Determination of adenosine nucleotides in cultured cells by ion-pairing liquid chromatography-electrospray ionization mass spectrometry

Advancements in Adenine Nucleotides Extraction and Quantification from a Single Drop of Human Blood

Adenine nucleotides (ANs)-adenosine 5'-triphosphate (ATP), adenosine 5'-diphosphate (ADP), and adenosine 5'-monophosphate (AMP)-are essential for energy transfer and the supply of countless processes within cellular metabolism. Their concentrations can be expressed as adenylate energy charge (AEC), a measure of cellular metabolic energy that directly correlates with the homeostasis of the organism. AEC index has broad diagnostic potential, as reduced ATP levels are associated to various conditions, such as inflammatory diseases, metabolic disorders, and cancer. We introduce a novel methodology for rapid isolation, purification, and quantification of ANs from a single drop of capillary blood. Of all the stationary phases tested, activated carbon proved to be the most efficient for the purification of adenine nucleotides, using an automated micro-solid phase extraction (µ-SPE) platform. An optimized µ-SPE method, coupled with RP-HPLC and a run time of 30 min, provides a reliable analytical framework for adenine nucleotide analysis of diverse biological samples. AN concentrations measured in capillary blood samples were 1393.1 µM, 254.8 µM, and 76.9 µM for ATP, ADP, and AMP molecules aligning with values reported in the literature. Overall, this study presents a streamlined and precise approach for analyzing ANs from microliters of blood, offering promising applications in clinical diagnostics.

Precursor Quantitation Methods for Next Generation Food Production

Food is essential for human survival. Nowadays, traditional agriculture faces challenges in balancing the need of sustainable environmental development and the rising food demand caused by an increasing population. In addition, in the emerging of consumers' awareness of health related issues bring a growing trend towards novel nature-based food additives. Synthetic biology, using engineered microbial cell factories for production of various molecules, shows great advantages for generating food alternatives and additives, which not only relieve the pressure laid on tradition agriculture, but also create a new stage in healthy and sustainable food supplement. The biosynthesis of food components (protein, fats, carbohydrates or vitamins) in engineered microbial cells often involves cellular central metabolic pathways, where common precursors are processed into different proteins and products. Quantitation of the precursors provides information of the metabolic flux and intracellular metabolic state, giving guidance for precise pathway engineering. In this review, we summarized the quantitation methods for most cellular biosynthetic precursors, including energy molecules and co-factors involved in redox-reactions. It will also be useful for studies worked on pathway engineering of other microbial-derived metabolites. Finally, advantages and limitations of each method are discussed.

Application of tetrahedral -deoxyribonucleic acid electrochemistry platform coupling aptazymes and hybridized hairpin reactions for the measurement of extracellular adenosine triphosphate in plants

Extracellular ATP (eATP) is an important biological signal transduction molecule. Although a variety of detection methods have been extensively used in ATP sensing and analysis, accurate detection of eATP remains difficult due to its extremely low concentration and spatiotemporal distribution. Here, an eATP measurement strategy based on tetrahedral DNA (T-DNA)-modified electrode sensing platform and hybridization chain reaction (HCR) combined with G-quadruplex/Hemin (G4/Hemin) DNAzyme dual signal amplification is proposed. In this strategy, ATP aptamer and RNA-cleaving DNAzyme were combined to form a split aptazyme. In the presence of ATP, this aptazyme hydrolyzes the cleaving substrate strand with high selectivity, releasing cleaved ssDNA, which are captured by the T-DNA assembled on the electrode surface, triggering an HCR on the electrode surface to form numerous linker sequences of the HCR dsDNA product. When G-quadruplex@AuNPs (G4) spherical nucleic acid enzymes (SNAzymes) with other linkers are used as nanocatalyst tags, they are captured by HCR dsDNA through sticky linkers present on the electrode surface. An amplified electrochemical redox current signal is generated through SNAzyme-mediated catalysis of H₂O₂, enabling easy detection of picomole amounts of ATP. Using this strategy, eATP levels released by tobacco suspension cells were accurately measured and the distribution and concentration of eATP released on the surface of an Arabidopsis leaf was determined.

Liquid Chromatography Methods for Separation of Polar and Charged Intracellular Metabolites for 13C Metabolic Flux Analysis

Accurate quantification of mass isotopolog distribution (MID) of intracellular metabolites is a key requirement for ¹³C metabolic flux analysis (¹³C-MFA). Liquid chromatography coupled with mass spectrometry (LC/MS) has emerged as a frontrunner technique that combines two orthogonal separation strategies. While metabolomics requires separation of monoisotopic peaks, ¹³C-MFA imposes additional demands for chromatographic separation as isotopologs of metabolites significantly add to the number of analytes. In this protocol chapter, we discuss two liquid chromatography methods, namely, reverse phase ion-pairing and hydrophilic interaction chromatography (HILIC) that together can separate a wide variety of metabolites that are typically used for ¹³C metabolic flux analysis.

Dynamic Inventory of Intermediate Metabolites of Cyanobacteria in a Diurnal Cycle

Cyanobacteria are gaining importance both as hosts for photoautotrophic production of chemicals and as model systems for studies of diurnal lifestyle. The proteome and transcriptome of cyanobacteria have been closely examined under diurnal growth, whereas the downstream effects on the intermediary metabolism have not received sufficient attention. The present study focuses on identifying the cellular metabolites whose inventories undergo dramatic changes in a fast-growing cyanobacterium, Synechococcus elongatus PCC 11801. We identified and quantified 67 polar metabolites, whose inventory changes significantly during diurnal growth, with some metabolites changing by 100-fold. The Calvin-Benson-Bassham cycle intermediates peak at midday to support fast growth. The hitherto unexplored γ-glutamyl peptides act as reservoirs of amino acids. Interestingly, several storage molecules or their precursors accumulate during the dark phase, dispelling the notion that all biosynthetic activity takes place in the light phase. Our results will guide metabolic modeling and strain engineering of cyanobacteria.

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We identify and quantify 67 polar intermediate metabolites in cyanobacteria via LC-MS

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A number of metabolites show large variations during the diurnal cycle

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Intermediates of the CBB cycle peak at midday, coinciding with peak in growth rate

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Gamma-glutamyl dipeptides identified as new storage compounds that peak at dawn

A microfluidic platform integrating paper adsorption-based sample clean-up and voltage-assisted liquid desorption electrospray ionization mass spectrometry for biological sample analysis

Clinical application of direct sampling electrospray ionization mass spectrometry (ESI-MS) remains limited due to problems associated with very "dirty" sample matrices. Herein we report on a microfluidic platform that allows direct mass spectrometric analysis of serum samples of microliter sizes. The platform integrates in-line paper adsorption-based sample clean-up and voltage assisted liquid desorption ESI-MS/MS (VAL DESI-MS/MS) to detect multiple targeted compounds of clinical interest. Adenosine monophosphate (AMP), adenosine diphosphate (ADP), and adenosine triphosphate (ATP) were selected as model analytes. Simultaneous quantification of these compounds in human serum samples was demonstrated. For all the three compounds, linear calibration curves were obtained in a concentration range from 0.20 to 20.0 μmol/L with r² values ≥ 0.996. Limits of detection were 0.019, 0.015, and 0.011 μmol/L for AMP, ADP, and ATP, respectively. Recovery was found in the range from 96.5% to 103.5% at spiking concentrations of 0.25 and 2.50 μmol/L. The results indicate that the proposed microfluidic mass spectrometric platform is robust and effective. It may have a potential in clinical analysis.

