Dynamics of l-valine in relation to the production of cyclosporin A by Tolypocladium inflatum

Summary We report the kinetics of endogenous l-valine in the fungus Tolypocladium inflatum, in an effort to understand the enhancing effect of externally supplemented l-valine on the production of the immunosuppressant cyclosporin A (CyA) in chemically defined medium. In a batch laboratory stirred reactor cultivation, the concentration of intracellular l-valine increased by up to four times between the end of the exponential phase and the beginning of the stationary phase when the medium was supplemented externally with 4 g/1 l-valine. The final CyA titre under these conditions was 710 mg/1 compared to only 130 mg/1 attained without l-valine supplementation. In contrast to substantial growth-associated production of CyA in unsupplemented culture, the formation of the immunosuppressant was prolonged during the stationary phase in l-valine-supplemented medium. As a result, the conversion yield of CyA on l-valine remained constant during the stationary phase at 0.27 g CyA/g l-valine.     

Improvement of l-valine production at high temperature in Brevibacterium flavum by overexpressing ilvEBNrC genes

Brevibacterium flavum ATCC14067 was engineered for l-valine production by overexpression of different ilv genes; the ilvEBN^rC genes from B. flavum NV128 provided the best candidate for l-valine production. In traditional fermentation, l-valine production reached 30.08 ± 0.92 g/L at 31°C in 72 h with a low conversion efficiency of 0.129 g/g. To further improve the l-valine production and conversion efficiency based on the optimum temperatures of l-valine biosynthesis enzymes (above 35°C) and the thermotolerance of B. flavum, the fermentation temperature was increased to 34, 37, and 40°C. As a result, higher metabolic rate and l-valine biosynthesis enzymes activity were obtained at high temperature, and the maximum l-valine production, conversion efficiency, and specific l-valine production rate reached 38.08 ± 1.32 g/L, 0.241 g/g, and 0.133 g g⁻¹ h⁻¹, respectively, at 37°C in 48 h fermentation. The strategy for enhancing l-valine production by overexpression of key enzymes in thermotolerant strains may provide an alternative approach to enhance branched-chain amino acids production with other strains.     

Factors enhancing l-valine production by the growth-limited l-isoleucine auxotrophic strain Corynebacterium glutamicum ΔilvA ΔpanB ilvNM13 (pECKAilvBNC)

Cell growth limitation is known to be an important condition that enhances l-valine synthesis in Corynebacterium glutamicum recombinant strains with l-isoleucine auxotrophy. To identify whether it is the limited availability of l-isoleucine itself or the l-isoleucine limitation-induced rel-dependent ppGpp-mediated stringent response that is essential for the enhancement of l-valine synthesis in growth-limited C. glutamicum cells, we deleted the rel gene, thereby constructing a relaxed (rel ⁻ ) C. glutamicum ΔilvA ΔpanB Δrel ilvNM13 (pECKAilvBNC) strain. Variations in enzyme activity and l-valine synthesis in rel ⁺ and rel ⁻ strains under conditions of l-isoleucine excess and limitation were investigated. A sharp increase in acetohydroxy acid synthase (AHAS) activity, a slight increase in acetohydroxyacid isomeroreductase (AHAIR) activity, and a dramatic increase in l-valine synthesis were observed in both rel ⁺ and rel ⁻ cells exposed to l-isoleucine limitation. Although the positive effect of induction of the stringent response on AHAS and AHAIR upregulation in cells was not confirmed, we found the stringent response to be beneficial for maintaining increased AHAS, dihydroxyacid dehydratase, and transaminase B activity and l-valine synthesis in cells during the stationary growth phase.     

Living cell reaction process forl-isoleucine andl-valine production

Summary A new process (Living Cell Reaction Process) forl-isoleucine production using viable, non-growing cells ofBrevibacterium flavum AB-07 was optimised using ethanol as the energy source and -ketobutyric acid (-KB) as precursor.l-valine also could be produced from glucose at high yield by this process. This process differs from the usual fermentation method in that non-growing cells are used, and the production ofl-isoleucine andl-valine were carried out under conditions of repressed cell division and growth. Minimal medium missing the essential growth factor, biotin was employed as the reaction mixture for the production ofl-isoleucine andl-valine. The productivity ofl-isoleucine andl-valine were 200 mmol·l^–1 · day^–1 (molecular yield to -KB: 95%) and 300 mmol · l^–1 · day^–1 (molecular yield to glucose: 80%) respectively. The content ofl-isoleucine andl-valine in total amino acids produced in the each mixture were 97% and 96% respectively.     

