Novel method for measurement of glutathione kinetics in neonates using liquid chromatography coupled to isotope ratio mass spectrometry

Preparative HPLC Separation of Underivatized Amino Acids for Isotopic Analysis

Single-compound analysis of stable or radioactive isotopes has found application in a number of fields ranging from archaeology to forensics. Often, the most difficult part of these analyses is the development of a method for isolating the compound(s) of interest, which can derive from a wide range of sample types including the hair, nails, and bone.Here we describe three complementary preparative HPLC techniques suitable for separating and isolating amino acids from bone collagen and hair keratin. Using preparative reversed-phase, ion-pair, or mixed-mode chromatography in aqueous carbon-free mobile phases, or those from which carbon can easily be removed, underivatized single amino acids can be isolated and further analyzed using mass spectrometric techniques.

A Solid-state Electrochemiluminescence Sensor for Detecting Glutathione with a Graphite-phase Carbon Nitride/Silica Modified Glassy Carbon Electrode

A solid-state electrochemiluminescence (ECL) sensor for the detection of reduced glutathione (GSH) based on a g-C₃N₄/SiO₂ modified glass carbon electrode (GCE) has been developed in this research. The g-C₃N₄, which is employed as a luminophore, is simply prepared and exhibits an excellent ECL response. Mesoporous silica hollow spheres (SiO₂) with a large specific surface area are introduced here to increase the loading amount of g-C₃N₄. Compared to a g-C₃N₄ modified GCE, the g-C₃N₄/SiO₂ modified GCE displays a much higher ECL intensity. A high enhancement effect on the ECL intensity of g-C₃N₄/SiO₂ modified GCE is obtained in the presence of GSH in the electrolyte. Moreover, the enhanced ECL intensity shows a good linear relationship to the GSH concentration in the range from 1.0 × 10^-7 to 5.0 × 10^-4 M, with a detection limit of 2.0 × 10^-8 M (6.1 ng/mL). Besides, the ECL sensor exhibits a good anti-interference ability and has been successfully applied in the detection of GSH in commercial samples. The proposed sensor provides a promising platform for life science.

Gas Source Isotope Ratio Mass Spectrometry (IRMS)

Gas source isotope ratio mass spectrometry is usually referred to as isotope ratio mass spectrometry (IRMS) or stable-isotope ratio mass spectrometry (SIRMS). IRMS is a conventional method for measuring isotope ratios and has benefited from more than 65 years of research and development. Modern mass spectrometers are all based on gas source isotope ratio mass spectrometry field mass separators. More recently, the development of high-resolution sector field devices has added a new dimension to IRMS. Modern instruments achieve a high sample throughput, which is a prerequisite, e.g., for ecosystem studies where usually a large number of samples needs to be analysed and high precision is required. IRMS is used specifically for the measurement of stable-isotope ratios of a limited number of elements (C, H, N, O and S) after transfer into a gaseous species. Si, Cl, Br and Se can be added to the list even though their applications are limited compared to the other isotope systems. A concise overview of the technical background is given here as well as numerous applications of this technique in earth and geosciences, paleoclimate research, cosmochemistry, environmental sciences and life sciences.

Performance of the wet oxidation unit of the HPLC isotope ratio mass spectrometry system for halogenated compounds

The performance of liquid chromatography-isotope ratio mass spectrometry (LC-IRMS) for polar halogenated compounds was evaluated. Oxidation capacity of the system was tested with halogenated acetic acids and halogenated aromatic compounds. Acetic acid (AA) was selected as a reference compound for complete oxidation and compared on the molar basis to the oxidation of other analytes. The isotope values were proofed with calibrated δ(13)C values obtained with an elemental analyzer (EA). Correct isotope values were obtained for mono- and dichlorinated, fluorinated, and tribrominated acetic acids and also for aniline, phenol, benzene, bromobenzene, chlorobenzene, 1,2-dichlorobenzene, 2,4,6-trichlorophenol, pentafluorophenol, and nitrobenzene. Incomplete oxidation of trichloroacetic acid (TCA) and trifluoroacetic acid (TFA) resulted in lower recovery compared to AA (37% and 24%, respectively) and in isotopic shift compared to values obtained with EA (TCA Δδ(13)C(EA/LC-IRMS) = 8.8‰, TFA Δδ(13)C(EA/LC-IRMS) = 6.0‰). Improvement of oxidation by longer reaction time in the reactor and increase in the concentration of sulfate radicals did not lead to complete combustion of TCA and TFA needed for δ(13)C analysis. To the best of our knowledge, this is the first time such highly chlorinated compounds were studied with the LC-IRMS system. This work provides information for method development of LC-IRMS methods for halogenated contaminants that are known as potential threats to public health and the environment.

