Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:2.6.1.1 (aspartate aminotransferase)
 21,665 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Mutant strains of Escherichia coli have been isolated in which the synthesis of 3-deoxy-d-arabinoheptulosonic acid 7-phosphate (DAHP) synthetase (phe) is derepressed, in addition to those enzymes of tyrosine biosynthesis previously shown to be controlled by the gene tyrR. The major enzyme of the terminal pathway of phenylalanine biosynthesis chorismate mutase-prephenate dehydratase is not derepressed in these strains. Genetic analysis of the mutants shows that the mutation or mutations causing derepression map close to previously reported tyrR mutations. A study of one of the mutations has shown it to be recessive to the wild-type allele in a diploid strain. It is proposed that the tyrR gene product is involved in the regulation of the synthesis of DAHP synthetase (phe) as well as the synthesis of DAHP synthetase (tyr), chorismate mutase-prephenate dehydrogenase, and transaminase A.
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PMID:Phenylalanine and tyrosine biosynthesis in Escherichia coli K-12: mutants derepressed for 3-deoxy-D-arabinoheptulosonic acid 7-phosphate synthetase (phe), 3-deoxy-D-arabinoheptulosonic acid 7-phosphate synthetase (tyr), chorismate mutase T-prephenate dehydrogenase, and transaminase A. 439 42

Tyrosine, added to the growth medium of a strain of Escherichia coli K-12 lacking transaminase B, repressed the tyrosine, phenylalanine, and tryptophan aminotransferase activities while leaving the aspartate aminotransferase activity unchanged. This suggested that the aspartate and the aromatic aminotransferase activities, previously believed to reside in the same protein, viz. transaminase A, are actually nonidentical. Further experiments showed that, upon incubation at 55 C, the aspartate aminotransferase of crude extracts was almost completely stable, whereas the tyrosine and phenylalanine activities were rapidly inactivated. Apoenzyme formation was faster, and apoenzyme degradation proceeded more slowly with aspartate aminotransferase than with tyrosine aminotransferase. Electrophoresis in polyacrylamide gels separated the aminotransferases. A more rapidly moving band contained tyrosine, phenylalanine, and tryptophan aminotransferases, and a slower band contained aspartate aminotransferase. A mutant of E. coli K-12 with low levels of aspartate aminotransferase exhibited unchanged levels of tyrosine aminotransferase. Thus, transaminase A appears to be made up of at least two proteins: one of broad specificity whose synthesis is repressed by tyrosine and another, specific for aspartate, which is not subject to repression by amino acids. The apparent molecular weights of both the aspartate and the aromatic aminotransferases, determined by gel filtration, were about 100,000.
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PMID:Nonidentity of the aspartate and the aromatic aminotransferase components of transaminase A in Escherichia coli. 440 56

There are at least two enzymes in adult human liver that transaminate tyrosine: cytoplasmic tyrosine aminotransferase (EC 2.6.1.5) and mitochondrial aspartate aminotransferase (EC 2.6.1.1). Total tyrosine aminotransferase activity in the supernatant fraction of adult human liver was 19.8 nmol of p-hydroxyphenylpyruvate formed per min/mg of protein as compared to 0.53 in fetuses of 12--22 weeks of gestational age and 2.0 in the newborn. The presence of specific tyrosine aminotransferase (EC 2.6.1.5) could be demonstrated by isoelectric focusing techniques in fetal human liver during the first trimester. No specific tyrosine aminotransferase could be detected in the placenta. Total tyrosine aminotransferase activity was elevated by dexamethasone and tyrosine administration to organ cultures of fetal liver.
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PMID:Tyrosine aminotransferase activity in human fetal liver. 610 40

Mink pseudodistemper, a recessive disease associated with high blood tyrosine levels, is an animal analogue of the human inborn error of metabolism, tyrosinemia II. Affected mink and man have eye and skin lesions. Affected mink have no hepatic tyrosine aminotransferase (TAT) activity, as measured immunologically and biochemically. Hepatic mitochondrial aspartate aminotransferase is increased to 188% of control. This new genetic animal model of TAT deficiency should allow new studies of tyrosine metabolism.
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PMID:Tyrosine aminotransferase deficiency in mink (Mustela vision): a model for human tyrosinemia II. 611 79

