Separation and analysis of 4'-epimeric UDP-sugars by borate high-performance liquid chromatography

Selective analysis of intracellular UDP-GlcNAc and UDP-GalNAc by hydrophilic interaction liquid chromatography-mass spectrometry

Uridine diphosphate-N-acetylglucosamine (UDP-GlcNAc) is one of the major nucleotide sugars in living organisms and serves as the key donor substrate for the post-translational modification of protein O-GlcNAcylation. It undergoes interconversion to its epimer uridine diphosphate-N-acetylgalactosamine (UDP-GalNAc), which acts as a sugar donor initiating mucin-type O-linked glycosylation. The intracellular levels of the two differ between the cell lines and largely fluctuate in response to metabolic perturbations, and recent studies have focused on the details of their biosynthesis or turnover. However, due to their similar chemical properties, sufficient resolution for the two epimers required non-volatile mobile phases that cannot be applied directly to a mass spectrometer. In this study, to implement simple liquid chromatography-mass spectrometry for UDP-GlcNAc and UDP-GalNAc, we optimized a condition of hydrophilic interaction liquid chromatography-mass spectrometry. We found that the use of ammonium hydroxide and an amide column with an optimized water-acetonitrile ratio, flow rate, and column temperature, provided complete separation of the two. The method allowed the analysis of intracellular levels, a stable isotope-labeled target, and patterns of product ion spectra in a single run with fewer sample preparation steps. The new method can be widely used for mass spectrometric analysis of UDP-GlcNAc and UDP-GalNAc.

Enzymatic assay for UDP-GlcNAc and its application in the parallel assessment of substrate availability and protein O-GlcNAcylation

O-linked N-acetylglucosaminylation (O-GlcNAcylation) is a ubiquitous and dynamic non-canonical glycosylation of intracellular proteins. Several branches of metabolism converge at the hexosamine biosynthetic pathway (HBP) to produce the substrate for protein O-GlcNAcylation, the uridine diphosphate N-acetylglucosamine (UDP-GlcNAc). Availability of UDP-GlcNAc is considered a key regulator of O-GlcNAcylation. Yet UDP-GlcNAc concentrations are rarely reported in studies exploring the HBP and O-GlcNAcylation, most likely because the methods to measure it are restricted to specialized chromatographic procedures. Here, we introduce an enzymatic method to quantify cellular and tissue UDP-GlcNAc. The method is based on O-GlcNAcylation of a substrate peptide by O-linked N-acetylglucosamine transferase (OGT) and subsequent immunodetection of the modification. The assay can be performed in dot-blot or microplate format. We apply it to quantify UDP-GlcNAc concentrations in several mouse tissues and cell lines. Furthermore, we show how changes in UDP-GlcNAc levels correlate with O-GlcNAcylation and the expression of OGT and O-GlcNAcase (OGA).

Enzyme-based assay for quantification of UDP-GlcNAc in cells and tissues

In this issue of Cell Reports Methods, Sunden et al. develop an enzymatic assay to measure UDP-GlcNAc levels from cells and tissue.1 By reporting on the level of the substrate itself, this approach can potentially enhance the fields' understanding of UDP-GlcNAc concentration under a variety of conditions.

Crystal structure of UDP-galactose 4-epimerase from the hyperthermophilic archaeon Pyrobaculum calidifontis

The crystal structure of a highly thermostable UDP-galactose 4-epimerase (GalE) from the hyperthermophilic archaeon Pyrobaculum calidifontis was determined at a resolution of 1.8Å. The asymmetric unit contained one subunit, and the functional dimer was generated by a crystallographic two-fold axis. Each monomer consisted of a Rossmann-fold domain with NAD bound and a carboxyl terminal domain. The overall structure of P. calidifontis GalE showed significant similarity to the structures of the GalEs from Escherichia coli, human and Trypanosoma brucei. However, the sizes of several surface loops were markedly smaller in P. calidifontis GalE than the corresponding loops in the other enzymes. Structural comparison revealed that the presence of an extensive hydrophobic interaction at the subunit interface is likely the main factor contributing to the hyperthermostability of the P. calidifontis enzyme. Within the NAD-binding site of P. calidifontis GalE, a loop (NAD-binding loop) tightly holds the adenine ribose moiety of NAD. Moreover, a deletion mutant lacking this loop bound NAD in a loose, reversible manner. Thus the presence of the NAD-binding loop in GalE is largely responsible for preventing the release of the cofactor from the holoenzyme.

