The FibroScan-aspartate aminotransferase score can stratify the disease severity in a Japanese cohort with fatty liver diseases.

This study aimed to prove that the FibroScan-aspartate aminotransferase (FAST) scores can be used to stratify disease severity in a Japanese cohort with fatty liver diseases [metabolic dysfunction-associated fatty liver disease (MAFLD) and nonalcoholic fatty liver disease (NAFLD)]. All the participants (n = 2254) underwent liver stiffness measurements and controlled attenuation parameter assessments. We compared the clinical characteristics of the patients with MAFLD and NAFLD using the FAST scores and explored the independent determinants of FAST scores ≥ 0.35, which indicated possible progressive disease. Overall, MAFLD was diagnosed in 789 patients (35.0%), while NAFLD was diagnosed in 618 (27.4%). The proportion of patients that had a condition that suggested progressive liver disease was higher in those with MAFLD than in those with NAFLD [68 (8.6%) vs 48 (7.7%)]. The area under the receiver-operating characteristic curve of the FAST score for diagnosing advanced fibrosis was 0.969 in MAFLD and 0.965 in NAFLD. Multivariate analyses determined that diabetes mellitus, alanine aminotransferase (ALT) levels, fatty liver index, and Fibrosis-4 index independently predict FAST scores ≥ 0.35 in patients with MAFLD. ALT levels had the strongest correlation with the FAST scores (p = 0.7817). The FAST score could stratify the disease severity in the Japanese cohort with fatty liver diseases.

Heterologous gene expression and characterization of recombinant aspartate aminotransferase from Geobacillus thermopakistaniensis.

Aspartate aminotransferase catalyzes the transfer of an amino group from l-aspartate to α-oxoglutarate. A gene encoding aspartate aminotransferase, ASTGt, from Geobacillus thermopakistaniensis was cloned and expressed in Escherichia coli. The purified recombinant ASTGt exhibited highest activity at 65 °C and pH 7.0. The activity was dependent on pyridoxal phosphate but not on any metal ions. Stoichiometry of purified ASTGt demonstrated that 0.1 pyridoxal phosphate was attached per subunit of the enzyme. Determination of molecular weight by gel filtration chromatography indicated that ASTGt existed in a dimeric form in solution. Thermostability experiments showed no significant change in activity even after 16 h incubation at 65 °C. ASTGt exhibited apparent Vmax and Km values of 120 µmol min-1 mg-1 and 1.5 mM, respectively, against l-aspartate. Substrate specificity experiments indicated the highest relative activity against aspartate (100%) followed by tyrosine (27%) and proline (16%). To the best of our knowledge, this is the first report on cloning and characterization of an AST from genus Geobacillus.

Characterization of five putative aspartate aminotransferase genes in the N2-fixing heterocystous cyanobacterium Anabaena sp. strain PCC 7120.

Aspartate and glutamate are two key amino acids used in biosynthesis of many amino acids that play vital role in cellular metabolism. Aspartate aminotransferases (AspATs) are required for channelling nitrogen (N(2)) between Glu and Asp in all life forms. Biochemical and genetic characterization of AspATs have been lacking in N(2)-fixing cyanobacteria. In this report, five putative AspAT genes (alr1039, all2340, alr2765, all4327 and alr4853) were identified in the N(2)-fixing heterocystous cyanobacterium Anabaena sp. PCC 7120. Five recombinant C-terminal hexahistidine-tagged AspATs (AspAT-H(6)) were overexpressed in Escherichia coli and purified to homogeneity. Biochemical analysis demonstrated that these five putative AspATs have authentic AspAT activity in vitro using aspartate as an amino donor. However, the enzymic activities of the five AspATs differed in vitro. Alr4853-H(6) showed the highest AspAT activity, while the enzymic activity for the other four AspATs ranged from 6.5 to 53.7 % activity compared to Alr4853 (100 %). Genetic characterization of the five AspAT genes was also performed by inactivating each individual gene. All of the five AspAT knockout mutants exhibited reduced diazotrophic growth, and alr4853 was further identified to be a Fox gene (requiring fixed N(2) for growth in the presence of oxygen). Four out of five P(aspAT)-gfp transcriptional fusions were constitutively expressed in both diazotrophic and nitrate-dependent growth conditions. Quantitative reverse transcriptase PCR showed that alr4853 expression was increased by 2.3-fold after 24 h of N(2) deprivation. Taken together, these findings add to our understanding of the role of AspATs in N(2)-fixing within heterocystous cyanobacteria.

