Uptake of L-glutamine into synaptosomes. Is the gamma-glutamyl cycle involved?

Mechanistic stoichiometric relationship between the rates of neurotransmission and neuronal glucose oxidation: Reevaluation of and alternatives to the pseudo-malate-aspartate shuttle model

The ~1:1 stoichiometry between the rates of neuronal glucose oxidation (CMRglc-ox-N) and glutamate (Glu)/γ-aminobutyric acid (GABA)-glutamine (Gln) neurotransmitter (NT) cycling between neurons and astrocytes (VNTcycle) has been firmly established. However, the mechanistic basis for this relationship is not fully understood, and this knowledge is critical for the interpretation of metabolic and brain imaging studies in normal and diseased brain. The pseudo-malate-aspartate shuttle (pseudo-MAS) model established the requirement for glycolytic metabolism in cultured glutamatergic neurons to produce NADH that is shuttled into mitochondria to support conversion of extracellular Gln (i.e., astrocyte-derived Gln in vivo) into vesicular neurotransmitter Glu. The evaluation of this model revealed that it could explain half of the 1:1 stoichiometry and it has limitations. Modifications of the pseudo-MAS model were, therefore, devised to address major knowledge gaps, that is, submitochondrial glutaminase location, identities of mitochondrial carriers for Gln and other model components, alternative mechanisms to transaminate α-ketoglutarate to form Glu and shuttle glutamine-derived ammonia while maintaining mass balance. All modified models had a similar 0.5 to 1.0 predicted mechanistic stoichiometry between VNTcycle and the rate of glucose oxidation. Based on studies of brain β-hydroxybutyrate oxidation, about half of CMRglc-ox-N may be linked to glutamatergic neurotransmission and localized in pre-synaptic structures that use pseudo-MAS type mechanisms for Glu-Gln cycling. In contrast, neuronal compartments that do not participate in transmitter cycling may use the MAS to sustain glucose oxidation. The evaluation of subcellular compartmentation of neuronal glucose metabolism in vivo is a critically important topic for future studies to understand glutamatergic and GABAergic neurotransmission.

The effect of ammonia and pH on brain γ-glutamyl transpeptidase in young rats

Acute hyperammonemia, induced by two consecutive injections of ammonium acetate (550 and 450 mg per kg b.wt.), decreased the activity of gamma-glutamyl transpeptidase (GGT) in most brain regions of 18- and 30-day-old rats. This decrease in the brain GGT activity was more pronounced in younger than in older rats. After the addition of NH4Cl to the incubation medium, the inhibitory action of NH4+ on this enzyme activity was also demonstrated in crude synaptosomal membranes at pH 7.4, but in a range of NH4+ concentrations many-times higher than those found in the plasma or brains of young hyperammonemic rats. Because similar concentrations of NH4+ stimulated the activity of the purified enzyme from rat kidney (mainly at pH 9.0), the inhibition of GGT activity in the young rat brain is probably mediated indirectly and not by a direct interaction of ammonia with the enzyme molecules.

Synaptosomal uptake of alpha-ketoglutarate and glutamine in thioacetamide-induced hepatic encephalopathy in rats

The kinetics of uptake of two astroglia-derived glutamate (GLU) precursors, alpha-ketoglutarate (alpha-KG) and glutamine (GLN) were determined in synaptosomes derived from rats with acute hepatic encephalopathy (HE) induced with a hepatotoxin, thioacetamide (TAA). TAA treatment increased by 33% Vmax for high affinity, low capacity alpha-KG uptake, without influencing its Km. The increase of the uptake capacity for alpha-KG may represent a response of the GLUergic nerve terminals to the decreased cerebral alpha-KG content, which during HE is associated with depressed activity of pyruvate carboxylase, an enzyme that replenishes alpha-KG in astrocytes. The result is thus consistent with the notion that HE affects the astroglial control of GLUergic neurotransmission. The Km and Vmax for the low affinity, high capacity GLN uptake was not affected by TAA treatment.

