Fast and Efficient Determination of Arginine, Symmetric Dimethylarginine, and Asymmetric Dimethylarginine in Biological Fluids by Hydrophilic-Interaction Liquid Chromatography–Electrospray Tandem Mass Spectrometry

Chronic renal failure alters endothelial function in cerebral circulation in mice

We examined structure, composition, and endothelial function in cerebral arterioles after 4 wk of chronic renal failure (CRF) in a well-defined murine model (C57BL/6J and apolipoprotein E knockout female mice). We also determined quantitative expression of endothelial nitric oxide synthase (eNOS), phosphorylated eNOS (on serine 1177 and threonine 495), and caveolin-1; quantitative expression of markers of vascular inflammation or oxidative stress [Rock-1, Rock-2, VCAM-1, and peroxisome proliferator-activated receptor-γ (PPARγ)]; and the plasma concentration of l-arginine and asymmetric dimethylarginine (ADMA). Our hypothesis was that endothelial function would be impaired in cerebral arterioles during CRF following either a decrease in NO production (through alteration of eNOS expression or regulation) or an increase in NO degradation (due to oxidative stress or vascular inflammation). Endothelium-dependent relaxation was impaired during CRF, but endothelium-independent relaxation was not. CRF had no effect on cerebral arteriolar structure and composition. Quantitative expressions of eNOS, eNOS phosphorylated on serine 1177, caveolin-1, Rock-1, Rock-2, and VCAM-1 were similar in CRF and non-CRF mice. In contrast, quantitative expression of PPARγ (which exercises a protective role on blood vessels) was significantly lower in CRF mice, whereas quantitative expression of eNOS phosphorylated on the threonine 495 (the inactive form of eNOS) was significantly higher. Lastly, the plasma concentration of ADMA (a uremic toxin and an endogenous inhibitor of eNOS) was elevated and plasma concentration of l-arginine was low in CRF. In conclusion, endothelial function is impaired in a mouse model of early stage CRF. These alterations may be related (at least in part) to a decrease in NO production.

Unilateral nephrectomy causes an abrupt increase in inflammatory mediators and a simultaneous decrease in plasma ADMA: a study in living kidney donors

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide synthases (NOS). Reducing inducible NOS activity in acute inflammation seems to be desirable. In vitro data show that ADMA increases in response to inflammatory mediators, yet the effect of acute inflammation in vivo is scarcely studied. The aim of the study was to evaluate ADMA plasma levels before, during, and after the acute (nonbacterial) inflammatory-like state. Plasma ADMA, l-arginine, C-reactive protein, and IL-6 were determined in 24 healthy subjects undergoing living related kidney donation before as well as 1, 6, 12, 24, 72, and 168 h thereafter. Six hours after nephrectomy, ADMA levels decreased compared with baseline (0.488 ± 0.075 vs. 0.560 ± 0.060 μmol/l, P < 0.05). This difference became even more marked 24 h after the operation (0.478 ± 0.083 μmol/l, P < 0.01 vs. baseline), when the proinflammatory cytokine IL-6 peaked. Seven days after unilateral nephrectomy, ADMA levels were elevated above baseline (0.63 ± 0.05 μmol/l, P < 0.001 vs. baseline). l-Arginine levels decreased already 1 h after nephrectomy (97.5 ± 22.5 μmol/l, P < 0.01 vs. baseline) and paralleled the change in ADMA thereafter. At the end of the observation period when inflammation markers were regressing, l-arginine levels were significantly elevated above baseline (160.6 ± 25.1 μmol/l, P < 0.001 vs. baseline). In summary, this is the first study showing that both ADMA and l-arginine decrease temporarily after unilateral nephrectomy coinciding with the increase in inflammatory mediators. The l-arginine/ADMA ratio, a surrogate for NO production capacity, was only altered for <24 h.

Low levels of asymmetric dimethylarginine in children with diabetes mellitus type I compared with healthy children

OBJECTIVE

Although high levels of asymmetric dimethylarginine (ADMA) are associated with an increased risk for vasculopathy in adults, elevated ADMA concentrations also have been found in healthy young children. Patients with diabetes mellitus type 1 (DM1) are at risk for vasculopathy, and because the function of ADMA in the development of vascular symptoms is incompletely understood, we investigated ADMA concentrations in pediatric patients with DM1 compared with healthy age- and sex-matched individuals.

STUDY DESIGN

This cross-sectional study included 85 pediatric patients with DM1 and 89 age- and sex-matched healthy controls.

RESULTS

ADMA concentrations were significantly lower in the patients with DM1 and were inversely correlated with hemoglobin A1c concentrations.

CONCLUSIONS

Besides its vasoprotective function, nitric oxide itself may exert oxidative stress by generating free radicals. In these circumstances, ADMA would protect the system from nitric oxide overproduction and perpetuation of oxidative stress. This theory is supported by the physiologically higher ADMA concentrations in healthy children. Thus, low ADMA concentrations in children with DM1 may be an indicator of impaired protection against oxidative stress.

