Covalently immobilize crude d-amino acid transaminase onto UiO-66-NH2 surface for d-Ala biosynthesis

Enzyme reaction has been accepted widely in numerous applications owing to the high efficiency and stereo-selectivity, as well as simple preparation by gene engineering. However, the fragility and complex purification process of the enzyme are long-standing problems which limit the large-scale application. One possible solution may be the enzyme immobilization. As one type of porous material with high loading capacity and designable functionality, Metal-Organic Frameworks (MOFs) are ideal choices for the immobilization of enzyme with a considerable interest in recent years. In this study, d-amino acid transaminase (DAT), an important enzyme for industrial synthesis of d-Ala, was covalently immobilized on the surface of a star MOFs material, UiO-66-NH_2. Interestingly, we found that the nanoscale hybrid enzyme UiO-66-NH₂-Gd-DAT not only maintained the high catalytic efficiency but also got rid of the interference of polluting enzymes, which meant that we could obtain efficient and stereo-selective immobilized enzyme without complex purification process. In general, our findings demonstrated that using UiO-66-NH₂ might be a promising strategy to immobilize enzyme and produce effective biocatalyst with high activity and stereo-selectivity.

The role of alanine synthesis and nitrate-induced nitric oxide production during hypoxia stress in Cucurbita pepo nectaries

Floral nectar is a sugary solution produced by nectaries to attract and reward pollinators. Nectar metabolites, such as sugars, are synthesized within the nectary during secretion from both pre-stored and direct phloem-derived precursors. In addition to sugars, nectars contain nitrogenous compounds such as amino acids; however, little is known about the role(s) of nitrogen (N) compounds in nectary function. In this study, we investigated N metabolism in Cucurbita pepo (squash) floral nectaries in order to understand how various N-containing compounds are produced and determine the role of N metabolism in nectar secretion. The expression and activity of key enzymes involved in primary N assimilation, including nitrate reductase (NR) and alanine aminotransferase (AlaAT), were induced during secretion in C. pepo nectaries. Alanine (Ala) accumulated to about 35% of total amino acids in nectaries and nectar during peak secretion; however, alteration of vascular nitrate supply had no impact on Ala accumulation during secretion, suggesting that nectar(y) amino acids are produced by precursors other than nitrate. In addition, nitric oxide (NO) is produced from nitrate and nitrite, at least partially by NR, in nectaries and nectar. Hypoxia-related processes are induced in nectaries during secretion, including lactic acid and ethanolic fermentation. Finally, treatments that alter nitrate supply affect levels of hypoxic metabolites, nectar volume and nectar sugar composition. The induction of N metabolism in C. pepo nectaries thus plays an important role in the synthesis and secretion of nectar sugar.

Self-Directed in Cell Production of Methionine Analogue Azidohomoalanine by Synthetic Metabolism and Its Incorporation into Model Proteins

Common protocols for the incorporation of noncanonical amino acids (ncAAs) into proteins require addition of the desired ncAA to the growth medium, its cellular uptake, and subsequent intracellular accumulation. This feeding scheme is generally suitable for small-scale proof-of-concept incorporation experiments. However, it is no general solution for orthogonal translation of ncAAs, as their chemical synthesis is generally tedious and expensive. Here, we describe a simple protocol that efficiently couples in situ semi-synthetic biosynthesis of L-azidohomoalanine and its incorporation into proteins at L-methionine (Met) positions. In our metabolically engineered Met-auxotrophic Escherichia coli strain, Aha is biosynthesized from externally added sodium azide and O-acetyl-L-homoserine as inexpensive precursors. This represents an efficient platform for expression of azide-containing proteins suitable for site-selective bioorthogonal strategies aimed at noninvasive protein modifications (Tornøe et al., J Org Chem 67:3057-3064, 2002; Kiick et al., Angew Chem Int Ed 39:2148-2152, 2000; Budisa, Angew Chem Int Ed Engl 47:6426-6463, 2004; van Hest, J Am Chem Soc 122:1282-1288, 2000).

Formylglycine, a post-translationally generated residue with unique catalytic capabilities and biotechnology applications

Formylglycine (fGly) is a catalytically essential residue found almost exclusively in the active sites of type I sulfatases. Formed by post-translational oxidation of cysteine or serine side chains, this aldehyde-functionalized residue participates in a unique and highly efficient catalytic mechanism for sulfate ester hydrolysis. The enzymes that produce fGly, formylglycine-generating enzyme (FGE) and anaerobic sulfatase-maturating enzyme (anSME), are as unique and specialized as fGly itself. FGE especially is structurally and mechanistically distinct, and serves the sole function of activating type I sulfatase targets. This review summarizes the current state of knowledge regarding the mechanism by which fGly contributes to sulfate ester hydrolysis, the molecular details of fGly biogenesis by FGE and anSME, and finally, recent biotechnology applications of fGly beyond its natural catalytic function.

Catabolism of exogenous lactate reveals it as a legitimate metabolic substrate in breast cancer

Lactate accumulation in tumors has been associated with metastases and poor overall survival in cancer patients. Lactate promotes angiogenesis and metastasis, providing rationale for understanding how it is processed by cells. The concentration of lactate in tumors is a balance between the amount produced, amount carried away by vasculature and if/how it is catabolized by aerobic tumor or stromal cells. We examined lactate metabolism in human normal and breast tumor cell lines and rat breast cancer: 1. at relevant concentrations, 2. under aerobic vs. hypoxic conditions, 3. under conditions of normo vs. hypoglucosis. We also compared the avidity of tumors for lactate vs. glucose and identified key lactate catabolites to reveal how breast cancer cells process it. Lactate was non-toxic at clinically relevant concentrations. It was taken up and catabolized to alanine and glutamate by all cell lines. Kinetic uptake rates of lactate in vivo surpassed that of glucose in R3230Ac mammary carcinomas. The uptake appeared specific to aerobic tumor regions, consistent with the proposed "metabolic symbiont" model; here lactate produced by hypoxic cells is used by aerobic cells. We investigated whether treatment with alpha-cyano-4-hydroxycinnamate (CHC), a MCT1 inhibitor, would kill cells in the presence of high lactate. Both 0.1 mM and 5 mM CHC prevented lactate uptake in R3230Ac cells at lactate concentrations at ≤ 20 mM but not at 40 mM. 0.1 mM CHC was well-tolerated by R3230Ac and MCF7 cells, but 5 mM CHC killed both cell lines ± lactate, indicating off-target effects. This study showed that breast cancer cells tolerate and use lactate at clinically relevant concentrations in vitro (± glucose) and in vivo. We provided additional support for the metabolic symbiont model and discovered that breast cells prevailingly take up and catabolize lactate, providing rationale for future studies on manipulation of lactate catabolism pathways for therapy.

