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At least 19 recordsLinked to original sources

PubMed1987

Ketose reductase activity in developing maize endosperm.

Ketose reductase (NAD-dependent polyol dehydrogenase EC 1.1.1.14) activity, which catalyzes the NADH-dependent reduction of fructose to sorbitol (d-glucitol), was detected in developing maize (Zea mays L.) endosperm, purified 104-fold from this tissue, and partially characterized. Product analysis by high performance liquid chromatography confirmed that the enzyme-catalyzed reaction was freely reversible. In maize endosperm, 15 days after pollination, ketose reductase activity was of the same order of magnitude as sucrose synthase activity, which produces fructose during sucrose degradation. Other enzymes of hexose metabolism detected in maize endosperm were present in activities of only 1 to 3% of the sucrose synthase activity. CaCl(2), MgCl(2), and MnCl(2) stimulated ketose reductase activity 7-, 6-, and 2-fold, respectively, but had little effect on NAD-dependent polyol dehydrogenation (the reverse reaction). The pH optimums for ketose reductase and polyol dehydrogenase reactions were 6.0 and 9.0, respectively. K(m) values were 136 millimolar fructose and 8.4 millimolar sorbitol. The molecular mass of ketose reductase was estimated to be 78 kilodaltons by gel filtration. It is postulated that ketose reductase may function to metabolize some of the fructose produced during sucrose degradation in maize endosperm, but the metabolic fate of sorbitol produced by this reaction is not known.

Journal Article↗

PubMed2001

Substrate-dependent chemoselective aldose-aldose and aldose-ketose isomerizations of carbohydrates promoted by a combination of calcium ion and monoamines.

Epimerization of aldoses at C-2 has been extensively investigated by using various metal ions in conjunction with diamines, monoamines, and aminoalcohols. Aldoses are epimerized at C-2 by a combination of alkaline-earth or rare-earth metal ions (Ca(2+), Sr(2+), Pr(3+), or Ce(3+)) and such monoamines as triethylamine. In particular, the Ca(2+)-triethylamine system proved effective in promoting aldose-ketose isomerization as well as C-2 epimerization of aldoses. 13C NMR studies using D-(1-(13)C)glucose and D-(1-(13)C)galactose with the CaCl(2) system in CD(3)OD revealed that the C-2 epimerization proceeds via stereospecific rearrangement of the carbon skeleton, or 1,2-carbon shift, and ketose formation proceeds partially through an intramolecular hydrogen migration or 1,2-hydride shift and, in part, via an enediol intermediate. These simultaneous aldose-aldose and aldose-ketose isomerizations showed interesting substrate-dependent chemoselectivity. Whereas the mannose-type aldoses having 2,3-erythro configuration (D-mannose, D-lyxose, and D-ribose) showed considerable resistance to both the C-2 epimerization and the aldose-ketose isomerization, the glucose-type sugars having 2,3-threo and 3,4-threo configurations, D-glucose and D-xylose, are mainly epimerized at C-2 and those having the 2,3-threo and 3,4-erythro configurations, D-galactose and D-arabinose, were mostly isomerized into 2-ketoses. These features are of potential interest in relevance to biomimic sugar transformations by metal ions.

Aldose-Ketose Isomerases↗

PubMed2001

In situ product removal of ketoses by immobilized 3-amino phenyl boronic acid: effect of immobilization method on pH profile.

The use of polymeric derivatives of phenylboronic acid (PBA) as an effective means for specific in situ product removal of ketoses from aldose-containing reaction mixtures, strongly depends on the retention of selective binding of ketoses exhibited by soluble PBA and 3-amino PBA, by their polymeric, water insoluble analogs. In this communication we demonstrate that immobilization chemistry has a strong effect on ketose preferred binding by polymeric PBA derivatives. Our results indicate that for the preparation of an effective and more specific adsorbent, 3-amino PBA should be coupled to the polymeric carrier via alkylamino chemistry and not via the commonly employed amido derivative. Immobilized alkylamino-PBA exhibited selective fructose and xylulose binding throughout glucose and xylose isomerization processes at the pH range of 7.0-8.0.

Boronic Acids↗

PubMed1984

Colorimetric method for the determination of ketoses using phenol-acetone-boric acid reagent (PABR).

