Isolation and characterization of the human tyrosine aminotransferase gene

Inborn errors of enzymes in glutamate metabolism

Glutamate is involved in a variety of metabolic pathways. We reviewed the literature on genetic defects of enzymes that directly metabolise glutamate, leading to inborn errors of glutamate metabolism. Seventeen genetic defects of glutamate metabolising enzymes have been reported, of which three were only recently identified. These 17 defects affect the inter-conversion of glutamine and glutamate, amino acid metabolism, ammonia detoxification, and glutathione metabolism. We provide an overview of the clinical and biochemical phenotypes of these rare defects in an effort to ease their recognition. By categorising these by biochemical pathway, we aim to create insight into the contributing role of deviant glutamate and glutamine levels to the pathophysiology. For those disorders involving the inter-conversion of glutamine and glutamate, these deviant levels are postulated to play a pivotal pathophysiologic role. For the other IEM however-with the exception of urea cycle defects-abnormal glutamate and glutamine concentrations were rarely reported. To create insight into the clinical consequences of disturbed glutamate metabolism-rather than individual glutamate and glutamine levels-the prevalence of phenotypic abnormalities within the 17 IEM was compared to their prevalence within all Mendelian disorders and subsequently all disorders with metabolic abnormalities notated in the Human Phenotype Ontology (HPO) database. For this, a hierarchical database of all phenotypic abnormalities of the 17 defects in glutamate metabolism based on HPO was created. A neurologic phenotypic spectrum of developmental delay, ataxia, seizures, and hypotonia are common in the inborn errors of enzymes in glutamate metabolism. Additionally, ophthalmologic and skin abnormalities are often present, suggesting that disturbed glutamate homeostasis affects tissues of ectodermal origin: brain, eye, and skin. Reporting glutamate and glutamine concentrations in patients with inborn errors of glutamate metabolism would provide additional insight into the pathophysiology.

Effects of acute handling stress on short-term central expression of orexigenic/anorexigenic genes in zebrafish

Physiological mechanisms driving stress response in vertebrates are evolutionarily conserved. These mechanisms involve the activation of both the hypothalamic-sympathetic-chromaffin cell (HSC) and the hypothalamic-pituitary-adrenal (HPA) axes. In fish, the reduction of food intake levels is a common feature of the behavioral response to stress but the central mechanisms coordinating the energetic response are not well understood yet. In this work, we explore the effects of acute stress on key central systems regulating food intake in fish as well as on total body cortisol and glucose levels. We show that acute stress induced a rapid increase in total body cortisol with no changes in body glucose, at the same time promoting a prompt central response by activating neuronal pathways. All three orexigenic peptides examined, i.e., neuropeptide y (npy), agouti-related protein (agrp), and ghrelin, increased their central expression level suggesting that these neuronal systems are not involved in the short-term feeding inhibitory effects of acute stress. By contrast, the anorexigenic precursors tested, i.e., cart peptides and pomc, exhibited increased expression after acute stress, suggesting their involvement in the anorexigenic effects.

Regulatory pathway analysis of coat color genes in Mongolian horses

BACKGROUND

Studies on the molecular genetics of horse skin pigmentation have typically focused on very few genes and proteins. In this study, we used Illumina sequencing to determine the global gene expression profiles in horses with white-colored coats and those with black-colored coats, with the goal of identifying novel genes that could regulate horse coat color.

RESULTS

Genes encoding ribosomal-associated proteins were highly expressed in horse skin. We found a total of 231 unigenes that were differentially expressed between horses with white coats and horses with black coats; 119 were down-regulated, and 112 were up-regulated. Many of the up-regulated genes in black horses, such as genes related to tyrosine metabolism, may directly regulate dark coat color. Keratin genes, MIA family genes, fatty acid-related genes, and melanoma-associated genes were also differentially regulated, which suggests that they may play important roles in coat color formation.

CONCLUSIONS

These findings show that the transcription profiles from white and black horse skin provide useful information to understand the genetics underlying the control of skin melanin synthesis in horses, which may enhance our knowledge of human skin diseases, such as melanoma and albinism.

Characteristics, tissue-specific expression, and hormonal regulation of expression of tyrosine aminotransferase in the avian female reproductive tract

Tyrosine aminotransferase (TAT) catalyzes the transamination of tyrosine to p-hydroxyphenylpyruvate. Accumulation of tyrosine in the body due to a genetic mutation in the TAT gene causes tyrosomia type II in humans. The TAT gene is regarded as a model for studying steroid-inducible factors regulating a variety of biological functions of TAT. However, little is known of the effects of estrogen on the expression of the TAT gene in chickens. Therefore, in the present study, we identified expression of the avian TAT gene in various organs. The results showed the TAT was detected predominantly in the liver and reproductive organs including testis, oviduct, and ovary. Specifically, TAT mRNA was expressed abundantly in the glandular and luminal epithelia of the oviducts in response to endogenous and exogenous estrogens which also induce dramatic morphological changes in the oviduct of chickens. In addition, target microRNAs of TAT (miR-1460, miR-1626-3p, miR-1690-5p, and miR-7442-3p) were found to modulate expression of the TAT gene. Especially, miR-1690-5p influenced TAT gene transcription by binding directly to its 3'-UTR region. Moreover, the expression of TAT was abundant in glandular epithelia of cancerous but not normal ovaries from laying hens. Taken together, our findings suggest that TAT plays an important role in the cytodifferentiation of oviducts in response to estrogen and in the progression of ovarian cancer in chickens.

