A liquid chromatography-mass spectrometry method to measure stable isotopic tracer enrichments of glycerol and glucose in human serum

LC/MS quantitative study of glucose by iodine attachment

We explored the potential of iodine attachment to improve the sensitivity of glucose measurement by LC/MS. After sample preparation, glucose was separated by normal phase chromatography, followed by anionization by I(-)-attachment prior to MS by post-column addition of a methanolic solution of iodoform. Iodine is capable of forming an anionic adduct with neutral monosaccharides in negative ion mode electrospray mass spectrometry. Quasi-molecular ions [M+I]- of glucose, and [6,6-(2)H2]glucose (abbreviated d2-glucose) internal standard were quantitated in selected ion monitoring (SIM) mode. Iodine attachment LC/MS analysis provided high sensitivity, superior to GC/MS. It greatly simplified sample preparation and increased throughput. The advantages of iodine attachment can be realized even on old mass spectrometers. A LOD of 50 pg glucose on column was achieved. Due to iodine's predisposition to sublimate, the iodoform concentration must be minimized, which adds complexity to method development. To optimize reagent concentration we developed an efficient and flexible gradient-based delivery platform. Strategy for method development with iodoform is given.

Analysis of tear glucose concentration with electrospray ionization mass spectrometry

We have developed a mass spectrometry-based method that allows one to accurately determine the glucose concentration of tear fluid. We used a 1 microL micro-capillary to collect tear fluid from the tear meniscus with minimal irritation of the eye. We analyzed the 1 muL volume of collected tear fluid with liquid-chromatography electrospray ionization mass spectrometry with the use of D-glucose-6,6-d2 as an internal standard. Repeated measurements and a recovery experiment on pooled, onion-induced tears showed that the analysis of the glucose in tears was precise (4% relative standard deviation) and provided 100% recovery. We found the tear glucose concentration of one fasting nondiabetic subject to be 13 to 51 microM while the onion-induced tear glucose concentration of a different nondiabetic subject to be 211 to 256 microM.

Detection of hydroxyethylstarch (HES) in human urine by liquid chromatography–mass spectrometry

The objective of this study was to establish the possibility of using liquid chromatography coupled to mass spectrometry for the detection of hydroxyethylstarch (a corn starch derived product) in urine as an alternative to the current time consuming GC-MS methods. Analyses were performed using an ion trap instrument after acidic hydrolysis. Ionization was carried out using atmospheric pressure chemical ionisation (APCI) operated in negative ionization mode and detection was performed using MS(2). The results indicate that the developed method can successfully be applied as a fast and reliable method for the detection and identification of hydroxyethylstarch.

Measurement of stable isotopic enrichment and concentration of long-chain fatty acyl-carnitines in tissue by HPLC-MS

We have developed a new method for the simultaneous measurements of stable isotopic tracer enrichments and concentrations of individual long-chain fatty acyl-carnitines in muscle tissue using ion-pairing high-performance liquid chromatography-electrospray ionization quadrupole mass spectrometry in the selected ion monitoring (SIM) mode. Long-chain fatty acyl-carnitines were extracted from frozen muscle tissue samples by acetonitrile/methanol. Baseline separation was achieved by reverse-phase HPLC in the presence of the volatile ion-pairing reagent heptafluorobutyric acid. The SIM capability of a single quadrupole mass analyzer allows further separation of the ions of interest from the sample matrixes, providing very clean total and selected ion chromatograms that can be used to calculate the stable isotopic tracer enrichment and concentration of long-chain fatty acyl-carnitines in a single analysis. The combination of these two separation techniques greatly simplifies the sample preparation procedure and increases the detection sensitivity. Applying this protocol to biological muscle samples proves it to be a very sensitive, accurate, and precise analytical tool.

Sensitive LC MS quantitative analysis of carbohydrates by Cs+ attachment

The development of a sensitive assay for the quantitative analysis of carbohydrates from human plasma using LC/MS/MS is described in this paper. After sample preparation, carbohydrates were cationized by Cs(+) after their separation by normal phase liquid chromatography on an amino based column. Cesium is capable of forming a quasi-molecular ion [M + Cs](+) with neutral carbohydrate molecules in the positive ion mode of electrospray ionization mass spectrometry. The mass spectrometer was operated in multiple reaction monitoring mode, and transitions [M + 133] --> 133 were monitored (M, carbohydrate molecular weight). The new method is robust, highly sensitive, rapid, and does not require postcolumn addition or derivatization. It is useful in clinical research for measurement of carbohydrate molecules by isotope dilution assay.

Compartmentation of Lactate Originating from Glycogen and Glucose in Cultured Astrocytes

Brain glycogen metabolism was investigated by employing isofagomine, an inhibitor of glycogen phosphorylase. Cultured cerebellar and neocortical astrocytes were incubated in medium containing [U-(13C)]glucose in the absence or presence of isofagomine and the amounts and percent labeling of intra- and extracellular metabolites were determined by mass spectrometry (MS). The percent labeling in glycogen was markedly decreased in the presence of isofagomine. Surprisingly, the percent labeling of intracellular lactate was also decreased demonstrating the importance of glycogen turnover. The decrease was limited to the percent labeling in the intracellular pool of lactate, which was considerably lower compared to that observed in the medium in which it was close to 100%. These findings indicate compartmentation of lactate derived from glycogenolysis and that derived from glycolysis. Inhibiting glycogen degradation had no effect on the percent labeling in citrate. However, the percent labeling of extracellular glutamine was slightly decreased in neocortical astrocytes exposed to isofagomine, indicating an importance of glycogen turnover in the synthesis of releasable glutamine. In conclusion, the results demonstrate that glycogen in cultured astrocytes is continuously synthesized and degraded. Moreover, it was found that lactate originating from glycogen is compartmentalized from that derived from glucose, which lends further support to a compartmentalized metabolism in astrocytes.