Kinetics and Mechanistic Study of Hydrolysis of Adenosine Monophosphate Disodium Salt (AMPNa2) in Acidic and Alkaline Media

Phosphate ester hydrolysis is essential in signal transduction, energy storage and production, information storage and DNA repair. In this investigation, hydrolysis of adenosine monophosphate disodium salt (AMPNa2) was carried out in acidic, neutral and alkaline conditions of pH ranging between 0.30-12.71 at 60°C. The reaction was monitored spectrophotometrically. The rate ranged between (1.20 ± 0.10) × 10-7 s-1 to (4.44 ± 0.05) × 10-6 s-1 at [NaOH] from 0.0008 M to 1.00M recorded a second-order base-catalyzed rate constant, kOH as 4.32 × 10-6 M-1 s-1. In acidic conditions, the rate ranged between (1.32 ± 0.06) × 10-7 s-1 to (1.67 ± 0.10) × 10-6 s-1 at [HCl] from 0.01 M to 1.00 M. Second-order acid-catalyzed rate constant, kH obtained was 1.62 × 10-6 M-1 s-1. Rate of reaction for neutral region, k0 was obtained from graphical method to be 10-7 s-1. Mechanisms were proposed to involve P-O bond cleavage in basic medium while competition between P-O bond and N-glycosidic cleavage was observed in acidic medium. In conclusion, this study has provided comprehensive information on the kinetic parameters and mechanism of cleavage of AMPNa2 which mimicked natural AMP cleavage and the action of enzymes that facilitate its cleavage.

Simultaneous determination of ribonucleoside and deoxyribonucleoside triphosphates in biological samples by hydrophilic interaction liquid chromatography coupled with tandem mass spectrometry

Information about the intracellular concentration of dNTPs and NTPs is important for studies of the mechanisms of DNA replication and repair, but the low concentration of dNTPs and their chemical similarity to NTPs present a challenge for their measurement. Here, we describe a new rapid and sensitive method utilizing hydrophilic interaction liquid chromatography coupled with tandem mass spectrometry for the simultaneous determination of dNTPs and NTPs in biological samples. The developed method showed linearity (R² > 0.99) in wide concentration ranges and could accurately quantify dNTPs and NTPs at low pmol levels. The intra-day and inter-day precision were below 13%, and the relative recovery was between 92% and 108%. In comparison with other chromatographic methods, the current method has shorter analysis times and simpler sample pre-treatment steps, and it utilizes an ion-pair-free mobile phase that enhances mass-spectrometric detection. Using this method, we determined dNTP and NTP concentrations in actively dividing and quiescent mouse fibroblasts.

Determination of ATP, ADP, and AMP Levels by Reversed-Phase High-Performance Liquid Chromatography in Cultured Cells

Cytoplasmic and mitochondrial Ca²⁺ signals couple cellular ATP production to activity-related energy demand. In order to accurately determine the bioenergetic effect of Ca²⁺ signals, cellular energy charge, i.e., the compound ratio of the phosphorylated adenine nucleotides AMP, ADP, and ATP, should be estimated. Reversed-phase high-performance liquid chromatography (RP-HPLC) allows the rapid separation and quantitation of these molecules. Here we describe a protocol applied in our laboratories to quantify ATP, ADP, and AMP nucleotides in cellular extracts.

Measuring Rates of ATP Synthesis

Here, we offer you a high-throughput assay to measure the ATP synthesis capacity in cells or isolated mitochondria. More specifically, the assay is linked to the mitochondrial' electron transport chain components of your interest being either through complex I (with or without a linkage to pyruvate dehydrogenase activity), through complex II, or through the electron transport flavoprotein and complex I (β-oxidation of fatty acids).

Quantitative Isotope-Dilution High-Resolution-Mass-Spectrometry Analysis of Multiple Intracellular Metabolites in Clostridium autoethanogenum with Uniformly 13C-Labeled Standards Derived from Spirulina

We have investigated the applicability of commercially available lyophilized spirulina ( Arthrospira platensis), a microorganism uniformly labeled with ¹³C, as a readily accessible source of multiple ¹³C-labeled metabolites suitable as internal standards for the quantitative determination of intracellular bacterial metabolites. Metabolites of interest were analyzed by hydrophilic-interaction liquid chromatography coupled with high-resolution mass spectrometry. Multiple internal standards obtained from uniformly (U)-¹³C-labeled extracts from spirulina were used to enable isotope-dilution mass spectrometry (IDMS) in the identification and quantification of intracellular metabolites. Extraction of the intracellular metabolites of Clostridium autoethanogenum using 2:1:1 chloroform/methanol/water was found to be the optimal method in comparison with freeze-thaw, homogenization, and sonication methods. The limits of quantification were ≤1 μM with excellent linearity for all of the calibration curves ( R² ≥ 0.99) for 74 metabolites. The precision and accuracy were found to be within relative standard deviations (RSDs) of 15% for 49 of the metabolites and within RSDs of 20% for all of the metabolites. The method was applied to study the effects of feeding different levels of carbon monoxide (as a carbon source) on the central metabolism and Wood-Ljungdahl pathway of C. autoethanogenum grown in continuous culture over 35 days. Using LC-IDMS with U-¹³C spirulina allowed the successful quantification of 52 metabolites in the samples, including amino acids, carboxylic acids, sugar phosphates, purines, and pyrimidines. The method provided absolute quantitative data on intracellular metabolites that was suitable for computational modeling to understand and optimize the C. autoethanogenum metabolic pathways active in gas fermentation.

Preparation and characterization of an aptamer-functionalized solid-phase microextraction fiber and its application in the selective monitoring of adenosine phosphates with liquid chromatography and tandem mass spectrometry

An aptamer with adenosine triphosphate as a ligand was immobilized onto the surface of a porous-polymer-coated fiber to obtain an aptamer-functionalized porous-polymer-coated solid-phase microextraction fiber. The fiber was observed with a crosslinked and porous morphological surface structure. It shows specific selectivity to adenosine triphosphate with a selectivity coefficient of 22.1 compared to the scrambled oligonucleotide functionalized fiber, and the selectivity factors over other analogues and reference compounds were from 6.1 to 77.5. When the fiber-based solid-phase microextraction was coupled with liquid chromatography and tandem mass spectrometry, detection limits of 2.7, 29, and 34 μg/L were achieved for adenosine triphosphate, adenosine diphosphate, and adenosine monophosphate, respectively. The spiking recoveries of 77.6-91.9% were achieved for trace adenosine phosphates in human serum sample. Furthermore, the fibers showed high stability and good reusability and could be used over 50 times for the real serum sample pretreatment without remarkable performance reduction.

