Overexpression of BCAT1 is a prognostic marker in gastric cancer

As one form of branched-chain amino-acid transaminase (BCAT) enzymes, It has been found that up-regulation of BCAT1 is associated with poor prognosis in numerous types of tumors, but studies on the role of BCAT1 expression in gastric cancer (GC) are rare. The aims of this study were to detect BCAT1 expression in GC and to analyze its association with prognosis of GC patients. Microarray experiments were performed on the Affymetrix U133 plus 2.0 GeneChip Array. The protein and messenger RNA levels of BCAT1 were validated by immunohistochemistry and real-time quantitative polymerase chain reaction in GC tissues and adjacent noncancerous tissues. Our study shows that the expression of BCAT1 significantly increased in human GC. Furthermore, it can also be found that BCAT1 overexpression was associated with TNM stage (P < .05), local invasion (P < .05), Lauren type (P < .05), tumor classification (P < .05), lymph node metastasis (P < .05), and presence of distant metastasis (P < .05). Kaplan-Meier survival analysis revealed that high BCAT1 expression predicted significantly worse overall survival (P < .05), whereas multivariate Cox regression analysis showed that BCAT1 affects GC independently. In conclusion, up-regulation of BCAT1 indicated a poor survival rate of GC and may serve as a useful marker for predicting the outcome of patients with GC.

[Expression and characterization of a novel ω-transaminase from Burkholderia phytofirmans PsJN]

Production of chiral amines and unnatural amino-acid using ω-transaminase can be achieved by kinetic resolution and asymmetric synthesis, thus ω-transaminase is of great importance in the synthesis of pharmaceutical intermediates. By genomic data mining, a putative ω-transaminase gene hbp was found in Burkholderia phytofirmans PsJN. The gene was cloned and over-expressed in Escherichia coli BL21 (DE3). The recombinant enzyme (HBP) was purified by Ni-NTA column and its catalytic properties and substrate profile were studied. HBP showed high relative activity (33.80 U/mg) and enantioselectivity toward β-phenylalanine (β-Phe). The optimal reaction temperature and pH were 40 ℃ and 8.0-8.5, respectively. We also established a simpler and more effective method to detect the deamination reaction of β-Phe by UV absorption method using microplate reader, and demonstrated the thermodynamic property of this reaction. The substrate profiling showed that HBP was specific to β-Phe and its derivatives as the amino donor. HBP catalyzed the resolution of rac-β-Phe and its derivatives, the products (R)-amino acids were obtained with about 50% conversions and 99% ee.

Ammonia upregulates kynurenine aminotransferase II mRNA expression in rat brain: a role for astrocytic NMDA receptors?

Kynurenine aminotransferase II (KAT-II) is the astrocytic enzyme catalyzing the synthesis of kynurenic acid (KYNA), an endogenous inhibitor of the α7-nicotinic receptor and the NMDA receptor (NMDAr). A previous study demonstrated an increase of KYNA synthesis in the brain of rats with thioacetamide (TAA)-induced acute liver failure. Here we show that TAA administration increases KAT-II expression in the rat cerebral cortex and the effect is mimicked in cerebral cortical astrocytes in culture treated with high (5 mM) concentration of ammonia. KAT-II expression in control and TAA-treated rats was increased by NMDAr antagonist memantine, and the effects of TAA and memantine appeared additive. In astrocytes, the NMDAr antagonist MK-801 raised KAT-II expression as well, while NMDA added alone had no effect. Glutamate decreased KAT-II mRNA level, which was attenuated by MK-801. The results suggest that stimulation of KAT-II expression during hepatic encephalopathy may be associated with a partial inactivation of astrocytic NMDAr by ammonia.

[Cloning, expression and characterization of N-acetylornithine aminotransferase from Corynebacterium crenatum and its effects on L-arginine fermentation]

N-Acetylornithine aminotransferase (EC 2.6.1.11, ACOAT) catalyzes the conversion of N-acetylglutamic semialdehyde to N-acetylornithine, the forth step involved in the L-arginine biosynthetic pathways. We studied the enzyme properties to set up reliable theoretical basis for the arginine fermentation optimization. ACOAT encoding gene argD was cloned from an industrial L-arginine producer Corynebacterium crenatum SYPA 5-5. Analysis of argD sequences revealed that only one ORF existed, which coded a peptide of 390 amino acids with a calculated molecular weight of 41.0 kDa. The argD gene from C. crenatum SYPA 5-5 was expressed both in Escherichia coli BL21 and C. crenatum SYPA. Then ACOAT was purified by Ni-NTA affinity chromatography and its specific enzyme activity was 108.2 U/g. Subsequently, the expression plasmid pJCtac-CcargD was transformed into C. crenatum SYPA and the specific activity of ACOAT was improved evidently in the recombinant C. crenatum CCD. Further fermentative character of CCD1 was also analyzed. The results showed that the L-arginine producing ability of the recombinant strain was 39.7 g/L improved by 14.7%.

Investigation of anticapsin biosynthesis reveals a four-enzyme pathway to tetrahydrotyrosine in Bacillus subtilis

Bacillus subtilis produces the antibiotic anticapsin as an L-Ala-L-anticapsin dipeptide precursor known as bacilysin, whose synthesis is encoded by the bacA-D genes and the adjacent ywfGH genes. To evaluate the biosynthesis of the epoxycyclohexanone amino acid anticapsin from the primary metabolite prephenate, we have overproduced, purified, and characterized the activity of the BacA, BacB, YwfH, and YwfG proteins. BacA is an unusual prephenate decarboxylase that avoids the typical aromatization of the cyclohexadienol ring by protonating C(8) to produce an isomerized structure. BacB then catalyzes an allylic isomerization, generating a conjugated dienone with a 295 nm chromophore. Both the BacA and BacB products are regioisomers of H(2)HPP (dihydro-4-hydroxyphenylpyruvate). The BacB product is then a substrate for the short chain reductase YwfH which catalyzes the conjugate addition of hydride at the C(4) olefinic terminus using NADH to yield the cyclohexenol-containing tetrahydro-4-hydroxyphenylpyruvate H(4)HPP. In turn, this keto acid is a substrate for YwfG, which promotes transamination (with L-Phe as amino donor), to form tetrahydrotyrosine (H(4)Tyr). Thus BacA, BacB, YwfH, and YwfG act in sequence in a four enzyme pathway to make H(4)Tyr, which has not previously been identified in B. subtilis but is a recognized building block in cyanobacterial nonribosomal peptides such as micropeptins and aeruginopeptins.

Barth syndrome presenting with acute metabolic decompensation in the neonatal period

We describe two patients affected by Barth syndrome. Their symptoms became manifest on respectively the third and first day of their lives. Clinical presentation included poor sucking, lethargy, hypotonia, hypothermia and cardiomyopathy. Laboratory findings such as hypoglycaemia, metabolic acidosis, elevated transaminases, hyperlactacidaemia and mild hyperammonaemia pointed to an inborn error of energy metabolism with possible mitochondrial involvement. Molecular analysis of the TAZ (G4.5) gene showed the c.877G > A mutation leading to the G197R amino acid substitution in patient 1, and the new splice donor c.829 + 1G > A genetic lesion in patient 2.

