Nerve Tissue‐Specific (GLUD2) and Housekeeping (GLUD1) Human Glutamate Dehydrogenases Are Regulated by Distinct Allosteric Mechanisms

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Abstract
Human glutamate dehydrogenase (GDH), an enzyme central to the metabolism of glutamate, is known to exist in housekeeping and nerve tissue‐specific isoforms encoded by the GLUD1 and GLUD2 genes, respectively. As there is evidence that GDH function in vivo is regulated, and that regulatory mutations of human GDH are associated with metabolic abnormalities, we sought here to characterize further the functional properties of the two human isoenzymes. Each was obtained in recombinant form by expressing the corresponding cDNAs in Sf9 cells and studied with respect to its regulation by endogenous allosteric effectors, such as purine nucleotides and branched chain amino acids. Results showed that L‐leucine, at 1.0 mM, enhanced the activity of the nerve tissue‐specific (GLUD2‐derived) enzyme by ∼1,600% and that of the GLUD1‐derived GDH by ∼75%. Concentrations of L‐leucine similar to those present in human tissues (∼0.1 mM) had little effect on either isoenzyme. However, the presence of ADP (10‐50 μM) sensitized the two isoenzymes to L‐leucine, permitting substantial enzyme activation at physiologically relevant concentrations of this amino acid. Nonactivated GLUD1 GDH was markedly inhibited by GTP (IC50 = 0.20 μM), whereas nonactivated GLUD2 GDH was totally insensitive to this compound (IC50 > 5,000 μM). In contrast, GLUD2 GDH activated by ADP and/or L‐leucine was amenable to this inhibition, although at substantially higher GTP concentrations than the GLUD1 enzyme. ADP and L‐leucine, acting synergistically, modified the cooperativity curves of the two isoenzymes. Kinetic studies revealed significant differences in the Km values obtained for α‐ketoglutarate and glutamate for the GLUD1‐ and the GLUD2‐derived GDH, with the allosteric activators differentially altering these values. Hence, the activity of the two human GDH is regulated by distinct allosteric mechanisms, and these findings may have implications for the biologic functions of these isoenzymes.