Quantification of sugars and sugar phosphates in Arabidopsis thaliana tissues using porous graphitic carbon liquid chromatography-electrospray ionization mass spectrometry

A new, validated HPLC-MS/MS method for the simultaneous determination of the anti-cancer agent capecitabine and its metabolites: 5'-deoxy-5-fluorocytidine, 5'-deoxy-5-fluorouridine, 5-fluorouracil and 5-fluorodihydrouracil, in human plasma

A rapid and selective liquid chromatography/tandem mass spectrometric method was developed for the simultaneous determination of capecitabine and its metabolites 5'-deoxy-5-fluorocytidine (5'-DFCR), 5'-deoxy-5-fluorouracil (5'-DFUR), 5-fluorouracil (5-FU) and dihydro-5-fluorouracil (FUH(2)) in human plasma. A 200 microL human plasma aliquot was spiked with a mixture of internal standards fludarabine and 5-chlorouracil. A single-step protein precipitation method was employed using 10% (v/v) trichloroacetic acid in water to separate analytes from bio-matrices. Volumes of 20 microL of the supernatant were directly injected onto the HPLC system. Separation was achieved on a 30 x 2.1 mm Hypercarb (porous graphitic carbon) column using a gradient by mixing 10 mm ammonium acetate and acetonitrile-2-propanol-tetrahydrofuran (1 : 3 : 2.25, v/v/v). The detection was performed using a Finnigan TSQ Quantum Ultra equipped with the electrospray ion source operated in positive and negative mode. The assay quantifies a range from 10 to 1000 ng/mL for capecitabine, from 10 to 5000 ng/mL for 5'-DFCR and 5'-DFUR, and from 50 to 5000 ng/mL for 5-FU and FUH(2) using a plasma sample of 200 microL. Correlation coefficients (r(2)) of the calibration curves in human plasma were better than 0.99 for all compounds. At all concentration levels, deviations of measured concentrations from nominal concentration were between -4.41 and 3.65% with CV values less than 12.0% for capecitabine, between -7.00 and 6.59% with CV values less than 13.0 for 5'-DFUR, between -3.25 and 4.11% with CV values less than 9.34% for 5'-DFCR, between -5.54 and 5.91% with CV values less than 9.69% for 5-FU and between -4.26 and 6.86% with CV values less than 14.9% for FUH(2). The described method was successfully applied for the evaluation of the pharmacokinetic profile of capecitabine and its metabolites in plasma of treated cancer patients.

Oligosaccharide analysis by graphitized carbon liquid chromatography-mass spectrometry

Structural analysis of complex mixtures of oligosaccharides using tandem mass spectrometry is regularly complicated by the presence of a multitude of structural isomers. Detailed structural analysis is, therefore, often achieved by combining oligosaccharide separation by HPLC with online electrospray ionization and mass spectrometric detection. A very popular and promising method for analysis of oligosaccharides, which is covered by this review, is graphitized carbon HPLC-ESI-MS. The oligosaccharides may be applied in native or reduced form, after labeling with a fluorescent tag, or in the permethylated form. Elution can be accomplished by aqueous organic solvent mixtures containing low concentrations of acids or volatile buffers; this enables online ESI-MS analysis in positive-ion or negative-ion mode. Importantly, graphitized carbon HPLC is often able to resolve many glycan isomers, which may then be analyzed individually by tandem mass spectrometry for structure elucidation. While graphitized carbon HPLC-MS for glycan analysis is still only applied by a limited number of groups, more users are expected to apply this method when databases which support structural assignment become available.

Retention studies of 2'-2'-difluorodeoxycytidine and 2'-2'-difluorodeoxyuridine nucleosides and nucleotides on porous graphitic carbon: development of a liquid chromatography-tandem mass spectrometry method

The development of a method for the separation of 2'-2'-difluorodeoxycytidine (gemcitabine, dFdC), 2'-2'-difluorodeoxyuridine (dFdU) and their mono-, di- and triphosphates using a porous graphitic carbon column (Hypercarb), without ion-pairing agent, is described. The retention of dFdC and dFdU could be controlled with an organic modifier (acetonitrile, CH(3)CN) and the retention of the anionic nucleotides with an eluting ion (bicarbonate). Separation of all analytes was achieved using a 0-25 mM ammonium bicarbonate gradient in CH(3)CN-H(2)O (15:85, v/v). Under these conditions, however, very long re-equilibration times were required. Injection of an acidic solution (100 microL 10% formic acid in H(2)O, v/v; 2.65 M) after running a gradient directly restored the separation capabilities of the column. Still, separation between the analytes slowly deteriorated over a period of months. These problems were solved by preconditioning the column with a pH buffered hydrogen peroxide (H(2)O(2)) solution (0.05% H(2)O(2) in CH(3)CN-H(2)O (15:85, v/v), pH 4) before starting an analytical run. The oxidation of the stationary phase with H(2)O(2) prevented its slow reduction, which most likely caused the decreasing retention times. The analytes were detected using tandem mass spectrometry.