Quantifying the cellular NAD+ metabolome using a tandem liquid chromatography mass spectrometry approach

INTRODUCTION

Nicotinamide adenine dinucleotide (NAD⁺) is an essential pyridine nucleotide that serves as a key hydride transfer coenzyme for several oxidoreductases. It is also the substrate for intracellular secondary messenger signalling by CD38 glycohydrolases, DNA repair by poly(adenosine diphosphate ribose) polymerase, and epigenetic regulation of gene expression by a class of histone deacetylase enzymes known as sirtuins. The measurement of NAD⁺ and its related metabolites (hereafter, the NAD⁺ metabolome) represents an important indicator of cellular function.

OBJECTIVES

A study was performed to develop a sensitive, selective, robust, reproducible, and rapid method for the concurrent quantitative determination of intracellular levels of the NAD⁺ metabolome in glial and oocyte cell extracts using liquid chromatography coupled to mass spectrometry (LC/MS/MS).

METHODS

The metabolites were separated on a versatile amino column using a dual HILIC-RP gradient with heated electrospray (HESI) tandem mass spectrometry detection in mixed polarity multiple reaction monitoring mode.

RESULTS

Quantification of 17 metabolites in the NAD⁺ metabolome in U251 human astroglioma cells could be achieved. Changes in NAD⁺ metabolism in U251 cell line, and murine oocytes under different culture conditions were also investigated.

CONCLUSION

This method can be used as a sensitive profiling tool, tailoring chromatography for metabolites that express significant pathophysiological changes in several disease conditions and is indispensable for targeted analysis.

A sensitive method for the separation and quantification of low-level adenine nucleotides using porous graphitic carbon-based liquid chromatography and tandem mass spectrometry

A liquid chromatography coupled to heated electrospray ionization/tandem mass spectrometry (LC-HESI-MS/MS) method was developed for the simultaneous quantitative analysis of low nanomolar level adenine nucleotides AMP, ADP, ATP, cyclic AMP (cAMP), and the nucleoside adenosine. For analyte retention and separation, reverse phase chromatography using porous graphitic carbon (PGC) was employed as it provided full resolution. The erratic chromatographic behaviour characteristic of PGC, including deterioration of analyte resolution and increased peak tailing (leading to decreased sensitivity), was mitigated by incorporating acidic equilibration within runs using a quaternary gradient. Analyte resolution and chromatographic sensitivity were still lost after a period of column inactivity; hence a pre-conditioning protocol was implemented between batches to regenerate the column. These column regeneration measures also allowed elution of AMP, ADP and ATP in the sequence of mono- to tri- nucleotides, differing from conventional reverse phase elution where analytes elute with decreasing polarity. This nucleotide elution sequence has the advantage of overcoming potential mis-annotation and inaccurate quantification of smaller nucleotides caused by in-source fragmentation of ATP. The method was validated in granulosa cell conditioned media, with the LLOQs ranging between 10-50nM for most analytes. To verify the method using biological samples, nucleotide secretion was measured in granulosa cell conditioned media under various treatments known to alter their levels. Moreover, the method was applied to cumulus-oocyte complex cell lysates to examine its linearity in a complex matrix.

Metabolomic signature of brain cancer

Despite advances in surgery and adjuvant therapy, brain tumors represent one of the leading causes of cancer-related mortality and morbidity in both adults and children. Gliomas constitute about 60% of all cerebral tumors, showing varying degrees of malignancy. They are difficult to treat due to dismal prognosis and limited therapeutics. Metabolomics is the untargeted and targeted analyses of endogenous and exogenous small molecules, which charact erizes the phenotype of an individual. This emerging "omics" science provides functional readouts of cellular activity that contribute greatly to the understanding of cancer biology including brain tumor biology. Metabolites are highly informative as a direct signature of biochemical activity; therefore, metabolite profiling has become a promising approach for clinical diagnostics and prognostics. The metabolic alterations are well-recognized as one of the key hallmarks in monitoring disease progression, therapy, and revealing new molecular targets for effective therapeutic intervention. Taking advantage of the latest high-throughput analytical technologies, that is, nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS), metabolomics is now a promising field for precision medicine and drug discovery. In the present report, we review the application of metabolomics and in vivo metabolic profiling in the context of adult gliomas and paediatric brain tumors. Analytical platforms such as high-resolution (HR) NMR, in vivo magnetic resonance spectroscopic imaging and high- and low-resolution MS are discussed. Moreover, the relevance of metabolic studies in the development of new therapeutic strategies for treatment of gliomas are reviewed.

A sandwich dipstick assay for ATP detection based on split aptamer fragments

Aptamer-based strip assay is an easy, highly efficient and low-cost detection method, which has been developed and easily applied to onsite detection. A new sensitive sandwich dipstick assay for adenosine triphosphate (ATP) detection was successfully developed based on specific recognition between split aptamer fragments and the target. In this method, the thiolated aptamer was first conjugated to the surface of gold nanoparticles (AuNPs), while the biotin-aptamer was immobilized on the surface of a nitrocellulose filter in the test line. In the presence of ATP, the thiol-aptamer/ATP/biotin-aptamer complexes were generated, which led to an obvious increase in optical signals at the test line. Under the optimal determination conditions, an excellent linear logarithmic response to the ATP concentration was obtained within the range of 0.5 μM to 5 mM. The limit of detection (LOD) of 0.5 μM was reached at a signal-to-noise ratio of 3. The dipstick assay showed a good average recovery of 96-108 % with the RSD of less than 20 % in urine samples. The proposed method exhibited high specificity against other nucleotides such as the uridine triphosphate (UTP), cytidine triphosphate (CTP), and guanosine triphosphate (GTP). The results indicated that the dipstick strip may be considered as an inexpensive screening tool for onsite ATP determination. Graphical Abstract A simple split aptamer fragments based sandwich-type dipstick assay was developed for ATP detection.

Overcoming stability challenges in the quantification of tissue nucleotides: determination of 2'-C-methylguanosine triphosphate concentration in mouse liver

A conventional, rapid and high throughput method for tissue extraction and accurate and selective LC-MS/MS quantification of 2'-C-methylguanosine triphosphate (2'-MeGTP) in mouse liver was developed and qualified. Trichloroacetic acid (TCA) was used as the tissue homogenization reagent that overcomes instability challenges of liver tissue nucleotide triphosphates due to instant ischemic degradation to mono- and diphosphate nucleotides. Degradation of 2'-MeGTP was also minimized by harvesting livers using in situ clamp-freezing or snap-freezing techniques. The assay also included a sample clean-up procedure using weak anion exchange solid phase extraction followed by ion exchange chromatography and tandem mass spectrometry detection. The linear assay range was from 50 to 10000 pmol/mL concentration in liver homogenate (250-50000 pmol/g in liver tissue). The method was qualified over three intraday batches for accuracy, precision, selectivity and specificity. The assay was successfully applied to pharmacokinetic studies of 2'-MeGTP in liver tissue samples after single oral doses of IDX184, a nucleotide prodrug inhibitor of the viral polymerase for the treatment of hepatitis C, to mice. The study results suggested that the clamp-freezing liver collection method was marginally more effective in preventing 2'-MeGTP degradation during liver tissue collection compared to the snap-freezing method.

