Glutamine-Glutamate Cycle Flux Is Similar in Cultured Astrocytes and Brain and Both Glutamate Production and Oxidation Are Mainly Catalyzed by Aspartate Aminotransferase

Leif Hertz,Douglas Rothman

doi:10.3390/biology6010017

Abstract

The glutamine-glutamate cycle provides neurons with astrocyte-generated glutamate/γ-aminobutyric acid (GABA) and oxidizes glutamate in astrocytes, and it returns released transmitter glutamate/GABA to neurons after astrocytic uptake. This review deals primarily with the glutamate/GABA generation/oxidation, although it also shows similarity between metabolic rates in cultured astrocytes and intact brain. A key point is identification of the enzyme(s) converting astrocytic α-ketoglutarate to glutamate and vice versa. Most experiments in cultured astrocytes, including those by one of us, suggest that glutamate formation is catalyzed by aspartate aminotransferase (AAT) and its degradation by glutamate dehydrogenase (GDH). Strongly supported by results shown in Table 1 we now propose that both reactions are primarily catalyzed by AAT. This is possible because the formation occurs in the cytosol and the degradation in mitochondria and they are temporally separate. High glutamate/glutamine concentrations abolish the need for glutamate production from α-ketoglutarate and due to metabolic coupling between glutamate synthesis and oxidation these high concentrations render AAT-mediated glutamate oxidation impossible. This necessitates the use of GDH under these conditions, shown by insensitivity of the oxidation to the transamination inhibitor aminooxyacetic acid (AOAA). Experiments using lower glutamate/glutamine concentration show inhibition of glutamate oxidation by AOAA, consistent with the coupled transamination reactions described here.

Highlights

In the adult brain neurons require metabolic collaboration with astrocytes in order to synthesize glutamate, the major excitatory transmitter, and its decarboxylation product γ-aminobutyric acid (GABA), the major inhibitory transmitter
Most authors assume that this process is catalyzed by glutamate dehydrogenase (GDH), but we have previously suggested that glutamate synthesis and degradation are coupled and both catalyzed by aspartate aminotransferase (AAT) [6,15,16]
The use of GDH during glutamate oxidation is supported by most, but not all, of the studies described below we suggest that this oxidation under most in vivo conditions is mainly catalyzed by AAT and that it provides the aspartate needed during glutamate synthesis

Summary

Introduction

In the adult brain neurons require metabolic collaboration with astrocytes in order to synthesize glutamate, the major excitatory transmitter, and its decarboxylation product γ-aminobutyric acid (GABA), the major inhibitory transmitter. Pyruvate carboxylation, which is active in astrocytes, but not in neurons, creates a new molecule of oxaloacetate, which after condensation with acetyl but not in neurons, creates a new molecule of oxaloacetate, which after condensation with acetyl coenzyme A, forms citrate that is metabolized in the TCA cycle to α-ketoglutarate (α-KG), which can coenzyme A, forms citrate that is metabolized in the TCA cycle to α-ketoglutarate (α-KG), which can leave the cycle to form glutamate (glu), catalyzed by aspartate aminotransferase (AAT). Further metabolism by the cytosolic and astrocyte-specific enzyme glutamine synthetase leads to the formation of glutamine (gln), which after transport to neurons is converted to transmitter glutamate or GABA in complex reactions (reviewed in [6]). 75%–80% is converted to Further metabolism by the cytosolic and astrocyte-specific enzyme glutamine synthetase leads to the formation of glutamine (gln), which after transport to neurons is converted to transmitter glutamate or GABA in complex reactions (reviewed in [6]). While extremely important functions of GDH in brain metabolism are presently being uncovered [23,24,26,28,35,36], they are unlikely to be related to its occasional and minor potential involvement in glutamate oxidation in the glutamine-glutamate cycle

The Glutamate-Glutamine Cycle in the Brain In Vivo

AOAA not tested

Suggested Pathway for Coupled Formation and Oxidation of Glutamate

Proposed

Findings

Conclusions