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)

The subunits of the dimeric enzyme aspartate aminotransferase have two domains: one large and one small. The active site lies in a cavity that is close to both the subunit interface and the interface between the two domains. On binding the substrate the domains close together. This closure completely buries the substrate in the active site and moves two arginine side-chains so they form salt bridges with carboxylate groups of the substrate. The salt bridges hold the substrate close to the pyridoxal 5'-phosphate cofactor and in the right position and orientation for the catalysis of the transamination reaction. We describe here the structural changes that produce the domain movements and the closure of the active site. Structural changes occur at the interface between the domains and within the small domain itself. On closure, the core of the small domain rotates by 13 degrees relative to the large domain. Two other regions of the small domain, which form part of the active site, move somewhat differently. A loop, residues 39 to 49, above the active site moves about 1 A less than the core of the small domain. A helix within the small domain forms the "door" of the active site. It moves with the core of the small domain and, in addition, shifts by 1.2 A, rotates by 10 degrees, and switches its first turn from the alpha to the 3(10) conformation. This results in the helix closing the active site. The domain movements are produced by a co-ordinated series of small changes. Within one subunit the polypeptide chain passes twice between the large and small domains. One link involves a peptide in an extended conformation. The second link is in the middle of a long helix that spans both domains. At the interface this helix is kinked and, on closure, the angle of the kink changes to accommodate the movement of the small domain. The interface between the domains is formed by 15 residues in the large domain packing against 12 residues in the small domain and the manner in which these residues pack is essentially the same in the open and closed structures. Domain movements involve changes in the main-chain and side-chain torsion angles in the residues on both sides of the interface. Most of these changes are small; only a few side-chains switch to new conformations.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:Domain closure in mitochondrial aspartate aminotransferase. 152 85

The ionization state of the phosphate group bound at the aspartate aminotransferase apoenzyme's active site has been investigated utilizing Fourier-transform infrared spectroscopy following the band corresponding to the symmetric stretching of the dianionic phosphate. Unlike free phosphate, when inorganic phosphate is bound at the enzyme's active site, the integrated intensity value of the dianionic band does not change with pH within the studied range, and this value is similar to that for free dianionic phosphate at pH 8.3. From these results, we propose a dianionic state for the phosphate ion bound to cytosolic aspartate aminotransferase throughout the pH range of 5.7-8.3. The presence of other anions such as acetate and chloride or the substrate aspartate and its analogues produces a pH-dependent phosphate removal from the active site which is favored at low pH values. Elimination of the charged primary amine at the active-site Lys-258, through formation of a Schiff base with pyridoxal or chemical modification by carbamylation, also produces a pH-independent phosphate release. These results are interpreted as Lys-258 together with the active-site alpha-helix and other residues may be involved in stabilizing phosphate as a dianion in the apoenzyme phosphate pocket which anchors the phosphate ester of pyridoxal phosphate in the holoenzyme. It is proposed that the dianionic phosphate contributes to the apoenzyme's thermal stability through formation of strong hydrogen bond and salt bridges with the amino acid residues forming the phosphate binding pocket with assistance of Lys-258, and other active-site cationic components.
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PMID:Inorganic phosphate binding and electrostatic effects in the active center of aspartate aminotransferase apoenzyme. 154 11

Recent studies indicate that in animals with marked cardiac hypertrophy, there is depressed function of Ca2+ sequestration by myocardial sarcoplasmic reticulum (SR) because of down regulation of the Ca(2+)-ATPase gene. However, in several animal models we have observed enhancement of myocardial Ca2+ sequestration in response to chronic cardiac stimulation. We tested the hypothesis that in animals with mild cardiac hypertrophy, there is enhanced Ca(2+)-cycling activity by the SR Ca2+ pump and Ca(2+)-release channel. Because creatine kinase activity is consistently decreased in cardiomyopathy, we also determined whether enhanced Ca2+ cycling was accompanied by down regulation or inhibition of the creatine kinase system. Mild cardiac hypertrophy was induced by volume overload; 2% salt was added to the diet of 2-week-old turkey poults for 4 weeks. Compared with age-matched controls, volume overload resulted in 14.3% increase in heart weight and 21.5% increase in heart-to-body weight ratios. The hypertrophied heart had approximately 20% increased activities of the SR Ca2+ pump and the SR Ca2+ channel. Net Ca2+ transport was increased by 16.5%. Compared with controls and in contrast to several other myocardial enzymes, creatine kinase activity was diminished in the hypertrophied hearts by 23% and creatine content was decreased by 8%. Differences between groups were not detected for lactate dehydrogenase, aspartate transaminase, and alanine transaminase. We concluded that an early adaptation of the myocardium undergoing hypertrophy in compensatory response to functional overload is an enhancement of Ca2+ cycling activity by the Ca2+ pump and Ca2+ channel of the SR.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:Effects of mild cardiac hypertrophy, induced by volume overload in turkeys, on myocardial sarcoplasmic reticulum calcium-pump and calcium-channel activities and on the creatine kinase system. 165 61