Simultaneous determination of 6 L-arginine metabolites in human and mouse plasma by using hydrophilic-interaction chromatography and electrospray tandem mass spectrometry

BACKGROUND

Macrophages and related cells are important cellular mediators of the innate immune system and play important roles in wound healing and fibrosis. Flux through different l-arginine metabolic pathways partially defines the functional behavior of macrophages. Methods to measure metabolites within the nitric oxide synthase/arginase pathways could provide insights into local and systemic inflammatory processes.

METHODS

A targeted metabolomics approach was developed by using hydrophilic-interaction liquid chromatography and electrospray ionization-tandem mass spectrometry to simultaneously measure l-arginine, asymmetric dimethylarginine, symmetric dimethylarginine, l-citrulline, l-ornithine, and l-proline in plasma from humans and mice.

RESULTS

All analytes were quantifiable in human and mouse plasma with a small volume (25 μL), minimal sample preparation, and no derivatization. Patients with high plasma concentrations of C-reactive protein and mice with acute inflammation induced by lipopolysaccharide had significant reductions of arginine metabolites in plasma compared with controls.

CONCLUSIONS

This new assay uses plasma metabolomic measurements to help provide new insights into metabolic changes coupled to the innate immune response. We identified significant changes in arginine metabolism in both humans and mice following an inflammatory stimulus. These changes were associated with decreased plasma arginine metabolite concentrations and increased methylated arginine concentrations.

Quantification of arginine and its metabolites in human erythrocytes using liquid chromatography-tandem mass spectrometry

Erythrocytes may affect several physiological processes because they are scavengers, vehicles, and (as recently highlighted) a producer of nitric oxide (NO). NO bioavailability is linked to arginine, its metabolic products ornithine and citrulline, and methylarginines. Here we describe a liquid chromatography-tandem mass spectrometry method for the simultaneous quantification of analytes involved in the Arg/NO metabolic pathway in erythrocytes. Calibration functions were linear, and the interday coefficients of variation were less than 10%. Limit of quantification values make this method suitable for low concentration samples. The method presented here allows easy sample preparation and provides a valuable tool for the evaluation of the Arg/NO metabolic pathway in erythrocytes.

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

PURPOSE

L-Arginine (ARG) is converted to nitric oxide (NO) and L-citrulline (CIT) by endothelial nitric oxide synthase which is competitively inhibited by asymmetric dimethylarginine (ADMA). We have developed a liquid chromatography-mass spectrometric method for the simultaneous determination of endogenous ARG, labeled ARG (¹⁵N₄-ARG), CIT, ADMA, and its inactive isomer, symmetric dimethylarginine (SDMA) in biological samples.

METHODS

Concentrations of unlabeled ARG, ¹⁵N₄-ARG, CIT, ADMA, and SDMA in EA.hy926 human endothelial cell lysate, cell incubation media, rat plasma or rat urine were measured by hydrophilic-interaction liquid chromatography electrospray tandem mass spectrometry. ¹³C₆-ARG, D₄-CIT and D₇-ADMA were used as internal standards for ARG and ¹⁵N₄-ARG, CIT, and dimethylarginines, respectively.

RESULTS

The calibration curves of ARG, ¹⁵N₄-ARG, CIT, ADMA, and SDMA were linear and independent of several sample matrices. Intra- and inter-day variabilities for the quantification of all the compounds were below 15% in quality control samples. Application of this method to determine the uptake as well as efflux of these compounds was illustrated through in vitro cell study by exposing human endothelial cells to ¹⁵N₄-ARG, which allowed the observation of generation of ¹⁵N₃-CIT and ¹⁵N₃-ARG in the cell lyate. Use of these isotopes adds insights into the cellular handling of endogenous vs. exogenous ARG. Application of this method for rat plasma and rat urine assays was demonstrated after ARG oral supplementation in rats.

CONCLUSION

An LC-MS/MS method was developed to quantify 6 ARG-related compounds simultaneously, utilizing 3 separate internal standards. This assay allows concurrent monitoring of uptake, efflux and metabolic processes when isotope-labeled ARG and CIT are measured, and can be applied for determination of these compounds in rat plasma and rat urine.

Elevated levels of asymmetric dimethylarginine in chronic heart failure: a pathophysiologic link between oxygen radical load and impaired vasodilator capacity and the therapeutic effect of allopurinol

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide-dependent vasodilation. In 113 patients with chronic heart failure (CHF) and 26 controls, ADMA level was studied in relation to peripheral blood flow and vasodilator capacity. Further, the effects of allopurinol on concentrations of reactive oxygen species (ROS) and ADMA and peripheral vasodilator capacity were tested in a double-blind design. ADMA level was found to be elevated in CHF patients as compared with controls and increased in parallel with New York Heart Association (NYHA) class and exercise capacity (all P < 0.0001). The level of ADMA predicted resting blood flow (P < 0.05) and postischemic vasodilator capacity (P < 0.001). Sixty eight patients died during the follow-up period. The level of ADMA predicted survival after multivariable adjustment (P = 0.04). Allopurinol reduced uric acid (UA) concentration (P < 0.001) and decreased ROS concentration (allantoin, P < 0.01). Allopurinol lowered ADMA concentration (P = 0.02); postischemic vasodilation as well as endothelium-dependent vasodilation (both P < 0.05) improved. ADMA may be a pathophysiologic factor that is modulated by ROS accumulation and contributes to impaired vascular regulation in CHF.