A versatile method for simultaneous stable carbon isotope analysis of DNA and RNA nucleotides by liquid chromatography/isotope ratio mass spectrometry

RATIONALE

Liquid chromatography/isotope ratio mass spectrometry (LC/IRMS) is currently the most accurate and precise technique for the measurement of compound-specific stable carbon isotope ratios ((13)C/(12)C) in biological metabolites, at their natural abundance. However, until now this technique could not be applied for the analysis of nucleic acids, the building blocks of the carriers of genetic information in living cells and viruses, DNA and RNA.

METHODS

Mixed-mode chromatography (MMC) was applied to obtain the complete separation of nine nucleotides (eight originating from DNA/RNA and one nucleotide (inosine monophosphate) that may serve as an internal standard) in a single run using LC/IRMS. We also developed and validated a method for DNA and RNA extraction and an enzymatic hydrolysis protocol for natural samples, which is compatible with LC/IRMS analysis as it minimizes the carbon blank. The method was used to measure the concentration and stable carbon isotope ratio of DNA and RNA nucleotides in marine sediment and in the common marine macro alga (Ulva sp.) at natural abundance levels as well as for (13)C-enriched samples.

RESULTS

The detection limit of the LC/IRMS method varied between 1.0 nmol for most nucleotides and 2.0 nmol for late-eluting compounds. The intraday and interday reproducibility of nucleotide concentration measurements was better than, respectively, 4.1% and 8.9% and for δ(13)C measurements better than, respectively, 0.3‰ and 0.5‰. The obtained nucleic acid concentrations and nucleic acid synthesis rates were in good agreement with values reported in the literature.

CONCLUSIONS

This new method gives reproducible results for the concentration and δ(13)C values of nine nucleotides. This solvent-free chromatographic method may also be used for other purposes, such as for instance to determine nucleotide concentrations using spectrophotometric detection. This sensitive method offers a new avenue for the study of DNA and RNA biosynthesis that can be applied in various fields of research.

Resuscitation of preterm infants with different inspired oxygen fractions

OBJECTIVE

To test the hypothesis that an initial fraction of inspired oxygen (FiO2) of 30% during resuscitation of preterm infants results in less oxidative stress and is associated with improved clinical outcomes compared with an FiO2 of 65%.

STUDY DESIGN

Preterm infants of gestational age <32 weeks (n = 193) were randomized to start resuscitation with either 30% oxygen (low-oxygen group) or 65% oxygen (high-oxygen group), after which the FiO2 was adjusted based on oxygen saturation values. The primary outcome was bronchopulmonary dysplasia (BPD) assessed at 36 weeks postmenstrual age. Secondary outcomes included major neonatal illnesses and markers of oxidative stress.

RESULTS

The median gestational age of included infants was 28(6)/7 weeks (IQR, 26(5)/7-30(3)/7 weeks). The incidence of BPD was not significantly different between the low-oxygen and high-oxygen groups (24% vs. 17%; P = .15). The FiO2 in both groups was adjusted to a mean of 40% by 7 minutes in the low-oxygen group and by 11 minutes in the high-oxygen group. No differences in markers of oxidative stress were noted between groups.

CONCLUSION

Initial supplementation of preterm infants with 30% oxygen during the fetal-to-neonatal transition is as safe as 65% oxygen, with no differences in oxidative stress markers or BPD.