?-Glutamylaminotransferase and Transglutaminase in Subcellular Fractions of Rat Cortex and in Cultured Astrocytes

The subcellular distribution of gamma-glutamylamino-transferase (EC 2.3.2.2) and transglutaminase (EC 2.3.2.1) has been investigated in rat brain tissue fractionated by a centrifugation and sedimentation technique. Neither of these enzymes was enriched in the synaptosomal fraction. Comparing the in vitro grown astrocytes with synaptosomes, we find that both of these enzymes may possibly be more important in the glial element of the synaptic region. gamma-Glutamylaminotransferase is most abundant in capillaries, confirming previous reports.

Toxicology of quinone-thioethers

Cytotoxicity associated with exposure to quinones has generally been attributed to either redox cycling, and the subsequent development of "oxidative stress," and/or to their interaction with cellular nucleophiles, such as protein and non-protein sulfhydryls. Glutathione (GSH) is the major non-protein sulfhydryl present in cells, and conjugation of potentially toxic electrophiles with GSH is usually associated with detoxication and excretion. However, this review discusses the biological (re)activity of quinone-thioethers. For example, quinone-thioethers are (1) capable of redox cycling (2) substrates for, and inhibitors of, a variety of enzymes (3) methemoglobinemic (4) potent nephrotoxicants (5) DNA reactive and (6) may contribute to quinone-mediated carcinogenicity and neurotoxicity. The ubiquitous nature of quinones, and the high intracellular concentrations of GSH, ensures that cells and tissues will be exposed to quinone-thioethers. The toxicological importance of quinone-thioethers in quinone-mediated toxicities therefore deserves further attention.

Transport of asparagine by rat brain synaptosomes: an approach to evaluate glutamine accumulation

Isolated rat brain synaptosomes accumulated L-asparagine with a Km value of 348 microM and a Vmax value of 3.7 nmol/mg of protein/min at 28 degrees C. Uptake of L-asparagine was inhibited by the presence of L-glutamine, whereas transport of L-glutamine was blocked by L-asparagine. Alanine, serine, cysteine, threonine, and, in particular, leucine were also inhibitory whereas alpha-(methylamino)isobutyrate, ornithine, lysine, arginine, and glutamate were much less effective blockers. Transport of L-asparagine had a substantial sodium-dependent component, whereas that of the D-stereoisomer was almost unaffected by the presence or absence of the cation. L-Asparagine was accumulated to a maximal gradient, [L-Asn]i/[L-Asn]o, of 20-30, and this value was reduced to 5-6 by withdrawal of sodium or addition of high [KCI]. A plot of log [Na+]o/[Na+]i against the log [L-Asn]i/[L-Asn]o had a slope close to I, which indicates that a single sodium ion is transported inward with each asparagine molecule. It is postulated that uptake of L-asparagine occurs, to a large extent, in cotransport with Na+ and that it utilizes the sodium chemical gradient and the membrane electrical potential as the source of energy. The similarity between the L-asparagine and L-glutamine transport systems and the reciprocal inhibition of influx of the two amino acids suggest that the same mechanism is responsible for glutamine accumulation. This could explain the high [Gln]i maintained by the brain in vivo.

The effect of acute and repeated hyperammonemia on gamma-glutamyl transpeptidase in homogenates and capillaries of various rat brain regions