Removal of asymmetric dimethylarginine during artificial liver support using fractionated plasma separation and adsorption

OBJECTIVE

Asymmetric dimethylarginine (ADMA) is the most potent endogenous nitric oxide synthase inhibitor. Elevated ADMA levels have been linked to increased mortality in different patient populations. Key regulation of ADMA levels mainly takes place in the liver. Hence, ADMA is elevated in liver disease. There is no specific pharmacological treatment to lower the elevated ADMA levels. Hemodialysis is of limited efficiency in removing ADMA as it is highly protein bound. Prometheus is an extracorporeal liver support system which allows the removal of protein-bound toxins. We assessed the efficiency of the Prometheus system in reducing high ADMA levels in patients with liver failure.

MATERIAL AND METHODS

We studied nine patients with acute-on-chronic liver failure and concomitant renal failure already necessitating hemodialysis. Seven patients needed intensive care treatment. Two consecutive sessions of Prometheus therapy of each 4 h were performed in all patients. ADMA and its structural isomer symmetrical dimethylarginine (SDMA) were determined using liquid chromatography-mass spectrometry.

RESULTS

ADMA levels correlated to model for end stage liver disease (MELD) score (r(s) = 0.62; p < 0.0001). Before Prometheus was started, levels of ADMA and SDMA were elevated (1.36 +/- 0.5 micromol/l and 1.90 +/- 0.4 micromol/l, respectively). During Prometheus treatments, plasma levels of ADMA dropped by a mean 25% (p < 0.0001) and SDMA levels by 22% (p < 0.0001). However, there was a significant rebound of ADMA levels between the two therapy sessions (p < 0.01).

CONCLUSIONS

This study shows for the first time that plasma levels of ADMA can be effectively lowered by an artificial liver support system (Prometheus). Effective elimination of ADMA might explain some of the beneficial clinical effects of these systems in patients with liver failure.

Analysis of arginine and methylated metabolites in human plasma by field amplified sample injection capillary electrophoresis tandem mass spectrometry

A CE ion trap tandem MS method was optimised for the analysis of arginine, monomethyl- and (symmetric and asymmetric) dimethylarginines in human plasma after a very reduced sample pretreatment step involving a simple protein precipitation with ACN. Several parameters affecting the analytes MS ionization and the capillary electrophoretic separation were carefully studied and optimised. The complete separation of arginine, monomethylarginine and symmetric and asymmetric dimethylarginine was obtained in formic acid BGE in short analysis time with high specificity due to MS(2) detection of specific analytes fragments. In order to achieve the detection sensitivity suitable for the analysis of asymmetric and symmetric dimethylarginine in human plasma, the field amplified sample injection was applied. Due to stacking effects, this methodology allowed to operate a consistent on-line preconcentration of the analytes before running the electrophoresis. The method was validated for linearity, repeatability, recovery and accuracy and applied to the quantitative analysis of arginine, monomethyl- and dimethylarginines in human plasma of healthy subjects.

SDMA is an early marker of change in GFR after living-related kidney donation

BACKGROUND

Early detection of changes in the glomerular filtration rate (GFR) is crucial in detecting acute kidney injury. There is burgeoning evidence from preclinical and clinical studies that symmetrical dimethylarginine (SDMA) correlates well with different parameters of renal function. In some studies, SDMA even outperformed creatinine as a marker of GFR. It is however unknown how fast SDMA is increasing after reduction in GFR. The aim of our study was therefore to determine the temporal change of SDMA in comparison with cystatin C after a defined reduction in GFR.

METHODS

Blood samples from 24 healthy living-related kidney donors (19 F/5 M), mean age 55.2 ± 8.3 years, were collected prior to donation of the kidney as well as 1, 6, 12, 24, 72 and 168 h after unilateral nephrectomy. SDMA levels were measured using a liquid chromatography-mass spectrometry-based method.

RESULTS

Within 6 h after unilateral nephrectomy, i.e. reduction of GFR by 50%, SDMA rose from 0.571 ± 0.120 to 0.659 ± 0.135 µmol/L (P < 0.001). Baseline cystatin C levels increased from 0.87 ± 0.16 to 1.07 ± 0.15 mg/L (P < 0.001). Also, serum creatinine rose significantly within 6 h after removal of one kidney from 65.4 ± 8.4 to 88.8 ± 10.2 µmol/L (P < 0.001).

DISCUSSION

SDMA might be a valuable and early marker of change in GFR in the clinical and experimental setting. Future studies will have to clarify whether sensitivity, specificity and temporal resolution of SDMA make it an attractive candidate for the assessment of renal function in both the experimental and clinical setting.

Circulating levels of osteopontin are closely related to glomerular filtration rate and cardiovascular risk markers in patients with chronic kidney disease

BACKGROUND

The pleiotropic cytokine osteopontin (OPN) is thought to be involved in the pathogenesis of atherosclerosis. However, the relationship between OPN and renal function, a cardiovascular risk factor itself, is not known. Therefore, we assessed the relationship between OPN plasma levels and renal function in patients at different stages of chronic kidney disease (CKD).

