Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UNIPROT:P17174 (aspartate aminotransferase)
 14,872 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Aspartate aminotransferase (mitochondrial isoenzyme from chicken) has been found to racemize very slowly dicarboxylic amino acid substrates in the presence of their cognate oxo acids [Kochhar, S. & Christen, P. (1988) Eur. J. Biochem. 175, 433-438]. Tyrosine, phenylalanine and alanine are racemized at the same rate although they undergo the transamination reaction 3-5 orders of magnitude more slowly than the dicarboxylic substrates. Similarly, the truncated enzyme aspartate aminotransferase-(27/32-410) catalyzes the racemization at the same rate as the native enzyme, while its rate of transamination is decreased to 3% of that of the native enzyme. Apparently, the rate-limiting step in racemization is not immediately linked to the transamination cycle. Decreasing the water concentration in the reaction medium by adding methanol at 0 degrees C drastically reduces the rate of racemization without affecting the rate of transamination. On the basis of these and additional kinetic data and the model of the three-dimensional structure of the active site, we conclude that a water molecule is responsible for the protonation of C alpha of the coenzyme-substrate intermediate from the wrong side. The diffusion of the water molecule into the interior of the enzyme appears to be the rate-limiting step in aspartate-aminotransferase-catalyzed racemization.
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PMID:Mechanism of racemization of amino acids by aspartate aminotransferase. 173 41

Aspartate aminotransferase from the archaebacterium Haloferax mediterranei was purified and found to be homogeneous. An average Mr of 66,000 was estimated. The native halophilic transaminase exhibited no maximum absorption at 410 nm, which indicates that the apo form is obtained by our purification procedure, and the molar absorption coefficient at 275 nm in 3.5 M-KCl (pH 7.8) was found to be 78.34 mM-1.cm-1. Plots of titration data show that 1 mol of halophilic aspartate aminotransferase binds 2 mol of pyridoxal 5'-phosphate. The halophilic transaminase behaved as a dimer with two similar subunits and had a maximum activity in the pH range 7.6-7.9 and at 65 degrees C in 3.5 M-KCl. By differential scanning calorimetry, the denaturation temperature of the halophilic holo- and apo-transaminase was determined to be 78.5 and 68.0 degrees C respectively at 3.3 M-KCl (pH 7.8). At low salt concentration the halophilic transaminase was inactivated, following first-order kinetics. The Km values for 2-oxoglutarate and L-aspartate, in 3 M-KCl (pH 7.8), were 0.75 mM and 12.6 mM respectively.
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PMID:Purification and characterization of aspartate aminotransferase from the halophile archaebacterium Haloferax mediterranei. 190 12

Aspartate aminotransferase from the archaebacterium Sulfolobus solfataricus binds pyridoxal 5' phosphate, via an aldimine bond, with Lys-241. This residue has been identified by reducing the enzyme in the pyridoxal form with sodium cyanoboro[3H]hydride and sequencing the specifically labeled peptic peptides. The amino acid sequence centered around the coenzyme binding site is highly conserved between thermophilic aspartate aminotransferases and differs from that found in mesophilic isoenzymes. An alignment of aspartate aminotransferase from Sulfolobus solfataricus with mesophilic isoenzymes, attempted in spite of the low degree of similarity, was confirmed by the correspondence between pyridoxal 5' phosphate binding residues. Using this alignment it was possible to insert the archaebacterial aspartate aminotransferase into a subclass, subclass I, of pyridoxal 5' phosphate binding enzymes comprising mesophilic aspartate aminotransferases, tyrosine aminotransferases and histidinol phosphate aminotransferases. These enzymes share 12 invariant amino acids most of which interact with the coenzyme or with the substrates. Some enzymes of subclass I and in particular aspartate aminotransferase from Sulfolobus solfataricus, lack a positively charged residue, corresponding to Arg-292, which in pig cytosolic aspartate aminotransferase interacts with the distal carboxylate of the substrates (and determines the specificity towards dicarboxylic acids). It was confirmed that aspartate aminotransferase from Sulfolobus solfataricus does not possess any arginine residue exposed to chemical modifications responsible for the binding of omega-carboxylate of the substrates. Furthermore, it has been found that aspartate aminotransferase from Sulfolobus solfataricus is fairly active when alanine is used as substrate and that this activity is not affected by the presence of formate. The KM value of the thermophilic aspartate aminotransferase towards alanine is at least one order of magnitude lower than that of the mesophilic analogue enzymes.
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PMID:The active site of Sulfolobus solfataricus aspartate aminotransferase. 195 27