Dipeptidyl peptidase IV activity in commercial solutions of human serum albumin

Due to the heterogeneous nature of commercial human serum albumin (cHSA), other components, such as the protease dipeptidyl peptidase IV (DPP-IV), possibly contribute to the therapeutic effect of cHSA. Here, we provide evidence for the first time that DPP-IV activity contributes to the formation of aspartate-alanine diketopiperazine (DA-DKP), a known immunomodulatory molecule from the N terminus of human albumin. cHSA was assayed for DPP-IV activity using a specific DPP-IV substrate and inhibitor. DPP-IV activity was assayed at 37 and 60°C because cHSA solutions are pasteurized at 60°C. DPP-IV activity in cHSA was compared with other sources of albumin such as a recombinant albumin (rHSA). In addition, the production of DA-DKP was measured by negative electrospray ionization/liquid chromatography mass spectrometry (ESI(-)/LCMS). Significant levels of DPP-IV activity were present in cHSA. This activity was abolished using a specific DPP-IV inhibitor. Fully 70 to 80% DPP-IV activity remained at 60°C compared with the 37°C incubate. No DPP-IV activity was present in rHSA, suggesting that DPP-IV activity is present only in HSA produced using the Cohn fractionation process. The formation of DA-DKP at 60°C was observed with the DPP-IV inhibitor significantly decreasing this formation. DPP-IV activity in cHSA results in the production of DA-DKP, which could account for some of the clinical effects of cHSA.

Knockout of SOD1 promotes conversion of selenocysteine to dehydroalanine in murine hepatic GPX1 protein

Se-dependent glutathione peroxidase-1 (GPX1) and Cu,Zn-superoxide dismutase (SOD1) are two major intracellular antioxidant enzymes. The purpose of this study was to elucidate the biochemical mechanisms for the 40% loss of hepatic GPX1 activity in SOD1(-/-) mice. Compared with the wild type (WT), the SOD1(-/-) mice showed no change in the total amount of GPX1 protein. However, their total enzyme protein exhibited 31 and 38% decreases (P<0.05) in the apparent k(cat) for hydrogen peroxide and tert-butylperoxide (at 2mM GSH), respectively. Most striking, mass spectrometry revealed two chemical forms of the 47th residue of GPX1: the projected native selenocysteine (Sec) and the Se-lacking dehydroalanine (DHA). The hepatic GPX1 protein of the SOD1(-/-) mice contained 38% less Sec and 77% more DHA than that of WT and showed aggravated dissociation of the tetramer structure. In conclusion, knockout of SOD1 elevated the conversion of Sec to DHA in the active site of hepatic GPX1, leading to proportional decreases in the apparent k(cat) and activity of the enzyme protein as a whole. Our data reveal a structural and kinetic mechanism for the in vivo functional dependence of GPX1 on SOD1 in mammals and provide a novel mass spectrometric method for the assay of oxidative modification of the GPX1 protein.

Genetics and regulation of the major enzymes of alanine synthesis in Escherichia coli

Genetic analysis of alanine synthesis in the model genetic organism Escherichia coli has implicated avtA, the still uncharacterized alaA and alaB genes, and probably other genes. We identified alaA as yfbQ. We then transferred mutations in several transaminase genes into a yfbQ mutant and isolated a mutant that required alanine for optimal growth. For cells grown with carbon sources other than pyruvate, the major alanine-synthesizing transaminases are AvtA, YfbQ (AlaA), and YfdZ (which we designate AlaC). Growth with pyruvate as the carbon source and multicopy suppression suggest that several other transaminases can contribute to alanine synthesis. Expression studies showed that alanine modestly repressed avtA and yfbQ but had no effect on yfdZ. The leucine-responsive regulatory protein (Lrp) mediated control by alanine. We purified YfbQ and YfdZ and showed that both are dimers with K(m)s for pyruvate within the intracellular range of pyruvate concentration.

Glycolysis and the tricarboxylic acid cycle are linked by alanine aminotransferase during hypoxia induced by waterlogging of Lotus japonicus

The role of nitrogen metabolism in the survival of prolonged periods of waterlogging was investigated in highly flood-tolerant, nodulated Lotus japonicus plants. Alanine production revealed to be a critical hypoxic pathway. Alanine is the only amino acid whose biosynthesis is not inhibited by nitrogen deficiency resulting from RNA interference silencing of nodular leghemoglobin. The metabolic changes that were induced following waterlogging can be best explained by the activation of alanine metabolism in combination with the modular operation of a split tricarboxylic acid pathway. The sum result of this metabolic scenario is the accumulation of alanine and succinate and the production of extra ATP under hypoxia. The importance of alanine metabolism is discussed with respect to its ability to regulate the level of pyruvate, and this and all other changes are discussed in the context of current models concerning the regulation of plant metabolism.

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.

Role of Bacillus subtilis BacB in the synthesis of bacilysin

Bacilysin is a non-ribosomally synthesized dipeptide antibiotic that is active against a wide range of bacteria and some fungi. Synthesis of bacilysin (l-alanine-[2,3-epoxycyclohexano-4]-l-alanine) is achieved by proteins in the bac operon, also referred to as the bacABCDE (ywfBCDEF) gene cluster in B. subtilis. Extensive genetic analysis from several strains of B. subtilis suggests that the bacABC gene cluster encodes all the proteins that synthesize the epoxyhexanone ring of l-anticapsin. These data, however, were not consistent with the putative functional annotation for these proteins whereby BacA, a prephenate dehydratase along with a potential isomerase/guanylyl transferase, BacB and an oxidoreductase, BacC, could synthesize l-anticapsin. Here we demonstrate that BacA is a decarboxylase that acts on prephenate. Further, based on the biochemical characterization and the crystal structure of BacB, we show that BacB is an oxidase that catalyzes the synthesis of 2-oxo-3-(4-oxocyclohexa-2,5-dienyl)propanoic acid, a precursor to l-anticapsin. This protein is a bi-cupin, with two putative active sites each containing a bound metal ion. Additional electron density at the active site of the C-terminal domain of BacB could be interpreted as a bound phenylpyruvic acid. A significant decrease in the catalytic activity of a point variant of BacB with a mutation at the N-terminal domain suggests that the N-terminal cupin domain is involved in catalysis.

Transfer of15N from oral lactose-ureide to lysine in normal adults

The metabolic fate of salvaged urea-nitrogen was explored in normal adults who had consumed a diet that provided 36 g protein/day for 7 days. We hypothesised that the colonic microflora utilise nitrogen derived from urea salvage to synthesise lysine in functionally significant amounts for the host. Oral lactose-[(15)N(15)N]ureide is resistant to digestion but is fermented by the colonic microflora to release (15)NH3, which can be used for amino acid synthesis. Prime and intermittent oral doses of lactose-[(15)N(15)N]ureide were ingested for 18 h, urine was collected every 3 h and stools were collected for a further 2 days. Amino acids were isolated from urine and from faecal bacterial protein and the enrichment measured. Compared with baseline values, there was significant enrichment (atoms per cent excess) in faecal bacterial glycine (0.0526), alanine (0.117), lysine (0.0875) and histidine (0.0487), and in urinary glycine (0.016), alanine (0.0144) and lysine (0.0098), but not hisitidine. These data show that the gastrointestinal bacteria can utilise urea-nitrogen in the formation of essential and non-essential amino acids that are available to the host. We estimate that on this low protein diet the amount of lysine from bacterial synthesis and available to the host may be 30 mg/kg/day. These data have important implications for our current perceptions for the dietary requirements for essential amino acids.