Quantitative analysis of intracellular nucleoside triphosphates and other polar metabolites using ion pair reversed-phase liquid chromatography coupled with tandem mass spectrometry

Simultaneous, quantitative determination of intracellular nucleoside triphosphates and other polar metabolites using liquid chromatography with electrospray ionization tandem mass spectrometry (LC-MS/MS) represents a bioanalytic challenge because of charged, highly hydrophilic analytes presented at a large concentration range in a complex matrix. In this study, an ion pair LC-MS/MS method using triethylamine (TEA)-hexafluoroisopropanol (HFIP) ion-pair mobile phase was optimized and validated for simultaneous and unambiguous determination of 8 nucleoside triphosphates (including ATP, CTP, GTP, UTP, dATP, dCTP, dGTP, and dTTP) in cellular samples. Compared to the the less volatile ion-pair reagent, triethylammonium acetate (100mM, pH 7.0), the combination of HFIP (100mM) and TEA (8.6mM) increased the MS signal intensity by about 50-fold, while retaining comparable chromatographic resolution. The isotope-labeled internal standard method was used for the quantitation. Lower limits of quantitation were determined at 0.5nM for CTP, UTP, dATP, dCTP, and dTTP, at 1nM for ATP, and at 5nM for GTP and dGTP. The intra- and inter-day precision and accuracy were within the generally accepted criteria for bioanalytical method validation (<15%). While the present method was validated for the quantitation of intracellular nucleoside triphosphates, it had a broad application potential for quantitative profiling of nucleoside mono- and bi-phosphates as well as other polar, ionic metabolic intermediates (including carbohydrate derivatives, carboxylic acid derivatives, co-acyl A derivatives, fatty acyls, and others) in biological samples.

Comprehensive quantitative analysis of purines and pyrimidines in the human malaria parasite using ion-pairing ultra-performance liquid chromatography-mass spectrometry

Targeted metabolite profiling has aided in the understanding of a variety of biological processes in the malaria parasite as well as in drug discovery. A fast and sensitive analytical method, based on ion-pairing reversed phase ultra-high performance liquid chromatography tandem mass spectrometry (IP-RP-UPLC-MS/MS), was optimized for the simultaneous analysis of intracellular levels of 35 purine and pyrimidine nucleobases, nucleosides, and nucleotides. This analytical method allows for chromatographic separation of highly polar metabolites using reverse phase chemistry within 15 min. The analytical performance of the method was evaluated and successfully applied to the quantification of purines and pyrimidines in Plasmodium falciparum and its host cell, the human erythrocyte. In addition, this method can be customized to include other related metabolites such as NADPH and NADP, among others.

Simultaneous determination of creatine phosphate, creatine and 12 nucleotides in rat heart by LC-MS/MS

A simple, rapid and sensitive LC-MS/MS method was developed and validated for simultaneous determination of creatine phosphate (CP), creatine (Cr) and 12 nucleotides in rat heart. The analytes, ATP, ADP, AMP, GTP, GDP, GMP, CTP, CDP, CMP, UTP, UDP, UMP, CP, Cr, were extracted from heart tissue with pre-cooled (0°C) methanol/water (1:1, v/v) and separated on a Hypersil Gold AQ C18 column (150mm×4.6mm, 3μm) using an isocratic elution with a mobile phase consisting of 2mmol/L ammonium acetate in water (pH 10.0, adjusted with ammonia). The detection was performed by negative ion electrospray ionization in selective reaction monitoring mode (SRM). In the assay, all the analytes showed good linearity over the investigated concentration range (r>0.99). The accuracy was between 80.7% and 120.6% and the precision expressed in RSD was less than 15.6%. This method was successfully applied to measure the concentrations of the 12 nucleotides, creatine phosphate and creatine in rat heart for the first time.

Analyzing clonal variation of monoclonal antibody-producing CHO cell lines using an in silico metabolomic platform

Monoclonal antibody producing Chinese hamster ovary (CHO) cells have been shown to undergo metabolic changes when engineered to produce high titers of recombinant proteins. In this work, we have studied the distinct metabolism of CHO cell clones harboring an efficient inducible expression system, based on the cumate gene switch, and displaying different expression levels, high and low productivities, compared to that of the parental cells from which they were derived. A kinetic model for CHO cell metabolism was further developed to include metabolic regulation. Model calibration was performed using intracellular and extracellular metabolite profiles obtained from shake flask batch cultures. Model simulations of intracellular fluxes and ratios known as biomarkers revealed significant changes correlated with clonal variation but not to the recombinant protein expression level. Metabolic flux distribution mostly differs in the reactions involving pyruvate metabolism, with an increased net flux of pyruvate into the tricarboxylic acid (TCA) cycle in the high-producer clone, either being induced or non-induced with cumate. More specifically, CHO cell metabolism in this clone was characterized by an efficient utilization of glucose and a high pyruvate dehydrogenase flux. Moreover, the high-producer clone shows a high rate of anaplerosis from pyruvate to oxaloacetate, through pyruvate carboxylase and from glutamate to α-ketoglutarate, through glutamate dehydrogenase, and a reduced rate of cataplerosis from malate to pyruvate, through malic enzyme. Indeed, the increase of flux through pyruvate carboxylase was not driven by an increased anabolic demand. It is in fact linked to an increase of the TCA cycle global flux, which allows better regulation of higher redox and more efficient metabolic states. To the best of our knowledge, this is the first time a dynamic in silico platform is proposed to analyze and compare the metabolomic behavior of different CHO clones.

Strategies for quantitation of endogenous adenine nucleotides in human plasma using novel ion-pair hydrophilic interaction chromatography coupled with tandem mass spectrometry

We present here a novel and highly sensitive ion-pair hydrophilic interaction chromatography-tandem mass spectrometry (IP-HILIC-MS/MS) method for quantitation of highly polar acid metabolites like adenine nucleotides. A mobile phase based on diethylamine (DEA) and hexafluoro-2-isopropanol (HFIP) and an aminopropyl (NH2) column were applied for a novel chromatographic separation for the determination of AMP, ADP and ATP in biological matrices. This novel IP-HILIC mechanism could be hypothesized by the ion-pairing reagent (DEA) in the mobile phase forming neutral and hydrophilic complexes with the analytes of polar organic acids. The IP-HILIC-MS/MS assay for adenine nucleotides was successfully validated with satisfactory linearity, sensitivity, accuracy, reproducibility and matrix effects. The lower limit of quantitation (LLOQ) at 2.00ng/mL obtained for ATP showed a least 10-fold higher sensitivity than previous LC-MS/MS assays except nano-LC-MS/MS assay. In summary, this novel IP-HILIC-MS/MS assay provides a sensitive method for nucleotides bioanalysis and shows great potential to determine a number of organic acids in biological matrices.