Aspartate aminotransferase undergoes major shifts in the conformational equilibrium of the protein matrix during transamination. The present study defines the two conformational states of the enzyme by crystallographic analysis, examines the conditions under which the enzyme crystallizes in each of these conformations, and correlates these conditions with the conformational behaviour of the enzyme in solution, as monitored by a fluorescent reporter group. Cocrystallization of chicken mitochondrial aspartate aminotransferase with inhibitors and covalent coenzymesubstrate adducts yields three different crystal forms. Unliganded enzyme forms triclinic crystals of the open conformation, the structure of which has been solved (space group P1) [Ford, G. C., Eichele, G. & Jansonius, J. N. (1980) Proc. Natl Acad. Sci. USA 77, 2559-2563; Kirsch, J. F., Eichele, G., Ford, G. C., Vincent, M. G., Jansonius, J. N., Gehring, H. & Christen, P. (1984) J. Mol. Biol. 174, 487-525]. Complexes of the enzyme with dicarboxylate ligands form monoclinic or orthorhombic crystals of the closed conformation. The results of structure determinations of the latter two crystal forms at 0.44 nm resolution are described here. In the closed conformation, the small domain has undergone a rigid-body rotation of 12-14 which closes the active-site pocket. Shifts in the conformational equilibrium of aspartate aminotransferase in solution, as induced by substrates, substrate analogues and specific dicarboxylic inhibitors, can be monitored by changes in the relative fluoresence yield of the enzyme labelled at Cys166 with monobromotrimethylammoniobimane. The pyridoxal and pyridoxamine forms of the labelled enzyme show the same fluorescence properties, whereas in the apoenzyme the fluorescence intensity is reduced by 30%. All active-site ligands, if added to the labelled pyridoxal enzyme at saturating concentrations, cause a decrease in the fluorescence intensity by 40-70% and a blue shift of maximally 5 nm. Comparison of the fluorescence properties of the enzyme in various functional states with the crystallographic data shows that both techniques probe the same conformational equilibrium. The conformational change that closes the active site seems to be ligand-induced in the reaction of the pyridoxal form of the enzyme and syncatalytic in the reverse reaction with the pyridoxamine enzyme.
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PMID:The open/closed conformational equilibrium of aspartate aminotransferase. Studies in the crystalline state and with a fluorescent probe in solution. 200 2

This study explored myocardial protective effects of allopurinol at various doses. Ninety patients undergoing coronary artery bypass or repair or replacement of cardiac valves were divided into three groups of 30 patients each in accordance with the amount of allopurinol administered to patients in each group. Patients in group I received no allopurinol, those in group II received low-dose allopurinol (total dose 1200 mg), and those in group III received high-dose allopurinol (total dose 2400 mg). Aspartate aminotransferase, cardiac isoenzyme of creatine kinase, and lactic dehydrogenase levels were measured up to 5 days after operation. Concentrations of allopurinol and oxypurinol were also measured before initiation of cardiopulmonary bypass and at the start and at the end of aortic crossclamping. Postoperative aspartate aminotransferase, creatine kinase, and lactate dehydrogenase 1 plus lactate dehydrogenase 2 levels in group III were significantly lower than those in groups I and II. Aspartate aminotransferase, creatine kinase, and lactate dehydrogenase 1 plus lactate dehydrogenase 2 levels in group II were lower than those in group I, without statistically significant differences. Plasma oxypurinol concentrations were significantly higher in group III than in group II. It was concluded that allopurinol had resultant high myocardial protective effects in dose-related fashion, but its effect might be attributed to oxypurinol levels formed by its degradation.
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PMID:A clinical trial of allopurinol (Zyloric) for myocardial protection. 200 10

