The role of glutamine synthetase and glutamate dehydrogenase in cerebral ammonia homeostasis

Abstract

In the brain, glutamine synthetase (GS), which is located predominantly in astrocytes, is largely responsible for the removal of both blood-derived and metabolically generated ammonia. Thus, studies with [(13)N]ammonia have shown that about 25 % of blood-derived ammonia is removed in a single pass through the rat brain and that this ammonia is incorporated primarily into glutamine (amide) in astrocytes. Major pathways for cerebral ammonia generation include the glutaminase reaction and the glutamate dehydrogenase (GDH) reaction. The equilibrium position of the GDH-catalyzed reaction in vitro favors reductive amination of α-ketoglutarate at pH 7.4. Nevertheless, only a small amount of label derived from [(13)N]ammonia in rat brain is incorporated into glutamate and the α-amine of glutamine in vivo. Most likely the cerebral GDH reaction is drawn normally in the direction of glutamate oxidation (ammonia production) by rapid removal of ammonia as glutamine. Linkage of glutamate/α-ketoglutarate-utilizing aminotransferases with the GDH reaction channels excess amino acid nitrogen toward ammonia for glutamine synthesis. At high ammonia levels and/or when GS is inhibited the GDH reaction coupled with glutamate/α-ketoglutarate-linked aminotransferases may, however, promote the flow of ammonia nitrogen toward synthesis of amino acids. Preliminary evidence suggests an important role for the purine nucleotide cycle (PNC) as an additional source of ammonia in neurons (Net reaction: L-Aspartate + GTP + H(2)O → Fumarate + GDP + P(i) + NH(3)) and in the beat cycle of ependyma cilia. The link of the PNC to aminotransferases and GDH/GS and its role in cerebral nitrogen metabolism under both normal and pathological (e.g. hyperammonemic encephalopathy) conditions should be a productive area for future research.

Fig. 1

HPLC profile of labeled metabolites in rat brain after a 20-minute infusion of [¹³N]ammonia. The [¹³N]ammonia in isotonic saline was administered intracarotidly at a rate of 0.15 ml/min into an adult male rat. At the end of the infusion the rat was sacrificed and the label in a deproteinized fraction (0.02 ml) of the brain was analyzed by HPLC by means of a cation exchange column as described in [21]. All counts in the HPLC profile were decay corrected back to an arbitrary time (start of HPLC). Modified from [21].

Fig. 2

Compartmentation of ammonia metabolism in the brain. Ammonia entering the brain by diffusion from either the blood or CSF is rapidly metabolized in the small compartment (astrocytes) predominately to glutamine. Ammonia generated endogenously in both the neuronal and astrocytic compartments is also rapidly incorporated into glutamine within the astrocytic compartment. For convenience the protonated form of ammonia (ammonium, NH₄⁺) is not shown in the brain compartments. The thickness of the arrows indicates the relative contribution of the GS reaction [NH₃ → glutamine Gln)] and GDH reaction [NH₃ → glutamate (Glu)] to the metabolism of endogenously produced ammonia and blood/CSF-derived ammonia. Under normal conditions, the GDH reaction plays a minor role in metabolizing brain ammonia. In the neurons the GDH reaction is a mechanism for net oxidative deamination of glutamate under normal conditions. The GDH reaction is therefore not shown as a means of removing ammonia in the neuronal compartment.

Fig. 3

Major routes for nitrogen metabolism in the brain and the central role of ammonia. Key to enzyme pathways: 1, GS; 2, glutaminase; 3, GDH; 4, AAT; 5, alanine and branched-chain amino acid (BCAA) aminotransferases; 6, net reaction of the PNC.