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PubMed1975

Activities of citrate synthase, NAD+-linked and NADP+-linked isocitrate dehydrogenases, glutamate dehydrogenase, aspartate aminotransferase and alanine aminotransferase in nervous tissues from vertebrates and invertebrates.

1. The activities of citrate synthase and NAD+-linked and NADP+-linked isocitrate dehydrogenases were measured in nervous tissue from different animals in an attempt to provide more information about the citric acid cycle in this tissue. In higher animals the activities of citrate synthase are greater than the sum of activities of the isocitrate dehydrogenases, whereas they are similar in nervous tissues from the lower animals. This suggests that in higher animals the isocitrate dehydrogenase reaction is far-removed from equilibrium. If it is assumed that isocitrate dehydrogenase activities provide an indication of the maximum flux through the citric acid cycle, the maximum glycolytic capacity in nervous tissue is considerably greater than that of the cycle. This suggest that glycolysis can provide energy in excess of the aerobic capacity of the tissue. 2. The activities of glutamate dehydrogenase are high in most nervous tissues and the activities of aspartate aminotransferase are high in all nervous tissue investigated. However, the activities of alanine aminotransferase are low in all tissues except the ganglia of the waterbug and cockroach. In these insect tissues, anaerobic glycolysis may result in the formation of alanine rather than lactate.

Alanine Transaminase↗
PubMed1993

Serum malondialdehyde and mitochondrial aspartate aminotransferase activity as markers of chronic alcohol intake and alcoholic liver disease.

Since lipid peroxidation is a well-know mechanism of alcohol-related liver damage, the aim of the present study was to assess the role of serum malondialdehyde (MDA), a secondary product of lipoperoxidation, in the detection of alcoholism and different stages of alcoholic liver disease and to correlate serum levels of malondialdehyde with other markers. Sixty-five patients with a mean alcohol intake of 151 gr/day, were divided into three groups: alcoholics with normal liver function (ANLF, 7 pts), non-cirrhotic alcoholic liver disease (NCALD, 26 pts) and alcoholic cirrhosis (ALC, 32 pts). The control group consisted of 15 healthy subjects. Serum MDA was measured by the thiobarbituric acid reaction test, and mitochondrial aspartate aminotransferase (mAST) with immunochemical assay. MDA had a higher sensitivity (70% vs 37.5%) and specificity (100% vs 93%) than mAST in detecting alcohol abuse, irrespective of the presence of liver disease. Serum MDA levels were significantly higher in all three groups than in controls (2.3 +/- 0.1 nmol/ml), the highest value being found in NCALD (4.6 +/- 0.4). Serum MDA levels were correlated with prothrombin time (p < 0.005) and blood alcohol levels (p < 0.05). mAST serum activity was also significantly higher in all three groups than in controls. A significant correlation was found between serum MDA and mAST only when the whole group was considered.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1987

Tyrosine 70 increases the coenzyme affinity of aspartate aminotransferase. A site-directed mutagenesis study.

The crucial step in enzymatic transamination is the tautomerization of aldimine/ketimine intermediates, formed between the pyridoxyl coenzyme and the amino/keto acid substrate, which is catalyzed primarily by the active site residue Lys-258 (Malcolm, B. A., and Kirsch, J. F. (1985) Biochem. Biophys. Res. Commun. 132, 915-921; W. L. Finlayson and J. F. Kirsch, in preparation). Tyr-70 is localized in close proximity to Lys-258 and, in addition, forms a hydrogen bond with the coenzyme phosphate. Tyr-70 has been postulated to have an important role in the tautomerization (Kirsch, J. F., Eichele, G., Ford, G. C., Vincent, M. G., Jansonius, J. N., Gehring, H., and Christen, P. (1984) J. Mol. Biol. 174, 497-525). This hypothesis has now been tested by the construction and analysis of a mutant Escherichia coli aspartate aminotransferase in which Tyr-70 has been changed to Phe (Y70F). Y70F retains at least 15% of the maximal activity of the wild type enzyme (WT) (kcat = 170 +/- 15 s-1 for WT versus greater than or equal to 26 +/- 3 s-1 for Y70F and shows increased Michaelis constants for both substrates (KmAsp = 2.5 +/- 0.4 mM; Km alpha Kg = 0.59 +/- 0.08 mM for WT versus KmAsp = 3.9 +/- 0.3 mM; Km alpha Kg = 2.70 +/- 0.02 mM for Y70F (where alpha Kg is alpha-ketoglutarate) ). The spectrophotometrically determined pK a values of the internal aldimines formed between pyridoxal 5'-phosphate (PLP) and Lys-258 are identical for WT and Y70F. In assays where excess L-aspartate and excess PLP are incubated with either WT or Y70F, the mutant enzyme converts the free PLP to free pyridoxamine 5'-phosphate 80-fold faster than WT (k = (3.75 +/- 0.23) X 10(-2)s-1 for Y70F versus (4.90 +/- 0.02) X 10(-4)s-1 for WT). Y70F also converts free pyridoxamine 5'-phosphate to free PLP faster than WT. Thus, Y70F dissociates coenzyme more readily than does WT. It therefore appears that the role of Tyr-70 is mainly in preventing the dissociation of the coenzyme from the enzyme. Tyr-70 does not function in an essential chemical step.

