Glutamate dehydrogenase (GDH) is a hexameric enzyme that catalyzes the reversible conversion of glutamate to α-ketoglutarate and ammonia while reducing NAD(P)⁺ to NAD(P)H. It is found in all living organisms serving both catabolic and anabolic reactions. In mammalian tissues, oxidative deamination of glutamate via GDH generates α-ketoglutarate, which is metabolized by the Krebs cycle, leading to the synthesis of ATP. In addition, the GDH pathway is linked to diverse cellular processes, including ammonia metabolism, acid-base equilibrium, redox homeostasis (via formation of fumarate), lipid biosynthesis (via oxidative generation of citrate), and lactate production. While most mammals possess a single GDH1 protein (hGDH1 in the human) that is highly expressed in the liver, humans and other primates have acquired, via duplication, an hGDH2 isoenzyme with distinct functional properties and tissue expression profile. The novel hGDH2 underwent rapid evolutionary adaptation, acquiring unique properties that enable enhanced enzyme function under conditions inhibitory to its ancestor hGDH1. These are thought to provide a biological advantage to humans with hGDH2 evolution occurring concomitantly with human brain development. hGDH2 is co-expressed with hGDH1 in human brain, kidney, testis and steroidogenic organs, but not in the liver. In human cerebral cortex, hGDH1 and hGDH2 are expressed in astrocytes, the cells responsible for removing and metabolizing transmitter glutamate, and for supplying neurons with glutamine and lactate. In human testis, hGDH2 (but not hGDH1) is densely expressed in the Sertoli cells, known to provide the spermatids with lactate and other nutrients. In steroid producing cells, hGDH1/2 is thought to generate reducing equivalents (NADPH) in the mitochondria for the biosynthesis of steroidal hormones. Lastly, up-regulation of hGDH1/2 expression occurs in cancer, permitting neoplastic cells to utilize glutamine/glutamate for their growth. In addition, deregulation of hGDH1/2 is implicated in the pathogenesis of several human disorders.

Mammalian glutamate dehydrogenase (GDH) is an allosterically regulated enzyme that is expressed widely. Its activity is potently inhibited by GTP and thought to be controlled by the need of the cell for ATP. In addition to this housekeeping human (h) GDH1, humans have acquired (via a duplication event) a highly homologous isoenzyme (hGDH2) that is resistant to GTP. Although transcripts of GLUD2, the gene encoding hGDH2, have been detected in human neural and testicular tissues, data on the endogenous protein are lacking. Here, we developed an antibody specific for hGDH2 and used it to study human tissues. Western blot analyses revealed, to our surprise, that endogenous hGDH2 is more densely expressed in testis than in brain. At the subcellular level, hGDH2 localized to mitochondria. Study of testicular tissue using immunocytochemical and immunofluorescence methods revealed that the Sertoli cells were strongly labeled by our anti-hGDH2 antibody. In human cerebral cortex, a robust labeling of astrocytes was detected, with neurons showing faint hGDH2 immunoreactivity. Astrocytes and Sertoli cells are known to support neurons and germ cells, respectively, providing them with lactate that largely derives from the tricarboxylic acid cycle via conversion of glutamate to alpha-ketoglutarate (GDH reaction). As hGDH2 is not subject to GTP control, the enzyme is able to metabolize glutamate even when the tricarboxylic acid cycle generates GTP amounts sufficient to inactivate the housekeeping hGDH1 protein. Hence, the selective expression of hGDH2 by astrocytes and Sertoli cells may provide a significant biological advantage by facilitating metabolic recycling processes essential to the supportive role of these cells.