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PubMedDate not supplied

[Usefulness of the integration between the values of the 1-hour blood xylose test and the hydrogen breath-test after xylose oral load for the indirect evaluation of mucosa damage].

Considering 50 children affected by sub-acute gastrointestinal diseases by severe growth disorders, we have compared the "one-hour blood xylose test" with the "xylose and lactose H2 breath test, looking for a relationship with the duodeno-jejunal mucosal damage. Finally the integration between the "one hour blood xylose test" and the "xylose H2 breath test" may be useful in order to compare more exactly the results of both xylose tests with the mucosal damage. Lactose H2 breath test seems less reliable for our purposes because of the possible presence of children with lactase deficiences, hardly comparable with the mucosal damage.

Breath Tests↗
PubMed1978

Evaluation of xylose absorption as measured in blood and urine: a one-hour blood xylose screening test in malabsorption.

The assessment of D-xylose absorption by measurement of urinary excretion is subject to several sources of error. An investigation has been made into the use of the blood levels obtained after a 5-g dose as an assessment of malabsorption. We have shown that optimal discrimination between normal and impaired xylose absorption is achieved by measuring blood xylose levels 1 hr after oral administration and then correcting the results to a constant body surface area. The reference range was 0.65 to 1.33 mmoles per liter for values corrected to 1.73m2. In a prospective study of the value of this modification in investigating malabsorption, the incidence of false-negatives and false-positives was found to be 4.8 and 2.2%, respectively. The test is of particular value in the elderly because surface area corrected 1-hr blood xylose levels are independent of age, eliminating the expected and observed declines in renal function and urinary xylose excretion in this age group.

False Negative Reactions↗
PubMed1996

Exploring the interaction between D-xylose isomerase and D-xylose by free energy calculation.

The numerical quadrature thermodynamic integration method is used to investigate enzyme-substrate interaction of D-xylose isomerase. A screening function for the coulombic interaction is introduced into the simulation to correct the effect of finite cut-off radius for the non-bonded interaction. The binding free energy difference for D-xylose with D-xylose isomerase and its N184D mutant has been calculated, and the result 3.9 +/- 1.2 kJ/mol agrees well with experimental data of 4.38 kJ/mol. In addition, the structure and dynamics of enzyme-substrate complex were simulated for mutant and wild-type enzyme, respectively. Analysis of the structures and intramolecular interactions of the complexes were found to be valuable for understanding the reaction mechanism of the enzyme D-xylose isomerase.

Aldose-Ketose Isomerases↗
PubMed1991

Complementation of the inability of Lactobacillus strains to utilize D-xylose with D-xylose catabolism-encoding genes of Lactobacillus pentosus.

The inability of two Lactobacillus strains to ferment D-xylose was complemented by the introduction of Lactobacillus pentosus genes encoding D-xylose isomerase, D-xylulose kinase, and a D-xylose catabolism regulatory protein. This result opens the possibility of using D-xylose fermentation as a food-grade selection marker for Lactobacillus spp.

Aldose-Ketose Isomerases↗
PubMed2003

Changing flux of xylose metabolites by altering expression of xylose reductase and xylitol dehydrogenase in recombinant Saccharomyces cerevisiae.

We changed the fluxes of xylose metabolites in recombinant Saccharomyces cerevisiae by manipulating expression of Pichia stipitis genes (XYL1 and XYL2) coding for xylose reductase (XR) and xylitol dehydrogenase (XDH), respectively. XYL1 copy number was kept constant by integrating it into the chromosome. Copy numbers of XYL2 were varied either by integrating XYL2 into the chromosome or by transforming cells with XYL2 in a multicopy vector. Genes in all three constructs were under control of the strong constitutive glyceraldehyde-3-phosphate dehydrogenase promoter. Enzymatic activity of XR and XDH in the recombinant strains increased with the copy number of XYL1 and XYL2. XR activity was not detected in the parent but was present at a nearly constant level in all of the transformants. XDH activity increased 12-fold when XYL2 was on a multicopy vector compared with when it was present in an integrated single copy. Product formation during xylose fermentation was affected by XDH activity and by aeration in recombinant S. cerevisiae. Higher XDH activity and more aeration resulted in less xylitol and more xylulose accumulation during xylose fermentation. Secretion of xylulose by strains with multicopy XYL2 and elevated XDH supports the hypothesis that D-xylulokinase limits metabolic flux in recombinant S. cerevisiae.