Evolution of two alanine glyoxylate aminotransferases in mosquito

In the mosquito, transamination of 3-HK (3-hydroxykynurenine) to XA (xanthurenic acid) is catalysed by an AGT (alanine glyoxylate aminotransferase) and is the major branch pathway of tryptophan metabolism. Interestingly, malaria parasites hijack this pathway to use XA as a chemical signal for development in the mosquito. Here, we report that the mosquito has two AGT isoenzymes. One is the previously cloned AeHKT [Aedes aegypti HKT (3-HK transaminase)] [Han, Fang and Li (2002) J. Biol. Chem. 277, 15781-15787], similar to hAGT (human AGT), which primarily catalyses 3-HK to XA in mosquitoes, and the other is a typical dipteran insect AGT. We cloned the second AGT from Ae. aegypti mosquitoes [AeAGT (Ae. aegypti AGT)], overexpressed the enzyme in baculovirus/insect cells and determined its biochemical characteristics. We also expressed hAGT for a comparative study. The new cloned AeAGT is highly substrate-specific when compared with hAGT and the previously reported AeHKT and Drosophila AGT, and is translated mainly in pupae and adults, which contrasts with AeHKT that is expressed primarily in larvae. Our results suggest that the physiological requirements of mosquitoes and the interaction between the mosquito and its host appear to be the driving force in mosquito AGT evolution.

Roles of phosphatidylinositol 3-kinase and osteopontin in steatosis and aminotransferase release by hepatocytes treated with methionine-choline-deficient medium

Feeding mice a methionine and choline-deficient (MCD) diet serves as an experimental animal model for nonalcoholic steatohepatitis (NASH). In the present study we examined the effect of exposing AML-12 hepatocytes to MCD culture medium in regard to mechanisms of steatosis and alanine amino-transferase (ALT) release. Cells exposed to MCD medium developed significant and progressive steatosis from 6 to 24 h and also had significantly increased loss of ALT into the medium at 18 and 24 hours of incubation. No increased oxidative injury or cell death was observed. Osteopontin (OPN) mRNA in cells and protein expression in medium were significantly increased during 6-24 hours of incubation. MCD medium treatment also resulted in activation of PI3-kinase by 30 minutes and its downstream target p-Akt within 1hour of incubation. Steatosis was associated with increased expression of microsomal triglyceride transfer protein (MTTP) mRNA and increased ALT release with over expression of ALT mRNA, all of which were completely prevented by inhibition of PI3-kinase (LY294002). Blocking OPN signaling by treating with anti-OPN or anti-beta3-integrin antibody prevented the increased ALT release while only partially prevented the increased ALT mRNA expression, but had no effect on either steatosis or MTTP expression. In conclusion, incubation of cultured hepatocytes with MCD medium results in cellular steatosis and OPN dependent ALT release. PI3-kinase plays a central role in signaling the MCD medium-induced steatosis and increased OPN expression, whereas OPN appears to play a role in signaling hepatocyte ALT release but not steatosis.

Molecular cloning, expression and characterization of pyridoxamine-pyruvate aminotransferase

Pyridoxamine-pyruvate aminotransferase is a PLP (pyridoxal 5'-phosphate) (a coenzyme form of vitamin B6)-independent aminotransferase which catalyses a reversible transamination reaction between pyridoxamine and pyruvate to form pyridoxal and L-alanine. The gene encoding the enzyme has been identified, cloned and overexpressed for the first time. The mlr6806 gene on the chromosome of a symbiotic nitrogen-fixing bacterium, Mesorhizobium loti, encoded the enzyme, which consists of 393 amino acid residues. The primary sequence was identical with those of archaeal aspartate aminotransferase and rat serine-pyruvate aminotransferase, which are PLP-dependent aminotransferases. The results of fold-type analysis and the consensus amino acid residues found around the active-site lysine residue identified in the present study showed that the enzyme could be classified into class V aminotransferases of fold type I or the AT IV subfamily of the alpha family of the PLP-dependent enzymes. Analyses of the absorption and CD spectra of the wild-type and point-mutated enzymes showed that Lys197 was essential for the enzyme activity, and was the active-site lysine residue that corresponded to that found in the PLP-dependent aminotransferases, as had been suggested previously [Hodsdon, Kolb, Snell and Cole (1978) Biochem. J. 169, 429-432]. The K(d) value for pyridoxal determined by means of CD was 100-fold lower than the K(m) value for it, suggesting that Schiff base formation between pyridoxal and the active-site lysine residue is partially rate determining in the catalysis of pyridoxal. The active-site structure and evolutionary aspects of the enzyme are discussed.

Development of a serum-free co-culture of human intestinal epithelium cell-lines (Caco-2/HT29-5M21)

Background

The absorptive and goblet cells are the main cellular types encountered in the intestine epithelium. The cell lineage Caco-2 is a model commonly used to reproduce the features of the bowel epithelium. However, there is a strong debate regarding the value of Caco-2 cell culture to mimick in vivo situation. Indeed, some authors report in Caco-2 a low paracellular permeability and an ease of access of highly diffusible small molecules to the microvilli, due to an almost complete lack of mucus. The HT29-5M21 intestinal cell lineage is a mucin-secreting cellular population. A co-culture system carried out in a serum-free medium and comprising both Caco-2 and HT29-5M21 cells was developed. The systematic use of a co-culture system requires the characterization of the monolayer under a given experimental procedure.

Results

In this study, we investigated the activity and localization of the alkaline phosphatase and the expression of IAP and MUC5AC genes to determine a correlation between these markers and the cellular composition of a differentiated monolayer obtained from a mixture of Caco-2 and HT29-5M21 cells. We observed that the culture conditions used (serum-free medium) did not change the phenotype of each cell type, and produced a reproducible model. The alkaline phosphatase expression characterizing Caco-2 cells was influenced by the presence of HT29-5M21 cells.

Conclusion

The culture formed by 75% Caco-2 and 25% HT29-5M21 produce a monolayer containing the two main cell types of human intestinal epithelium and characterized by a reduced permeability to macromolecules.

Expression of alanine:glyoxylate aminotransferase gene from Saccharomyces cerevisiae in Ashbya gossypii

Two plasmids containing an autonomously replicating sequence from Saccharomyces cerevisiae were constructed. Using these vectors, the AGX1 gene encoding alanine:glyoxylate aminotransferase (AGT) from S. cerevisiae, which converts glyoxylate into glycine but is not present in Ashbya gossypii, was expressed in A. gossypii. Geneticin-resistant transformants with the plasmid having the kanamycin resistance gene under the control of the translation elongation factor 1 alpha (TEF) promoter and terminator from A. gossypii were obtained with a transformation efficiency of approximately 10-20 transformants per microgram of plasmid DNA. The specific AGT activities of A. gossypii pYPKTPAT carrying the AGX1 gene in glucose- and rapeseed-oil-containing media were 40 and 160 mU mg-1 of wet mycelial weight, respectively. The riboflavin concentrations of A. gossypii pYPKTPAT carrying AGX1 gene in glucose- and rapeseed-oil-containing media were 20 and 150 mg l-1, respectively. In the presence of 50 mM glyoxylate, the riboflavin concentration and the specific riboflavin concentration of A. gossypii pYPKTPAT were 2- and 1.3-fold those of A. gossypii pYPKT without the AGX1 gene.

Isethionate as a product from taurine during nitrogen-limited growth of Klebsiella oxytoca TauN1

Klebsiella oxytoca TauN1 represents a group of isolates which utilise taurine (2-aminoethanesulfonate) quantitatively as a sole source of combined nitrogen for aerobic growth. During growth, a compound is excreted, which has now been identified as isethionate (2-hydroxyethanesulfonate). An ion-chromatographic separation of isethionate was developed to quantify the putative isethionate, whose identity was confirmed by matrix-assisted, laser-desorption ionisation time-of-flight mass spectrometry. Strain TauN1 utilised taurine (and excreted isethionate) concomitantly with growth. Cell-free extracts contained inducible taurine transaminase, which yielded sulfoacetaldehyde. A soluble, NADP-dependent isethionate dehydrogenase converted sulfoacetaldehyde to isethionate. The enzyme was partially purified and it apparently belonged to the family of short-chain alcohol dehydrogenases.