Because S-adenosylmethionine promotes synthesis of hepatic glutathione in chronic liver disease and is well tolerated in man, we investigated its use as an antidote to acetaminophen hepatotoxicity in two mouse models. In C57Bl6 mice, deaths were abolished by S-adenosylmethionine given within 1 hr of 3.3 mmol/kg body wt acetaminophen (0 of 32 vs. 13 of 49, p less than 0.005) and reduced if given 2 to 5 hours after acetaminophen administration (4 of 42 vs. 13 of 49, p less than 0.01). Mixed disulfate/tosylate salt of S-adenosylmethionine abolished mortality in C3H mice given 2 mmol/kg body wt acetaminophen (0 of 24 vs. 4 of 18; p less than 0.05). In both mouse models, S-adenosylmethionine reduced depletion of plasma (median = 20.8 mumol/L vs. 14.6 mumol/L) and liver glutathione (198% vs. 100%; p less than 0.05), liver damage and release of AST after acetaminophen administration. Pretreatment with buthionine sulfoximine, which inhibits glutathione synthesis, abolished the beneficial effect of S-adenosylmethionine on survival and plasma glutathione level. S-adenosylmethionine reduces acetaminophen hepatotoxicity by metabolism of the active moiety to glutathione. This benefit may last as long as 5 hr after acetaminophen ingestion.
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PMID:S-adenosylmethionine protects against acetaminophen hepatotoxicity in two mouse models. 173 33

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

Twenty of 320 patients with Wilson's disease initially presented with chemical and laboratory features of chronic active hepatitis, confirmed histologically in 17. When first seen, cirrhosis was present in all 20 and was complicated by ascites and/or jaundice in 11. Within 1 week to 8 years of the onset of over liver disease the diagnosis of Wilson's disease was established, and treatment with D-penicillamine was promptly initiated in 19 patients. One man refused treatment and died 4 months later. Treated patients received D-penicillamine or trientine for a total of 264 patient-years (median, 14 patient-years). Abnormal water retention, for which salt restriction and diuretics were added to penicillamine or trientine, disappeared in all but 1 of the patients so affected. Symptomatic improvement and virtually normal levels of serum albumin, bilirubin, aspartate aminotransferase, and alanine aminotransferase followed within 1 year in the majority of subjects. One woman died after 9 months of treatment. Two patients, who became noncompliant with the therapeutic regimen after 9 and 17 years of successful pharmacological treatment, required liver transplants. These results indicate that the prognosis of specifically treated Wilsonian chronic active hepatitis is very good in spite of the presence of cirrhosis.
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PMID:Prognosis of Wilsonian chronic active hepatitis. 199 98

Male and female Sprague-Dawley rats were administered the sodium salt of monochloroacetic acid (SMCA) by oral gavage for a period of 90 consecutive days. Dosage levels of 15, 30, 60 or 120 mg/kg per day were employed. SMCA clearly induced toxicity in both females and males, with the greatest severity in the male animals. Both the liver and kidneys were identified as target organs. At 120 mg/kg per day, 30% of females and 80% of the males died, most within the first 2 days of treatment. Hemorrhagic and congested lungs (possibly a postmortem change) were seen in the early deaths (1-3 days) whereas liver lesions were observed in later deaths. In addition, there was nephrotoxicity as evidenced by elevated creatinine, blood calcium (BCAL), and blood urea nitrogen (BUN) levels. Hepatotoxicity was indicated by increases in the serum levels of alanine aminotransferase (ALT) and aspartate aminotransferase (AST). Both organs showed increased organ-to-body weight ratios. Microscopic examination revealed a significant (P less than or equal to 0.001) increase in chronic renal nephropathy and increased splenic pigmentation at 60 mg/kg per day in the males. Based on the observation of toxicity at all treatment levels in males, a lowest observed adverse effect level (LOAEL) of 15 mg/kg per day is proposed for a 90-day exposure to SMCA by oral gavage to the Sprague--Dawley rat.
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PMID:Ninety-day toxicity study of sodium monochloroacetate in Sprague-Dawley rats. 203 Dec 51

