Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:2.6.1.1 (aspartate aminotransferase)
21,665 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Aspartate aminotransferase (AspAT, EC 2.6.1.1) from the halophilic archaebacterium Haloferax mediterranei was purified [Muriana, Alvarez-Ossorio and Relimpio (1991) Biochem. J. 278, 149-154] and further characterization of the effects of temperature on the activity and stability of the halophilic AspAT were carried out. The halophilic transaminase is most active at 65 degrees C and stable at high temperatures, under physiological or nearly physiological conditions (3.5 M KCl, pH 7.8). Thermal inactivation (60-85 degrees C) of the halophilic AspAT followed first-order kinetics, 2-oxoglutarate causing a shift of the thermal inactivation curves to higher temperatures. The salt concentration affected the thermal stability of the halophilic transaminase at 60 degrees C, suggesting that disruption of hydrophobic interactions may play an important role in the decreased thermal stability of the enzyme.
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PMID:Further thermal characterization of an aspartate aminotransferase from a halophilic organism. 813 56

The strictly conserved active site residue, Asp222, which forms a hydrogen-bonded salt bridge with the pyridine nitrogen atom of the pyridoxal 5' phosphate (PLP) co-factor of aspartate aminotransferase (AATase), was replaced with alanine (D222A) in the Escherichia coli enzyme. The D222A mutant exhibits non-hyberbolic saturation behavior with amino acid substrates which appear as apparent negative cooperativity in steady-state kinetic analyses. Single turnover progress curves for D222A are well described by the sum of two exponentials, contrasting with the monophasic kinetics of the wild-type enzyme. An active/inactive heterodimer containing the D222A mutation retains this biphasic kinetic response, proving that the observed cooperativity is not the result of induced allostery. The anomalous behavior is explained by a hysteretic kinetic model involving two slowly interconverting enzyme forms, only one of which is catalytically competent. The slow functional transition between the two forms has a half-life of approximately 10 mins. Preincubation of the mutant with the dicarboxylic inhibitor maleate shifts the equilibrium population of the enzyme towards the catalytically active form, suggesting that the slow transition is related to the domain closure known to occur upon association of this inhibitor with the wild-type enzyme. The importance of Asp222 in the chemical steps of transamination is confirmed by the approximately 10(5)-fold decrease in catalytic competence in the D222A mutant, and by the large primary C alpha-deuterium kinetic isotope effect (6.7 versus 2.2 for the wild-type). The transamination activity of the D222A mutant is enhanced 4- to 20-fold by reconstitution with the co-factor analog N-methylpyridoxal-5'-phosphate (N-MePLP), and the C alpha-proton abstraction step is less rate determining, as evidenced by the decrease in the primary kinetic isotope effect from 6.7 to 2.3. These results suggest that the conserved interaction between the protonated pyridine nitrogen of PLP and the negatively charged carboxylate of Asp222 is important not only for efficient C alpha-proton abstraction, but also for conformational transitions concomitant with the transamination process.
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PMID:Characterization of the apparent negative co-operativity induced in Escherichia coli aspartate aminotransferase by the replacement of Asp222 with alanine. Evidence for an extremely slow conformational change. 817 90

A relationship between plasma membrane fatty acid binding protein (FABPpm), a putative membrane transporter for long-chain fatty acids, and the mitochondrial isoform of aspartate aminotransferase (m-AspAT) has been reported. Accordingly, we have compared the chemical and immunological properties of rat liver m-AspAT with those of rat liver FABPpm isolated by two procedures: 1) detergent solubilization of the membranes followed by purification via fatty acid affinity chromatography (FABP-1) or 2) salt extraction of the membranes and subsequent purification by high-performance liquid chromatography (HPLC; FABP-2). Comparison of the three protein preparations revealed no differences with respect to NH2-terminal amino acid sequence, amino acid composition, peptides from tryptic digests, AspAT enzymatic activity, isoelectric point, mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), retention on five different HPLC columns, and immunoprecipitation and immunoblotting of SDS-PAGE separated proteins with polyclonal antisera. Examination of the proteins by nondenaturing PAGE showed a consistent second band in FABP-1 and FABP-2 not always present in m-AspAT. However, whenever present, this band was immunoreactive with antibodies to both m-AspAT and FABP-1. Hence, FABP-1 and FABP-2 are indistinguishable from one another. They are also at least closely related, if not identical, to m-AspAT.
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PMID:Comparison of plasma membrane FABP and mitochondrial isoform of aspartate aminotransferase from rat liver. 823 19

