Determination of 4-aminobutyric acid, aspartate, glutamate and glutamine and their 13C stable-isotopic enrichment in brain tissue by gas chromatography-mass spectrometry

Stable isotope dilution mass spectrometric assay for PRPP using enzymatic procedures

5-Phosphoribosyl-1-pyrophosphate (PRPP) is an important regulator of de novo purine synthesis. A method for the measurement of PRPP in erythrocytes was designed, which is based on the determination of [(13)C(5)]glutamate derived from [(13)C(5)]glutamine following the utilization of PRPP by the action of amidophosphoribosyltransferase. The present study describes a gas chromatographic-mass spectrometric method for determination of [(13)C(5)]glutamate using [(13)C(2)]glutamate as an internal standard. The methods involved purification by anion-exchange chromatography using a BondElut SAX and derivatization with isobutyl chlorocarbonate in water-methanol-pyridine. Quantitation was performed by selected ion monitoring of the protonated molecular ions in the chemical ionization mode. The intra-day reproducibility in the amounts of [(13)C(5)]glutamate determined was in good agreement with the actual amounts added in erythrocytes. A linear relationship was found between the amount of PRPP added and the amount of [(13)C(5)]glutamate formed from [(13)C(5)]glutamine using amidophosphoribosyltransferase.

Analysis of glutamine, glutamate, pyroglutamate, and GABA in cerebrospinal fluid using ion pairing HPLC with positive electrospray LC/MS/MS

A simple and sensitive method for the separation and quantitation of glutamine, glutamate, pyroglutamate, and gamma-aminobutyric acid (GABA) in cerebrospinal fluid (CSF) is presented. The method utilizes ion pairing with heptafluorobutyric acid (HFBA) to achieve HPLC separation with detection by positive ESI LC/MS/MS. The method does not require extraction or derivatization, utilizes a heavy labeled internal standard for each analyte, and allows for rapid throughput with a 5 min run time. The method was developed with particular attention taken to prevent conversion between analytes known to occur under certain conditions. The lower limit of quantitation is 7.8 ng/ml for all analytes, and the intra-day and inter-day accuracy (%RE) and precision (%R.S.D.) are defined for all analytes. The method was developed as a sensitive, selective, and robust method to investigate the excitatory and inhibitory neurotransmitters (glutamate and GABA) as biomarkers in drug development.

Liquid chromatographic determination of gamma-aminobutyric acid in cerebrospinal fluid using 2-hydroxynaphthaldehyde as derivatizing reagent

gamma Aminobutyric acid (GABA) was determined by precolumn derivatization with 2-hydroxynaphthaldehyde and elution was made using Phenomenex C(18), 5 microm column with methanol: water (62:38 v/v) and UV detection at 330 nm. In a mixture containing glycine, l-lysine and tyramine GABA separated completely. A number of amines and amino acids tested did not affect the response of GABA. A linear calibration curve was obtained for GABA in the range of 1.2-28.0 microg/ml with detection limit of 2.8 ng/injection (5 microl). The method was used for the determination of GABA in cerebral spinal fluid (CSF) samples and gave results of 19.0 to 22.4 microg/m1 with coefficient of variation 2.4%

Determination of glutamine and glutamic acid in mammalian cell cultures using tetrathiafulvalene modified enzyme electrodes

Tetrathiafulvalene (TTF) mediated amperometric enzyme electrodes have been developed for the monitoring of L-glutamine and L-glutamic acid in growing mammalian cell cultures. The detection of glutamine was accomplished by a coupled enzyme system comprised of glutaminase plus glutamate oxidase, while the detection of glutamic acid was carried out by a single enzyme, glutamate oxidase. The appropriate enzyme(s) were immoblized on the Triton-X treated surface of tetrathiafulvalene modified carbon paste electrodes by adsorption, in conjunction with entrapment by an electrochemically deposited copolymer film of 1,3-phenylenediamine and resorcinol. Operating conditions for the glutamine enzyme electrode were optimized with respect to the amount of enzymes immoblized, pH, temperature and mobile phase flow rate for operation in a flow injection (FIA) system. When applied to glutamine and glutamic acid measurements in mammalian cell culture in FIA, the results obtained with enzyme electrodes were in excellent agreement with those determined by enzymatic analysis.

Flow injection determination of glutamate in human serum and rat brain samples with immobilized glutamate oxidase and glutamate dehydrogenase reactors

Two methods are proposed for the determination of regional concentrations of glutamate in the rat brain as well as in human serum. Glutamate oxidase was immobilized on non-porous glass beads and glutamate dehydrogenase was immobilized on glass derivatives. These supports were employed for the construction of Single Bead String Reactors and Packed Bed Reactors, respectively, which in turn were linked to Flow Injection Analysis systems with either photometric or fluorometric detection. Analytical working curves are linear in the range 1-200 mumol/l for packed bed reactors and 10-500 mmol/l for single bead string reactors. The samples were pretreated depending on their origin and the applied measuring system. Optimal dilution factors were established for the two techniques. Optimal dilution ratios were established and the influence of several added substances was investigated. Recovery and method comparison studies including high performance liquid chromatography verified the accuracy of the proposed methods. Results from within-day and between-day measurements gave relative standard deviations of 4.7 and 5.9% for serum samples and 2.5 and 4.0% for brain samples, respectively.

