REGULATION OF THE GLUTAMATE-DEHYDROGENASE ACTIVITY IN RAT ISLETS OF LANGERHANS AND ITS CONSEQUENCE ON INSULIN RELEASE

Kinetic properties of glutamate dehydrogenase (GDH) and the effects on its activity of several putative modulators were examined in mitochon drial extracts of rat pancreatic islets. In the presence of 40 mmol/L NH4Cl and 0.1 mmol/L NADH, stepwise elevation of the 2-oxoglutarate co ncentration from 0.005 to 0.05 mmol/L increased glutamate formation, w hereas further increases led to a progressive decrease of the reaction velocity. Adenosine diphosphate (ADP) at 0.1 mmol/L partially and at 1 mmol/L completely reversed the inhibitory effect of 2-oxoglutarate. The sensitivity to activation by either ADP or leucine was dependent o n 2-oxoglutarate concentrations. At higher concentrations of the latte r, greater amounts of the activators were needed to attain maximal eff ect. In the absence of allosteric activators, sulfate or phosphate at 20 mmol/L partially released the inhibitory effect of 2-oxoglutarate l evels and increased the maximal velocity (Vmax) for the reaction. In t he presence of 0.1 mmol/L ADP. both anions prevented the inhibition by higher concentrations of 2-oxoglutarate, whereas with 1 mmol/L ADP th eir only effect was a slight increase in the Vmax. Mg2+ and naturally occurring polyamines decreased glutamate formation in a dose-dependent manner; with 0.1 mmol/L ADP, inhibition was seen at all 2-oxoglutarat e concentrations studied, whereas with 1 mmol/L ADP, it was noticeable at substrate concentrations higher than 0.5 mmol/L. This inhibitory e ffect on GDH activity was partially attenuated by sulfate. Addition of either 2 mmol/L spermidine or extra magnesium (final 2.5 or 5 mmol/L) to the perifusion buffer markedly attenuated the insulin release elic ited by alpha-ketoisocaproate. It is suggested that naturally occurrin g polyamines, magnesium, and phosphate act as physiological modulators of GDH activity in pancreatic beta cells. It is also postulated that these compounds may be important in the regulation of insulin release induced by non-carbohydrate nutrient secretagogues. Copyright (C) 1994 by W.B. Saunders Company