METHODS

We studied 49 non-diabetic and non-smoking patients with primary kidney disease at different CKD stages (K/DOQI 1-5). True glomerular filtration rate (GFR) in patients was assessed using the inulin-clearance technique. To examine the role of an abrupt change in GFR on circulating OPN, 15 living related kidney donors were studied before and after unilateral nephrectomy. Twenty matched non-smoking healthy subjects served as controls.

RESULTS

OPN plasma levels in patients with CKD stage 1 (i.e. GFR above 90 mL min(-1) 1.73 m(-2)) were comparable with controls. OPN levels increase in a linear fashion with declining GFR (r = -0.9, P < 0.0001), so that the increase in OPN mirrors the severity of renal impairment. After unilateral nephrectomy, circulating OPN increased significantly in parallel to the decrease in GFR. We found a direct association between OPN and other markers of renal function (serum-creatinine, homocysteine and symmetric dimethylarginine,) as well as with cardiovascular risk factors such as asymmetric dimethylarginine (r = 0.36, P = 0.0213).

CONCLUSION

There is a close inverse association between GFR and circulating OPN in patients with CKD. Furthermore, OPN plasma levels correlate with established cardiovascular risk markers in patients with CKD. Assessment of renal function is important for the interpretation of OPN levels in patients with atherosclerotic disease.

The biochemistry, measurement and current clinical significance of asymmetric dimethylarginine

Asymmetric dimethylarginine (ADMA) is an endogenous competitive inhibitor of nitric oxide synthase and an important cause of endothelial dysfunction. Its increased plasma concentration is associated with a variety of traditional cardiovascular risk factors, and may mediate their effects on the vascular endothelium. ADMA is also an independent predictor of cardiovascular events and mortality, and predicts outcomes in critically ill patients in the intensive care unit. This work has provided insights into the role of ADMA as an endogenous regulator of nitric oxide synthesis. At present there is no specific therapy to modify ADMA concentration, but increasing interest and work on protein arginine methyltransferases and dimethylarginine dimethylaminohydrolase, which synthesize and metabolize ADMA, respectively, might provide novel therapeutic targets.

Red wine decreases asymmetric dimethylarginine via SIRT1 induction in human endothelial cells

To investigate the effect of three red wines (RWs) from different growing areas and made from different grapes on asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthase, in young and senescent human endothelial cells (ECs). All RWs decreased ADMA levels, but 2-fold concentration of German RW was necessary to reach the same effect on ADMA compared to Italian RW and French RW without affecting the cell viability and morphology. The ADMA-lowering effect of RW was increased in senescent compared to young cells, accompanied by enhanced activity of the metabolizing enzyme: dimethylarginine dimethylaminohydrolase (DDAH) II, whereas the same amount in the upregulated protein expression of DDAH II and the downregulated protein expression of the synthesizing enzyme: protein arginine methyltransferase 1 was revealed. These effects were associated with decreased 8-iso-prostaglandin F(2alpha) and peroxynitrite formation, enhanced protein expression of NAD(+)-dependent class III histone deacetylase sirtuin (SIRT) 1, and downregulated protein expression of histone senescence factor p53. Blockade of SIRT1 activity abolished the effect of red wine on ADMA. These data are the first demonstration that RW by activating SIRT1 impairs synthesis and increases metabolism of ADMA. This effect of RW is accentuated in senescent cells probably due to enhanced DDAH activity.

Dimethylarginine dimethylaminohydrolase regulation: a novel therapeutic target in cardiovascular disease

Asymmetric dimethylarginine (ADMA), an endogenous methylated form of the amino acid L-arginine, inhibits the activity of the enzyme endothelial nitric oxide synthase, with consequent reduced synthesis of nitric oxide. ADMA is metabolised to L-citrulline and dimethylamine by the enzyme dimethylarginine dimethylaminohydrolase (DDAH). The modulation of DDAH activity and expression plays a pivotal role in regulating intracellular ADMA concentrations, with important effects on vascular homeostasis. For example, impairment in DDAH activity, resulting in elevated ADMA concentrations and reduced nitric oxide synthesis, can promote the onset and progression of atherosclerosis in experimental models. This review discusses the current role of ADMA and DDAH in vascular health and disease, the techniques used to assess DDAH activity and expression, and the results of recent studies on pharmacological and biological agents modulating DDAH activity and expression. Suggestions for future basic and clinical research directions are also discussed.

The effects of G-CSF-induced mobilization of progenitor cells are limited by ADMA

OBJECTIVE

Progenitor cells (PC) are thought to induce angiogenesis, and thereby, PC may help to improve ventricular performance in patients with ischemic heart disease (IHD). However, mobilization of progenitor cells by application of G-CSF gives inconsistent clinical effects. The aim of the present study was to assess pathophysiologic effects of progenitor cell mobilization.