Aldehyde Reductase↗
PubMed1996

Effects of oral lactose and xylose loads on blood glucose, galactose, xylose, and insulin values in healthy calves and calves with diarrhea.

OBJECTIVE: 2 hypotheses were tested: calves with acute, mild diarrhea digest lactose less efficiently than healthy calves, and they are in a catabolic state, which influences plasma glucose concentration after glucose absorption. DESIGN: Clinical study; 2 treatments with 10 repetitions/treatment. ANIMALS: 20 preruminant Brown Swiss and Simmental Red Holstein calves; 10 calves with mild diarrhea, and 10 age-matched healthy calves. PROCEDURE: Blood metabolite and hormone concentrations were determined before and after an oral lactose load. Plasma xylose concentration was determined after an oral xylose load. III calves were tested 1 day after the onset of diarrhea. RESULTS: Calves with diarrhea (cryptosporidia, coronavirus) had lower preprandial concentrations of plasma glucose, insulin-like growth factor 1, and 3,5,3-triiodothyronine (P < 0.01) and a higher concentration of free fatty acids (P < 0.03) than did healthy calves. After the oral lactose and xylose loads, blood galactose and plasma xylose concentrations were lower in ill calves (P = 0.10 and P = 0.07, respectively). In calves with diarrhea, there was a larger increase of plasma glucose concentration (P = 0.12) and a smaller increase of plasma insulin concentration (P = 0.04) above baseline values after lactose ingestion. CONCLUSIONS: Lactose digestion is slightly impaired in calves with mild diarrhea. Calves with acute diarrhea are in a catabolic state and, therefore, respond with a larger increase of plasma glucose concentration to a given amount of absorbed glucose than do healthy calves. CLINICAL-RELEVANCE: Plasma glucose concentration is not a reliable measure for glucose absorption in animals that are in a catabolic state.

Acute Disease↗
PubMed2003

Direct production of D-arabinose from D-xylose by a coupling reaction using D-xylose isomerase, D-tagatose 3-epimerase and D-arabinose isomerase.

Klebsiella pneumoniae 40bXX, a mutant strain that constitutively produces D-arabinose isomerase (D-AI), was isolated through a series of repeated subcultures from the parent strain on a mineral salt medium supplemented with L-Xylose as the sole carbon source. D-AI could be efficiently immobilized on chitopearl beads. The optimum temperature for the activity of the immobilized enzyme was 40 degrees C and the enzyme was stable up to 50 degrees C. The D-Al was active at pH 10.0 and was stable in the range of pH 6.0-11.0. The enzyme required manganese ions for maximum activity. Three immobilized enzymes, D-xylose isomerase (D-XI), D-tagatose 3-epimerase (D-TE and D-AI were used for the preparation of D-arabinose from D-xylose in a coupling reaction. After completion of the reaction, degradation of D-xylulose was carried out by Saccharomyces cerevisiae. The reaction mixture containing D-Xylose, D-ribulose and the product was then separated by ion exchange column chromatography. After crystallization, the product was checked by HPLC, IR spectroscopy, NMR spectroscopy and optical rotation measurements. Finally, 2.0 g of D-arabinose could be obtained from 5 g of the substrate.

Journal Article↗
PubMed1997

The reaction pathway of the isomerization of D-xylose catalyzed by the enzyme D-xylose isomerase: a theoretical study.

