Simultaneous bioanalysis of L-arginine, L-citrulline, and dimethylarginines by LC-MS/MS

Comparison of two methods for dimethylarginines quantification

Objectives

Both dimethylarginines are widely bound to chronic kidney disease (CKD). This study was focused to validate published LC-MS/MS method and compared the measured data with an immunoassay.

Design and methods

The analysis was performed on a Dionex UltiMate 3000 UHPLC-Standard (Thermo Fisher Scientific, Waltham, Massachusetts, USA) with an amaZon SL ion trap (Bruker, Billerica, Massachusetts, USA). Comparison was evaluated by using Passing Bablok regression and Bland Altman plot. Healthy volunteers (n = 40) were used for validation and as control group to patients group (n = 40) with different stages of CKD.

Results

The results in healthy controls determined by the LC-MS/MS (ELISA) method were 0.52 ± 0.0892 with 95 % CI: 0.49-0.55 (0.61 ± 0.1213 with 95 % CI: 0.57-0.64) μmol/L for AD MA and 0.56 ± 0.0810 with 95 % CI: 0.53-0.58 (0.62 ± 0.0752 with 95 % CI: 0.57-0.65) μmol/L for SDMA. In the same way, the patient group values determined by the LC-MS/MS (ELISA) method were 0.82 ± 0.1604 with 95 % CI: 0.75-0.88 (1.06 ± 0.3002 with 95 % CI: 0.94-1.19) μmol/L and 2.14 ± 0.8778 with 95 % CI: 1.47-2.58 (1.65 ± 0.5160 with 95 % CI: 1.40-1.98) μmol/L for ADMA and SDMA, respectively. The correlation between the methods, expressed as the Spearman correlation coefficient (R), was 0.858 (0.8059) for ADMA (p < 0.0001) and 0.895 (0.9607) for SDMA (p < 0.0001).

Conclusions

ADMA levels determined by the immunoassay were almost 30 % overestimated, in contrast to SDMA levels, which were 3 % underestimated. According to our findings, a better correlation could be obtained by simple sample dilution.

Per/polyfluoroalkyl substances modulate plasmid transfer of antibiotic resistance genes: A balance between oxidative stress and energy support

Emerging contaminants can accelerate the transmission of antibiotic resistance genes (ARGs) from environmental bacteria to human pathogens via plasmid conjugation, posing a great challenge to the public health. Although the toxic effects of per/polyfluoroalkyl substances (PFAS) as persistent organic pollutants have been understood, it is still unclear whether and how PFAS modulate the transmission of ARGs. In this study, we for the first time reported that perfluorooctanoic acid (PFOA), perfluorododecanoic acid (PFDoA) and ammonium perfluoro (2-methyl-3-oxahexanoate) (GenX) at relatively low concentrations (0.01, 0.1 mg/L) promoted the conjugative transfer of plasmid RP4 within Escherichia coli, while the plasmid conjugation was inhibited by PFOA, PFDoA and GenX at relatively high concentrations (1, 10 mg/L). The non-unidirectional conjugation result was ascribed to the co-regulation of ROS overproduction, enhanced cell membrane permeability, shortage of energy support as well as l-arginine pool depletion. Taking the well-known PFOA as an example, it significantly enhanced the conjugation frequency by 1.4 and 3.4 times at relatively low concentrations (0.01, 0.1 mg/L), respectively. Exposure to PFOA resulted in enhanced cell membrane permeability and ROS overproduction in donor cells. At high concentrations of PFOA (1, 10 mg/L), although enhanced oxidative stress and cell membrane permeability still occurred, the ATP contents in E. coli decreased, which contributed to the inhibited conjugation. Transcriptome analysis further showed that the expression levels of genes related to arginine biosynthesis (argA, argC, argF, argG, argI) and transport (artJ, artM, artQ) pathways were significantly increased. Intracellular l-arginine concentration deficiency were observed at high concentrations of PFOA. With the supplementary exogenous arginine, it was demonstrated that arginine upregulated conjugation transfer- related genes (trfAp, trbBp) and restores the cell number of transconjugants in PFOA-treated group. Therefore, the inhibited conjugation at high concentrations PFOA were attributed to the shortage of ATP and the depletion of L-arginine pool. These findings provide important insights into the effect environmental concentrations of PFAS on the conjugative transfer of ARGs, and update the regulation mechanism of plasmid conjugation, which is critical for the management of antibiotic resistance in aquatic environments.

An Optimized MRM-Based Workflow of the l-Arginine/Nitric Oxide Pathway Metabolites Revealed Disease- and Sex-Related Differences in the Cardiovascular Field

Clinical data indicate that low circulating l-homoarginine (HArg) concentrations are associated with cardiovascular (CV) disease, CV mortality, and all-cause mortality. A high number of LC-based analytical methods for the quantification of HArg, in combination with the l-arginine (Arg)-related pathway metabolites, have been reported. However, these methods usually consider a limited panel of analytes. Thus, in order to achieve a comprehensive picture of the Arg metabolism, we described an improved targeted metabolomic approach based on a multiple reaction monitoring (MRM) mass spectrometry method for the simultaneous quantification of the Arg/nitric oxide (NO) pathway metabolites. This methodology was then employed to quantify the plasma concentrations of these analytes in a cohort of individuals with different grades/types of coronary artery disease (CAD) in order to increase knowledge about the role of HArg and its associated metabolites in the CV field. Our results showed that the MRM method here implemented is suitable for the simultaneous assessment of a wide panel of amino acids involved in the Arg/NO metabolic pathway in plasma samples from patients with CV disease. Further, our findings highlighted an impairment of the Arg/NO metabolic pathway, and suggest a sex-dependent regulation of this metabolic route.

Evaluation of Renal Biomarkers, Including Symmetric Dimethylarginine, following Gentamicin-Induced Proximal Tubular Injury in the Rat

Symmetric dimethylarginine (SDMA) is an excretory renal function biomarker shown to correlate well with glomerular filtration rate in dogs, cats, humans, and rats. The objectives of this study were to determine utility of serum SDMA as a renal biomarker in a rat model of gentamicin-induced renal injury and to provide validation of a commercially available SDMA immunoassay for rat serum. Rats were randomly assigned to one of three dose levels of gentamicin (20, 50, or 100 mg/kg) or a vehicle control group and dosed once daily by subcutaneous injection for either four or ten days. Serum and urine renal biomarker evaluation, including serum SDMA, hematologic and serum biochemical analysis, urinalysis, and histologic examination of kidney, were performed. Before biologic validation, analytic validation of the SDMA immunoassay for rat serum was performed, including assessment of assay accuracy, precision, analytical sensitivity, linearity, analyte stability, and interference testing. Among markers of excretory renal function, SDMA and serum creatinine increased earliest and at the lowest gentamicin concentrations and were significantly increased in both the 50- and 100- mg/kg dose levels in the four- and ten-dose treatment groups compared with controls. Time- and dose-dependent increases were noted for all urinary biomarkers investigated in this study, with microalbumin being most responsive and osteopontin least responsive for detection of gentamicin-induced injury across dose levels and schedules investigated. The SDMA immunoassay met all set quality requirements assessed in analytical validation. This study is the first to investigate performance of serum SDMA compared with other excretory renal function markers in a rat gentamicin acute toxicity model. In this study, serum SDMA was an earlier biomarker for detection of gentamicin-induced toxicity than serum cystatin C, BUN, and creatinine clearance. The SDMA immunoassay provides a reliable commercially available assay for future renal investigations in rat models.