Simultaneous determination of 19 intracellular nucleotides and nucleotide sugars in Chinese Hamster ovary cells by capillary electrophoresis

Twelve nucleotides and seven nucleotide sugars in Chinese Hamster ovary (CHO) cells were determined by capillary electrophoresis (CE). The CE operating conditions of buffer pH value, ion strength, capillary temperature, polymer additive and cell extraction method were investigated. Optimum separation was achieved with 40 mM sodium tetraborate buffer (pH 9.5) containing 1% (w/v) polyethylene glycol (PEG) at a capillary temperature of 22 degrees C. Acetonitrile and chloroform were used for intracellular extraction. This method can be used to monitor intracellular carbohydrate metabolism.

Influence of chronological aging on the survival and nucleotide content of Saccharomyces cerevisiae cells grown in different conditions: occurrence of a high concentration of UDP-N-acetylglucosamine in stationary cells grown in 2% glucose

Saccharomyces cerevisiae cells (strain W303) grown in a minimal medium (containing 2% or 0.1% glucose) until exponential or stationary phase, were subjected to chronological aging in water, and yeast viability and nucleotide content were analyzed along several days of nutrient starvation. Cells collected in exponential phase (whether grown in the presence of 0.1% or 2% glucose) were viable up to five days and thereafter the viability decreased linearly with a half-survival rate of around eight days. ATP and other nucleoside triphosphates decreased similarly in both cases. Cells collected in stationary phase, and transferred to water, behaved differently whether grown in 0.1% or in 2% glucose, with a half-survival life of around nine and 28 days respectively. A double mutant in glycogen synthase (gsy1delta gsy2delta) and its isogenic wild-type strain, grown to stationary phase in 2% glucose, presented a similar half-survival life of around eight days. The W303 cells grown to stationary phase in the presence of 2% glucose showed a 7-fold increase of UDP-N-acetylglucosamine (UDP-GlcNAc) as compared with the level present in the cells grown in any of the other three metabolic situations. The nature of UDP-GlcNAc was established by MALDI-TOF ionization analysis. It is also worth noting that the rate of decay of NAD+ was lower than that of ATP in any of the situations here considered.

Diabetes and the accompanying hyperglycemia impairs cardiomyocyte calcium cycling through increased nuclear O-GlcNAcylation

Diabetic cardiomyopathy is characterized by impaired cardiac contractility leading to poor myocardial performance. We investigated the role that the hexosamine pathway, and especially altered nuclear O-Glc-NAcylation, plays in the development of diabetic cardiomyopathy. Incubating neonatal rat cardiomyocytes in high glucose (25 mM) resulted in prolonged calcium transients when compared with myocytes incubated in normal glucose (5.5 mM), which is consistent with delayed myocardial relaxation. High glucose-treated myocytes also exhibited reduced sarcoendoplasmic reticulum Ca(2+)-ATPase 2a (SERCA2a) mRNA and protein expression, decreased SERCA2a promoter activity, and increased O-GlcNAcylation of nuclear proteins compared with myocytes treated with normal glucose. Exposure of myocytes to 8 mM glucosamine or an adenovirus expressing O-GlcNAc-transferase (OGT) resulted in prolonged calcium transient decays and significantly reduced SERCA2a protein levels, whereas treatment with an adenovirus encoding O-GlcNAcase (GCA) resulted in improved calcium transients and SERCA2a protein levels in myocytes exposed to high glucose. Effects of elevated glucose or altered O-GlcNAcylation were also observed on essential transcription factors involved in cardiomyocyte function. High glucose-treated myocytes (with or without OGT adenovirus) exhibited increased levels of O-GlcNAcylated specificity protein 1 compared with control myocytes, whereas infecting high glucose-treated myocytes with GCA adenovirus reduced the degree of specificity protein 1 Glc-NAcylation. Treatment of myocytes with 25 mM glucose, 8 mM glucosamine, or OGT adenovirus also significantly reduced levels of myocytes enhancer factor-2A protein compared with control myocytes, whereas infection with GCA adenovirus resulted in improved myocytes enhancer factor-2 expression. Our results suggest that the hexosamine pathway, and O-GlcNAcylation in particular, is important in impaired cardiac myocyte function and the development of diabetic cardiomyopathy.