A blue native polyacrylamide gel electrophoretic technology to probe the functional proteomics mediating nitrogen homeostasis in Pseudomonas fluorescens.

As glutamate and ammonia play a pivotal role in nitrogen homeostasis, their production is mediated by various enzymes that are widespread in living organisms. Here, we report on an effective electrophoretic method to monitor these enzymes. The in gel activity visualization is based on the interaction of the products, glutamate and ammonia, with glutamate dehydrogenase (GDH, EC: 1.4.1.2) in the presence of either phenazine methosulfate (PMS) or 2,6-dichloroindophenol (DCIP) and iodonitrotetrazolium (INT). The intensity of the activity bands was dependent on the amount of proteins loaded, the incubation time and the concentration of the respective substrates. The following enzymes were readily identified: glutaminase (EC: 3.5.1.2), alanine transaminase (EC: 2.6.1.2), aspartate transaminase (EC: 2.6.1.1), glycine transaminase (EC: 2.6.1.4), ornithine oxoacid aminotransferase (EC: 2.6.1.13), and carbamoyl phosphate synthase I (EC: 6.3.4.16). The specificity of the activity band was confirmed by high pressure liquid chromatography (HPLC) following incubation of the excised band with the corresponding substrates. These bands are amenable to further molecular characterization by a variety of analytical methods. This electrophoretic technology provides a powerful tool to screen these enzymes that contribute to nitrogen homeostasis in Pseudomonas fluorescens and possibly in other microbial systems.

Isolation, purification, and characterization of phenylpyruvate transaminating enzymes of Erwinia carotovora.

Enzymes of Erwinia carotovora that transaminate phenylpyruvate were isolated, purified, and characterized. Two aromatic aminotransferases (PAT1 and PAT2) and an aspartic aminotransferase (PAT3) were found. According to gel filtration, these enzymes have molecular weights of 76, 75, and 78 kDa. The enzymes consist of two identical subunits of molecular weights of 31.4, 31, and 36.5 kDa, respectively. The isoelectric points of PAT1, PAT2, and PAT3 were determined as 3.6, 3.9, and 4.7, respectively. The enzyme preparations considerably differ in substrate specificity. All three of the enzymes productively interacted with the following amino acids: L-aspartic acid, L-leucine (except PAT3), L-isoleucine (except PAT3), L-serine, L-methionine, L-cysteine, L-phenylalanine, L-tyrosine, and L-tryptophane. The aromatic aminotransferases display higher specificity to the aromatic amino acids and the leucine-isoleucine pair, whereas the aspartic aminotransferase displays higher specificity to L-aspartic acid and relatively low specificity to the aromatic amino acids. The aspartic aminotransferase does not use L-leucine or L-isoleucine as a substrate. PAT1, PAT2, and PAT3 show the highest activity at pH 8.9 and at 48, 53, and 58°C, respectively.

Diagnostic performance of ethyl glucuronide in hair for the investigation of alcohol drinking behavior: a comparison with traditional biomarkers.