Developing a robust, fast and reliable measurement method for the analysis of methylarginine derivatives and related metabolites

Background

Nitric oxide (NO) plays an important role in endothelial homeostasis. Asymmetric dimethyl arginine (ADMA), L-N monomethyl arginine (L-NMMA) and symmetric dimethyl arginine (SDMA), which are derivatives of methylarginine, directly or indirectly reduce NO production. Therefore, these metabolites are an important risk factor for various diseases, including cardiovascular diseases. Numerous methods have been developed for the measurement of methylarginine derivatives, but various difficulties have been encountered. This study aimed to develop a reliable, fast and cost-effective method for the analysis and measurement of methylarginine derivatives (ADMA, SDMA, L-NMMA) and related metabolites (arginine, citrulline, homoarginine, ornithine), and to validate this method according to Clinical and Laboratory Standards Institute (CLSI) protocols.

Methods

For the analysis of ADMA, SDMA, L-NMMA, arginine, homoarginine, citrulline, ornithine, 200 Âµl of serum were precipitated with methanol, and subsequently derivatized with a butanol solution containing 5% acetyl chloride. Butyl derivatives were separated using a C18 reverse phase column with a 5 min run time. Detection of analytes was achieved by utilising the specific fragmentation patterns identified through tandem mass spectrometry.

Results

The method was linear for ADMA, SDMA, L-NMMA, ornithine, arginine, homoarginine and citrulline in the ranges of 0.023–6.0, 0.021–5.5, 0.019–5.0, 0.015–250, 0.015–250, 0.019–5 and 0.015–250 µM, respectively. The inter-assay CV% values for all analytes was less than 9.8%.

Conclusions

Data obtained from method validation studies shows that the developed method is highly sensitive, precise and accurate. Short analysis time, cost-effectiveness, and multiplexed analysis of these metabolites, with the same pretreatment steps, are the main advantages of the method.

Electrophoretic Determination of Symmetric and Asymmetric Dimethylarginine in Human Blood Plasma with Whole Capillary Sample Injection

Asymmetric and symmetric dimethylarginines are toxic non-coded amino acids. They are formed by post-translational modifications and play multifunctional roles in some human diseases. Their determination in human blood plasma is performed using capillary electrophoresis with contactless conductivity detection. The separations are performed in a capillary covered with covalently bonded PAMAPTAC polymer, which generates anionic electroosmotic flow and the separation takes place in the counter-current regime. The background electrolyte is a 750 mM aqueous solution of acetic acid with pH 2.45. The plasma samples for analysis are treated by the addition of acetonitrile and injected into the capillary in a large volume, reaching 94.5% of the total volume of the capillary, and subsequently subjected to electrophoretic stacking. The attained LODs are 16 nm for ADMA and 22 nM for SDMA. The electrophoretic resolution of both isomers has a value of 5.3. The developed method is sufficiently sensitive for the determination of plasmatic levels of ADMA and SDMA. The determination does not require derivatization and the individual steps in the electrophoretic stacking are fully automated. The determined plasmatic levels for healthy individuals vary in the range 0.36-0.62 µM for ADMA and 0.32-0.70 µM for SDMA.

Response of exercise-onset vasodilator kinetics to L-citrulline supplementation during different phases of the menstrual cycle

The purpose of this study was to determine whether L-citrulline (CIT) supplementation during the follicular and luteal phases of the menstrual cycle would present differential effects on vasodilator kinetics in dynamically contracting muscle. Twenty-four women were studied during the follicular (day 15 after onset of menses, n = 13) or the luteal phase (day 25 after onset of menses, n = 11). Supplementation with CIT (6g/day) or placebo occurred 7-days prior to testing in a crossover design across two menstrual cycles. Forearm vascular conductance (FVC) was calculated from blood flow and mean arterial pressure measured continuously during handgrip exercise performed at 10% maximal grip strength. FVC was calculated for each duty cycle (contract:relax, 1:2s) and expressed as a change from baseline (ΔFVC) before being fit with a monoexponential model. Amplitude of the ΔFVC response and the number of duty cycles for ΔFVC to reach 63% of steady-state amplitude (τΔFVC) were derived from the model. Analysis of variance showed no difference in the amplitude of ΔFVC between CIT and placebo (p = .45) or between menstrual cycle phases (p = .11). Additionally, τΔFVC was not different (p = .35) between CIT and placebo in women tested during the follicular (6 ± 3 versus 5 ± 3 duty cycles) or luteal phase (9 ± 1 versus 8 ± 1 duty cycles) although τΔFVC was found to be slower for women tested during the luteal as compared to the follicular phase (8 ± 4 versus 5 ± 3 duty cycles, p = .02). These results indicate that exercise-onset vasodilator kinetics is unaltered with CIT supplementation in young healthy women irrespective of menstrual cycle phase.

Metabolomic changes of the multi (-AGC-) kinase inhibitor AT13148 in cells, mice and patients are associated with NOS regulation

INTRODUCTION

To generate biomarkers of target engagement or predictive response for multi-target drugs is challenging. One such compound is the multi-AGC kinase inhibitor AT13148. Metabolic signatures of selective signal transduction inhibitors identified in preclinical models have previously been confirmed in early clinical studies. This study explores whether metabolic signatures could be used as biomarkers for the multi-AGC kinase inhibitor AT13148.

OBJECTIVES

To identify metabolomic changes of biomarkers of multi-AGC kinase inhibitor AT13148 in cells, xenograft / mouse models and in patients in a Phase I clinical study.

METHODS

HILIC LC-MS/MS methods and Biocrates AbsoluteIDQ™ p180 kit were used for targeted metabolomics; followed by multivariate data analysis in SIMCA and statistical analysis in Graphpad. Metaboanalyst and String were used for network analysis.

RESULTS

BT474 and PC3 cells treated with AT13148 affected metabolites which are in a gene protein metabolite network associated with Nitric oxide synthases (NOS). In mice bearing the human tumour xenografts BT474 and PC3, AT13148 treatment did not produce a common robust tumour specific metabolite change. However, AT13148 treatment of non-tumour bearing mice revealed 45 metabolites that were different from non-treated mice. These changes were also observed in patients at doses where biomarker modulation was observed. Further network analysis of these metabolites indicated enrichment for genes associated with the NOS pathway. The impact of AT13148 on the metabolite changes and the involvement of NOS-AT13148- Asymmetric dimethylarginine (ADMA) interaction were consistent with hypotension observed in patients in higher dose cohorts (160-300 mg).

CONCLUSION

AT13148 affects metabolites associated with NOS in cells, mice and patients which is consistent with the clinical dose-limiting hypotension.

A surrogate analyte-based LC-MS/MS method for the determination of 5-hydroxytryptamine, kynurenine and tryptophan

Aim: The metabolism of tryptophan (TRP) through kynurenine (KYN) and 5-hydroxytryptamine (5-HT) pathways is linked to various diseases such as neurological diseases and cancer. The levels of 5-HT, KYN, TRP can be used as indicators for the diagnosis of various diseases in clinical and scientific research. Experimental: Since 5-HT, KYN, TRP are both endogenous molecules in biological samples, it is difficult to obtain a 'real blank sample'. A surrogate analyte-based LC-MS/MS method was chosen, using 5-HT-d4, KYN-d4 and TRP-d5 as surrogate analytes to replace the authentic analytes 5-HT, KYN and TRP, respectively. Theophylline was selected as the internal standard (IS). Results: The method was applied to quantification 5-HT, KYN and TRP of plasma, liver, colon, brain and verified to be acceptable in terms of linearity, precision, accuracy, matrix effect, recovery efficiency and stability.