Branched-chain fatty acid biosynthesis in a branched-chain amino acid aminotransferase mutant of Staphylococcus carnosus

Fatty acid biosynthesis by a mutant strain of Staphylococcus carnosus deficient in branched-chain amino acid aminotransferase (IlvE) activity was analysed. This mutant was unable to produce the appropriate branched-chain alpha-ketoacid precursors for branched-chain fatty acid biosynthesis from the amino acids valine, isoleucine, and leucine, and required the short branched chain acids 2-methylbutanoic acid or 2-methylpropanoic acid for growth in a defined medium. The isoleucine related metabolites, alpha-keto-beta-methylvaleric acid and 2-methylbutanal also served as growth factors. Growth in rich medium and growth in defined medium supplemented with 2-methylpropanoic acid lead to extensive alteration of the fatty acid composition in the cell membrane. In rich medium, a change from 51.7% to 17.1% anteiso-C15:0, and from 3.6% to 33.9% iso-C14:0 fatty acids as compared to the wild-type strain was observed. Despite the deficiency in IlvE activity, the mutant strain was still able to produce the short chain carboxylic acids, 3-methylbutanoic acid and 2-methylpropanoic acid when cultivated in rich medium. Supplementation experiments employing deuterated glucose induced the valine biosynthetic pathway for 2-methylpropanoic acid production, revealing that the IlvE protein plays an important, but not essential role in the biosynthesis of branched-chain fatty acids and secondary metabolites in S. carnosus.

Rapamycin successfully treats post-transplant autoimmune hepatitis

Rapamycin (Rapa), one of the newer immunosuppressants has been found to control and prevent autoimmune features in animal models. This is the first report describing the successful control of post-transplant autoimmune hepatitis (AIH) with Rapa. Post-transplant AIH is diagnosed in the presence of raised transaminases, elevated immunoglobulin G, presence of autoantibodies and histologic changes consistent with AIH on liver biopsy. It may represent a recurrence of the original AIH that led to transplantation or present as a de novo AIH after liver transplant. Post-transplant AIH has conventionally been treated with Prednisolone (Pred) and Azathioprine (AZA). In this report, tailoring of immunosuppression after diagnosis of post-transplant AIH is described with special emphasis on those treated successfully with Rapa. Fifteen of 21 patients responded to treatment with an increase in dose of Pred and addition of AZA or Mycophenolate Mofetil (MMF) to calcineurin inhibitor. Five non-responders and one other patient with post-transplant AIH were treated with addition of Rapa. All six responded to treatment but drug was withdrawn in one patient. Adverse events were minimal. Rapa may prove to be an important addition in the control of autoimmune liver disease.

Exacerbation of oral erosive lichen planus by combination of interferon and ribavirin therapy for chronic hepatitis C

Hepatitis C virus (HCV) induces extrahepatic manifestations such as oral lichen planus (OLP) as well as chronic liver diseases. The treatment of HCV-related chronic liver disease has evolved from the use of a single agent, mainly interferon (IFN), to the combination of IFN and ribavirin. We present a case of erosive OLP, cutaneous lichen planus (CLP), and leukoplakia of the vocal cord in a man with chronic hepatitis C infection treated with IFN and ribavirin. A 65-year-old man suffered from OLP before undergoing combination of IFN and ribavirin therapy for chronic hepatitis C. He was initially treated with IFNbeta (6 million units (MU) /day for 2 weeks), then a combination of IFNalpha-2b (6 MU/day for 2 weeks and 3 times a week for 14 weeks) and ribavirin (400-600 mg/day). The OLP lesion was not aggravated by application of steroids during the 7 weeks after the treatment, but after 18 weeks, the combination of IFN and ribavirin was stopped because of aggravation of the OLP. Elevated aminotransferase levels returned to normal during the therapy. But 7 weeks after discontinuation, aminotransferase levels rose to 10 times the normal range. Five months after discontinuation, the papules of CLP appeared. Eight months after discontinuation, the OLP erosion had gradually reduced, but some erosion remained. Aminotransferase levels were decreased, but serum HCV RNA had not disappeared. Caution should be exercised when IFN or ribavirin therapy is given to chronic hepatitis C patients with prior erosive OLP.

Aromatic amino acid aminotransferase activity and indole-3-acetic acid production by associative nitrogen-fixing bacteria

In this work, we report the detection of aromatic amino acid aminotransferase (AAT) activity from cell-free crude extracts of nine strains of N(2)-fixing bacteria from three genera. Using tyrosine as substrate, AAT activity ranged in specific activity from 0.084 to 0.404 micromol min(-1)mg(-1). When analyzed under non-denaturating PAGE conditions; and using tryptophan, phenylalanine, tyrosine, and histidine as substrates Pseudomonas stutzeri A15 showed three isoforms with molecular mass of 46, 68 and 86 kDa, respectively; Azospirillum strains displayed two isoforms which molecular mass ranged from 44 to 66 kDa and Gluconacetobacter strains revealed one enzyme, which molecular mass was estimated to be much more higher than those of Azospirillum and P. stutzeri strains. After SDS-PAGE, some AAT activity was lost, indicating a differential stability of proteins. All the strains tested produced IAA, especially with tryptophan as precursor. Azospirillum strains produced the highest concentrations of IAA (16.5-38 microg IAA/mg protein), whereas Gluconacetobacter and P. stutzeri strains produced lower concentrations of IAA ranging from 1 to 2.9 microg/mg protein in culture medium supplemented with tryptophan. The IAA production may enable bacteria promote a growth-promoting effect in plants, in addition to their nitrogen fixing ability.

Anti-oxidant activities of fucosterol from the marine algae Pelvetia siliquosa

The anti-oxidant activities of fucosterol isolated from the marine algae Pelvetia siliquosa were investigated. Fucosterol exhibited a significant decrease in serum transaminase activities elevated by hepatic damage induced by CCl4-intoxication in rats. Fucosterol inhibited the sGOT and sGPT activities by 25.57 and 63.16%, respectively. Fucosterol showed the increase in the anti-oxidant enzymes such as hepatic cytosolic superoxide dismutase (SOD), catalase and glutathione peroxidase (GSH-px) activities by 33.89, 21.56 and 39.24%, respectively, in CCl4-intoxicated rats. These results suggest that fucosterol possess not only the anti-oxidant, but also the hepatoprotective activities in rats.

Mss51p promotes mitochondrial Cox1p synthesis and interacts with newly synthesized Cox1p

The post-transcriptional role of Mss51p in mitochondrial gene expression is of great interest since MSS51 mutations suppress the respiratory defect caused by shy1 mutations. SHY1 is a Saccharomyces cerevisiae homolog of human SURF1, which when mutated causes a cytochrome oxidase assembly defect. We found that MSS51 is required for expression of the mitochondrial reporter gene ARG8(m) when it is inserted at the COX1 locus, but not when it is at COX2 or COX3. Unlike the COX1 mRNA-specific translational activator PET309, MSS51 has at least two targets in COX1 mRNA. MSS51 acts in the untranslated regions of the COX1 mRNA, since it was required to synthesize Arg8p when ARG8(m) completely replaced the COX1 codons. MSS51 also acts on a target specified by the COX1 coding region, since it was required to translate either COX1 or COX1:: ARG8(m) coding sequences from an ectopic COX2 locus. Mss51p was found to interact physically with newly synthesized Cox1p, suggesting that it could coordinate Cox1p synthesis with insertion into the inner membrane or cytochrome oxidase assembly.