BACKGROUND: Ethyl glucuronide (EtG) in hair has emerged as a useful biomarker for detecting alcohol abuse and monitoring abstinence. However, there is a need to establish a reliable cutoff value for the detection of chronic and excessive alcohol consumption. METHODS: One hundred and twenty-five subjects were classified as teetotalers, low-risk drinkers, at-risk drinkers, or heavy drinkers. The gold standard for subjects' classifications was based on a prospective daily alcohol self-monitoring log. Subjects were followed for a 3-month period. The EtG diagnostic performance was evaluated and compared with carbohydrate-deficient transferring (CDT) and the activities of aspartate aminotransferase, alanine aminotransferase, and γ-glutamyl-transferase (γGT). RESULTS: A cutoff of >9 pg/mg EtG in hair, suggesting an alcohol consumption of >20/30 g (at-risk drinkers), and a cutoff of >25 pg/mg, suggesting a consumption of >60 g (heavy drinkers), were determined by receiver operating characteristic analysis. The EtG diagnostic performance was significantly better (P < 0.05) than any of the traditional biomarkers alone. EtG, as a single biomarker, yielded a stronger or similar diagnostic performance in detecting at-risk or heavy drinkers, respectively, than the best combination of traditional biomarkers (CDT and γGT). The combination of EtG with traditional biomarkers did not improve the diagnostic performance of EtG alone. EtG demonstrated a strong potential to identify heavy alcohol consumption, whereas the traditional biomarkers failed to do so. EtG was not significantly influenced by gender, body mass index, or age. CONCLUSION: Hair EtG definitively provides an accurate and reliable diagnostic test for detecting chronic and excessive alcohol consumption. The proposed cutoff values can serve as reference for future cutoff recommendations for clinical and forensic use.

Cloning, expression and characterization of a new aspartate aminotransferase from Bacillus subtilis B3.

In the present study, we report the identification of a new gene from the Bacillus subtilis B3 strain (aatB3), which comprises 1308 bp encoding a 436 amino acid protein with a monomer molecular weight of 49.1 kDa. Phylogenetic analyses suggested that this enzyme is a member of the Ib subgroup of aspartate aminotransferases (AATs; EC 2.6.1.1), although it also has conserved active residues and thermostability characteristic of Ia-type AATs. The Asp232, Lys270 and Arg403 residues of AATB3 play a key role in transamination. The enzyme showed maximal activity at pH 8.0 and 45 °C, had relatively high activity over an alkaline pH range (pH 7.0-9.0) and was stable up to 50 °C. AATB3 catalyzed the transamination of five amino acids, with L-aspartate being the optimal substrate. The K(m) values were determined to be 6.7 mM for L-aspartate, 0.3 mM for α-ketoglutarate, 8.0 mM for L-glutamate and 0.6 mM for oxaloacetate. A 32-residue N-terminal amino acid sequence of this enzyme has 53% identity with that of Bacillus circulans AAT, although it is absent in all other AATs from different organisms. Further studies on AATB3 may confirm that it is potentially beneficial in basic research as well as various industrial applications.

Purification, crystallization and preliminary X-ray analysis of the aspartate aminotransferase of Plasmodium falciparum.

Aspartate aminotransferases (EC 2.6.1.1) catalyse the conversion of aspartate and alpha-ketoglutarate to oxaloacetate and glutamate in a reversible manner. Thus, the aspartate aminotransferase of Plasmodium falciparum (PfAspAT) plays a central role in the transamination of amino acids. Recent findings suggest that PfAspAT may also play a pivotal role in energy metabolism and the de novo biosynthesis of pyrimidines. While therapeutics based upon the inhibition of other proteins in these pathways are already used in the treatment of malaria, the advent of multidrug-resistant strains has limited their efficacy. The presence of PfAspAT in these pathways may offer additional opportunities for the development of novel therapeutics. In order to gain a deeper understanding of the function and role of PfAspAT, it has been expressed and purified to homogeneity. The successful crystallization of PfAspAT, the collection of a 2.8 A diffraction data set and initial attempts to solve the structure using molecular replacement are reported.

Biochemical characterization of stage-specific isoforms of aspartate aminotransferases from Trypanosoma cruzi and Trypanosoma brucei.