Simultaneous determination of eight arginine-related metabolites in cellular extracts using liquid chromatography-tandem mass spectrometry

A simple, sensitive, and rapid liquid chromatography (LC)-tandem mass spectrometry (MS/MS) method was developed for the simultaneous determination of arginine and its pathway-related metabolites (ornithine, proline, citrulline, glutamate, agmatine, spermidine, and spermine) in cellular extracts. Cells were lysed and cellular proteins precipitated by the addition of acetonitrile followed by ultra-sonication. Supernatants were analyzed using a Chromolith High Resolution RP-18 endcapped column (100 × 4.6 mm, 1.15 μm, 150 Å), with mobile phases of 0.1% formic acid solution and 0.1% formic acid in acetonitrile. Detection was carried out in multiple reaction monitoring (MRM) mode. Calibration curves showed linearity (r² > 0.99) for all metabolites over the calibration ranges used. The intra- and inter-day precision was less than 13.5%, and the accuracy was between 91.3 and 114.7%. The method developed in this study was successfully applied to measure arginine and its pathway-related metabolites, which are related to nitric oxide synthase/arginase pathways in mouse bone marrow-derived dendritic cells (BMDCs). The ability to simultaneously measure arginine and its pathway-related metabolites is valuable for better understanding local and systemic inflammatory processes.

A Citrulline-Based Translational Population System Toxicology Model for Gastrointestinal-Related Adverse Events Associated With Anticancer Treatments

Gastrointestinal (GI)-related adverse events (AEs) are commonly observed in the clinic during cancer treatments. Citrulline is a potentially translatable biomarker of GI AEs. In this study, irinotecan-induced citrulline changes were studied for a range of doses and schedules in rats. A translational system toxicology model for GI AEs using citrulline was then developed based on new experimental data and parameters from a literature intestinal cell dynamic model. With the addition of feedback-development and tolerance-development mechanisms, the model well captured the plasma citrulline profiles after irinotecan treatment in rats. Subsequently, the model was translated to humans and predicted the observed GI AE dynamics in humans including dose-scheduling effect using the cytotoxic and feedback parameters estimated in rats with slight calibrations. This translational toxicology model could be used for other antineoplastic drugs to simulate various clinical dosing scenarios before human studies and mitigate potential GI AEs.

Analysis of Arginine Metabolism Using LC-MS and Isotopic Labeling

Arginine metabolism is linked to several important metabolic processes, and reprogramming of arginine metabolism occurs in various physiological and pathological conditions. Here we describe a method, using a LC-MS-based metabolomics and ¹⁵N₄-arginine tracing approach, to quantitatively analyze arginine metabolism. This method can reliably quantify the abundance of important intermediates and fluxes of major metabolic reactions in arginine metabolism in a variety of cultured mammalian cell models.

A rapid and sensitive HPLC-MS/MS method for determination of endogenous creatine biosynthesis precursors in plasma of children with viral myocarditis

A simple, rapid and sensitive HPLC-MS/MS method for simultaneous determination of 4 of amino acids, guanidinoacetic acid, S-adenosylmethionine and S-adenosylhomocysteine in human plasma was developed and validated. The method requires no tedious sample preparation, derivatization reagents or ion-pairing reagents. Samples were prepared by combining plasma with a chilled mixture of acetonitrile (ACN) and water, followed by centrifugation and diluting the supernatant with 2 volumes of water. Analytes were detected with multiple reaction monitoring using a positive scan mode with electrospray ionization (ESI). In the assay, all the analytes showed good linearity over the investigated concentration range (r > 0.99). The accuracy expressed in relative error (RE) was between -5.0% and 13.2%, and the precision expressed in coefficient of variation (CV) ranged from 0.6% to 14.7%. In the two spiked levels (low and high), the averaged recoveries of analytes were between 45.0% and 110.9% and the recovery of internal standard was 92.0%. This method was successfully applied to studying the concentration changes of endogenous creatine (Cr) synthesis precursors in the plasma of children with viral myocarditis after intravenous administration of phosphocreatine (PCr).

Reactive oxygen species (ROS)-responsive biomaterials mediate tissue microenvironments and tissue regeneration

ROS-responsive biomaterials alleviate the oxidative stress in tissue microenvironments, promoting tissue regeneration and disease therapy.

Independent spectral characteristics of functionalized silver nanoparticles for colorimetric assay of arginine and spermine in biofluids

A colorimetric assay for selective and sensitive detection of arginine and spermine using 6-ATT-AgNPs as a probe.

Plasma asymmetric dimethylarginine concentrations are not related to differences in maximal oxygen uptake in endurance trained and untrained men

NEW FINDINGS

What is the central question of this study? Is there an association of plasma concentration of asymmetric dimethylarginine, which is related to exercise capacity in patients with cardiovascular diseases, with oxygen delivery and subsequently exercise capacity in healthy subjects in the absence of the potentially confounding influence of inflammation and oxidative stress? What is the main finding and its importance? Plasma asymmetric dimethylarginine concentrations are not related to exercise capacity in healthy subjects, while O₂ delivery in the working skeletal muscle during the maximal graded-exercise test is not associated with any of the l-arginine analogues. ADMA alone does not play a crucial role in local muscle perfusion and in maintaining exercise capacity.

ABSTRACT

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide (NO) synthesis that could limit oxygen (O₂ ) delivery in the working skeletal muscles by altering endothelium-dependent vasodilatation. Exercise capacity is associated with plasma ADMA concentrations in patients with cardiovascular diseases, but this issue has still not been investigated in healthy subjects. We aimed to determine whether plasma ADMA concentrations were negatively associated with exercise capacity in young healthy male subjects. Ten men with maximal oxygen uptake ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:mrow><mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mi>max</mml:mi> </mml:mrow> </mml:msub> </mml:math> ) > 65 mL kg^-1  min^-1 were included in the high exercise capacity group (HI-FIT), and 10 men with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:mrow><mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mi>max</mml:mi> </mml:mrow> </mml:msub> </mml:math>  < 45 mL kg^-1  min^-1 were included in the low exercise capacity group (LO-FIT). Plasma ADMA and other l-arginine analogue concentrations were measured before and after a maximal graded-exercise test by liquid chromatography-tandem mass spectrometry. Microvascular O₂ delivery during exercise was estimated through the pattern from the sigmoid model of muscle deoxygenation in the vastus lateralis measured by near infrared spectroscopy. <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:mrow><mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mi>max</mml:mi> </mml:mrow> </mml:msub> </mml:math> was 60% higher in the HI-FIT group (median: 70.2 mL kg^-1  min^-1 ; IQR: 68.0-71.9 mL kg^-1  min^-1 ) than in the LO-FIT group (median: 43.8 mL kg^-1  min^-1 ; IQR: 34.8-45.3 mL kg^-1  min^-1 ). Plasma ADMA concentrations did not differ between the LO-FIT and HI-FIT groups before (0.50 ± 0.06 vs. 0.54 ± 0.07 μmol L^-1 , respectively) and after the maximal incremental exercise test (0.49 ± 0.08 vs. 0.55 ± 0.03 μmol L^-1 , respectively). There was no significant association of plasma ADMA concentrations with the pattern of local muscle deoxygenation and exercise capacity. Exercise capacity and microvascular O₂ delivery are not related to plasma ADMA concentrations in young healthy male subjects. Our findings show that ADMA does not play a crucial role in local muscle perfusion and in maintaining exercise capacity without pathological conditions.

