Metabolism of 15NH3 in organotypic cerebellar explants and cultured astrocytes: studies with gas chromatography-mass spectrometry

Interrelationships of leucine and glutamate metabolism in cultured astrocytes

The aim was to study the extent to which leucine furnishes alpha-NH2 groups for glutamate synthesis via branched-chain amino acid aminotransferase. The transfer of N from leucine to glutamate was determined by incubating astrocytes in a medium containing [15N]leucine and 15 unlabeled amino acids; isotopic abundance was measured with gas chromatography-mass spectrometry. The ratio of labeling in both [15N]glutamate/[15N]leucine and [2-15N]glutamine/[15N]leucine suggested that at least one-fifth of all glutamate N had been derived from leucine nitrogen. At the same time, enrichment in [15N]leucine declined, reflecting dilution of the 15N label by the unlabeled amino acids that were in the medium. Isotopic abundance in [15N]isoleucine increased very quickly, suggesting the rapidity of transamination between these amino acids. The appearance of 15N in valine was more gradual. Measurement of branched-chain amino acid transaminase showed that the reaction from leucine to glutamate was approximately six times more active than from glutamate to leucine (8.72 vs. 1.46 nmol/min/mg of protein). However, when the medium was supplemented with alpha-ketoisocaproate (1 mM), the ketoacid of leucine, the reaction readily ran in the "reverse" direction and intraastrocytic [glutamate] was reduced by approximately 50% in only 5 min. Extracellular concentrations of alpha-ketoisocaproate as low as 0.05 mM significantly lowered intracellular [glutamate]. The relative efficiency of branched-chain amino acid transamination was studied by incubating astrocytes with 15 unlabeled amino acids (0.1 mM each) and [15N]glutamate. After 45 min, the most highly labeled amino acid was [15N]alanine, which was closely followed by [15N]leucine and [15N]isoleucine. Relatively little 15N was detected in any other amino acids, except for [15N]serine. The transamination of leucine was approximately 17 times greater than the rate of [1-14C]leucine oxidation. These data indicate that leucine is a major source of glutamate nitrogen. Conversely, reamination of alpha-ketoisocaproate, the ketoacid of leucine, affords a mechanism for the temporary "buffering" of intracellular glutamate.

A multiple mass spectral line method for determining positional specific activities in stable isotope-labeled amino acids

A method for determining the position and enrichment of isotope labels in amino acids using gas chromatography/mass spectrometry is described. [alpha-15N]- and [epsilon-15N]lysine, [1-13C]- and [15N]alanine and -leucine, and [1-13C]-, [2-13C]-, [3-13C]-, and [4-13C]aspartic acid were investigated. Standards for each isotope label were prepared and analyzed under scan conditions, and line pairs characteristic for the label were identified. The standards were reanalyzed under selective ion monitoring conditions to verify the behavior of the line pairs. Mixtures of amino acids containing different isotope labels or the same label in different positions were prepared and analyzed under selective ion monitoring conditions. Enrichments were determined with high precision and relative errors ranging from 0.14 to 36%.

Glutathione turnover in cultured astrocytes: studies with [15N]glutamate

The incorporation of [15N]glutamic acid into glutathione was studied in primary cultures of astrocytes. Turnover of the intracellular glutathione pool was rapid, attaining a steady state value of 30.0 atom% excess in 180 min. The intracellular glutathione concentration was high (20-40 nmol/mg protein) and the tripeptide was released rapidly into the incubation medium. Although labeling of glutathione (atom% excess) with [15N]glutamate occurred rapidly, little accumulation of 15N in glutathione was noted during the incubation compared with 15N in aspartate, glutamine, and alanine. Glutathione turnover was stimulated by incubating the astrocytes with diethylmaleate, an electrophile that caused a partial depletion of the glutathione pool(s). Diethylmaleate treatment also was associated with significant reductions of intraastrocytic glutamate, glycine, and cysteine, i.e., the constituents of glutathione. Glutathione synthesis could be stimulated by supplementing the steady-state incubation medium with 0.05 mM L-cysteine, such treatment again partially depleting intraastrocytic glutamate and causing significant reductions of 15N labeling of both alanine and glutamine, suggesting that glutamate had been diverted from the synthesis of these amino acids and toward the formation of glutathione. The current study underscores both the intensity of glutathione turnover in astrocytes and the relationship of this turnover to the metabolism of glutamate and other amino acids.