An 11-month-old female Vietnamese pot-bellied pig was examined for severe dehydration and neurologic signs including disorientation, ataxia, blindness, and involuntary twitching of the muscles of the neck and head. Biochemical analyses of serum revealed hypernatremia, hyperchloremia, hyperkalemia, azotemia, hyperphosphatemia, hyperalbuminemia, and high activities of aspartate transaminase and creatine kinase. A diagnosis of salt toxicosis/water deprivation was made. Medical management consisted of intravenous administration of a high-sodium crystalloid solution, anti-inflammatory drugs, and other supportive care. Sodium concentration of fluids administered intravenously was adjusted to be slightly less than the pig's serum sodium concentration so that the serum sodium concentration was reduced gradually over 48 hours. Resolution of clinical signs was rapid and the pig was discharged after 8 days of hospitalization. Fourteen days after the initial examination, the pig appeared healthy except for visual deficits. Historically, prognosis with conventional treatment of salt toxicosis/water deprivation is poor; however, this alternative approach to treating this condition appears promising.
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PMID:High-sodium crystalloid solution for treatment of hypernatremia in a Vietnamese pot-bellied pig. 883 48

The therapeutic effect of most immunosuppressive agents is unspecific and therefore often limited by an increased risk of infection by viral, bacterial or fungal organisms as well as by an increased incidence of malignant neoplasms. This short review includes the most commonly used immunosuppressants such as corticosteroids, azathioprine, methotrexate, cyclophosphamide and cyclosporine. The most common risks of long-term corticosteroid treatment are Cushing-like changes, decreased glucose tolerance and the usually benign steroid diabetes. Also clinically important is osteoporosis, since it can be prevented by physical training, calcium supplementation and treatment with vitamin D if necessary. Although there is still no proof of a significantly increased risk of peptic ulcer during steroid therapy, patients may develop gastrointestinal hemorrhage and even perforation without producing pain while being treated with corticosteroids. Mineralocorticoid effects, such as salt and water retention, are seen only with hydrocortisone and prednisone, whereas with synthetic steroids such as dexamethasone, sodium retention is absent despite their strong antiphlogistic activity. The most important side effect of the cytotoxic agents azathioprine, methotrexate and cyclophosphamide is marrow suppression. Due to the high turnover of neutrophils, patients most frequently suffer neutropenia rather than thrombocytopenia or anemia. Neutropenia, as well as impaired humoral and cellular immune mechanisms, are responsible for increased susceptibility to bacterial, viral or parasitic diseases during immunosuppressive therapy. Hepatotoxicity has been reported among patients receiving azathioprine (cholestatic hepatitis) and methotrexate (elevated AST levels and, rarely, liver fibrosis or cirrhosis). Cyclophosphamide causes hemorrhagic cystitis in a substantial proportion of patients, as well as an increased incidence of urothelial neoplasms. Both these side effects may be prevented by Mesna. The most important side effects of cyclosporine are acute and chronic nephrotoxicity usually associated with significantly elevated plasma levels of the drug. It must be borne in mind that severe nephrotoxicity may occur in patients receiving cyclosporine and ketoconazole together, since the latter may inappropriately increase the plasma cyclosporine level.
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PMID:[Immunosuppression--a tightrope walk between iatrogenic harm and therapy]. 892 65

Cysteinyl-leukotrienes can cause cholestasis and liver damage when administered at nanomolar concentrations. Using the isolated and perfused rat liver we analyzed whether S-adenosyl-L-methionine (SAMe) may protect this organ against the noxious effects of leukotriene-D4 (LTD4). We observed that a 2 nmol bolus of this compound decreased bile flow (-12.6% +/- 1.6%, P < .02), and bile salt excretion (-23.5% +/- 2.2%, P < .02; both compared with baseline values), caused the release of glutamic-oxaloacetic transaminase (GOT) and lactic dehydrogenase (LDH) to the hepatic effluent, and increased significantly the perfusion pressure as compared with a control group not receiving LTD4 (6.0 +/- 1.1 vs. 0.2 +/- 0.02 mm hg, respectively; P < .001). The cholestatic effect of LTD4 was attenuated by infusion of SAMe which, at rates of 67 and 100 microg/min, totally prevented the decrease in bile salt excretion. Likewise, in SAMe infused livers, the release to the effluent of GOT and LDH was lower than in the group receiving LTD4 only, and was even lower than in the control group. We also found that the increase in perfusion pressure induced by LTD4 was prevented by SAMe in a dose-dependent manner. Of interest, SAMe increased the biliary excretion of the eicosanoid in a dose-related fashion. We conclude that SAMe reverts the cholestatic, cytotoxic, and hemodynamic effects of LTD4 on the liver, and that these protective effects might be partly because of a stimulation of the biliary excretion of the leukotriene.
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PMID:S-adenosyl-L-methionine protects the liver against the cholestatic, cytotoxic, and vasoactive effects of leukotriene D4: a study with isolated and perfused rat liver. 925 42