Tyrosinemia type II: Novel mutations in TAT in a boy with unusual presentation

Tyrosinemia type II is a rare autosomal recessive disorder caused by deficiency of tyrosine aminotransferase (TAT). It may occur with ocular and cutaneous symptoms with or without mental retardation, but epileptic seizure is a rare presentation of this disease. Herein we report the clinical, biochemical and genetic features of a 4-year-old boy who presented with afebrile seizure and photophobia. Genomic DNA was obtained from peripheral blood leukocytes from the whole family. Sequencing analysis was performed using the MiSeq next-generation sequencing platform. Sequencing of TAT indicated two new homozygous mutations p.L312P (c.935T>C) and p.T408M (c.1223C>T) for the proband and his asymptomatic sister. During a 2 year follow-up period, the patient had overall poor compliance with protein-restricted diet, but his asymptomatic sister had good compliance with the diet. Cognitive function of the patient worsened steadily, but his asymptomatic sister maintained normal mental status. Tyrosinemia type II should be considered in the differential diagnosis of children presenting with epileptic seizure and photophobia; furthermore, early diagnosis and protein-restricted regimen are important to reduce the risk of long-term complications of tyrosinemia type II such as mental disability.

Relaxed evolution in the tyrosine aminotransferase gene tat in old world fruit bats (Chiroptera: Pteropodidae)

Frugivorous and nectarivorous bats fuel their metabolism mostly by using carbohydrates and allocate the restricted amounts of ingested proteins mainly for anabolic protein syntheses rather than for catabolic energy production. Thus, it is possible that genes involved in protein (amino acid) catabolism may have undergone relaxed evolution in these fruit- and nectar-eating bats. The tyrosine aminotransferase (TAT, encoded by the Tat gene) is the rate-limiting enzyme in the tyrosine catabolic pathway. To test whether the Tat gene has undergone relaxed evolution in the fruit- and nectar-eating bats, we obtained the Tat coding region from 20 bat species including four Old World fruit bats (Pteropodidae) and two New World fruit bats (Phyllostomidae). Phylogenetic reconstructions revealed a gene tree in which all echolocating bats (including the New World fruit bats) formed a monophyletic group. The phylogenetic conflict appears to stem from accelerated TAT protein sequence evolution in the Old World fruit bats. Our molecular evolutionary analyses confirmed a change in the selection pressure acting on Tat, which was likely caused by a relaxation of the evolutionary constraints on the Tat gene in the Old World fruit bats. Hepatic TAT activity assays showed that TAT activities in species of the Old World fruit bats are significantly lower than those of insectivorous bats and omnivorous mice, which was not caused by a change in TAT protein levels in the liver. Our study provides unambiguous evidence that the Tat gene has undergone relaxed evolution in the Old World fruit bats in response to changes in their metabolism due to the evolution of their special diet.

Novel and recurrent mutations in the TAT gene in Tunisian families affected with Richner-Hanhart syndrome

Tyrosinemia type II, also designated as oculocutaneous tyrosinemia or Richner-Hanhart syndrome (RHS), is a very rare autosomal recessive disorder. In the present study, we report clinical features and molecular genetic investigation of the tyrosine aminotransferase (TAT) gene in two young patients, both born to consanguineous unions between first-degree cousins. These two unrelated families originated from Northern and Southern Tunisia. The clinical diagnosis was based on the observation of several complications related to Richner-Hanhart syndrome: recurrent eye redness, tearing and burning pain, photophobia, bilateral pseudodendritic keratitis, an erythematous and painful focal palmo-plantar hyperkeratosis and a mild delay of mental development. The diagnosis was confirmed by biochemical analysis. Sequencing of the TAT gene revealed the presence of a previously reported missense mutation (c.452G>A, p.Cys151Tyr) in a Tunisian family, and a novel G duplication (c.869dupG, p.Trp291Leufs 6). Early diagnosis of RHS and protein-restricted diet are crucial to reduce the risk and the severity of long-term complications of hypertyrosinemia such as intellectual disability.

Tyrosine aminotransferase contributes to benzylisoquinoline alkaloid biosynthesis in opium poppy

Tyrosine aminotransferase (TyrAT) catalyzes the transamination of L-Tyr and α-ketoglutarate, yielding 4-hydroxyphenylpyruvic acid and L-glutamate. The decarboxylation product of 4-hydroxyphenylpyruvic acid, 4-hydroxyphenylacetaldehyde, is a precursor to a large and diverse group of natural products known collectively as benzylisoquinoline alkaloids (BIAs). We have isolated and characterized a TyrAT cDNA from opium poppy (Papaver somniferum), which remains the only commercial source for several pharmaceutical BIAs, including codeine, morphine, and noscapine. TyrAT belongs to group I pyridoxal 5'-phosphate (PLP)-dependent enzymes wherein Schiff base formation occurs between PLP and a specific Lys residue. The amino acid sequence of TyrAT showed considerable homology to other putative plant TyrATs, although few of these have been functionally characterized. Purified, recombinant TyrAT displayed a molecular mass of approximately 46 kD and a substrate preference for L-Tyr and α-ketoglutarate, with apparent K(m) values of 1.82 and 0.35 mm, respectively. No specific requirement for PLP was detected in vitro. Liquid chromatography-tandem mass spectrometry confirmed the conversion of L-Tyr to 4-hydroxyphenylpyruvate. TyrAT gene transcripts were most abundant in roots and stems of mature opium poppy plants. Virus-induced gene silencing was used to evaluate the contribution of TyrAT to BIA metabolism in opium poppy. TyrAT transcript levels were reduced by at least 80% in silenced plants compared with controls and showed a moderate reduction in total alkaloid content. The modest correlation between transcript levels and BIA accumulation in opium poppy supports a role for TyrAT in the generation of alkaloid precursors, but it also suggests the occurrence of other sources for 4-hydroxyphenylacetaldehyde.