METHODS AND RESULTS

PC levels (CD34+/CD117+) were counted in 8 patients with severe coronary heart disease and angina pectoris symptoms refractory to conventional therapy during G-CSF treatment (5 μg/kg/d) on days 2, 5, 8, at the end of hospitalization (day 10-12) and after 142±33 days of follow-up. Levels of asymmetric dimethylarginine (ADMA; inhibitor of eNOS) and symmetric dimethylarginine (SDMA) were determined at each occasion and correlated with leukocyte count, systemic nitrite levels, myeloperoxidase (MPO) expression in leukocytes, and urine levels of 8-iso-prostaglandin F2α. Isolated CD34+ cells and endothelial cell cultures were used for functional experiments. G-CSF therapy induced leukocytosis and a rise in CD34+ cell levels. Amounts of MPO positive leukocytes and ADMA levels increased significantly during the treatment phase. ADMA levels correlated to MPO activity (r=0.78; p=0.001) and were inversely related to nitrate levels. In contrast, 8-iso-prostaglandin F2α and amounts of SDMA did not change. Culturing endothelial cells in the presence of myeloperoxidase caused an increase in endothelial ADMA synthesis, which was prevented by application of the antioxidant trolox.

CONCLUSIONS

Leukocytosis associated with increased MPO activity during G-CSF therapy appears to be responsible for the systemic release of ADMA, which impairs eNOS activity. Thus, increased MPO and ADMA levels seem to counteract the potential beneficial effects of PC mobilization.

Plasma levels of asymmetric dimethylarginine in premature neonates: its possible involvement in developmental programming of chronic diseases

AIM

The endothel dysfunction in early life may play a role in developmental programming of cardiovascular morbidity. The changes of dimethylarginines' plasma levels during the first month among preterm infants and their determinants had been investigated in our study.

METHODS

Twenty preterm infants of healthy mothers were studied. Mean (+/-SD) birth weight and gestational age were 919.5 +/- 235.5 g and 26.7 +/- 1.6 weeks, respectively. Blood samples were taken by venipuncture at the 3rd, 7th, 14th, 21st and 28th days. Plasma concentrations of L-arginine, asymmetric and symmetric dimethylarginine (SDMA) were measured by liquid chromatography-mass spectrometry method, evaluated by multivariate linear regression analysis.

RESULTS

L-arginine (p < 0.001) and asymmetric dimethylarginine (ADMA) levels (p < 0.001) were positively associated with postnatal age. ADMA levels were negatively correlated with gestational age (p = 0.007), dopamine-need on the 3rd day of life (p = 0.015) and late infection (p = 0.038). The higher birth weight was associated with higher L-arginine (p = 0.052) and ADMA (p = 0.002) concentrations. The dopamine-need on the 7th day of life had a significant effect on postnatal elevation of SDMA levels (p = 0.035).

CONCLUSION

The progressive increase of ADMA levels described by our study among preterm infants suggests that early endothel dysfunction may take part in developmental programming of chronic adult diseases.

Identification and quantification of the atypical metabolite ornithine-lactam in human plasma by liquid chromatography-tandem mass spectrometry (LC-MS/MS)

In the late 1970s the atypical metabolite of ornithine, ornithine-lactam, has been observed in urine samples of patients suffering from hyperornithinemia. However, not least due to insufficient analytical methods, until now there are no data available about ornithine-lactam in human plasma. Here, we describe a new method, which is, for the first time, suitable to identify and quantify ornithine-lactam in human EDTA-plasma. The method was validated according to the requirements of the FDA guidance for bioanalytical method validation. The analytes were extracted on mixed mode cation exchange SPE columns, separated on a silica analytical HPLC column working in the HILIC mode and detected on a tandem mass spectrometer equipped with an ESI ion source. As internal standard newly synthesized stable isotope labeled D(6)-ornithine-lactam was used. The calibration function was linear in the range of 0.1-5 microM. Intra- and inter-day precision and accuracy was better than 14% at all concentration levels. In EDTA-plasma samples from 30 volunteers ornithine-lactam concentrations ranging from 0.136 to 0.653 microM were determined. These concentrations correlated significantly (p<0.001, R(2)=0.784) to those of ornithine in EDTA-plasma.

Determination of dimethylarginine levels in rats using HILIC-MS/MS: an in vivo microdialysis study

Nitric oxide (NO) is one of the most important mediators and neurotransmitters and its levels change under pathological conditions. NO production may be regulated by endogenous nitric oxide synthase (NOS) inhibitors, in particular asymmetric dimethylarginine (ADMA). Most of the interest is focused on ADMA, since this compound is present in plasma and urine and accumulation of ADMA has been described in many disease states but little is known about cerebrospinal fluid (CSF) concentrations of this compound and of its structural isomer symmetric dimethylarginine (SDMA). To determine the levels of methylarginines, we here present a new hydrophilic interaction chromatography (HILIC)-MS/MS method for the precise determination of these substances in CSF from microdialysis samples of rat prefrontal cortex (PFC). The method requires only minimal sample preparation and features isotope-labelled internal standards.