Aspartate Aminotransferases↗
PubMed1981

[Blood mineral (Ca, Mg, Na K, Pa) levels and aspartate aminotransferase, leucine aminopeptidase, creatine phosphokinase and alkaline phosphatase activity and blood glucose levels in cattle with hypomagnesemia prior to and following infusion of a 500 ml solution containing 60 g of Mg adipate and 60 g of Ca gluconate].

The levels of various compounds in blood serum and blood plasma were tested in 14 heads of cattle with hypomagnesiaemia (x = 0.76 +/- 0.29 mg/100 ml), prior to and following infusion of 500 ml of a calcium-magnesium solution (containing 60 g of magnesium adipate and 60 g of calcium gluconate). Concomitant hypocalcaemia (6.1 mg/100 ml) was recordable only from one animal. Most of the potassium as well as all Pa and protein values in blood serum were physiologically normal, while sodium ws somewhat reduced. The glucose level in blood plasma was increased in the animal with concomitant hypocalcaemia and in one animal with poor recovery potential. The drops in magnesium and calcium levels in blood serum, following infusion, in cattle with hypomagnesiaemia was very similar to that in clinically intact cattle. Aspartate-aminotransferase and creatinine-phosphokinase activities of the serum were increased in all cattle, but leucine-aminopeptidase activity in only some of them. The activity of alkaline phosphatase of the blood serum was physiologically normal.

Adipates↗
PubMed2005

Identification by microarray analysis of aspartate aminotransferase and glutamine synthetase as glucocorticoid target genes in a mouse Schwann cell line.

Schwann cells have been identified as targets for glucocorticoids. Besides genes implicated in the myelination process, the target genes of glucocorticoids have not been identified in these cells. For that purpose, we performed microarray analysis on MSC80 (mouse Schwann cells) treated with a synthetic glucocorticoid, dexamethasone. These cells express a functional glucocorticoid receptor (GR), but none of the other steroid receptors. This allowed us to identify genes specifically regulated by GR in the absence of the mineralocorticoid receptor. Among the 5000 genes analyzed, 12 were at least two-fold upregulated and 91 genes were at least two-fold down-regulated upon treatment with dexamethasone. Because of their potential role in Schwann cell homeostasis, we selected, for further analysis, the upregulated genes encoding glutamine synthetase (GS) and cytosolic aspartate aminotransferase (cAspAT). These genes play a crucial role in the glutamate cycle which was shown to be vital in neuron-astrocyte cross-talk in the central nervous system. Their activation was confirmed by semi-quantitative and real-time PCR. A detailed analysis of cAspAT promoter activity revealed that the mechanism of regulation by GR in Schwann cells differs from that in hepatoma cells, suggesting a cell-specific regulation. The transactivation potency of the two Glucocorticoid Responsive Units (GRU) present in the cAspAT promoter seems to be dependent on the levels of the GR in MSC80 cells. Furthermore, we show that an increase in GR levels under certain circumstances could considerably potentiate the effects of glucocorticoids on the cAspAT promoter via synergistic activation of both GRU, To the opposite, an enhancement in GR levels did not further potentiate Dex-activation of the GS promoter, showing a differential mechanism of action of GR in the context of both promoters.

Animals↗
PubMed1994

Tricarboxylic acid cycle in rat brain synaptosomes. Fluxes and interactions with aspartate aminotransferase and malate/aspartate shuttle.