TAT gene mutation analysis in three Palestinian kindreds with oculocutaneous tyrosinaemia type II; characterization of a silent exonic transversion that causes complete missplicing by exon 11 skipping

Deficiency of the hepatic cytosolic enzyme tyrosine aminotransferase (TAT) causes marked hypertyrosinaemia leading to painful palmoplantar hyperkeratoses, pseudodendritic keratitis and variable mental retardation (oculocutaneous tyrosinaemia type II or Richner-Hanhart syndrome). Parents may therefore seek prenatal diagnosis, but this is not possible by biochemical assays as tyrosine does not accumulate in amniotic fluid and TAT is not expressed in chorionic villi or amniocytes. Molecular analysis is therefore the only possible approach for prenatal diagnosis and carrier detection. To this end, we sought TAT gene mutations in 9 tyrosinaemia II patients from three consanguineous Palestinian kindreds. In two kindreds (7 patients), the only potential abnormality identified after sequencing all 12 exons and exon-intron boundaries was homozygosity for a silent, single-nucleotide transversion c.1224G > T (p.T408T) at the last base of exon 11. This was predicted to disrupt the 5' donor splice site of exon 11 and result in missplicing. However, as TAT is expressed exclusively in liver, patient mRNA could not be obtained for splicing analysis. A minigene approach was therefore used to assess the effect of c.1224G > T on exon 11 splicing. Transfection experiments with wild-type and c.1224G > T mutant minigene constructs demonstrated that c.1224G > T results in complete exon 11 skipping, illustrating the utility of this approach for confirming a putative splicing defect when cDNA is unavailable. Homozygosity for a c.1249C > T (R417X) exon 12 nonsense mutation (previously reported in a French patient) was identified in both patients from the third kindred, enabling successful prenatal diagnosis of an unaffected fetus using chorionic villous tissue.

The narrow substrate specificity of human tyrosine aminotransferase--the enzyme deficient in tyrosinemia type II

Human tyrosine aminotransferase (hTATase) is the pyridoxal phosphate-dependent enzyme that catalyzes the reversible transamination of tyrosine to p-hydrophenylpyruvate, an important step in tyrosine metabolism. hTATase deficiency is implicated in the rare metabolic disorder, tyrosinemia type II. This enzyme is a member of the poorly characterized Igamma subfamily of the family I aminotransferases. The full length and truncated forms of recombinant hTATase were expressed in Escherichia coli, and purified to homogeneity. The pH-dependent titration of wild-type reveals a spectrum characteristic of family I aminotransferases with an aldimine pK(a) of 7.22. I249A mutant hTATase exhibits an unusual spectrum with a similar aldimine pK(a) (6.85). hTATase has very narrow substrate specificity with the highest enzymatic activity for the Tyr/alpha-ketoglutarate substrate pair, which gives a steady state k(cat) value of 83 s(-1). In contrast there is no detectable transamination of aspartate or other cosubstrates. The present findings show that hTATase is the only known aminotransferase that discriminates significantly between Tyr and Phe: the k(cat)/K(m) value for Tyr is about four orders of magnitude greater than that for Phe. A comparison of substrate specificities of representative Ialpha and Igamma aminotransferases is described along with the physiological significance of the discrimination between Tyr and Phe by hTATase as applied to the understanding of the molecular basis of phenylketonuria.

Clinical and mutational investigations of tyrosinemia type II in Northern Tunisia: identification and structural characterization of two novel TAT mutations

Tyrosinemia type II or Richner-Hanhart Syndrome (RHS) is an autosomal recessive disorder characterized by keratitis, palmoplantar keratosis, mental retardation, and elevated blood tyrosine levels. The disease is due to a deficiency of hepatic cytosolic tyrosine aminotransferase (TATc), an enzyme involved in the tyrosine catabolic pathway. Because of the high rate of consanguinity this disorder seems to be relatively common among the Arab and Mediterranean populations. RHS is characterized by inter and intrafamilial phenotypic variability. A large spectrum of mutations within TATc gene has been shown to be responsible for RHS. In the present study, we report the clinical features and the molecular investigation of RHS in three unrelated consanguineous Tunisian families including 7 patients with confirmed biochemical diagnosis of tyrosinemia type II. Mutation analyses were performed and two novel missense mutations were identified (C151Y) and (L273P) within exon 5 and exon 8, respectively. The 3D-structural characterization of these mutations provides evidence of defective folding of the mutant proteins, and likely alteration of the enzymatic activity. Phenotype variability was observed even among individuals sharing the same pathogenic mutation.

Comparison of the tyrosine aminotransferase cDNA and genomic DNA sequences of normal mink and mink affected with tyrosinemia type II

Type II tyrosinemia, designated Richner-Hanhart syndrome in humans, is a hereditary metabolic disorder with autosomal recessive inheritance characterized by a deficiency of tyrosine aminotransferase activity. Mutations occur in the human tyrosine aminotransferase gene, resulting in high levels of tyrosine and disease. Type II tyrosinemia occurs in mink, and our hypothesis was that it would also be associated with mutation(s) in the tyrosine aminotransferase gene. Therefore, the transcribed cDNA and the genomic tyrosine aminotransferase gene were sequenced from normal and affected mink. The gene extended over 11.9 kb and had 12 exons coding for a predicted 454-amino-acid protein with 93% homology with human tyrosine aminotransferase. FISH analysis mapped the gene to chromosome 8 using the Mandahl and Fredga (1975) nomenclature and chromosome 5 using the Christensen et al. (1996) nomenclature. The hypothesis was rejected because sequence analysis disclosed no mutations in either cDNA or introns that were associated with affected mink. This suggests that an unlinked gene regulatory mutation may be the cause of tyrosinemia in mink.