Expression, purification, crystallization and preliminary crystallographic analysis of phosphoserine aminotransferase from Bacillus alcalophilus

Phosphoserine aminotransferase (PSAT; EC 2.6.1.52) from Bacillus alcalophilus, an obligatory alkalophile with optimum growth at pH 10.6, was overexpressed in Escherichia coli, purified and crystallized under two different conditions using the hanging-drop vapour-diffusion method. Crystals were obtained using trisodium citrate dihydrate or PEG 400 as a precipitating agent. Crystals grown in the presence of trisodium citrate belong to the orthorhombic space group C222(1), with unit-cell parameters a = 105.6, b = 136.6, c = 152.0 A, and those grown in the presence of PEG 400 belong to the orthorhombic space group P2(1)2(1)2, with unit-cell parameters a = 143.7, b = 84.3, c = 67.4 A. Complete data sets were collected to 1.7 and 1.6 A resolution, respectively, at 100 K using synchrotron radiation. Analysis of the structure of B. alcalophilus PSAT may reveal structural features that contribute to enzyme adaptability at high pH values.

L-cysteine sulphinate, endogenous sulphur-containing amino acid, inhibits rat brain kynurenic acid production via selective interference with kynurenine aminotransferase II

In the present study the effect of endogenous sulphur-containing amino acids, L-cysteine sulphinate, L-cysteate, L-homocysteine sulphinate and L-homocysteate, on the production of glutamate receptor antagonist, kynurenic acid (KYNA), was evaluated. The experiments comprised the measurements of (a). KYNA synthesis in rat cortical slices and (b). the activity of KYNA biosynthetic enzymes, kynurenine aminotransferases (KATs). All studied compounds reduced KYNA production and inhibited the activity of KAT I and/or KAT II, thus acting most probably intracellularly. L-Cysteine sulphinate in very low, micromolar concentrations selectively affected the activity of KAT II, the enzyme catalyzing approximately 75% of KYNA synthesis in the brain. L-Cysteine sulphinate potency was higher than other studied sulphur-containing amino acids, than L-aspartate, L-glutamate, or any other known KAT II inhibitor. Thus, L-cysteine sulphinate might act as a modulator of KYNA formation in the brain.

Tissue expression and translational control of rat kynurenine aminotransferase/glutamine transaminase K mRNAs

Kynurenic acid (KA) is an endogenous glutamate receptor antagonist at the level of the different ionotropic glutamate receptors. One of the enzymes responsible for the production of KA, kynurenine aminotransferase I (KATI), also catalyses the reversible transamination of glutamine to oxoglutaramic acid (GTK, EC 2.6.1.15). The enzyme exists in a cytosolic and in a mitochondrial form because of the presence of two different KATI mRNAs coding for a protein respectively with and without leader sequence targeting the protein into mitochondria. We have cloned from a phage library of rat kidney cDNA four new KATI cDNAs containing different 5' untranslated regions (UTRs). One of the transcripts (+14KATI cDNA) contains an alternative site of initiation of translation. The tissue distribution of the different transcripts was studied by RT-PCR. The study demonstrated that several KATI mRNAs are constitutively expressed in ubiquitous manner, while +14KATI mRNA is present only in kidney. The translational efficiency of the different transcripts was studied in vitro and enzymatic activities were measured in transiently transfected Cos-1 cells. Each KATI mRNA exhibits a different in vitro translational efficiency, which corresponds to different levels of KAT enzymatic activity in transfected cells. Both findings correlate with the predicted accessibility of the ribosomal binding sites of the different mRNAs. The structure of the rat KATI/GTK gene was also studied. The expression of several KATI mRNAs with different 5'UTRs represents an interesting example of transcriptional/translational control on the expression of pyridoxal phosphate (PLP)-dependent aminotransferases.

Simultaneous synthesis of enantiomerically pure (R)-1-phenylethanol and (R)-alpha-methylbenzylamine from racemic alpha-methylbenzylamine using omega-transaminase/alcohol dehydrogenase/glucose dehydrogenase coupling reaction

A simultaneous synthesis of (R)-1-phenylethanol and (R)-alpha-methylbenzylamine from racemic alpha-methylbenzylamine was achieved using an omega-transaminase, alcohol dehydrogenase, and glucose dehydrogenase in a coupled reaction. Racemic alpha-methylbenzylamine (100 mM) was converted to 49 mM (R)-1-phenylethanol (> 99% ee) and 48 mM (R)-alpha-methylbenzylamine (> 98% ee) in 18 h at 37 degrees C. This method was also used to overcome product inhibition of omega-TA by the ketone product in the kinetic resolution of racemic amines at high concentration.

Expression of kynurenine aminotransferase in the subplate of the rat and its possible role in the regulation of programmed cell death

The neurons of the transient subplate zone, considered important for the prenatal development of the cerebral cortex, were shown here to express kynurenine aminotransferase (KAT)-I from embryonic day (E) 16 until postnatal day (P) 7 in the rat. No other cells of brain tissue exerted KAT-I immunoreactivity during this period. From P3 on, the neurons of the subplate gave rise to KAT-I immunoreactive, varicose axons, which entered the thalamus and terminated around thalamic nerve cells that are devoid of KAT-I immunoreactivity. Other subplate markers displayed a different expression pattern during development. Thus, subplate neurons displayed parvalbumin (PV) immuno-reactivity from E16 to P10 and an intense NPY immunoreaction from P7 to P1. They also exhibited nitric oxide synthase immunoreactivity from E16 to P10, whereas on the surface of the subplate neurons, the alpha7 subunit of the nicotinic acetylcholine receptor (nAChR) was present from P1 to P10. The cells of Cajal-Retzius were nAChR-immunoreactive during this period. Between P1 and P7, the perikarya of subplate neurons also showed an intense immuno-reaction with the N-methyl-D-aspartate (NMDA) receptor subtype R2A. After the first postnatal week, many of the KAT-I positive subplate neurons display a gradual decrease of immunoreactivity and undergo programmed cell death. Since KAT-I persists in the subplate through the period E16-P7, we conclude that KAT-I is a useful and reliable subplate marker in the rat. Since it is assumed that migration of nerve cells is regulated by NMDA receptors, and since kynurenic acid--the only naturally occurring NMDA receptor antagonist--is synthesized by KAT, we suggest that a temporary breakdown of the delicate equilibrium between NMDA and KAT might induce abnormal neuronal migration, giving rise to developmental abnormalities.

1-Methyl-4-phenylpyridinium and 3-nitropropionic acid diminish cortical synthesis of kynurenic acid via interference with kynurenine aminotransferases in rats

The aim of the present study was to evaluate the effect of mitochondrial inhibitors, 1-methyl-4-phenylpyridinium (MPP(+)) and 3-nitropropionic acid (3-NPA), on the brain production of endogenous glutamate antagonist, kynurenic acid (KYNA). MPP(+) and 3-NPA dose-dependently impaired the synthesis of KYNA in rat cortical slices. Enzymatic studies revealed that MPP(+) inhibits in a concentration-dependent manner the activity of kynurenine aminotransferase II (KAT II), but not the activity of kynurenine aminotransferase I (KAT I). 3-NPA impaired the activity of both enzymes, KAT I and KAT II. Thus, MPP(+)- and 3-NPA-evoked neurotoxicity may be at least partially associated with the depletion of KYNA.