Evaluation of mobile phase, ion pairing, and temperature influence on an HILIC-MS/MS method for L-arginine and its dimethylated derivatives detection

Asymmetric N(G),-N(G)-dimethylarginine (ADMA) increases in diseases such as renal failure, diabetes mellitus, and hypercholesterolemia. The feasibility and utility of a hydrophilic interaction chromatography (HILIC) method for the separation of free L-arginine (Arg), ADMA, and symmetric N(G),-N(G')-dimethylarginine (SDMA) on a typical silica column were explored and the impact of some experimental parameters on the chromatographic behavior of these analytes was investigated. The effect of water and TFA content in mobile phase and of column temperature was investigated during the development of a fast and simple HILIC-MS/MS method that might be suitable for the quantification of free Arg, ADMA, and SDMA in plasma for routine analysis. Our results show that a good compromise between efficiency and peak shape with acceptable retention and total chromatographic run time is achieved using an ACN/water (90:10) mobile phase with TFA% as additive ranging from 0.015 to 0.025% and column temperature ranging from 25 to 30 degrees C.

Development and validation of a fast quantitative method for plasma dimethylarginines analysis using liquid chromatography-tandem mass spectrometry

OBJECTIVES

The aim of this work was to implement a fast, accurate and simple method to quantify plasma ADMA and SDMA, in a run time suitable for routine analysis.

DESIGN AND METHODS

We developed and validated a hydrophilic interaction chromatographic method coupled to tandem mass spectrometry (HILIC-MS/MS) for separation and simultaneous quantification of Arginine (Arg) and its dimethylarginines, ADMA and SDMA, with a short run time (less than 5 min) using a small volume of human plasma (0.02 mL).

RESULTS

Correlation coefficients (r) of the calibration curves ranged from 0.9926 to 0.9984. Within-day and between-day imprecision (CV%) and inaccuracy (%), carry-over and recovery were also evaluated for validation. Preliminary data of Arg, ADMA and SDMA from 30 apparently healthy subjects and type 2 diabetic patients (n=33) with and without kidney dysfunction were calculated and some statistical differences occurred among them (p<0.05).

CONCLUSIONS

Data from calibration curves and quality controls reveal that the method is accurate and precise. Healthy subjects and diabetic patients' values are in agreement with those reported in other studies.

Possible indirect detection of rHuEPO administration in human urine by high-performance liquid chromatography tandem mass spectrometry

The present study is based on the assumption that changes in an ADMA-DDAH-NOS (ADMA-asymmetrical dimethylarginine; DDAH-dimethyl-arginine dimethylaminohydrolase; NOS-nitric oxide synthase) system could be employed as indirect markers for recombinant human erythropoietin (rHuEPO) administration in doping control. We assessed a predictive value of four proposed new markers for rHuEPO abuse. Preliminary data showed that concentrations of ADMA, symmetrical dimethylarginine (SDMA), citrulline and arginine in human urine were increased after administration of a single intravenous erythropoietin injection (2000 U day(-1), Epocrine, St-Petersburg, Russia). The study of variations of ADMA, SDMA, arginine and citrulline levels before and after rHuEPO administration was performed with two healthy male volunteers. Urine samples were collected before rHuEPO administration and urinary concentrations of ADMA and SDMA were determined at 10.0-40 microg mL(-1) and of arginine and citrulline at 0.5-10 microg mL(-1). A single dose injection of rHuEPO caused an increase in ADMA, SDMA, arginine and citrulline concentrations up to 40-270 microg mL(-1), 40-240 microg mL(-1), 10-60 microg mL(-1) and 12-140 microg mL(-1), respectively. These preliminary results indicated that an indirect approach could be used as a pre-screening of urine samples in order to decrease the number of samples with a low probability of rHuEPO abuse and, thus, save costs and human workload.

Direct analysis of un-derivatized asymmetric dimethylarginine (ADMA) and L-arginine from plasma using mixed-mode ion-exchange liquid chromatography-tandem mass spectrometry

A high-throughput analytical method was developed for the measurement of asymmetric dimethylarginine (ADMA) and L-arginine (ARG) from plasma using LC/MS/MS. The sample preparation was simple and only required microfiltration prior to analysis. ADMA and ARG were assayed using mixed-mode ion-exchange chromatography which allowed for the retention of the un-derivatized compounds. The need for chromatographic separation of ADMA from symmetric dimethylarginine (SDMA) was avoided by using an ADMA specific product ion. As a result, the analytical method only required a total run time of 2 min. The method was validated by linearity, with r2>or=0.995 for both compounds, and accuracy, with no more than 7% deviation from the theoretical value. The estimated limit of detection and limit of quantification were suitable for clinical evaluations. The mean values of plasma ADMA and ARG taken from healthy volunteers (n=15) were 0.66+/-0.12 and 87+/-35 microM, respectively; the mean molar ratio of ARG to ADMA was 142+/-81.

Determination of ornithine in human plasma by hydrophilic interaction chromatography–tandem mass spectrometry

The amino acid ornithine (Orn) acts as a vital part in the physiologically fundamental urea cycle. As such, it is a main intermediate in the catabolic breakdown as well as in the synthesis of arginine and is involved in many other metabolic pathways with potential clinical implications. We here describe a LC-MS-MS method for the detection of Orn in human plasma which is fast, easy and precise. The sample preparation comprises only protein precipitation and the addition of the isotopic labeled I.S. The analytes are separated by hydrophilic interaction chromatography (HILIC) in less than 4min on a silica column with an isocratic mobile phase consisting of 0.1% trifluoroacetic acid in water and acetonitrile in the ratio of 25:75. Orn and its I.S. are detected and quantified by APCI tandem mass spectrometry. The calibration function is linear from 7.5 to 205 micromol/l and covers the range of concentrations found in patients undergoing different clinical interventions. The quantification results are independent with regard to the biological matrix analyzed. The intra-day and inter-day relative standard deviations are 1.1% and 3.5%, respectively. As an application of the described method in clinical investigations, we report arginine and ornithine plasma concentration results from an arginine supplementation study enrolling healthy volunteers and patients suffering from hypercholesterolemia. After oral dosing of 110 mg/kg arginine, ornithine plasma concentrations rose from 54 to 148 micromol/l after 2h and were back to baseline after 24h. However, arginine to ornithine ratios kept constant during the complete observation time.