Heart-cutting two-dimensional gas chromatography in combination with isotope ratio mass spectrometry for the characterization of the wax fraction in plant material

Gas chromatography coupled to isotope ratio mass spectrometry after on-line combustion (GC-C-IRMS) and high temperature conversion (GC-HTC-IRMS) is used for compound specific isotope ratio determination. This determination can only be performed successfully if the target solutes are fully resolved from other compounds. A new instrumental set-up consisting of heart-cutting two-dimensional GC based on capillary flow technology and a low thermal mass GC oven in combination with an isotope ratio mass spectrometer is presented. Capillary flow technology was also used in all column and interface connections for robust and leak-free operation. The new configuration was applied to the characterization of wax compounds in tobacco leaf and corresponding smoke samples. It is demonstrated that high accuracy is obtained, both in the determination of δ(13)C and δ(2)H values, allowing the study of biosynthesis and delivery mechanisms of naturally occurring compounds in tobacco.

Resuscitation of very preterm infants with 30% vs. 65% oxygen at birth: study protocol for a randomized controlled trial

Background

Resuscitation at birth with 100% oxygen is known to increase the oxidative burden with concomitant deleterious effects. Although fractions of inspired oxygen (FiO₂) < 100% are widely used in preterm infants, starting resuscitation at a (too) low FiO₂ may result in hypoxia. The objective of this study is to compare the safety and efficacy of resuscitating very preterm infants with an initial FiO₂ of 30% versus 65%.

Methods/design

In this double-blind, randomized controlled trial, 200 very preterm infants with a gestational age < 32 weeks will be randomized to start resuscitation after birth with either 30% or 65% oxygen. The FiO₂ will be adjusted based on oxygen saturation measured by pulse oximetry (SpO₂) and pulse rate (which should be over 100 beats per minute) in order to achieve a target SpO₂ of 88–94% at 10 min of life. The FiO₂ and pulse oximetry data will be continuously recorded.

The primary outcome is survival without bronchopulmonary dysplasia, as assessed by a physiological test at 36 weeks postmenstrual age. The secondary outcomes include the time to achieve SpO₂ > 88%, Apgar score at 5 min, cumulative O₂ exposure, oxidative stress (as determined by glutathione synthesis and oxidative stress markers), retinopathy of prematurity, brain injury and neurodevelopmental outcome at 2 years of age.

This study will provide insight into determining the appropriate initial FiO₂ to start resuscitation of very preterm infants.

Trial registration

http://www.trialregister.nl, NTR243.

High-precision mass spectrometric analysis using stable isotopes in studies of children

The use of stable isotopes combined with mass spectrometry (MS) provides insight into metabolic processes within the body. Herein, an overview on the relevance of stable isotope methodology in pediatric research is presented. Applications for the use of stable isotopes with MS cover carbohydrate, fat, and amino acid metabolism as well as body composition, energy expenditure, and the synthesis of specific peptides and proteins, such as glutathione and albumin. The main focus of these studies is on the interactions between nutrients and the endogenous metabolism within the body and how these factors affect the health of a growing infant. Considering that the early imprinting of metabolic processes hugely impacts metabolism (and thus functional outcome) later in life, research in this area is important and is advancing rapidly. The major fluxes on a metabolic level are the synthesis and breakdown rates. They can be quantified using kinetic tracer analysis and mathematical modeling. Organic MS and isotope ratio mass spectrometry (IRMS) are the two most mature techniques for the isotopic analysis of compounds. Introduction of the samples is usually done by coupling gas chromatography (GC) to either IRMS or MS because it is the most robust technique for specific isotopic analysis of volatile compounds. In addition, liquid chromatography (LC) is now being used more often as a tool for sample introduction of both volatile and non-volatile compounds into IRMS or MS for (13)C isotopic analyses at natural abundances and for (13)C-labeled enriched compounds. The availability of samples is often limited in pediatric patients. Therefore, sample size restriction is important when developing new methods. Also, the availability of stable isotope-labeled substrates is necessary for measurements of the kinetics and concentrations in metabolic studies, which can be a limiting factor. During the last decade, the availability of these substrates has increased. Furthermore, improvements in the accuracy, precision, and sensitivity of existing techniques (such as GC/IRMS) and the development of new techniques (such as LC/IRMS) have opened up new avenues for tackling these limitations.

