Expression of human GLUD2 glutamate dehydrogenase in human tissues: Functional implications
Glutamate dehydrogenase (GDH), a mitochondrial enzyme with a key metabolic role, exists in the human in hGDH1 and hGDH2 isoforms encoded by the GLUD1 and GLUD2 genes, respectively. It seems that GLUD1 was retroposed to the X chromosome where it gave rise to GLUD2 via random mutations and natural selection. Of these, evolutionary Gly456Ala substitution dissociated hGDH2 from GTP control, while replacement of Arg443 by Ser drastically modified basal activity, heat stability, optimal pH, allosteric regulation and migration pattern in SDS-PAGE, thus suggesting an effect on enzyme's conformation. While GLUD2-specific transcripts have been detected in human brain, retina and testis, data on the endogenous hGDH2 protein are lacking. Given the housekeeping nature of hGDH1 and its high homology to hGDH2, the specific detection of hGDH2 in tissues presents a challenge. To develop an antibody specific for hGDH2, we considered that an epitope containing the Arg443Ser change was an attractive target. We accordingly used a peptide that corresponds to residues 436-447, with Ser at position 443, to immunize rabbits and succeeded in raising a polyclonal antibody specific for hGDH2. Western blots showed that human testis contained equal amounts of hGDH2 and hGDH1 and that both isoproteins localized to the mitochondrial fraction. In human brain, however, hGDH2 expression was lower than that of hGDH1. Immuno-histochemical studies on human testis and cerebral cortex, showed punctuate, organelle-like hGDH2 immuno-labeling in sertoli cells and in astrocytes, respectively, consistent with the mitochondrial localization of the enzyme. Similar studies in kidney revealed that hGDH2 is expressed in epithelial cells of the proximal convoluted tubule. As hGDH2 can metabolize glutamate at relatively low pH without the GTP constrain, it may function efficiently under conditions of relative acidification that prevail in astrocytes following glutamate uptake. Similarly, in the kidney, hGDH2 could contribute to enhanced excretion of ammonia under acidosis.