Determination of Nucleotides and Sugar Nucleotides Involved in Protein Glycosylation by High-Performance Anion-Exchange Chromatography: Sugar Nucleotide Contents in Cultured Insect Cells and Mammalian Cells

We have developed a simple and highly sensitive HPLC method for determination of cellular levels of sugar nucleotides and related nucleotides in cultured cells. Separation of 9 sugar nucleotides (CMP-Neu5Ac, CMP-Neu5Gc, CMP-KDN, UDP-Gal, UDP-Glc, UDP-GalNAc, UDP-GlcNAc, GDP-Fuc, GDP-Man) and 12 nucleotides (AMP, ADP, ATP, CMP, CDP, CTP, GMP, GDP, GTP, UMP, UDP, and UTP) was examined by reversed-phase HPLC and high-performance anion-exchange chromatography (HPAEC). Although the reversed-phase HPLC, using an ion-pairing reagent, gave a good separation of the 12 nucleotides, it did not separate sufficiently the sugar nucleotides for quantification. On the other hand, the HPAEC method gave an excellent and reproducible separation of all nucleotides and sugar nucleotides with high sensitivity and reproducibility. We applied the HPAEC method to determine the intracellular sugar nucleotide levels of cultured Spodoptera frugiperda (Sf9) and Trichoplusia ni (High Five, BTN-TN-5B1-4) insect cells, and compared them with those in Chinese hamster ovary (CHO-K1) cells. Sf9 and High Five cells showed concentrations of UDP-GlcNAc, UDP-Gal, UDP-Glc, GDP-Fuc, and GDP-Man equal to or higher than those in CHO cells. CMP-Neu5Ac was detected in CHO cells, but it was not detected in Sf9 and High Five cells. In conclusion, the newly developed HPAEC method could provide valuable information necessary for generating sialylated complex-type N-glycans in insect or other cells, either native or genetically manipulated.

Glutamine metabolism to glucosamine is necessary for glutamine inhibition of endothelial nitric oxide synthesis

L-Glutamine is a physiological inhibitor of endothelial NO synthesis. The present study was conducted to test the hypothesis that metabolism of glutamine to glucosamine is necessary for glutamine inhibition of endothelial NO generation. Bovine venular endothelial cells were cultured for 24 h in the presence of 0, 0.1, 0.5 or 2 mM D-glucosamine, or of 0.2 or 2 mM L-glutamine with or without 20 microM 6-diazo-5-oxo-L-norleucine (DON) or with 100 microM azaserine. Both DON and azaserine are inhibitors of L-glutamine:D-fructose-6-phosphate transaminase (isomerizing) (EC 2.6.1.16), the first and rate controlling enzyme in glucosamine synthesis. Glucosamine at 0.1, 0.5 and 2 mM decreased NO production by 34, 45 and 56% respectively compared with controls where glucosamine was lacking. DON (20 microM) and azaserine (100 microM) blocked glucosamine synthesis and prevented the inhibition of NO generation by glutamine. Neither glutamine nor glucosamine had an effect on NO synthase (NOS) activity, arginine transport or cellular tetrahydrobiopterin and Ca(2+) levels. However, both glutamine and glucosamine inhibited pentose cycle activity and decreased cellular NADPH concentrations; these effects of glutamine were abolished by DON or azaserine. Restoration of cellular NADPH levels by the addition of 1 mM citrate also prevented the inhibiting effect of glutamine or glucosamine on NO synthesis. A further increase in cellular NADPH levels by the addition of 5 mM citrate resulted in greater production of NO. Collectively, our results demonstrate that the metabolism of glutamine to glucosamine is necessary for the inhibition of endothelial NO generation by glutamine. Glucosamine reduces the cellular availability of NADPH (an essential cofactor for NOS) by inhibiting pentose cycle activity, and this may be a metabolic basis for the inhibition of endothelial NO synthesis by glucosamine.