The glycemic response does not reflect the in vivo starch digestibility of fiber-rich wheat products in healthy men

Starchy food products differ in the rate of starch digestion, which can affect their metabolic impact. In this study, we examined how the in vivo starch digestibility is reflected by the glycemic response, because this response is often used to predict starch digestibility. Ten healthy male volunteers [age 21 ± 0.5 y, BMI 23 ± 0.6 kg/m² (mean ± SEM)] participated in a cross-over study, receiving three different meals: pasta with normal wheat bran (PA) and bread with normal (CB) or purple wheat bran (PBB). Purple wheat bran was added in an attempt to decrease the rate of starch digestion. The meals were enriched in ¹³C and the dual isotope technique was applied to calculate the rate of appearance of exogenous glucose (RaE). The ¹³C-isotopic enrichment of glucose in plasma was measured with GC/combustion/isotope ratio MS (IRMS) and liquid chromatography/IRMS. Both IRMS techniques gave similar results. Plasma glucose concentrations [2-h incremental AUC (iAUC)] did not differ between the test meals. The RaE was similar after consumption of CB and PBB, showing that purple wheat bran in bread does not affect in vivo starch digestibility. However, the iAUC of RaE after men consumed PA was less than after they consumed CB (P < 0.0001) despite the similar glucose response. To conclude, the glycemic response does not always reflect the in vivo starch digestibility. This could have implications for intervention studies in which the glycemic response is used to characterize test products.

Preparative HPLC separation of underivatized amino acids for isotopic analysis

Single-compound analysis of stable or radio-isotopes has found application in a number of fields ranging from archaeology to forensics. Often, the most difficult part of these analyses is the development of a method for isolating the compounds of interest.Here, we describe three complementary preparative HPLC procedures suitable for separating and isolating single amino acids from bone collagen or hair keratin with minimal isotopic contamination. Using preparative reversed-phase, ion-pair, or mixed-mode chromatography of underivatized amino acids in aqueous mobile phases, single amino acids can be isolated and further analyzed using mass spectrometric techniques.

The role of liquid chromatography and flow injection analyses coupled to isotope ratio mass spectrometry for studying human in vivo glucose metabolism

Under most physiological conditions, glucose, or carbohydrate (CHO), homeostasis is tightly regulated. In order to mechanistically appraise the origin of circulating glucose (e.g. via either gluconeogenesis, glycogenolysis or oral glucose intake), and its regulation and oxidation, the use of stable isotope tracers is now a well-accepted analytical technique. Methodologically, liquid chromatography coupled to isotope ratio mass spectrometry (LC/IRMS) can replace gas chromatography coupled to combustion-isotope ratio mass spectrometry (GC/C/IRMS) for carrying out compound-specific (13)C isotopic analysis. The LC/IRMS approach is well suited for studying glucose metabolism, since the plasma glucose concentration is relatively high and the glucose can readily undergo chromatography in an aqueous mobile phase. Herewith, we report two main methodological approaches in a single instrument: (1) the ability to measure the isotopic enrichment of plasma glucose to assess the efficacy of CHO-based treatment (cocoa-enriched) during cycling exercise with healthy subjects, and (2) the capacity to carry out bulk isotopic analysis of labeled solutions, which is generally performed with an elemental analyzer coupled to IRMS. For plasma samples measured by LC/IRMS the data show a isotopic precision SD(δ(13)C) and SD(APE) of 0.7 ‰ and 0.001, respectively, with δ(13)C and APE values of -25.48 ‰ and 0.06, respectively, being generated before and after tracer administration. For bulk isotopic measurements, the data show that the presence of organic compounds in the blank slightly affects the δ(13)C values. Despite some analytical limitations, we clearly demonstrate the usefulness of the LC/IRMS especially when (13)C-glucose is required during whole-body human nutritional studies.

