Effect of starvation or streptozotocin-diabetes on phosphate-activated glutaminase of different rat brain regions

Phosphate-activated glutaminase (PAG) was assayed in homogenates of brain cerebellum, hippocampus or striatum from normal, starved for 48 h or 120 h or streptozotocin-diabetic rats. Only the hippocampal enzyme was increased (47%) by diabetes. Starvation had no effect in any of the regions studied. PAG of synaptosomes or of non-synaptosomal mitochondria from the hippocampus was also increased by 48% and 22% respectively in diabetes. PAG of synaptosomes from the cortex, the cerebellum, or the striatum or of the non-synaptosomal mitochondria from the cortex were not affected by diabetes or prolonged (120h) starvation. A suggestion is presented that peripheral insulin, indirectly, may regulate PAG activity in a specific region of the rat brain. © 1992 Plenum Publishing Corporation

Leucine: Effector of phosphate activated glutaminase in rat cerebral cortex

Phosphate activated glutaminase (PAG) was assayed in whole homogenate and synaptosomes of cerebral cortex from normal or fasted for 120 h rats. The specific activity (s.a.) of PAG was found diminished by 25% in the whole homogenate from the fasted animals compared to the normal. On the contrary, fasting did not affect PAG s.a. of the synaptosomal fraction. Reconstitution experiments showed that when the deproteinized supernatant of the 12,500 g centrifugation from the fasted rats was added to the synaptosomes from either fed or fasted animals the PAG activity was diminished but there was no change when the corresponding supernatant from the fed animals was added to the synaptosomes from both conditions. When leucine at 5mM was added to the homogenate or to synaptosomes from fed or fasted animals the s.a. of PAG was significantly decreased. Even in the presence of aminooxyacetate the effect of leucine was observed. Branched chain amino acids i.e. leucine, isoleucine and valine at 0.5 mM each added to synaptosomes again decreased PAG activity. The addition of ketone bodies had no effect. It is suggested that leucine, because PAG has been implicated in the supply of transmitter glutamate, might be an important regulator of the pool of this neurotransmitter. © 1991 Plenum Publishing Corporation

Effects of fasting and diabetes on some enzymes and transport of glutamate in cortex slices or synaptosomes from rat brain

Phosphate-activated glutaminase (PAG) and glutamic acid decarboxylase (GAD) were assayed in homogenates and synaptosomes obtained from starved (48 hr or 120 hr) and diabetic (streptozotocin) rat brain cortex. Glutamine synthetase (GS) was assayed in homogenates, microsomal and soluble fractions, from brain cortex of similarly treated rats. l-Glutamate uptake and exit rates were determined in cortex slices and synaptosomes under the same conditions. The specific activity (s.a.) of PAG, a glutamate producing enzyme, decreased (50%) in the homogenate after 120-hr starvation. In synaptosomes it decreased (25%) only after 48-hr starvation. The s.a of GAD and GS, which are glutamate-consuming enzymes, were progressively increased with time of starvation, reaching 39% and 55% respectively after 120 hr. GS in the microsomes or the soluble fraction and GAD in the synaptosomes showed no change in s.a. under these conditions. Diabetes increased (40%) microsomal GS s.a. and decreased GAD s.a. (18%) in the homogenate. The l-glutamate uptake rate was decreased (48%) by diabetes in slices but not in synaptosomes. It is suggested that a) enzymes of the glutamate system respond differently in different subcellular fractions towards diabetes or deprivation of food and b) diabetes may affect the uptake system in glial cells but not in neurons. © 1988 Plenum Publishing Corporation