Three genes encoding putative aspartate aminotransferases (ASATs) were identified in the Trypanosoma cruzi genome. Two of these ASAT genes, presumably corresponding to a cytosolic and mitochondrial isoform, were cloned and expressed as soluble His-tagged proteins in Escherichia coli. The specific activities determined for both T. cruzi isozymes were notably higher than the values previously reported for Trypanosoma brucei orthologues. To confirm these differences, T. brucei mASAT and cASAT were also expressed as His-tagged enzymes. The kinetic analysis showed that the catalytic parameters of the new recombinant T. brucei ASATs were very similar to those determined for T. cruzi orthologues. The cASATs from both parasites displayed equally broad substrate specificities, while mASATs were highly specific towards aspartate/2-oxoglutarate. The subcellular localization of the mASAT was confirmed by digitonin extraction of intact epimastigotes. At the protein level, cASAT is constitutively expressed in T. brucei, whereas mASAT is down-regulated in the bloodstream forms. By contrast in T. cruzi, mASAT is expressed along the whole life cycle, whereas cASAT is specifically induced in the mammalian stages. Similarly, the expression of malate dehydrogenases (MDHs) is developmentally regulated in T. cruzi: while glycosomal MDH is only expressed in epimastigotes and mitochondrial MDH is present in the insect and mammalian stages. Taken together, these findings provide evidence for a metabolically active mitochondrion in the mammalian stages of T. cruzi, and suggest that the succinate excreted by amastigotes more likely represents a side product of an at least partially operative Krebs cycle, than an end product of glycosomal catabolism.

The aspartate aminotransferase family in conifers: biochemical analysis of a prokaryotic-type enzyme from maritime pine.

Plant aspartate aminotransferase (AAT, EC 2.6.1.1) plays a key role in primary nitrogen assimilation, the transfer of reducing equivalents and the interchanges of carbon and nitrogen pools between subcellular compartments. We investigated the AAT family in conifers using maritime pine as the experimental model. Genes for cytosolic, mitochondrial and two plastidic isoenzymes (eukaryotic- and prokaryotic-types) were identified and their deduced amino acid sequences compared. The primary structure of the eukaryotic-type enzymes is quite well conserved, whereas the prokaryotic-type AAT is highly divergent (15% of identity). These molecular data were confirmed by the absence of immunological cross-reactivity between the two types of native AATs. The mature prokaryotic-type polypeptide was overexpressed in Escherichia coli, and the native enzyme was purified to apparent homogeneity and its molecular properties determined. The fully active recombinant holoenzyme showed highest catalytic activity at 50-60 degrees C and was moderately thermostable, retaining about 50% of its activity after incubation at 70 degrees C for 5-10 min. The presence of pyridoxal 5'-phosphate significantly increased the thermostability of the enzyme. These molecular characteristics were exploited to develop a rapid protocol for the purification of this prokaryotic-type enzyme from pine cotyledons. The results will be useful for studying aspartate and amino acid metabolism in trees.

[Expression, purification and enzymatic characterization of Thermus thermophilus HB8 aspartate aminotransferase in Escherichia coli].

To obtain thermostable aspartate aminotransferase, the gene aspC from an extremely thermophilic bacterium, Thermus thermophilus HB8 was cloned, and its product was overexpressed in Escherichia coli BL21 (DE3) and Rosetta (DE3). The expression in Rosetta (DP3) was more efficient. The optimum reactive pH was 7, and the recombinant enzyme activity changed little when incubated in the buffer of pH8 - 10 on 37 degrees C for 1 h. The optimum reactive temprature was 75 degrees C, and the recombinant enzyme was more stable on the temperature of 25 - 55 degrees C. The half life of recombinant enzyme on 65 degrees C was 3.5 h, on 75 degrees C was 2.5 h. KmKG was 7.559 mmol/L, VmaxKG was 0.086 mmol/(L x min), KmAsp was 2.031 mmol/L, VmaxAsp was 0.024 mmol/(L x min). Ca2+, Fe3+, Mn2+ inhibited enzyme activity softly.

[Subcellular localization, purification, and various catalitic properties of aspartate aminotransferase from Spirodela polyrhiza].

Intracellular distribution of aspartate aminotransferase (AAT) in Spirodela polyrhiza (Lemnaceae), strain SJ, has been studied by differential centrifugation. The bulk of the enzyme (73% of total cellular content) was localized in the cytoplasm and 24% activity was localized in chloroplasts. Purified cytoplasmic and chloroplastic isozymes differed by their affinity for substrates. The reaction balance was shifted towards direct and reverse transamination in the cytoplasm and chloroplast, respectively. Competitive inhibition of AAT by excessive substrates and enzyme affinity modulation by certain intermediates of the tricarboxylic acid cycle (isocitrate, succinate, and citrate) were observed. Possible involvement of AAT isozymes in the coordination of carbon and nitrogen metabolism through the regulation of 2-oxoglutarate synthesis and utilization in different cellular compartments is discussed.