A role for cytosolic isocitrate dehydrogenase as a negative regulator of glucose signaling for insulin secretion in pancreatic ß-cells

Cytosolic NADPH may act as one of the signals that couple glucose metabolism to insulin secretion in the pancreatic ß-cell. NADPH levels in the cytoplasm are largely controlled by the cytosolic isoforms of malic enzyme and isocitrate dehydrogenase (IDHc). Some studies have provided evidence for a role of malic enzyme in glucose-induced insulin secretion (GIIS) via pyruvate cycling, but the role of IDHc in ß-cell signaling is unsettled. IDHc is an established component of the isocitrate/α-ketoglutarate shuttle that transfers reducing equivalents (NADPH) from the mitochondrion to the cytosol. This shuttle is energy consuming since it is coupled to nicotinamide nucleotide transhydrogenase that uses the mitochondrial proton gradient to produce mitochondrial NADPH and NAD(+) from NADP(+) and NADH. To determine whether flux through IDHc is positively or negatively linked to GIIS, we performed RNAi knockdown experiments in ß-cells. Reduced IDHc expression in INS 832/13 cells and isolated rat islet ß-cells resulted in enhanced GIIS. This effect was mediated at least in part via the KATP-independent amplification arm of GIIS. IDHc knockdown in INS 832/13 cells did not alter glucose oxidation but it reduced fatty acid oxidation and increased lipogenesis from glucose. Metabolome profiling in INS 832/13 cells showed that IDHc knockdown increased isocitrate and NADP(+) levels. It also increased the cellular contents of several metabolites linked to GIIS, in particular some Krebs cycle intermediates, acetyl-CoA, glutamate, cAMP and ATP. The results identify IDHc as a component of the emerging pathways that negatively regulate GIIS.

Separation of nucleotides by hydrophilic interaction chromatography using the FRULIC-N column

A stationary phase composed of silica-bonded cyclofructan 6 (FRULIC-N) was evaluated for the separation of four cyclic nucleotides, six nucleoside monophosphates, four nucleoside diphosphates, and five nucleoside triphosphates via hydrophilic interaction chromatography (HILIC) in both isocratic and gradient conditions. The gradient conditions gave significantly better separations by narrowing peak widths. Sixteen out of nineteen nucleotides were baseline separated on the FRULIC-N column in one run. Unlike other known HILIC stationary phases, there can be dual-retention mechanisms unique to this media. Traditional hydrogen bonding/dipolar interactions can be supplemented by dynamic ion interaction effects for anionic analytes. This occurs because the FRULIC-N stationary phase is able to bind certain buffer cations. The extent of the ion interaction is tunable, in comparison to stationary phases with embedded charged groups, where the inherent ionic properties are fixed. The best mobile phase conditions were determined by varying the organic modifier (acetonitrile) content, as well as salt type/concentration and electrolyte pH. The thermodynamic characteristic of the FRULIC-N column was investigated by evaluating the column temperature effect on retention and utilizing van't Hoff plots. This study shows that there is a greater entropic contribution for the retention of nucleotide di and triphosphates, whereas there is a greater enthalphic contribution for the cyclic nucleotides with the FRULIC-N column.

Inosine monophosphate dehydrogenase expression and activity are significantly lower in kidney transplant recipients with diabetes mellitus

BACKGROUND

Inosine 5'-monophosphate dehydrogenase (IMPDH) is a target of the immunosuppressive drug, mycophenolic acid (MPA). A 12-hour clinical pharmacokinetic and pharmacodynamic study was conducted to compare IMPDH1 and IMPDH2 gene expression, IMPDHI and IMPDHII protein levels, and enzyme activity between kidney transplant recipients with respect to diabetes status.

METHODS

Nondiabetic (ND, n = 11) and diabetic (D, n = 9) kidney transplant recipients and on nontransplant nondiabetic (n = 10) and diabetic (n = 10) volunteers were included in the study.

RESULTS

Area under the effect curve values for gene expression: IMPDH1 [ND: 22.1 (13.8-31.3) versus D: 4.5 (2.3-6.5), P < 0.001] and IMPDH2 [ND: 15.3 (11.0-21.7) versus D: 6.1 (4.6-8.6), P < 0.001], protein level: IMPDHI [ND: 1.0 (0.5-1.3) versus 0.5 (0.4-0.7), P = 0.002] and IMPDHII [ND: 1.0 (0.6-1.6) versus D: 0.7 (0.6-0.8) P < 0.001] and enzyme activity [ND: 180 (105-245) versus D: 29.9 (15.3-35.6) µmole·s(-1)·mole(-1) adenosine monophosphate, P < 0.001] was significantly lower in transplant recipients with diabetes. Similar results were observed in nontransplanted volunteers. Kinetic studies of MPA-mediated suppression of IMPDH activity in nontransplanted individuals revealed an approximately 2.5-fold lower half-maximum effective concentration (EC50) for diabetic as compared with nondiabetic [ND: 50.2 (49.8-50.7) versus D: 15.8 (15.6-16.3) nmole/L, P = 0.004] volunteers. This difference was not related to several IMPDH gene variants.

CONCLUSIONS

This study indicates a significantly lower IMPDH gene expression, protein level, and enzyme activity in diabetic patients. Further clinical studies in a larger number of patients are warranted to verify whether MPA dosing must be optimized for kidney transplant recipients with diabetes mellitus.

Effect of mobile phase pH on the retention of nucleotides on different stationary phases for high-performance liquid chromatography

The main aim of the present investigation was to study the retention and separation of eight nucleotides with the use of seven stationary phases in RP HPLC mode. Two octadecyl columns were used; however, aminopropyl, alkylamide, cholesterol, alkyl-phosphate, and phenyl were also studied. Special attention was paid to the mobile phase buffer pH, since it appears that this factor is very influential in the case of chromatographic separation of nucleotides. The retention of nucleotides was greater for mobile phase pH 4.0 in comparison with pH 7.0 (except for octadecyl and phenyl packing). This is a consequence of protonization of polar groups present on the stationary phase surface. It was proved that aminopropyl, alkyl phosphate, alkylamide packing materials are not suitable for the resolution of nucleotides. On the other hand, cholesterol and phenyl stationary phases are alternatives for conventional octadecyl phases. Application of cholesterol packing allows separation of small, polar nucleotides, which is impossible to achieve in the case of octadecyl phase. Moreover, a phenyl support allows separation of nucleotides in a shorter time in comparison with octadecyl packing.