Development of Isocratic RP-HPLC Method for Separation and Quantification of L-Citrulline and L-Arginine in Watermelons

Watermelons (Citrullus lanatus) are known to have sufficient amino acid content. In this study, watermelons grown and consumed in Malaysia were investigated for their amino acid content, L-citrulline and L-arginine, by the isocratic RP-HPLC method. Flesh and rind watermelons were juiced, and freeze-dried samples were used for separation and quantification of L-citrulline and L-arginine. Three different mobile phases, 0.7% H₃P0₄, 0.1% H₃P0₄, and 0.7% H₃P0₄ : ACN (90 : 10), were tested on two different columns using Zorbax Eclipse XDB-C₁₈ and Gemini C₁₈ with a flow rate of 0.5 mL/min and a detection wavelength at 195 nm. Efficient separation with reproducible resolution of L-citrulline and L-arginine was achieved using 0.1% H₃P0₄ on the Gemini C₁₈ column. The method was validated and good linearity of L-citrulline and L-arginine was obtained with R² = 0.9956, y = 0.1664x + 2.4142 and R² = 0.9912, y = 0.4100x + 3.4850, respectively. L-citrulline content showed the highest concentration in red watermelon of flesh and rind juice extract (43.81 mg/g and 45.02 mg/g), whereas L-arginine concentration was lower than L-citrulline, ranging from 3.39 to 11.14 mg/g. The isocratic RP-HPLC method with 0.1% H₃P0₄ on the Gemini C₁₈ column proved to be efficient for separation and quantification of L-citrulline and L-arginine in watermelons.

Quantitative Analysis of l-Arginine, Dimethylated Arginine Derivatives, l-Citrulline, and Dimethylamine in Human Serum Using Liquid Chromatography-Mass Spectrometric Method

ABSTRACT

Nitric oxide (NO) is a small molecule involved in the regulation of many physiological processes. It plays a crucial role in the regulation of nervous system, immune and inflammatory responses, and blood flow. NO is synthesized by nitric oxide synthase (NOS) during two-step oxidation of l-arginine to l-citrulline. Intermediates and derivatives of NO metabolism, such as l-arginine, l-citrulline, asymmetrical dimethylarginine (ADMA), symmetrical dimethylarginine (SDMA), and dimethylamine (DMA), are investigated as potential biomarkers. In this article, we present a novel analytical method that allowed for simultaneous analysis of l-arginine, ADMA, SDMA, l-citrulline, and DMA, in a single-step extraction and derivatization using benzoyl chloride. In brief, aliquots of serum were mixed with internal standard solution mixture (50 µM D6-DMA, 20 µM D7-ADMA, and 100 µM D7-arginine) and 0.025 M borate buffer, pH 9.2 (10:1:5). The derivatization process was performed at 25 °C for 5 min using 10% benzoyl chloride. A reverse phase column was used for chromatographic separation. Quantitation was performed using following ions (m/z): 279.1457, 286.1749, 307.1717, 314.2076, 280.1297, 150.0919, and 156.1113 for l-arginine, D7-arginine, ADMA, SDMA, D7-ADMA, l-citrulline, DMA, and D6-DMA, respectively. The method was validated, and its assay linearity, accuracy and precision, recovery, and limits of detection (1.7 µM l-arginine, 0.03 µM ADMA, 0.02 µM SDMA, 0.36 µM l-citrulline, 0.06 µM DMA) and quantification (3.2 µM l-arginine, 0.08 µM ADMA, 0.05 µM SDMA, 1.08 µM l-citrulline, 0.19 µM DMA) were determined. The method is sensitive, reliable, repeatable, and reproducible. It can be applied in the routine clinical/diagnostic laboratory.

GRAPHICAL ABSTRACT

Sex-specific eNOS activity and function in human endothelial cells

Clinical and epidemiological data show that biological sex is one of the major determinants for the development and progression of cardiovascular disease (CVD). Impaired endothelial function, characterized by an imbalance in endothelial Nitric Oxide Synthase (eNOS) activity, precedes and accelerates the development of CVD. However, whether there is any sexual dimorphism in eNOS activity and function in endothelial cells (ECs) is still unknown. Here, by independently studying human male and female ECs, we found that female ECs expressed higher eNOS mRNA and protein levels both in vitro and ex vivo. The increased eNOS expression was associated to higher enzymatic activity and nitric oxide production. Pharmacological and genetic inhibition of eNOS affected migratory properties only in female ECs. In vitro angiogenesis experiments confirmed that sprouting mostly relied on eNOS-dependent migration in female ECs. At variance, capillary outgrowth from male ECs was independent of eNOS activity but required cell proliferation. In this study, we found sex-specific differences in the EC expression, activity, and function of eNOS. This intrinsic sexual dimorphism of ECs should be further evaluated to achieve more effective and precise strategies for the prevention and therapy of diseases associated to an impaired endothelial function such as CVD and pathological angiogenesis.

Cellular interactions between L-arginine and asymmetric dimethylarginine: Transport and metabolism

This study was aimed to examine the effect of L-arginine (ARG) exposure on the disposition of asymmetric dimethylarginine (ADMA) in human endothelial cells. Although the role of ADMA as an inhibitor of endothelial nitric oxide synthase (eNOS) is well-recognized, cellular interactions between ARG and ADMA are not well-characterized. EA.hy926 human vascular endothelial cells were exposed to ¹⁵N₄-ARG, and the concentrations of ¹⁵N₄-ARG and ADMA in the cell lysate and incubation medium were determined by a liquid chromatography-electrospray tandem mass spectrometry (LC-MS/MS) assay. Nitric oxide (NO) production was estimated by utilizing cumulative nitrite concentration via a fluorometric assay. Cells incubated with ¹⁵N₄-ARG exhibited enhanced nitrite production as well as ¹⁵N₄-ARG cellular uptake. These changes were accompanied by a decrease in cellular ADMA level and increase in extracellular ADMA level, indicating an efflux of endogenous ADMA from the cell. The time courses of ADMA efflux as well as nitrite accumulation in parallel with ¹⁵N₄-ARG uptake were characterized. Following preincubation with ¹⁵N₄-ARG and D₇-ADMA, the efflux of cellular ¹⁵N₄-ARG and D₇-ADMA was significantly stimulated by high concentrations of ARG or ADMA in the incubation medium, demonstrating trans-stimulated cellular transport of these two amino acids. D₇-ADMA metabolism was inhibited in the presence of added ARG. These results demonstrated that in addition to an interaction at the level of eNOS, ARG and ADMA may mutually influence their cellular availability via transport and metabolic interactions.

Arginine/Asymmetric Dimethylarginine Ratio in Colorectal Surgery

BACKGROUND

Asymmetric dimethylarginine (ADMA) is a naturally occurring amino acid that inhibits the effects of nitric oxide synthetases, producing considerable effects on the vascular system. Recent studies have suggested that increased ADMA is a major contributor to endothelial dysfunction, resulting in significant effects on cardiovascular and renal function.

METHODS

The study sample consisted of five male and 11 female patients scheduled for elective laparoscopic colorectal surgery; patients were between 41 and 77 years of age. Venous whole blood (8 - 10 mL) was collected from patients prior to colorectal surgery, as well as on postoperative day 1 and 2. Arginine, citrulline, ADMA, and symmetric dimethylarginine levels were measured in collected blood using liquid chromatography coupled to mass spectrometry.