Dexamethasone responsive element in the rat Na, K-ATPase beta1 gene coding region

The Na, K-ATPase plays an essential role in active alveolar epithelial fluid resorption. In fetal and adult alveolar epithelial cells, glucocorticoids (GC) increase Na, K-ATPase activity, mRNA levels, and transcription rate of the beta(1) subunit. In this study, we describe a glucocorticoid responsive element (GRE) in the coding region of the rat Na, K-ATPase beta(1) gene in a rat lung epithelial cell line. Transient transfection experiments with the beta(1) subunit coding region with or without the 5' and 3' untranslated regions demonstrated responsiveness to dexamethasone induction and also identified a GRE at +434 in exon IV. The +434 GRE conferred dexamethasone responsiveness in a heterologous thymidine kinase promoter irrespective of its orientation to the beta(1) promoter. Transcriptional upregulation by dexamethasone was abolished in +434 mutants. Electrophoretic mobility shift assays (EMSA) demonstrated specific binding of nuclear proteins to the +434 GRE and the presence of the GC receptor. This specific binding was inhibited by a GRE previously described in the rat Na, K-ATPase beta(1) gene at -631. In conclusion, we identified a GRE at +434 in the exon IV of the rat Na, K-ATPase beta(1) gene.

Dexamethasone stimulates transcription of the Na+-K+-ATPase beta1 gene in adult rat lung epithelial cells

Na+-K+-ATPase plays an essential role in active alveolar epithelial fluid resorption. In fetal and adult alveolar epithelial cells, glucocorticoids (GC) increase Na+-K+-ATPase activity and mRNA levels. We sought to define the mechanism of Na+-K+-ATPase gene upregulation by GC. In a rat alveolar epithelial cell line (RLE), dexamethasone (Dex) increased beta1-subunit Na+-K+-ATPase mRNA expression two- to threefold within 3 h after exposure to the GC. The increased gene expression was due to increased transcription as demonstrated by nuclear run-on assays, whereas mRNA stability remained unchanged. Transient transfection of 5' deletion mutants of a beta1 promoter-reporter construct demonstrated a 1.5- to 2.2-fold increase in promoter activity by Dex. All of the 5' deletion constructs contained partial or palindromic GC regulatory elements (GRE) and responded to GC. The increased expression of promoter reporter was inhibited by RU-486, a GC receptor (GR) antagonist, suggesting the involvement of GR. The palindromic GRE at -631 demonstrated Dex induction in a heterologous promoter construct. Gel mobility shift assays using RLE nuclear extracts demonstrated specific binding to this site and the presence of GR. We conclude that GC directly stimulate transcription of Na+-K+-ATPase beta1 gene expression in adult rat lung epithelial cells through a GR-dependent mechanism that can act at multiple sites.

Distribution of tyrosine aminotransferase activity in mink (Mustela vison)

The distribution of the enzyme tyrosine aminotransferase in tissues of mink, Mustela vison, was investigated. High levels of enzymatic activity were detected only in liver, documenting the hepatic-specific nature of this enzyme in this species. Further studies disclosed that tyrosine aminotransferase is not absent from non-hepatic tissues because of the lack of the use of a stabilized buffer, sensitivity to temperature, or due to the presence of an inhibitor. Collectively, these results suggest that the enzymatic assay of tyrosine aminotransferase will be unlikely to be an efficacious approach for identifying mink that are heterozygous for the autosomal recessive deficiency of this enzyme that is common in dark mink.

Conserved as well as divergent regulatory elements account for expression of the human and rodent phenylalanine hydroxylase genes

We have uncovered a fundamental difference in the regulation of the rodent and the human phenylalanine hydroxylase (PAH) genes: expression of human PAH is independent of glucocorticoids and/or cAMP in contrast to the mouse gene which is not only highly inducible but dependent upon hormones for expression. Nevertheless, the two genes do exhibit similarities: DNaseI hypersensitive sites are identically located in the regulatory regions, and the sequences around these sites are partially conserved and associated with regulatory elements sharing similar function. In transient transfections, the human proximal promoter is tissue-specific and presents significant activity compared to the extremely low and ubiquitous activity of the mouse promoter. DNA fragments corresponding to the two upstream hypersensitive sites of both genes have enhancer activity that depends upon the liver-enriched transcription factor binding sites for hepatocyte nuclear factor (HNF) 1 and/or CCAAT/enhancer binding protein (C/EBP). While expression of the rodent gene relies upon two modules in the HSIII enhancer, one activated by HNF1 and C/EBP and the other required for the hormone response, the human equivalent has conserved only the liver-specific transcription factor binding module. Even though the more proximal enhancer is not necessary for full reporter gene activity in transient transfection assays in Pah-expressing hepatoma cells, this enhancer could be required in both species for activation during development.

Cloning and characterization of a coronatine-regulated tyrosine aminotransferase from Arabidopsis

In plants, the phytotoxin coronatine, which is an analog of the octadecanoids 12-oxo-phytodienoic acid and/or jasmonic acid, gives rise to a number of physiological responses similar to those of octadecanoids. To further elucidate the physiological role of these compounds, the differential RNA display technique was used to isolate a number of novel octadecanoid-inducible genes expressed in coronatine-treated Arabidopsis. Among these, a cDNA clone was identified that was similar to known tyrosine aminotransferases (TATs). The function was verified with the expressed recombinant protein. In Arabidopsis, the protein is present as a multimer of 98 kD, with a monomer of an apparent molecular mass of 47 kD. TAT mRNA could be induced within 2 h by various octadecanoids and by wounding of the plants. Accumulation of the TAT protein and a 5- to 7-fold increase in its enzymatic activity was observed 7 to 9 h after application of octadecanoids, coronatine, or wounding. The potential role of TAT in the defense response to herbivores and pathogens is discussed.