Avaliação hematológica e dosagem bioquímica de ALT, AST e creatinina em elefante-marinho-do-sul, Mirounga leonina (linnaeus, 1758), encontrado no litoral de Salvador, Bahia / Haematological analysis and clinical chemistry of ALT, AST and creatinine of a southern elephant seal, Mirounga leonina (linnaeus, 1758), found on the Salvador coast, Bahia

O Centro de Resgate de Mamíferos Aquáticos (CRMA) tem trabalhado desde 1999 com o resgate e a reabilitação de cetáceos e pinípedes em situação de encalhe no litoral do Estado da Bahia. O presente trabalho objetiva apresentar e avaliar os resultados de hemogramas e dosagens séricas de alanina aminotranferase (ALT), aspartato aminotransferase (AST) e creatinina realizados em um exemplar de elefante-marinho-do-sul, Mirounga leonina (LINNAEUS, 1758), encontrado no dia 11 de fevereiro de 2002 na praia da Barra, litoral de Salvador, BA. Tratava-se de um filhote macho, com 137cm de comprimento e peso aproximado de 49kg, com estado nutricional debilitado, um ferimento no lado direito na altura da escápula. O exemplar permaneceu em acompanhamento clínico por 56 dias, sendo vermifugado com Febendazole, o ferimento suturado e tratado com álcool iodado, Nitrofurazona solução e Tartarato de Ketanserina, simultaneamente com Enrofloxacina 10%, Potenay®, Complexo B e Benerva®.No dia 16 seguinte apresentou um quadro de conjuntivite unilateral direita, sendo tratado até o final de sua estadia com pomada oftálmica à base de Cloridrato de Cloranfenicol. Durante este período colheu-se um total de seis amostras de sangue, sendo três para a realização de hemograma e as demais para dosagem de ALT, AST e creatinina. De acordo com os resultados dos eritrogramas o filhote desenvolveu um quadro de anemia, classificada como microcítica e normocrômica. Com relação aos leucogramas, destacou-se linfopenia, eosinopenia e monocitopenia, possivelmente influenciado pelas condições de estresse e o manejo. Constatou-se nos exames bioquimicos valores diminuídos para a AST e creatinina, porém não representando a existência de um quadro patológico de significância clínica.

Since 1999 the Aquatic Mammals Rescue Center - AMRC has been working in the rescue and rehabilitation of stranded cetaceans and pinnipeds on the coast of Bahia, Brazil. This paper presents and analyses the blood cells count and clinical chernistry of alanine aminotransferase (ALT), aspartate aminotransferase (AST) and creatinine of a southern elephant seal, Mirounga leonina (LINNAEUS, 1758), found on February the 11 th at Barra Beach, Salvador, BA. The specimen was an orphan male calf, with 137cm of length and estimated weight of 49kg. It presemed bad nutritional conditions and a shark bite on the right shoulder area. Clinical management was performed for 56 days, anthelrnintic Febendazole was utilized, and the bite was treated with iodined alcohol, Nitrofurazone solution and Kethanserin, simultaneously with Enrofloxacin 10%, Potenay®, Vitamin B Complex and Benerva®. On the 16th the animal presented a right unilateral conjuntivitis, treated with Cloranphenicol oftalmic pomade until the end of its stay in the captive. During this period a total of six blood samples were collected, three for total blood counts and the others for the biochernistry determination of ALT, AST and creatinine. According to the haematological analysis the seal developed an anaemia which was classified as microcytic and normochromic. Lymphopenia, eosinopenia and monocytopenia were also observed, possibly due to its handling and stress conditions. The clinical chemistry presented low values for AST and creatinine, although this did not represent the existence of any pathologic context or disease with clinical significance.

Isolation, purification, and crystallization of aspartate aminotransferase from wheat grain.