Rats having a protein-free diet available ad libitum were fed a daily casein meal at the beginning of either the light- or the dark-phase of the day. A control group received a mixed-diet ad libitum. In all three groups, daily food ingestion was the same and casein corresponded to 12% of total intake. Liver activities of alanine, aspartate, ornithine and tyrosine aminotransferase, ornithine decarboxylase and serine dehydratase were assessed. In mixed-fed controls, all activities were low. Tyrosine aminotransferase and ornithine decarboxylase exhibited clear circadian rhythms of low amplitude. Feeding casein as a concentrated meal had no effect on aspartate aminotransferase. It depressed alanine aminotransferase and serine dehydratase activities. Tyrosine aminotransferase and ornithine decarboxylase exhibited rapid and strong stimulatory responses but, within 12 hours, returned to levels similar to those observed in mixed-fed controls. Ornithine aminotransferase was increased in the group receiving the casein meal during the light phase. It is concluded that the capacity for amino acid catabolism remains low in separately-fed animals, and that only tyrosine and especially ornithine, which may become limiting for urea synthesis, are actively metabolized. Thus, when high fluxes of amino acids reach the liver following the absorption of the casein meal, more amino acids are available for incorporation into newly synthesized proteins.
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PMID:Activity of several enzymes of amino acid catabolism in the liver of rats fed protein as a meal. 613 52

Of 33 components analyzed in overnight fasting serum from 30 patients with alcoholic liver cirrhosis, portal hypertension, and bleeding esophageal varices, total serum bile acids, gamma-glutamyltransferase, prealbumin, and tyrosine were the most frequently abnormal 'liver tests'. Total serum bile acids correlated significantly with bilirubin, immunoglobulin M, threonine, glycine, methionine, and tyrosine. Gamma-glutamyltransferase correlated with aspartate aminotransferase, glutamine, and alanine. Prealbumin correlated with albumin and immunoglobulins G and A. Tyrosine correlated with total bile acids, orosomucoid, and 10 amino acids. The amino acid ratio of valine + isoleucine + leucine to tyrosine + phenylalanine was lowered in all patients. It is concluded that the clinical picture and pattern of serum components in patients with alcoholic liver disease are influenced by many complex pathophysiological mechanisms.
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PMID:Total serum bile acids, gamma-glutamyl transferase, prealbumin, and tyrosine: sensitive serum markers of hepatic dysfunction in alcoholic liver cirrhosis. 614 23

In order to study whether hormone-sensitive tyrosine aminotransferase exists in tissues other than liver, we have devised means to separate the liver-specific enzyme from other enzymes that transaminate tyrosine and to distinguish between the authentic enzyme and the principal "pseudotyrosine aminotransferases," which are the isoenzymes of aspartate aminotransferase. We accomplish this by suppressing proteolysis of the authentic enzyme using a buffer of pH 8.0 containing 0.1 M potassium chloride; enzyme extracted from liver in this buffer migrates as a single peak during chromatography on hydroxylapatite and represents the undegraded native form. A much smaller peak of tyrosine aminotransferase activity elutes at higher ionic strength and corresponds to a mixture of mitochondrial aspartate aminotransferase and partially degraded tyrosine aminotransferase. Cytosolic aspartate aminotransferase, in contrast, adsorbs weakly to the hydroxylapatite column and transaminates tyrosine very poorly although it readily utilizes monoiodotyrosine. The aspartate aminotransferase isoenzymes separate completely from tyrosine aminotransferase during chromatography on DEAE-Sepharose CL-6B. By combining these techniques with the use of specific antibodies, we show that brain, heart, and kidney do not contain tyrosine aminotransferase. Furthermore, we locate both isoenzymes of aspartate aminotransferase on polyacrylamide gels and show that both react histochemically as tyrosine aminotransferases when monoiodotyrosine is used as substrate. Use of these techniques, therefore, permits unambiguous identification of tyrosine aminotransferase and its separation from the background of nonspecific transamination.
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PMID:Organ specificity of glucocorticoid-sensitive tyrosine aminotransferase. Separation from aspartate aminotransferase isoenzymes. 614 85