The effect of hyperammonemia of varying degree and duration on the gamma-glutamyl-transpeptidase (GGT) activity was studied in the homogenates and capillaries of different brain regions of the rat. "Acute" hyperammonemia (750 and 600 mg of ammonium acetate per kg b.w. were injected i.p. at 30 min interval, and the animals were decapitated immediately), in which blood ammonia was increased 14-fold, and brain ammonia six-fold above the control level, produced a 20% increase of the enzyme activity in cerebellum, and a 17% decrease in gyrus dentatus, but had no effect in the frontal cortex and the CA1 and CA3 regions of hippocampus. "Subchronic" hyperammonemia (two injections of 600 mg ammonium acetate/kg were given at 24 h intervals, and tissue samples were removed 24 h later), that was accompanied by only a 60% increase of blood or brain ammonia, increased the activity in cerebellum to 38% above control, but produced no effect in the other brain regions. "Chronic" hyperammonemia (three injections of 600 mg ammonium acetate/kg at 24 h intervals and excision of tissue samples 30 min after the last injection), in which blood and brain ammonia were, respectively, 60 and 100% higher than in control animals, elevated the GGT activity in the cerebellum by 57%, in CA1 by 15%, and in CA3 by 21%, but produced no effect in the frontal cortex or gyrus dentatus. By contrast, "chronic" hyperammonemia produced a 30% increase of GGT activity in cerebral cortical capillaries, but only a 10% increase in hippocampal capillaries, and no change in cerebellar capillaries. The results suggest that, hyperammonemia of relatively long duration may contribute to the enhancement of brain GGT activity observed in chronic forms of hepatic encephalopathy. However, ammonia does not appear to activate the enzyme directly.

Glutathione, gamma-glutamyl transpeptidase, and the mercapturic acid pathway as modulators of 2-bromohydroquinone oxidation

Glutathione (GSH) conjugates of 2-bromohydroquinone are more difficult to oxidize than the parent hydroquinone. Hydrolysis catalyzed by gamma-glutamyl transpeptidase (gamma-GT), however, results in the formation of the corresponding cysteine conjugate which is more readily oxidized than the parent hydroquinone. N-Acetylation of the cysteine conjugate to yield the mercapturate regenerates a compound that is more stable to oxidation than either the parent quinol or its cysteine conjugate. Thus, 2-bromohydroquinone oxidation is exquisitely modulated via GSH conjugation and its metabolism through the mercapturic acid pathway. The ability of gamma-GT to facilitate quinol oxidation by catalyzing the formation of the labile cysteine conjugate may have important biological consequences. Cells exhibiting high gamma-GT activity may be predisposed to the potentially toxic effects of quinol-thioethers.

Synthesis of transmitter glutamate and the glial-neuron interrelationship

Glutamate in glutamatergic neurons exists in a cytosolic pool, as well as a transmitter pool, which is assumed to be localized in synaptic vesicles. Transmitter glutamate released from glutamatergic neurons is taken up by both neurons and glial cells, giving rise to a flux of glutamate from neurons to astrocytes. In astrocytes, glutamine is formed from glutamate by the glial-specific enzyme glutamine synthetase (EC 6.3.1.2). Glutamine diffuses back to neurons, where glutamate is formed by phosphate-activated glutaminase (EC 3.5.1.2). However, this cycle is not stoichiometric, and glutamine obtained from glial cells cannot replenish all transmitter glutamate lost from neurons. 2-Oxoglutarate is another putative precursor for transmitter glutamate. Net synthesis of citric acid cycle intermediates is dependent on carbon dioxide fixation to pyruvate, catalyzed by pyruvate carboxylase (EC 6.4.1.1). Since this enzyme is exclusively glial, a net flow of citric acid cycle intermediates from glial cells to neurons probably exists. The quantitative contribution of each transmitter precursor may not be the same in different regions of the brain and may vary with the metabolic state of the neuron. The pool of transmitter glutamate is most likely regulated by the activity of glutamate-forming enzymes in the nerve terminal, and/or by uptake/release of glutamate and glutamate precursors through the synaptosomal plasma membrane.

Changes in glutamate-related enzyme activities in the striatum of the rat following lesion of corticostriatal fibres

The behaviour of enzymes putatively involved in glutamate/aspartate transmitter metabolism (glutamate dehydrogenase, aspartate amino-transferase, alanine aminotransferase, gamma-glutamyl-transpeptidase) was studied in the striatum 3, 7, 14 days and 7 weeks after mechanical destruction of corticostriatal fibres. For a period of up to seven days after unilateral lesion, enzyme activities were significantly diminished (by up to 13% based on protein) in the ipsilateral striatum as compared to the striatum of the intact side. Later, the enzyme activities in the ipsilateral striatum recovered. After seven weeks, an increase was observed for glutamate dehydrogenase activity, whereas the activity of alanine aminotransferase showed a transient rise enzyme levels is interpreted as being attributable to the destruction of nerve endings which are considered to be glutamatergic, interfering with various compensating processes (e.g. glial cell proliferation) which occur with advancing times after lesion.

