Aromatic amino acid transaminase in rat intestine
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The transamination of aromatic l-amino acids (5-hydroxytryptophan, tryptophan, tyrosine, phenylalanine and kynurenine) was shown to be catalysed by enzyme preparations from rat small intestine. On the basis of the partial purification and characterization of these aromatic amino acid transaminases, it is suggested that rat small intestine contains several kinds of aromatic amino acid transaminases.Keywords:
Transamination
Transaminase
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Abstract The degradation of l‐ and d‐aromatic amino acids has been studied in Candida maltosa . The following main metabolites were indentified: indolelactate was the degradation product of tryptophan, phenylpyruvate and phenyllactate were that of phenylalanine and p ‐hydroxyphenylpyruvate and p ‐hydroxyphenyllactate resulted from degradation of tyrosine. The first step of utilization of l‐aromatic amino acids was found to be a transamination. The cells contained increased aromatic aminotransferase activity if grown on medium supplemented with one of the three l‐aromatic amino acids. Three aromatic aminotransferases were separated by DEAE‐cellulose chromatography. The pathway for degradation of d‐aromatic amino acids involves deamination as the initial step. The enzyme was characterized to be a general d‐amino acid oxidase capable of utilizing various d‐amino acids. Synthesis of this enzyme was constitutive. Common products of both enzymatic reactions with the aromatic amino acids were the corresponding aromatic pyruvates. Formation of the aromatic lactate derivatives was catalyzed by an aromatic lactate dehydrogenase. This enzyme possessed a high substrate specificity for p ‐hydroxyphenylpyruvate, phenylpyruvate, and indolepyruvate and was synthesized constitutively.
Transamination
Aromatic amine
Deamination
Mimosine
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Kinetic studies of the transport of aromatic amino acids by Pseudomonas aeruginosa revealed the existence of two high-affinity transport systems which recognized the three aromatic amino acids. From competition data and studies on the exchange of preformed aromatic amino acid pools, the first transport system was found to be functional with phenylalanine, tyrosine, and tryptophan (in order of decreasing activity), whereas the second system was active with tryptophan, phenylalanine, and tyrosine. The two systems also transported a number of aromatic amino acid analogues but not other amino acids. Mutants defective in each of the two and in both transport systems were isolated and described. When the amino acids were added at low external concentrations to cells growing logarithmically in glucose minimal medium, the tryptophan pool very quickly became saturated. Under identical conditions, phenylalanine and tyrosine each accumulated in the intracellular pool of P. aeruginosa at a concentration which was 10 times greater than that of tryptophan.
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Aromatic amino acids (phenylalanine, tyrosine and tryptophan) were heated at 300°C under nitrogen and volatile compounds generated were examined. Twelve compounds in which many of them have aromatic rings were identified in the volatiles from thermal degradation of phenylalanine. Tyrosine and tryptophan produced some phenols and indoles, respectively, besides several compounds. Formation mechanisms of some compounds were also discussed.
Degradation
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A rapid, high-throughput screening methodology has been developed for the determination of transaminase activity. This pH based, colorimetric assay can also be used to scale reactions directly from 100 microL screening scale to 25 mL development scale. Additionally, three techniques have been developed to drive transamination reactions toward complete conversion. The first method uses lactate dehydrogenase to remove the inhibitory pyruvate keto acid by-product from the reaction and drive reaction equilibrium toward the desired amine. The second method is a single enzyme system, and uses a large excess of isopropylamine to drive the transamination. Method three requires only a catalytic amount of amine donor, as an amino acid dehydrogenase is employed to regenerate the amine donor in situ using ammonia. All three systems have been demonstrated for the production of optically pure methylbenzylamine from acetophenone. An enantiomeric excess of >99% was achieved for both the R- and S-methylbenzylamine products.
Transamination
Isopropylamine
Transaminase
Acetophenone
Enantiomeric excess
Deamination
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The inner membrane protein YddG of Escherichia coli is a homologue of the known amino acid exporters RhtA and YdeD. It was found that the yddG gene overexpression conferred resistance upon E. coli cells to the inhibiting concentrations of l-phenylalanine and aromatic amino acid analogues, dl-p-fluorophenylalanine, dl-o-fluorophenylalanine and dl-5-fluorotryptophan. In addition, yddG overexpression enhanced the production of l-phenylalanine, l-tyrosine or l-tryptophan by the respective E. coli-producing strains. On the other hand, the inactivation of yddG decreased the aromatic amino acid accumulation by these strains. The cells of the E. colil-phenylalanine-producing strain containing overexpressed yddG accumulated less l-phenylalanine inside and exported the amino acid at a higher rate than the cells of the isogenic strain containing wild-type yddG. Taken together, these results indicate that YddG functions as an aromatic amino acid exporter.
Strain (injury)
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The transamination of aromatic l-amino acids (5-hydroxytryptophan, tryptophan, tyrosine, phenylalanine and kynurenine) was shown to be catalysed by enzyme preparations from rat small intestine. On the basis of the partial purification and characterization of these aromatic amino acid transaminases, it is suggested that rat small intestine contains several kinds of aromatic amino acid transaminases.
Transamination
Transaminase
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Citations (15)
Aromatic amino acids (phenylalanine, tyrosine and tryptophan) were heated at 300°C under nitrogen and volatile compounds generated were examined. Twelve compounds in which many of them have aromatic rings were identified in the volatiles from thermal degradation of phenylalanine. Tyrosine and tryptophan produced some phenols and indoles, respectively, besides several compounds. Formation mechanisms of some compounds were also discussed.
Degradation
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Transamination
Decarboxylation
Hydroxylation
Phenylalanine hydroxylase
Phenylacetic acid
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Transamination
Transaminase
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