The flux through different segments of the tricarboxylic acid cycle was measured in rat brain synaptosomes with gas chromatography-mass spectrometry using either deuterated glutamine or [13C]aspartate. The flux between 2-oxoglutarate and oxaloacetate was estimated to be 3.14 and 4.97 nmol/min/mg protein with and without glucose, respectively. These values were 3-5-fold faster than the flux between oxaloacetate and 2-oxoglutarate (0.92 nmol/min per mg protein) measured in the presence of glucose. The pattern of intermediates labeling suggests that the overall rate-controlling reaction involves either citrate synthase or pyruvate dehydrogenase but not 2-oxoglutarate or isocitrate dehydrogenase. The enrichment in [3,3,4,4-2H4]glutamate from [2,3,3,4,4-2H5]glutamine was as rapid as in [2,3,3,4,4-2H5]glutamate, which indicates that the aspartate aminotransferase reaction is severalfold faster than the flux through the tricarboxylic acid cycle. [13C]Aspartate was rapidly converted to [13C]malate, suggesting that in intact synaptosomes aspartate entry into the mitochondrion is very slow. The finding that aspartate is taken up by mitochondria as malate, along with the observed high enrichment in [3-2H]malate (from [2,3,3,4,4-2H5]glutamine), is consistent with the substantial synaptosomal activity of the malate/aspartate shuttle.

Animals↗
PubMed1999

Non-Enzymatic Glycosylation (or Glycation) and Inhibition of the Pig Heart Cytosolic Aspartate Aminotransferase by Glyceraldehyde 3-Phosphate.

Glyceraldehyde 3-phosphate (Glyc3P), a glycolytic intermediate, non-enzymatically glycosylated (or glycated) and inhibited the pig heart cytoplasmic aspartate aminotransferase (cAAT). Glyc3P (5.0 mM) decreased cAAT activity by 47% after 1 min at 23 degrees C. cAAT activity remained unchanged after a 24 h incubation with either glucose 6-phosphate (5.0 mM) or ribose 5-phosphate (5.0 mM). Increasing the incubation pH from 6.4 to 7.8 or the incubation temperature from 23 degrees C to 50 degrees C enhanced Glyc3P's inhibitory effect on cAAT activity. Glyc3P (250-500 µM) decreased the thermal stability of cAAT as evidenced by lowering the T(m) or temperature that caused a 50% irreversible loss of cAAT activity (69 degrees C, control; 58.5 degrees C, 500 µM Glyc3P). Glyc3P decreased cAAT amino group content and increased glycation products, which were measured by adduct formation, fluorescence and protein crosslinking.

Journal Article↗
PubMed1989

Estimation of free energy barriers in the cytoplasmic and mitochondrial aspartate aminotransferase reactions probed by hydrogen-exchange kinetics of C alpha-labeled amino acids with solvent.

The existence of the postulated quinonoid intermediate in the cytoplasmic aspartate amino-transferase catalyzed transamination of aspartate to oxaloacetate was probed by determining the extent of transfer of tritium from the C alpha position of tritiated L-aspartate to pyridoxamine 5'-phosphate in single turnover experiments in which washout from the back-reaction was obviated by product trapping. The maximum amount of transferred tritium observed was 0.7%, consistent either with a mechanism in which a fraction of the net transamination reaction proceeds through a quinonoid intermediate or with a mechanism in which this intermediate is formed off the main reaction pathway. It is shown that transfer of labeled hydrogen from the amino acid to cofactor cannot be used to differentiate a stepwise from a concerted transamination mechanism. The amount of tritium transferred is a function of the rate constant for torsional equilibration about the epsilon-amino group of Lys-258, the presumptive abstractor of the C alpha proton; the relative rate constants for hydrogen exchange with solvent versus cofactor protonation; and the tritium isotope effect on this ratio. The free energy barriers facing the covalent intermediate between aldimine and keto acid product (i.e., ketimine and possibly quinonoid) were evaluated relatively by comparing the rates of C alpha-hydrogen exchange in starting amino acid with the rates of keto acid formation. The value of theta (= kexge/kprod) was found to be 2.6 for the reaction of cytoplasmic isozyme with aspartate and ca. 0.5 for that of the mitochondrial form with glutamate.

Animals↗