Quantification of ADMA: analytical approaches

Mass Spectrometry-Based Evaluation of the Bland-Altman Approach: Review, Discussion, and Proposal

Reliable quantification in biological systems of endogenous low- and high-molecular substances, drugs and their metabolites, is of particular importance in diagnosis and therapy, and in basic and clinical research. The analytical characteristics of analytical approaches have many differences, including in core features such as accuracy, precision, specificity, and limits of detection (LOD) and quantitation (LOQ). Several different mathematic approaches were developed and used for the comparison of two analytical methods applied to the same chemical compound in the same biological sample. Generally, comparisons of results obtained by two analytical methods yields different quantitative results. Yet, which mathematical approach gives the most reliable results? Which mathematical approach is best suited to demonstrate agreement between the methods, or the superiority of an analytical method A over analytical method B? The simplest and most frequently used method of comparison is the linear regression analysis of data observed by method A (y) and the data observed by method B (x): y = α + βx. In 1986, Bland and Altman indicated that linear regression analysis, notably the use of the correlation coefficient, is inappropriate for method-comparison. Instead, Bland and Altman have suggested an alternative approach, which is generally known as the Bland-Altman approach. Originally, this method of comparison was applied in medicine, for instance, to measure blood pressure by two devices. The Bland-Altman approach was rapidly adapted in analytical chemistry and in clinical chemistry. To date, the approach suggested by Bland-Altman approach is one of the most widely used mathematical approaches for method-comparison. With about 37,000 citations, the original paper published in the journal The Lancet in 1986 is among the most frequently cited scientific papers in this area to date. Nevertheless, the Bland-Altman approach has not been really set on a quantitative basis. No criteria have been proposed thus far, in which the Bland-Altman approach can form the basis on which analytical agreement or the better analytical method can be demonstrated. In this article, the Bland-Altman approach is re-valuated from a quantitative bioanalytical perspective, and an attempt is made to propose acceptance criteria. For this purpose, different analytical methods were compared with Gold Standard analytical methods based on mass spectrometry (MS) and tandem mass spectrometry (MS/MS), i.e., GC-MS, GC-MS/MS, LC-MS and LC-MS/MS. Other chromatographic and non-chromatographic methods were also considered. The results for several different endogenous substances, including nitrate, anandamide, homoarginine, creatinine and malondialdehyde in human plasma, serum and urine are discussed. In addition to the Bland-Altman approach, linear regression analysis and the Oldham-Eksborg method-comparison approaches were used and compared. Special emphasis was given to the relation of difference and mean in the Bland-Altman approach. Currently available guidelines for method validation were also considered. Acceptance criteria for method agreement were proposed, including the slope and correlation coefficient in linear regression, and the coefficient of variation for the percentage difference in the Bland-Altman and Oldham-Eksborg approaches.

An updated systematic review and meta-analysis of the effect of statins on asymmetric dimethylarginine

OBJECTIVES

We conducted an updated systematic review and meta-analysis of the effect of statins on serum or plasma concentrations of the endogenous inhibitor of endothelial nitric oxide synthase, asymmetric N^G,N^G-dimethyl-l-arginine (ADMA).

METHODS

A systematic literature search was conducted in the electronic databases PubMed, Web of Science, and Scopus, from inception to July 2021. Risk of bias and certainty of evidence were assessed using the Joanna Briggs Institute Critical Appraisal Checklist for analytical studies and GRADE, respectively.

RESULTS

In 23 studies, reporting 25 treatment arms in 845 participants (mean age 53 years, 57% males, treatment duration 4-48 weeks), statins significantly reduced ADMA concentrations (SMD = -0.39, 95% CI -0.62 to -0.16, p = 0.001; moderate certainty of evidence). The extreme heterogeneity observed was substantially reduced in study subgroups of specific class and individual statins, regional areas, and analytical methods for ADMA concentrations. There was no publication bias. In sensitivity analysis, the corresponding SMD values were not substantially modified when individual studies were sequentially removed. Significant associations were observed, in meta-regression, between the SMD and publication year (t = -3.25, p = 0.003), but not baseline cholesterol concentrations.