Different pathways of the metal-induced isomerization of D-xylose to D-xylulose are investigated and compared in detail using energy minimization and molecular dynamics simulation. Two theoretical models are constructed for the reaction: in vacuum and in the enzyme D-xylose isomerase. The vacuum model is constructed based on the X-ray structure of the active site of D-xylose isomerase. It contains the atoms directly involved in the reaction and is studied using a semi-empirical molecular orbital method (PM3). The model in the enzyme includes the effects of the enzyme environment on the reaction using a combined quantum mechanical and molecular mechanical potential. For both models, the structures of the reactants, products, and intermediate complexes along the isomerization pathway are optimized. The effects of the position of the "catalytic Mg2+ ion" on the energies of the reactions are studied. The results indicate: 1) in vacuum, the isomerization reaction is favored when the catalytic metal cation is at site A, which is remote from the substrate; 2) in the enzyme, the catalytic metal cation, starting from site A, moves and stays at site B, which is close to the substrate; analysis of the charge redistribution of the active site during the catalytic process shows that the metal ion acts as a Lewis acid to polarize the substrate and catalyze the hydride shift; these results are consistent with previous experimental observations; and 3) Lys183 plays an important role in the isomerization reaction. The epsilon-NH3+ group of its side chain can provide a proton to the carboxide ion of the substrate to form a hydroxyl group after the hydride shift step. This role of Lys183 has not been suggested before. Based on our calculations, we believe that this is a reasonable mechanism and consistent with site-directed mutation experiments.

Aldose-Ketose Isomerases↗
PubMed1996

Contributions of XylR CcpA and cre to diauxic growth of Bacillus megaterium and to xylose isomerase expression in the presence of glucose and xylose.

Bacillus megaterium shows diauxic growth in minimal medium containing glucose and xylose. We have examined the influence of three elements that regulate xyl operon expression on diauxic growth and expression of a xylA-lacZ fusion. xylA is 13-fold repressed during growth on glucose. Induction occurs at the onset of the lag phase after glucose is consumed. Inactivation of xylR yields a two-fold increase in expression of xylA on glucose. Deletion of the catabolite responsive element (cre) has a more pronounced effect, reducing glucose repression from 13-fold in the wild type to about 2.5-fold. When xylR and cre are inactivated together a residual two-fold repression of xylA is found. Inactivation of xylR affects diauxic growth by shortening the lag phase from 70 to 40 min. In-frame deletion of ccpA results in the loss of diauxic growth, an increase in doubling time and simultaneous use of both sugars. In contrast, a strain with an inactivated cre site in xylA exhibits diauxic growth without an apparent lag phase on glucose and xylose, whereas fructose and xylose are consumed simultaneously.

Aldose-Ketose Isomerases↗
PubMed2000

Double oxidation of D-xylose to D-glycero -pentos-2,3-diulose (2,3-diketo-D-xylose) by pyranose dehydrogenase from the mushroom Agaricus bisporus.

Pyranose dehydrogenase purified to homogeneity from the mycelia of the basidiomycete fungus Agaricus bisporus catalyzed the oxidation of D-xylose at C-2 to D-threo-pentos-2-ulose (2-keto-D-xylose) and successively at C-3 to D-glycero-pentos-2,3-diulose (2,3-diketo-D-xylose) using 1,4-benzoquinone as an electron acceptor. The sites of oxidation were deduced from the spectroscopic analysis (MS, NMR) of the N,N-diphenylhydrazone derivatives of the reaction products.

Agaricus↗
PubMed1996

Ethanolic fermentation of xylose with Saccharomyces cerevisiae harboring the Thermus thermophilus xylA gene, which expresses an active xylose (glucose) isomerase.

The Thermus thermophilus xylA gene encoding xylose (glucose) isomerase was cloned and expressed in Saccharomyces cerevisiae under the control of the yeast PGK1 promoter. The recombinant xylose isomerase showed the highest activity at 85 degrees C with a specific activity of 1.0 U mg-1. A new functional metabolic pathway in S. cerevisiae with ethanol formation during oxygen-limited xylose fermentation was demonstrated. Xylitol and acetic acid were also formed during the fermentation.

