
authors.lastname: Kulik
authors.affiliation: Tytgat Institute for Liver and Intestinal Research, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.
authors.forename: Jacqueline L M
authors.initials: JL
authors.lastname: Vermeulen
authors.affiliation: Tytgat Institute for Liver and Intestinal Research, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.
authors.forename: Suzanne
authors.initials: S
authors.lastname: Duijst
authors.affiliation: Heart Failure Center, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.
authors.forename: Jan M
authors.initials: JM
authors.lastname: Ruijter
authors.affiliation: Department of Radiology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.
authors.forename: Jurgen H
authors.initials: JH
authors.lastname: Runge
authors.affiliation: Department of General Surgery, Maastricht University, Maastricht, The Netherlands.
authors.forename: Nicolaas E P
authors.initials: NE
authors.lastname: Deutz
authors.affiliation: Department of Anatomy & Embryology, Maastricht University, Maastricht, The Netherlands.
authors.forename: S Eleonore
authors.initials: SE
authors.lastname: Koehler
authors.affiliation: Tytgat Institute for Liver and Intestinal Research, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.|Department of Anatomy & Embryology, Maastricht University, Maastricht, The Netherlands.
authors.forename: Wouter H
authors.initials: WH
authors.lastname: Lamers
mesh_terms: D000641:Ammonia / Q000378:metabolism*; D000818:Animals; D001639:Bicarbonates / Q000378:metabolism; D005974:Glutamate-Ammonia Ligase / Q000502:physiology*; D005973:Glutamine / Q000378:metabolism; D008658:Inactivation, Metabolic; D008099:Liver / Q000378:metabolism; D051379:Mice
chemical_list: D001639:Bicarbonates; D005973:Glutamine; C027043:ammonium bicarbonate; D000641:Ammonia; D005974:Glutamate-Ammonia Ligase