CONCLUSION

Statin treatment significantly lowers ADMA concentrations. This effect is independent of baseline cholesterol. Prospective studies are required to determine whether ADMA-lowering mediates, at least partly, the protective effects of statins against atherosclerotic cardiovascular disease. (PROSPERO registration number: CRD42021275123).

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.

Circulating biomarkers of nitric oxide bioactivity and impaired muscle vasoreactivity to exercise in adults with uncomplicated type 1 diabetes

AIMS/HYPOTHESIS

Early compromised endothelial function challenges the ability of individuals with type 1 diabetes to perform normal physical exercise. The exact mechanisms underlying this vascular limitation remain unknown, but may involve either formation or metabolism of nitric oxide (NO), a major vasodilator, whose activity is known to be compromised by oxidative stress.

METHODS

Muscle microvascular reactivity (near-infrared spectroscopy) to an incremental exhaustive bout of exercise was assessed in 22 adults with uncomplicated type 1 diabetes (HbA_1c 64.5 ± 15.7 mmol/mol; 8.0 ± 1.4%) and in 21 healthy individuals (18-40 years of age). NO-related substrates/metabolites were also measured in the blood along with other vasoactive compounds and oxidative stress markers; measurements were taken at rest, at peak exercise and after 15 min of recovery. Demographic characteristics, body composition, smoking status and diet were comparable in both groups.

RESULTS

Maximal oxygen uptake was impaired in individuals with type 1 diabetes compared with in healthy participants (35.6 ± 7.7 vs 39.6 ± 6.8 ml min^-1 kg^-1, p < 0.01) despite comparable levels of habitual physical activity (moderate to vigorous physical activity by accelerometery, 234.9 ± 160.0 vs 280.1 ± 114.9 min/week). Compared with non-diabetic participants, individuals with type 1 diabetes also displayed a blunted exercise-induced vasoreactivity (muscle blood volume at peak exercise as reflected by ∆ total haemoglobin, 2.03 ± 5.82 vs 5.33 ± 5.54 μmol/l; interaction 'exercise' × 'group', p < 0.05); this was accompanied by lower K⁺ concentration (p < 0.05), reduced plasma L-arginine (p < 0.05)-in particular when HbA_1c was high (mean estimation: -4.0, p < 0.05)-and lower plasma urate levels (p < 0.01). Nonetheless, exhaustive exercise did not worsen lipid peroxidation or other oxidative stress biomarkers, and erythrocytic enzymatic antioxidant resources were mobilised to a comparable extent in both groups. Nitrite and total nitrosation products, which are potential alternative NO sources, were similarly unaltered. Graphical abstract CONCLUSIONS/INTERPRETATION: Participants with uncomplicated type 1 diabetes displayed reduced availability of L-arginine, the essential substrate for enzymatic nitric oxide synthesis, as well as lower levels of the major plasma antioxidant, urate. Lower urate levels may reflect a defect in the activity of xanthine oxidase, an enzyme capable of producing NO from nitrite under hypoxic conditions. Thus, both canonical and non-canonical NO production may be reduced. However, neither of these changes exacerbated exercise-induced oxidative stress.

TRIAL REGISTRATION

clinicaltrials.gov NCT02051504.

Pre-Analytical and Clinical Validation of a Dried Blood Spot Assay for Asymmetric Dimethylarginine and L-Arginine