Recombinant tyrosine aminotransferase from Trypanosoma cruzi: structural characterization and site directed mutagenesis of a broad substrate specificity enzyme

The gene encoding tyrosine aminotransferase (TAT, EC 2.6.1.5) from the parasitic protozoan Trypanosoma cruzi was amplified from genomic DNA, cloned into the pET24a expression vector and functionally expressed as a C-terminally His-tagged protein in Escherichia coli BL21(DE3)pLysS. Purified recombinant TAT exhibited identical electrophoretic and enzymatic properties as the authentic enzyme from T. cruzi. Both recombinant and authentic T. cruzi TATs were highly resistant to limited tryptic cleavage and contained no disulfide bonds. Comprehensive analysis of its substrate specificity demonstrated TAT to be a broad substrate aminotransferase, with leucine, methionine as well as tyrosine, phenylalanine, tryptophan and alanine being utilized efficiently as amino donors. Valine, isoleucine and dicarboxylic amino acids served as poor substrates while polar aliphatic amino acids could not be transaminated. TAT also accepted several 2-oxoacids, including 2-oxoisocaproate and 2-oxomethiobutyrate, in addition to pyruvate, oxaloacetate and 2-oxoglutarate. The functionality of the expression system was confirmed by constructing two variants; one (Arg389) being a completely inactive enzyme; the other (Arg283) retaining its full activity, as predicted from the recently solved three-dimensional structure of T. cruzi TAT. Thus, only one of the two strictly conserved arginines which are essential for the enzymatic activity of subfamily Ialpha aspartate and aromatic aminotransferases is critical for T. cruzi's TAT activity.

The molecular structure of hyperthermostable aromatic aminotransferase with novel substrate specificity from Pyrococcus horikoshii

Aromatic amino acid aminotransferase (ArATPh), which has a melting temperature of 120 degrees C, is one of the most thermostable aminotransferases yet to be discovered. The crystal structure of this aminotransferase from the hyperthermophilic archaeon Pyrococcus horikoshii was determined to a resolution of 2.1 A. ArATPh has a homodimer structure in which each subunit is composed of two domains, in a manner similar to other well characterized aminotransferases. By the least square fit after superposing on a mesophilic ArAT, the ArATPh molecule exhibits a large deviation of the main chain coordinates, three shortened alpha-helices, an elongated loop connecting two domains, and a long loop transformed from an alpha-helix, which are all factors that are likely to contribute to its hyperthermostability. The pyridine ring of the cofactor pyridoxal 5'-phosphate covalently binding to Lys(233) is stacked parallel to F121 on one side and interacts with the geminal dimethyl-CH/pi groups of Val(201) on the other side. This tight stacking against the pyridine ring probably contributes to the hyperthermostability of ArATPh. Compared with other ArATs, ArATPh has a novel substrate specificity, the order of preference being Tyr > Phe > Glu > Trp > His>> Met > Leu > Asp > Asn. Its relatively weak activity against Asp is due to lack of an arginine residue corresponding to Arg(292)* (where the asterisk indicates that this is a residues supplied by the other subunit of the dimer) in pig cytosolic aspartate aminotransferase. The enzyme recognizes the aromatic substrate by hydrophobic interaction with aromatic rings (Phe(121) and Tyr(59)*) and probably recognizes acidic substrates by a hydrophilic interaction involving a hydrogen bond network with Thr(264)*.

Crystal structure of Trypanosoma cruzi tyrosine aminotransferase: substrate specificity is influenced by cofactor binding mode

The crystal structure of tyrosine aminotransferase (TAT) from the parasitic protozoan Trypanosoma cruzi, which belongs to the aminotransferase subfamily Igamma, has been determined at 2.5 A resolution with the R-value R = 15.1%. T. cruzi TAT shares less than 15% sequence identity with aminotransferases of subfamily Ialpha but shows only two larger topological differences to the aspartate aminotransferases (AspATs). First, TAT contains a loop protruding from the enzyme surface in the larger cofactor-binding domain, where the AspATs have a kinked alpha-helix. Second, in the smaller substrate-binding domain, TAT has a four-stranded antiparallel beta-sheet instead of the two-stranded beta-sheet in the AspATs. The position of the aromatic ring of the pyridoxal-5'-phosphate cofactor is very similar to the AspATs but the phosphate group, in contrast, is closer to the substrate-binding site with one of its oxygen atoms pointing toward the substrate. Differences in substrate specificities of T. cruzi TAT and subfamily Ialpha aminotransferases can be attributed by modeling of substrate complexes mainly to this different position of the cofactor-phosphate group. Absence of the arginine, which in the AspATs fixes the substrate side-chain carboxylate group by a salt bridge, contributes to the inability of T. cruzi TAT to transaminate acidic amino acids. The preference of TAT for tyrosine is probably related to the ability of Asn17 in TAT to form a hydrogen bond to the tyrosine side-chain hydroxyl group.

