Comparison of the phosphate-dependent glutaminase obtained from rat brain and kidney

Glutaminase in microglia: A novel regulator of neuroinflammation

Neuroinflammation is the inflammatory responses that are involved in the pathogenesis of most neurological disorders. Glutaminase (GLS) is the enzyme that catalyzes the hydrolysis of glutamine to produce glutamate. Besides its well-known role in cellular metabolism and excitatory neurotransmission, GLS has recently been increasingly noticed to be up-regulated in activated microglia under pathological conditions. Furthermore, GLS overexpression induces microglial activation, extracellular vesicle secretion, and neuroinflammatory microenvironment formation, which, are compromised by GLS inhibitors in vitro and in vivo. These results indicate that GLS has more complicated implications in brain disease etiology than what are previously known. In this review, we introduce GLS isoforms, expression patterns in the body and the brain, and expression/activities regulation. Next, we discuss the metabolic and neurotransmission functions of GLS. Afterwards, we summarize recent findings of GLS-mediated microglial activation and pro-inflammatory extracellular vesicle secretion, which, in turns, induces neuroinflammation. Lastly, we provide a comprehensive discussion for the involvement of microglial GLS in the pathogenesis of various neurological disorders, indicating microglial GLS as a promising target to treat these diseases.

An appraisal of the in vivo role of phosphate as a modulator of urinary ammonium and titratable acid excretion in the acidotic rabbit

Previous studies have shown that the intravenous infusion of inorganic phosphate increased urinary ammonium excretion 8- to 10-fold in the acidotic rabbit. This was considered to be a very important observation at the time and to be unique to the rabbit. While investigating this finding, we discovered that the formol titration procedure, used to measure urinary ammonium by this research group, is subject to interference by phosphate, casting doubt on the validity of the urinary ammonium excretion data reported by them in the literature. In order to re-assess the importance of phosphate as a potential modulator of urinary ammonium excretion in the acidotic rabbit, renal net acid excretion studies were carried out in phosphate-loaded acidotic animals. We observed that while urinary ammonium excretion increased significantly (p < 0.05) after 50 min of phosphate infusion, the maximum concentrations excreted were substantially less than previously reported in the literature. However, through its urinary buffering capacity, we observed that inorganic phosphate, via an experimentally induced phosphaturia, could substantially enhance titratable acid excretion. Contrary to earlier reports, we demonstrated that phosphate plays a relatively minor in vivo modulator role in enhancing renal net acid excretion through the vehicle of ammonium during acute metabolic acidosis in the hyperphosphataemic rabbit. The findings reported in this study constitute an important update on ammonia metabolism in the acidotic rabbit.

A tale of two glutaminases: homologous enzymes with distinct roles in tumorigenesis

Many cancer cells exhibit an altered metabolic phenotype, in which glutamine consumption is upregulated relative to healthy cells. This metabolic reprogramming often depends upon mitochondrial glutaminase activity, which converts glutamine to glutamate, a key precursor for biosynthetic and bioenergetic processes. Two isozymes of glutaminase exist, a kidney-type (GLS) and a liver-type enzyme (GLS2 or LGA). While a majority of studies have focused on GLS, here we summarize key findings on both glutaminases, describing their structure and function, their roles in cancer and pharmacological approaches to inhibiting their activities.

Glutaminases

Mammalian glutaminases catalyze the stoichiometric conversion of L-glutamine to L-glutamate and ammonium ions. In brain, glutaminase is considered the prevailing pathway for synthesis of the neurotransmitter pool of glutamate. Besides neurotransmission, the products of glutaminase reaction also fulfill crucial roles in energy and metabolic homeostasis in mammalian brain. In the last years, new functional roles for brain glutaminases are being uncovered by using functional genomic and proteomic approaches. Glutaminases may act as multifunctional proteins able to perform different tasks: the discovery of multiple transcript variants in neurons and glial cells, novel extramitochondrial localizations, and isoform-specific proteininteracting partners strongly support possible moonlighting functions for these proteins. In this chapter, we present a critical account of essential works on brain glutaminase 80 years after its discovery. We will highlight the impact of recent findings and thoughts in the context of the glutamate/glutamine brain homeostasis.

Loss of function mutation of the Slc38a3 glutamine transporter reveals its critical role for amino acid metabolism in the liver, brain, and kidney

Glutamine, the most abundant amino acid in mammals, is critical for cell and organ functions. Its metabolism depends on the ability of cells to take up or release glutamine by transporters located in the plasma membrane. Several solute carrier (SLC) families transport glutamine, but the SLC38 family has been thought to be mostly responsible for glutamine transport. We demonstrate that despite the large number of glutamine transporters, the loss of Snat3/Slc38a3 glutamine transporter has a major impact on the function of organs expressing it. Snat3 mutant mice were generated by N-ethyl-N-nitrosurea (ENU) mutagenesis and showed stunted growth, altered amino acid levels, hypoglycemia, and died around 20 days after birth. Hepatic concentrations of glutamine, glutamate, leucine, phenylalanine, and tryptophan were highly reduced paralleled by downregulation of the mTOR pathway possibly linking reduced amino acid availability to impaired growth and glucose homeostasis. Snat3-deficient mice had altered urea levels paralleled by dysregulation of the urea cycle, gluconeogenesis, and glutamine synthesis. Mice were ataxic with higher glutamine but reduced glutamate and gamma-aminobutyric acid (GABA) levels in brain consistent with a major role of Snat3 in the glutamine-glutamate cycle. Renal ammonium excretion was lower, and the expression of enzymes and amino acid transporters involved in ammoniagenesis were altered. Thus, SNAT3 is a glutamine transporter required for amino acid homeostasis and determines critical functions in various organs. Despite the large number of glutamine transporters, loss of Snat3 cannot be compensated, suggesting that this transporter is a major route of glutamine transport in the liver, brain, and kidney.

