Regulation of human glutamate dehydrogenases: Implications for glutamate, ammonia and energy metabolism in brain
Version of Record online: 30 NOV 2001
Copyright © 2001 Wiley-Liss, Inc.
Journal of Neuroscience Research
Volume 66, Issue 5, pages 899–908, 1 December 2001
How to Cite
Plaitakis, A. and Zaganas, I. (2001), Regulation of human glutamate dehydrogenases: Implications for glutamate, ammonia and energy metabolism in brain. J. Neurosci. Res., 66: 899–908. doi: 10.1002/jnr.10054
- Issue online: 30 NOV 2001
- Version of Record online: 30 NOV 2001
- Manuscript Accepted: 6 AUG 2001
- Manuscript Received: 19 JUL 2001
- Association for the Advancement of Research and Treatment of Neurologic Disorders of Crete
- glutamate dehydrogenase;
Glutamate dehydrogenase (GDH) catalyzes the oxidative deamination of glutamate to α-ketoglutarate using NAD or NADP as cofactors. In mammalian brain, GDH is located predominantly in astrocytes, where it is probably involved in the metabolism of transmitter glutamate. The exact mechanisms that regulate glutamate fluxes through this pathway, however, have not been fully understood. In the human, GDH exists in heat-resistant and heat-labile isoforms, encoded by the GLUD1 (housekeeping) and GLUD2 (nerve tissue-specific) genes, respectively. These forms differ in their catalytic and allosteric properties. Kinetic studies showed that the Km value for glutamate for the nerve tissue GDH is within the range of glutamate levels in astrocytes (2.43 mM), whereas for the housekeeping enzyme, this value is significantly higher (7.64 mM; P < 0.01). The allosteric activators ADP (0.1–1.0 mM) and L-leucine (1.0–10.0 mM) induce a concentration-dependent enzyme stimulation that is proportionally greater for the nerve tissue-specific GDH (up to 1,600%) than for the housekeeping enzyme (up to 150%). When used together at lower concentrations, ADP (10–50 mM) and L-leucine (75–200 μM) act synergistically in stimulating GDH activity. GTP exerts a powerful inhibitory effect (IC50 = 0.20 mM) on the housekeeping GDH; in contrast, the nerve tissue isoenzyme is resistant to GTP inhibition. Thus, although the housekeeping GDH is regulated primarily by GTP, the nerve tissue GDH activity depends largely on available ADP or L-leucine levels. Conditions associated with enhanced hydrolysis of ATP to ADP (e.g., intense glutamatergic transmission) are likely to activate nerve tissue-specific GDH leading to an increased glutamate flux through this pathway. © 2001 Wiley-Liss, Inc.