Tyrosine aminotransferase catalyzes the final step of methionine recycling in Klebsiella pneumoniae

An aminotransferase which catalyzes the final step in methionine recycling from methylthioadenosine, the conversion of alpha-ketomethiobutyrate to methionine, has been purified from Klebsiella pneumoniae and characterized. The enzyme was found to be a homodimer of 45-kDa subunits, and it catalyzed methionine formation primarily using aromatic amino acids and glutamate as the amino donors. Histidine, leucine, asparagine, and arginine were also functional amino donors but to a lesser extent. The N-terminal amino acid sequence of the enzyme was determined and found to be almost identical to the N-terminal sequence of both the Escherichia coli and Salmonella typhimurium tyrosine aminotransferases (tyrB gene products). The structural gene for the tyrosine aminotransferase was cloned from K. pneumoniae and expressed in E. coli. The deduced amino acid sequence displayed 83, 80, 38, and 34% identity to the tyrosine aminotransferases from E. coli, S. typhimurium, Paracoccus denitrificans, and Rhizobium meliloti, respectively, but it showed less than 13% identity to any characterized eukaryotic tyrosine aminotransferase. Structural motifs around key invariant residues placed the K. pneumoniae enzyme within the Ia subfamily of aminotransferases. Kinetic analysis of the aminotransferase showed that reactions of an aromatic amino acid with alpha-ketomethiobutyrate and of glutamate with alpha-ketomethiobutyrate proceed as favorably as the well-known reactions of tyrosine with alpha-ketoglutarate and tyrosine with oxaloacetate normally associated with tyrosine aminotransferases. The aminotransferase was inhibited by the aminooxy compounds canaline and carboxymethoxylamine but not by substrate analogues, such as nitrotyrosine or nitrophenylalanine.

Isolation and partial characterization of a broad specificity aminotransferase from Leishmania mexicana promastigotes

A broad specificity aminotransferase (BSAT), with high activity with both, aromatic amino acids and aspartate as substrates, was purified to homogeneity from promastigotes of Leishmania mexicana by a method involving chromatography on DEAE-cellulose, Red-120-Sepharose and Mono Q, and gel filtration on Sephacryl S-200. The purified enzyme showed a single band in SDS-polyacrylamide gel electrophoresis, with an apparent molecular mass of 45 kDa. Since the apparent molecular mass of the native enzyme, determined by gel filtration, was 90 kDa, the native enzyme is a dimer of similar subunits. The amino acid composition was determined, as well as the sequence of four internal peptides obtained by tryptic digestion. Two of these peptides, consisting of 49 amino acid residues in total, showed high similarity (57%) with corresponding sequences of plant aspartate aminotransferases, whereas they had only 33% identity with the aromatic aminotransferase of Escherichia coli, and 16% identity with the tyrosine aminotransferase from the related parasite Trypanosoma cruzi. The BSAT contained only one 1/2 Cys residue per monomer. The optimal pH for the enzyme reaction, with tyrosine and alpha-oxoglutarate as substrates, was 7.0. The apparent Km values for tyrosine, phenylalanine, tryptophan and glutamate, with oxaloacetate as co-substrate, were 1.3, 0.9, 0.9 and 171.8 mM, respectively; the value for aspartate with alpha-oxoglutarate as co-substrate was 2.5 mM, and that for alanine with alpha-oxoglutarate as co-substrate was 216 mM. The values for pyruvate, alpha-oxoglutarate and oxaloacetate, with tyrosine as co-substrate, were 5.6, 0.71 and 0.12 mM, respectively. These results suggest that the enzyme is a broad-specificity aminotransferase, able to transaminate the aromatic amino acids, aspartate, and to a lower extent alanine, with high sequence similarity to aspartate aminotransferases.

Jasmonic acid-dependent and -independent signaling pathways control wound-induced gene activation in Arabidopsis thaliana

Plant response to mechanical injury includes gene activation both at the wound site and systemically in nondamaged tissues. The model developed for the wound-induced activation of the proteinase inhibitor II (Pin2) gene in potato (Solanum tuberosum) and tomato (Lycopersicon esculentum) establishes the involvement of the plant hormones abscisic acid and jasmonic acid (JA) as key components of the wound signal transduction pathway. To assess in Arabidopsis thaliana the role of these plant hormones in regulating wound-induced gene expression, we isolated wound- and JA-inducible genes by the differential mRNA display technique. Their patterns of expression upon mechanical wounding and hormonal treatments revealed differences in the spatial distribution of the transcripts and in the responsiveness of the analyzed genes to abscisic acid and JA. A correlation can be established between sensitivity to JA and the accumulation of the transcripts in systemic tissues upon wounding. A comparative study of the wound response in wild-type and JA-insensitive coi1 mutant plants indicated that in A. thaliana wound signals are transmitted via at least two different pathways. One of them does not involve JA as a mediator and is preferentially responsible for gene activation in the vicinity of the wound site, whereas the other requires JA perception and activates gene expression throughout the aerial part of the plant.

Cloning and expression of human tyrosine aminotransferase cDNA

Complementary DNA clones encoding human tyrosine aminotransferase (TAT) were isolated by screening a normal adult woman liver lambda gt11 library with rat TAT cDNA. The largest isolated cDNA is 2051 bp long (EMBL accession number X55675). This cDNA was subcloned downstream of the cytomegalovirus promoter in the pCMV vector for transfection into human cervical carcinoma HeLa cells. Expression of the TAT cDNA resulted in the synthesis of a protein with a molecular mass of approximately 50 kDa, as assessed by Western analysis, a value which is in close agreement with the predicted molecular weight of 50,399, for a deduced sequence of 454 amino acids. The expressed protein catalyzed specifically the conversion of L-[14C]tyrosine into p-[14C]hydroxyphenylpyruvate. The availability of a functional TAT cDNA provides a useful tool for detailed study of the structure-function relationship of the enzyme and its mutated derivatives.