Asymmetric dimethylarginine (ADMA) inhibits nitric oxide (NO) synthesis. It is a risk marker for cardiovascular events and mortality in patients with cardiometabolic diseases and in population-based studies. Plasma or serum analysis of ADMA may be hampered by pre-analytical sample handling. We validated a dried blood spot (DBS) assay for ADMA and L-arginine and show here that this assay has excellent variabilities and reproducibilities. Filter paper is impregnated with the arginase inhibitor nor-NOHA (N^ω-hydroxy-nor-Arginine) to avoid L-arginine degradation. Clinical validation of this DBS assay confirms elevated ADMA concentration in hemodialysis patients as compared to healthy controls, higher ADMA concentrations in men versus women, and elevated L-arginine concentration in subjects supplemented with L-arginine. The DBS assay was used in a cohort study involving 100 primarily healthy subjects in the Andean region to assess the impact of chronic intermittent hypoxia on ADMA and L-arginine; ADMA DBS concentration at sea level was prospectively associated with pulmonary hypertension after six months of exposure to 3500 m. In a cohort of 753 individuals, L-arginine/ADMA ratio significantly decreased with increasing number of traditional cardiovascular risk factors. Analysis of ADMA and L-arginine in DBS is a reliable and reproducible method for quantitation of these markers in field studies.

Intraplatelet L-Arginine-Nitric Oxide Metabolic Pathway: From Discovery to Clinical Implications in Prevention and Treatment of Cardiovascular Disorders

Despite the development of new drugs and other therapeutic strategies, cardiovascular disease (CVD) remains still the major cause of morbidity and mortality in the world population. A lot of research, performed mostly in the last three decades, revealed an important correlation between "classical" demographic and biochemical risk factors for CVD, (i.e., hypercholesterolemia, hyperhomocysteinemia, smoking, renal failure, aging, diabetes, and hypertension) with endothelial dysfunction associated directly with the nitric oxide deficiency. The discovery of nitric oxide and its recognition as an endothelial-derived relaxing factor was a breakthrough in understanding the pathophysiology and development of cardiovascular system disorders. The nitric oxide synthesis pathway and its regulation and association with cardiovascular risk factors were a common subject for research during the last decades. As nitric oxide synthase, especially its endothelial isoform, which plays a crucial role in the regulation of NO bioavailability, inhibiting its function results in the increase in the cardiovascular risk pattern. Among agents altering the production of nitric oxide, asymmetric dimethylarginine-the competitive inhibitor of NOS-appears to be the most important. In this review paper, we summarize the role of L-arginine-nitric oxide pathway in cardiovascular disorders with the focus on intraplatelet metabolism.

Is Asymmetric Dimethylarginine a Useful Biomarker in Children With Carbon Monoxide Poisoning?

OBJECTIVE

Carbon monoxide poisoning (COP) is the leading cause of mortality and morbidity due to poisoning worldwide. Because children are affected more quick and severely from COP, they may require a longer treatment period, even if carboxyhemoglobin (CO-Hb) and/or lactate levels return to normal. Therefore, a new marker that predicts the duration of treatment and the final outcomes of COP is needed.

METHODS

This case control study was conducted on 32 carbon monoxide-poisoned patients younger than 18 years who had been admitted to pediatric emergency department. The control group included age- and sex-matched 30 healthy children. Blood samples were obtained for analysis of arterial blood gases, CO-Hb percent, methemoglobine, lactate, and asymmetric dimethylarginine (ADMA).

RESULTS

Asymmetric dimethylarginine levels were significantly increased (P < 0.05) in patients with COP on admission and after the treatment when compared with controls (1.36 [0.89-6.94], 1.69 [0.76-7.81], 1.21 [0.73-3.18] nmol/L, respectively). There was no positive correlation between CO-Hb and ADMA levels on admission and at 6 hours (P = 0.903, r = 0.218, P = 0.231, r = 0.022, respectively). Positive correlation was found between lactate and CO-Hb levels on admission (P = 0.018, r = 0.423).

CONCLUSIONS

This study showed that ADMA levels were still high after 6 hours of 100% oxygen therapy in children with COP, even CO-Hb and/or lactate levels return to normal range. On the basis of these results, we consider that ADMA may be a useful biomarker in patient with COP.