Simultaneous, quantitative analysis of UDP-N-acetylglucosamine, UDP-N-acetylgalactosamine, UDP-glucose and UDP-galactose in human peripheral blood cells, muscle biopsies and cultured mesangial cells by capillary zone electrophoresis

The aim of this study was to develop and evaluate a capillary zone electrophoretic (CE) procedure for the accurate quantification of the UDP-hexosamines as well as for the corresponding UDP-hexoses in samples from various biological origins. Testing different buffer conditions, voltages, capillary dimensions and temperatures, optimal results were achieved with a 90 mM borate buffer, pH 9.0, at 18 degrees C and 15.5 kV in an uncoated fused-silica capillary of 50 cm x 50 microm and a detection wavelength of lambda = 262 nm. The total procedure, i.e., including variations of the sample preparation, showed coefficients of variation for the peak areas between 4. 1% and 10.4% in mesangial cells (n = 7) and between 7.8 and 10.3% (n = 6) in leukocytes for the components of interest. To improve precision, an internal standard was used for calibration. The limit of detection for all compounds is an absolute amount of 180 fmol, sufficient for the precise analysis of UDP-sugars in a limited amount of biological samples, such as human leukocytes (obtained from a 10 mL blood sample), muscle biopsies (< or = 100 mg), and mesangial kidney cells (ca. 2.5 x 10(5) cells). This reproducible, quantitative analysis of all four UDP-sugars from various biomedically relevant origins by CZE is a definite improvement over the generally used high performance liquid chromatography (HPLC) procedures. The CZE method allows the study of the flux through the hexosamine pathway in diabetes mellitus and other diseases in a simple, quantitative and accurate way.

HPLC analysis of hexosamine phosphates in biological samples

Galactosamine is quickly metabolized to galactosamine 1-phosphate in rats treated with this compound. An HPLC method to quantify hexosamine phosphates in biological samples is described, modified from the o-phthaldialdehyde amino acid analysis procedure. o-Phthaldialdehyde derivatives of hexosamines and hexosamine-phosphates can be eluted from a reverse-phase column at different retention times, with a total analysis time of 30 min and without overlapping with free amino acids at physiological concentrations. The standard curves are linear between 1 and 40 nmol. This simple method is more selective and sensitive than previous enzymatic analyses of hexosamine phosphorylation.

Biochemical pharmacology and analysis of fluoropyrimidines alone and in combination with modulators