Effect of dietary protein restriction on renal ammonia metabolism

Dietary protein restriction has multiple benefits in kidney disease. Because protein intake is a major determinant of endogenous acid production, it is important that net acid excretion change in parallel during protein restriction. Ammonia is the primary component of net acid excretion, and inappropriate ammonia excretion can lead to negative nitrogen balance. Accordingly, we examined ammonia excretion in response to protein restriction and then we determined the molecular mechanism of the changes observed. Wild-type C57Bl/6 mice fed a 20% protein diet and then changed to 6% protein developed an 85% reduction in ammonia excretion within 2 days, which persisted during a 10-day study. The expression of multiple proteins involved in renal ammonia metabolism was altered, including the ammonia-generating enzymes phosphate-dependent glutaminase (PDG) and phosphoenolpyruvate carboxykinase (PEPCK) and the ammonia-metabolizing enzyme glutamine synthetase. Rhbg, an ammonia transporter, increased in expression in the inner stripe of outer medullary collecting duct intercalated cell (OMCDis-IC). However, collecting duct-specific Rhbg deletion did not alter the response to protein restriction. Rhcg deletion did not alter ammonia excretion in response to dietary protein restriction. These results indicate 1) dietary protein restriction decreases renal ammonia excretion through coordinated regulation of multiple components of ammonia metabolism; 2) increased Rhbg expression in the OMCDis-IC may indicate a biological role in addition to ammonia transport; and 3) Rhcg expression is not necessary to decrease ammonia excretion during dietary protein restriction.

Diphenylarsinic acid promotes degradation of glutaminase C by mitochondrial Lon protease

Glutaminase C (GAC), a splicing variant of the kidney-type glutaminase (KGA) gene, is a vital mitochondrial enzyme protein that catalyzes glutamine to glutamate. Earlier studies have shown that GAC proteins in the human hepatocarcinoma cell line, HepG2, were down-regulated by diphenylarsinic acid (DPAA), but the mechanism by which DPAA induced GAC protein down-regulation remained poorly understood. Here, we showed that DPAA promoted GAC protein degradation without affecting GAC transcription and translation. Moreover, DPAA-induced GAC proteolysis was mediated by mitochondrial Lon protease. DPAA insolubilized 0.5% Triton X-100-soluble GAC protein and promoted the accumulation of insoluble GAC in Lon protease knockdown cells. DPAA destroyed the native tetrameric GAC conformation and promoted an increase in the unassembled form of GAC when DPAA was incubated with cell extracts. Decreases in the tetrameric form of GAC were observed in cells exposed to DPAA, and decreases occurred prior to a decrease in total GAC protein levels. In addition, decreases in the tetrameric form of GAC were observed independently with Lon protease. Mitochondrial heat shock protein 70 is known to be an indispensable protein that can bind to misfolded proteins, thereby supporting degradation of proteins sensitive to Lon protease. When cells were incubated with DPAA, GAC proteins that can bind with mtHsp70 increased. Interestingly, the association of mtHsp70 with GAC protein increased when the tetrameric form of GAC was reduced. These results suggest that degradation of native tetrameric GAC by DPAA may be a trigger in GAC protein degradation by Lon protease.

Glutaminase immunoreactivity and enzyme activity is increased in the rat dorsal root ganglion following peripheral inflammation

Following inflammation, primary sensory neurons in the dorsal root ganglion (DRG) alter the production of several proteins. Most DRG neurons are glutamatergic, using glutaminase as the enzyme for glutamate production, but little is known about glutaminase following inflammation. In the present study, adjuvant-induced arthritis (AIA) was produced in rats with complete Freund's adjuvant into the hindpaw. At 7 days of AIA, DRG were examined with glutaminase immunohistochemistry, Western blot immunoreactivity, and enzyme activity. Image analysis revealed that glutaminase was elevated most in small-sized neurons (21%) (P < 0.05). Western blot analysis revealed a 19% increase (P < 0.05) in total glutaminase and 21% in mitochondrial glutaminase (P < 0.05). Glutaminase enzyme activity was elevated 29% (P < 0.001) from 2.20 to 2.83 moles/kg/hr. Elevated glutaminase in primary sensory neurons could lead to increased glutamate production in spinal primary afferent terminals contributing to central sensitization or in the peripheral process contributing to peripheral sensitization.

Systems biology of the metabolic network regulated by the Akt pathway

Cancer has been proposed as an example of systems biology disease or network disease. Accordingly, tumor cells differ from their normal counterparts more in terms of intracellular network dynamics than single markers. Here we shall focus on a recently recognized hallmark of cancer, the deregulation of cellular energetics. The constitutive activation of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway has been confirmed as an essential step toward cell transformation. We will consider how the effects of Akt activation are connected with cell metabolism; more precisely, we will review existing metabolic models and discuss the current knowledge available to construct a kinetic model of the most relevant metabolic processes regulated by the PI3K/Akt pathway. The model will enable a systems biology approach to predict the metabolic targets that may inhibit cell growth under hyper activation of Akt.

The activity of phosphate-dependent glutaminase from the rat small intestine is modulated by ADP and is dependent on integrity of mitochondria

The effect of adenine nucleotides and phosphate on rat small intestine phosphate-dependent glutaminase (PDG) activity was investigated in intact mitochondria. Disruption of the integrity of mitochondria by sonication or freeze-thawing resulted in loss of enzyme activity. ADP was the strongest adenine nucleotide activator of the enzyme giving a V(max) that was over 5-fold of that for AMP or ATP. The sigmoid activation curve of PDG by ADP became hyperbolic in presence ATP. ADP also lowered the K(m) for glutamine and increased V(max) and these effects were further enhanced by the presence of ATP. Activation of PDG by phosphate and ADP was not completely additive suggesting some antagonism between the activators. There was no clear relationship between changing ATP/ADP ratios and PDG activity in presence of a constant concentration of phosphate. However, ratios of approximately 1:4 and 4:1 gave the highest and lowest activities, respectively. The pH dependence of PDG activity was affected by phosphate concentration and results suggest that the divalent ion is the activating species.

Brain glutaminases

Glutaminase is considered as the main glutamate producer enzyme in brain. Consequently, the enzyme is essential for both glutamatergic and gabaergic transmissions. Glutamine-derived glutamate and ammonia, the products of glutaminase reaction, fulfill crucial roles in energy metabolism and in the biosynthesis of basic metabolites, such as GABA, proteins and glutathione. However, glutamate and ammonia are also hazardous compounds and danger lurks in their generation beyond normal physiological thresholds; hence, glutaminase activity must be carefully regulated in the mammalian brain. The differential distribution and regulation of glutaminase are key factors to modulate the metabolism of glutamate and glutamine in brain. The discovery of novel isoenzymes, protein interacting partners and subcellular localizations indicate new functions for brain glutaminase. In this short review, we summarize recent findings that point consistently towards glutaminase as a multifaceted protein able to perform different tasks. Finally, we will highlight the involvement of glutaminase in pathological states and its consideration as a potential therapeutic target.