Production of L -alanine by metabolically engineered Escherichia coli

Escherichia coli W was genetically engineered to produce L: -alanine as the primary fermentation product from sugars by replacing the native D: -lactate dehydrogenase of E. coli SZ194 with alanine dehydrogenase from Geobacillus stearothermophilus. As a result, the heterologous alanine dehydrogenase gene was integrated under the regulation of the native D: -lactate dehydrogenase (ldhA) promoter. This homologous promoter is growth-regulated and provides high levels of expression during anaerobic fermentation. Strain XZ111 accumulated alanine as the primary product during glucose fermentation. The methylglyoxal synthase gene (mgsA) was deleted to eliminate low levels of lactate and improve growth, and the catabolic alanine racemase gene (dadX) was deleted to minimize conversion of L: -alanine to D: -alanine. In these strains, reduced nicotinamide adenine dinucleotide oxidation during alanine biosynthesis is obligately linked to adenosine triphosphate production and cell growth. This linkage provided a basis for metabolic evolution where selection for improvements in growth coselected for increased glycolytic flux and alanine production. The resulting strain, XZ132, produced 1,279 mmol alanine from 120 g l(-1) glucose within 48 h during batch fermentation in the mineral salts medium. The alanine yield was 95% on a weight basis (g g(-1) glucose) with a chiral purity greater than 99.5% L: -alanine.

Alanine production in an H+-ATPase- and lactate dehydrogenase-defective mutant of Escherichia coli expressing alanine dehydrogenase

Previously, we reported that pyruvate production was markedly improved in TBLA-1, an H(+)-ATPase-defective Escherichia coli mutant derived from W1485lip2, a pyruvate-producing E. coli K-12 strain. TBLA-1 produced more than 30 g/l pyruvate from 50 g/l glucose by jar fermentation, while W1485lip2 produced only 25 g/l pyruvate (Yokota et al. in Biosci Biotechnol Biochem 58:2164-2167, 1994b). In this study, we tested the ability of TBLA-1 to produce alanine by fermentation. The alanine dehydrogenase (ADH) gene from Bacillus stearothermophilus was introduced into TBLA-1, and direct fermentation of alanine from glucose was carried out. However, a considerable amount of lactate was also produced. To reduce lactate accumulation, we knocked out the lactate dehydrogenase gene (ldhA) in TBLA-1. This alanine dehydrogenase-expressing and lactate dehydrogenase-defective mutant of TBLA-1 produced 20 g/l alanine from 50 g/l glucose after 24 h of fermentation. The molar conversion ratio of glucose to alanine was 41%, which is the highest level of alanine production reported to date. This is the first report to show that an H(+)-ATPase-defective mutant of E. coli can be used for amino acid production. Our results further indicate that H(+)-ATPase-defective mutants may be used for fermentative production of various compounds, including alanine.

Metabolomic alterations in elicitor treated Silybum marianum suspension cultures monitored by nuclear magnetic resonance spectroscopy

A comprehensive metabolomic profiling of Silybum marianum (L.) Gaernt cell cultures elicited with yeast extract or methyl jasmonate for the production of silymarin was carried out using one- and two-dimensional nuclear magnetic resonance spectroscopy. With these techniques we were able to detect both temporal quantitative variations in the metabolite pool in yeast extract-elicited cultures and qualitative differences in cultures treated with the two types of elicitors. Yeast extract and methyl jasmonate caused a metabolic reprogramming that affected amino acid and carbohydrate metabolism; upon elicitation sucrose decreased and glucose levels increased, these changes being dependent on "de novo" protein synthesis. Also dependent on protein synthesis were the increase seen in alanine and glutamine in elicited cultures. Yeast extract differentially acted on threonine and valine metabolism and promoted accumulation of choline and alpha-linolenic acid in cells thus suggesting its action on membranes and the involvement of the octadecanoid pathway in the induction of silymarin in S. marianum cultures. Phenylpropanoid metabolism was altered by elicitation but, depending on elicitor, different phenylpropanoid profile was produced. The results obtained in this study will permit in the future to identify candidate components of the signalling pathway involved in the stimulation of the constitutive pathway of silymarin.

Probing the oxygen-binding site of the human formylglycine-generating enzyme using halide ions

The catalytic residue in sulfatases is a unique formylglycine that is post-translationally generated by oxidation of a cysteine or serine precursor. Molecular oxygen oxidizes the cysteine precursor in eukaryotic sulfatases, a reaction that is catalysed by the formylglycine-generating enzyme FGE. Previously, FGE was crystallized in complex with a chloride ion which, based on its similar polarizability and hydrophobicity, indicates the site of molecular oxygen binding. Here, two structures of FGE in complex with bromide and iodide were determined in order to further delineate the volume and stereochemical restraints of the oxygen-binding site for potential reaction intermediates. Anomalous difference density maps unambiguously assigned the nature of the halide ions. Unexpectedly, data collected at a wavelength of 1.54 A from the iodide-containing crystal and data collected at a wavelength of 0.8 A from a bromide-containing crystal were sufficient for SIRAS phasing.

L-valine production with pyruvate dehydrogenase complex-deficient Corynebacterium glutamicum

Corynebacterium glutamicum was engineered for the production of L-valine from glucose by deletion of the aceE gene encoding the E1p enzyme of the pyruvate dehydrogenase complex and additional overexpression of the ilvBNCE genes encoding the L-valine biosynthetic enzymes acetohydroxyacid synthase, isomeroreductase, and transaminase B. In the absence of cellular growth, C. glutamicum DeltaaceE showed a relatively high intracellular concentration of pyruvate (25.9 mM) and produced significant amounts of pyruvate, L-alanine, and L-valine from glucose as the sole carbon source. Lactate or acetate was not formed. Plasmid-bound overexpression of ilvBNCE in C. glutamicum DeltaaceE resulted in an approximately 10-fold-lower intracellular pyruvate concentration (2.3 mM) and a shift of the extracellular product pattern from pyruvate and L-alanine towards L-valine. In fed-batch fermentations at high cell densities and an excess of glucose, C. glutamicum DeltaaceE(pJC4ilvBNCE) produced up to 210 mM L-valine with a volumetric productivity of 10.0 mM h(-1) (1.17 g l(-1) h(-1)) and a maximum yield of about 0.6 mol per mol (0.4 g per g) of glucose.

Inhibition of Rhizomucor miehei and Candida rugosa lipases by d-glucose in esterification between l-alanine and d-glucose

A detailed kinetic study of the esterification of D-glucose with L-alanine catalyzed by lipases from Rhizomucor miehei (RML) and Candida rugosa (CRL) showed that both lipases follow the Ping-Pong Bi-Bi mechanism, in which L-alanine and D-glucose bind in subsequent steps releasing water and L-alanyl-D-glucose, with competitive substrate inhibition by D-glucose at higher concentrations leading to the formation of dead-end lipase.D-glucose complexes. An attempt to obtain the best fit of this kinetic model through curve fitting yielded good approximates of the apparent values of four important kinetic parameters: for RML-k(cat)=0.29+/-0.028x10(-3) M h(-1) mg(-1), K(m L-alanine)= 4.9+/-0.51x10(-3) M, K(m D-glucose)=0.21+/-0.018x10(-3) M, and K(i D-glucose)=1.76+/-0.19x10(-3) M; for CRL-k(cat)= 0.75+/-0.08x10(-3) M h(-1) mg(-1), K(m L-alanine)=56.2+/-5.7x10(-3) M, K(m D-glucose)=16.2+/-1.8x10(-3) M, and K(i D-glucose) =21.0+/-1.9x10(-3) M.