RESULTS

Data from the amino-acid blood analysis demonstrated that the levels of L-citrulline and L-arginine decreased on postoperative days 1, coinciding with an increase of ADMA levels. The overall result of these changes was a decrease in the arginine to ADMA ratio by 21% from the preoperative period to postoperative day 2 (P = 0.02).

CONCLUSIONS

The data collected through this study demonstrate a significant increase in systemic ADMA levels following major laparoscopic colorectal surgery.

Inhibition of the Polyamine Synthesis Pathway Is Synthetically Lethal with Loss of Argininosuccinate Synthase 1

Argininosuccinate synthase 1 (ASS1) is the rate-limiting enzyme for arginine biosynthesis. ASS1 expression is lost in a range of tumor types, including 50% of malignant pleural mesotheliomas. Starving ASS1-deficient cells of arginine with arginine blockers such as ADI-PEG20 can induce selective lethality and has shown great promise in the clinical setting. We have generated a model of ADI-PEG20 resistance in mesothelioma cells. This resistance is mediated through re-expression of ASS1 via demethylation of the ASS1 promoter. Through coordinated transcriptomic and metabolomic profiling, we have shown that ASS1-deficient cells have decreased levels of acetylated polyamine metabolites, together with a compensatory increase in the expression of polyamine biosynthetic enzymes. Upon arginine deprivation, polyamine metabolites are decreased in the ASS1-deficient cells and in plasma isolated from ASS1-deficient mesothelioma patients. We identify a synthetic lethal dependence between ASS1 deficiency and polyamine metabolism, which could potentially be exploited for the treatment of ASS1-negative cancers.

Determination of Asymmetric and Symmetric Dimethylarginine in Serum from Patients with Chronic Kidney Disease: UPLC-MS/MS versus ELISA

Asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide (NO) synthesis, and its structural isomer symmetric dimethylarginine (SDMA) are uremic toxins accumulating in chronic kidney disease (CKD) patients. The objective of this study was to develop and validate a robust UPLC-MS/MS method for the simultaneous determination of ADMA and SDMA in human serum. Chromatographic separation after butyl ester derivatization was achieved on an Acquity UPLC BEH C18 column, followed by tandem mass spectrometric detection. After validation, the applicability of the method was evaluated by the analysis of serum samples from 10 healthy controls and 77 CKD patients on hemodialysis (CKD5HD). Both ADMA (0.84 ± 0.19 µM vs. 0.52 ± 0.07 µM) and SDMA concentrations (2.06 ± 0.82 µM vs. 0.59 ± 0.13 µM) were significantly (p < 0.001) elevated in CKD5HD patients compared to healthy controls. In general, low degrees of protein binding were found for both ADMA and SDMA. In addition, an established commercially available ELISA kit was utilized on the same samples (n = 87) to compare values obtained both with ELISA and UPLC-MS/MS. Regression analysis between these two methods was significant (p < 0.0001) but moderate for both ADMA (R = 0.78) and SDMA (R = 0.72).

Development, validation, and comparison of four methods to simultaneously quantify l-arginine, citrulline, and ornithine in human plasma using hydrophilic interaction liquid chromatography and electrospray tandem mass spectrometry

To understand the role of l-arginine depletion in impaired nitric oxide synthesis in disease, it is important to simultaneously quantify arginine, citrulline, and ornithine in the plasma. Because the three amino acids are endogenous analytes, true blank matrix for them is not available. It is necessary and valuable to compare the performance of different approaches due to lack of regulatory clarity for validation. A two-step sample preparation method using methanol as protein precipitation reagent was developed in this study is used for sample preparation. Because true blank matrix for endogenous analytes is not available, water as blank matrix, 1% BSA in PBS as blank matrix, surrogate analyte, and background subtraction were designed to establish successful quantification methods. Four methods to simultaneously quantify arginine, citrulline, and ornithine in human plasma using hydrophilic interaction liquid chromatography and electrospray tandem mass spectrometry were developed, validated, and compared. The developed two-step sample preparation method using methanol as protein precipitation reagent in this study needs less time and provides higher recovery comparing with other approaches. Three of the four methods, water as blank matrix, 1% BSA in PBS as blank matrix, and surrogate analyte, have been successful in fulfilling all the criteria, while background subtraction has failed. Results of the measured concentrations in 97 human plasma samples using the three methods show that the difference between any two methods or among the three methods presents 100% of samples with less than 20% for all the three amino acids and majority of them are under 10%. The developed two-step sample preparation method using methanol as protein precipitation reagent is simple and convenient. Three of the four methods are fully validated and the validation is successful. The BSA functioned effectively as a blank matrix for these three amino acids, considering cost, data quality, matrix similarity, and practicality.

Dose-Dependent Effects of L-Arginine on PROP Bitterness Intensity and Latency and Characteristics of the Chemical Interaction between PROP and L-Arginine

Genetic variation in the ability to taste the bitterness of 6-n-propylthiouracil (PROP) is a complex trait that has been used to predict food preferences and eating habits. PROP tasting is primarily controlled by polymorphisms in the TAS2R38 gene. However, a variety of factors are known to modify the phenotype. Principle among them is the salivary protein Ps-1 belonging to the basic proline-rich protein family (bPRP). Recently, we showed that oral supplementation with Ps-1 as well as its related free amino acids (L-Arg and L-Lys) enhances PROP bitterness perception, especially for PROP non-tasters who have low salivary levels of Ps-1. Here, we show that salivary L-Arg levels are higher in PROP super-tasters compared to medium tasters and non-tasters, and that oral supplementation with free L-Arg enhances PROP bitterness intensity as well as reduces bitterness latency in a dose-dependent manner, particularly in individuals with low salivary levels of both free L-Arg and Ps-1 protein. Supplementation with L-Arg also enhanced the bitterness of caffeine. We also used 1H-NMR spectroscopy and quantum-mechanical calculations carried out by Density Functional Theory (DFT) to characterize the chemical interaction between free L-Arg and the PROP molecule. Results showed that the -NH2 terminal group of the L-ArgH+ side chain interacts with the carbonyl or thiocarbonyl groups of PROP by forming two hydrogen bonds with the resulting charged adduct. The formation of this PROP•ArgH+ hydrogen-bonded adduct could enhance bitterness intensity by increasing the solubility of PROP in saliva and its availability to receptor sites. Our data suggest that L-Arg could act as a 'carrier' of various bitter molecules in saliva.

Arginine deprivation using pegylated arginine deiminase has activity against primary acute myeloid leukemia cells in vivo

The strategy of enzymatic degradation of amino acids to deprive malignant cells of important nutrients is an established component of induction therapy of acute lymphoblastic leukemia. Here we show that acute myeloid leukemia (AML) cells from most patients with AML are deficient in a critical enzyme required for arginine synthesis, argininosuccinate synthetase-1 (ASS1). Thus, these ASS1-deficient AML cells are dependent on importing extracellular arginine. We therefore investigated the effect of plasma arginine deprivation using pegylated arginine deiminase (ADI-PEG 20) against primary AMLs in a xenograft model and in vitro. ADI-PEG 20 alone induced responses in 19 of 38 AMLs in vitro and 3 of 6 AMLs in vivo, leading to caspase activation in sensitive AMLs. ADI-PEG 20-resistant AMLs showed higher relative expression of ASS1 than sensitive AMLs. This suggests that the resistant AMLs survive by producing arginine through this metabolic pathway and ASS1 expression could be used as a biomarker for response. Sensitive AMLs showed more avid uptake of arginine from the extracellular environment consistent with their auxotrophy for arginine. The combination of ADI-PEG 20 and cytarabine chemotherapy was more effective than either treatment alone resulting in responses in 6 of 6 AMLs tested in vivo. Our data show that arginine deprivation is a reasonable strategy in AML that paves the way for clinical trials.