Fixative composition alters distributions of immunoreactivity for glutaminase and two markers of nociceptive neurons, Nav1.8 and TRPV1, in the rat dorsal root ganglion

Most, if not all, dorsal root ganglion (DRG) neurons use the neurotransmitter glutamate. There are, however, conflicting reports of the percentages of DRG neurons that express glutaminase (GLS), the enzyme that synthesizes glutamate, ranging from 30% to 100% of DRG neurons. Defining DRG neuron populations by the expression of proteins like GLS, which indicates function, is routinely accomplished with immunolabeling techniques. Proper characterization of DRG neuron populations relies on accurate detection of such antigens. It is known intuitively that fixation can alter immunoreactivity (IR). In this study, we compared the effects of five formaldehyde concentrations between 0.25% and 4.0% (w/v) and five picric acid concentrations between 0.0% and 0.8% (w/v) on the IR of GLS, the voltage-gated sodium channel 1.8 (Na(v)1.8), and the capsaicin receptor TRPV1. We also compared the effects of five incubation time lengths from 2 to 192 hr, in primary antiserum on IR. Lowering formaldehyde concentration elevated IR for all three antigens, while raising picric acid concentration increased Na(v)1.8 and TRPV1 IR. Increasing IR improved detection sensitivity, which led to higher percentages of labeled DRG neurons. By selecting fixation conditions that optimized IR, we found that all DRG neurons express GLS, 69% of neurons express Na(v)1.8, and 77% of neurons express TRPV1, indicating that some previous studies may have underestimated the percentages of DRG neurons expressing these proteins. This manuscript contains online supplemental material at http://www.jhc.org. Please visit this article online to view these materials.

New insights into brain glutaminases: beyond their role on glutamatergic transmission

The synthesis of glutamate in brain must be exquisitely regulated because of its harmful potential giving rise to excitotoxic damage. In this sense, a stringent control based on multiple regulatory mechanisms should be expected to be exhibited by the biosynthetic enzymes responsible of glutamate generation, to assure that glutamate is only synthesized at the right place and at the right time. Glutaminase is considered as the main glutamate-producer enzyme in brain. Recently, novel glutaminase isoforms and extramitochondrial locations for these proteins have been discovered in the brain of mammals: identifying the function of each isozyme is essential for understanding the role of glutaminases in cerebral function. In addition, the interactome of glutaminases is starting to be uncovered adding a new level of regulatory complexity with important functional consequences, including selective and regulated targeting to concrete cellular locations. Finally, recent progress has identified glutaminase to be also present in astrocytes which precludes its classical consideration as a neuron-specific enzyme.

A novel glutaminase isoform in mammalian tissues

The synthesis of neurotransmitter glutamate in brain is mainly carried out by glutaminase enzymes. This synthesis must be exquisitely regulated because of its harmful potential giving rise to excitotoxic damage. It is noteworthy that two glutaminase isozymes coded by different genes are expressed in the brain of mammals. The need for two genes and two isozymes to support the single process of glutamate synthesis is unexplained, and identifying the role of each glutaminase is an important factor in understanding glutamate-mediated neurotransmission. Multiple transcripts for glutaminase genes and simultaneous expression of glutaminase isoforms have been reported in mammalian tissues and cells. The recent discovery of protein interacting partners widens the possibilities of regulatory mechanisms controlling these biosynthetic enzymes. The expression of distinct isozymes and binding partners may represent the biochemical and molecular basis to achieve fine-tuning control of glutamate synthesis in different cell types or developmental states. In this review, we will briefly summarize recent works on glutaminase proteins in mammals, with particular emphasis on brain studies. We present convergent evidence supporting the existence of a novel glutaminase isozyme in mammalian tissues.

Novel form of phosphate activated glutaminase in cultured astrocytes and human neuroblastoma cells, PAG in brain pathology and localization in the mitochondria

A novel form of phosphate activated glutaminase (PAG), catalyzing the synthesis of glutamate from glutamine, has been detected in cultured astrocytes and SH-SY5Y neuroblastoma cells. This enzyme form is different from that of the kidney and liver isozymes. In these cells we found high enzyme activity, but no or very weak immunoreactivity against the kidney type of PAG, and no immunoreactivity against the liver type. PAG was also investigated in brain under pathological conditions. In patients with Down's syndrome the immunoreactivity in the frontoparietal cortex was significantly reduced. The findings leading to our conclusion of a functionally active PAG on the outer face of the inner mitochondrial membrane are discussed, and a model is presented.

Co-localization of glutamic acid decarboxylase and phosphate-activated glutaminase in neurons of lateral reticular nucleus in feline thalamus

Immunohistochemical methods were used to label singly and/or in combination glutamic acid decarboxylase (GAD, the sole synthesizing enzyme for the inhibitory neurotransmitter gamma-aminobutyric acid) and phosphate-activated glutaminase (GLN, a synthesizing enzyme for glutamate) in neurons of lateral reticular nucleus (LRN) of thalamus of adult cats. (1) GAD- and GLN-immunoreactivity (IR) exhibited matching regional patterns of organization within LRN. (2) GAD- and GLN-IR co-localized within most if not all LRN neuronal cell bodies as shown by light microscopy. (3) GAD- and GLN-IR had distinct subcellular localizations in LRN neurons as shown by correlative light/electron microscopy. LRN neurons are important conceptual models where strongly inhibitory cells receive predominant excitatory glutamatergic afferents (from neocortex). Consistent with known actions of intermediary astrocytes, LRN neurons demonstrate GLN enrichment synergistically coupled with glutamatergic innervation to supplement the glutamate pool for GABA synthesis (via GAD) and for metabolic utilization (via the GABA shunt/tricarboxylic acid cycle) but not, apparently, for excitatory neurotransmission.

Intestinal glutaminase activity is increased in liver cirrhosis and correlates with minimal hepatic encephalopathy

BACKGROUND/AIMS

We performed the current study to assess the intestinal activity of enterocyte phosphate-activated glutaminase (PAG) in cirrhosis.

METHODS

Forty-nine cirrhotic patients and 36 control subjects underwent endoscopic duodenal biopsies. Minimal hepatic encephalopathy (MHE) was evaluated using three psychometric tests. Oral glutamine challenge (OGC) was performed and MELD, Child-Pugh and the presence of esophageal varices were recorded. PAG was measured by enzymatic methods. Cerebral magnetic resonance spectroscopy was performed in 10 cirrhotics.

RESULTS

PAG was found to be higher in cirrhotics than control subjects 2.4+/-1.51 vs. 0.68+/-0.57IU/mg protein (P<0.001). PAG was also increased in patients with MHE and correlated with MELD, INR, esophageal varices and serum bile acids. A negative correlation was observed between PAG activity and intra-cerebral choline/creatine ratio (r=-0.67; P=0.035) and a positive correlation with glutamine plus glutamate/creatine ratio (r=0.78; P=0.007). In multivariate analysis using backward logistic regression, presence of MHE was the only variable independently related to altered enterocyte PAG.