High-affinity transport of L-glutamine by a plasma membrane preparation from rat brain

Plasma membrane vesicles prepared from rat brain contain a saturable, high-affinity transport system for L-glutamine that exhibits the following characteristics: (1) The rate of L-glutamine transport is linear up to 200 micrograms/mL membrane protein. (2) Transport of [3H]-L-glutamine is linear with time for at least 10 min, is significantly reduced by lowering the assay temperature to 4 degrees C, and is essentially abolished by the addition of excess unlabeled L-glutamine. (3) The transport rate is optimal in the range of pH 7.4-8.2. (4) The system exhibits a Km for L-glutamine of approximately 1.7 microM and a Vmax of approximately 46 pmol/(min.mg of protein). (5) The system is not highly dependent upon the addition of monovalent or divalent cations. (6) Inhibitor studies reveal that the amino acid amides exhibit the highest affinity for the system and that there is a high specificity for the L-isomers.

Changes in the activity of gamma-glutamyl transpeptidase induced by kainic acid and surgical lesions of the hippocampal formation in young rats

To study possible functional involvement of gamma-glutamyl transpeptidase (GGT) in glutamate transmitter metabolism we lesioned putative glutamatergic structures of the rat hippocampal formation by intracerebroventricular (i.c.v.) injection of kainic acid (KA) or by surgical CA3 axotomy. Unilateral injection of KA into the left lateral cerebral ventricle of 30-day-old rats resulted in decreased GGT activity in hippocampal areas CA3, Ca1 ipsilaterally, and in the contralateral area CA1, four hours after the induction of the chemical lesion. Four days after the injection, the enzyme activity was decreased in all hippocampal areas with the exception of the contralateral dentate gyrus. Four days after bilateral i.c.v. injection of KA, lower GGT levels were found than was seen after bilateral surgical lesion of the CA3 pyramidal cell axons (Schaffer's collaterals). The surgical lesion was followed by a decrease of GGT only in the stratum pyramidale and stratum radiatum of area CA1. In contrast to the effects in 30-day-old rats, unilateral i.c.v. injection of KA on postnatal day 12 did not alter the GGT activity in any studied hippocampal area presumably because of incomplete maturation of structures required for KA vulnerability.

Changes of cerebral gamma glutamyltransferase activities after treatment with exogenous inducers

The activity of gamma-glutamyltransferase localized in isolated brain synaptic membranes- and microvessels-enriched fractions was assayed after treatment of rats with either phenobarbital or ethanol. Phenobarbital increased the activity of gamma-glutamyltransferase in microvessels, without alteration of synaptic membranes activity. An increase of enzyme activity was also obtained after a chronic intoxication with ethanol. These results suggest that the isoform of gamma-glutamyltransferase localized in brain microvessels may respond to exogenous inducers.

Uptake and release for glutamine and glutamate in a crude synaptosomal fraction from rat brain

[14C]Glutamine uptake in a crude synaptosomal (P2) fraction, (representing the sum of [14C]glutamine accumulated and [14C]glutamate formed by hydrolysis), is distinct from glutamate uptake. Glutamine uptake is Na+-independent and unaffected by the Na+-K+-ATPase inhibitor ouabain, whereas glutamate uptake is Na+-dependent and inhibited by ouabain. The uptake of both glutamine and glutamate is unaffected by the gamma-glutamyltransferase inhibitor, Acivicin. This indicates that glutamine uptake is not mediated by a carrier, as distinct from that of glutamate, and also not linked to gamma-glutamyl-transferase. Na+ affects the distribution of glutamine-derived glutamate by increasing the synaptosomal content and reducing that of the medium. When glutamate release from synaptosomes preloaded with [14C]glutamate is measured by superfusion technique in order to prevent reuptake, Na+ has been found to inhibit release in a non-depolarizing medium (Ringer buffer with no Ca2+) of the [14C]glutamate as well as of endogenous glutamate. The specific activity of the [14C]glutamine-derived glutamate in the incubation medium is much higher than that in the synaptosomes, indicating that there exists a readily releasable pool of newly formed glutamate in addition to another pool. The latter glutamate pool is partially reduced by Na+.