High-throughput liquid chromatographic-tandem mass spectrometric determination of arginine and dimethylated arginine derivatives in human and mouse plasma

The balance between nitric oxide (NO) and vasoconstrictors like endothelin is essential for vascular tone and endothelial function. L-Arginine is converted to NO and L-citrulline by NO synthase (NOS). Asymmetric dimethylarginine (ADMA) and symmetric dimethylarginine (SDMA) are endogenous inhibitors of NO formation. ADMA is degraded by dimethylamino dimethylhydrolases (DDAHs), while SDMA is exclusively eliminated by the kidney. In the present article we report a LC-tandem MS method for the simultaneous determination of arginine, ADMA, and SDMA in plasma. This method is designed for high sample throughput of only 20-mul aliquots of human or mouse plasma. The analysis time is reduced to 1.6 min by LC-tandem MS electrospray ionisation (ESI) in the positive mode. The mean plasma levels of l-arginine, ADMA, and SDMA were 74+/-19 (SD), 0.46+/-0.09, and 0.37+/-0.07 microM in healthy humans (n=85), respectively, and 44+/-14, 0.72+/-0.23, and 0.19+/-0.06 microM in C57BL/6 mice. Also, the molar ratios of arginine to ADMA were different in man and mice, i.e. 166+/-50 and 85+/-22, respectively.

Determination of cerebrospinal fluid concentrations of arginine and dimethylarginines in patients with subarachnoid haemorrhage

Elevated cerebrospinal fluid (CSF) concentrations of asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthase (NOS), are assumed to be related to delayed vasospasm after subarachnoid haemorrhage (SAH). However, data on CSF concentrations of L-arginine, ADMA and its structural isomer symmetric dimethylarginine (SDMA) are very sparse in humans. We here present a new hydrophilic interaction chromatography-tandem mass spectrometry (HILIC-MS-MS) method for the precise determination of these substances in CSF. The method requires only minimal sample preparation and features isotope labeled internal standards. First data of patients with SAH showed that on the day of admission CSF concentration values of L-arginine and ADMA were not significantly different from controls, but increased markedly during the course of the hospital stay. The decrease of the L-arginine to ADMA ratio points to a progressive impairment of the NO production rate in the brain after SAH which is confirmed by a simultaneous decrease in nitrate and nitrite concentrations in CSF.

Elevated plasma asymmetric dimethyl-L-arginine levels are linked to endothelial progenitor cell depletion and carotid atherosclerosis in rheumatoid arthritis

OBJECTIVE

Similarities between rheumatoid arthritis (RA) and atherosclerosis include endothelial dysfunction (an antecedent of plaque formation) and depletion of circulating bone marrow-derived endothelial progenitor cells. This study was undertaken to test the hypothesis that endothelial progenitor cell depletion and subclinical atherosclerosis in RA may be related to accumulation of an endogenous inhibitor of nitric oxide (NO) synthesis, asymmetric dimethyl-L-arginine.

METHODS

We studied 30 patients with active RA and 20 age- and sex-matched healthy controls. Exclusion criteria were clinically evident atherosclerosis, traditional risk factors, hyperhomocysteinemia, and renal dysfunction. The blood endothelial progenitor cell count was assayed by flow cytometry and expressed as a percentage of lymphocytes. Plasma L-arginine, asymmetric dimethyl-L-arginine, and symmetric dimethyl-L-arginine were measured with liquid chromatography-mass spectrometry. Mean carotid intima-media thickness (IMT) was assessed by B-mode ultrasound.

RESULTS

In RA patients, we found elevated levels of asymmetric dimethyl-L-arginine (mean +/- SD 0.49 +/- 0.07 micromoles/liter versus 0.40 +/- 0.07 micromoles/liter in controls; P < 0.001), a depressed endothelial progenitor cell count (0.039 +/- 0.025% versus 0.063 +/- 0.035%; P < 0.05), and increased IMT (0.65 +/- 0.13 mm versus 0.55 +/- 0.10 mm; P < 0.01), with no differences in levels of L-arginine or symmetric dimethyl-L-arginine. The endothelial progenitor cell count was inversely correlated with the level of asymmetric dimethyl-L-arginine. IMT was positively related to the ratio of asymmetric dimethyl-L-arginine to L-arginine and negatively related to the endothelial progenitor cell count, in univariate and multivariate analyses.

CONCLUSION

Plasma asymmetric dimethyl-L-arginine levels are elevated in RA patients free of cardiovascular disease or risk factors. Asymmetric dimethyl-L-arginine accumulation may contribute to endothelial progenitor cell depletion via depressed NO-dependent endothelial progenitor cell mobilization and/or survival, with consequent impairment of endothelial progenitor cell-mediated endothelial repair, which can promote atherogenesis in RA.

