Neurochemistry International 43 (2003) 493–499 Glutamine synthetase activity and glutamine content in brain: modulation by NMDA receptors and nitric oxide Elena Kosenko, Marta Llansola, Carmina Montoliu, Pilar Monfort, Regina Rodrigo, Mariluz Hernandez-Viadel, Slaven Erceg, Ana M. Sánchez-Perez, Vicente Felipo * Laboratory of Neurobiology, Instituto de Investigaciones Citológicas, Fundación Valenciana de Investigaciones Biomédicas, Amadeo de Saboya 4, 46010 Valencia, Spain Received 13 September 2002; received in revised form 17 October 2002; accepted 21 October 2002 Abstract Acute intoxication with large doses of ammonia leads to rapid death. The main mechanism for ammonia elimination in brain is its reaction with glutamate to form glutamine. This reaction is catalyzed by glutamine synthetase and consumes ATP. In the course of studies on the molecular mechanism of acute ammonia toxicity, we have found that glutamine synthetase activity and glutamine content in brain are modulated by NMDA receptors and nitric oxide. The main findings can be summarized as follows. Blocking NMDA receptors prevents ammonia-induced depletion of brain ATP and death of rats but not the increase in brain glutamine, indicating that ammonia toxicity is not due to increased activity of glutamine synthetase or formation of glutamine but to excessive activation of NMDA receptors. Blocking NMDA receptors in vivo increases glutamine synthetase activity and glutamine content in brain, indicating that tonic activation of NMDA receptors maintains a tonic inhibition of glutamine synthetase. Blocking NMDA receptors in vivo increases the activity of glutamine synthetase assayed in vitro, indicating that increased activity is due to a covalent modification of the enzyme. Nitric oxide inhibits glutamine synthetase, indicating that the covalent modification that inhibits glutamine synthetase is a nitrosylation or a nitration. Inhibition of nitric oxide synthase increases the activity of glutamine synthetase, indicating that the covalent modification is reversible and it must be an enzyme that denitrosylate or denitrate glutamine synthetase. NMDA mediated activation of nitric oxide synthase is responsible only for part of the tonic inhibition of glutamine synthetase. Other sources of nitric oxide are also contributing to this tonic inhibition. Glutamine synthetase is not working at maximum rate in brain and its activity may be increased pharmacologically by manipulating NMDA receptors or nitric oxide content. This may be useful, for example, to increase ammonia detoxification in brain in hyperammonemic situations. © 2003 Elsevier Science Ltd. All rights reserved. Keywords: Glutamine synthetase; Brain glutamine; NMDA receptors; Nitric oxide; Hyperammonemia; Hepatic encephalopathy 1. Introduction Glutamine synthetase catalyzes the reaction between am- monia and glutamate to form glutamine. This reaction con- sumes a molecule of ATP: Glutamate + NH 4 + + ATP → Glutamine + ADP + P i We became interested in this reaction because we are studying the molecular mechanism(s) of ammonia toxicity and this reaction is the main mechanism for ammonia elim- ination in brain. * Corresponding author. Tel.: +34-96-3391250; fax: +34-96-3601453. E-mail address: vfelipo@ochoa.fib.es (V. Felipo). Ammonia is a normal product of degradation of proteins and other compounds, but at high concentrations ammonia is toxic and leads to functional disturbances of the central nervous system. To avoid the toxic effects of ammonia it is usually detoxified in the liver by incorporation into urea that is eliminated in urine. However, when the liver fails, ammo- nia detoxification does not occur properly and the levels of ammonia in blood and tissues increase, leading to hyperam- monemia. There are two main types of hyperammonemia: (1) chronic moderate hyperammonemia, as occurs in liver cirrhosis, which leads to altered cerebral function and is responsible for the neurological alterations in different hy- perammonemic states and for some of the neurological 0197-0186/03/$ – see front matter © 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0197-0186(03)00039-1