A procedure for isolation and purification of aspartate aminotransferase from wheat grain includes chromatography on DEAE cellulose, acidification-alkalization, precipitation with protamine sulfate, fractionation with ammonium sulfate, and chromatography on hydroxyapatite. The yield of protein was 27% with 95% purity. Crystals of the enzyme (0.05 x 0.025 x 0.015 mm3) were obtained from ammonium sulfate solution.

Aspartate aminotransferase generates proagonists of the aryl hydrocarbon receptor.

The aryl hydrocarbon receptor (AHR) binds planar aromatic compounds and up-regulates the transcription of a battery of xenobiotic-metabolizing enzymes. To identify proteins involved in the biosynthesis of endogenous AHR ligands, we screened extracts of various mouse tissues for AHR signaling activity. We found heart extract to activate AHR and identified the active component to be the enzyme aspartate aminotransferase (EC 2.6.1.1). We demonstrate that this transaminase can activate AHR signaling by converting l-tryptophan to indole-3-pyruvate. In turn, indole-3-pyruvate spontaneously reacts in aqueous solution to form a large number of compounds that act as agonists of AHR. Tyrosine and the serotonin-precursor 5-hydroxytryptophan also activate AHR signaling in combination with aspartate aminotransferase, suggesting that 4-hydroxyphenylpyruvate and 5-hydroxyindolepyruvate also act as proagonists of AHR. This study demonstrates that the known tryptophan metabolic-intermediate indole-3-pyruvate is a proagonist of AHR that reacts in aqueous solution to form a variety of AHR agonists.

An aspartate aminotransferase of Wolbachia endobacteria from Onchocerca volvulus is recognized by IgG1 antibodies from residents of endemic areas.

Wolbachia are intracellular alpha-proteobacteria, closely related to Rickettsia, that infect various arthropods and filarial parasites. In the present study, the cDNA encoding the aspartate aminotransferase (AspAT) of Wolbachia from the human pathogenic filarial parasite Onchocerca volvulus (Ov-WolAspAT) was identified. At the amino acid level, the identity of the Ov-WolAspAT was 56% to Rickettsia prowazekii AspAT and 54% to the AspAT of the nitrogen-fixing bacterium Sinorhizobium meliloti, but the highest degree of identity was found to the putative AspAT of Wolbachia from Brugia malayi and Drosophila melanogaster (85%). All of these bacterial AspATs are members of the AspAT subclass Ib. A 35 kDa fragment of the Ov-WolAspAT was expressed in Escherichia coli, and immunolocalization using polyclonal antibodies against this antigen revealed that Ov-WolAspAT is present in a considerable proportion of the Wolbachia from O. volvulus, as well as in the endobacteria of several other filarial parasites. Western blot analysis using recombinant Ov-WolAspAT as antigen showed that IgG1 antibodies were present in 70 (51%) individuals living in areas endemic for O. volvulus, B. malayi or Wuchereria bancrofti and no IgG4 or IgE antibodies were found. Among 40 sera of persons from Uganda and Liberia who were putatively not infected with human filarial parasites, 11 (28%) individuals presented IgG1 antibodies, while none of the 33 sera from healthy Europeans and none of the 14 sera from patients with proven Rickettsia or Brucella infections reacted with the antigen. These results also show that an intracellular protein of Wolbachia endobacteria (WolAspAT) acts as antigen in human filariasis.

Crystallization and preliminary X-ray analysis of the Mj0684 gene product, a putative aspartate aminotransferase, from Methanococcus jannaschii.

A putative aspartate aminotransferase from the hyperthermophilic archaeon Methanococcus jannaschii encoded by the Mj0684 gene has been overexpressed in Escherichia coli and crystallized at 296 K using the sitting-drop vapour-diffusion method. The crystals belong to space group P4(1)2(1)2 (or P4(3)2(1)2), with unit-cell parameters a = b = 111.87, c = 60.86 A. They diffract to 2.2 A resolution using Cu Kalpha X-rays. The asymmetric unit contains a single subunit of the recombinant Mj0684 gene product, giving a corresponding V(M) of 2.25 A(3) Da(-1) and a solvent content of 45.3% by Volume. An X-ray diffraction data set has been collected to 2.2 A at 295 K.