Review: Current applications and challenges for liquid chromatography coupled to isotope ratio mass spectrometry (LC/IRMS)

High-precision isotope analysis is recognized as an essential research tool in many fields of study. Until recently, continuous flow isotope ratio mass spectrometry (CF-IRMS) was available via an elemental analyzer or a gas chromatography inlet system for compound-specific analysis of light stable isotopes. In 2004, however, an interface that coupled liquid chromatography with IRMS (LC/IRMS) became commercially available for the first time. This brought the capability for new areas of application, in particular enabling compound-specific δ(13)C analysis of non-volatile, aqueous soluble, compounds from complex mixtures. The interface design brought with it several analytical constraints, however, in particular a lack of compatibility with certain types of chromatography as well as limited flow rates and mobile phase compositions. Routine LC/IRMS methods have, however, been established for measuring the δ(13)C isotopic ratios of underivatized individual compounds for application in archeology, nutrition and physiology, geochemistry, hydrology, soil science and food authenticity. Seven years after its introduction, we review the technical advances and constraints, methodological developments and new applications of liquid chromatography coupled to isotope ratio mass spectrometry.

High-temperature reversed-phase liquid chromatography coupled to isotope ratio mass spectrometry

Compound-specific isotope analysis (CSIA) by liquid chromatography coupled to isotope ratio mass spectrometry (LC/IRMS) has until now been based on ion-exchange separation. In this work, high-temperature reversed-phase liquid chromatography was coupled to, and for the first time carefully evaluated for, isotope ratio mass spectrometry (HT-LC/IRMS) with four different stationary phases. Under isothermal and temperature gradient conditions, the column bleed of XBridge C(18) (up to 180 °C), Acquity C(18) (up to 200 °C), Triart C(18) (up to 150 °C), and Zirchrom PBD (up to 150 °C) had no influence on the precision and accuracy of δ(13) C measurements, demonstrating the suitability of these columns for HT-LC/IRMS analysis. Increasing the temperature during the LC/IRMS analysis of caffeine on two C(18) columns was observed to result in shortened analysis time. The detection limit of HT-RPLC/IRMS obtained for caffeine was 30 mg L(-1) (corresponding to 12.4 nmol carbon on-column). Temperature-programmed LC/IRMS (i) accomplished complete separation of a mixture of caffeine derivatives and a mixture of phenols and (ii) did not affect the precision and accuracy of δ(13)C measurements compared with flow injection analysis without a column. With temperature-programmed LC/IRMS, some compounds that coelute at room temperature could be baseline resolved and analyzed for their individual δ(13)C values, leading to an important extension of the application range of CSIA.

Strong anion-exchange liquid chromatography coupled with isotope ratio mass spectrometry using a Liquiface interface

The introduction of liquid chromatography coupled with isotope ratio mass spectrometry (LC/IRMS) as an analytical tool for the measurement of isotope ratios in non-volatile analytes has somewhat simplified the analytical cycle from sample collection to analysis mainly due to the avoidance of the extensive sample processing and derivatisation that were necessary for gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). Here we test the performance of coupling strong anion exchange to IRMS using only the second commercially available interface; the Liquiface. The system was modified from installation specification to improve peak resolution in the interface and maintain peak separation from the column to the mass spectrometer. The system performance was assessed by the determination of sensitivity, accuracy and precision attained from carbohydrate separations. The system performed satisfactorily after modifications, resulting in maintenance of peak resolution from column to mass spectrometer. The sensitivity achieved suggested that approximately 150 ng carbon could be analysed with acceptable precision (<0.3 per thousand). Accuracy was maintained in the interface as determined by correlation with offline techniques, resulting in regression coefficient of r(2) = 0.98 and a slope of 0.99. The average precision achieved for the separation of seven monosaccharides was 0.36 per thousand. The integration of a carbonate removal device limited the effect of background carbon perturbations in the mass spectrometer associated with eluent gradients, and the coupling of strong anion-exchange chromatography with IRMS was successfully achieved using the Liquiface.

Glutathione synthesis rates in early postnatal life

Preterm infants have diminished antioxidant defenses. Glutathione (GSH), the main intracellular antioxidant, increases upon amino acid (AA) administration in preterm infants, without an accompanying rise of the fractional synthesis rate of GSH (FSRGSH) This study investigated the mechanism behind this increased GSH concentration by determining GSH synthesis in the first days after birth using stable isotope techniques in very low-birth-weight (VLBW) infants receiving i.v. AAs. Advanced oxidized protein products (AOPPs) were determined to quantify oxidative stress. Eighteen infants (birth weight 989 +/- 241 g, gestational age of 27/7 +/- 1/7 weeks) were studied either on postnatal day 1 or 2 (7 or 31 h postnatally, respectively). Concentration of GSH increased with postnatal age (1.45 +/- 0.48 mM versus 1.99 +/- 0.40 mM, p = 0.019). FSRGSH was not significantly different, but the absolute synthesis rate of GSH (ASRGSH) tended to be higher in the infants studied on day 2 [8.1 +/- 2.7 mg/(kg . d) versus 10.6 +/- 2.4 mg/(kg . d), p = 0.054]. AOPP concentrations were not different between groups. In conclusion, GSH concentration in VLBW infants increases significantly after birth. A concomitant increased synthesis rate was not found, suggesting that GSH consumption decreases upon AA administration.