Estimation of nitric oxide synthase activity via liquid chromatography/tandem mass spectrometric assay determination of 15N3 -citrulline in biological samples

RATIONALE

We showed that the metabolite peaks of (15)N(3) -citrulline ((15)N(3) -CIT) and (15)N(3) -arginine ((15)N(3) -ARG) could be detected when (15) N(4) -ARG was metabolized by nitric oxide synthase (NOS) in endothelial cells. The usefulness of these metabolites as potential surrogate indices of nitric oxide (NO) generation is evaluated.

METHODS

A hydrophilic-interaction liquid chromatography/electrospray tandem mass spectrometric assay (LC/MS/MS) was utilized for the simultaneous analysis of (15)N(4) -ARG, ARG, CIT, (15)N(3) -CIT and (15)N(3) -ARG. (15)N(3) -CIT and (15)N(3) -ARG from impurities of (15)N(4) -ARG were determined and corrected for the calculation of their concentration. (15)N(4) -ARG-derived NO, i.e., (15)NO formation was determined by analyzing (15)N-nitrite accumulation by another LC/MS/MS assay.

RESULTS

After EA.hy926 human endothelial cells were challenged with (15)N(4) -ARG for 2 hours, the peak intensities of (15)N(3) -CIT and (15)N(3) -ARG significantly increased with (15)N(4) -ARG concentration and positively correlated with (15)N-nitrite production. The estimated Km values were independent of the metabolite (i.e., (15)N(3) -CIT, (15)N(3) -CIT+(15)N(3) -ARG or (15) N-nitrite) used for calculation. However, after correction for its presence as a chemical contaminant of (15)N(4) -ARG, (15)N(3) -ARG was only a marginal contributor for the estimation of NOS activity.

CONCLUSIONS

These data suggest that the formation of (15)N(3) -CIT can be used as an indicator of NOS activity when (15)N(4) -ARG is used as a substrate. This approach may be superior to the radioactive (14)C-CIT method which can be contaminated by (14)C-urea, and to the (14)N-nitrite method which lacks sensitivity.

Is arginine/asymetric dimethylarginine ratio depletion an indicator of insufficient resuscitation in a porcine model of hemorrhage-reperfusion?

BACKGROUND

Hemorrhagic shock leads to a complex cascade of metabolic and hormonal processes that may result in hypoperfusion, end organ damage, and death even when blood pressure is restored. Studies have shown that morbidity and mortality could be attributable to a diminished availability of endothelial-derived nitric oxide (eNO). It is unclear whether adequate levels of citrulline (CIT) and arginine (ARG)--the precursors of eNO synthesis--are available to sustain the eNO needed to maintain adequate perfusion in severe shock. An indirect measure of eNO is the ratio between the levels of ARG and its inhibitor asymmetric dimethylarginine (ARG/ADMA). The purpose of the study was to identify the temporal impact of the ARG/ADMA ratio, ARG, CIT, and ADMA in response to hemorrhage and crystalloid fluid resuscitation by the use of a porcine model of severe hemorrhagic shock.

METHODS

Hemorrhagic shock was induced in Yorkshire cross pigs by mimicking a bleeding pattern of rapid uncontrolled hemorrhage to achieve a shed volume of 30 mL/kg, a 50% decrease in mean arterial pressure, and an oxygen debt of >60 mL/kg. Normal saline, up to 2 times the shed blood volume, was started 1 hour after the start of hemorrhage with the goal of restoring mean arterial pressure to >50 mm Hg. Hemodynamics, blood gas measurements, and plasma samples were obtained at baseline, 1 hour after the start of hemorrhage, and 1 hour after resuscitation. Amino acids were measured by liquid chromatography coupled to mass spectrometry.

RESULTS

During hemorrhage, a distinct subset of pigs was better able to tolerate ischemia than the rest. These pigs required less resuscitation, had evidence of better organ perfusion, and exhibited less of an increase in interleukin-6 (IL-6) after resuscitation. Compared with their less-tolerant counterparts, this group had a greater increase in CIT above baseline (analysis of variance, P < .05) with hemorrhage. ARG levels were similar and remained stable with hemorrhage, which indicated the similar availability of substrate for eNO synthesis but differences in the quantity produced in response to the blood volume loss. With crystalloid fluid resuscitation, ARG levels and ARG/ADMA decreased (analysis of variance, P < .05), whereas CIT remained increased in the group less able to tolerate hemorrhage. ARG/ADMA decreased proportional to greater oxygen debt during hemorrhage and greater IL-6 levels with fluid resuscitation.

CONCLUSION

Our results suggest that a sufficient decrease in MAP during hemorrhagic shock is associated with a subsequent increase in IL-6, persisting impairment of end organ perfusion, and evidence of ongoing eNO deficit and an increase in ADMA despite resuscitation. The ARG/ADMA ratio reflects both of these parameters and corresponds to the increase in IL-6 and persistent ischemia after resuscitation. We propose that the mechanism of IL-6 increase in trauma derives from eNO deficiency, and the ARG/ADMA ratio more accurately depicts the pathologic mechanism responsible for increased morbidity and mortality in trauma.

Recent advances in hydrophilic interaction chromatography for quantitative analysis of endogenous and pharmaceutical compounds in plasma samples

There is an increasing need for new analytical methods that can handle a large number of analytes in complex matrices. Hydrophilic interaction chromatography (HILIC) has recently been demonstrated as an important supplement to reversed-phase liquid chromatography for polar analytes, particularly endogenous compounds. With the increasing popularity of HILIC, progressively more polar phases with diverse functional groups have been developed. In addition, the coupling of HILIC to mass spectrometry offers the advantages of improved sensitivity by employing an organic-rich mobile phase. This article reviews recent applications of HILIC for the analysis of endogenous and pharmaceutical compounds in plasma samples. Furthermore, based on recent studies, we provide a discussion of column selection, sample pretreatment for HILIC analysis, and detection sensitivity.

Identification and quantitation of biomarkers for radiation-induced injury via mass spectrometry

Biomarker identification and validation for radiation exposure is a rapidly expanding field encompassing the need for well defined animal models and advanced analytical techniques. The resources within the consortium, Medical Countermeasures Against Radiological Threats (MCART), provide a unique opportunity for accessing well defined animal models that simulate the key sequelae of the acute radiation syndrome and the delayed effects of acute radiation exposure. Likewise, the use of mass spectrometry-based analytical techniques for biomarker discovery and validation enables a robust analytical platform that is amenable to a variety of sample matrices and considered the benchmark for biomolecular identification and quantitation. Herein, the authors demonstrate the use of two targeted mass spectrometry approaches to link established MCART animal models to identified metabolite biomarkers. Circulating citrulline concentration was correlated to gross histological gastrointestinal tissue damage, and retinoic acid production in lung tissue was established to be reduced at early and late time points post high dose irradiation. Going forward, the use of mass spectrometry-based metabolomics coupled to well defined animal models provides the unique opportunity for comprehensive biomarker discovery.