CONCLUSIONS

Enterocyte PAG is increased in cirrhotic patients and correlates with MHE. These data support a possible role for intestinal glutaminase in the pathogenesis of hepatic encephalopathy (HE) and could be a new target for future therapies.

Distribution and morphological characterization of phosphate-activated glutaminase-immunoreactive neurons in cat visual cortex

Phosphate-activated glutaminase (PAG) is the major enzyme involved in the synthesis of the excitatory neurotransmitter glutamate in cortical neurons of the mammalian cerebral cortex. In this study, the distribution and morphology of glutamatergic neurons in cat visual cortex was monitored through immunocytochemistry for PAG. We first determined the specificity of the anti-rat brain PAG polyclonal antibody for cat brain PAG. We then examined the laminar expression profile and the phenotype of PAG-immunopositive neurons in area 17 and 18 of cat visual cortex. Neuronal cell bodies with moderate to intense PAG immunoreactivity were distributed throughout cortical layers II-VI and near the border with the white matter of both visual areas. The largest and most intensely labeled cells were mainly restricted to cortical layers III and V. Careful examination of the typology of PAG-immunoreactive cells based on the size and shape of the cell body together with the dendritic pattern indicated that the vast majority of these cells were pyramidal neurons. However, PAG immunoreactivity was also observed in a paucity of non-pyramidal neurons in cortical layers IV and VI of both visual areas. To further characterize the PAG-immunopositive neuronal population we performed double-stainings between PAG and three calcium-binding proteins, parvalbumin, calbindin and calretinin, to determine whether GABAergic non-pyramidal cells can express PAG, and neurofilament protein, a marker for a subset of pyramidal neurons in mammalian neocortex. We here present PAG as a neurochemical marker to map excitatory cortical neurons that use the amino acid glutamate as their neurotransmitter in cat visual cortex.

Bacterial expression, purification, and characterization of rat kidney-type mitochondrial glutaminase

The human gene that encodes the kidney-type glutaminase (KGA) spans 84-kb, contains 19 exons, and encodes two alternatively spliced mRNAs. Various segments of the rat KGA cDNA were PCR amplified and cloned into a bacterial expression vector to determine whether the N- and C- terminal ends of the glutaminase protein were essential for activity. A recombinant glutaminase, lacking the coding sequence contained in exon 1, was found to be fully active. In contrast, proteins that lacked sequences from exons 1 and 2 and exons 1-3 were inactive. An additional construct that corresponded to the sequence encoded by exons 2-14 also retained full activity. Both of the fully active, truncated proteins were purified to apparent homogeneity using an incorporated N-terminal His(6)-tag and Ni(2+)-affinity chromatography. The K(M) values for glutamine of the native and recombinant forms of glutaminase were nearly identical. However, the two truncated forms of the glutaminase exhibit the characteristic phosphate activation profile only when dialyzed into a buffer lacking phosphate. Dialysis versus 10mM Tris-phosphate was sufficient to form an active tetramer. Thus, the deleted N-terminal sequence may contribute to the phosphate-dependent oligomerization and activation of the native glutaminase.

A mutant Bacillus subtilis gamma-glutamyltranspeptidase specialized in hydrolysis activity

gamma-Glutamyltranspeptidase (GGT) catalyzes the hydrolysis of gamma-glutamyl compounds and the transfer of their gamma-glutamyl moieties to amino acids and peptides. The transpeptidation activity of Bacillus subtilis GGT is about 10-fold higher than its hydrolysis activity. In B. subtilis GGT, substitution of Asp-445 with Ala abolished its transpeptidation activity. The specific activity for hydrolysis of D445A GGT was 40.2% of that of the wild-type GGT. The K(m) value for L-glutamine was 15.3 mM. D445A GGT was salt tolerant like the wild-type GGT. These results indicate that D445A GGT will be highly useful as a 'glutaminase' in food industry.

Purification and characterisation of a glutaminase from Debaryomyces spp

A glutaminase was purified from the cell-free extract of Debaryomyces spp. CECT 11815 by protamine sulphate treatment and several chromatographic procedures including anion exchange chromatography and gel filtration. The purified enzyme consisted of two subunits, with molecular masses of 65 and 50 kDa, respectively. Activity was optimal at 40 degrees C and pH 8.5, and the Km value for L-glutamine was 4.5 mM. The glutaminase exhibited activity against L-gamma-Glu-methyl ester, L-gamma-Glu-hydrazide, and L-albiziin, while L-asparagine, CBZ-L-Gln, CBZ-L-Gln-Gly, glutathione, L-gamma-Glu-pNA and L-gamma-Glu-AMC were not hydrolysed. The enzyme was not affected by PMSF, DTT and EDTA. However, the enzyme was inhibited by sulfhydryl group reagents, DON, L-albizziin, L-asparagine and high concentrations of L-glutamine and ammonium, while L-aspartate did not affect the activity. Phosphate and acetate did not produce any significant effect on the glutaminase activity, but it was slightly stimulated by lactate and borate.

Overexpression, purification, and characterization of glutaminase-interacting protein, a PDZ-domain protein from human brain

A human brain cDNA clone coding for a novel PDZ-domain protein of 124 amino acids has been previously isolated in our laboratory. The protein was termed GIP (glutaminase-interacting protein) because it interacts with the C-terminal region of the human brain glutaminase L. Here we report the heterologous expression of GIP as a histidine-tagged fusion protein in Escherichia coli cells. The induction conditions (temperature and isopropyl beta-d-thiogalactopyranoside concentrations) were optimized in such a way that GIP accounted for about 20% of the total E. coli protein. A simple and rapid procedure for purification was developed, which yielded 17 mg of purified GIP per liter of bacterial cell culture. The apparent molecular mass of the protein by SDS-PAGE was 16 kDa, whereas in native form it was determined to be 28 kDa, which suggests dimer formation. The nature and integrity of the recombinant protein were verified by mass spectrometry analysis. The functionality of the GIP protein was tested with an in vitro activity assay: after being pulled down with glutathione S-transferase-glutaminase, GIP was revealed by Western blot using anti-GIP antibodies. Furthermore, the glutaminase activity in crude rat liver extracts was inhibited by the presence of recombinant purified GIP protein.