The 2-aminoethylphosphonate-specific transaminase of the 2-aminoethylphosphonate degradation pathway

The 2-aminoethylphosphonate transaminase (AEPT; the phnW gene product) of the Salmonella enterica serovar Typhimurium 2-aminoethylphosphonate (AEP) degradation pathway catalyzes the reversible reaction of AEP and pyruvate to form phosphonoacetaldehyde (P-Ald) and L-alanine (L-Ala). Here, we describe the purification and characterization of recombinant AEPT. pH rate profiles (log V(m) and log V(m)/K(m) versus pH) revealed a pH optimum of 8.5. At pH 8.5, K(eq) is equal to 0.5 and the k(cat) values of the forward and reverse reactions are 7 and 9 s(-1), respectively. The K(m) for AEP is 1.11 +/- 0.03 mM; for pyruvate it is 0.15 +/- 0.02 mM, for P-Ald it is 0.09 +/- 0.01 mM, and for L-Ala it is 1.4 +/- 0.03 mM. Substrate specificity tests revealed a high degree of discrimination, indicating a singular physiological role for the transaminase in AEP degradation. The 40-kDa subunit of the homodimeric enzyme is homologous to other members of the pyridoxalphosphate-dependent amino acid transaminase superfamily. Catalytic residues conserved within well-characterized members are also conserved within the seven known AEPT sequences. Site-directed mutagenesis demonstrated the importance of three selected residues (Asp168, Lys194, and Arg340) in AEPT catalysis.

Induction of expression of branched-chain aminotransferase and alpha-keto acid dehydrogenase in rat tissues during lactation

This study was designed to determine the effect of lactation and weaning on the gene expression of branched-chain aminotransaminase (BCAT) and branched-chain alpha-keto acid dehydrogenase (BCKD) in different tissues of the lactating rat. BCAT activity increased in mammary tissue during lactation and was 6-fold higher than in virgin rats. This increase was associated with an increase in protein levels measured by immunoblot analysis, and with an increase in BCAT mitochondrial (BCATm) mRNA concentration. Twenty-four hours after weaning, BCAT activity, protein concentration, and mRNA levels in the dam decreased. BCAT activity, protein enzyme levels, and BCATm mRNA concentration in muscle were higher in weaning rats than in lactating rats. BCAT cytosolic (BCATc) mRNA was not expressed in mammary tissue, and there was no BCATc enzyme detected by Western blot in any physiological state. Mammary tissue BCKD activity increased and was active (dephosphorylated) during the lactation period. The level of enzyme also increased and the mRNA level for the E2 subunit in mammary tissue was 10-fold higher than the virgin values. Hepatic enzyme activity increased during weaning, and this was associated with the protein level and with the mRNA level of the E2 subunit. Muscle BCKD activity and protein content were the lowest of all tissues, and the E2 subunit mRNA level was barely detected by Northern blot analysis. The results suggest gene regulation of the two main catabolic enzymes of the branched-chain amino acid metabolism during lactation.

Substrate inhibition mode of omega-transaminase from Vibrio fluvialis JS17 is dependent on the chirality of substrate

Substrate inhibition is a common phenomenon in enzyme chemistry, which is observed only with a fast-reacting substrate enantiomer. We report here for the first time substrate inhibition of an enantioselective enzyme by both substrate enantiomers. The enantioselective substrate inhibition, i.e., different mode of inhibition by each substrate enantiomer, of (S)-specific omega-transaminase was found with various chiral amines. A kinetic model based on ping-pong bi-bi mechanism has been developed and kinetic parameters were measured. The kinetic model reveals that the inhibition by (R)-amine results from formation of Michaelis complex with enzyme-pyridoxal 5'-phosphate, whereas the inhibition by (S)-amine results from the formation of the complex with enzyme-pyridoxamine 5'-phosphate. Substrate inhibition constants (K(SI)) of each (S)-enantiomer of four chiral amines showed a linear correlation with those of cognate (R)-amines. Such a correlation was also found between the K(SI) values and Michaelis constants of (S)-amines. These correlations indicate that recognition mechanisms and active site structures of both enzyme-pyridoxal 5'-phosphate, enzyme-pyridoxamine 5'-phosphate are similar. Taken together with the results, high propensity for non-productive substrate binding strongly suggests that binding pockets of the omega-transaminase is loosely defined, which accounts for the enantioselective substrate inhibition.

Effect of alcohol withdrawal on liver transaminase levels and markers of liver fibrosis

BACKGROUND AND AIM

Acute alcohol withdrawal causes changes in hepatic blood flow and metabolism that may result in liver damage. This study aims to assess liver function tests and markers of hepatic fibrogenesis following alcohol withdrawal in alcoholics with clinically compensated liver disease.

METHODS

Serial liver function tests and clinical assessments were performed on 22 male alcoholics during alcohol withdrawal. Plasma tissue inhibitor of metalloproteinase 1 (TIMP1), an inhibitor of collagen degradation, and plasma amino-terminal procollagen III peptide (PIIINP), a collagen precursor molecule, were measured in these alcoholics and in 11 control subjects.

RESULTS

Transaminase levels did not change significantly over 7 days when all subjects were analyzed together. However, 32% of subjects showed a marked transaminase rise. These subjects did not differ from the others in baseline characteristics or short-term outcome, but had a greater benzodiazepine requirement. Only one subject consumed paracetamol (acetaminophen; 1-2 g/day). He had the largest transaminase rise. By comparing PIIINP assays, intact PIIINP concentration appears to increase following alcohol withdrawal. The TIMP1 levels were elevated in alcoholic subjects, but did not change following withdrawal.

CONCLUSIONS

Increasing PIIINP suggests that hepatic fibrogenesis increases, or hepatic clearance falls, during acute alcohol withdrawal. The TIMP1 elevation in these alcoholics suggests that the inhibition of collagen degradation occurs while liver disease is still compensated. The period following alcohol withdrawal may be a time of marked increased susceptibility to paracetamol. The biochemical changes we observed were not associated with adverse short-term outcome, but the cumulative effect after repeated episodes of abrupt withdrawal may be of concern.

Expression and identification of the RfbE protein from Vibrio cholerae O1 and its use for the enzymatic synthesis of GDP-D-perosamine

The 4-amino-6-deoxy-monosaccharide D-perosamine is an important element in the glycosylation of interesting cell products, such as antibiotics and lipopolysaccharides (LPS) of Gram-positive and Gram-negative bacteria. The biosynthetic pathway of the precursor molecule, GDP-D-perosamine, in Vibrio cholerae O1 starts with an isomerisation of fructose-6-phosphate catalyzed by the bifunctional enzyme phosphomannose isomerase-guanosine diphosphomannose pyrophosphorylase (RfbA; E.C. 2.7.7.22) creating the intermediate mannose-6-phosphate, which is subsequently converted by the phosphomanno-mutase (RfbB; E.C. 5.4.2.8) and further by RfbA to GDP-D-mannose, to GDP-4 keto-6-deoxymannose by a 4,6-dehydratase (RfbD; E.C. 4.2.1.47) and finally to GDP-D-perosamine by an aminotransferase (RfbE; E.C. not yet classified). We cloned the rfbD and the rfbE genes of V. cholerae O1 in Escherichia coli expression vectors. Both biosynthetic enzymes were overproduced in E. coli BL21 (DE3) and their activities were analyzed. The enzymatic conversion from GDP-D-mannose to GDP-D-perosamine was optimized and the final product, GDP-D-perosamine, was purified and identified by nuclear magnetic resonance, mass spectrometry, and chromatography. The catalytically active form of the GDP-4-keto-6-deoxy-D-mannose-4-aminotransferase seems to be a tetramer of 170 kDa. The His-tag RfbE fusion protein has a Km of 0.06 mM and a Vmax value of 38 nkat/mg protein for the substrate GDP-4-keto-6-deoxy-D-mannose. The Km and Vmax values for the cosubstrate L-glutamate were 0.1 mM and 42 nkat/mg protein, respectively. The intention of this work is to establish a basis for both the in vitro production of GDP-D-perosamine and for an in vivo perosaminylation system in a suitable bacterial host, preferably E. coli.