Mass spectrometric quantification of L-arginine and its pathway related substances in biofluids: the road to maturity

The amino acid L-arginine together with its metabolites and related substances is in the center of many biologically important pathways, especially the urea cycle and the nitric oxide (NO) synthesis. Therefore, the concentrations of these substances in various biological fluids are of great interest as predictive markers for health and disease. Yet, they provide major analytical difficulties as they are very polar in nature and therefore not easily to be separated on standard reversed phase HPLC stationary phases. Furthermore, as endogenous substances, no analyte-free matrix is available, a fact that results in complicated calibration procedures. This review evaluates the analytical literature for the determination of L-arginine, symmetric dimethylarginine, asymmetric dimethylarginine, monomethylarginine, L-citrulline, L-ornithine, L-homoarginine, agmatine and dimethylguanidinovaleric acid in biological fluids. Papers are discussed, which were published since 2007 and describe methods applying capillary electrophoresis (CE), gas chromatography (GC), reversed phase HPLC or polar phase HPLC, coupled to mass spectrometric quantification. Nowadays, many carefully developed and validated methods for L-arginine and its related substances are available to the scientific community. The use of stable isotope labeled internal standards enables high precision and accuracy in mass spectrometry-based quantitative analysis.

AMP-activated protein kinase regulates L-arginine mediated cellular responses

Background

Our prior study revealed the loss in short-term L-Arginine (ARG) therapeutic efficacy after continuous exposure; resulting in tolerance development, mediated by endothelial nitric oxide synthase (eNOS) down-regulation, secondary to oxidative stress and induced glucose accumulation. However, the potential factor regulating ARG cellular response is presently unknown.

Method

Human umbilical vein endothelial cells were incubated with 100 μM ARG for 2 h in buffer (short-term or acute), or for 7 days in culture medium and challenged for 2 h in buffer (continuous or chronic), in the presence or absence of other agents. eNOS activity was determined by analyzing cellular nitrite/nitrate (NO₂^–/NO₃^–), and AMP-activated protein kinase (AMPK) activity was assayed using SAMS peptide. ¹³C₆ glucose was added to medium to measure glucose uptake during cellular treatments, which were determined by LC-MS/MS. Cellular glucose was identified by o-toluidine method. Superoxide (O₂^•–) was identified by EPR-spin-trap, and peroxynitrite (ONOO^–) was measured by flow-cytometer using aminophenyl fluorescein dye.

Results

Short-term incubation of cells with 100 μM ARG in the presence or absence of 30 μM L-N^G-Nitroarginine methyl ester (L-NAME) or 30 μM AMPK inhibitor (compound C, CMP-C) increased cellular oxidative stress and overall glucose accumulation with no variation in glucose transporter-1 (GLUT-1), or AMPK activity from control. The increase in total NO₂^–/NO₃^– after 2 h 100 μM ARG exposure, was suppressed in cells co-incubated with 30 μM CMP-C or L-NAME. Long-term exposure of ARG with or without CMP-C or L-NAME suppressed NO₂^–/NO₃^–, glucose uptake, GLUT-1, AMPK expression and activity below control, and increased overall cellular glucose, O₂^•– and ONOO^–. Gluconeogenesis inhibition with 30 μM 5-Chloro-2-N-2,5-dichlorobenzenesulfonamido-benzoxazole (CDB) during ARG exposure for 2 h maintained overall cellular glucose to control, but increased cellular glucose uptake. Continuous co-incubation with CDB and ARG increased NO₂^–/NO₃^–, glucose uptake, GLUT-1, AMPK expression and activity, and maintained overall cellular glucose, O₂^•– and ONOO^– to control conditions.

Conclusion

The present study provides the fundamental evidence for AMPK as the primary modulator of ARG cellular responses and for regulating the mode of glucose accumulation during short-term and continuous ARG treatments.

Advances in MALDI mass spectrometry in clinical diagnostic applications

The concept of matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) was first reported in 1985. Since then, MALDI MS technologies have been evolving, and successfully used in genome, proteome, metabolome, and clinical diagnostic research. These technologies are high-throughput and sensitive. Emerging evidence has shown that they are not only useful in qualitative and quantitative analyses of proteins, but also of other types of biomolecules, such as DNA, glycans, and metabolites. Recently, parallel fragmentation monitoring (PFM), which is a method comparable to selected reaction monitoring, has been reported. This highlights the potentials of MALDI-TOF/TOF tandem MS in quantification of metabolites. Here we critically review the applications of the major MALDI MS technologies, including MALDI-TOF MS, MALDI-TOF/TOF MS, SALDI-TOF MS, MALDI-QqQ MS, and SELDI-TOF MS, to the discovery and quantification of disease biomarkers in biological specimens, especially those in plasma/serum specimens. Using SELDI-TOF MS as an example, the presence of systemic bias in biomarker discovery studies employing MALDI-TOF MS and its possible solutions are also discussed in this chapter. The concepts of MALDI, SALDI, SELDI, and PFM are complementary to each other. Theoretically, all these technologies can be combined, leading to the next generation of the MALDI MS technologies. Real applications of MALDI MS technologies in clinical diagnostics should be forthcoming.

Quantification of citrulline by parallel fragmentation monitoring--a novel method using graphitized carbon nanoparticles and MALDI-TOF/TOF mass spectrometry

BACKGROUND

Selected reaction monitoring (SRM) is a reliable mass spectrometry (MS)-based technique for quantification of small molecules. However, it is not applicable to matrix-assisted laser desorption/ionization time-of-flight/time-of-flight tandem MS (MALDI-TOF/TOF MS) instruments. This work presents a novel comparable MALDI-TOF/TOF MS technique, "Parallel Fragmentation Monitoring" (PFM), for high-throughput quantification of citrulline.

METHOD

Calibrator/sample solutions were spiked with internal standard that was a stable isotopic analog with 1 mass unit heavier than citrulline. Both citrulline and internal standard were isolated and fragmented in parallel by MALDI-TOF/TOF MS in the presence of graphitized carbon nanoparticles as matrix. The ratio of the peak intensities of the selected fragment of citrulline to that of internal standard was used to calibrate/calculate the concentrations of citrulline in samples.

RESULTS

Linear calibration curves were obtained in the range of 10-250 μmol/l citrulline with correlation coefficients ≥0.997. Stored calibration curve and batch-specific calibration curve produced highly similar measurement values. Within- and between-day CVs were 3.1-8.7% and 3.5-10.6%, respectively, illustrating the reliability and robustness of PFM.

CONCLUSION

Using citrulline for proof-of-concept, we have developed the PFM technique with tremendous potential for high-throughput quantification of amino acids and other small molecules.

A new derivatization method coupled with LC-MS/MS to enable baseline separation and quantification of dimethylarginines in human plasma from patients to receive on-pump CABG surgery

Asymmetric dimethylarginine (ADMA) is an inhibitor of nitric oxide synthase and a risk factor for cardiovascular events. We have developed a new derivatization method to enable baseline separation of the regio-isomers, ADMA, and symmetric dimethylarginine (SDMA), within 15 min on a C18 reverse phase column. Reacting naphthalene-2,3-dicarboxaldehyde with ADMA and SDMA in the presence of 2-mercaptoethanol produces corresponding 2,3-dihydro-benzo[f]isoindol-1-ones that are more stable than previously reported ortho-phthaldialdehyde and 2-mercaptoethanol derivatives. LC-MS/MS quantitation of these derivatives can be used to determine ADMA and SDMA concentrations in the plasma of patients to receive on-pump coronary artery bypass grafting (CABG) surgery. The LOD, LOQ and lower LOQ (LLOQ) of this method were determined to be 2.6, 8.7, and 25 nM for ADMA, and 2.5, 8.3, and 25 nM for SDMA, respectively, with consumption of only 50 μL of plasma. The relative standard deviations and relative errors of the intraday and interday determinations, as measurements of reproducibility and accuracy, are all within 15%. The ADMA and SDMA concentrations in patient plasma are 298.1 ± 11.2 nM (mean ± S.E.M., n = 123) and 457.7 ± 19.8 nM (mean ± S.E.M., n = 123), respectively. Upon unblinding of our clinical trial, these predetermined values might explain patient clinical outcomes associated with on-pump CABG surgery, as ADMA is known to inhibit nitric oxide production. Furthermore, this derivatization reaction in conjunction with LC-MS/MS analysis may open a venue to explore alternative chemical labeling modes for LC-MS/MS applications, such as analysis of other amino acids, metabolites, and peptides containing primary amine group(s).