Aldose-Ketose Isomerases↗
PubMedDate not supplied

[Specific features of fermentation of D-xylose and D-glucose by xylose-assimilating yeasts].

The ability to assimilate D-glucose and D-xylose was studied in 21 yeast species of the following genera: Candida, Kluyveromyces, Pachysolen, Pichia, and Torulopsis. All the cultures fermented D-glucose with the formation of ethanol. During the assimilation of D-xylose, ethanol was produced by P. stipitis and C. shehatae, whereas xylitol was produced by C. didensiae, C. intermediae, C. parapsilosis, C. silvanorum, C. tropicalis, K. fragilis, K. marxianus, P. guillermondii, and T. molishiama. The yeast P. tannophilus produced comparable amounts of both alcohols. The possible use of xylose-assimilating yeasts for the production of xylitol and ethanol is discussed.

Ethanol↗
PubMed1991

Cloning and expression in Saccharomyces cerevisiae of the NAD(P)H-dependent xylose reductase-encoding gene (XYL1) from the xylose-assimilating yeast Pichia stipitis.

The XYL1 gene of the yeast Pichia stipitis has been isolated from a genomic library using a specific cDNA probe, and its nucleotide (nt) sequence has been determined. In the 5' noncoding region of the P. stipitis XYL1 gene a TATAAA element (known to be necessary for transcription initiation in most yeast genes) is found at nt -81, and two CCAAT recognition motifs (often referred to as the CCAAT box) are present at nt -146 and -106. The XYL1 encodes a polypeptide of 35,927 Da that constitutes a NADH/NADPH-dependent xylose reductase (XR). The enzyme is part of the xylose-xylulose pathway that is absent or only weakly expressed in Saccharomyces cerevisiae. Extensive homology is found to the N terminus of the XR of Pachysolen tannophilus and Candida shehatae. None of the known cofactor binding domains found in many NAD-dependent dehydrogenases are present in the protein. Transformants of S. cerevisiae containing XYL1 of P. stipitis synthesize an active XR. Fusion of XYL1 with the prokaryotic tac promoter leads to a gene that can be expressed in S. cerevisiae and Escherichia coli.

Aldehyde Reductase↗
PubMed1992

Heteropolysaccharide Formation by Arthrobacter viscosus Grown on Xylose and Xylose Oligosaccharides.

Arthrobacter viscosus NRRL B-1973 produces its viscous extracellular polysaccharides (EPS) when grown on media containing xylose or enzymatic xylan hydrolysates. Crude EPS formation from xylose averaged 12 g/liter when initial culture pH was adjusted to 8.0 and total nitrogen was limited to 0.03%. Purified EPS from pentose and hexose substrates were analyzed for their monosaccharide, acetyl, and uronic acid components, intrinsic viscosities, and average molecular masses. Differences were apparent in degrees of acetylation, molecular masses, and intrinsic viscosities of the heteropolysaccharides produced on different carbon sources.

Journal Article↗
PubMed1992

Molecular mechanics simulations of a conformational rearrangement of D-xylose in the active site of D-xylose isomerase.

A proposed reaction mechanism for the enzyme D-xylose isomerase involves the ring opening of the cyclic substrate with a subsequent conformational rearrangement to an extended open-chain form. Restrained energy minimization was used to simulate the rearrangement. In the ring-opening step, the substrate energy function was gradually altered from a cyclic to an open-chain form, with energy minimization after each change. The protein/sugar contact energy did not increase significantly during the process, showing that there was no steric hindrance to ring opening. The conformational rearrangement involves an alteration in the coordination of the substrate to metal ion [1], which was induced by gradually changing restraints on metal/ligand distances. By allowing varying amounts of flexibility in the protein and examining a simplified model system, the interactions of the sugar with metal ion [1] and its immediate ligands were found to be the most important contributors to the energy barrier for the change. Only small changes in the positions of protein atoms were required. The energy barrier to the rearrangement was estimated to be less than the Arrhenius activation energy for the enzymatic reaction. This is in accordance with experimental indications that the isomerization step is rate determining.

