Molecular basis of human glutamate dehydrogenase regulation under changing energy demands

Glutamate dehydrogenase (GDH), an enzyme central to glutamate metabolism, is located in the mitochondria although there is evidence for extramitochondrial localization of GDH. In the human, housekeeping and nerve tissue-specific isoforms, encoded by the GLUD1 and GLUD2 genes, have been identified. The two isoenzymes differ markedly in their baseline activities, allosteric regulation, and thermal stability. GTP potently inhibits GLUD1-derived GDH (IC(50) = 0.2 muM), whereas the GLUD2-derived isoenzyme is resistant to this compound. The GLUD2-derived GDH shows low basal activity and has the capacity to be activated fully by ADP or L-leucine. We used molecular biological tools to study the subcellular localization of GLUD1-derived GDH in cultured cells and the molecular basis of its regulation. COS7 cells, transfected with a GLUD1-pEGFP-N3 vector, revealed a GFP fluorescence pattern nearly identical to that of the mitochondrial marker pDsRed2-Mito. Site-directed mutagenesis of GLUD1 gene showed that replacement of Gly456 by Ala made the enzyme resistant to GTP (IC(50) = 2.8 +/- 0.15 microM) without altering its regulation by ADP. Substitution of Ser for Arg443 rendered the enzyme virtually inactive at its basal state, but fully responsive to ADP activation. The Arg443Ser mutant was more active at pH 7.0 than at pH 8.0. The Gly456Ala change therefore dissociated GLUD2-derived GDH function from GTP, whereas the Arg443Ser change made enzyme regulation possible without this inhibitor. These properties may allow the brain isoenzyme to function well under conditions of intracellular acidification and increased turnover of ATP to ADP, as occurs in synaptic astrocytes during excitatory transmission.