Quantification of L-arginine, asymmetric dimethylarginine and symmetric dimethylarginine in human plasma: a step improvement in precision by stable isotope dilution mass spectrometry

The amino acid L-arginine and its metabolites ADMA and SDMA are important markers for a range of diseases in humans. Increased levels of ADMA and SDMA in plasma point to endothelial dysfunction, hypertension, renal impairment and other pathological states. We present here a method to quantify L-arginine, ADMA and SDMA in human plasma, which is suitable to support clinical research in this field. Sample preparation consisted only of protein precipitation and the analytes were separated using a silica based HILIC column. The analytes were detected by ESI MS/MS, providing high selectivity and sensitivity. The calibration functions were linear in the ranges of 7.5-150 μmol/l for l-arginine, 0.15-3 μmol/l for ADMA and 0.2-4 μmol/l for SDMA. These ranges cover the concentrations encountered in healthy and pathological human plasma. The method employs (13)C(6)-arginine, D(7)-ADMA and, for the first time in LC-MS/MS, D(6)-SDMA as internal standards for L-arginine, ADMA and SDMA. Therefore, matrix independency and a high intra-day precision of 0.82% for L-arginine, 2.12% for ADMA and 2.83% for SDMA, were achieved at basal plasma concentrations. The respective inter-day precision values were 4.01% for l-arginine, 3.77% for ADMA and 3.86% for SDMA.

Reference intervals for plasma L-arginine and the L-arginine:asymmetric dimethylarginine ratio in the Framingham Offspring Cohort

L-arginine, as a precursor of NO synthesis, has attracted much scientific attention in recent years. Experimental mouse models suggest that L-arginine supplementation can retard, halt, or even reverse atherogenesis. In human studies, supplementation with L-arginine improved endothelium-dependent vasodilation. However, L-arginine levels are best interpreted in the context of levels of asymmetric dimethylarginine (ADMA), a competitive inhibitor of NO synthase. Thus, reference limits for circulating L-arginine and the L-arginine:ADMA ratio may help to determine the nutritional state of individuals at high cardiovascular risk in light of increased ADMA levels. We defined reference limits for plasma L-arginine in 1141 people and for the L-arginine:ADMA ratio in 1138 relatively healthy individuals from the Framingham Offspring Cohort. Plasma L-arginine and ADMA concentrations were determined by using a stable isotope-based LC-MS/MS method. The reference limits (2.5th and 97.5th percentiles) for plasma L-arginine were 41.0 μmol/L (95% CI = 39.5-42.5 μmol/L) and 114 μmol/L (95% CI = 112-115 μmol/L), whereas corresponding reference limits (2.5th and 97.5th percentiles) for the L-arginine:ADMA ratio were 74.3 μmol/L (95% CI = 71.1-77.3 μmol/L) and 225 μmol/L (95% CI = 222-228 μmol/L). Plasma L-arginine was positively associated with the estimated glomerular filtration rate (eGFR) and blood glucose levels, whereas the L-arginine:ADMA ratio was positively associated with eGFR and diastolic blood pressure but inversely associated with homocysteine and (log)C-reactive protein. We report reference levels for plasma L-arginine and for the L-arginine:ADMA ratio that may be helpful for evaluation of the effects of L-arginine supplementation in participants with an impaired L-arginine/NO pathway.

Continuous exposure to L-arginine induces oxidative stress and physiological tolerance in cultured human endothelial cells

The therapeutic benefits of L-arginine (ARG) supplementation in humans, often clearly observed in short-term studies, are not evident after long-term use. The mechanisms for the development of ARG tolerance are not known and cannot be readily examined in humans. We have developed a sensitive in vitro model using a low glucose/low arginine culture medium to study the mechanisms of ARG action and tolerance using two different human endothelial cells, i.e., Ea.hy926 and human umbilical venous endothelial cells. Cultured cells were incubated with different concentrations of ARG and other agents to monitor their effects on endothelial nitric oxide synthase (eNOS) expression and function, as well as glucose and superoxide (O2(·-) ) accumulation. Short-term (2 h) exposure to at least 50 μM ARG moderately increased eNOS activity and intracellular glucose (p < 0.05), with no change in eNOS mRNA or protein expression. In contrast, 7-day continuous ARG exposure suppressed eNOS expression and activity. This was accompanied by increase in glucose and O2(·-) accumulation. Co-incubation with 100 μM ascorbic acid, 300 U/ml polyethylene glycol-superoxide dismutase (PEG-SOD), 100 μM L-lysine or 30 μM 5-chloro-2-(N-2,5-dichlorobenenesulfonamido)-benzoxazole (a fructose-1,6-bisphosphatase inhibitor) prevented the occurrence of cellular ARG tolerance. Short-term co-incubation of ARG with PEG-SOD improved cellular nitrite accumulation without altering cellular ARG uptake. These studies suggest that ARG-induced oxidative stress may be a primary causative factor for the development of cellular ARG tolerance.

Intracellular L-arginine concentration does not determine NO production in endothelial cells: implications on the "L-arginine paradox"

We examined the relative contributory roles of extracellular vs. intracellular L-arginine (ARG) toward cellular activation of endothelial nitric oxide synthase (eNOS) in human endothelial cells. EA.hy926 human endothelial cells were incubated with different concentrations of (15)N(4)-ARG, ARG, or L-arginine ethyl ester (ARG-EE) for 2h. To modulate ARG transport, siRNA for ARG transporter (CAT-1) vs. sham siRNA were transfected into cells. ARG transport activity was assessed by cellular fluxes of ARG, (15)N(4)-ARG, dimethylarginines, and L-citrulline by an LC-MS/MS assay. eNOS activity was determined by nitrite/nitrate accumulation, either via a fluorometric assay or by(15)N-nitrite or estimated (15)N(3)-citrulline concentrations when (15)N(4)-ARG was used to challenge the cells. We found that ARG-EE incubation increased cellular ARG concentration but no increase in nitrite/nitrate was observed, while ARG incubation increased both cellular ARG concentration and nitrite accumulation. Cellular nitrite/nitrate production did not correlate with cellular total ARG concentration. Reduced (15)N(4)-ARG cellular uptake in CAT-1 siRNA transfected cells vs. control was accompanied by reduced eNOS activity, as determined by (15)N-nitrite, total nitrite and (15)N(3)-citrulline formation. Our data suggest that extracellular ARG, not intracellular ARG, is the major determinant of NO production in endothelial cells. It is likely that once transported inside the cell, ARG can no longer gain access to the membrane-bound eNOS. These observations indicate that the "L-arginine paradox" should not consider intracellular ARG concentration as a reference point.