Tissue cholestasis is a histologic feature in some patients with alcoholic liver disease, but its significance is unknown. We studied prospectively the clinical, laboratory, and histologic findings of 306 chronic male alcoholics in whom liver tissue was available. Tissue cholestasis permitted identification of two groups: group I, absent or mild cholestasis (239 patients), and group II, moderate to severe cholestasis (67 patients). Statistical evaluation was performed by Student's t test and regression analyses. In patients with tissue cholestasis, 97% had elevated serum cholylglycine levels, while only 61% had significant jaundice (serum bilirubin greater than 5 mg/dl). In patients without tissue cholestasis, 66% had elevated serum cholylglycine and 13.5% jaundice. Highly significant statistical correlations (P less than 0.0001) were found between cholestasis and malnutrition, prothrombin time, AST, alkaline phosphatase, bilirubin, Maddrey's discriminant function, serum cholylglycine level, albumin, and histologic severity score. In group I, 54% survived 60 months versus 22% in group II (P less than 0.0001). Highly significant statistical correlations (P less than 0.0001) were noted between serum cholylglycine levels and the parameters enumerated earlier, but not with survival. We conclude that tissue cholestasis is a highly significant prognostic indicator of outcome in alcoholic hepatitis and is more consistently associated with bile salt retention than jaundice.
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PMID:Prognostic significance of cholestatic alcoholic hepatitis. VA Cooperative Study Group #119. 236 44

Primary human hepatocytes were used to study bile salt hepatotoxicity and the hepatoprotective potential of ursodeoxycholate in vitro. Hepatocytes were obtained by collagenase perfusion of healthy human liver tissue and were treated with glycochenodeoxycholate for 24 hr 1 day after plating. Clear signs of cytotoxicity were observed at concentrations of about 100 mumol/L glycochenodeoxycholate. Toxicity was determined by release of alkaline phosphatase, gamma-glutamyl transferase, AST, ALT or lactate dehydrogenase into the culture medium, by measuring DNA synthesis of the cultured liver cells and by testing the viability of the hepatocytes using trypan-blue dye exclusion. Addition of ursodeoxycholate, which by itself proved to be of little toxicity, significantly reduced the hepatotoxic effects of glycochenodeoxycholate: 72% +/- 6% of the cells survived treatment with 500 mumol/L glycochenodeoxycholate alone, but addition of 100 mumol/L ursodeoxycholate increased the survival rate to 87% +/- 4% (p less than 0.05). Moreover, all enzymes tested were secreted at a significantly lower level when ursodeoxycholate was present. Similarly, the cellular DNA synthesis was maintained at significantly higher levels as a result of ursodeoxycholate treatment. We conclude that (a) primary human hepatocytes are a suitable model for studying hepatotoxicity of bile salts in vitro, (b) ursodeoxycholate reduces hepatotoxicity of other bile salts and (c) ursodeoxycholate can act hepatoprotectively by itself (i.e., alteration of the metabolism of other bile salts is not necessarily required).
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PMID:Ursodeoxycholate reduces hepatotoxicity of bile salts in primary human hepatocytes. 240 54

A reinvestigation of the effects of pH and salt concentration on the proton and dicarboxylic acid dissociation constants of pig heart aspartate aminotransferase shows that both anions and cations were involved concomitantly, both as stoichiometric reactants and bioenergetically. Equations are presented which can be used experimentally, to determine the numbers of salt ions (their thermodynamic stoichiometries) involved in biochemical equilibria such as proton and ligand dissociations from macromolecules. These equations were used to reinvestigate the effects of salts on the chromophoric pKa of the enzyme prosthetic group, the interaction of the enzyme with dicarboxylic acids, and the overall equilibrium for the transamination half-reaction.
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PMID:Thermodynamic bookkeeping: a reinvestigation of proton and dicarboxylic acid binding to aspartate aminotransferase. 254 52


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