The use of multiple mass spectral line pairs for enhanced precision in isotope enrichment studies of 15N-labeled amino acids

A method for enhancing the precision in the calculation of isotope enrichment for 15N-labeled amino acids is presented. The method utilizes multiple line pairs for the calculation of isotope enrichment. Using multiple line pairs allows the evaluation of calibration curves for nonlinear behavior and permits differentiation among sites containing more than one labeled nitrogen. The increase in precision is related to the number of isotopically shifted line pairs used in calculating the isotopic enrichment and varies with the amino acid of interest.

Astrocyte metabolism of [15N]glutamine: implications for the glutamine-glutamate cycle

The metabolism of glutamine was studied in cultured astrocytes by incubating these cells with [2-15N]-glutamine and using gas chromatography-mass spectrometry to quantitate the transfer of 15N to other amino acids. We found that astrocytes simultaneously synthesize and consume [2-15N]glutamine, with the respective synthetic and utilization rates being approximately equal (ca. 13.0 nmol min-1 mg protein-1). Considerable 15N was transferred to alanine and a significant amount to the essential amino acids leucine, tyrosine, and phenylalanine, the latter process denoting active reamination of cognate ketoacids. A net export of alanine into the medium was noted. Astrocyte glutamine utilization appeared to be mediated via both the phosphate-activated glutaminase (PAG) pathway and the glutamine aminotransferase pathway, the activity of which was about half that of PAG. The glutamine concentration in the incubation medium determined whether net synthesis or utilization of this amino acid occurred. When glutamine was omitted from the medium, net synthesis occurred. When it was present at a high (5 mM) level, net consumption was observed. At a physiologic (0.5 mM) concentration, neither net synthesis nor consumption was noted, although the 15N data indicated that glutamine was actively metabolized. An implication of this work is that astrocytes clearly are capable of both synthesizing and utilizing glutamine, and current concepts of a glutamate-glutamine cycle functioning stoichiometrically between astrocytes and neurons may be an oversimplification.

Glucose and synaptosomal glutamate metabolism: studies with [15N]glutamate

The metabolism of [15N]glutamate was studied with gas chromatography-mass spectrometry in rat brain synaptosomes incubated with and without glucose. [15N]Glutamate was taken up rapidly by the preparation, reaching a steady-state level in less than 5 min. 15N was incorporated predominantly into aspartate and, to a much lesser extent, into gamma-aminobutyrate. The amount of [15N]ammonia formed was very small, and the enrichment of 15N in alanine and glutamine was below the level of detection. Omission of glucose substantially increased the rate and amount of [15N]aspartate generated. It is proposed that in synaptosomes (a) the predominant route of glutamate nitrogen disposal is through the aspartate aminotransferase reaction; (b) the aspartate aminotransferase pathway generates 2-oxoglutarate, which then serves as the metabolic fuel needed to produce ATP; (c) utilization of glutamate via transamination to aspartate is greatly accelerated when flux through the tricarboxylic acid cycle is diminished by the omission of glucose; (d) the metabolism of glutamate via glutamate dehydrogenase in intact synaptosomes is slow, most likely reflecting restriction of enzyme activity by some unknown factor(s), which suggests that the glutamate dehydrogenase reaction may not be near equilibrium in neurons; and (e) the activities of alanine aminotransferase and glutamine synthetase in synaptosomes are very low.