Mixed-mode chromatography/isotope ratio mass spectrometry

Liquid chromatography coupled to molecular mass spectrometry (LC/MS) has been a standard technique since the early 1970s but liquid chromatography coupled to high-precision isotope ratio mass spectrometry (LC/IRMS) has only been available commercially since 2004. This development has, for the first time, enabled natural abundance and low enrichment delta(13)C measurements to be applied to individual analytes in aqueous mixtures creating new opportunities for IRMS applications, particularly for the isotopic study of biological molecules. A growing number of applications have been published in a range of areas including amino acid metabolism, carbohydrates studies, quantification of cellular and plasma metabolites, dietary tracer and nucleic acid studies. There is strong potential to extend these to new compounds and complex matrices but several challenges face the development of LC/IRMS methods. To achieve accurate isotopic measurements, HPLC separations must provide baseline-resolution between analyte peaks; however, the design of current liquid interfaces places severe restrictions on compatible flow rates and in particular mobile phase compositions. These create a significant challenge on which reports associated with LC/IRMS have not previously focused. Accordingly, this paper will address aspects of chromatography in the context of LC/IRMS, in particular focusing on mixed-mode separations and their benefits in light of these restrictions. It aims to provide an overview of mixed-mode stationary phases and of ways to improve high aqueous separations through manipulation of parameters such as column length, temperature and mobile phase pH. The results of several practical experiments are given using proteogenic amino acids and nucleosides both of which are of noted importance in the LC/IRMS literature. This communication aims to demonstrate that mixed-mode stationary phases provide a flexible approach given the constraints of LC/IRMS interface design and acts as a practical guide for the development of new chromatographic methods compatible with LC/IRMS applications.

Analysis of [U-13C6]glucose in human plasma using liquid chromatography/isotope ratio mass spectrometry compared with two other mass spectrometry techniques

The use of stable isotope labelled glucose provides insight into glucose metabolism. The 13C-isotopic enrichment of glucose is usually measured by gas chromatography/mass spectrometry (GC/MS) or gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). However, in both techniques the samples must be derivatized prior to analysis, which makes sample preparation more labour-intensive and increases the uncertainty of the measured isotopic composition. A novel method for the determination of isotopic enrichment of glucose in human plasma using liquid chromatography/isotope ratio mass spectrometry (LC/IRMS) has been developed. Using this technique, for which hardly any sample preparation is needed, we showed that both the enrichment and the concentration could be measured with very high precision using only 20 microL of plasma. In addition, a comparison with GC/MS and GC/IRMS showed that the best performance was achieved with the LC/IRMS method making it the method of choice for the measurement of 13C-isotopic enrichment in plasma samples.

High-dose cysteine administration does not increase synthesis of the antioxidant glutathione preterm infants

OBJECTIVE

Our aim was to evaluate whether administration of additional cysteine is safe and stimulates glutathione synthesis in preterm infants in early life.

METHODS

We conducted a prospective, randomized, clinical trial with infants with birth weights of <1500 g (N = 20). The infants were assigned randomly to receive either a standard dose (45 mg/kg per day) or a high dose (81 mg/kg per day) of cysteine. Intakes of other amino acids were similar, providing a total protein intake of 2.4 g/kg per day in both groups. We recorded base requirements in the first 6 days of life. On postnatal day 2, we conducted a stable isotope study to determine glutathione concentrations and synthesis rates in erythrocytes.

RESULTS

Base requirements were higher in the high-dose cysteine group on days 3, 4, and 5. Despite an 80% increase in cysteine intake, plasma cystine concentrations did not increase. Glutathione concentrations and synthesis rates did not increase with additional cysteine administration.