A purplish pink color developed when ketose solution (100 microliters) was mixed with phenol-acetone-boric acid reagent (0.5 ml 5% phenol, 2% acetone, 4% boric acid) and then treated with 96% sulfuric acid (1.4 ml). The absorbance of the reaction mixture was measured at 568 nm after 60 min at 37 degrees C. This method allowed the simple determination of 3-50 nmol of D-fructose with coefficient of variation 7.8% for 3 nmol and 2.8% for 50 nmol. Carbohydrates other then ketoses did not interfere with this reaction. The influence of various chemicals on the colorimetric reaction and applicability of the method for determination of ketoses in natural products are presented.

Acetone↗

PubMed1993

Theoretical examination of the mechanism of aldose-ketose isomerization.

Two mechanisms for an aldose-ketose isomerization have been examined using high level ab initio and semiempirical molecular orbital methods. The proton transfer pathway via an enediol intermediate is shown to be favored in the absence of a metal ion, while the hydride transfer pathway becomes favored in the presence of a metal ion. Our calculations explain why the proton transfer pathway is operative in most aldose-ketose isomerization reactions. These calculations also provide further support for the previously proposed metal ion-mediated hydride transfer mechanism for xylose isomerase.

Aldehydes↗

PubMed1976

Structural functions of taste in the sugar series VII: taste properties of ketoses.

The preferable structural feature for sweetness is a lipophilic moiety, e.g., a five- or six-membered ring with a polar substituent containing an A-H/B system outside the ring. In ketoses, this unit is probably the 1,2-glycol. If a third feature (the lipophilic, gamma, site) is required for the attainment of optimum sweetness and if it is C-6 in ketoses, then this site can accommodate quite a large constituent, both above and below the plane of the ring. The removal of the hydroxyl group from the C-6 hydroxymethyl substituent to yield the 7-deoxy derivatives causes bitterness, thus implicating the primary hydroxymethyl group with bitterness.Therefore, the creation of lipophilic site(s) in the sugar ring causes the realignment of the sugar molecule on the taste receptor surface. The distrubance of the proposed A-H/B system, e.g., the removal of the C-2 hydroxyl group, causes the ring alpha-glycol unit (most likely the ring C-3 and C-4 hydroxyl groups) to function as the A-H/B system.

Binding Sites↗

PubMed1987

Isolation of ketoses or nonreducing sugars from a sugar mixture containing aldoses by aniline treatment.

A method for the isolation of ketoses and nonreducing sugars from sugar mixtures by the addition of aniline, in the presence of acetic acid as a catalyst, is described. The aldoses react readily to form aniline derivatives, which are then removed using standard procedures: extraction with organic solvents, and treatment with activated charcoal and with a cation-exchange resin. The ketoses and nonreducing sugars are retained in aqueous solution.

Acetates↗

PubMed1999

Metabolism of D-glycero-D-manno-heptitol, volemitol, in polyanthus. Discovery Of a novel ketose reductase1

Volemitol (D-glycero-D-manno-heptitol, alpha-sedoheptitol) is an unusual seven-carbon sugar alcohol that fulfills several important physiological functions in certain species of the genus Primula. Using the horticultural hybrid polyanthus (Primula x polyantha) as our model plant, we found that volemitol is the major nonstructural carbohydrate in leaves of all stages of development, with concentrations of up to 50 mg/g fresh weight in source leaves (about 25% of the dry weight), followed by sedoheptulose (D-altro-2-heptulose, 36 mg/g fresh weight), and sucrose (4 mg/g fresh weight). Volemitol was shown by the ethylenediaminetetraacetate-exudation technique to be a prominent phloem-mobile carbohydrate. It accounted for about 24% (mol/mol) of the phloem sap carbohydrates, surpassed only by sucrose (63%). Preliminary 14CO2 pulse-chase radiolabeling experiments showed that volemitol was a major photosynthetic product, preceded by the structurally related ketose sedoheptulose. Finally, we present evidence for a novel NADPH-dependent ketose reductase, tentatively called sedoheptulose reductase, in volemitol-containing Primula species, and propose it as responsible for the biosynthesis of volemitol in planta. Using enzyme extracts from polyanthus leaves, we determined that sedoheptulose reductase has a pH optimum between 7.0 and 8.0, a very high substrate specificity, and displays saturable concentration dependence for both sedoheptulose (apparent Km = 21 mM) and NADPH (apparent Km = 0.4 mM). Our results suggest that volemitol is important in certain Primula species as a photosynthetic product, phloem translocate, and storage carbohydrate.

Journal Article↗

PubMedDate not supplied

[Fructose-1,6-diphosphate aldolase and ketose-1-phosphate aldolase isoenzyme spectra in the kidneys normally and in uranyl acetate poisoning].