Analysing overexpression of l-valine biosynthesis genes in pyruvate-dehydrogenase-deficient Corynebacterium glutamicum

l-Valine biosynthesis was analysed by comparing different plasmids in pyruvate-dehydrogenase-deficient Corynebacterium glutamicum strains in order to achieve an optimal production strain. The plasmids contained different combinations of the genes ilvBNCDE encoding for the l-valine forming pathway. It was shown that overexpression of the ilvBN genes encoding acetolactate synthase is obligatory for efficient pyruvate conversion and to prevent l-alanine as a by-product. In contrast to earlier studies, overexpression of ilvE encoding transaminase B is favourable in pyruvate-dehydrogenase-negative strains. Its amplification enhanced l-valine formation and avoided extra- and intracellular accumulation of ketoisovalerate.     

Comparative study of the enzymatic synthesis of l-valine by purified enzymes,crude extract,intact or permeabilized cells from Bacillus megaterium

Summary A detailed study on the reductive amination of -ketoisovalerate to l-valine by l-valine dehydrogenase using glucose dehydrogenase as an NADH regeneration enzyme was performed. The presence of both enzyme activities in Bacillus megaterium ATCC 39 118 permitted a direct and systematic comparison of the performances (initial l-valine production rate, productivity, molar conversion yield) of different types of conversion systems: purified enzymes or crude extract and whole cells, intact or permeabilized. A maximal l-valine productivity of 8 mmol·l^–1 · h^–1 was obtained using purified enzymes which constituted the most efficient system with a maximal rate of 0.87 mol · ml^–1 · min^–1 and a molar conversion yield of 0.91. Permeabilized cells were also an attractive system because of their easy preparation and of the good performances attained.Offprint requests to: F. Monot     

Influence of l-isoleucine and pantothenate auxotrophy for l-valine formation in Corynebacterium glutamicum revisited by metabolome analyses

The effect of different amounts of supplemented l-isoleucine and pantothenate has been analysed with the auxotrophic strain Corynebacterium glutamicum ΔilvA ΔpanB, showing that the final biomass concentration of this preliminary l-valine production strain can be controlled by the amount of added l-isoleucine. One gramme cell dry weight is formed from 48 μmol l-isoleucine. Different amounts of available pantothenate affect the intracellular pyruvate concentration. By limiting pantothenate supplementation from 0.8 to 0.1 μM, a 35-fold increase of cytoplasmic pyruvate up to 14.2 mM can be observed, resulting in the increased formation of l-valine, l-alanine and organic acids in the presence of low pantothenate concentrations. These findings can be used to redirect the carbon flux from glycolysis via pyruvate to the TCA cycle towards the desired product l-valine.     

Regulation of acetohydroxy acid synthase in Corynebacterium glutamicum during fermentation of α-ketobutyrate to l-isoleucine

Summary Corynebacterium glutamicum ATCC 13 032 produces 13 g/l l-isoleucine from 200 mM -ketobutyrate as a synthetic precursor. In fed batch cultures up to 19 g/l l-isoleucine is formed. For optimal conversion the addition of 0.3 mM l-valine plus 0.3 mM l-leucine to the fermentation medium is required. The affinity constants for the acetohydroxy acid synthase (AHAS) were determined. (This enzyme directs the flow of -ketobutyrate plus pyruvate towards l-isoleucine and that of two moles of pyruvate to l-valine and l-leucine, respectively.) For -ketobutyrate the K _m is 4.8×10^-3 M, and V _max 0.58 U/mg, for pyruvate the K _m is 8.4×10^-3 M, and V _max 0.37 U/mg. Due to these characteristics the presence of high -ketobutyrate concentrations apparently results in a l-valine, l-leucine deficiency. This in turn leads to a derepression of the AHAS synthesis from 0.03 U/mg to 0.29 U/mg and high l-isoleucine production is favoured. The derepression of the AHAS synthesis induced by the l-valine, l-leucine shortage was directly proven with a l-valine, l-leucine, l-isoleucine auxotrophic mutant where the starvation of each amino acid resulted in an increased AHAS level. This is in accordance with the fact that only one AHAS enzyme could be verified by chromatographic and electrophoretic separations as being responsible for the synthesis of all three branched-chain amino-acids.     