CONCLUSIONS

Administration of a high dose of cysteine (81 mg/kg per day) to preterm infants seems clinically safe but does not stimulate glutathione synthesis, compared with a lower dose (45 mg/kg per day). Further research is required to determine whether there is significant benefit associated with cysteine supplementation.

Simultaneous analysis of (13)C-glutathione as its dimeric form GSSG and its precursor [1-(13)C]glycine using liquid chromatography/isotope ratio mass spectrometry

Determination of glutathione kinetics using stable isotopes requires accurate measurement of the tracers and tracees. Previously, the precursor and synthesized product were measured with two separate techniques, liquid chromatography/isotope ratio mass spectrometry (LC/IRMS) and gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). In order to reduce sample volume and minimize analytical effort we developed a method to simultaneously determine (13)C-glutathione as its dimeric form (GSSG) and its precursor [1-(13)C]glycine in a small volume of erythrocytes in one single analysis. After having transformed (13)C-glutathione into its dimeric form GSSG, we determined both the intra-erythrocytic concentrations and the (13)C-isotopic enrichment of GSSG and glycine in 150 microL of whole blood using liquid chromatography coupled to LC/IRMS. The results show that the concentration (range of micromol/mL) was reliably measured using cycloleucine as internal standard, i.e. with a precision better than 0.1 micromol/mL. The (13)C-isotopic enrichment of GSSG and glycine measured in the same run gave reliable values with excellent precision (standard deviation (sd) <0.3 per thousand) and accuracy (measured between 0 and 5 APE). This novel method opens up a variety of kinetic studies with relatively low dose administration of tracers, reducing the total cost of the study design. In addition, only a minimal sample volume is required, enabling studies even in very small subjects, such as preterm infants.

A versatile method for stable carbon isotope analysis of carbohydrates by high-performance liquid chromatography/isotope ratio mass spectrometry

We have developed a method to analyze stable carbon isotope ((13)C/(12)C) ratios in a variety of carbohydrates using high-performance liquid chromatography/isotope ratio mass spectrometry (HPLC/IRMS). The chromatography is based on strong anion-exchange columns with low strength NaOH eluents. An eluent concentration of 1 mM resulted in low background signals and good separation of most of the typical plant neutral carbohydrates. We also show that more strongly bound carbohydrates such as acidic carbohydrates can be separated by inclusion of NO(3) (-) as an inorganic pusher ion in the eluent. Analyses of neutral carbohydrate concentrations and their stable carbon isotope ratios are shown for plant materials and marine sediment samples both at natural abundance and for (13)C-enriched samples. The main advantage of HPLC/IRMS analysis over traditional gas chromatography based methods is that no derivatization is needed resulting in simple sample treatment and improved accuracy and reproducibility.

Isotope Ratio Mass Spectrometry

Isotope Ratio Mass Spectrometry (IRMS) is a specialized technique used to provide information about the geographic, chemical, and biological origins of substances. The ability to determine the source of an organic substance stems from the relative isotopic abundances of the elements which comprise the material. Because the isotope ratios of elements such as carbon, hydrogen, oxygen, sulfur, and nitrogen can become locally enriched or depleted through a variety of kinetic and thermodynamic factors, measurement of the isotope ratios can be used to differentiate between samples which otherwise share identical chemical compositions. Several sample introduction methods are now available for commercial isotope ratio mass spectrometers. Combustion is most commonly used for bulk isotopic analysis, whereas gas and liquid chromatography are predominately used for the real-time isotopic analysis of specific compounds within a mixture. Here, highlights of advances in instrumentation and applications within the last three years are provided to illustrate the impact of this rapidly growing area of research. Some prominent new applications include authenticating organic food produce, ascertaining whether or not African elephants are guilty of night-time raids on farmers' crops, and linking forensic drug and soil samples from a crime scene to a suspected point of origin. For the sake of brevity, we focus this Minireview on the isotope ratio measurements of lighter-elements common to organic sources; we do not cover the equally important field of inorganic isotope ratio mass spectrometry.