Surface-assisted laser desorption/ionization mass spectrometric detection of biomolecules by using functional single-walled carbon nanohorns as the matrix

A surface-assisted laser desorption/ionization time-of-flight mass spectrometric (SALDI-TOF MS) method was developed for the analysis of small biomolecules by using functional single-walled carbon nanohorns (SWNHs) as matrix. The functional SWNHs could transfer energy to the analyte under laser irradiation for accelerating its desorption and ionization, which led to low matrix effect, avoided fragmentation of the analyte, and provided high salt tolerance. Biomolecules including amino acids, peptides, and fatty acids could successfully be analyzed with about 3- and 5-fold higher signals than those obtained using conventional matrix. By integrating the advantages of SWNHs and the recognition ability of aptamers, a selective approach was proposed for simultaneous capture, enrichment, ionization, and MS detection of adenosine triphosphate (ATP). This method showed a greatly improved detection limit (1.0 μM) for the analysis of ATP in complex biological samples. This newly designed protocol not only opened a new application of SWNHs, but also offered a new technique for selective MS analysis of biomolecules based on aptamer recognition systems.

Fast determination of adenosine 5'-triphosphate (ATP) and its catabolites in royal jelly using ultraperformance liquid chromatography

To obtain insight into the metabolic regulation of adenosine 5'-triphosphate (ATP) in royal jelly and to determine whether ATP and its catabolites can be used as objective parameters to evaluate the freshness and quality of royal jelly (RJ), a rapid ultraperformance liquid chromatography (UPLC) method has been developed for feasible separation and quantitation of ATP and its catabolites in RJ, namely, adenosine 5'-diphosphate (ADP), adenosine 5'-monophosphate (AMP), inosine monophosphate (IMP), inosine (HxR), and hypoxanthine (Hx). The analytes in the sample were extracted using 5% precooled perchloric acid. Chromatographic separation was performed on a Waters Acquity UPLC system with a Waters BEH Shield RP18 column and gradient elution based on a mixture of two solvents: solvent A, 50 mM phosphate buffer (pH 6.5); and solvent B, acetonitrile. The recoveries were in the range of 86.0-102.3% with RSD of no more than 3.6%. The correlation coefficients of six analytes were high (r(2) ≥ 0.9988) and within the test ranges. The limits of detection and quantification for the investigated compounds were lower, at 0.36-0.68 and 1.22-2.30 mg/kg, respectively. The overall intra- and interday RSDs were no more than 1.8%. The developed method was successfully applied to the analysis of the analytes in samples. The results showed that ATP in RJ sequentially degrades to ADP, AMP, IMP, HxR, and Hx during storage.

Quantitation of intracellular purine intermediates in different Corynebacteria using electrospray LC-MS/MS

Intermediates of the purine biosynthesis pathway play key roles in cellular metabolism including nucleic acid synthesis and signal mediation. In addition, they are also of major interest to the biotechnological industry as several intermediates either possess flavor-enhancing characteristics or are applied in medical therapy. In this study, we have developed an analytical method for quantitation of 12 intermediates from the purine biosynthesis pathway including important nucleotides and their corresponding nucleosides and nucleobases. The approach comprised a single-step acidic extraction/quenching procedure, followed by quantitative electrospray LC-MS/MS analysis. The assay was validated in terms of accuracy, precision, reproducibility, and applicability for complex biological matrices. The method was subsequently applied for determination of free intracellular pool sizes of purine biosynthetic pathway intermediates in the two Gram-positive bacteria Corynebacterium glutamicum and Corynebacterium ammoniagenes. Importantly, no ion pair reagents were applied in this approach as usually required for liquid chromatography analysis of large classes of diverse metabolites.

Mass spectrometry in the quantitative analysis of therapeutic intracellular nucleotide analogs

Nucleoside analogs are widely used in anti-cancer, anti-(retro)viral, and immunosuppressive therapy. Nucleosides are prodrugs that require intracellular activation to mono-, di-, and finally triphosphates. Monitoring of these intracellular nucleotides is important to understand their pharmacology. The relatively involatile salts and ion-pairing agents traditionally used for the separation of these ionic analytes limit the applicability of mass spectrometry (MS) for detection. Both indirect and direct methods have been developed to circumvent this apparent incompatibility. Indirect methods consist of de-phosphorylation of the nucleotides into nucleosides before the actual analysis. Various direct approaches have been developed, ranging from the use of relatively volatile or very low levels of regular ion-pairing agents, hydrophilic interaction chromatography (HILIC), weak anion-exchange, or porous graphitic carbon columns to capillary electrophoresis and matrix-assisted light desorption--time of flight (MALDI-TOF) MS. In this review we present an overview of the publications describing the quantitative analysis of therapeutic intracellular nucleotide analogs using MS. The focus is on the different approaches for their direct analysis. We conclude that despite the technical hurdles, several useful MS-compatible chromatographic approaches have been developed, enabling the use of the excellent selectivity and sensitivity of MS for the quantitative analysis of intracellular nucleotides.

Simultaneous determination of nucleosides and nucleotides in dietary foods and beverages using ion-pairing liquid chromatography-electrospray ionization-mass spectrometry

A method using ion-pairing liquid chromatography-electrospray ionization (ESI)-mass spectrometry (MS) was developed for the simultaneous determination of 23 types of purine or pyrimidine nucleosides and nucleotides in dietary foods and beverages. Dihexylammonium acetate (DHAA) was used as an ion-pairing agent and an ultra performance liquid chromatography (UPLC) system with a reversed-phase column and a gradient program was employed for the separation of nucleosides and nucleotides. Positive-ion ESI-MS was applied for the detection of nucleosides, and negative-ion ESI-MS was used for nucleotides. Lower limits of quantitation ranged from 0.02 micromol/L (UMP and AMP) to 1.3 micromol/L (CDP). The present method was validated, and sufficient reproducibility and accuracy was obtained for the quantitative measurement of the 23 types of nucleosides and nucleotides. The method was subsequently applied to their determination in a range of Japanese foods and beverages that are considered to contain significant amounts of umami flavor compounds. Because dietary purine nucleosides and nucleotides are known to be related to hyperuricemia and gout, the determination of their concentrations in dietary foods is useful for both evaluating umami flavor and assessing the effects of dietary food on purine metabolism.