Kinetics and localization of brain phosphate activated glutaminase

The cellular concentration of phosphate, the main activator of phosphate activated glutaminase (PAG) is rather constant in brain and kidney. The enzyme activity, however, is modulated by a variety of compounds affecting the binding of phosphate, such as glutamate, calcium, certain long chain fatty acids, fatty acyl CoA derivatives, members of the tricarboxylic acid cycle and protons (Kvamme et al. [2000] Neurochem. Res. 25:1407-1419). Therefore, the kinetic and allosteric properties of the enzyme are essential for regulating the enzyme activity in situ, especially because the enzymically active pool of PAG is assumed to have an external localization in the inner mitochondrial membrane, being exposed to cytosolic variation in the content of effectors. This has largely been overlooked. A hypothetical model for the allosteric interactions based on the sequential induced fit allosteric model by Koshland et al. ([1966] Biochemistry 5:365-385) is presented. Furthermore, it has been generally accepted that there exist only two isoforms of PAG, the kidney PAG that is similar to brain PAG, and the liver PAG. Therefore, the immunoreactivity of brain cells against kidney PAG antibodies has been considered a measure of PAG protein. Gomez-Fabre et al. ([2000] Biochem. J. 345:365-375) recently found, however, that a PAG mRNA from human breast cancer ZR75 cells is present in human brain and liver, but not in the kidney. We observed only traces of PAG immunoreactivity in cultured astrocytes and cultured neuroblastoma cells, regardless whether antibodies against the C- and N-termini of kidney PAG or antibodies against liver PAG were used, but considerable enzyme activity, demonstrating hitherto unknown isoforms of PAG (Torgner et al. [2001] FEBS Lett. 268(Suppl 1):PS2-031).

Phosphate-activated glutaminase and mitochondrial glutamine transport in the brain

A review of the properties of purified and tissue bound phosphate activated glutaminase (PAG) in brain and kidney (pig and rat) is presented, based on kinetic, electron microscopic and immunocytochemical studies. PAG is a mitochondrial enzyme and two pools can be separated, a soluble and membrane associated one. Intact mitochondria appear to express PAG accessible only to the outer phase of the inner mitochondrial membrane. This PAG has properties similar to that of the membrane fraction and polymeric form of purified enzyme. PAG in the soluble fraction has properties similar to that of the monomeric form of purified enzyme and is assumed to be dormant due to the high matrix concentration of the inhibitor glutamate. A hypothetical model for the localization of PAG in the mitochondria is presented. The activity of PAG in vivo is assumed to be regulated by cytosolic glutamate and other compounds, that affect the activation by phosphate. Glutamine is transported into brain and kidney mitochondria by a protein catalyzed energy requiring process, which may be mediated by more than one protein. There is no correlation between glutamine hydrolysis and transport.

Isolation, characterization and expression of a human brain mitochondrial glutaminase cDNA

Various cDNAs that encode overlapping portions of the full-length human brain glutaminase (GA) cDNA were cloned and sequenced. The overall nucleotide sequence of hGA has a very high degree of identity with that of the rat kidney-type GA cDNA (77.4%) and the known portion of the cDNA that encodes the 5.0-kb porcine GA mRNA (81.1%). The identity is even more remarkable at the amino acid level, particularly in the C-terminal half where the three proteins share a 99.7% sequence identity. The hGA cDNA encodes a 73,427-Da protein that contains an N-terminal mitochondrial targeting signal and retains the primary proteolytic cleavage site characterized for the cytosolic precursor of the rat renal mitochondrial glutaminase. The entire coding region was assembled through the use of unique restriction sites and cloned into a baculovirus. Sf9 cells infected with the recombinant virus express high levels of properly processed and active glutaminase. Thus, expression of the isolated hGA cDNA should provide a means to purify large amounts of the mitochondrial glutaminase, a protein that catalyzes a key reaction in the metabolism of glutamine and the synthesis of important excitatory and inhibitory neurotransmitters.

Localization of phosphate dependent glutaminase in ascites fluid of ovarian cancer patient

Phosphate dependent glutaminase was purified from ascites fluid of ovarian cancer patients. The purified enzyme showed a final specific activity of 110 unit / mg protein with 72 fold purification and 21% yield. Purified enzyme gives one dark band of Mr approximately 65.5 KD and two light bands of Mr approximately 47.5 KD and approximately 45 KD respectively on 10% SDS-PAGE. One major immunoreactive band was found in trans-immunoblot analysis using antibodies against rat kidney and ascites fluid glutaminase raised in rabbit and mice respectively. Phosphate dependent glutaminase enzyme purified from mitochondria of malignant and non malignant ovarian tissue also showed bands of same molecular weight on 10% SDS-PAGE and gave same immunoreactive bands in trans-immunoblot like the purified glutaminase from ascites fluid. This result was confirmed by using the specific activity stain for glutaminase, which indicates that same enzyme activity is probably due to leakage of the same enzyme from malignant tissue into the ascites fluid. The purified enzyme from human peritoneal fluid showed a high specificity toward glutamine, therefore is a true glutaminase. Moreover, ascites fluid taken from patients of different age group with different stages of ovarian carcinoma revealed the presence of same glutaminase on 10% SDS-PAGE, and exhibited immunoreaction on ELISA, trans-immunoblot and dot immunoblot analysis.

Postembedding immunogold labelling reveals subcellular localization and pathway-specific enrichment of phosphate activated glutaminase in rat cerebellum

Phosphate activated glutaminase is a key enzyme in glutamate synthesis. Here we have employed a quantitative and high-resolution immunogold procedure to analyse the cellular and subcellular expression of this enzyme in the cerebellar cortex. Three main issues were addressed. First, is phosphate activated glutaminase exclusively or predominantly a mitochondrial enzyme, as biochemical data suggest? Second, to what extent is the mitochondrial content of glutaminase dependent on cell type and transmitter identity? Third, can individual neurons maintain a subcellular segregation of mitochondria with different glutaminase content? An attempt was also made to disclose the intramitochondrial localization of glutaminase, and to correlate the content of this enzyme with that of glutamate and glutamine in the same mitochondria (by use of triple labelling). Antisera to the N- and C-termini of glutaminase revealed strong labelling of the putatively glutamatergic mossy fibre terminals. The vast majority of gold particles (approximately 96%) was associated with the mitochondria. Equally high labelling intensities were found in mitochondria of perikarya and dendrites in the pontine nuclei, a major source of mossy fibres. The level of glutaminase immunoreactivity in parallel and climbing fibres (which like the mossy fibres are thought to use glutamate as transmitter) was only about 20% of that in mossy fibres, and similar to that in non-glutamatergic neurons (Purkinje and Golgi cells). Glial cell mitochondria were devoid of specific glutaminase labelling and revealed a much lower glutamate:glutamine ratio than did the mitochondria of mossy fibres. As to the submitochondrial localization of glutaminase, immunogold particles were often found to be aligned with the cristae, suggesting an association of the enzyme with the inner mitochondrial membrane. However, the existence of a glutaminase pool in the mitochondrial matrix could not be excluded. The outer mitochondrial membrane was unlabelled. The present study provides quantitative evidence for a substantial heterogeneity in the mitochondrial content of glutaminase. This heterogeneity applies not only to neurons with different transmitter signatures, but also to different categories of glutamatergic pathways. In terms of the routes involved, the synthesis of transmitter glutamate may be less uniform than previously expected.