Long-Term Intermittent Work at High Altitude: Right Heart Functional and Morphological Status and Associated Cardiometabolic Factors

Background: Living at high altitude or with chronic hypoxia implies functional and morphological changes in the right ventricle and pulmonary vasculature with a 10% prevalence of high-altitude pulmonary hypertension (HAPH). The implications of working intermittently (day shifts) at high altitude (hypobaric hypoxia) over the long term are still not well-defined. The aim of this study was to evaluate the right cardiac circuit status along with potentially contributory metabolic variables and distinctive responses after long exposure to the latter condition. Methods: A cross-sectional study of 120 healthy miners working at an altitude of 4,400-4,800 m for over 5 years in 7-day commuting shifts was designed. Echocardiography was performed on day 2 at sea level. Additionally, biomedical and biochemical variables, Lake Louise scores (LLSs), sleep disturbances and physiological variables were measured at altitude and at sea level. Results: The population was 41.8 ± 0.7 years old, with an average of 14 ± 0.5 (range 5-29) years spent at altitude. Most subjects still suffered from mild to moderate symptoms of acute mountain sickness (mild was an LLS of 3-5 points, including cephalea; moderate was LLS of 6-10 points) (38.3%) at the end of day 1 of the shift. Echocardiography showed a 23% mean pulmonary artery pressure (mPAP) >25 mmHg, 9% HAPH (≥30 mmHg), 85% mild increase in right ventricle wall thickness (≥5 mm), 64% mild right ventricle dilation, low pulmonary vascular resistance (PVR) and fairly good ventricle performance. Asymmetric dimethylarginine (ADMA) (OR 8.84 (1.18-66.39); p < 0.05) and insulin (OR: 1.11 (1.02-1.20); p < 0.05) were associated with elevated mPAP and were defined as a cut-off. Interestingly, the correspondence analysis identified association patterns of several other variables (metabolic, labor, and biomedical) with higher mPAP. Conclusions: Working intermittently at high altitude involves a distinctive pattern. The most relevant and novel characteristics are a greater prevalence of elevated mPAP and HAPH than previously reported at chronic intermittent hypobaric hypoxia (CIHH), which is accompanied by subsequent morphological characteristics. These findings are associated with cardiometabolic factors (insulin and ADMA). However, the functional repercussions seem to be minor or negligible. This research contributes to our understanding and surveillance of this unique model of chronic intermittent high-altitude exposure.

Asymmetric and Symmetric Dimethylarginine as Risk Markers for Total Mortality and Cardiovascular Outcomes: A Systematic Review and Meta-Analysis of Prospective Studies

Background

A growing number of studies linked elevated concentrations of circulating asymmetric (ADMA) and symmetric (SDMA) dimethylarginine to mortality and cardiovascular disease (CVD) events. To summarize the evidence, we conducted a systematic review and quantified associations of ADMA and SDMA with the risks of all-cause mortality and incident CVD in meta-analyses accounting for different populations and methodological approaches of the studies.

Methods

Relevant studies were identified in PubMed until February 2015. We used random effect models to obtain summary relative risks (RR) and 95% confidence intervals (95%CIs), comparing top versus bottom tertiles. Dose-response relations were assessed by restricted cubic spline regression models and potential non-linearity was evaluated using a likelihood ratio test. Heterogeneity between subgroups was assessed by meta-regression analysis.

Results

For ADMA, 34 studies (total n = 32,428) investigating associations with all-cause mortality (events = 5,035) and 30 studies (total n = 30,624) investigating the association with incident CVD (events = 3,396) were included. The summary RRs (95%CI) for all-cause mortality were 1.52 (1.37–1.68) and for CVD 1.33 (1.22–1.45), comparing high versus low ADMA concentrations. Slight differences were observed across study populations and methodological approaches, with the strongest association of ADMA being reported with all-cause mortality in critically ill patients. For SDMA, 17 studies (total n = 18,163) were included for all-cause mortality (events = 2,903), and 13 studies (total n = 16,807) for CVD (events = 1,534). High vs. low levels of SDMA, were associated with increased risk of all-cause mortality [summary RR (95%CI): 1.31 (1.18–1.46)] and CVD [summary RR (95%CI): 1.36 (1.10–1.68) Strongest associations were observed in general population samples.

Conclusions

The dimethylarginines ADMA and SDMA are independent risk markers for all-cause mortality and CVD across different populations and methodological approaches.