Activities of both aldolases were distinctly decreased in acute kidney insufficiency, caused by administration of uranyl acetate. The decrease in the activity of fructose-1,6-diphosptate aldolase was more distinct which led to alteration in ratio between fructose -1,6-diphosphate and ketose-1-phosphate aldolases. The isozyme spectra of the enzymes were also altered. Isozymes of fructose-1,6-diphosphate aldolase AB2 and of ketose-1-phosphate aldolase form 22 possessed the same electrophoretic mobility but their relative activity was increased 2-3-fold under conditions of the acute kidney insufficiency. At the same time, the more electrophoretically mobile isozymes were not detected.

Acetates↗

PubMed1993

Capillary zone electrophoresis of p-aminobenzoic acid derivatives of aldoses, ketoses and uronic acids.

Aldoses, ketoses and uronic acids were derivatized with p-aminobenzoic acid and separated as their borate complexes by capillary zone electrophoresis, using a capillary tube of fused silica containing 150 mM borate buffer, pH 10.0, as carrier. The electrophoretic mobilities of 22 carbohydrates were determined and found to increase with increasing stability of the borate complexes formed. Besides the number of hydroxyl groups and the presence of substituents, complex stability depended most strongly on the configuration of the three vicinal hydroxyl groups at C2, C3 and C4. On-column UV monitoring at 285 nm allowed the detection of glucose with a lower mass detection limit of 15 fmol and a concentration sensitivity of 4 microM. Reproducible quantification of carbohydrates was achieved at least in the concentration range of 0.1-10 mM in reaction solutions by the relative peak area method, using cinnamic acid as internal standard. The method was applied successfully to the determination of the monosaccharide composition of polysaccharides extracted from Radix althaeae.

4-Aminobenzoic Acid↗

PubMedDate not supplied

Determination of aldoses and ketoses by GC-MS using differential derivatisation.

A method has been established by which to determine aldoses and ketoses in plant material simultaneously. Monosaccharides were extracted by sonication with 80% ethanol and sugar oximes formed by treatment of the resultant extract with hydroxylamine and pyridine at 90 degrees C. After reaction, one aliquot of the product was derivatised with acetic anhydride at 90 degrees C, whilst a second aliquot was silylated with HMDS and TMCS at 80 degrees C. Both reaction mixtures were analysed by GC-MS in the SIM mode. Quantivation was linear within the range 1-4 microg/mL and the detection limit for monosaccharides was 5-25 ng/mL. The absolute recoveries were between 73.0 and 90.2% and the RSDs were 3.1-10.0%. This method was applied to analyse the free monosaccharides in Lyceum barbarum L.; eight monosaccharides were present in amounts between 0.26 and 368.65 microg/mg.

Carbohydrates↗

PubMed2001

Crystal structure of the NADP(H)-dependent ketose reductase from Bemisia argentifolii at 2.3 A resolution.

Polyhydric alcohols are widely found in nature and can be accumulated to high concentrations as a protection against a variety of environmental stresses. It is only recently, however, that these molecules have been shown to be active in protection against heat stress, specifically in the use of sorbitol by the silverleaf whitefly, Bemisia argentifolii. We have determined the structure of the enzyme responsible for production of sorbitol in Bemisia argentifolii, NADP(H)-dependent ketose reductase (BaKR), to 2.3 A resolution. The structure was solved by multiwavelength anomalous diffraction (MAD) using the anomalous scattering from two zinc atoms bound in the structure, and was refined to an R factor of 21.9 % (R(free)=25.1 %). BaKR belongs to the medium-chain dehydrogenase family and its structure is the first for the sorbitol dehydrogenase branch of this family. The enzyme is tetrameric, with the monomer having a very similar fold to the alcohol dehydrogenases (ADHs). Although the structure determined is for the apo form, a phosphate ion in the active site marks the likely position for the adenyl phosphate of NADP(H). The catalytic zinc ion is tetrahedrally coordinated to Cys41, His66, Glu67 and a water molecule, in a modification of the zinc site usually found in ADHs. This modified zinc site seems likely to be a conserved feature of the sorbitol dehydrogenase sub-family. Comparisons with other members of the ADH family have also enabled us to model a ternary complex of the enzyme, and suggest how structural differences may influence coenzyme binding and substrate specificity in the reduction of fructose to sorbitol.

Alcohol Oxidoreductases↗

PubMed1990

Mechanism for aldose-ketose interconversion by D-xylose isomerase involving ring opening followed by a 1,2-hydride shift.