Fed-batch two-phase production of alanine by a metabolically engineered Escherichia coli

DL-Alanine was produced from glucose in an Escherichia coli pfl pps poxB ldhA aceEF pTrc99A-alaD strain which lacked pyruvate-formate lyase, phosphoenolpyruvate (PEP) synthase, pyruvate oxidase, lactate dehydogenase, components of the pyruvate dehydogenase complex and over-produced alanine dehydrogenase (ALD). A two-phase process was developed with cell growth under aerobic conditions followed by alanine production under anaerobic conditions. Using the batch mode, cells grew to 5.3 g/l in 9 h with the accumulation of 6-10 g acetate/l, and under subsequent anaerobic conditions achieved 34 g alanine/l in 13 h with a yield of 0.86 g/g glucose. Using the fed-batch mode at micro = 0.15 h(-1), only about 1 g acetate/l formed in the 25 h required for the cells to reach 5.6 g/l, and 88 g alanine/l accumulated during the subsequent 23 h. This fed-batch process attained an alanine volumetric productivity of 4 g/lh during the production phase, and a yield that was essentially 1 g/g.

Short-term fasting, seizure control and brain amino acid metabolism

The ketogenic diet is an effective treatment for seizures, but the mechanism of action is unknown. It is uncertain whether the anti-epileptic effect presupposes ketosis, or whether the restriction of calories and/or carbohydrate might be sufficient. We found that a relatively brief (24 h) period of low glucose and low calorie intake significantly attenuated the severity of seizures in young Sprague-Dawley rats (50-70 gms) in whom convulsions were induced by administration of pentylenetetrazole (PTZ). The blood glucose concentration was lower in animals that received less dietary glucose, but the brain glucose level did not differ from control blood [3-OH-butyrate] tended to be higher in blood, but not in brain, of animals on a low-glucose intake. The concentration in brain of glutamine increased and that of alanine declined significantly with low-glucose intake. The blood alanine level fell more than that of brain alanine, resulting in a marked increase ( approximately 50%) in the brain:blood ratio for alanine. In contrast, the brain:blood ratio for leucine declined by about 35% in the low-glucose group. When animals received [1-(13)C]glucose, a metabolic precursor of alanine, the appearance of (13)C in alanine and glutamine increased significantly relative to control. The brain:blood ratio for [(13)C]alanine exceeded 1, indicating that the alanine must have been formed in brain and not transported from blood. The elevated brain(alanine):blood(alanine) could mean that a component of the anti-epileptic effect of low carbohydrate intake is release of alanine from brain-to-blood, in the process abetting the disposal of glutamate, excess levels of which in the synaptic cleft would contribute to the development of seizures.

A general binding mechanism for all human sulfatases by the formylglycine-generating enzyme

The formylglycine (FGly)-generating enzyme (FGE) uses molecular oxygen to oxidize a conserved cysteine residue in all eukaryotic sulfatases to the catalytically active FGly. Sulfatases degrade and remodel sulfate esters, and inactivity of FGE results in multiple sulfatase deficiency, a fatal disease. The previously determined FGE crystal structure revealed two crucial cysteine residues in the active site, one of which was thought to be implicated in substrate binding. The other cysteine residue partakes in a novel oxygenase mechanism that does not rely on any cofactors. Here, we present crystal structures of the individual FGE cysteine mutants and employ chemical probing of wild-type FGE, which defined the cysteines to differ strongly in their reactivity. This striking difference in reactivity is explained by the distinct roles of these cysteine residues in the catalytic mechanism. Hitherto, an enzyme-substrate complex as an essential cornerstone for the structural evaluation of the FGly formation mechanism has remained elusive. We also present two FGE-substrate complexes with pentamer and heptamer peptides that mimic sulfatases. The peptides isolate a small cavity that is a likely binding site for molecular oxygen and could host reactive oxygen intermediates during cysteine oxidation. Importantly, these FGE-peptide complexes directly unveil the molecular bases of FGE substrate binding and specificity. Because of the conserved nature of FGE sequences in other organisms, this binding mechanism is of general validity. Furthermore, several disease-causing mutations in both FGE and sulfatases are explained by this binding mechanism.

Stereoselective lipases from Burkholderia sp., cloning and their application to preparation of methyl (R)-N-(2,6-dimethylphenyl)alaninate, a key intermediate for (R)-Metalaxyl

Two microbial strains (referred to as MC 16-3 and 99-2-1) that produce extracellular lipases were isolated from soil samples and identified as Burkholderia species. The lipases were partially purified by isopropyl alcohol precipitation and gave molecular weight of 33kDa. The lipases were characterized in terms of stereoselectivity with racemic methoxyethyl (R,S)-N-(2,6-dimethylphenyl)alaninate and the genes encoding the proteins have been identified by homology alignment of lipases reported belonging to I.2 subfamily and their complete DNA sequences were determined. The lipases will be useful for the preparation of methyl (R)-N-(2,6-dimethylphenyl)alaninate, a key intermediate for the synthesis of (R)-Metalaxyl, which is one of the best-selling fungicides.

Ammonia toxicity under hyponatremic conditions in astrocytes: de novo synthesis of amino acids for the osmoregulatory response

This study investigates how the metabolic activity and de novo synthesis of amino acids from glucose correlate with changes in intracellular organic osmolytes involved in astrocytic volume regulation during hyperammonemia and hyponatremia. Multinuclear (1H-, 31P-, 13C-) NMR spectra were recorded to quantify water-soluble metabolites, the cellular energy state, as well as the incorporation of [1-(13)C]glucose into amino acids of primary astrocyte cultures. Myo-inositol levels were strongly decreased already at 3h after treatment with NH4Cl; other intracellular osmolytes, such as hypotaurine and choline-containing compounds were also decreased, along with a concomitant increase of both the total concentration and the amount of newly synthesized glutamine, alanine, and glutathione. During ammonia stress, the decrease of organic osmolytes compensated in part for increased intracellular osmolarity caused by amino acid synthesis. Hypotonic conditions alone also lowered the content of organic osmolytes including cellular amino acids, but much less than in hyperammonemia. This was due to impaired mitochondrial metabolic activity via the Krebs cycle, which also enhanced ammonia-induced ATP decrease. However, the changes in the sum of organic osmolytes were not significantly different after ammonia-treatment in hypoosmotic compared to anisoosmotic media, suggesting that the decrease of cellular organic osmolytes may not adequately compensate for the increased intracellular osmolarity caused by amino acids under hyponatremia. Therefore, the ammonia-induced release of osmolytes is an early process in response to increased intracellular osmolarity evoked by increased glutamine and alanine as a consequence of stimulated metabolic activity. The imperfect correlation of changes in astrocytic glutamine, other organic osmolytes, and the cellular energy state under hyperammonemic stress in isoosmotic and hypoosmotic media, however, point to additional mechanisms contributing to astrocyte dysfunction in hyperammonemic states, which are independent from glutamine formation.

De novo alanine synthesis by bacteroids of Mesorhizobium loti is not required for nitrogen transfer in the determinate nodules of Lotus corniculatus

Deletion of both alanine dehydrogenase genes (aldA) in Mesorhizobium loti resulted in the loss of AldA enzyme activity from cultured bacteria and bacteroids but had no effect on the symbiotic performance of Lotus corniculatus plants. Thus, neither indeterminate pea nodules nor determinate L. corniculatus nodules export alanine as the sole nitrogen secretion product.

