Glutamate dehydrogenase of nuclear and extra-nuclear compartments of Chang's liver cells

Evaluation of sub-cellular distribution of glutamate dehydrogenase (GDH) in Drosophila melanogaster larvae

Glutamate dehydrogenase (GDH) enzyme was conventionally known as a mitochondrial marker. However, subsequently it was reported to be present in the nuclei as well. So far, the nuclear distribution of GDH has been reported in a number of organisms including yeast, rat, cow, chicken. However, the sub-cellular distribution of GDH, illustrated by in situ methods still remains elusive. Here, by assaying the GDH activity and by immuno-blotting using anti-GDH antibody in the fractionated nuclear and cytoplasmic fractions of Drosophila larvae, we demonstrate the cytoplasmic distribution of GDH. This observation was further supported by in situ immunostaining of salivary gland, Malpighian tubules and eye imaginal discs of Drosophila larvae. Collectively, our results demonstrate that in Drosophila larvae, GDH is not found in the nucleus, but is localized exclusively in the cytoplasm.

Enhanced [3H] Glutamate Binding in the Cerebellum of Insulin-Induced Hypoglycaemic and Streptozotocin-Induced Diabetic Rats

AIM

Energy deprivation causes neuronal death affecting the cognitive and memory ability of an individual. The kinetic parameters of glutamate dehydrogenase (GDH), the enzyme involved in the production of glutamate, was studied in the cerebellum and liver and the binding parameters of glutamate receptors in the cerebellum of insulin-induced hypoglycaemic and streptozotocin-induced diabetic rats were studied to reveal the role of glutamate excitotoxicity.

METHODS

A single intrafemoral dose of streptozotocin was administered to induce diabetes. Hypoglycaemia was induced by appropriate doses of insulin subcutaneously in control and diabetic rats. The kinetic parameters V (max) and K (m) of GDH were studied spectrophotometrically at different substrate concentrations of alpha-ketoglutarate. Glutamate receptor binding assay was done with different concentrations of [3H] Glutamate.

RESULTS

The GDH enzyme assay showed a significant increase (P < 0.001) in the V (max) of the enzyme in the cerebellum of hypoglycaemic and diabetic rat groups when compared to control. The V (max) of hypoglycaemic groups was significantly increased (P < 0.001) when compared to diabetic group. In the liver, the V (max) of GDH was significantly increased (P < 0.001) in the diabetic and diabetic hypoglycaemia group when compared to control. The V (max) of GDH increased significantly (P < 0.001) in the diabetic hypoglycaemic rats compared to diabetic group, whereas the control hypoglycaemic rats showed a significant decrease in V (max) (P < 0.001) when compared to diabetic and diabetic hypoglycaemic rats. The K (m) showed no significant change amongst the groups in cerebellum and liver. Scatchard analysis showed a significant increase (P < 0.001) in B (max) in the cerebellum of hypoglycaemic and diabetic rats when compared to control. The B (max) of hypoglycaemic rats significantly increased (P < 0.001) when compared to diabetic group. In hypoglycaemic groups, B (max) of the control hypoglycaemic rats showed a significant increase (P < 0.001) compared to diabetic hypoglycaemic rats. The K (d) of the diabetic group decreased significantly (P < 0.01) when compared to control and control hypoglycaemic rats. There was a significant decrease (P < 0.05) in the K (d) of diabetic hypoglycaemic group when compared to the control hypoglycaemic rats.

CONCLUSION

Our studies demonstrated the increased enzyme activity in the hypoglycaemic rats with increased production of extracellular glutamate. The present study also revealed increased binding parameters of glutamate receptors reflecting an increased receptor number with increase in the affinity. This increased number of receptors and the increased glutamate production will lead to glutamate excitotoxicity and neuronal degeneration which has an impact on the cognitive and memory ability. This has immense clinical significance in the management of diabetes and insulin therapy.

The subcellular localization of glutamate dehydrogenase (GDH): is GDH a marker for mitochondria in brain?

Glutamate dehydrogenase (GDH, EC 1.4.1.2) has long been used as a marker for mitochondria in brain and other tissues, despite reports indicating that GDH is also present in nuclei of liver and dorsal root ganglia. To examine whether GDH can be used as a marker to differentiate between mitochondria and nuclei in the brain, we have measured GDH by enzymatic activity and on immunoblots in rat brain mitochondria and nuclei which were highly enriched by density-gradient centrifugation methods. The activity of GDH was enriched in the nuclear fraction as well as in the mitochondrial fraction, while the activities of other "mitochondrial" enzymes (fumarase, NAD-isocitrate dehydrogenase and pyruvate dehydrogenase complex) were enriched only in the mitochondrial fraction. Immunoblots using polyclonal antibodies against bovine liver GDH confirmed the presence of GDH in the rat brain nuclear and mitochondrial fractions. The GDH in these two subcellular fractions had a very similar molecular weight of 56,000 daltons. The mitochondrial and nuclear GDH differed, however, in their susceptibility to solubilization by detergents and salts. The mitochondrial GDH could be solubilized by extraction with low concentrations of detergents (0.1% Triton X-100 and 0.1% Lubrol PX), while the nuclear GDH could be solubilized only by elevated concentrations of detergents (0.3% each) plus KCl (greater than 150 mM). Our results indicate that GDH is present in both nuclei and mitochondria in rat brain. The notion that GDH may serve as a marker for mitochondria needs to be re-evaluated.

Mitochondrial and nuclear glutamate dehydrogenases in Chinese hamster ovary cells in culture

Nuclear glutamate dehydrogenase (EC 1.4.1.3) activity has been demonstrated in Chinese hamster ovary cells. Some characteristics of this enzyme have been examined and compared with those of the mitochondrial glutamate dehydrogenase from the same source. Differences were detected in the extent of the activation by inorganic phosphate, in the pH versus activity curves, in the affinity of the two enzymes for the cofactor NAD+ and in the electrophosretic mobility. A different rate of decay of the two enzymes has been observed in cells grown in the presence of chloramphenicol. Immunological studies show that, as in ox liver, the nuclear enzyme has specific antigenic determinants besides those in common with mitochondrial glutamate dehydrogenase. Finally, experiments of thermal inactivation indicate a higher stability of the mitochondrial enzyme.

Purification and properties of NADP-dependent glutamate dehydrogenase from yeast nuclear fractions

1. NADP-dependent glutamate dehydrogenase (EC 1.4.1.4) extracted from nuclear fractions of Saccharomyces cerevisiae was partially purified. The final purification achieved was over 100-fold over the initial extract. 2. Cellulose acetate electrophoresis shows that the preparation is close to homogeneity and that the enzyme is slightly more anionic than cytoplasmic glutamate dehydrogenase. 3. The response of the nuclear activity to variation of pH, of inorganic phosphate and other electrolyte concentration and of the concentration of the reaction substrates has been investigated. Several differences were detected in comparison with cytoplasmic glutamate dehydrogenase.