Question

How does the degradation of glutamate to glutamine relate to glycolysis and the TCA cycle?

Answer 1

The glutamine-glutamate cycle provides neurons with astrocyte-generated glutamate/-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 -ketoglutarate to glutamate and vice versa. Most experiments in cultured astrocytes, including those by one of us, suggest that glutamate formation is catalyzed by a spart at eaminotransferase (AAT) and its degradation by glutamate dehydrogenase (GDH). Strongly supported , 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 -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.

The interaction between glycolysis and glutaminolysis is mainly determined by two factors:

1. the existence of a residual active glycerol 3-phosphate shuttle.
2. the asscociation of PGM within the glycolytic enzyme complex.

In tumor cells cytosolic hydrogen is transported into the mitochondria via two different shuttle systems: the malate-aspartate-shuttle and the glycerol 3-phosphate shuttle.
The malate-aspartate-shuttle is part of the glutaminolytic pathway.
The glycerol 3-phosphate shuttle depends on the expression of cytosolic glycerol 3-phosphate.

In cells without cytosolic glycerol 3-phosphate dehydrogenase activity, such as MCF-7 cells, transformed oval cells and transformed NIH 3T3 cells the glycerol 3-phosphate shuttle is inactive and glutamine consumption increases with increasing glucose consumption.