Effects of halothane on the synthesis of neurotransmitter amino acids in mouse brain

The effects of halothane on the synthesis of the three major neuroactive amino acids (gamma-aminobutyric acid, aspartate and glutamate) and glutamine, which is closely related metabolically, were investigated in mouse brain using a labelled precursor ([13C]glucose) and a gas chromatography-mass spectrometry system. The ratios of newly synthesized amino acids were increased relative to baseline values when animals were exposed to 1% halothane, and decreased when they were exposed to 2% halothane. These findings suggest that halothane affects the synthesis of neurotransmitter amino acids in a concentration-dependent manner, without discrimination between excitatory and inhibitory amino acids.

The effects of halothane on the contents of putative transmitter amino acids in whole rat brain

The effects of halothane on brain amino acid contents were investigated in rat by using a gas chromatography-mass spectrometry system. The contents of glycine and glutamate did not show any consistent changes in 1% or 2% halothane, but increased in 3% or 4% halothane. The levels of gamma-aminobutyric acid were not altered in any concentrations of halothane. The levels of aspartate increased in a dose-dependent manner up to 3% halothane, but returned to the control level in 4% halothane. These findings suggest that halothane brings the alteration of brain amino acid contents by affecting their synthesis and catabolism.

Use of stable isotopes and gas chromatography-mass spectrometry in the study of different pools of neurotransmitter amino acids in brain slices

A method was developed for simultaneous determination of endogenous and newly synthesized neurotransmitter amino acids (4-aminobutyric acid, glutamate and aspartate) and glutamine in brain in vitro. Brain slices were incubated in artificial cerebrospinal fluid in the presence of 13C-labeled precursors (glucose, pyruvate or acetate). After the incubation, the slices were homogenized in cold 80% ethanol and the supernatants were evaporated to dryness. The resultant residues were derivatized with N-methyl-N-(tert.-butyldimethylsilyl)trifluoroacetamide and analyzed by capillary gas chromatography-mass spectrometry in the electron-impact mode. N(O)-tert.-Butyldimethylsilyl derivatives of the naturally occurring amino acids, their 13C-enriched counterparts and deuterated internal standards were detected as their [M-57]+ fragments using selected-ion monitoring. The method was shown applicable to studying compartmentation of neurotransmitter amino acids.

Determination of [U-13C]glucose turnover into various metabolite pools for the differential diagnosis of lactic acidemias

Stable-isotope tracer experiments performed in vitro are evaluated for their utility in differentiating between pyruvate dehydrogenase and cytochrome oxidase deficiencies, two of several enzyme defects commonly associated with the lactic acidemias. Fibroblasts of enzyme-deficient individuals and of age-matched controls are grown in medium containing [U-13C]glucose. Direct analysis of cells and conditioned culture medium provides only minor differences in the organic acid/amino acid GC-MS profiles, making differentiation of enzyme defects difficult by this method. However, differences have been found in the glucose turnover into various cell metabolites, making differentiation of these two enzyme defects possible. The cellular pool of glutamic acid experiences 13C-enrichment in both the control and cytochrome oxidase deficient lines, but not in the pyruvate dehydrogenase-deficient line. The cellular pool of an unknown, possibly an aminopentose sugar, on the other hand, experiences 13C-enrichment in the pyruvate dehydrogenase and control lines, but not in the cytochrome oxidase line. These observations, as well as other differences in the extent of enrichment into various metabolite pools, suggest that this stable-isotope approach, in vitro, is feasible and may allow these two enzyme defects to be differentiated in a definitive manner. Such stable-isotope experiments are easy to carry out with cultured cells and are inexpensive. Applications of the technique to other genetic disorders might be appropriate.

Effects of pharmacological manipulations on basal and newly synthesized levels of GABA, glutamate, aspartate and glutamine in mouse brain cortex

Concentrations of basal and newly synthesized inhibitory (gamma-aminobutyric acid, GABA) and excitatory (glutamate and aspartate) neurotransmitter amino acids and glutamine were determined in mouse brain cortex. Isotopic enrichment following an intravenous infusion of a stable-labeled precursor, [13C6]D-glucose, was used to estimate the newly synthesized amino acid content. Effects of various pharmacological agents (valproate, aminooxyacetic acid, 3-mercaptopropionic acid, N-methyl-D-aspartate, and 2-amino-7-phosphonohepatanoic acid) were evaluated. The effects of 3-mercaptopropionic acid (an inhibitor of glutamate decarboxylase, a GABA-synthesizing enzyme) were restricted to the GABAergic system. On the other hand, N-methyl-D-aspartate (an agonist of a glutamate receptor subtype) was selective for the glutamate-glutamine system, and its effects were prevented by its selective antagonist, 2-amino-7-phosphonoheptanoic acid. In some cases, divergent effects were observed on basal and new amino acids. This suggested that basal and new amino acids may represent different compartments. The anticonvulsant drug valproate caused an increase in basal but a decrease in newly synthesized GABA. Aminooxyacetic acid caused a dramatic increase in basal GABA without affecting the newly synthesized GABA. This approach may be useful in studying compartmentation and fluxes of neurotransmitters.