| Glutamate Dehydrogenase 1 (GLUD1) and Glutamate Dehydrogenase 2 (GLUD2) encode closely related mitochondrial glutamate dehydrogenases that catalyze the reversible oxidative deamination of glutamate to α‑ketoglutarate and ammonia, positioning these enzymes at a key junction between amino acid catabolism, the tricarboxylic acid cycle, and nitrogen handling in liver, brain, and endocrine tissues. The proteins share a conserved hexameric allosteric dehydrogenase architecture, with catalytic sites that bind glutamate and NAD(H)/NADP(H) and regulatory sites that respond to activators such as ADP and leucine and inhibitors such as GTP, allowing GLUD1/2 activity to be tightly tuned by cellular energy charge and amino acid availability and thereby to adjust anaplerotic flux of carbon into the TCA cycle and mitochondrial redox state. In pancreatic β cells, GLUD1 activity couples amino acid metabolism to insulin secretion: oxidative deamination of glutamate increases α‑ketoglutarate supply to the TCA cycle, enhances ATP production, and promotes closure of ATP‑sensitive K⁺ channels and depolarization, so gain‑of‑function GLUD1 mutations that impair GTP‑mediated inhibition cause the hyperinsulinism–hyperammonemia (HI/HA) syndrome, characterized by leucine‑sensitive hyperinsulinemic hypoglycemia and chronically elevated plasma ammonia due to increased renal ammoniagenesis. Patients with HI/HA carry dominant activating GLUD1 variants that increase enzyme responsiveness to leucine and reduce GTP inhibition, and they display protein‑triggered hypoglycemia, fasting hypoglycemia, and persistent mild hyperammonemia, while brain involvement with developmental delay and epilepsy in some cases is consistent with enhanced glutamate dehydrogenase activity also affecting neuronal glutamate handling and neurotransmission. Regulation of GLUD1 and GLUD2 in nerve tissue is complex, with transcriptional control, alternative promoters, and post‑translational modulation integrating signals from energy status, amino acid supply, and calcium, so that neuronal and glial GDH tune glutamate oxidation versus recycling in concert with glutamine synthetase and transaminases to maintain excitatory neurotransmitter pools and mitochondrial energy production. GLUD1 and GLUD2 function as allosterically regulated mitochondrial dehydrogenases that convert glutamate and NAD(P)⁺ into α‑ketoglutarate, reduced pyridine nucleotides, and ammonia, linking amino acid flux to TCA cycle anaplerosis, insulin secretion, nitrogen balance, and neuronal energy metabolism, and GLUD1 gain‑of‑function mutations in HI/HA syndrome illustrate how disruption of this regulatory system drives a defined endocrine–metabolic disease phenotype. |