The active site and mechanism of D-xylose isomerase have been probed by determination of the crystal structures of the enzyme bound to various substrates, inhibitors and cations. Ring-opening is an obligatory first step of the reaction and is believed to be the rate-determining step for the aldose to ketose conversion. The structure of a complex with a cyclic thio-glucose has been determined and it is concluded that this is an analogue of the Michaelis complex. At -10 degrees C substrates in crystals are observed in the extended chain form. The absence of an appropriately situated base for either the cyclic or extended chain forms from the substrate binding site indicates that the isomerisation does not take place by an enediol or enediolate mechanism. Binding of a trivalent cation places an additional charge at the active site, producing a substrate complex that is analogous to a possible transition state. Of the two binding sites for divalent cations, [1] is permanently occupied under catalytic conditions and is co-ordinated to four carboxylate groups. In the absence of substrate it is exposed to solvent, and in the Michaelis complex analogue, site [1] is octahedrally coordinated, with ligands to O-3 and O-4 of the thiopyranose. In the complex with an open-chain substrate it remains octahedrally co-ordinated, with ligands to O-2 and O-4. Binding at a second cation site [2] is also necessary for catalysis and this site is believed to bind Co2+ more strongly than site [1]. This site is octahedrally co-ordinated to three carboxylate groups (bidentate co-ordination to one of them), an imidazole and a solvent molecule. It is proposed that during the hydride shift the C-O-1 and C-O-2 bonds of the substrate are polarized by the close approach of the site [2] cation. In the transition-state analogue this cation is observed at a site [2'], 1.0 A from site [2] and about 2.7 A from O-1 and O-2 of the substrate. It is likely that co-ordination of the cation to O-1 and O-2 would be concomitant with ionisation of the sugar hydroxyl group. The polarisation of C-O-1 and C-O-2 is assisted by the co-ordination of O-2 to cation [1] and O-1 to a lysine side-chain.(ABSTRACT TRUNCATED AT 400 WORDS)

Aldose-Ketose Isomerases↗

PubMed1999

Molecular basis for thermoprotection in Bemisia: structural differences between whitefly ketose reductase and other medium-chain dehydrogenases/reductases.

The silverleaf whitefly (Bemisia argentifolii, Bellows and Perring) accumulates sorbitol as a thermoprotectant in response to elevated temperature. Sorbitol synthesis in this insect is catalyzed by an unconventional ketose reductase (KR) that uses NADPH to reduce fructose. A cDNA encoding the NADPH-KR from adult B. argentifolii was cloned and sequenced to determine the primary structure of this enzyme. The cDNA encoded a protein of 352 amino acids with a calculated molecular mass of 38.2 kDa. The deduced amino acid sequence of the cDNA shared 60% identity with sheep NAD(+)-dependent sorbitol dehydrogenase (SDH). Residues in SDH involved in substrate binding were conserved in the whitefly NADPH-KR. An important structural difference between the whitefly NADPH-KR and NAD(+)-SDHs occurred in the nucleotide-binding site. The Asp residue that coordinates the adenosyl ribose hydroxyls in NAD(+)-dependent dehydrogenases (including NAD(+)-SDH), was replaced by an Ala in the whitefly NADPH-KR. The whitefly NADPH-KR also contained two neutral to Arg substitutions within four residues of the Asp to Ala substitution. Molecular modeling indicated that addition of the Arg residues and loss of the Asp decreased the electric potential of the adenosine ribose-binding pocket, creating an environment favorable for NADPH-binding. Because of the ability to use NADPH, the whitefly NADPH-KR synthesizes sorbitol under physiological conditions, unlike NAD(+)-SDHs, which function in sorbitol catabolism.

Aldose-Ketose Isomerases↗

PubMed2001

Mechanistic imperatives for aldose-ketose isomerization in water: specific, general base- and metal ion-catalyzed isomerization of glyceraldehyde with proton and hydride transfer.