Studies on substrate utilisation in l-valine-producing Corynebacterium glutamicum strains deficient in pyruvate dehydrogenase complex

The pyruvate dehydrogenase complex was deleted to increase precursor availability in Corynebacterium glutamicum strains overproducing l-valine. The resulting auxotrophy is treated by adding acetate in addition glucose for growth, resulting in the puzzling fact of gluconeogenic growth with strongly reduced glucose uptake in the presence of acetate in the medium. This result was proven by intracellular metabolite analysis and labelling experiments. To increase productivity, the SugR protein involved in negative regulation of the phosphotransferase system, was inactivated, resulting in enhanced consumption of glucose. However, the surplus in substrate uptake was not converted to l-valine; instead, the formation of up to 289 μM xylulose was observed for the first time in C. glutamicum. As an alternative to the genetic engineering solution, a straightforward process engineering approach is proposed. Acetate limitation resulted in a more efficient use of acetate as cosubstrate, shown by an increased biomass yield Y _X/Ac and improved l-valine formation.     

Synthesis of the penicillin precursor, δ-(l-α-aminoadipyl)-l-cysteinyl-d-valine (ACV), by an immobilized enzyme preparation

Summary -(l--Aminoadipyl)-l-cysteinyl-d-valine (ACV) synthetase activity has been partially-purified from cell-free extracts of Streptomyces clavuligerus by ammonium sulfate precipitation. The salt precipitated enzyme was immobilized on an anion exchange resin and synthesis of ACV was observed by exposing the immobilized enzyme preparation to a reaction mixture containing l--aminoadipic acid, l-valine and l-cysteine in the presence of appropriate cofactors. Reaction mixtures containing l--aminobutyric acid(aB) in place of l-valine synthesized the ACV analog ACaB. Immobilized ACV synthetase can be reused, and after six cycles of reaction, 28.9% of original activity remains.     

Metabolism of amino acid amides in Pseudomonas putida ATCC 12633

The metabolism of the natural amino acid l-valine, the unnatural amino acids d-valine, and d-, l-phenyglycine (d-, l-PG), and the unnatural amino acid amides d-, l-phenylglycine amide (d-, l-PG-NH₂) and l-valine amide (l-Val-NH₂) was studied in Pseudomonas putida ATCC 12633. The organism possessed constitutive l-amidase activities towards l-PG-NH₂ and l-Val-NH₂, both following the same pattern of expression, suggesting the involvement of similarly regulated enzymes, or a common enzyme. Quite surprisingly, growth in mineral media with l-PG-NH₂ resulted in variable, long lag phases of growth and strongly reduced l-amidase activities. Conversion of d-PG-NH₂ into d-PG and l-PG also occurred and could be attributed to the presence of an inducible d-amidase and the racemization of the amino acid amide in combination with l-amidase activity, respectively. The further degradation of l-PG and d-PG involved constitutive l-PG aminotransferase and inducible d-PG dehydrogenase activities, respectively, both with a high degree of enantioselectivity. Amino acid racemase activity for d- and l-PG was not detected. Correspondence to: L. Dijkhuizen     

Effect of l-azetidine 2-carboxilic acid on the activity of the general amino-acid permease from Saccharomyces cerevisiae var. ellipsoideus