Characterization of aspartate aminotransferase isoenzymes from leaves of Lupinus albus L. cv Estoril.

Two aspartate aminotransferase (EC 2.6.1.1) isoenzymes (AAT-1 and AAT-2) from Lupinus albus L. cv Estoril were separated, purified, and characterized. The molecular weight, pI value, optimum pH, optimum temperature, and thermodynamic parameters for thermal inactivation of both isoenzymes were obtained. Studies of the kinetic mechanism, and the kinetics of product inhibition and high substrate concentration inhibition, were performed. The effect of some divalent ions and irreversible inhibitors on both AAT isoenzymes was also studied. Native PAGE showed a higher molecular weight for AAT-2 compared with AAT-1. AAT-1 appears to be more anionic than AAT- 2, which was suggested by the anion exchange chromatography. SDS-PAGE showed a similar sub-unit molecular weight for both isoenzymes. The optimum pH (between 8.0 and 9.0) and temperature (60-65 degrees C) were similar for both isoenzymes. In the temperature range of 45-65 degrees C, AAT-2 has higher thermostability than AAT-1. Both isoenzymes showed a high affinity for keto-acid substrates, as well as a higher affinity to aspartate than glutamate. Manganese ions induced an increase in both AAT isoenzymes activities, but no cooperative effect was detected. Among the inhibitors tested, hydroxylamine affected both isoenzymes activity by an irreversible inhibition mechanism.

Kynurenine aminotransferase and glutamine transaminase K of Escherichia coli: identity with aspartate aminotransferase.

The present study describes the isolation of a protein from Escherichia coli possessing kynurenine aminotransferase (KAT) activity and its identification as aspartate aminotransferase (AspAT). KAT catalyses the transamination of kynurenine and 3-hydroxykynurenine to kynurenic acid and xanthurenic acid respectively, and the enzyme activity can be easily detected in E. coli cells. Separation of the E. coli protein possessing KAT activity through various chromatographic steps led to the isolation of the enzyme. N-terminal sequencing of the purified protein determined its first 10 N-terminal amino acid residues, which were identical with those of the E. coli AspAT. Recombinant AspAT (R-AspAT), homologously expressed in an E. coli/pET22b expression system, was capable of catalysing the transamination of both l-kynurenine (K(m)=3 mM; V(max)=7.9 micromol.min(-1).mg(-1)) and 3-hydroxy-dl-kynurenine (K(m)=3.7 mM; V(max)=1.25 micromol.min(-1).mg(-1)) in the presence of pyruvate as an amino acceptor, and exhibited its maximum activity at temperatures between 50-60 degrees C and at a pH of approx. 7.0. Like mammalian KATs, R-AspAT also displayed high glutamine transaminase K activity when l-phenylalanine was used as an amino donor (K(m)=8 mM; V(max)=20.6 micromol.min(-1).mg(-1)). The exact match of the first ten N-terminal amino acid residues of the KAT-active protein with that of AspAT, in conjunction with the high KAT activity of R-AspAT, provides convincing evidence that the identity of the E. coli protein is AspAT.

Effects of substrate structural analogues on the enzymatic activities of aspartate aminotransferase isoenzymes.

Aspartate aminotransferase (AAT, EC 2.6.1.1) catalyses the transamination of L-asparate to oxaloacetate. It has been reported that AAT from different plant sources can catalyse the transamination of other compounds structurally similar to the natural substrates. Specificity and kinetic studies were performed with two aspartate aminotransferase isoenzymes (AAT-1 and AAT-2) from leaves of Lupinus albus L. cv Estoril using different amino donors and acceptors. Both isoenzymes showed residual activity for some of the substrates tested. Competitive inhibition was found with most of the structural analogues which is typical of a ping-pong bi-bi kinetic mechanism. It was found that both isoenzymes can use 2-amino-4-methoxy-4-oxobutanoic acid as amino donor. AAT-2 uses 2-amino-4-methoxy-4-oxobutanoic acid at a similar rate as L-aspartate but AAT-1 uses this substrate at a slower rate. The use of this amino donor by AAT isoenzymes has not been reported previously, and our results indicate structural differences between both isoenzymes.