An enzymatic fluorimetric assay for glucose-6-phosphate: application in an in vitro Warburg-like effect

Recently, there has been a resurgence of interest in the regulatory role of cell metabolism in tumor biology and immunology. To assess changes in metabolite levels in cell populations and tissues, especially from small clinical samples, highly sensitive assays are required. Based on the reaction of glucose-6-phosphate (G6P) and the diaphorase-resazurin amplifying system, we have developed a fluorescence methodology to measure G6P concentrations in cell extracts. In this approach, G6P is oxidized by G6P dehydrogenase in the presence of NADP+, and the stoichiometrically generated NADPH is then amplified by the diaphorase cycling system to produce a highly fluorescent molecule-resorufin. The limit of detection (LOD) of the assay is 10 pmol. The assay has a Z' factor of 0.81. Its usefulness is demonstrated by experiments in which the pyruvate kinase inhibitor, phenylalanine, is added to cells. After 2h of incubation at 37 degrees C, G6P levels rose by 20%, thereby illustrating an in vitro Warburg-like effect on cell metabolism.

Influence of electrosorption, solvent, temperature, and ion polarity on the performance of LC-ESI-MS using graphitic carbon for acidic oligosaccharides

Porous graphitic carbon (PGC) emerges as an ideal stationary phase for LC-ESI-MS of complex oligosaccharides. Therefore, we studied the factors influencing detection and elution of charged oligosaccharides from PGC columns coupled to an ESI source. Electrosorption by the carbon surface leads to total retention of very acidic glycans on instruments where voltage is applied to the spray needle. This problem can be eliminated by thorough electrical grounding. A point of general importance is the influence of ionic strength on the elution and peak shape of glycans containing several carboxylic acid groups in the form of sialic acids or uronic acids. Solvent pH had a marginal effect on the ionization efficiency in both ion polarities, but the content of organic solvent strongly influenced signal intensity of acidic glycans in the negative mode. As a consequence, detection in the positive ion mode appears preferable when neutral and charged glycans shall be quantitated in the same sample. While retention of neutral glycans is not affected by pH, sialylated species are retained somewhat stronger at acidic pH resulting in a larger spread of the entire elution range of N-glycans. Remarkably, retention of glycans on PGC increased at higher temperatures.

Metabolite profiling of Calvin cycle intermediates by HPLC-MS using mixed-mode stationary phases

SUMMARY

A sensitive and robust mixed-mode high performance liquid chromatography-tandem mass spectrometry method was developed for the qualitative and quantitative determination of sugar phosphates, which are notoriously difficult to separate using reversed-phase materials. Sugar phosphates were separated on a Primesep SB column by gradient elution using aqueous ammonium formate and acetonitrile as mobile phases. Target analytes were identified by their precursor/product ions and retention times. Quantitative analysis was performed in negative ionization/multiple reaction monitoring mode with five different time segments. The method was validated by spiking authentic sugar phosphate standards into complex plant tissue extracts. Standard curves of neat authentic standards and spiked extracts were generated for concentrations in the low picomole to nanomole range, with correlation coefficients of R(2) > 0.991, and the degree of ion suppression in the presence of a plant matrix was calculated for each analyte. Analyte recoveries, which were determined by including known quantities of authentic standards in the sugar phosphate extraction protocol, ranged from 40.0% to 57.4%. The analytical reproducibility was assessed by determining the coefficient of variance based on repeated extractions/measurements (<20%). The utility of our method is demonstrated with two types of applications: profiling of Calvin cycle intermediates in (i) dark-adapted and light-treated tobacco leaves, and in (ii) antisense plants expressing reduced levels of the Calvin cycle enzymes glyceraldehyde-3-phosphate dehydrogenase or ribulose-1,5-bisphosphate carboxylase/oxygenase (comparison with wild-type controls). The broader applicability of our method is illustrated by profiling sugar phosphates extracted from the leaves of five taxonomically diverse plants.

Hydrophilic interaction chromatography/electrospray mass spectrometry analysis of carbohydrate-related metabolites from Arabidopsis thaliana leaf tissue

This work describes the development and application of an on-line liquid chromatography/mass spectrometry (LC/MS) method using hydrophilic interaction chromatography (HILIC) coupled to negative ion mode electrospray ionisation ion trap mass spectrometry (ESI-MS) for the analysis of highly polar carbohydrate-related metabolites commonly found in plants, ranging from reducing and non-reducing sugars and sugar alcohols to sugar phosphates. Using this method, separation and detection of a mixture of eight authentic standard compounds containing glucose (Glc), sucrose (Suc), raffinose, verbascose, mannitol, maltitol, glucose-6-phosphate (Glc6P) and trehalose-6-phosphate (Tre6P) were achieved in less than 15 min. The method is rapid, robust, selective, and sensitive, with limits of detection (LODs) ranging from 0.2 microM obtained for neutral sugars, to 1.0 microM obtained for sugar alcohols, and 2.0 microM obtained for negatively charged sugar phosphates. We have studied the negative ion collision-induced dissociation (CID) fragmentation behaviour of the non-reducing raffinose family oligosaccharides (RFOs) raffinose, stachyose, and verbascose. Mainly Bi and Ci glycosidic and Ai cross-ring structurally informative cleavages are observed. We have applied this HILIC/ESI-MS method for the analysis of Arabidopsis thaliana wild-type Columbia-0 (Col-0) and its starchless phosphoglucomutase mutant (pgm1) leaf extracts. The method was used to quantify Glc, Suc, raffinose, and Glc6P in A. thaliana extracts. Data obtained using this HILIC/ESI-MS method were compared with those obtained using a comparable porous graphitic carbon-based LC/ESI-MS method.