Activation of Innate Immunity in Nonhuman Primates Following Intraportal Administration of Adenoviral Vectors

The innate immune response to intraportally infused adenoviral vector was evaluated in rhesus monkeys. A first-generation adenovirus-expressing lacZ (Ad-lacZ) was administered at a dose just below that which causes severe morbidity. The response to vector was evaluated for the initial 24 h following infusion. Clinical findings during this time were primarily limited to petechiae, consistent with the development of thrombocytopenia and biochemical evidence of disseminated intravascular coagulation. Serum transaminases were elevated and a lymphopenia developed. Tracking of fluorescent-labeled vector demonstrated distribution to macrophages and dendritic cells of the spleen and Kupffer cells of the liver. A systemic release of the cytokine IL-6 occurred soon after vector infusion. Analysis of splenic cells revealed acute activation of macrophages and dendritic cells followed by massive apoptosis. Bone marrow cultures demonstrated normal erythroid and primitive progenitors with a significant decrease in myeloid progenitors. Similar findings, except the abnormality in bone marrow cultures, were observed in monkeys who received an identical dose of Ad-lacZ in which vector genes were inactivated with psoralen and UV irradiation. These data suggest that inadvertent targeting of antigen-presenting cells following intraportal infusion of vector leads to a systemic cytokine syndrome which may be triggered by the viral capsid proteins.

1,8-cineole protects against liver failure in an in-vivo murine model of endotoxemic shock

The effects of 1,8-cineole on D-galactosamine/lipopolysaccharide (GalN/LPS)-induced shock model of liver injury was investigated in mice. The co-administration of GalN (700 mg kg(-1), i.p.) and LPS (5 microg kg(-1), i.p.) greatly elevated serum concentrations of tumour necrosis factor-alpha (TNF-alpha), alanine aminotransferase and aspartate aminotransferase, and induced massive hepatic necrosis and lethality in 100% of control mice. Pretreatment with 1,8-cineole (400 mg kg(-1), p.o.) and dexamethasone (1 mg kg(-1), s.c.), 60 min before GalN/LPS, offered complete protection (100%) against the lethal shock and acute elevation in serum TNF-alpha and serum transaminases. Hepatic necrosis induced by GalN/LPS was also greatly reduced by both 1,8-cineole and dexamethasone treatment. The results indicate that 1,8-cineole protects mice against GalN/LPS-induced liver injury through the inhibition of TNF-alpha production, and suggest that 1,8-cineole may be a promising agent to combat septic-shock-associated pathologies.

Functional synergism between the most common polymorphism in human alanine:glyoxylate aminotransferase and four of the most common disease-causing mutations

The autosomal recessive disorder primary hyperoxaluria type 1 (PH1) is caused by a deficiency of the liver-specific pyridoxal-phosphate-dependent enzyme alanine:glyoxylate aminotransferase (AGT). Numerous mutations and polymorphisms in the gene encoding AGT have been identified, but in only a few cases has the causal relationship between genotype and phenotype actually been demonstrated. In this study, we have determined the effects of the most common naturally occurring amino acid substitutions (both normal polymorphisms and disease-causing mutations) on the properties, especially specific catalytic activity, of purified recombinant AGT. The results presented in this paper show the following: 1) normal human His-tagged AGT can be expressed at high levels in Escherichia coli and purified in a correctly folded, dimerized and catalytically active state; 2) presence of the common P11L polymorphism decreases the specific activity of purified recombinant AGT by a factor of three; 3) AGTs containing four of the most common PH1-specific mutations (G41R, F152I, G170R, and I244T) are all soluble and catalytically active in the absence of the P11L polymorphism, but in its presence all lead to protein destabilization and aggregation into inclusion bodies; 4) naturally occurring and artificial amino acid substitutions that lead to peroxisome-to-mitochondrion AGT mistargeting in mammalian cells also lead to destabilization and aggregation in E. coli; and 5) the PH1-specific G82E mutation abolishes AGT catalytic activity by interfering with cofactor binding, as does the artificial K209R mutation at the putative site of cofactor Shiff base formation. These results are discussed in the light of the high allelic frequency ( approximately 20%) of the P11L polymorphism and its importance in determining the phenotypic manifestations of mutations in PH1.

In vivo analysis of mutated initiation codons in the mitochondrial COX2 gene of Saccharomyces cerevisiae fused to the reporter gene ARG8m reveals lack of downstream reinitiation

To examine normal and aberrant translation initiation in Saccharomyces cerevisiae mitochondria, we fused the synthetic mitochondrial reporter gene ARG8m to codon 91 of the COX2 coding sequence and inserted the chimeric gene into mitochondrial DNA (mtDNA). Translation of the cox2(1-91)::ARG8m mRNA yielded a fusion protein precursor that was processed to yield wild-type Arg8p. Thus mitochondrial translation could be monitored by the ability of mutant chimeric genes to complement a nuclear arg8 mutation. As expected, translation of the cox2(1-91)::ARG8m mRNA was dependent on the COX2 mRNA-specific activator PET111. We tested the ability of six triplets to function as initiation codons in both the cox2(1-91)::ARG8m reporter mRNA and the otherwise wild-type COX2 mRNA. Substitution of AUC, CCC or AAA for the initiation codon abolished detectable translation of both mRNAs, even when PET111 activity was increased. The failure of these mutant cox2(1-91)::ARG8m genes to yield Arg8p demonstrates that initiation at downstream AUG codons, such as COX2 codon 14, does not occur even when normal initiation is blocked. Three mutant triplets at the site of the initiation codon supported detectable translation, with efficiencies decreasing in the order GUG, AUU, AUA. Increased PET111 activity enhanced initiation at AUU and AUA codons. Comparisons of expression, at the level of accumulated product, of cox2(1-91)::ARG8m and COX2 carrying these mutant initiation codons revealed that very low-efficiency translation can provide enough Cox2p to sustain significant respiratory growth, presumably because Cox2p is efficiently assembled into stable cytochrome oxidase complexes.

Inducing effect of diamines on transcription of the cephamycin C genes from the lat and pcbAB promoters in Nocardia lactamdurans

The diamines putrescine, cadaverine, and diaminopropane stimulate cephamycin biosynthesis in Nocardia lactamdurans, in shake flasks and fermentors, without altering cell growth. Intracellular levels of the P7 protein (a component of the methoxylation system involved in cephamycin biosynthesis) were increased by diaminopropane, as shown by immunoblotting studies. Lysine-6-aminotransferase and piperideine-6-carboxylate dehydrogenase activities involved in biosynthesis of the alpha-aminoadipic acid precursor were also greatly stimulated. The diamine stimulatory effect is exerted at the transcriptional level, as shown by low-resolution S1 protection studies. The transcript corresponding to the pcbAB gene and to a lesser extent also the lat transcript were significantly increased in diaminopropane-supplemented cultures, whereas transcription from the cefD promoter was not affected. Coupling of the lat and pcbAB promoters to the reporter xylE gene showed that expression from the lat and pcbAB promoters was increased by addition of diaminopropane in Streptomyces lividans. Intracellular accumulation of diamines in Nocardia may be a signal to trigger antibiotic production.

Fenofibrate modifies transaminase gene expression via a peroxisome proliferator activated receptor alpha-dependent pathway

Fibrates modify the expression of genes implicated in lipoprotein and fatty acid metabolism via the peroxisome proliferator-activated receptor alpha(PPARalpha), leading to reductions in serum triglycerides and cholesterol. The expression of certain genes regulated by PPARalpha have been shown to be modified in a species dependent manner. Aspartate aminotransferase (AspAT or GOT) and alanine aminotransferase (AlaAT or GPT) are enzymes involved in intermediate metabolism in all cells and in hepatic gluconeogenesis. These enzymes are also widely used as serum markers of possible tissue damage. This study investigated whether fenofibrate could modify the expression of liver AspAT and/or AlaAT and thus possibly alter transaminase levels independently of a cytotoxic effect. In human Hep G2 cells, fenofibrate increased cytosolic AspAT (cAspAT) activity by 40% and AlaAT activity by 100%, as well as both mRNAs. Nuclear run on assays showed that this effect was, at least in part, transcriptional. Increases in mRNA were also observed in human hepatocyte cultures at concentrations of the drug attained in patients. In C57BL/6 mice, fenofibrate decreased cAspAT and cAlaAT mRNA, while these effects were abolished in PPARalpha knock-out mice. In conclusion, fenofibrate has been shown to modify cAspAT and AlaAT gene expression in a species and PPARalpha dependent manner. This is the first demonstration that cAspAT and AlaAT activities may be pharmacologically altered, independently of a toxic phenomenon.