Novel, highly robust method of carbohydrate pre-purification by two-dimensional liquid chromatography prior to liquid chromatography/mass spectrometry or gas chromatography/mass spectrometry

We describe a novel two-dimensional liquid chromatography (2D-LC) method for fast and robust isolation and concentration of low abundant carbohydrates (sorbitol, glycerol) from biological matrices (plasma and urine). Off-line pre-purified fractions, enriched by analyte of interest, were analyzed by liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS-MS). Initial 2D-LC automated sample pre-purification improved MS detection, eliminated matrix effects, and achieved high sensitivity (picogram detection limit) with a 6 min runtime and increased column lifetime. Using this method we have analyzed more than 1300 samples from biological matrices without column replacement.

The liquid chromatography mass spectrometry approach to measure amino acid isotope ratios

PURPOSE OF REVIEW

Although the application of liquid chromatography mass spectrometry has seen a rapid development in the world of clinical science since the introduction of atmospheric pressure ionization, its potential is still hardly recognized in the field of stable isotope biomedical research, as indicated by the small number of publications on this topic. Nevertheless, considering the polar nature of many biological substrates of interest as well as the non-destructive nature of electrospray ionization, a liquid chromatography mass spectrometry approach simplifies mass spectrometric spectra, allows the application of multiple labelled tracers, and provides an easy on-line sample processing, thus minimizing labour while at the same time providing concentration data and good reproducibility in low quantities of sample.

RECENT FINDINGS

These features make it an excellent tool to perform both clinical and animal experiments that usually generate large numbers of samples, which when processed through the classic approach employing gas chromatography combustion isotope ratio mass spectrometry systems would require much more work and time and thus analytical costs.

SUMMARY

The present paper is intended to introduce the functionality, limitations and options of representatives from the present family of modern liquid chromatography mass spectrometry systems into the field of stable isotope biomedical research, with a focus on amino acid metabolism.

Normalization of skeletal muscle glycogen synthesis and glycolysis in rosiglitazone-treated Zucker fatty rats: an in vivo nuclear magnetic resonance study

The aim of this study was to characterize insulin-stimulated skeletal muscle glucose metabolism in Zucker fatty rats and to provide insight into the therapeutic mechanism by which rosiglitazone increases insulin-stimulated glucose disposal in these rats. Metabolic parameters were measured using combined in vivo (13)C nuclear magnetic resonance (NMR) spectroscopy to measure skeletal muscle glucose uptake and its distributed fluxes (glycogen synthesis and glycolysis), and (31)P NMR was used to measure simultaneous changes in glucose-6-phosphate (G-6-P) during a euglycemic-hyperinsulinemic clamp in awake Zucker fatty rats. Three groups of Zucker fatty rats (fatty rosiglitazone [FRSG], fatty control [FC], lean control [LC]) were treated for 7 days before the experiment (3 mg/kg rosiglitazone or vehicle via oral gavage). Rates of glycolysis and glycogen synthesis were assessed after treatment by monitoring 1,6-(13)C(2) glucose label incorporation into 1-(13)C glycogen, 3-(13)C lactate, and 3-(13)C alanine during a euglycemic ( approximately 7-8 mmol/l)-hyperinsulinemic (10 mU. kg(-1). min(-1)) clamp. The FRSG group exhibited a significant increase in insulin sensitivity, reflected by an increased whole-body glucose disposal rate during the clamp (24.4 +/- 1.9 vs. 17.6 +/- 1.4 and 33.2 +/- 2.0 mg. kg(-1). min(-1) in FRSG vs. FC [P < 0.05] and LC [P < 0.01] groups, respectively). The increased insulin-stimulated glucose disposal in the FRSG group was associated with a normalization of the glycolytic flux (52.9 +/- 9.1) to LC (56.2 +/- 16.6) versus FC (18.8 +/- 8.6 nmol. g(-1). min(-1), P < 0.02) and glycogen synthesis flux (56.3 +/- 11.5) to LC (75.2 +/- 15.3) versus FC (16.6 +/- 12.8 nmol. g(-1). min(-1), P < 0.05). [G-6-P] increased in the FRSG and LC groups versus baseline during the clamp (13.0 +/- 11.1 and 16.9 +/- 5.8%, respectively), whereas [G-6-P] in the FC group decreased (-23.3 +/- 13.4%, P < 0.05). There were no differences between groups in intramyocellular glucose, as measured by biochemical assay. These data suggest that the increased insulin-stimulated glucose disposal in muscle after rosiglitazone treatment can be attributed to a normalization of glucose transport and metabolism.