After more than three decades since their introduction, fluoropyrimidines, especially FUra, are still a mainstay in the treatment of various solid malignancies. The antitumor effects of fluoropyrimidines are dependent upon metabolic activation. FdUMP, FUTP and FdUTP were identified as the key cytotoxic metabolites that interfere with the proper function of thymidylate synthase and nucleic acids. The relevance of these metabolites is cell-type specific. Recently, fluorouridine diphospho sugars have been detected, but the precise function of this class of metabolites is currently unknown. In mammalian systems fluoropyrimidines and their natural counterparts share the same metabolic pathways since the substrate properties in enzyme-catalyzed reactions are frequently comparable. Ongoing studies indicate that the metabolism and action of fluoropyrimidines exhibit circadian rhythms, which appear to be due to variations in the activity of metabolizing enzymes. Essential for the expanding knowledge of the pathways and effects of fluoropyrimidines has been the constant improvement of analytical methods. These include ligand binding techniques, numerous dedicated HPLC systems and 19F-NMR. Because the overall response rates achieved with fluoropyrimidines are modest, strategies based on biochemical modulation have been devised to enhance their therapeutic index. Biochemical modulators include a wide range of various compounds with different modes of action. In recently completed clinical trials, combinations of FUra with leucovorin, a precursor for 5,10-methylene tetrahydrofolate, or with levamisole, an anthelminthic with immunomodulatory activity, appeared to be superior to FUra alone. At the preclinical level combinations of fluoropyrimidines with, e.g. interferons or L-histidinol were demonstrated to be interesting candidates for further testing. The future therapeutic utility of fluoropyrimidines will depend on both the improvement of combination regimens currently used in the treatment of cancer patients and the judicious clinical implementation of promising experimental modulation strategies. Moreover, novel fluoropyrimidines with superior pharmacological properties may become important as part of or instead of modulation approaches.

Isocratic high-performance liquid chromatographic determination of the concentration and specific radioactivity of phosphoenolpyruvate and uridine diphosphate glucose in tissue extracts

A rapid and efficient isocratic high-performance liquid chromatographic method for studying the metabolism of phosphoenolpyruvate and uridine diphosphate glucose (UDPG) has been developed. For each compound this method can measure tissue concentrations in the range 0.1-1000 nmol/g of tissue and determine specific radioactivity. All measurements can be performed in 200 mg of tissue. The recoveries of uridine diphosphate [6-3H]glucose and phosphoenol[1-14C]pyruvate from liver tissue homogenates were 97 and 99%, respectively. Following intra-arterial infusion of [6-3H]glucose and [U-14C]lactate in conscious rat, the concentration and specific radioactivity of phosphoenolpyruvate and UDPG were determined in rat liver. The method may be applied to experimentation in small animals using radiolabelled precursors in order to quantitate in vivo the glycogenic and gluconeogenic fluxes.

UDP-glucosamine as a substrate for dolichyl monophosphate glucosamine synthesis

The sugar nucleotide analogue UDP-glucosamine was found to function as a sugar donor in microsomal preparations of both chick-embryo cells and rat liver, yielding dolichyl monophosphate glucosamine (Dol-P-GlcN). This was characterized by t.l.c. and retention by DEAE-cellulose. Glucosamine was the only water-soluble product released on mild acid hydrolysis. Dol-P-GlcN did not serve as substrate by transferring its glucosamine moiety to dolichol-linked oligosaccharide. Competition experiments between UDP-[3H]glucose and UDP-glucosamine showed Dol-P-[3H]glucose synthesis to be depressed by 56 or 73% in microsomes from chick-embryo cells and rat liver respectively. The concentrations of the UDP-sugars in this experiment were comparable with those occurring in galactosamine-metabolizing liver. These findings suggest that Dol-P-GlcN, formed as a metabolite of D-galactosamine, may interfere with Dol-P-dependent reactions.

45Calcium uptake during the transition from reversible to irreversible liver injury induced by D-galactosamine in vivo

The hepatic uptake of 45calcium (45Ca) was studied in rats after administration of D-galactosamine (3 mmoles per kg, i.v.). In contrast to measurements of the hepatic calcium content, 45Ca uptake served as a dynamic rather than a static indicator of calcium homeostasis during the transition from reversible to irreversible liver injury which occurs between 3 and 4 hr after injection of the hepatotoxin. 45Ca uptake during a 1 hr-labeling period increased from 25 to 100% above control between 3 and 4 hr and subsequently remained at this level. The rise in 45Ca uptake and in hepatic calcium content occurred 2 to 3 hr after the D-galactosamine-induced depletion of UTP, UDP-galactose, UDP-glucose and UDP-glucuronate. The level of UDP-glucuronate was the earliest to recover. The enhanced 45Ca uptake was associated with hepatic glycogen breakdown and with an increased SGPT activity in plasma. Inhibition of RNA polymerase II by alpha-amanitin (0.5 mg per kg, i.p.) and of dolichol-dependent protein glycosylation as well as ganglioside synthesis by tunicamycin (2 mg per kg, i.p.) were used to imitate two of the early actions of D-galactosamine and indicated that an interference with either process can lead to an enhanced uptake of 45Ca into the liver in vivo. Uridine, at a dose replenishing uracil nucleotide pools after their depletion by D-galactosamine, prevented or reversed the rise in 45Ca uptake. The antiinflammatory steroid dexamethasone, injected prior to or simultaneously with D-galactosamine also protected against the loss of calcium homeostasis and the development of liver injury.(ABSTRACT TRUNCATED AT 250 WORDS)