During the transamination reaction of mitochondrial aspartate aminotransferase, transfer of tritium from the alpha-position of glutamate to the pro-S position of C4' of pyridoxamine 5'-phosphate was detected. A fast mixing and quenching device had to be used in order to reduce the number of transamination cycles undergone by the enzyme and thus to minimize the accompanying exchange of label with water. The extent of transfer of label (mean value 1.5%; range 0.8-4%) indicates that the 1,3-prototropic shift follows a stepwise rather than a concerted mechanism and that a single acid/base group is responsible for the proton transfer. The actual extent of proton transfer has to be much higher because the rate of alpha-tritium exchange with solvent was only approximately 10% of that of the turnover of unlabeled substrate, reflecting either an isotope effect or a retention of the tritium label in the reaction center during tautomerization. Under the assumption of an isotope effect, the actual transfer may be estimated to be 13%. This value is consistent with the notion of Lys-258 acting as the proton transferring group in which case the maximal value of transfer in an active site not accessible to solvent during the 1,3-prototropic shift would be 33%. However, alternative mechanisms involving Tyr-70 or a water molecule enclosed in the active site serving as acid/base group cannot be excluded on the basis of the present results. Furthermore, in these investigations aspartate aminotransferase was found to catalyze also the exchange of tritium from the beta-position of glutamate, though at a rate 350 times slower than that of the alpha-exchange.
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PMID:Transfer of C alpha-hydrogen of glutamate to coenzyme of aspartate aminotransferase during transamination reaction. 615 79

The plasma levels of corticosterone, insulin and glucagon, and the concomitant changes in the levels of several liver enzymes and metabolites were measured in intact rats in the basal state during 24 hours and under conditions of food deprivation and hypoxia. The levels of the following enzymes and metabolites were examined: phosphoenolpyruvate carboxykinase, glucose-6-phosphatase, pyruvate kinase, phosphofructokinase, glutamic-oxaloacetic transaminase, glutamic-pyruvic transaminase, glucose, glucose-6-phosphate, glycogen, fructose-6-phosphate, hexokinase, tyrosine amino-transferase and tryptophan oxygenase. During food deprivation, the increased gluconeogenesis is possibly a result of glucagon activity. In contrast, however, during hypoxia the increase in gluconeogenesis seems to be a result of the higher plasma level of corticosterone. During starvation, the insulin concentration dropped steadily and came close to zero.
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PMID:Plasma concentrations of glucose, corticosterone, glucagon and insulin and liver content of metabolic substrates and enzymes during starvation and additional hypoxia in the rat. 703 Aug 99

Phenylalanine and tyrosine were metabolized by the perfused rat heart via a mitochondrial aminotransferase. When L-[alanyl-2,3-3H]phenylalanine and L-[alanyl-2,3-3H]tyrosine were used, release of 3H2O was progressive over 2 h of perfusion. Metabolism of L-[U-14C]phenylalanine to 14CO2 or production of 3H2O from L-[ring-2,6-3H]phenylalanine or L-[ring-2,6-3H]tyrosine was not detected. Although 3H2O production from L-[alanyl-2,3-3H]phenylalanine was rapid, net production of phenylpyruvate or other metabolites of phenylalanine was negligible. As a result, use of aromatic amino acids as monitors of protein turnover in heart muscle was validated. Production of 3H2O from L-[alanyl-2,3-3H]phenylalanine was catalyzed by a mitochondrial enzyme, which is thought to be aspartate aminotransferase (EC 2.6.1.1). The rate of 3H2O production by both intact and detergent-treated mitochondria exceeded that of phenylpyruvate by a factor of 10 and occurred in the absence of alpha-ketoglutarate. These data provide an explanation for the production of 3H2O from L-[alanyl-2,3-3H]phenylalanine by perfused rat heart without the concomitant production of [3H]phenylpyruvate.
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PMID:Use of aromatic amino acids as monitors of protein turnover. 724 35


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