Liquid and gas chromatography coupled to isotope ratio mass spectrometry for the determination of 13C-valine isotopic ratios in complex biological samples

On-line gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) is commonly used to measure isotopic ratios at natural abundance as well as for tracer studies in nutritional and medical research. However, high-precision (13)C isotopic enrichment can also be measured by liquid chromatography-isotope ratio mass spectrometry (LC-IRMS). Indeed, LC-IRMS can be used, as shown by the new method reported here, to obtain a baseline separation and to measure (13)C isotopic enrichment of underivatised amino acids (Asp, Thr-Ser, Glu, Pro, Gly, Ala, Cys and Val). In case of Val, at natural abundance, the SD(delta(13)C) reported with this method was found to be below 1 per thousand . Another key feature of the new LC-IRMS method reported in this paper is the comparison of the LC-IRMS approach with the conventional GC-C-IRMS determination. To perform this comparative study, isotopic enrichments were measured from underivatised Val and its N(O, S)-ethoxycarbonyl ethyl ester derivative. Between 0.0 and 1.0 molar percent excess (MPE) (delta(13)C= -12.3 to 150.8 per thousand), the calculated root-mean-square (rms) of SD was 0.38 and 0.46 per thousand and the calculated rms of accuracy was 0.023 and 0.005 MPE, respectively, for GC-C-IRMS and LC-IRMS. Both systems measured accurately low isotopic enrichments (0.002 atom percent excess (APE)) with an SD (APE) of 0.0004. To correlate the relative (delta(13)C) and absolute (atom%, APE and MPE) isotopic enrichment of Val measured by the GC-C-IRMS and LC-IRMS devices, mathematical equations showing the slope and intercept of the curves were established and validated with experimental data between 0.0 to 2.3 MPE. Finally, both GC-C-IRMS and LC-IRMS instruments were also used to assess isotopic enrichment of protein-bound (13)C-Val in tibial epiphysis in a tracer study performed in rats. Isotopic enrichments measured by LC-IRMS and GC-C-IRMS were not statistically different (p>0.05). The results of this work indicate that the LC-IRMS was successful for high-precision (13)C isotopic measurements in tracer studies giving (13)C isotopic enrichment similar to the GC-C-IRMS but without the step of GC derivatisation. Therefore, for clinical studies requiring high-precision isotopic measurement, the LC-IRMS is the method of choice to measure the isotopic ratio.

Glutathione synthesis rates after amino acid administration directly after birth in preterm infants

BACKGROUND

The availability of glutathione, the main intracellular antioxidant, is compromised in preterm neonates. A possible explanation is the low availability of substrate for synthesis, because many neonatologists are reluctant to administer amino acids in the direct postnatal period for fear of intolerance.

OBJECTIVE

The objective of the study was to determine the effects of amino acid administration directly after birth on glutathione synthesis rates and markers of oxidative stress.

DESIGN

Premature infants (<1500 g) received from birth onward either dextrose (control group; n = 10) or dextrose plus 2.4 g amino acids . kg (- 1) . d(-1) (intervention group; n = 10). On postnatal day 2, [1-(13)C]glycine was administered to determine glutathione fractional synthesis rates (FSR(GSH)) and absolute synthesis rates (ASR(GSH)) in erythrocytes. In plasma, advanced oxidized protein products and dityrosine, both markers of oxidative stress, were measured. The results are expressed as means +/- SDs.

RESULTS

The FSR(GSH) was not different between groups: 44 +/- 6 and 48 +/- 9%/d in the control and intervention groups, respectively (P = 0.28). The concentration of erythrocyte glutathione was higher (P < 0.001) in the intervention group (2.28 +/- 0.35 mmol/L) than in the control group (1.73 +/- 0.37 mmol/L). ASR(GSH) values were 6.5 +/- 1.5 and 11.3 +/- 1.9 mg . kg(-1) . d(-1) in the control and intervention groups, respectively (P < 0.001). Advanced oxidized protein products and dityrosine concentrations were not significantly different between groups.

CONCLUSIONS

Amino acid administration directly after birth increases ASR(GSH) in preterm infants. Our data are consistent, however, with higher glutathione concentrations rather than a higher FSR(GSH). Greater availability of glutathione, nevertheless, did not decrease markers of oxidative stress.