The active site residue lysine 258 of chicken mitochondrial aspartate aminotransferase was replaced with a histidine residue by means of site-directed mutagenesis. The mutant protein was expressed in Escherichia coli and purified to homogeneity. Addition of 2-oxoglutarate to its pyridoxamine form changed the coenzyme absorption spectrum (lambda max = 330 nm) to that of the pyridoxal form (lambda max = 330/392 nm). The rate of this half-reaction of transamination (kcat = 4.0 x 10(-4)s-1) is five orders of magnitude slower than that of the wild-type enzyme. However, the reverse half-reaction, initiated by addition of aspartate or glutamate to the pyridoxal form of the mutant enzyme, is only three orders of magnitude slower than that of the wild-type enzyme, kmax of the observable rate-limiting elementary step, i.e. the conversion of the external aldimine to the pyridoxamine form, being 7.0 x 10(-2)s-1. Aspartate aminotransferase (Lys258----His) thus represents a pyridoxal-5'-phosphate-dependent enzyme with significant catalytic competence without an active site lysine residue. Apparently, covalent binding of the coenzyme, i.e. the internal aldimine linkage, is not essential for the enzymic transamination reaction, and a histidine residue can to some extent substitute for lysine 258 which is assumed to act as proton donor/acceptor in the aldimine-ketimine tautomerization.
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PMID:Aspartate aminotransferase with the pyridoxal-5'-phosphate-binding lysine residue replaced by histidine retains partial catalytic competence. 210 17

Aminotransferase activities were measured in the serum of two- to three-year-old Thoroughbred fillies and colts during a four week period of peak training for flat racing. Aspartate aminotransferase (AspAT, EC 2.6.1.1), mitochondrial aspartate aminotransferase (m-AspAT) and alanine aminotransferase (AlaAT, EC 2.6.1.2) activities in serum were measured and the relative proportions of apoenzyme and holoenzyme were determined. The aminotransferase activities were increased only slightly immediately following exercise. This small and immediate post exercise increase in activity did not vary greatly over the period of peak training. Measured in the presence of exogenous pyridoxal 5'-phosphate, mean enzyme activities (iu/litre at 30 degrees C) before exercise were: AspAT, 291; m-AspAT, 13; AlaAT, 18. After exercise they were: AspAT, 317; m-AspAT, 16; AlaAT, 23. Nearly all of the AspAT activity was present in the holoenzyme form (94 per cent holoenzyme) indicating excellent vitamin B6 status in these animals. Paradoxically, the AlaAT in serum from the same highly trained Thoroughbred horses was poorly saturated with pyridoxal phosphate, with nearly half of the AlaAT in most horses present in the inactive apoenzyme form (61 per cent that of holoenzyme). It is critical therefore, that exogenous pyridoxal phosphate be included in aminotransferase assays to determine the amounts of enzyme release into the peripheral circulation.
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PMID:Effects of exercise on serum amino-transferase activity and pyridoxal phosphate saturation in Thoroughbred racehorses. 236 10