Liquid chromatographic methods for the determination of endogenous nucleotides and nucleotide analogs used in cancer therapy: a review

Endogenous ribonucleotides and deoxyribonucleotides play a crucial role in cell function. The determination of their levels is of fundamental interest in numerous applications such as energy metabolism, biochemical processes, or in understanding the mechanism of nucleoside analog compounds. Nucleoside analogs are widely used in anticancer therapy. Their mechanisms of action are related to their structural similarity with natural nucleotides. Numerous assays have been described for the determination of endogenous nucleotides or anticancer nucleotide analogs in different matrices such as cellular cultures, tissue or peripheral blood mononuclear cells. The determination of these compounds is challenging due to the large difference of concentrations between ribonucleotides and deoxyribonucleotides, the presence of numerous endogenous interferences in complex matrices and the high polarity of the molecules due to the phosphate moiety. The extraction was generally performed at low temperature and was based on protein precipitation using acid or solvent mixture. This first phase could be coupled with extraction or cleaning step of the supernatant. Liquid chromatography coupled with UV detection and based on ion-exchange chromatography using non-volatile high salt concentrations was largely described for the quantification of nucleotides. However, the development of LC-MS and LC-MS/MS during the last ten years has constituted a sensitive and specific tool. In this case, analytical column was mostly constituted by graphite or C18 stationary phase. Mobile phase was usually based on a mixture of ammonium buffer and acetonitrile and in several assays included a volatile ion-pairing agent. Mass spectrometry detection was performed either with positive or negative electrospray mode according to compounds and mobile phase components. The purpose of the current review is to provide an overview of the most recent chromatographic assays (over the past ten years) developed for the determination of endogenous nucleotides and nucleotide analogs used in cancer therapy. We focused on sample preparation, chromatographic separation and quantitative considerations.

Development and application of an LC-MS/MS method for measuring the effect of (partial) agonists on cAMP accumulation in vitro

Cyclic-adenosine monophosphate (cAMP) plays an important role in cell signalling and is widely used as a marker for receptor activation and as a target for treating various diseases. In this paper we present the development and validation of a new method for the determination of cAMP and ATP (adenosine triphosphate) and other nucleotides in a biological system by combining zwitterionic hydrophilic interaction liquid chromatography (HILIC) and tandem mass spectrometry (MS/MS). The HILIC-MS/MS method was developed for the simultaneous quantitative analysis of cAMP and ATP, and was validated by assessment of linearity (over a range from 0.5 to 100nM for cAMP and 50 nM to 50 microM for ATP (r(2)>0.999)), resolution, limit of detection (0.5 and 50 nM for cAMP and ATP, respectively) and reproducibility. Furthermore, the method was validated and applied in vitro to determine cAMP accumulation in biological samples. The effect of several dopamine D(2) (partial) agonists and antagonists on cAMP accumulation was assessed by determination of the cAMP/ATP ratio in cells transfected with the human dopamine D(2L) receptor. Quinpirole, dopamine and ropinirole produced agonist effects on cAMP accumulation, with a potency of quinpirole>ropinirole>dopamine. Lisuride, terguride and bifeprunox were found to be partial agonists with efficacies of lisuride>terguride>bifeprunox. As expected, haloperidol, (-)-sulpiride and LY-741626 were antagonists. These results demonstrate that the present analytical method was robust, fast, sensitive, and selective. Moreover, it showed utility in determining cAMP/ATP in biological systems and the ability to study the effect of (partial) agonists and antagonists which makes it a useful tool for drug discovery.

Simultaneous analysis of eight nucleoside triphosphates in cell lines by liquid chromatography coupled with tandem mass spectrometry

In this study, we developed a new method for the simultaneous determination of eight endogenous ribonucleoside triphosphates and deoxyribonucleoside triphosphates based on a combination of a selective sample preparation and an ion-pair liquid chromatography-electrospray tandem mass spectrometry. The sample preparation was based on a protein precipitation coupled with a solid phase extraction using a weak-anion-exchange cartridge. The analytical separation of the nucleotides was achieved on a porous graphitic carbon stationary phase with a binary elution gradient program employing ion-pairing reagents (diethylamine and hexylamine) and organic eluent (methanol). The triple quadrupole mass spectrometer operated in both negative and positive multiple reaction monitoring modes. The calibration assay used the stable isotope labelled analogs of each compounds as standard. Standard calibrations were from 0.25 to 10pmol injected according to deoxyribonucleotides and from 12.5 to 3000pmol injected according to ribonucleotides. The within-run precision of the assay was less than 14.5% and the between-run precision was less than 12.4% for each analytes. Assay accuracy was in the range of 92.3-107.6%. This method allows the determination of NTP and dNTP pools from lysats of several cell lines or peripheral blood mononuclear cell from patient. Assays were performed with different preparation of cells to confirm the quality and the relevance of the described method.

HILIC LC-MS for the determination of 2'-C-methyl-cytidine-triphosphate in rat liver

A very accurate and selective LC-MS/MS method was developed and validated for the quantification of 2'-C-modified nucleoside triphosphate in liver tissue samples. An efficient pretreatment procedure of liver tissue samples was developed, using a fully automated SPE procedure with 96-well SPE plate (weak anion exchange sorbent, 30 mg). Nucleotide hydrophilic interaction chromatography has been performed on an aminopropyl column (100 mm x 2.0 mm, 3 microm) using a gradient mixture of ACN and ACN/water (5:95 v/v) with 20 mM ammonium acetate at pH 9.45 as mobile phase at 300 microL/min flow rate. The 2'-C-modified nucleoside triphosphate was detected in the negative ESI mode in multiple reaction monitoring (MRM) mode. Calibration curve was linear over the 0.05-50 microM concentration range. Satisfying results, confirming the high reliability of the established LC-MS/MS method, were obtained for intraday precision (CV = 2.5-9.1%) and accuracy (92.6-94.8%) and interday precision (CV = 9.6-11.5%) and accuracy (94.4-102.4%) as well as for recovery (82.0-112.6%) and selectivity. The method has been successfully applied for pharmacokinetic studies of 2'-C-methyl-cytidine-triphosphate in liver tissue samples.

Online cleanup of accelerated solvent extractions for determination of adenosine 5'-triphosphate (ATP), adenosine 5'-diphosphate (ADP), and adenosine 5'-monophosphate (AMP) in royal jelly using high-performance liquid chromatography

Determination of the levels of adenosine 5'-triphosphate (ATP), adenosine 5'-diphosphate (ADP), and adenosine 5'-monophosphate (AMP) in royal jelly is important for the study of its pharmacological activities, health benefits, and adenosine phosphate degradation. In this study was developed a novel method to determine ATP, ADP, and AMP levels in royal jelly using accelerated solvent extraction (ASE) followed by online cleanup and high-performance liquid chromatography (HPLC) with diode array detection (DAD). The optimum extraction conditions were obtained using an 11 mL ASE cell, ethanol/water (5:5 v/v) as the extraction solvent, 1500 psi, 80 degrees C, a 5 min static time, and a 60% flush volume. Optimum separation of the three compounds was achieved in <25 min using a Waters XBridge Shield RP18 column with 0.05 mol L(-1) NH(4)H(2)PO(4) (pH 5.70) and acetonitrile as the mobile phase. Detection was performed at 257 nm. The method was sensitive (LOD <or= 0.32 mg L(-1)), repeatable (RSD <or= 4.5%), accurate (recovery rates of 87.6-94.2% with RSD <or= 5.4), and precise (intraday RSD <or= 8.5%, interday RSD <or= 3.4%). The ASE extraction procedures developed here were compared with the classical adenosine phosphate extraction procedures (perchloric acid). The results indicate that the two techniques are similar in terms of recovery and reproducibility, but when other factors such as extraction time, environmental protection, and worker's health are considered, ASE is preferable to the classical extraction method. With this ASE-HPLC method, a minisurvey of ATP, ADP, and AMP levels in 15 samples of royal jelly of different origins was performed. Sample results indicated that the AMP concentration was 24.2-2214.4 mg kg(-1), whereas ATP and ADP were not detectable or present only at low levels.