Molecular cloning and expression of a cDNA sequence encoding histidinol phosphate aminotransferase from Nicotiana tabacum

A Nicotiana tabacum cDNA sequence encoding histidinol phosphate aminotransferase (HPA) was isolated by functional complementation of an Escherichia coli histidine auxotroph (UTH780). The enzymatic assay has confirmed that the isolated cDNA encodes a functional HPA protein. Amino acid sequence alignment of the HPA protein from N. tabacum, Saccharomyces cerevisiae and E. coli revealed that, despite the low degree of identity, some residues were found to be highly conserved. The predicted protein contains a transit peptide sequence at the amino-terminal end, suggesting a chloroplastic localization of the HPA enzyme. Western blot analysis demonstrated that the deduced HPA protein and the mature HPA protein have an apparent molecular mass of about 45 kDa and 40 kDa respectively. Gene copy number estimation by Southern analysis indicates the presence of at least two genes per haploid genome coding for this protein in Nicotiana sp. From northern analysis results, the gene seems to be highly expressed in green tissues and the detected transcript showed a single band of expected molecular size.

Serum transaminase elevations as indicators of hepatic injury following the administration of drugs

During the preclinical, early clinical, late-stage clinical, and postmarketing phases of the pharmaceutical discovery and development process, one important aspect of drug safety assessment involves monitoring for possible drug-induced hepatic injury. Hepatic injuries vary in nature from direct, intrinsic effects that are observed in most recipients and more than one species to rare idiosyncratic responses seen only in a few clinical subjects. Histological types of injuries vary from hepatocellular to hepatobiliary with multiple cellular effects characteristic of each type. Of the various clinical laboratory markers for hepatic injury, serum transaminases, especially alanine aminotransferase (ALT), are the most universally important indicators for studies ranging from early preclinical animal testing to postmarketing patient monitoring. This review examines the characteristics of hepatic toxicity that result in serum ALT changes, the differences in the etiology of hepatic responses which govern when liver injury is most likely to be detected during the four phases of the drug discovery and development process, and those modulating factors which affect the utility of ALT as a dependable marker of hepatic injury in clinical populations. The paper concludes with a summary of some ancillary methods for early preclinical screening such as in vitro metabolism and toxicity assays, gene and protein expression analysis, and some strategies for enhancing the probability for the early detection of idiosyncratic hepatotoxic responses which are infrequent but significant factors in the safety assessment process.

Induction by peroxisome proliferators and triiodothyronine of serine:pyruvate/alanine:glyoxylate aminotransferase of rat liver

In rat liver, a single serine:pyruvate/alanine:glyoxylate aminotransferase (SPT or SPT/AGT) gene is transcribed from two transcription initiation sites. Transcription from the upstream site generates the mRNA encoding the precursor for mitochondrial SPT (pSPTm) and is markedly enhanced by the administration of glucagon or cAMP. In this report we show the increase in the downstream transcript, the peroxisomal SPT (SPTp) mRNA, caused by peroxisome proliferators and triiodothyronine (T3). In the case of T3, the pSPTm mRNA was also increased 72 h after a single administration of the hormone in addition to an earlier increase in SPTp mRNA.

Developmental modulation of cysteine conjugate beta-lyase/glutamine transaminase K/kynurenine aminotransferase mRNA in rat brain

Cysteine conjugate beta-lyase/glutamine transaminase K/kynurenine aminotransferase (CS-lyase/GTK/KAT) is a tri-functional enzyme found in several organs, including the brain. Kynurenine aminotransferase is important in tryptophan metabolism in the CNS, producing kynurenic acid, a NMDA receptor antagonist and neuroprotective. Tryptophan not metabolised via kynurenine aminotransferase may form quinolinic acid, a NMDA receptor agonist and neurotoxin. Kynurenic acid co-treatment blocks quinolinic acid induced lesions in the CNS in rat. In many conditions exhibiting neurodegeneration (i.e. Huntington's, Parkinsonism, Down's syndrome) quinolinic acid and/or kynurenic acid concentrations are altered, suggesting the ratio of these chemicals may be important in neurodegeneration. We have investigated the developmental modulation of CS-lyase/GTK/KAT mRNA in rat brain. CS-lyase/GTK/KAT mRNA was measured in 14, 21, 28, 35, 42 day post-natal and adult rats. While many regions demonstrated a steady increase to adult levels, two other profiles were seen. Five regions rapidly reached adult levels of the mRNA, while two peaked above the adult level before falling back. This provides evidence that expression of the CS-lyase/GTK/KAT gene is physiologically modulated, and provides the basis for further investigation into the mechanism of control. Artificial modulation could possibly be used to alter levels of the neuroprotectant kynurenic acid in neurodegeneration.

Purification and characterization of polyamine aminotransferase of Arthrobacter sp. TMP-1

Polyamine aminotransferase of Arthrobacter sp. TMP-1 was induced by 1,3-diaminopropane (DAP), N-3-aminopropyl-1,3-diaminopropane (norspermidine), spermidine, and spermine, but not by putrescine. The enzyme was purified to homogeneity. Its molecular weight and subunit size were 129,000 and 64,000, respectively. Its absorption spectrum had maxima at 280 and 420 nm and a shoulder at about 350 nm, and changes were observed upon the addition of DAP, putrescine, and sodium borohydride. The spectrum and its changes indicated that the enzyme contained pyridoxal-5'-phosphate as the coenzyme. The coenzyme content was found to be 1 mol per mol of subunit. DAP, putrescine, norspermidine, spermidine, and spermine were active amino donors and gave relative rates of 100, 73, 24, 30, and 23%, respectively. Pyruvate was the most active amino acceptor, while 2-ketoglutarate and oxaloacetate were inert. The equilibrium constant of the DAP-pyruvate transamination was 0.34. DAP was suggested to be a minor product of the norspermidine-pyruvate reaction.

Identification of stsC, the gene encoding the L-glutamine:scyllo-inosose aminotransferase from streptomycin-producing Streptomycetes

Eight new genes, strO-stsABCDEFG, were identified by sequencing DNA in the gene cluster that encodes proteins for streptomycin production of Streptomyces griseus N2-3-11. The StsA (calculated molecular mass 43.5 kDa) and StsC (45.5 kDa) proteins - together with another gene product, StrS (39.8 kDa), encoded in another operon of the same gene cluster - show significant sequence identity and are members of a new class of pyridoxal-phosphate-dependent aminotransferases that have been observed mainly in the biosynthetic pathways for secondary metabolites. The aminotransferase activity was demonstrated for the first time by identification of the overproduced and purified StsC protein as the L-glutamine:scyllo-inosose aminotransferase, which catalyzes the first amino transfer in the biosynthesis of the streptidine subunit of streptomycin. The stsC and stsA genes each hybridized specifically to distinct fragments in the genomic DNA of most actinomycetes tested that produce diaminocyclitolaminoglycosides. In contrast, only stsC, but not stsA, hybridized to the DNA of Streptomyces hygroscopicus ssp. glebosus, which produces the monoaminocyclitol antibiotic bluensomycin; this suggests that both genes are specifically used in the first and second steps of the cyclitol transamination reactions. Sequence comparison studies performed with the deduced polypeptides of the genes adjacent to stsC suggest that the enzymes encoded by some of these genes [strO (putative phosphatase gene), stsB (putative oxidoreductase gene), and stsE (putative phosphotransferase gene)] also could be involved in (di-)aminocyclitol synthesis.