Twenty-eight Norwegian Red Cattle dairy cows were fed silage ad libitum and restricted amounts of concentrates. Blood samples were collected before morning feeding, once or twice weekly, from 2 weeks before to 12 weeks after calving. Parameters of liver function, carbohydrate status and fertility were recorded in order to assess their interrelationships. Eight cows were treated for clinical ketosis. Four of these had to be treated 2 or 3 times. Aspartate aminotransferase and bilirubin showed the highest within-animal coefficients of correlation with acetoacetate. Analysis of variance revealed a significant effect of carbohydrate status (indicated by plasma acetoacetate levels) on the levels of aspartate aminotransferase, glutamate dehydrogenase and sorbitol dehydrogenase, though only a small part of the total variation was explained by this factor. The estimated volume density of liver fat in the 4th week of lactation averaged 6.0 +/- 6.4% (+/- SD) ranging from 0.1-25.1%. Liver fat content at this stage of lactation was not significantly correlated with other indicators of liver function or carbohydrate status. Cows treated for clinical ketosis had significantly lower plasma progesterone values at the time of first ketosis treatment than untreated multiparous cows. The frequency of high progesterone values (greater than 3 ng/ml) being significantly lower in treated than in untreated cows during the period from 3-5 weeks post partum, though not at later stages. In conclusion, the results revealed a significant relationship between carbohydrate status and liver function, and also between clinical ketosis and luteal function.
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PMID:Variations in parameters of liver function and plasma progesterone related to underfeeding and ketosis in a dairy herd. 259 86

Aspartate aminotransferase, alanine aminotransferase, gamma-glutamyltransferase, and alkaline phosphatase activities in the blood serum of women taking the oral contraceptive preparation Microgynon through extended periods were raised; the activity of cholinesterase was simultaneously reduced. In rats liver homogenates ethynylestradiol, one of the active components of Microgynon, acted as an inducer of gamma-glutamyltransferase and alkaline phosphatase while leaving aspartate aminotransferase and alanine aminotransferase unaffected, but reduced the level of cholinesterase. Norgestrel, the other active component of the preparation, suppressed the biosynthesis of gamma-glutamyltransferase and alkaline phosphatase while leaving aspartate aminotransferase, alanine aminotransferase and cholinesterase levels unaffected. A mixture of ethynylestradiol plus norgestrel in the mass proportion occurring in Microgynon produced the same effects upon gamma-glutamyltransferase and alkaline phosphatase as ethynylestradiol alone. Estradiol, the parent hormone of ethynylestradiol, lacked the inducing capability of the latter while ethynylpropargyl chloride induced gamma-glutamyltransferase and alkaline phosphatase so it was concluded the inducing effect of ethynylestradiol must be ascribed to the ethynyl radical. Progesterone, the parent of norgestrel, shared the latter's suppressive activity for gamma-glutamyltransferase and alkaline phosphatase biosynthesis, and behaved like its derivative towards the other enzymes.
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PMID:Changes of activities of some transferases, alkaline phosphatase and cholinesterase in the blood of women using oral contraceptives and in vitro influence of these agents on tissular enzyme levels in rat liver. 260 59

The excitatory amino acids, aspartate and glutamate, have been proposed as retinal neurotransmitters. Aspartate aminotransferase (AAT) is an enzyme which is involved in the routine metabolism of these amino acids and may be involved in the specific synthesis of glutamate and/or aspartate for use as a neurotransmitter. On the basis of the hypothesis that increased levels of aspartate aminotransferase may reflect a transmitter role for aspartate and/or glutamate, we have localized aspartate aminotransferase in the guinea pig and cynamolgus monkey retinas with light and electron microscopic immunohistochemical techniques. AAT-like immunoreactivity is localized to the cones of guinea pig retina and to monkey rods. Both species contain a subpopulation of immunoreactive amacrine cells as well as a subpopulation of immunoreactive cells in the ganglion cell layer. Immunostaining is seen in bipolar cells and terminals in the monkey but not in the guinea pig retina. We have performed quantitative analysis of the immunoreactive staining in the outer plexiform layer and described the synaptic organization of immunoreactive processes in the inner plexiform layer (IPL). Labeled amacrine processes in both species form synaptic contacts predominantly to and from bipolar terminals in the inner third of the IPL and to and from other amacrine and small unidentified processes in the outer portion of the IPL. The majority of labeled bipolar terminals in the monkey retina are seen in the inner third of the IPL where they synapse exclusively onto amacrine processes. Labeled bipolar terminals in the outer third of the IPL occasionally synapse onto ganglion processes.
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PMID:Aspartate aminotransferase-like immunoreactivity in the guinea pig and monkey retinas. 285 36


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