Addition of the l-proline analogue l-azetidine 2-carboxylic acid to growing cultures of Saccharomyces cerevisiae var. ellipsoideus promoted fast deactivation of the general aminoacid permease, measured as l-valine uptake, without an immediate decrease in the growth rate. Cells preincubated with the analogue for 3 h were unable to restore either growth ability or general aminoacid permease activity in analogue-free medium. Eadie-Hofstee plots of l-valine uptake in the presence of the analogue are consistent with a strong reduction in the number of active molecules of the general amino-acid permease located in the plasma membrane. Inhibitory effects on protein synthesis were seen after preincubations of the yeast with the analogue for 3 h although a 30 min preincubation had no effect.Abbreviations GAP general amino-acid permease - AZC l-azetidine 2-carboxylic acid - YNB yeast nitrogen base - YE Yeast extract     

Purification and properties of thermostable N-acylamino acid racemase from Amycolatopsis sp. TS-1-60

Thermostable N-acylamino acid recemase from Amycolatopsis sp. TS-1-60, a rare actinomycete strain selected for its ability to grow on agar plates incubated at 40° C, was purified to homogeneity and characterized. The relative molecular mass (M _r) of the native enzyme and the subunit was estimated to be 300 000 and 40 000 on gel filtration chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis respectively. The isoelectric point (pI) of the enzyme was 4.2. The optimum temperature and pH were 50° C and 7.5 respectively. The enzyme was stable at 55° C for 30 min. The enzyme catalyzed the racemization of optically active N-acylamino acids such as N-acetyl-l-or d-methionine, N-acetyl-l-valine, N-acetyl-l-tyrosine and N-chloroacetyl-l-valine. In addition, the enzyme also catalyzed the recemization of the dipeptide l-alanyl-l-methionine. By contrast, the optically active amino acids, N-alkyl-amino acids and methyl and athyl ester derivatives of N-acetyl-d- and l-methionine were not racemized. The apparent K _m values for N-acetyl-l-methionine and N-acetyl-d-methionine were calculated to be 18.5 mM and 11.3 mM respectively. The enzyme activity was markedly enhanced by the addition of divalent metal ions such as Co²⁺, Mn²⁺ and Fe²⁺ and was inhibited by addition of EDTA and P-chloromercuribenzoic acid. The similarity between the NH₂-terminal amino acid sequence of the enzyme and that of Streptomyces atratus Y-53 [Tokuyama et al. (1994) Appl Microbiol Biotechnol 40:835–840] was above 80%.     

Effect ofd-valine and cytosine arabinoside on [3H]thymidine incorporation in rat and rabbit epididymal epithelial cell cultures

Summary Epithelial cell enriched primary cultures were established from the rat and the rabbit epididymis. Epithelial cell aggregates, obtained after pronase digestion of minced epididymis, attached to the culture dish and after 72 h in vitro spread out to form discrete patches of cells. These cells have an epithelioid morphology and form a monolayer of closely apposed polygonal cells where DNA synthesis, as judged by [³H]thymidine uptake, is very low. Inl-valine medium the nonepithelial cell contamination was no more than 10% in rat and rabbit epididymal primary cultures. The labeling index of rat epididymal cells cultured ind-valine medium was significantly lower than that of cells cultured inl-valine medium. In contrast, the labeling index of rabbit epididymal cells cultured ind-valine medium was significantly higher than that of cells cultured inl-valine medium. Cytosine arabinoside decreased the number of labeled cells in bothl-valine andd-valine cultures. From these results, it appears thatd-valine is a selective agent for rat epididymal epithelial cells, but not for rabbit epithelial cells, and that cytosine arabinoside is a simple and effective means to control the proliferation of fibroblast-like cells in both rat and rabbit epididymal cell cultures. This research was sponsored by grants from the National Institute of Child Health and Human Development, Bethesda, MD (HD-03820, HD-11816, HD-05797), and the Mellon Foundation.     