Kynurenic acid and kynurenine aminotransferase in heart

Kynurenic acid (KYNA) is a tryptophan metabolite and represents the only known endogenous compound acting as an antagonist to excitatory amino acid receptors in the mammalian CNS. Blocking of these receptors in CNS by KYNA affects cardiac function. As it is not known whether human heart is able to synthesize this neuromodulatory amino acid, we investigated the biosynthesizing enzyme of kynurenine aminotransferase (KAT) in the human heart and compared the activity with that of the human brain. The activities of heart and brain KATs were assayed by the conversion of L-kynurenine (L-KYN) to KYNA and quantitated by HPLC with fluorescence detection. Using either pyruvate or 2-oxoglutarate as cosubstrates, heart KAT was found to have a shallow pH optimum between 8 and 9. Highest heart KAT activity was seen in the presence of 2-oxoglutarate, followed by pyruvate. 2-oxoadipate, and 2-oxoisocaproate. Kinetic analyses, performed at pH 8.5, and using various concentrations of L-KYN (from 0.125 to 22.8 mM) in the presence of 2-oxoglutarate (1 and 5 mM) or pyruvate (5 mM) revealed apparent K(m) values in the millimolar range, for L-KYN 1.5, 27, and 20 mM, respectively. Heart KAT activities were compared with those in human brain KAT I and KAT II showing different pH optima 7.4 and 9.6, respectively. In contrast to brain KAT I, heart KAT activity was not inhibited by an excess of 2 mM L-tryptophan, L-glutamine, or L-phenylalanine at pH 9.6, as well as at pH 8 or 7.4. Our study demonstrates that human heart is capable of synthesizing KYNA from low concentrations of L-KYN selectively. A shallow pH optimum of KAT activity, i.e. between 8.0 and 9.0, pronounced 2-oxoacid specificity, and a lack of sensitivity to inhibition by L-glutamine, L-phenylalanine, and L-tryptophan indicate that the heart KAT system displays enzymatic characteristics different from those of human brain KAT I or KAT II. Fluctuation of L-KYN and 2-oxoacid levels may markedly influence the KYNA synthesis and subsequent KYNA effect on cardiac activity. KYNA synthesis in the human heart suggests a neurophysiologic role. Our studies from the basis for purification and further characterization of KAT protein in human heart as well as for physiologic studies.

Nucleotide sequence of the Bacillus licheniformis ATCC 10716 dat gene and comparison of the predicted amino acid sequence with those of other bacterial species

The gene encoding the D-aminotransferase from Bacillus licheniformis was cloned and the complete DNA sequence was determined. The deduced D-aminotransferase protein sequence, consists of 283 amino acids and shows a high degree of homology with other Bacillus D-aminotransferases, branched chain aminotransferase of Escherichia coli and the 4-amino-benzoate-4-deoxychorismate lyase of Bacillus subtilis and Escherichia coli.

Molecular cloning, characterization and expression of cDNA encoding phosphoserine aminotransferase involved in phosphorylated pathway of serine biosynthesis from spinach

Phosphoserine aminotransferase (PSA) catalyzes the conversion of phosphohydroxypyruvate to phosphoserine in the phosphorylated pathway of serine biosynthesis. A cDNA clone encoding PSA was isolated from the cDNA library of spinach (Spinacia oleracea L.) green leaves. Determination of the nucleotide sequence revealed the presence of an open reading frame encoding 430 amino acids, exhibiting 38-50% homology with the amino acid sequences of bacterial, yeast and animal PSA. It contains an N-terminal extension of ca. 60 amino acids in addition to the sequences from other organisms. The general features of plastidic transit peptide are observed in this N-terminal sequence, suggesting the plastid localization of the PSA protein encoded by this cDNA. The bacterial expression of the cDNA could functionally rescue the auxotrophy of serine in the serC- mutant, Escherichia coli KL282. The enzymatic activity of PSA was demonstrated in vitro in the extracts of E. coli over-expressing the cDNA. Southern blot analysis indicated the presence of a couple of related genes (Psa) in the spinach genome. RNA blot hybridization suggested the preferential expression of the Psa gene in the roots of green seedlings and in the suspension cells cultured under a dark condition.

Induction of L-lysine epsilon-aminotransferase by L-lysine in Streptomyces clavuligerus, producer of cephalosporins

L-Lysine epsilon-aminotransferase (LAT) catalyzes the first reaction in the two-step conversion of L-lysine (Lys) to 1-alpha-aminoadipic acid (Aaa), a direct precursor of cephalosporins (including cephamycin C) in Streptomyces clavuligerus. Previous work showed that addition of Lys to chemically defined medium improved antibiotic production. We show that in S. clavuligerus cultures supplemented with high concentrations of Lys, Lys enhances antibiotic production by a dual effect, i.e. as a substrate of LAT thus providing Aaa and also as an inducer of LAT yielding even more Aaa. On the other hand, LAT is not induced by Aaa.

Expression of a recoded nuclear gene inserted into yeast mitochondrial DNA is limited by mRNA-specific translational activation

Genetic code differences prevent expression of nuclear genes within Saccharomyces cerevisiae mitochondria. To bridge this gap a synthetic gene, ARG8m, designed to specify an arginine biosynthetic enzyme when expressed inside mitochondria, has been inserted into yeast mtDNA in place of the COX3 structural gene. This mitochondrial cox3::ARG8m gene fully complements a nuclear arg8 deletion at the level of cell growth, and it is dependent for expression upon nuclear genes that encode subunits of the COX3 mRNA-specific translational activator. Thus, cox3::ARG8m serves as a mitochondrial reporter gene. Measurement of cox3::ARG8m expression at the levels of steady-state protein and enzymatic activity reveals that glucose repression operates within mitochondria. The levels of this reporter vary among strains whose nuclear genotypes lead to under- and overexpression of translational activator subunits, in particular Pet494p, indicating that mRNA-specific translational activation is a rate-limiting step in this organellar system. Whereas the steady-state level of cox3::ARG8m mRNA was also glucose repressed in an otherwise wild-type strain, absence of translational activation led to essentially repressed mRNA levels even under derepressing growth conditions. Thus, the mRNA is stabilized by translational activation, and variation in its level may be largely due to modulation of translation.

Metabolic engineering of cephalosporin biosynthesis in Streptomyces clavuligerus

The biosynthesis of beta-lactams is one of the most thoroughly studied antibiotic pathways. The availability of the characteristics and the time profiles of activities of enzymes involved in the biosynthesis allows one to critically evaluate the potential rate-limiting steps in its production. Our approach to understanding the control of beta-lactam biosynthesis has been pursued using a two-stage strategy: (1) to predict the rate-limiting steps using a kinetic model and (2) to relax the rate-limiting steps by engineering the biosynthetic pathway or by altering the kinetic parameters of the predicted key rate-limiting enzyme. Kinetic analysis of the pathway dynamics of cephamycin C production in Streptomyces clavuligerus was performed using data obtained from wild type. Sensitivity analysis revealed that the availability of precursor alpha-aminoadipic acid and activity of ACV synthetase were the potential rate-limiting steps. Relaxation of the precursor limitation was accomplished by integration of an additional copy of the gene encoding lysine-epsilon-aminotransferase (lat) into the chromosome. The recombinant strain showed an increased level of cephamycin C production as expected. The intracellular levels of different intermediates in the pathway in batch cultures were analyzed.