The role of glutaminase in the small intestine

Glutaminase is the enzyme which hydrolyses glutamine, the main respiratory fuel of the intestine, to yield glutamate and ammonia. Glutaminase has a central role in intestinal metabolism: the products of the reaction catalyzed by glutaminase can be transaminated, catabolized to yield energy or used for the biosynthesis of pyrimidine nucleotides. Experimental treatments which deprive the intestine of glutamine induce intestinal atrophy. In this review, attention is paid to the role of glutaminase in intestinal metabolism. Background information on the structure, kinetics and distribution of glutaminase precede a discussion of the metabolism of glutamine within the intestine. In closing, we review the factors known to regulate glutaminase activity and emphasise that the regulation of glutaminase within the intestine is poorly understood.

Glutamine utilization by rat neutrophils: presence of phosphate-dependent glutaminase

The capacity of rat neutrophils to utilize glutamine was investigated by 1) determination of oxygen consumption in the presence of glucose or glutamine, 2) measurement of maximal activity of phosphate-dependent glutaminase, 3) Northern blot, Western blot, and immunocytochemical detection of glutaminase, and 4) measurement of glutamine utilization and also production of ammonia, glutamate, aspartate, alanine, and lactate and decarboxylation of [U-14C]glutamine in cells incubated for 1 h. The rate of respiration by isolated neutrophils in the absence of added substrate was 5.0 nmol x min(-1) x 10(7) cells(-1). Maximal activity of phosphate-dependent glutaminase was 56 nmol x min(-1) x mg protein(-1) in freshly obtained neutrophils; the Michaelis-Menten constant was 3.5 mM for glutamine. This enzyme activity was inhibited by 2 mM glutamate, 2 mM oxoglutarate, and 2 mM NH4Cl. The presence of glutaminase protein (65 kDa) was confirmed by Western blot and immunocytochemical detection and the presence of the mRNA (6.0 kb) by Northern blot analysis. Glutamine was utilized by neutrophils incubated for 1 h at a rate of 12.8 nmol x min(-1) x mg protein(-1) when the amino acid was added to the medium at 2 mM, which is three to four times higher than the physiological concentration. In the presence of 0.5 mM glutamine, the amino acid was utilized at a rate of 2.9 nmol x min(-1) x mg protein(-1). The addition of 0.5 mM glutamate to the incubation medium caused a marked reduction (by 70%) in glutamine utilization by neutrophils. Glucose was utilized at 7.7 nmol x min(-1) x mg protein(-1) when cells were incubated in 5 mM glucose. The conversion of [U-14C]glutamine to 14CO2 was very low: <1% was totally oxidized. The formation of ammonia was approximately 27% of glutamine utilization, and the conversion of glutamine to glutamate, aspartate, alanine, and lactate accounted for approximately 84.6% of the total amino acid utilized by neutrophils. In this study, evidence is presented that, in addition to lymphocytes and macrophages, neutrophils also utilize glutamine.

Increased production of extracellular glutamate by the mitochondrial glutaminase following neuronal death

Elevated extracellular concentrations of the excitatory transmitter glutamate are an important cause of neuronal death in a variety of disorders of the nervous system. The concentrations and rates of clearance and production of extracellular glutamate were measured in the medium of primary cultures from mouse neocortex containing neurons, astrocytes, or both cell types. Measurements were performed in the presence and absence of 2 mM glutamine with or without neuronal injury caused by 5-h exposure to hypoxia or 500 microM N-methyl-D-aspartate or a freeze-thaw cycle. High rates of glutamate generation (0.5-0.8 microM/min in the 0.4-ml culture well) occurred if neurons were both damaged and exposed to glutamine. Intact neurons or glia exposed to glutamine generated only small amounts of glutamate (0.03 microM/min). Glutamate generation by damaged neurons was dependent on the presence of glutamine, activated by phosphate, and inhibited by 6-diazo-5-oxo-L-norleucine and p-chloromercuriphenylsulfonic acid (pCMPS), strongly implicating the mitochondrial glutaminase. Following 5-h exposure to 500 microM N-methyl-D-aspartate, the glutaminase was localized to fragments of damaged neurons and was accessible to inhibition by the membrane-impermeant pCMPS. The glutaminase activity from damaged neurons is sufficient to account for the neurotoxic concentrations of glutamate in hypoxic mixed neuronal-glial cultures exposed to 2 mM glutamine. Finally, pCMPS is neuroprotective and also prevents the increased rate of generation of glutamate observed in neuronal cultures after prolonged exposure to glutamine. The cumulative data indicate the following: 1) excitotoxic neuronal death activates the hydrolysis of extracellular glutamine by the mitochondrial glutaminase, and 2) the glutaminase in damaged neurons is sufficient to cause neuronal death in in vitro models of neuronal injury.

Role of the N-terminal 118 amino acids in the processing of the rat renal mitochondrial glutaminase precursor

Rat renal mitochondrial glutaminase (GA) is synthesized as a 74-kDa cytosolic precursor that is translocated into mitochondria and processed via a 72-kDa intermediate to yield a 3:1 ratio of mature 66- and 68-kDa subunits, respectively. The 66-kDa subunit is derived by removal of a 72-amino-acid presequence. The structural determinants necessary for translocation and proteolytic processing were further delineated by characterizing the processing of different chimeric constructs formed by fusing various segments of the N-terminal sequence of the GA precursor to chloramphenicol acetyl transferase (CAT). GA1-118 CAT is translocated and processed in isolated rat liver mitochondria or cleaved by purified mitochondrial processing peptidase (MPP) to yield an intermediate peptide and two mature subunits that are analogous to the products of processing of the GA precursor. The two reactions also occur with kinetics which are similar to those observed for processing of the GA precursor. Thus, all of the information required for the translocation and synthesis of the mature subunits of GA reside in the N-terminal 118 amino acids of the GA precursor. In contrast, GA1-72 CAT, a construct that contains the GA presequence fused to CAT, is apparently translocated and processed less efficiently. It yields only two peptides that are analogous to the intermediate and 68 kDa forms of GA. In addition, GA1-31 CAT associates with mitochondria but is not proteolytically processed and GA1-31,73-118 CAT is slowly translocated and processed to a single peptide that is analogous to the 66 kDa form of GA. The latter results suggest that the MPP cleavage reactions which yield the GA intermediate and the 66-kDa subunit depend primarily on information that is present C-terminal to the respective sites of cleavage.