Deacetylation ofN-acetylthienamycin to thienamycin by a cell-free extract ofStreptomyces cattleya,the thienamycin producer

Summary A cell-free extract from the thienamycin producer,Streptomyces cattleya, has been found to deacetylate the co-product,N-acetylthienamycin. The pH optimum of the reaction is 7.5. Due to the lability ofN-acetylthienamycin, we used thed andl forms of the synthetic substrateN-chloroacetylvaline. We found that the enzyme is anl-deacetylase, has a molecular weight of 58 000, is stable up to 40°C, acts optimally at 45°C, is stable at pH 5–8, is not activated by divalent metal ions and is inhibited by Hg⁺⁺, Cu⁺⁺ andp-chloromercuribenzoate. This is the first report of an extract from a carbapenem producer which carries out the deacetylation ofN-acetylthienamycin, suggesting that the acetylated derivative is a precursor of thienamycin.Abbreviations THM thienamycin - N-AcTHM N-acetylthienamycin - CFE cell-free extract - N-Cl-Ac-l-Val N-chloroacetyl-l-valine - N-Cl-Ac-d-Val N-chloroacetyl-d-valine     

Characteristics of long-term human epithelial cell cultures derived from normal human fetal kidney

Summary Epithelial cell cultures were prepared from normal human fetal kidney and established in long-term culture. The growth characteristics and production of keratin, and alkaline phosphatase (AP) and gamma-glutamyl transpeptidase (GGT) activities were compared in a modified minimal essential medium (mMEM),d-valine-containing modified alpha-MEM (mALPHA) andl-valine mALPHA. The mean number of cumulative population doublings (CPDL) was significantly (P<0.001) enhanced with thel-valine mALPHA (40.8 CPDL) over that achievable in mMEM (14.2 CPDL) ord-valine mALPHA (18.3 CPDL) media. In all three media, greater than 95% of the cells in culture produced keratin throughout the life span of these cultures. Surface-associated fibronectin was absent in these cell cultures. AP and GGT activities increased as a function of subpassage and time in culture, with the greatest activity in thel-valine mALPHA. The expression of these renal cell-associated functions suggests that these cells in culture are proximal tubule epithelial cells. The conditions and procedures described in this paper can provide a human kidney epithelial cell culture system for studying human renal function, metabolism, cytotoxicity, genotoxicity, and transformation. Research was supported by a NIEHS (ES 3101) grant to S. M. D’Ambrosio and a NCI grant (CA21104) to J. E. Trosko.     

Effect of pyruvate dehydrogenase complex deficiency on <Emphasis Type="SmallCaps">l</Emphasis>-lysine production with <Emphasis Type="Italic">Corynebacterium glutamicum</Emphasis>

Intracellular precursor supply is a critical factor for amino acid productivity of Corynebacterium glutamicum. To test for the effect of improved pyruvate availability on l-lysine production, we deleted the aceE gene encoding the E1p enzyme of the pyruvate dehydrogenase complex (PDHC) in the l-lysine-producer C. glutamicum DM1729 and characterised the resulting strain DM1729-BB1 for growth and l-lysine production. Compared to the host strain, C. glutamicum DM1729-BB1 showed no PDHC activity, was acetate auxotrophic and, after complete consumption of the available carbon sources glucose and acetate, showed a more than 50% lower substrate-specific biomass yield (0.14 vs 0.33 mol C/mol C), an about fourfold higher biomass-specific l-lysine yield (5.27 vs 1.23 mmol/g cell dry weight) and a more than 40% higher substrate-specific l-lysine yield (0.13 vs 0.09 mol C/mol C). Overexpression of the pyruvate carboxylase or diaminopimelate dehydrogenase genes in C. glutamicum DM1729-BB1 resulted in a further increase in the biomass-specific l-lysine yield by 6 and 56%, respectively. In addition to l-lysine, significant amounts of pyruvate, l-alanine and l-valine were produced by C. glutamicum DM1729-BB1 and its derivatives, suggesting a surplus of precursor availability and a further potential to improve l-lysine production by engineering the l-lysine biosynthetic pathway. This study is dedicated to Prof. Dr. Hermann Sahm on the occasion of his 65th birthday.     