Coexpression of Formylglycine-Generating Enzyme Is Essential for Synthesis and Secretion of Functional Arylsulfatase A in a Mouse Model of Metachromatic Leukodystrophy

Metachromatic leukodystrophy (MLD) is a lysosomal storage disorder involving inherited deficiency of arylsulfatase A (ASA). The disease is characterized by progressive demyelination and widespread deposition of sulfatide in both the central and peripheral nervous systems. Direct injection of viral vector through the blood-brain barrier is a possible gene therapy approach to MLD. However, to treat all brain cells, it is essential to secrete a sufficient amount of functional ASA from limited numbers of transduced cells. In the present study, we tested the utility of formylglycine-generating enzyme (FGE) for overexpression of functional ASA. FGE is a posttranslational modifying enzyme essential for activating multiple forms of sulfatases including ASA. COS-7 cells were transfected with ASA- and FGE-expressing plasmids. ASA activity was increased up to 20-fold in cell lysates and 70-fold in conditioned medium by coexpression of FGE. Intravenous injection of the expression plasmids into MLD knockout mice by a hydrodynamics-based procedure resulted in a significant synergistic increase in ASA activity both in liver and serum. Blot hybridization analysis of FGE mRNA demonstrated that the expression of endogenous FGE was particularly low in human brain. Our results suggest, on the basis of cross-correction of ASA deficiency, that coexpression of FGE is essential for gene therapy of MLD.

Molecular basis for multiple sulfatase deficiency and mechanism for formylglycine generation of the human formylglycine-generating enzyme

Sulfatases are enzymes essential for degradation and remodeling of sulfate esters. Formylglycine (FGly), the key catalytic residue in the active site, is unique to sulfatases. In higher eukaryotes, FGly is generated from a cysteine precursor by the FGly-generating enzyme (FGE). Inactivity of FGE results in multiple sulfatase deficiency (MSD), a fatal autosomal recessive syndrome. Based on the crystal structure, we report that FGE is a single-domain monomer with a surprising paucity of secondary structure and adopts a unique fold. The effect of all 18 missense mutations found in MSD patients is explained by the FGE structure, providing a molecular basis of MSD. The catalytic mechanism of FGly generation was elucidated by six high-resolution structures of FGE in different redox environments. The structures allow formulation of a novel oxygenase mechanism whereby FGE utilizes molecular oxygen to generate FGly via a cysteine sulfenic acid intermediate.

Physiological Function and Metabolism of Free D-Alanine in Aquatic Animals

Aquatic crustaceans and some bivalve mollusks contain a large amount of free D-alanine (up to 100 mumol/g wet wt.) in their tissues. Under high salinity stress, crustaceans and bivalve mollusks largely accumulate D- and L-alanine irrespective of species examined, together with L-glutamine, L-proline, and glycine of which increases are species dependent. These data indicate that D-alanine is one of the major compatible osmolytes responsible for the intracellular isosmotic regulation in the tissues of crustaceans and bivalves. Alanine racemase has been proven to catalyze the interconversion of D- and L-alanine in these invertebrates. The enzyme has been isolated to homogeneity from the muscle of black tiger prawn Penaeus monodon and its cDNA has been cloned from the muscle and hepatopancreas of kuruma prawn Penaeus japonicus for the first time in eukaryotes other than yeast. Several fish species fed on crustaceans and mollusks contain D-amino acid and D-aspartate oxidases that catalyze the decomposition of D-amino acids. A cDNA of D-amino acid oxidase has been cloned from the hepatopancreas of omnivorous common carp Cyprinus carpio. During oral administration of free D-alanine to carp, the activity and mRNA of D-amino acid oxidase increased rapidly in hepatopancreas and the increases were highest in intestine followed by hepatopancreas and kidney. These data suggest that D-amino acid oxidase is inducible in carp and an important enzyme responsible for the efficient utilization of carbon skeleton of D-alanine in their feeds.

bac genes for recombinant bacilysin and anticapsin production in Bacillus host strains

The genes encoding the biosynthesis of the dipeptide bacilysin and its antibiotic constituent anticapsin were isolated from several strains of Bacillus subtilis as well as B. amyloliquefaciens and B. pumilus. The ywfBCDEF genes of B. subtilis 168 were shown to carry the biosynthetic core functions and were renamed bacABCDE. Mutation of the bacD gene or transformation of the bacABC genes into a B. subtilis Delta (ywfA-bacABCDE) deletion mutant led to the accumulation of anticapsin, which was fourfold higher after transformation of the bacABC genes into a bacD mutant. The genes bacD and bacE proved to encode the functions of amino acid ligation and self-protection to bacilysin, respectively. Amplification of the bacABCDE gene cluster in a bacAB gene-deficient host strain of B. amyloliquefaciens resulted in a tenfold bacilysin overproduction. Some host strains required distinct glucosamine and yeast extract supplements in order to prevent suicidal effects of the recombinant antibiotic production. The bac genes from different Bacillus species revealed the same arrangement and 72.6-88.6% of sequence identity.

Glutamine Synthesis in the Developing Porcine Placenta1

Glutamine plays a vital role in fetal carbon and nitrogen metabolism and exhibits the highest fetal:maternal plasma ratio among all amino acids in pigs. Such disparate glutamine levels between mother and fetus suggest that glutamine may be actively synthesized and released into the fetal circulation by the porcine placenta. We hypothesized that branched-chain amino acid (BCAA) metabolism in the placenta plays an important role in placental glutamine synthesis. This hypothesis was tested by studying conceptuses from gilts on Days 20, 30, 35, 40, 45, 50, 60, 90, or 110 of gestation (n = 6 per day). Placental tissue was analyzed for amino acid concentrations, BCAA transport, BCAA degradation, and glutamine synthesis as well as the activities of related enzymes (including BCAA transaminase, branched-chain alpha-ketoacid dehydrogenase, glutamine synthetase, glutamate-pyruvate transaminase, and glutaminase). On all days of gestation, rates of BCAA transamination were much greater than rates of branched-chain alpha-ketoacid decarboxylation. The glutamate generated from BCAA transamination was primarily directed to glutamine synthesis and, to a much lesser extent, alanine production. Placental BCAA transport, BCAA transamination, glutamine synthesis, and activities of related enzymes increased markedly between Days 20 and 40 of gestation, as did glutamine in fetal allantoic fluid. Accordingly, placental BCAA levels decreased after Day 20 of gestation in association with a marked increase in BCAA catabolism and concentrations of glutamine. There was no detectable catabolism of glutamine in pig placenta throughout pregnancy, which would ensure maximum output of glutamine by this tissue. These novel results demonstrate glutamine synthesis from BCAAs in pig placentae, aid in explaining the abundance of glutamine in the fetus, and provide valuable insight into the dynamic role of the placenta in fetal metabolism and nutrition.

Aerobic production of alanine by Escherichia coli aceF ldhA mutants expressing the Bacillus sphaericus alaD gene

Alanine was produced from glucose in an Escherichia coli aceF ldhA double mutant strain that contained the pTrc99A- alaD plasmid expressing Bacillus sphaericus alanine dehydrogenase. The aceF gene encodes one of the proteins of the pyruvate dehydrogenase complex, and therefore this strain required acetate as an additional carbon source. The ldhA gene encodes fermentative lactate dehydrogenase, a competitor of alanine dehydrogenase for the substrate pyruvate. Fermentations included an oxygenated growth phase followed by an oxygen-limited alanine production phase. The lowest value for the mass transfer coefficient ( k(L)a) studied during the production phase yielded the greatest alanine. With feeding of glucose and NH(4)Cl, 32 g/l alanine accumulated in 27 h with a yield of 0.63 g alanine generated per gram glucose consumed.