In vitro characterization of the mitochondrial processing and the potential function of the 68-kDa subunit of renal glutaminase

Rat renal mitochondrial glutaminase (GA) is initially synthesized in primary cultures of proximal tubule cells as a 74-kDa precursor and is processed via a 72-kDa intermediate to generate a heterotetrameric enzyme which contains three 66-kDa subunits and one 68-kDa subunit (Perera, S. Y., Chen, T. C., and Curthoys, N. P. (1990) J. Biol. Chem. 265, 17764-17770). The two mature subunits may be derived by either of two possible mechanisms: 1) alternative proteolytic processing or 2) initial synthesis of the 66-kDa subunit followed by its covalent modification to generate the 68-kDa subunit. An in vitro system was utilized to further characterize this unique processing pathway and to investigate the potential function of the 68-kDa subunit. In vitro transcription and translation of the GA cDNA yields a single 74-kDa precursor. Upon incubation with isolated rat liver mitochondria, the precursor is translocated into the mitochondria and processed via a 72-kDa intermediate to yield a 3:1 ratio of the 66- and 68-kDa subunits, respectively. The kinetics of the in vitro processing reaction also closely approximate the kinetics observed in cultured cells. Mitochondrial processing is blocked by o-phenanthroline, an inhibitor of the matrix processing peptidase (MPP). The 72-amino acid presequence of the 66-kDa subunit contains a large proportion of basic amino acids. Two-dimensional gel electrophoresis of mature GA established that the 68-kDa subunit is slightly more basic than the 66-kDa subunit. In addition, incubation of the 74-kDa precursor with purified MPP yields equimolar amounts of the two mature peptides. A cDNA construct, p delta GA, was created which lacks the nucleotides that encode the amino acid residues 32 through 72 of GA. When transcribed and translated in vitro, p delta GA yields a 70-kDa precursor. This precursor is processed by mitochondria to a single mature subunit with a M of 66 kDa. This observation suggests that the 68-kDa subunit is not produced by covalent modification of the 66-kDa subunit and further supports the conclusion that the two mature subunits of GA are produced by alternative processing reactions which can be catalyzed by MPP. However, the yield of products obtained in intact mitochondria may be determined by some unidentified accessory factor. Submitochondrial fractionation of imported GA and delta GA precursors suggest that the 68-kDa subunit may function to retain the mature GA within the mitochondrial matrix.

In vivo activity of glutaminase in the brain of hyperammonaemic rats measured by 15N nuclear magnetic resonance

The in vivo activity of phosphate-activated glutaminase (PAG) was measured in the brain of hyperammonaemic rat by 15N n.m.r. Brain glutamine was 15N-enriched by intravenous infusion of 15NH4+ until the concentration of [5-15N]glutamine reached 6.1 mumol/g. Further glutamine synthesis was inhibited by intraperitoneal injection of methionine-DL-sulphoximine, an inhibitor of glutamine synthetase, and the infusate was changed to 14NH4+ during observation of decrease in brain [5-15N]glutamine due to PAG and other glutamine utilization pathways. Progressive decrease in brain [5-15N]glutamine, PAG-catalysed production of 15NH4+ and its subsequent assimilation into glutamate by glutamate dehydrogenase were monitored in vivo by 15N n.m.r. Brain [5-15N]glutamine (15N enrichment of 0.35-0.50) decreased at a rate of 1.2 mumol/h per g of brain. The in vivo PAG activity, determined from the observed rate and the quantity of 15NH4+ produced and subsequently assimilated into glutamate and aspartate, was 0.9-1.3 mumol/h per g. This activity is less than 1.1% of the reported activity in vitro measured in rat brain homogenate at a 10 mM concentration of the activator Pi. Inhibition by ammonia (brain level 1.4 mumol/g) alone does not account for the observed low activity in vivo. The result strongly suggests that, in intact brain, PAG activity is maintained at a low level by a suboptimal in situ concentration of Pi and the strong inhibitory effect of glutamate. The observed PAG activity in vivo is lower than the reported in vivo activity of glutamate decarboxylase which converts glutamate into gamma-aminobutyrate (GABA). The result suggests that PAG-catalysed hydrolysis of glutamine is not the sole provider of glutamate used for GABA synthesis.

Regulation of phosphate-activated glutaminase (PAG) by glutamate analogues

The ability of structural analogues of glutamate (GLU) to modulate phosphate activated glutaminase (PAG) was assessed in the present series of studies. A number of GLU receptor agonists and antagonists were tested for their ability to inhibit synaptosomal PAG activity. PAG activity was determined by measuring GLU formation from 0.5 mM glutamine (GLN) in the presence of 10 mM phosphate. GLU analogues at 5-10 mM were found to significantly inhibit PAG activity. It was determined that PAG inhibition occurred regardless of whether the GLU analogues were receptor agonists or antagonists, however, PAG inhibition was influenced by analogue chain length, isomeric form and substituent substitution. The glutamate uptake blockers, dihydrokainic acid and DL-threo-beta-hydroxyaspartic acid were relatively weak inhibitors of PAG (< 25% inhibition) as were the receptor agonists, ibotenic acid and (+-)cis-2,3-piperidine-dicarboxylic acid. Other GLU analogues produced inhibition of PAG in the range of 40-70%. PAG inhibition by GLU analogues did not appear to differ substantially among the brain regions evaluated (cortex, striatum and hippocampus). The endogenous amino acids, glycine, taurine and N-acetylaspartic acid, also significantly inhibited PAG activity in the 5-10 mM range. The noncompetitive NMDA antagonists, (+)MK801 and ketamine, at a concentration of 5 mM, significantly stimulated PAG activity 1.5-2 fold over control values. The activation of PAG by (+)MK801 was dose-related, stereoselective and appeared to result from a synergistic interaction with phosphate to enhance substrate (GLN) binding to PAG.(ABSTRACT TRUNCATED AT 250 WORDS)