Analysis of adenosine phosphates in HepG-2 cell by a HPLC-ESI-MS system with porous graphitic carbon as stationary phase

A high performance liquid chromatography coupled with electrospray ionization/mass spectrometry method was developed for the determination of adenosine 5'-monophosphate (AMP), adenosine 5'-diphosphate (ADP), and adenosine 5'-triphosphate (ATP) in the extract of HepG-2 cells. The chromatographic conditions were optimized by using porous graphitic carbon as the stationary phase for the retention and separation of the AMP, ADP and ATP. Negative-ion mode ESI-MS in basic mobile phase was applied to improve the method sensitivity. An external calibration method with linear ranges from 0.22 to 57.80 microM for AMP, from 0.59 to 117.37 microM for ADP, and from 0.49 to 98.81 microM for ATP was used for quantitative analysis. The levels of ATP, ADP, and AMP in HepG-2 cells treated with benzo[a]pyrene with different time periods were determined. Total adenine nucleotides and the energy charge potential were calculated for the investigation of the effect of benzo[a]pyrene on cell energy metabolism.

Simultaneous quantification of free nucleotides in complex biological samples using ion pair reversed phase liquid chromatography isotope dilution tandem mass spectrometry

A new sensitive and accurate analytical method has been developed for quantification of intracellular nucleotides in complex biological samples from cultured cells of different microorganisms such as Saccharomyces cerevisiae, Escherichia coli, and Penicillium chrysogenum. This method is based on ion pair reversed phase liquid chromatography electrospray ionization isotope dilution tandem mass spectrometry (IP-LC-ESI-ID-MS/MS. A good separation and low detection limits were observed for these compounds using dibutylamine as volatile ion pair reagent in the mobile phase of the LC. Uniformly (13)C-labeled isotopes of nucleotides were used as internal standards for both extraction and quantification of intracellular nucleotides. The method was validated by determining the linearity, sensitivity, and repeatability.

AMPK activation by long chain fatty acyl analogs

The antidiabetic efficacy of first-line insulin sensitizers (e.g., metformin, glitazones) is accounted for by activation of AMP-activated protein kinase (AMPK). Long chain fatty acids (LCFA) activate AMPK, but their putative antidiabetic efficacy is masked by their beta-oxidized or esterified lipid products. Substituted alpha,omega-dicarboxylic acids of 14-18 carbon atoms in length (MEDICA analogs) are not metabolized beyond their acyl-CoA thioesters, and may therefore simulate AMPK activation by LCFA while avoiding LCFA turnover into beta-oxidized or esterified lipid products. MEDICA analogs are shown here to activate AMPK and some of its downstream targets in vivo, in cultured cells and in a cell-free system consisting of the (alpha(1)beta(1)gamma(1))AMPK recombinant and LKB1-MO25-STRAD (AMPK-kinase) recombinant proteins. AMPK activation by MEDICA is accompanied by normalizing the hyperglycemia-hyperinsulinemia of diabetic db/db mice in vivo with suppression of hepatic glucose production in cultured liver cells. Activation of AMPK by MEDICA or LCFA is accounted for by (a) decreased intracellular ATP/AMP ratio and energy charge by the free acid, (b) activation of LKB1 phosphorylation of AMPK(Thr172) by the acyl-CoA thioester. The two activation modes are complementary since LKB1/AMPK activation by the CoA-thioester is fully evident under conditions of excess AMP. MEDICA analogs may expand the arsenal of AMPK activators used for treating diabetes type 2.

Quantitative profiling of nucleotides and related phosphate-containing metabolites in cultured mammalian cells by liquid chromatography tandem electrospray mass spectrometry

A method has been developed for the quantitative profiling of over twenty nucleotides and related phosphorylated species using ion-pair reversed-phase liquid chromatography hyphenated to negative ion tandem electrospray mass spectrometry. The influence of mobile phase pH and ion-pairing agent concentration were assessed to optimise separation and peak shapes. Full quantitative analysis was obtained for the nucleotides by reference to structurally related calibration standards. The developed method was applied to profile changes in nucleotides and related compounds in monolayer cultured Chinese hamster ovary (CHO) cells expressing the beta(2) adrenoceptor when exposed to pharmacological stimuli. These experiments demonstrate the potential of the LC-MS/MS method to detect changes in nucleotide drug targets as well as the simultaneous monitoring of levels of other nucleotides.

Simultaneous quantification of GMP, AMP, cyclic GMP and cyclic AMP by liquid chromatography coupled to tandem mass spectrometry

Phosphodiesterases are drug targets for treating various diseases. Inhibition of these can increase cAMP and cGMP levels, which can affect a variety of physiological responses. Here we report a new method for determining PDE activity by combining high-performance liquid chromatography and tandem mass spectrometry. Characteristic peaks of the substrates, cGMP or cAMP and products, GMP or AMP, were identified in positive-ion electrospray ionization using multiple reaction monitoring. The method can be applied to determine activity of PDE inhibitors. Our results showed that this new method was fast, sensitive and highly reproducible.

Simultaneous quantitative analysis of metabolites using ion-pair liquid chromatography-electrospray ionization mass spectrometry

We have developed an analytical method, consisting of ion-pair liquid chromatography coupled to electrospray ionization mass spectrometry (IP-LC-ESI-MS), for the simultaneous quantitative analysis of several key classes of polar metabolites, like nucleotides, coenzyme A esters, sugar nucleotides, and sugar bisphosphates. The use of the ion-pair agent hexylamine and optimization of the pH of the mobile phases were critical parameters in obtaining good retention and peak shapes of many of the above-mentioned polar and acidic metabolites that are impossible to analyze using standard reversed-phase LC/MS. Optimum conditions were found when using a gradient from 5 mM hexylamine in water (pH 6.3) to 90% methanol/10% 10 mM ammonium acetate (pH 8.5). The IP-LC-ESI-MS method was extensively validated by determining the linearity (R2 > 0.995), sensitivity (limit of detection 0.1-1 ng), repeatability, and reproducibility (relative standard deviation <10%). The IP-LC-ESI-MS method was shown to be a useful tool for microbial metabolomics, i.e., the comprehensive quantitative analysis of metabolites in extracts of microorganisms, and for the determination of the energy charge, i.e., the cellular energy status, as an overall quality measure for the sample workup and analytical protocols.