Feedback inhibition of homoserine dehydrogenase and threonine deaminase in the genusBifidobacterium

Feedback inhibition of crude and purified extracts of homoserine dehydrogenase and threonine deaminase activities in the genusBifidobacterium was studied. Homoserine dehydrogenase was partially or completely inhibited byl-threonine, and a marked inhibitory effect byl-isoleucine on threonine deaminase was observed. In the speciesBifidobacterium cuniculi high levels ofl-valine reversed the inhibitory effect ofl-isoleucine. The -aminobutyric acid-resistant mutant Ru 326/106 of the speciesB. ruminale, overproducer ofl-isoleucine, had a derepressed homoserine dehydrogenase and a lesser feedback inhibition byl-threonine. Homoserine dehydrogenase appeared to be in bifids specifically NAD dependent. The regulatory mechanisms of aspartate family amino acid biosynthesis in bifidobacteria was discussed.     

Therlt11 andraec1 mutants ofArabidopsis thaliana lack the activity of a basic-amino-acid transporter

The concentration dependence of the influx ofl-lysine in excised roots ofArabidopsis thaliana seedlings was analyzed for the wild-type (WT) and two mutants,rlt11 andraec1, which had been selected as resistant to lysine plus threonine, and to S-2-aminoethyl-l-cysteine, respectively. In the WT three components were resolved: (i) a high-affinity, low-capacity component [K _m = 2.2 M;V _max = 23 nmol·(g FW)^–1·h^–1]; (ii) a low-affinity, high-capacity component [K _m = 159 M;V _max = 742 nmol·(g FW)^–1·h^–1]; (iii) a component which is proportional to the external concentration, with a constant of proportionalityk = 104 nmol·(g FW)^–1 h^–1];·mM^–1. The influx ofl-lysine in the mutants was lower than in the WT, notably in the concentration range 0.1–0.4 mM, where it was only 7% of that in the WT. In both mutants the reduced influx could be fully attributed to the absence of the low-affinity (high-K _m ) component. This component most likely represents the activity of a specific basic-amino-acid transporter, since it was inhibited by several other basic amino acids (arginine, ornithine, hydroxylysine, aminoethylcysteine) but not byl-valine. The high-affinity uptake ofl-lysine may be due to the activity of at least two general amino acid transporters, as it was inhibitable byl-valine, and could be further dissected into two components with a high affinity (K _i = 1–5 M; and a low affinity (K _i = 0.5–1mM) forl-valine, respectively. Therlt11 andraecl mutant have the same phenotype and the corresponding loci were mapped on chromosome 1, but it is not yet clear whether they are allelic.Abbreviations AEC S-2-aminoethyl-l-cysteine - K _i equilibrium constant - WT wild-type     

Removal of <Emphasis Type="SmallCaps">l</Emphasis>-alanine from the production of <Emphasis Type="SmallCaps">l</Emphasis>-2-aminobutyric acid by introduction of alanine racemase and <Emphasis Type="SmallCaps">d</Emphasis>-amino acid oxidase

l-2-Aminobutyric acid can be synthesized in a transamination reaction from l-threonine and l-aspartic acid as substrates by the action of threonine deaminase and aromatic aminotransferase, but the by-product l-alanine was produced simultaneously. A small amount of l-alanine increased the complexity of the l-2-aminobutyric acid recovery process because of their extreme similarity in physical and chemical properties. Acetolactate synthase has been introduced to remove the pyruvate intermediate for reducing the l-alanine concentration partially. To eliminate the remnant l-alanine, alanine racemase of Bacillus subtilis in combination with d-amino acid oxidase of Rhodotorula gracilis or Trigonopsis variabilis respectively was introduced into the reaction system for the l-2-aminobutyric acid synthesis. l-Alanine could be completely removed by the action of alanine racemase of B. subtilis and d-amino acid oxidase of R. gracilis; thereby, high-purity l-2-aminobutyric acid was achieved. The results revealed that alanine racemase could discriminate effectively between l-alanine and l-2-aminobutyric acid, and selectively catalyzed l-alanine to d-alanine reversibly. d-Amino acid oxidase then catalyzed d-alanine to pyruvate stereoselectively. Furthermore, this method was also successfully used to remove the by-product l-alanine in the production of other neutral amino acids such as l-tertiary leucine and l-valine, suggesting that multienzymatic whole-cell catalysis can be employed to provide high purity products.