Pioglitazone improves endothelial function in non-diabetic patients with coronary artery disease

OBJECTIVE

To test the hypothesis that pioglitazone, a peroxisome proliferator-activated receptor-gamma agonist, will improve endothelial function in non-diabetic subjects with coronary artery disease, we conducted a prospective study to evaluate the effect of this medication on the brachial artery vasomotor function and circulating markers of endothelial activation.

METHODS

Baseline characteristics were collected. After initial endothelial function assessment, patients were treated with pioglitazone hydrochloride 30 mg daily. The medication was continued for 12 weeks and endothelial function was reassessed as well as the inflammatory markers. The study medication then was stopped, and all the tests were repeated 12 weeks later.

RESULTS

Seventeen subjects completed all three-study phases. Mean age was 58 (range: 36-77 years). Compared with the baseline, the endothelium-dependent vasodilation improved significantly with the treatment (p < 0.001) from 4.4 +/- 3.9 to 8.4 +/- 4.1%, a relative increase of 91%. After withdrawal of treatment, the endothelium-dependent vasodilation returned towards baseline values. There was no change in endothelium-independent vasodilatation (12.27 +/- 6.35 to 13.9 +/- 9.23%, to 12.42 +/- 5.35%, p = 0.177). The urine asymmetric dimethlyarginine levels decreased significantly with the treatment, but also returned to the initial values after the wash-out period (1.27 +/- 0.5 micromol/ml to 0.97 +/- 0.3 micromol/ml to 1.34 +/- 0.5 micromol/ml, p = 0.017). No difference in the lipid profile, C-reactive protein, erythrocyte sedimentation rate, or fibrinogen levels was seen.

CONCLUSION

Pioglitazone rapidly improves endothelial function in non-diabetic patients with coronary artery disease. This improvement is associated with a change in mean urinary asymmetric dimethylarginine levels, although a cause and effect cannot be determined from this investigation.

An overview of plasma concentrations of asymmetric dimethylarginine (ADMA) in health and disease and in clinical studies: methodological considerations

Recent studies among patients including those with known coronary disease demonstrate that small elevations in asymmetric dimethylarginine (ADMA) concentrations in plasma are predictive of adverse outcomes. The precision of current methodologies for quantitation of ADMA such as HPLC, MS and ELISA is discussed with respect to many reports which appear to over-estimate ADMA levels and quote broad concentration ranges. While plasma ADMA concentrations tend to increase with age, the mean for a healthy population is between 0.4 and 0.6 microM. ADMA levels may fluctuate in normal subjects, and this needs to be considered in light of the relatively small differences in ADMA concentration between healthy normal subjects and patients.

Chromatographic-mass spectrometric methods for the quantification of L-arginine and its methylated metabolites in biological fluids

L-Arginine (Arg) and its methylated metabolites play a major role in the synthesis of the cell signaling molecule nitric oxide (NO). Arg serves as a substrate for the enzyme NO synthase (NOS), which produces NO, whereas monomethylarginine (L-NMMA) and asymmetric dimethylarginine (ADMA) act as competitive inhibitors of NOS. Symmetric dimethylarginine (SDMA) has virtually no inhibitory effect on NOS activity, but shares the pathway for cell entry and transport with Arg and ADMA. Accurate and reliable quantification of these substances in various biological fluids is essential for scientific research in this field. In this review, chromatographic-mass spectrometric methods for Arg and its methylated metabolites ADMA and SDMA are discussed. Mass spectrometric detection provides an intrinsic higher selectivity than detection by means of UV absorbance or fluorescence. Taking advantage of the high selectivity, approaches involving mass spectrometric detection require less laborious sample preparation and produce reliable results. A consensus emerges that the concentration values in plasma of young healthy volunteers are about 65 microM for Arg, 0.4 microM for ADMA and 0.5 microM for SDMA.

No benefit of hemodiafiltration over hemodialysis in lowering elevated levels of asymmetric dimethylarginine in ESRD patients

BACKGROUND

It has been suggested that hemodiafiltration (HDF) is more efficient than hemodialysis (HD) in lowering plasma levels of the endogenous nitric oxide synthase inhibitor asymmetric dimethylarginine (ADMA), which is a strong and independent predictor of overall mortality in ESRD patients.

METHODS/PATIENTS

Twenty ESRD patients (11 women) were studied during both a single online HDF session and a single HD session. In each patient, ADMA, L-arginine, SDMA, beta(2)-microglobulin and urea were measured at several time points.

RESULTS

Although HDF was clearly superior to HD in decreasing plasma beta(2)-microglobulin, there was no difference in the elimination characteristics of ADMA. However, HDF but not HD eliminated the nitric oxide synthase substrate L-arginine, making HD superior in increasing the L-arginine/ADMA ratio.

CONCLUSION

Neither HD nor HDF sufficiently removes the putative uremic toxin ADMA. The clinical significance of HD better improving the L-arginine/ADMA ratio (parameter of NO production) as compared to HDF needs to be determined.