The primary role of Tyr225 in the aspartate aminotransferase mechanism is to provide a hydrogen bond to stabilize the 3'O- functionality of bound pyridoxal phosphate. The strength of this hydrogen bond is perturbed by replacement of Tyr225 with 3-fluoro-L-tyrosine (FlTyr) by in vitro transcription/translation. This mutant enzyme exhibits kcat/values that are near to those of wild type enzyme; however, the kcat/vs pH profile is much sharper with similar pKas of approximately 7.5 for both the ascending and descending limbs. The pKas are assigned to the endocyclic proton of the internal aldimine and to the bridging hydrogen bond, respectively. The pKas in the kcat vs pH profile of 7.2 and 8.7 are assigned to the epsilon-NH3+ of lysine 258 and to the endocyclic protons of the ketimine complex, respectively. Arginine 292 forms a salt bridge with the beta-COOH of the substrate, aspartate. An improvement on the earlier attempt to invert the substrate charge specificity via R292D mutation-induced arginine transaminase activity [Cronin, C. N., & Kirsch, J. F. (1988) Biochemistry 27, 4572-4579] is described. Here Arg292 is replaced with homoglutamate (R292hoGlu). This construct exhibits 6.8 x 10(4)-fold greater activity for the cationic substrate D,L-[Calpha-3H]-alpha-amino-beta-guanidinopropionic acid (D,L-[Calpha-3H]AGPA) than does wild type enzyme. The gain in selectivity for this substrate is at least 4500-fold greater than that achieved in the 1988 experiment, i.e., [(kcat/KM)R292hoGlu/(kcat/KM)WT (D,L-[Calpha-3H]AGPA)] >/= 4500 x [(kcat/KM)R292D/(kcat/KM)WT (L-arginine)]. The value of (kcat/KM)R292D is 0.43 M-1 s-1 with L-Arg while (kcat/KM)R292hoGlu is 29 M-1 s-1 with D,L-[Calpha-3H]AGPA (it is assumed that the D-enantiomer is unreactive). The latter value is the lower limit because of the uncertain value of 3H kinetic isotope effect.
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PMID:Noncoded amino acid replacement probes of the aspartate aminotransferase mechanism. 926 32

Haematological, biochemical and pathological effects in rats produced by the salt-tolerant plant Avicennia marina given at oral doses of 1 or 4 g kg(-1) for three consecutive days or 0.5 g kg(-1) day(-1) for 28 consecutive days are reported. No overt behavioral changes, moribundity or mortality were seen in either of the two experiments. A dose of 1 g kg(-1) did not affect significantly either body or liver weights. However, at a dose of 4 g kg(-1) the extract reduced both body and liver weights. The extract at both doses significantly increased leucocyte (mainly neutrophil) counts but did not affect significantly erythrocyte counts, haemoglobin concentration or the haematocrit. Except for a slight, but statistically significant, decrease in plasma glucose concentration and an increase in Na, Ca, Cu, Mg and cholesterol concentrations and aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities, the extract exerted no significant effects on plasma biochemistry. The treatment produced dose-related mild cellular degeneration in the liver and congestion in the central veins. There were also prominent Kupffer's cells and monocellular infiltrations. In the kidneys there was shrinkage and cellular degeneration of glomeruli and patches of medullary haemorrhage. In the spleen a slight activation of the germinal centre in the white pulp was noted. Subchronic treatment with the extract did not affect significantly the body and liver weights, the water intake, faecal and urinary output, leucocyte and erythrocyte counts, haemoglobin or haematocrit. There was a significant decrease in the number of platelets and an increase in the number of neutrophils. No significant changes in plasma biochemistry were observed, except for a 15% increase in AST activity. Subchronic treatment produced a significant reduction in glutathione concentration, amounting to about 20%. Histopathological findings after the subchronic treatment were similar in nature but milder than those seen after the acute treatment.
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PMID:Toxicological studies on the leaves of Avicennia marina (mangrove) in rats. 957 Jun 93