Aldose-Ketose Isomerases↗
PubMed1992

Regulation of the Bacillus subtilis W23 xylose utilization operon: interaction of the Xyl repressor with the xyl operator and the inducer xylose.

A crude protein extract of Bacillus subtilis W23 contains a sequence-specific DNA binding activity for the xyl operator as detected by the gel mobility shift assay. A xylR determinant encoded on a multicopy plasmid leads to increased expression of this binding activity. In situ footprinting analysis of the protein-DNA complex in a polyacrylamide gel shows that the xyl operator is sequence-specifically bound and protected from cleavage by copper-phenanthroline at 26 phosphodiester bonds on each strand. Quantitative competition assays for repressor binding reveal that a 25 bp synthetic xyl operator cloned into a polylinker is bound with the same affinity as the operator in the wild-type xyl regulatory region. This confirms that no additional sites in the wild-type sequence contribute to repressor binding. The xyl operator consists of ten palindromic base pairs flanking five central non-palindromic base pairs. A mutational analysis shows that the sequence of the central base pairs contributes to recognition by the repressor protein and that the spacing of the palindromic elements is crucial for repressor binding. An operator half site is not bound by the repressor. In vivo and in vitro induction studies suggest that, of several structurally similar sugars, xylose is the only molecular inducer of the Xyl repressor.

Bacillus subtilis↗
PubMed2001

Structural and functional properties of aldose xylose reductase from the D-xylose-metabolizing yeast Candida tenuis.

The primary structure of the aldose xylose reductase from Candida tenuis (CtAR) is shown to be 39% identical to that of human aldose reductase (hAR). The catalytic tetrad of hAR is completely conserved in CtAR (Tyr51, Lys80, Asp46, His113). The amino acid residues involved in binding of NADPH by hAR (D.K. Wilson, et al., Science 257 (1992) 81-84) are 64% identical in CtAR. Like hAR the yeast enzyme is specific for transferring the 4-pro-R hydrogen of the coenzyme. These properties suggest that CtAR is a member of the aldo/keto reductase superfamily. Unlike hAR the enzyme from C. tenuis has a dual coenzyme specificity and shows similar specificity constants for NADPH and NADH. It binds NADP(+) approximately 250 times less tightly than hAR. Typical turnover numbers for aldehyde reduction by CtAR (15-20 s(-1)) are up to 100-fold higher than corresponding values for hAR, probably reflecting an overall faster dissociation of NAD(P)(+) in the reaction catalyzed by the yeast enzyme.

Aldehyde Reductase↗
PubMed1992

Protein engineering of xylose (glucose) isomerase from Actinoplanes missouriensis. 2. Site-directed mutagenesis of the xylose binding site.

Site-directed mutagenesis in the active site of xylose isomerase derived from Actinoplanes missouriensis is used to investigate the structural and functional role of specific residues. The mutagenesis work together with the crystallographic studies presented in detail in two accompanying papers adds significantly to the understanding of the catalytic mechanism of this enzyme. Changes caused by introduced mutations emphasize the correlation between substrate specificity and cation preference. Mutations in both His 220 and His 54 mainly affect the catalytic rate constant, with catalysis being severely reduced but not abolished, suggesting that both histidines are important, but not essential, for catalysis. Our results thus challenge the hypothesis that His 54 acts as an obligatory catalytic base for ring opening; this residue appears instead to be implicated in governing the anomeric specificity. With none of the active site histidines acting as a catalytic base, the role of the cations in catalyzing proton transfer is confirmed. In addition, Lys 183 appears to play a crucial part in the isomerization step, by assisting the proton shuttle. Other residues also are important but to a lesser extent. The conserved Lys 294 is indirectly involved in binding the activating cations. Among the active site aromatic residues, the tryptophans (16 and 137) play a role in maintaining the general architecture of the substrate binding site while the role of Phe 26 seems to be purely structural.

Actinomycetales↗