Native polyacrylamide gel electrophoresis of membrane proteins: glutaminase detection after in situ specific activity staining

A new procedure for the analysis and detection of phosphate-activated glutaminase (EC 3.5.1.2) by native electrophoresis has been developed. The method is based on the in situ detection of glutaminase activity in two different systems of native polyacrylamide gradient gels, containing 3-(3-cholamidopropyl)-dimethyl-ammonio-1-propane sulfonate (CHAPS) or Triton X-100 as nondenaturant detergents. Crude Triton X-100 extracts of mitochondria were resolved by electrophoresis. The enzyme was specifically revealed by incubation of the gel with glutamine and coupling the oxidation of the glutamate formed to the reduction of a tetrazolium dye, in the presence of glutamate dehydrogenase trapped in a 1% agar solid overlay. Both Ehrlich ascitic cell and mouse kidney glutaminases were resolved by native electrophoresis and specifically detected with the activity staining. Moreover, the redox-cycling staining was tested in solution, showing linearity with the amount of glutamate or glutaminase activity present. The method described could be a useful tool for native polyacrylamide gel electrophoresis of membrane proteins.

Glutamate production in islets of Langerhans: properties of phosphate-activated glutaminase

Homogenates of rat pancreas, pancreatic islets, and HIT-T15 cells (a clonal line derived from B cells) catalyzed the breakdown of glutamine to glutamate. This activity was markedly stimulated by the addition of orthophosphate and was much greater in homogenates from islets and the B-cell-derived clonal cell line than in those from whole pancreas. Islet glutaminase was half-maximally stimulated with 40 mmol/L phosphate. Kinetic analyses of the rates of glutamine hydrolysis showed that the Vmax for the reaction increased with the increase in phosphate concentration, whereas the Km for glutamine (2.6 +/- 0.2 mmol/L) was unaltered. The pH optimum for enzyme activity was 8.0 to 8.5 at all phosphate concentrations studied. Glutamine breakdown was enhanced by adenosine triphosphate ([ATP] approximately 100% at 10 mmol/L) and citrate (approximately 30% at 10 mmol/L), but it was unaffected by malate, 2-oxoglutarate, lactate, and ammonia. Glutamate significantly inhibited glutamine hydrolysis. Freshly isolated islets had a low content of both glutamate and glutamine. After culturing for 1 hour in an amino acid-containing medium, the concentrations of glutamine and glutamate increased. Subsequent perifusion without amino acids caused a loss of glutamine and a concomitant increase in glutamate level. Perifusion with 1 mmol/L glutamine led to an increase in both internal glutamine and glutamate. The addition to the perifusion medium of either 10 mmol/L glutamine, 10 mmol/L orthophosphate, or both substantially enhanced insulin release evoked by 10 mmol/L leucine.(ABSTRACT TRUNCATED AT 250 WORDS)

Glutamine catabolism by heart muscle. Properties of phosphate-activated glutaminase

1. Rat heart homogenates catabolized glutamine in the presence of rotenone, an inhibitor of the respiratory chain. 2. The reaction was markedly stimulated by phosphate and inhibited by glutamate, but not greatly affected by ammonia. 3. Glutamine breakdown was enhanced by lactate, malate, citrate and ATP, and almost completely blocked by 1 mM-N-ethylmaleimide. 4. The Km for glutamine measured in homogenates supplemented with either 50 mM- or 100 mM-phosphate and at pH 7.3 or 8.0 was about 4 mM, whereas the Vmax was larger at higher anion concentrations and at the more alkaline pH. 5. Activity of glutaminase was 3-fold greater in isolated mitochondria than in muscle homogenates. Distribution of the activity was the same as that of a mitochondrial marker, cytochrome c oxidase. 6. Properties of glutaminase with respect to its dependence on the concentrations of phosphate, glutamine and H+ were the same in intact and broken mitochondria and very similar to those in homogenates. However, the specific activity of the enzyme was considerably smaller in frozen-thawed than in intact organelles. 7. It is concluded that heart mitochondria possess a kidney-type phosphate-activated glutaminase that can serve as a source of myocardial glutamate.

Biosynthesis and processing of mitochondrial glutaminase in HTC hepatoma cells

Rat HTC hepatoma cells were used to characterize the biosynthesis and processing of the renal isoenzyme of the mitochondrial glutaminase. Immunoblot analysis indicated that mitochondria isolated from HTC cells contained two prominent glutaminase peptides of 68 and 65 kDa and two minor peptides of 61 and 58 kDa. When the cells were labelled with [35S]methionine, the glutaminase-specific antibodies precipitated the same four polypeptides. However, when labelled in the presence of 5 microM-carbonyl cyanide m-chlorophenylhydrazone, an uncoupler of oxidative phosphorylation, only a 72 kDa cytoplasmic precursor of the mitochondrial glutaminase was immunoprecipitated. A comparison of the peptides generated by partial proteolysis of the precursor and the fully processed peptides indicates significant structural similarity. A 71 kDa form of the glutaminase was also observed when HTC cells were pulse-labelled for 2-6 min with [35S]methionine. Pulse-chase experiments indicate that the cytoplasmic precursor is quantitatively converted into the mature forms of the glutaminase. In addition, the observed kinetics established that the 71 kDa peptide is a true intermediate in the import of the mitochondrial glutaminase.

Distribution of phosphate-activated glutaminase in the human cerebral cortex

Phosphate-activated glutaminase (PAG), which catalyses conversion of glutamine to glutamate, is a potential marker for glutamatergic, and possibly GABA, neurons in the central nervous system. A polyclonal antibody, raised in rabbits against rat brain PAG, was applied to postmortem human brain tissue to reveal the distribution of PAG in the cerebral cortex. PAG immunoreactivity was observed in pyramidal and non-pyramidal neurons but not in glial cells. In the neocortex, large to medium-sized pyramidal neurons in layers III and V were stained most intensely, while the majority of smaller pyramidal cells were labeled either lightly or moderately. Such modified pyramids as the giant Betz cells, the large pyramidal cells of Meynert, and the solitary cells of Ramón y Cajal were also stained intensely. Fusiform cells in layer VI showed moderate to intense labeling. A number of cortical non-pyramidal neurons of various sizes stained moderately to intensely. These included large basket cells which were identified by their characteristic morphology and size in primary cortical areas. Pyramidal cells in the hippocampal formation as well as basket cells of the stratum oriens stained moderately to intensely. Since pyramidal cells are believed to be glutamatergic and large basket cells GABAergic, these results suggest that PAG plays a role in generating not only transmitter glutamate, but also GABA precursor glutamate.