The deuterium enrichment of dihydroxyacetone obtained from the aldose-ketose isomerization of D,L-glyceraldehyde in D(2)O at 25 degrees C was determined by (1)H NMR spectroscopy from the integrated areas of the signals for the alpha-CH(2) and alpha-CHD groups of the product. One mole equivalent of deuterium is incorporated into the product when the isomerization is carried out in 150 mM pyrophosphate buffer at pD 8.4, but only 0.6 mol equiv of deuterium is incorporated into the product of isomerization in the presence of 0.01 M deuterioxide ion, so that 40% of the latter isomerization reaction proceeds by the intramolecular transfer of hydride ion. Several pathways were identified for catalysis of the isomerization of glyceraldehyde to give dihydroxyacetone. The isomerization with hydride transfer is strongly catalyzed by added Zn(2+). Deprotonation of glyceraldehyde is rate-determining for isomerization with proton transfer, and this proton-transfer reaction is catalyzed by Brønsted bases. Proton transfer also occurs by a termolecular pathway with catalysis by the combined action of Brønsted bases and Zn(2+). These results show that there is no large advantage to the spontaneous isomerization of glyceraldehyde in alkaline solution with either proton or hydride transfer, and that effective catalytic pathways exist to stabilize the transition states for both of these reactions in water. The existence of separate enzymes that catalyze the isomerization of sugars with hydride transfer and the isomerization of sugar phosphates with proton transfer is proposed to be a consequence of the lack of any large advantage to reaction by either of these pathways for the corresponding nonenzymatic isomerization in water.

Aldose-Ketose Isomerases↗

PubMed2004

The interaction of per-O-acetylated acyclic 1-(1-butylindol-3-yl)-1-deoxy-ketoses with silylated uracil.

The per-O-acetylated open chain derivatives of 1-(1-butylindol-3-yl)-1-deoxy-1-L-sorbose and 1-(1-butylindol-3-yl)-1-deoxy-L-tagatose, which are readily available by alkaline degradation of 1-butylascorbigen followed by acetylation, were used in a nucleoside-type synthesis. The interaction of these ketoses derivatives with bis-(trimethylsilyl)-uracil yielded in each case a mixture of (E)-2,4,5,6-tetra-O-acetyl-1-(1-butylindol-3-yl)-1,3-dideoxy-3-(uracil-1-yl)-L-xylo-hexa-1-enitol and (E)-2,4,5,6-tetra-O-acetyl-1-(1-butylindol-3-yl)-1,3-dideoxy-3-(uracil-1-yl)-L-lyxo-hexa-1-enitol, which were separated by preparative HPLC. The deacetylation of each of these compounds by MeONa in MeOH produced a mixture of 1-(1-butylindol-3-yl)-1,3-dideoxy-4-O-methyl-3-(uracil-1-yl)-alpha-L-sorbopyranose and 1-(1-butylindol-3-yl)-1,3-dideoxy-4-O-methyl-3-(uracil-1-yl)-beta-D-fructopyranose, which were also separated by HPLC, the structures were confirmed by NMR.

Ketoses↗

PubMed1987

Ketose induced respiratory inhibition in isolated hepatocytes.

The addition of 10 mM fructose or 10 mM tagatose to a suspension of hepatocytes caused respiratory inhibition, whereas no change in oxygen uptake was observed following the addition of glucose. However, incubations in the presence of fructose showed a high, aerobic glycolytic activity. Tagatose is phosphorylated to tagatose 1-phosphate but is not further metabolized by cell free liver extract. Moreover, the addition of fructose to glucagon treated cells also caused the Crabtree-like effect. The concentration of adenine nucleotides and inorganic phosphate (Pi) in the mitochondrial and cytosolic compartments during incubation (time 30 min) was determined by the digitonin fractionation procedure. In the presence of 10 mM fructose or tagatose, the total adenine nucleotide pools decreased by 40%; however, glucose produced no change. The addition of ketoses diminished the asymmetric distribution of extramitochondrial (ATP/ADP)e ratio and intramitochondrial (ATP/ADP)i ratio. At the same time the total mitochondrial Pi fell from 17 mM to 6-7 mM. The mitochondrial membrane potential (-161 mV) in the presence of fructose showed no changes during the 30 min experimental period. An increase in the NADH/NAD+ ratio was observed. These results suggest that in hepatocytes the inhibition of respiration is not necessarily linked with the enhanced aerobic glycolysis, by competition for common substrates.

Adenine Nucleotides↗

PubMed1977

The possible utilization of the 1-amino-1-deoxy-2-ketose compounds between amino acids and carbohydrates as foodstuff integrators.

The behavior of the primary products (1-amino-1-deoxy-2-ketose-derivatives) from amino acids and monosaccharides in the Maillard reaction to the action of pepsin, trypsin, chymostrypsin and of heating has been investigated, in order to test the possibility of their utilization as foodstuff integrators. The behavior is very different from the corresponding behavior of the original amino acids.

Amino Acids↗