Multiple sulfatase deficiency is caused by mutations in the gene encoding the human C(alpha)-formylglycine generating enzyme

C(alpha)-formylglycine (FGly) is the catalytic residue in the active site of eukaryotic sulfatases. It is posttranslationally generated from a cysteine in the endoplasmic reticulum. The genetic defect of FGly formation causes multiple sulfatase deficiency (MSD), a lysosomal storage disorder. We purified the FGly generating enzyme (FGE) and identified its gene and nine mutations in seven MSD patients. In patient fibroblasts, the activity of sulfatases is partially restored by transduction of FGE encoding cDNA, but not by cDNA carrying an MSD mutation. The gene encoding FGE is highly conserved among pro- and eukaryotes and has a paralog of unknown function in vertebrates. FGE is localized in the endoplasmic reticulum and is predicted to have a tripartite domain structure.

Amino acid turnover by elongating cattle blastocysts recovered on days 14-16 after insemination

Blastocyst elongation from day 14 to day 16 after insemination coincides with a major phase of embryo loss in cattle. Protein synthesis, reflected in protein content, increases markedly over this period but little is known about the amino acid requirement of elongating blastocysts at this time. Cattle blastocysts produced in vivo were recovered on days 14-16 after insemination and cultured individually for up to 8 h in synthetic oviduct fluid containing a physiological mixture of amino acids plus 1 mmol glutamine l(-1) and 0.1% (w/v) polyvinyl alcohol (SOFaaPVA). After 1, 4 and 8 h in culture, an aliquot of culture medium was removed and the rate of amino acid depletion or production was calculated per unit of protein and per hour of culture. Amino acids were depleted or produced at different rates. Arginine was depleted from the medium at a significant rate (P < 0.05) during all culture periods. Alanine and glutamate were produced at a significant rate (P < 0.05) during all culture periods. The rate of alanine production was significantly greater (P < 0.05) in blastocysts recovered on day 14 compared with days 15 or 16 after insemination. Alanine production and arginine depletion tended to be greater in smaller embryos recovered on day 14 compared with larger and later stage embryos, indicating that earlier stage embryos may have higher metabolic activity than later stage embryos. Qualitatively, the pattern of amino acid consumption and production during elongation was similar to that shown from the zygote to early blastocyst stage.

Synthesis and incorporation of [6,7]-selenatryptophan into dihydrofolate reductase

Until recently, the only selenium containing amino acid which could be used to completely substitute for a wild type amino acid was selenomethionine (SeMet). In the last decade the preparation of SeMet containing proteins has proved to be valuable tools in the determination of three-dimensional structure by multiwavelength anomalous diffraction (MAD) techniques. The potential utility of a selenium containing tryptophan analog, beta-seleno[3,2-b]pyrrolyl-L-alanine ([4,5]selenatryptophan), has recently been demonstrated in the literature. This finding shows promise for the bioincorporation of its positional isomer, beta-selenolo[2,3-b]pyrrolyl-L-alanine ([6,7]selenatryptophan), thereby adding to the essential arsenal of selenium-containing amino acids for use in the characterization of proteins. The synthesis of [6,7]selenatryptophan by enzymatic biotransformation with tryptophan synthase from selenolo[2,3-b]pyrrole was carried out as well as its characterization by NMR spectroscopy and thin layer chromatography. Selenatryptophyl dihydrofolate reductase ([6,7]SeTrp-DHFR) was then synthesized in vivo, purified, and found to exhibit no perturbations to enzymatic activity.

Room-temperature synthesis of L-alanine using the alanine dehydrogenase of the hyperthermophilic archaeon Archaeoglobus fulgidus

Alanine dehydrogenase from the hyperthermophilic archaeon Archaeoglobus fulgidus was used at room temperature for batch synthesis of L-alanine by the reductive amination of pyruvate. The reaction mixture included yeast formate dehydrogenase for regeneration of NADH with formate as electron donor. The synthesis of L-alanine at room temperature was accompanied by no detectable loss of alanine dehydrogenase activity over 139 h and > or =99% consumption of pyruvate. The total number of enzyme turnovers was 5.1 million. This work demonstrates the potential utility of novel hyperthermostable enzymes that can be both very active and highly stable at moderate temperature.

Troglitazone induces a cellular acidosis by inhibiting acid extrusion in cultured rat mesangial cells

We studied the effect of troglitazone on cellular acid-base balance and alanine formation in isolated rat mesangial cells. Mesangial cells were grown to confluency in RPMI 1640 media on 30-mm chambers used to monitor both cellular pH using the pH-sensitive dye 2'7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein and metabolic acid production as well as glutamine metabolism. Troglitazone (10 microM) induced a spontaneous cellular acidosis (6.95 +/- 0.02 vs. 7.47 +/- 0.04, respectively; P < 0.0001) but without an increase in lactic acid production. Alanine production was reduced 64% (P < 0.01) consistent with inhibition of the glutamate transamination. These findings pointed to a decrease in acid extrusion rather than an increase in acid production as the underlying mechanism leading to the cellular acidosis. To test their acid extrusion capabilities, mesangial cells were acid loaded with NH and then allowed to recover in Krebs-Henseleit media or in Krebs-Henseleit media minus bicarbonate (HEPES substituted), and the recovery response (Delta pH(i)/min) was monitored. In the presence of 10 microM troglitazone, the recovery response to the NH acid load was virtually eliminated in the bicarbonate-buffered media (0.00 +/- 0.001 vs. 0.06 +/- 0.02 pH(i)/min, P < 0.0001 vs. control) and reduced 75% in HEPES-buffered media (0.01 +/- 0.01 vs. 0.04 +/- 0.02 pH(i)/min, P < 0.002 vs. control). These results show that troglitazone induces a spontaneous cellular acidosis resulting from a reduction in cellular acid extrusion.

The simultaneous biosynthesis and uptake of amino acids by Lactococcus lactis studied by (13)C-labeling experiments

Uniformly (13)C labeled glucose was fed to a lactic acid bacterium growing on a defined medium supplemented with all proteinogenic amino acids except glutamate. Aspartate stemming from the protein pool and from the extracellular medium was enriched with (13)C disclosing a substantial de novo biosynthesis of this amino acid simultaneous to its uptake from the growth medium and a rapid exchange flux of aspartate over the cellular membrane. Phenylalanine, alanine, and threonine were also synthesized de novo in spite of their presence in the growth medium.

Modulation of cyanoalanine synthase and O-acetylserine (thiol) lyases A and B activity by beta-substituted alanyl and anion inhibitors

The reaction mechanisms of three enzymes belonging to a single gene family are compared: a cyanoalanine synthase and two isoforms of O-acetylserine (thiol) lyase (O-ASTL) isolated from spinach (Spinacea oleracea L. cv. Medina). O-ASTL represents a major regulatory point in the S-assimilatory pathway, and the related cyanoalanine synthase, which is specific to the mitochondrial compartment, has evolved an independent function of cyanide detoxification. All three enzymes catalysed both the cysteine synthesis and cyanoalanine synthesis reactions although with different efficiencies, and which may be explained by a single amino acid substitution in the substrate-binding pocket of the enzyme. Substituted alanine and nucleophillic inhibitors caused predominantly non-competitive inhibition, indicating binding to both E- and F-forms of the enzyme in a bi-bi ping-pong kinetic model. Michaelis-Menten kinetics were observed when the alanyl substrate was varied in the presence and absence of inhibitors. The use of alanyl inhibitors has shown that the alanyl half-cycle of both the cysteine synthesis and cyanoalanine synthesis reactions of cyanoalanine synthase and O-acetylserine (thiol) lyases are similar. This is in contrast to the results observed with nucleophillic inhibitors, which have shown that the mechanisms of anion binding and processing differ between cyanoalanine synthase and O-ASTLs.