The objective was to evaluate the rationale for liver needle biopsy versus blood liver functional tests in monitoring the incidence of hepatotoxicity in Egyptian rheumatoid arthritic patients treated with gold compounds. Forty patients (12 males, 28 females) were randomly selected out of 258 Egyptian rheumatoid arthritic patients treated with sodium auro-thiomalate during the past 4 years. The minimum duration of treatment was 40 weeks. The methods used were firstly, liver function tests (serum glutamic-oxaloacetic transaminase, serum glutamic-pyruvic transaminase, total serum bilirubin and total serum albumin) before, weekly during and after administration of sodium auro-thiomalate. Secondly, a needle liver biopsy was conducted by using the tru-cut needle. Then liver histology was graded according to Roenigk for grading liver toxicity. Viral hepatitis markers (hepatitis B surface antigen, anti-hepatitis C virus were done for monitoring viral hepatitis. Finally, the liver tissue contents of heavy metals were counted in the cases that showed grade IIIB histological changes. The results showed that none of the studied cases developed any clinically significant liver disease during the course of chrysotherapy. Blood liver function tests were of normal value throughout the course of drug administration. According to Roenigk grading, 20 patients (50%) showed grade I liver changes, and the other 20 patients showed liver changes of grades II and III (four grade II, eight grade IIIA, and another eight grade IIIB). None of the patients showed grade IV liver changes. It was concluded that blood liver tests are not the most sensitive methods to detect hepatotoxicity in gold-receiving Egyptian rheumatoid arthritic patients. Needle liver biopsy is not superior in detecting liver toxicity, compared with routine laboratory liver function tests, because of its complications. Rheumatoid arthritic patients with a potential risk of clinically significant liver disease should not be exposed to the risk of gold salt therapy. Pretreatment HLA-DR genetic typing may be a good detector for rheumatoid arthritic patients with potential risk of hepatotoxicity.
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PMID:Liver toxicity profile in gold-treated Egyptian rheumatoid arthritis patients. 960 32

Aminotransferase reversibly catalyzes the transamination reaction by a ping-pong bi-bi mechanism with pyridoxal 5'-phosphate (PLP) as a cofactor. Various kinds of aminotransferases developing into catalysts for particular substrates have been reported. Among the aminotransferases, aromatic amino acid aminotransferase (EC 2.6.1. 57) catalyzes the transamination reaction with both acidic substrates and aromatic substrates. To elucidate the multiple substrate recognition mechanism, we determined the crystal structures of aromatic amino acid aminotransferase from Paracoccus denitrificans (pdAroAT): unliganded pdAroAT, pdAroAT in a complex with maleate as an acidic substrate analog, and pdAroAT in a complex with 3-phenylpropionate as an aromatic substrate analog at 2.33 A, 2. 50 A and 2.30 A resolution, respectively. The pdAroAT molecule is a homo-dimer. Each subunit has 394 amino acids and one PLP and is divided into small and large domains. The overall structure of pdAroAT is essentially identical to that of aspartate aminotransferase (AspAT) which catalyzes the transamination reaction with only an acidic amino acid. On binding the acidic substrate analog, arginine 292 and 386 form end-on salt bridges with carboxylates of the analog. Furthermore, binding of the substrate induces the domain movement to close the active site. The recognition mechanism for the acidic substrate analog in pdAroAT is identical to that observed in AspAT. Binding of the aromatic substrate analog causes reorientation of the side-chain of the residues, lysine 16, asparagine 142, arginine 292* and serine 296*, and changes in the position of water molecules in the active site to form a new hydrogen bond network in contrast to the active site structure of pdAroAT in the complex with an acidic substrate analog. Consequently, the rearrangement of the hydrogen bond network can form recognition sites for both acidic and aromatic side-chains of the substrate without a conformational change in the backbone structure in pdAroAT.
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PMID:Crystal structures of Paracoccus denitrificans aromatic amino acid aminotransferase: a substrate recognition site constructed by rearrangement of hydrogen bond network. 966 48


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