Structure of genes that encode isozymes of aspartate aminotransferase in Panicum miliaceum L., a C4 plant

The cytosolic and mitochondrial isozymes of aspartate aminotransferase (AspAT) function in the C4 photosynthetic cycle in NAD-malic enzyme-type C4 plants and are expressed at high levels in mesophyll cells and bundle sheath cells, respectively. We constructed a genomic library from Panicum miliaceum, a NAD-malic enzyme-type C4 plant, and cloned the genes for these isozymes. The sequence of the cloned gene for cytosolic AspAT spans 7800 bp and consists of 12 exons. The sequence of the cloned gene for mitochondrial AspAT spans 9000 bp and consists of 10 exons. The results of primer-extension analysis suggest that transcription may be initiated from multiple adjacent sites. Both genes have significant GC-rich regions around the site of initiation of transcription, and these regions showed no CpG suppression. The 5'- flanking regions of both genes include several short sequences similar to the regulatory elements found in other genes for components of the photosynthetic machinery. In particular, the cytosolic AspAT gene contains sequences similar to nuclear protein-binding sites in other mesophyll-expressed C4 photosynthetic genes and the mitochondrial AspAT gene contains elements for light-sensitive and constitutive expression of a bundle sheath-expressed gene. The results of Southern analysis indicated that there are at least two genes that encode each isozyme in the genome of P. miliaceum. A comparison of intron-insertion positions between AspAT genes of plants and animals revealed that several introns are located at identical positions. On the basis of a phylogenetic tree among AspATs and tyrosine aminotransferase, we have shown that the introns of aminotransferase genes antedate the divergence of eubacteria, archaebacteria, and eukaryotes.

Purification and partial structural and kinetic characterization of tyrosine aminotransferase from epimastigotes of Trypanosoma cruzi

Tyrosine aminotransferase was purified to homogeneity from epimastigotes of Trypanosoma cruzi by a method involving chromatography on DEAE-cellulose, gel filtration on Sephacryl S-200 and chromatography on Mono Q in an f.p.l.c. system. The purified enzyme showed a single band in SDS/PAGE, with an apparent molecular mass of 45 kDa. Since the apparent molecular mass of the native enzyme, determined by gel filtration, is 91 kDa, the native enzyme is a dimer of similar subunits. The amino-acid composition was determined, as well as the sequences of three internal peptides obtained by CNBr cleavage at Met residues. Both criteria suggest considerable similarity with the tyrosine aminotransferases from rat and from human liver. The enzyme contains nine 1/2 Cys residues, three free and the others forming three disulphide bridges. The enzyme is not N-glycosylated. The isoelectric point is 4.6-4.8. The optimal pH for the reaction of the enzyme with tyrosine as a substrate is 7.0. The apparent Km values for tyrosine, phenylalanine and tryptophan, with pyruvate as a co-substrate, were 6.8, 17.9 and 21.4 mM, respectively, whereas those for pyruvate, alpha-oxoglutarate and oxaloacetate, with tyrosine as a substrate, were 0.5, 38 and 16 mM respectively. The purified tyrosine aminotransferase acts as an alanine aminotransferase as well and the activity seems to reside in the same enzyme molecule. The results suggest that the enzyme is a general aromatic-amino-acid transaminase, with high sequence similarity to tyrosine aminotransferases from rat and human liver.

Isolation and characterization of a gene from Trypanosoma cruzi encoding a 46-kilodalton protein with homology to human and rat tyrosine aminotransferase

The complete sequence of a gene encoding a 46-kDa protein of Trypanosoma cruzi is presented. The first ATG complies with the consensus sequence for initiation of translation. A single band of 2 kb was highlighted by hybridizing a probe from the 46-kDa protein gene to a Northern filter containing total T. cruzi RNA. The gene is present in 50-80 copies per cell and most of them are contained in 2 tandem arrays on large T. cruzi chromosomes (> 2000 kb). A strong homology with rat and human tyrosine aminotransferase was detected. Homology with a Trypanosoma brucei retrotransposon was found in the nonsense strand of the intergenic region.

Aminotransferases: demonstration of homology and division into evolutionary subgroups

A total of 150 amino acid sequences of vitamin B6-dependent enzymes are known to date, the largest contingent being furnished by the aminotransferases with 51 sequences of 14 different enzymes. All aminotransferase sequences were aligned by using algorithms for sequence comparison, hydropathy patterns and secondary structure predictions. The aminotransferases could be divided into four subgroups on the basis of their mutual structural relatedness. Subgroup I comprises aspartate, alanine, tyrosine, histidinol-phosphate, and phenylalanine aminotransferases; subgroup II acetylornithine, ornithine, omega-amino acid, 4-aminobutyrate and diaminopelargonate aminotransferases; subgroup III D-alanine and branched-chain amino acid aminotransferases, and subgroup IV serine and phosphoserine aminotransferases. (N-1) Profile analysis, a more stringent application of profile analysis [Gribskov, M., McLachlan, A. D. and Eisenberg, D. (1987) Proc. Natl Acad. Sci. USA 84, 4355-4358], established the homology among the enzymes of each subgroup as well as among all subgroups except subgroup III. However, similarity of active-site segments and the hydropathy patterns around invariant residues suggest that subgroup III, though most distantly related, might also be homologous with the other aminotransferases. On the basis of the comprehensive alignment, a new numbering of amino acid residues applicable to aminotransferases (AT) in general is proposed. In the multiply aligned sequences, only four out of a total of about 400 amino acid residues proved invariant in all 51 sequences, i.e. Gly(314AT)197, Asp/Glu(340AT)222, Lys(385AT)258 and Arg(562AT)386, the number not in parentheses corresponding to the structure of porcine cytosolic aspartate aminotransferase. Apparently, the aminotransferases constitute a group of homologous proteins which diverged into subgroups and, with some exceptions, into substrate-specific individual enzymes already in the universal ancestor cell.

Identification and characterization of new human medium reiteration frequency repeats

We report nine new families of human medium reiteration frequency interspersed repetitive elements (MER elements). They were identified by computer-assisted analyses. Six of them were independently confirmed as repetitive families by DNA-DNA hybridization, and the number of elements for each of these families was estimated by plaque hybridization assay. The involvement of some of the reported MER elements in genetic rearrangements is demonstrated.

