Regulation of glutaminase by exogenous glutamate, ammonia and 2-oxoglutarate in synaptosomal enriched preparation from rat brain

Kinetic properties of glutaminase from cerebral cortex

The rates of phosphate activated glutaminase activity in finely homogenised cerebral cortex and synaptosomes were measured. Activity was 25-50% higher at pH 7.0 than at pH 8.0. Glutamate inhibited activity with a Ki of 2-3 mM while aspartate had little effect. Calcium (1 mM) activated the enzyme but magnesium was without action. The pH profiles of the effects of these modulators of glutaminase activity in these finely ground preparations showed that all agents were more effective at pH 7.0 than at pH 8.0.

Comparative investigations of glutaminase development in cerebral cortex of chick embryo and in primary cultures of neurons and glial cells

Glutaminase activity was determined in pure cultures of neurons, glial cells and in mixed cultures obtained from chick embryo brain. The development of this enzyme was observed periodically over time and compared to its evolution in corresponding cerebral hemispheres during embryonic and postnatal development. The specific activity of brain glutaminase increased between the twelfth and sixteenth day of embryogenesis. A similar increase was observed in cultures of neuroblasts during the corresponding period of time, although the activity in culture was about one-third lower than in vivo. In contrast to neurons, there was no significant increase of glutaminase activity in glial cells before the fifteenth day of culture. The enzyme level in glial cells between the thirteenth and fifteenth days of culture was approximately 25% of that in 7- and 8-day-old neurons. The different development of glutaminase activity in neurons and glial cells was demonstrated in both pure and mixed cultures. The results support the hypothesis that there is a glutamine shunt from glial cells to neurons.

Feline generalized penicillin epilepsy: changes of glutamic acid and taurine parallel the progressive increase in excitability of the cortex

A coinciding temporal sequence of electrophysiological and biochemical correlates of developing generalized penicillin epilepsy in cats may indicate a "cause and effect" relationship between the two phenomena. After intramuscular injection of penicillin, in the pre-epileptic state prior to the onset of spike-and-wave discharge, the cortical content of glutamic acid decreases. This change occurs when an increased amplitude of visual evoked potentials in association cortex heralds the approach of spike-and-wave activity. The decrease of glutamic acid and that of aspartic acid occur in parallel with an almost stoichiometric increase of glutamine, gamma-aminobutyric acid (GABA), or both, while taurine levels in the pre-epileptic state remain near normal. As the pre-epileptic progresses to the epileptic state, characterized by generalized 4-5 cycles/s spike-and-wave discharges, a failure of the glial capture mechanisms for taurine and glutamate appears to occur, since both amino acids are lost from the tissue and glutamine levels fall while GABA levels are maintained or become elevated but increasingly at the expense of aspartic acid. A presumed increase in interstitial glutamic acid concentration possibly in combination with subsequent failure of GABA inhibition appears the most plausible explanation for the increasing hyperexcitability during the development of feline generalized penicillin epilepsy.

Calcium stimulation of glutamine hydrolysis in synaptosomes from rat brain

Calcium stimulates the hydrolysis of glutamine in synaptosomes prepared from rat brain both by the sucrose- (12) and the Ficoll/sucrose-gradient techniques (13). The calcium activation is phosphate-dependent and maximal effect is obtained at a calcium concentration of 0.5-1.0 mM. It is reduced by increasing the numbers of synaptosomes in the incubation mixture, and abolished by the product inhibitors of glutaminase, glutamate and ammonia, but unaffected by the uncoupler 2,4-dinitrophenol which inhibits the mitochondrial proton pump. Moreover, since the hydrolysis of glutamine is mediated by glutaminase (EC 3.5.1.2), and calcium does not activate the purified enzyme, an indirect phosphate-dependent effect of calcium on glutaminase is most likely. Calcium activates preferentially the N-ethylmaleimide insensitive fraction of glutaminase. The calcium activation is not dependent on synaptosomal membranes as it is found in synaptosomes subject to previous freezing. It is also found in isolated synaptosomal mitochondria and is thus a property of nerve endings. The calcium activation of glutaminase is unaffected by potassium in depolarizing concentrations, and may not be directly involved in the neurotransmission processes, but possibly in replenishing depleted stores of transmitter glutamate.

Properties of phosphate activated glutaminase in astrocytes cultured from mouse brain

Astrocytes in primary cultures contain a relatively high activity of phosphate activated glutaminase, although it is significantly lower than that of synaptosomal enriched preparations. The relatively high glutaminase activity in the astrocytes appears not to be caused by substrate induction, since a 10-fold variation in the glutamine concentration of the culture medium does not affect the activity. Of the reaction products, only glutamate inhibits astrocytic glutaminase whereas that of synaptosomal enriched preparations is inhibited by both glutamate and ammonia. Similar to the synaptosomal enzyme, glutaminase in astrocytes is inhibited about 50% by N-ethylmaleimide, indicating N-ethylmaleimide-sensitive and -insensitive compartments of the enzyme. Calcium activates glutaminase in astrocytes as in synaptosomes, by promoting phosphate activation. Except for the lower activity and the lack of effect of ammonia, the properties of the astroglial glutaminase has been found to be no different from that of the synaptosomal one. The relatively unrestrained astroglial glutaminase may, however, argue against the concept of a glutamine cycle operating in a stoichiometric manner.