Relation between glutamine, branched-chain amino acids, and protein metabolism

The branched-chain amino acids (BCAAs; valine, isoleucine, and leucine) are the major nitrogen source for glutamine and alanine synthesis in muscle. Synthesis of glutamine, alanine, and BCAA use is activated in critical illnesses such as in sepsis, cancer, and trauma. The use of glutamine often exceeds its synthesis, resulting in the lack of glutamine in plasma and tissues. In critical illness, resynthesis of BCAA from branched-chain keto acids is activated, particularly in hepatic tissue. The BCAA released to circulation may be used for protein synthesis or synthesis of alanine and glutamine. Glutamine and/or alanine infusion has an inhibitory effect on the breakdown of body proteins and decreases BCAA catabolism in postabsorptive control, endotoxemic, and irradiated rats. Decreased protein breakdown also was observed when glutamine synthesis was activated by ammonia infusion. In conclusion some favorable effects of BCAA supply can be explained by its role in the synthesis of glutamine and some positive effects of glutamine exogenous supply can be explained by its effect on metabolism of BCAA.

Effects of potassium and glutamine on metabolism of glucose in astrocytes

The metabolic effects of extracellular glutamine (2.5 mM) or high potassium (25 mM) on glucose metabolism were studied in cultured cerebellar astrocytes. High potassium caused an increased glycolytic flux and an increase in glutamine release. Exposure to glutamine increased glycolytic flux and alanine formation, indicating that glutamine uptake is an energy requiring process. The effects of glutamine and high potassium on glycolytic flux were additive. Formation of metabolites from [1-13C]glucose and [2-13C]acetate confirmed the effects of glutamine and high potassium on glycolytic metabolism. In the presence of extracellular glutamine, analysis of the 13C labeling patterns of citrate and glutamine indicated a decrease in the cycling ratio and/or pyruvate carboxylation and glutamine synthesis from [1-13C]glucose did occur, but was decreased. Exposure to high potassium led to extracellular accumulation of acetate, presumably through non-enzymatic decarboxylation of pyruvate.

Characterization of posttranslational formylglycine formation by luminal components of the endoplasmic reticulum

C(alpha)-formylglycine is the key catalytic residue in the active site of sulfatases. In eukaryotes formylglycine is generated during or immediately after sulfatase translocation into the endoplasmic reticulum by oxidation of a specific cysteine residue. We established an in vitro assay that allowed us to measure formylglycine modification independent of protein translocation. The modifying enzyme was recovered in a microsomal detergent extract. As a substrate we used ribosome-associated nascent chain complexes comprising in vitro synthesized sulfatase fragments that were released from the ribosomes by puromycin. Formylglycine modification was highly efficient and did not require a signal sequence in the substrate polypeptide. Ribosome association helped to maintain the modification competence of nascent chains but only after their release efficient modification occurred. The modifying machinery consists of soluble components of the endoplasmic reticulum lumen, as shown by differential extraction of microsomes. The in vitro assay can be performed under kinetically controlled conditions. The activation energy for formylglycine formation is 61 kJ/mol, and the pH optimum is approximately 10. The activity is sensitive to the SH/SS equilibrium and is stimulated by Ca(2+). Formylglycine formation is efficiently inhibited by a synthetic sulfatase peptide representing the sequence directing formylglycine modification. The established assay system should make possible the biochemical identification of the modifying enzyme.

Differences in neurotransmitter synthesis and intermediary metabolism between glutamatergic and GABAergic neurons during 4 hours of middle cerebral artery occlusion in the rat: the role of astrocytes in neuronal survival

Astrocytes are intimately involved in both glutamate and gamma-aminobutyric acid (GABA) synthesis, and ischemia-induced disruption of normal neuroastrocytic interactions may have important implications for neuronal survival. The effects of middle cerebral artery occlusion (MCAO) on neuronal and astrocytic intermediary metabolism were studied in rats 30, 60, 120, and 240 minutes after MCAO using in vivo injection of [1-13C]glucose and [1,2- 13C]acetate combined with ex vivo 13C magnetic resonance spectroscopy and high-performance liquid chromatography analysis of the ischemic core (lateral caudoputamen and lower parietal cortex) and penumbra (upper frontoparietal cortex). In the ischemic core, both neuronal and astrocytic metabolism were impaired from 30 minutes MCAO. There was a continuous loss of glutamate from glutamatergic neurons that was not replaced as neuronal glucose metabolism and use of astrocytic precursors gradually declined. In GABAergic neurons astrocytic precursors were not used in GABA synthesis at any time after MCAO, and neuronal glucose metabolism and GABA-shunt activity declined with time. No flux through the tricarboxylic acid cycle was found in GABAergic neurons at 240 minutes MCAO, indicating neuronal death. In the penumbra, the neurotransmitter pool of glutamate coming from astrocytic glutamine was preserved while neuronal metabolism progressively declined, implying that glutamine contributed significantly to glutamate excitotoxicity. In GABAergic neurons, astrocytic precursors were used to a limited extent during the initial 120 minutes, and tricarboxylic acid cycle activity was continued for 240 minutes. The present study showed the paradoxical role that astrocytes play in neuronal survival in ischemia, and changes in the use of astrocytic precursors appeared to contribute significantly to neuronal death, albeit through different mechanisms in glutamatergic and GABAergic neurons.

Encapsulation of PEG-urease/PEG-AlaDH enzyme system in erythrocyte

No intravenously injectable enzyme preparate containing urease as an alternetive to hemodialysis, hemoperfusion and CAPD systems in patients having chronic renal failure has been encountered in literature. In this study, it has been aimed to convert blood urea to alanine by using PEG-urease/PEG-AlaDH enzyme pair encapsulated within living erythrocyte. In this system, urea is decomposed into NH3 and HCO3- and the ammonia released is converted into alanine by reacting pyruvate under the catalytic action of alaninedehydrogenase. The production of pyruvate and NADH by erythrocyte required in the second stage of the reaction will make the process a feasible and ceaseless one. The success of the system will enable the renal patients with diabetes mellitus. Urease and AlaDH were covalently immobilized on activated PEG. PEG-urease/PEG-AlaDH were encapsulated in erythrocyte (1/1)(v/v) by using slow dialysis methods. The activity of enzyme system, encapsulation yield and hemogram analysis were determined for each sample.

Increased islet DNA synthesis and glucose-derived lipid and amino acid production in association with beta-cell hyperproliferation in normoglycaemic 60 % pancreatectomy rats

AIMS/HYPOTHESIS

Glycaemia does not change following a 60 % pancreatectomy in rats because of enhanced beta-cell function and proliferation (so-called beta-cell adaptation). We previously studied these rats 4 weeks after surgery and showed hypersensitization of glucose-induced insulin secretion because of increased glucokinase activity. In this study of 60 % pancreatectomy rats 5 days after surgery, when beta-cell proliferation increased threefold, we investigated whether increases in glucose metabolism enhance the production of glucose-derived lipid, amino acids and DNA.