Developmental changes in gamma-glutamyl transpeptidase activity in nervous tissues with reference to amino acid transmission processes

In homogenate supernatants of hippocampal formation and cerebellum of the rat, gamma-glutamyl transpeptidase (gamma-GTP) activities increased about 6 times from postnatal day 6 (0.178 +/- 0.02 and 0.187 +/- 0.007 U/g wet wt., respectively) to day 100. In dorsal root ganglia (0.183 +/- 0.003 U/g at day 6) and superior cervical ganglia (0.188 +/- 0.019 U/g at day 6) in which apparently amino acidergic transmission processes do not occur, enzyme activities were seen to go up 4 times and 2.4 times, respectively. Based on protein, enzyme activities in both brain material and dorsal root ganglia showed a similar pattern, whereas the activity increase in superior cervical ganglia was somewhat gradual (1.4 times from day 6 to day 100). Postnatal changes in gamma-GTP activities indicated a functional correlation with the maturation of amino acidergic structures. Kainic acid added to hippocampal extracts (0.45 and 5.0 mM) and, for comparison, to those of kidney (5.0 mM) did not yield any statistically significant effect on gamma-GTP activity.

Changes in cortical amino acids during electrical kindling in rats

The effect of kindling rats by electrical stimulation of the frontal cortex for approx. 3 months on the concentrations of amino acids (taurine, aspartate, glutamate, glutamine and GABA) in the cerebral cortex has been studied, as well as the release of endogenous amino acids from kindled slices of brain in vitro. In these kindled rats, killed 1 week after the last shock, there were no changes in any concentrations of amino acids. However, when cortical slices, prepared from either the control or kindled rats, were stimulated in vitro by exposure to veratrine, more endogenous glutamate and aspartate were released from the kindled tissue than from the control. The neurotransmitter amino acids, glutamate and aspartate, may be involved in the permanent effects of electrical kindling.

Developmental change of endogenous glutamate and gamma-glutamyl transferase in cultured cerebral cortical interneurons and cerebellar granule cells, and in mouse cerebral cortex and cerebellum in vivo

The developmental change of endogenous glutamate, as correlated to that of gamma-glutamyl transferase and other glutamate metabolizing enzymes such as phosphate activated glutaminase, glutamate dehydrogenase and aspartate, GABA and ornithine aminotransferases, has been investigated in cultured cerebral cortex interneurons and cerebellar granule cells. These cells are considered to be GABAergic and glutamatergic, respectively. Similar studies have also been performed in cerebral cortex and cerebellum in vivo. The developmental profiles of endogenous glutamate in cultured cerebral cortex interneurons and cerebellar granule cells corresponded rather closely with that of gamma-glutamyl transferase and not with other glutamate metabolizing enzymes. In cerebral cortex and cerebellum in vivo the developmental profiles of endogenous glutamate, gamma-glutamyl transferase and phosphate activated glutaminase corresponded with each other during the first 14 days in cerebellum, but this correspondence was less good in cerebral cortex. During the time period from 14 to 28 days post partum the endogenous glutamate concentration showed no close correspondence with any particular enzyme. It is suggested that gamma-glutamyltransferase regulates the endogenous glutamate concentration in cultured neurons. The enzyme may also be important for regulation of endogenous glutamate in brain in vivo and particularly in cerebellum during the first 14 days post partum. Gamma-glutamyl transferase in cultured neurons and brain tissue in vivo appears to be devoid of maleate activated glutaminase.