Point mutations in the tyrosine aminotransferase gene in tyrosinemia type II

Tyrosinemia type II (Richner-Hanhart syndrome, RHS) is a disease of autosomal recessive inheritance characterized by keratitis, palmoplantar hyperkeratosis, mental retardation, and elevated blood tyrosine levels. The disease results from deficiency in hepatic tyrosine aminotransferase (TAT; L-tyrosine:2-oxoglutarate aminotransferase, EC 2.6.1.5), a 454-amino acid protein encoded by a gene with 12 exons. To identify the causative mutations in five TAT alleles cloned from three RHS patients, chimeric genes constructed from normal and mutant TAT alleles were tested in directing TAT activity in a transient expression assay. DNA sequence analysis of the regions identified as nonfunctional revealed six different point mutations. Three RHS alleles have nonsense mutations at codons 57, 223, and 417, respectively. One "complex" RHS allele carries a GT----GG splice donor mutation in intron 8 together with a Gly----Val substitution at amino acid 362. A new splice acceptor site in intron 2 of the fifth RHS allele leads to a shift in reading frame.

Structure of the human type II 3 beta-hydroxysteroid dehydrogenase/delta 5-delta 4 isomerase (3 beta-HSD) gene: adrenal and gonadal specificity

While classical 3 beta-hydroxysteroid dehydrogenase/delta 5-delta 4 isomerase deficiency (3 beta-HSD) is a known cause of adrenal hyperplasia resulting in ambiguous genitalia and adrenal insufficiency at birth, nonclassical or late-onset 3 beta-HSD deficiency is found in an important proportion of women with androgen excess. We have previously isolated and sequenced the cDNA and gene for the human type I 3 beta-HSD, which represents the main species expressed in the placenta and skin. Recently, we isolated, sequenced, and expressed the functional cDNA encoding type II 3 beta-HSD, which is the predominant 3 beta-HSD expressed in human adrenals and gonads. The present study describes the isolation and complete sequence of the corresponding type II 3 beta-HSD gene, which is the form most likely responsible for human 3 beta-HSD deficiency. The structural gene contains four exons of 57, 231, 165, and 1,214 bp, respectively, separated by introns of 128, 3,383, and 2,162 bp. DNA sequence analysis of the 5'-flanking region reveals the existence of two putative TATA boxes situated 28 and 140 nucleotides upstream from the transcription start site whereas two putative CAAT boxes are located 57 and 38 nucleotides upstream from the TATA boxes, respectively. A restriction fragment length pattern specific for each gene has been characterized. The present findings should provide the tools required for detailed analysis of the molecular basis of 3 beta-HSD deficiency as well as of normal sex steroid biosynthesis.

Isolation and characterization of active N-terminal truncated apo- and holoenzyme of mammalian liver tyrosine aminotransferase

Limited proteolysis was used to probe the structure of the apo- and holoenzyme of rat liver tyrosine aminotransferase. Both were subjected to trypsinolysis and the major fragments were isolated and characterized. Trypsin cleaves the apoenzyme after residues Arg57, Lys64, and Lys71 and the holoenzyme after Arg37 and Lys38. The difference in the accessibility of the enzyme deprived or associated with pyridoxal 5'-phosphate reflects two distinct conformations. The activity, the affinity for the ligands and the thermostability of the purified truncated enzyme forms are similar to those of the native apo- and holoenzyme. A model for the domain structure of mammalian tyrosine aminotransferase and a mechanism for its rapid turnover are proposed.

Expression of mammalian tyrosine aminotransferase in Saccharomyces cerevisiae and Escherichia coli. Purification to homogeneity and characterization of the enzyme overproduced in the bacteria

Rat liver tyrosine aminotransferase has been expressed in Saccharomyces cerevisiae and Escherichia coli. In yeast, the extent of production is 20-fold higher than that in rat liver after induction by dexamethasone, and reaches 250-fold higher in an E. coli strain carrying the T7 RNA polymerase transcription system. About 250 mg pure and homogeneous enzyme was obtained from 50 g transformed E. coli cells. Determination of Mr and pI, as well as analysis of N- and C-terminal amino acids, suggest that the isolated protein is native. The catalytic properties, similar to those of the enzyme from rat liver, confirm that it is fully active and that post-translational modifications in the mammalian cells are not essential for activity. Pyridoxal 5'-phosphate strongly protects the enzyme against thermal inactivation. After denaturation, 10 thiol groups, out of 16 in the polypeptide chain, react with 5,5'-dithiobis(2-nitrobenzoic acid) whereas only five or six are accessible under native conditions. Two thiols are rapidly modified with concomitant inactivation of the apoenzyme, but pyridoxal 5'-phosphate partially protects them in the holoenzyme. The results are interpreted in the light of the structure/function relationship in this enzyme.

Prostacyclin concentrations in haemolytic uraemic syndrome after acute shigellosis in children

The role of prostacyclin in the pathogenesis of haemolytic uraemic syndrome was evaluated in 11 children with acute shigellosis. Plasma concentrations of 6-keto prostaglandin, F1 alpha, a stable metabolite of prostacyclin, were measured by radioimmunoassay during acute illness, early convalescence, and after clinical recovery. Its concentration was low during acute illness in each patient, returning to normal concentrations or above at the time of the last sample. These results suggest that plasma prostacyclin may be involved in the development of the syndrome.

Outcome in tyrosinaemia type II

Tyrosinaemia type II was diagnosed in a boy with failure to thrive and in his sister on neonatal screening. On diet the outcome, at 12 and 10 years respectively, has been excellent in respect of oculocutaneous sequelae, growth, and psychomotor development, contrasting with the generally unfavourable outcome in most reported cases.

Heteroduplex between an EMBL3 genomic clone and a lambda gt11 cDNA clone provides internal size standards

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