METHODS

Isolated islets from 60 % pancreatectomy and sham-operated control rats 5 days or 4 weeks after surgery were studied.

RESULTS

Five days after 60 % pancreatectomy surgery, islet glucose phosphorylation increased threefold, but overall glucose usage increased only twofold. The glucose-6-phosphate (G6P) concentration thus doubled, resulting in a sixfold increase in G6P metabolism through the pentose phosphate shunt (PPS). The pentose phosphate shunt generates ribose-5-phosphate for nucleotide synthesis, and DNA synthesis doubled in the partial pancreatectomy islets. In contrast, partial pancreatectomy rats 4 weeks after surgery had a smaller increase in glucokinase activity and their islet glucose-6-phosphate concentration and pentose phosphate shunt activity were equal to that of the control rats. DNA synthesis and beta-cell proliferation, based on BrdU incorporation were close to normal. Another consequence of the heightened glucose metabolism in the 5-day partial pancreatectomy islets was twofold increase in production of glucose-derived lipid and the amino acids, alanine and glutamate.

CONCLUSIONS/INTERPRETATION

The enhanced glucokinase activity in 60 % pancreatectomy rats supports the compensatory beta-cell hyperproliferation by increasing production of glucose-derived DNA, lipids and amino acids.

Effects of ammonia exposition on glioma cells: changes in cell volume and organic osmolytes studied by diffusion-weighted and high-resolution NMR spectroscopy

NH(4)Cl (10 mM) caused a sustained increase in the cell volume in immobilized, perfused F98 glioma cells to approx. 125% of control after 3 h, as measured by diffusion-weighted (1)H NMR spectroscopy. Concomitantly, the glutamine (Gln) concentration increased by 130%, accompanied by a marked decrease in cytosolic osmolytes, i.e. myo-inositol and taurine, determined from (1)H NMR spectra of PCA extracts. Inhibition of Gln synthetase partially prevented the increase in water content. While losses of organic osmolytes are also observed under hypotonic conditions, the rapid cell swelling is followed by the regulatory cell volume decrease (RVD), and is accompanied by decreased cytosolic Gln. We suggest that the rise in intracellular osmolarity, which is attributed to NH(4)Cl metabolism to Gln, but also to alanine (Ala), is not compensated by the release of other osmolytes, and causes cell swelling without RVD.

Inhibition of muscle glutamine formation in hypercatabolic patients

Glutamine is synthesized primarily in skeletal muscle, and enables transfer of nitrogen to the liver, as well as serving other functions. There is increasing evidence for beneficial clinical effects of glutamine supplementation in critically ill patients. However, the response of endogenous glutamine formation to severe stress is poorly understood. The rates of net protein balance, leucine oxidative decarboxylation, and alanine and glutamine synthesis de novo were determined in leg skeletal muscle of 20 severely burned patients and 19 normal controls in the post-absorptive state. Patients were studied at 14+/-5 days post-burn, and their mean burn size was 66+/-18% of total body surface area. Methods were based on the leg arteriovenous balance technique in combination with biopsies of the vastus lateralis muscle. In the post-absorptive state, patients with severe burns, as compared with healthy control subjects, exhibited accelerated muscle loss (+150%) (i.e. proteolysis minus synthesis) and leucine oxidative decarboxylation (+117%), and depletion of the intramuscular free glutamine pool (-63%). The average rate of glutamine synthesis de novo was decreased by 48%, whereas net alanine synthesis de novo was increased by 174%, in skeletal muscle of burned patients. In conclusion, in severely hypercatabolic burned patients, muscle glutamine formation was suppressed, whereas alanine was the major vehicle for inter-organ nitrogen transport. These changes account for a decreased glutamine availability during prolonged severe stress.

(13)C MR spectroscopy study of lactate as substrate for rat brain

In order to address the question whether lactate in blood can serve as a precursor for cerebral metabolites, fully awake rats were injected intravenously with [U-(13)C]lactate or [U-(13)C]glucose followed 15 min later by decapitation. Incorporation of label from [U-(13)C]glucose was seen mainly in glutamate, GABA, glutamine, aspartate, alanine and lactate. More label was found in glutamate than glutamine, underscoring the predominantly neuronal metabolism of pyruvate from [U-(13)C]glucose. It was estimated that the neuronal metabolism of acetyl CoA from glucose accounts for at least 66% and the glial for no more than 34% of the total glucose consumption. When [U-(13)C]lactate was the precursor, label incorporation was similar to that observed from [U-(13)C]glucose, but much reduced. Plasma analysis revealed the presence of approximately equal amounts of [1,2,3-(13)C]- and [1,2-(13)C]glucose, showing gluconeogenesis from [U-(13)C]lactate. It was thus possible that the labeling seen in the cerebral amino acids originated from labeled glucose, not [U-(13)C]lactate. However, the presence of significantly more label in [U-(13)C]- than in [2,3-(13)C]alanine demonstrated that [U-(13)C]lactate did indeed cross the blood-brain barrier, and was metabolized further in the brain. Furthermore, contributions from pyruvate carboxylase (glial enzyme) were detectable in glutamine, glutamate and GABA, and were comparatively more pronounced in the glucose group. This indicated that relatively more pyruvate from lactate than glucose was metabolized in neurons. Surprisingly, the same amount of lactate was synthesized via the tricarboxylic acid cycle in both groups, indicating transfer of neurotransmitters from the neuronal to the astrocytic compartment, as previous studies have shown that this lactate is synthesized primarily in astrocytes. Taking into consideration that astrocytes take up glutamate more avidly than GABA, it is conceivable that neuronal lactate metabolism was more prominent in glutamatergic neurons.

Purification and characterization of the alanine aminotransferase from the hyperthermophilic Archaeon pyrococcus furiosus and its role in alanine production

Alanine aminotransferase (AlaAT) was purified from cell extracts of the hyperthermophilic archaeon Pyrococcus furiosus by multistep chromatography. The enzyme has an apparent molecular mass of 93.5 kDa, as estimated by gel filtration, and consists of two identical subunits of 46 kDa, as deduced by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and the gene sequence. The AlaAT displayed a broader substrate specificity than AlaATs from eukaryal sources and exhibited significant activity with alanine, glutamate, and aspartate with either 2-oxoglutarate or pyruvate as the amino acceptor. Optimal activity was found in the pH range of 6. 5 to 7.8 and at a temperature of over 95 degrees C. The N-terminal amino acid sequence of the purified AlaAT was determined and enabled the identification of the gene encoding AlaAT (aat) in the P. furiosus genome database. The gene was expressed in Escherichia coli, and the recombinant enzyme was purified. The pH and temperature dependence, molecular mass, and kinetic parameters of the recombinant were indistinguishable from those of the native enzyme from P. furiosus. The k(cat)/K(m) values for alanine and pyruvate formation were 41 and 33 s(-1) mM(-1), respectively, suggesting that the enzyme is not biased toward either the formation of pyruvate, or alanine. Northern analysis identified a single 1.2-kb transcript for the aat gene. In addition, both the aat and gdh (encoding the glutamate dehydrogenase) transcripts appear to be coregulated at the transcriptional level, because the expression of both genes was induced when the cells were grown on pyruvate. The coordinated control found for the aat and gdh genes is in good agreement with these enzymes acting in a concerted manner to form an electron sink in P. furiosus.