The role of astrocytes in hepatic encephalopathy

The Alzheimer type II astrocyte change is the distinctive morphologic alteration in brain of humans and experimental animals succumbing to hepatic encephalopathy (HE). Whether this change is a primary event in the pathogenesis of HE or whether it is secondary to injury of some other component(s) of the CNS has not been clarified. Studies in a rat model of HE have revealed early reactive changes in astrocytes characterized by cytoplasmic hypertrophy. During the later phases, degenerative changes ensue corresponding to the Alzheimer type II change observed by light microscopy. In view of the role of astrocytes in ammonia detoxification and the importance of ammonia in the pathogenesis of HE, we have suggested that the initial astrocytic changes are the morphological correlates of ammonia detoxification. We have speculated that the later degenerative alterations could lead to failure by astrocytes to carry out key functions (e.g., neurotransmitter uptake, ion regulation, and the like) and contribute the development of the encephalopathy. Recently, the potential involvement of astrocytes in HE has been further investigated, using primary astrocyte cultures. Exposure of cultures to ammonia at clinically relevant concentrations has shown morphologic changes closely resembling those observed in experimental HE in vivo. These deleterious effects can partly be prevented by raising cyclic AMP levels in cells. Other potential toxins (octanoic acid, phenol) have shown pathologic changes as well. Although some alterations were common to all three, they each possessed distinctive pathological effects. A synergistic interaction has also been demonstrated with these toxins. Functional studies of ammonia-treated astrocytes have shown the following: With low doses or short-term exposure, the uptakes of K+, glutamate, and GABA remained unchanged or slightly increased, whereas with higher doses or longer treatment, those activities diminished. A fall in ATP values occurred with prolonged ammonia treatment. Preliminary findings have shown no significant derangements in the beta-adrenergic receptor, except for a slight decrease in receptor affinity. However, cyclic AMP production was diminished following stimulation with isoproterenol. A slight rise in the number of benzodiazepine receptors was found. These studies indicate that profound changes occur in astrocytes following exposure to ammonia and other putative toxins. It is proposed that toxins and factors involved in the precipitation of HE do so by affecting astroglial properties. Derangements in such properties may lead to glial dysfunction (primary gliopathy), resulting in an encephalopathic state.

[15N]aspartate metabolism in cultured astrocytes. Studies with gas chromatography-mass spectrometry

The metabolism of 2.5 mM-[15N]aspartate in cultured astrocytes was studied with gas chromatography-mass spectrometry. Three primary metabolic pathways of aspartate nitrogen disposition were identified: transamination with 2-oxoglutarate to form [15N]glutamate, the nitrogen of which subsequently was transferred to glutamine, alanine, serine and ornithine; condensation with IMP in the first step of the purine nucleotide cycle, the aspartate nitrogen appearing as [6-amino-15N]adenine nucleotides; condensation with citrulline to form argininosuccinate, which is cleaved to yield [15N]arginine. Of these three pathways, the formation of arginine was quantitatively the most important, and net nitrogen flux to arginine was greater than flux to other amino acids, including glutamine. Notwithstanding the large amount of [15N]arginine produced, essentially no [15N]urea was measured. Addition of NaH13CO3 to the astrocyte culture medium was associated with the formation of [13C]citrulline, thus confirming that these cells are capable of citrulline synthesis de novo. When astrocytes were incubated with a lower (0.05 mM) concentration of [15N]aspartate, most 15N was recovered in alanine, glutamine and arginine. Formation of [6-amino-15N]adenine nucleotides was diminished markedly compared with results obtained in the presence of 2.5 mM-[15N]aspartate.

Utilization of [15N]glutamate by cultured astrocytes

The metabolism of 0.25 mM-[15N]glutamic acid in cultured astrocytes was studied with gas chromatography-mass spectrometry. Almost all 15N was found as [2-15N]glutamine, [2-15N]glutamine, [5-15N]glutamine and [15N]alanine after 210 min of incubation. Some incorporation of 15N into aspartate and the 6-amino position of the adenine nucleotides also was observed, the latter reflecting activity of the purine nucleotide cycle. After the addition of [15N]glutamate the ammonia concentration in the medium declined, but the intracellular ATP concentration was unchanged despite concomitant ATP consumption in the glutamine synthetase reaction. Some potential sources of glutamate nitrogen were identified by incubating the astrocytes for 24 h with [5-15N]glutamine, [2-15N]glutamine or [15N]alanine. Significant labelling of glutamate was noted with addition of glutamine labelled on either the amino or the amide moiety, reflecting both glutaminase activity and reductive amination of 2-oxoglutarate in the glutamate dehydrogenase reaction. Alanine nitrogen also is an important source of glutamate nitrogen in this system.