Substrate properties of 5-fluorouridine diphospho sugars detected in hepatoma cells

Nucleotide sugars derived from 5-fluorouridine were studied in cultured AS-30D hepatoma cells as well as in kinetic enzyme assays in vitro in comparison with the physiologic uridine diphospho sugars. Hepatoma cells converted 5-fluoro [14C]uridine to 5-fluorouridine diphospho (FUDP) glucose, FUDP-galactose, FUDP-N-acetylglucosamine, FUDP-N-acetylgalactosamine, and trace amounts of FUDP-glucuronate, as analyzed by different systems of high-performance liquid chromatography. 5-Fluoro[14C]uridine and [14C]uridine, at concentrations of 5 microM in the culture medium, were phosphorylated by the cells during 60 min to similar amounts of FUTP and UTP, respectively, while the synthesis of [14]FUDP-sugars was reduced to 14% as compared to that of [14C]UDP-sugars. FUDP-sugars, synthesized by chemical and enzymatic procedures, were assayed in vitro as substrates for enzymes of UDP-sugar metabolism. Km and V values in a range comparable to that of the respective UDP-sugars were determined for FUDP-sugars in the reactions catalyzed by UDP-glucose pyrophosphorylase, galactose-1-phosphate uridylyltransferase, UDP-glucose 4-epimerase, UDP-N-acetylglucosamine 2-epimerase, glycogen synthase, and UDP-glucose dehydrogenase. Our experiments in hepatoma cells and with enzymes in vitro have revealed additional reactions of FUDP-sugar metabolism demonstrating a metabolite pattern analogous to that of UDP-sugars. The amounts of FUDP-sugars formed relative to UDP-sugars in intact cells were smaller than suggested on the basis of their kinetic comparison in vitro.

D-glucosamine-induced changes in nucleotide metabolism and growth of colon-carcinoma cells in culture

Human colon-carcinoma cells were exposed to D-glucosamine at 2.5, 5 and 10 mM, concentrations that were growth-inhibitory but not cytocidal in the presence of a physiological glucose concentration. Labelling of these HT-29 cells with D-[14C]-glucosamine, followed by nucleotide analyses, demonstrated that UDP-N-acetyl-hexosamines represented the major intracellular nucleotide pool and the predominant metabolite of the amino sugar. D-[14C]Glucosamine was not a precursor of UDP-glucosamine. After 4h exposure to D-glucosamine (2.5 mM), the pool of UDP-N-acetylhexosamines was increased more than 6-fold, whereas UTP and CTP were markedly decreased. UDP-glucuronate content increased by more than 2-fold, whereas purine nucleotide content was little altered. Uridine (0.1 mM) largely reversed the decrease in UTP, CTP, UDP-glucose and UDP-galactose, while intensifying the expansion of the UDP-N-acetylhexosamine pool. Uridine did not reverse the D-glucosamine-induced retardation of growth in culture. A 50% decrease in growth also persisted when uridine and cytidine, cytidine alone, or UDP, were added together with D-glucosamine. The growth-inhibitory effect of the amino sugar could therefore be best correlated with the quantitative change in the pattern of sugar nucleotides, and, in particular, with the many-fold increase in UDP-N-acetylglucosamine and UDP-N-acetylgalactosamine.