The level of circulating PGC1a in cardiovascular disease

The level of peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC1a) in human blood plasma was investigated. Samples of healthy individuals (n=34) and patients with cardiovascular diseases (n=110), including aortic aneurysm (n=69), aortic stenosis (n=25) and patients without aortic pathologies were analyzed. In patients the PGC1a concentration was higher than that in healthy persons, and tended to decrease with age. Elevated concentrations of lactic acid, total homocysteine and asymmetric dimethylarginine in the blood of patients suggested a parallel development of endothelial and secondary mitochondrial dysfunction. However, concentrations of lactic and pyruvic acids exceeding reference limit were associated with the decrease in the PGC1a level

Measurement of arginine metabolites: regulators of nitric oxide metabolism

Arginine is the substrate for nitric oxide synthases (NOS), and arginine availability regulates the production of nitric oxide. Through the activity of methyltransferases, arginine can be methylated to form monomethylarginine (NMMA), asymmetrical dimethylarginine (ADMA), and symmetrical dimethylarginine (SDMA). NMMA and ADMA directly inhibit NOS, whereas SDMA inhibits the cellular import of arginine through the cationic amino acid transporter. Increased levels of methylarginine compounds have been associated with many diseases including atherosclerosis, renal failure, pulmonary hypertension, and preeclampsia. Previous HPLC methods to measure these molecules rely on derivatization with ortho-phthalaldehyde, which is unstable and requires immediate pre- or post-column reactions. We have identified a new fluorometric agent that is stable for at least 1 week and provides chromatographic properties that facilitate separation of these chemically similar compounds by reverse phase chromatography.

Oxidative stress in diabetes: implications for vascular and other complications

In recent decades, oxidative stress has become a focus of interest in most biomedical disciplines and many types of clinical research. Increasing evidence shows that oxidative stress is associated with the pathogenesis of diabetes, obesity, cancer, ageing, inflammation, neurodegenerative disorders, hypertension, apoptosis, cardiovascular diseases, and heart failure. Based on these studies, an emerging concept is that oxidative stress is the “final common pathway” through which the risk factors for several diseases exert their deleterious effects. Oxidative stress causes a complex dysregulation of cell metabolism and cell–cell homeostasis; in particular, oxidative stress plays a key role in the pathogenesis of insulin resistance and β-cell dysfunction. These are the two most relevant mechanisms in the pathophysiology of type 2 diabetes and its vascular complications, the leading cause of death in diabetic patients.

Heat-assisted extraction for the determination of methylarginines in serum by CE

Methylarginines (MAs) are potent vasoconstrictors that have been reported to be present at elevated concentrations in the blood of patients suffering from cardiovascular disease. To determine the diagnostic potential of MAs for cardiovascular disease, a method capable of rapidly quantifying their endogenous concentrations from serum samples is required. To that end, a heat-assisted extraction method was developed. Serum was first rapidly heated, causing it to congeal into a gel, and then subjected to solid-liquid extraction. The extraction solution was then derivatized with a fluorogenic dye and analyzed by CE-LIF to permit quantitation of the MAs. This heat-assisted extraction procedure allowed a no-net-dilution extraction of the analytes to be performed into a solvent compatible with the subsequent CE analysis. This enabled direct detection of low-abundance analytes, such as MAs, without the need for a preconcentration step. This sample preparation method was compared with a commonly used solid-phase extraction method for MA analysis. Endogenous MA concentrations determined by both the heating and solid-phase extraction methods were found to be in good agreement with each other and with values previously reported in the literature.

Cardiovascular risk in autoimmune disorders: role of asymmetric dimethylarginine

Mounting evidence indicates that cardiovascular events are a main cause of excessive mortality of autoimmune disorders like type I diabetes mellitus and rheumatic diseases. Inflammation and endothelial dysfunction, independent predictors to cardiovascular disease, are hallmarks of autoimmunity. Asymmetric dimethylarginine (ADMA), an endogenous nitric oxide synthase inhibitor, can cause or contribute to the inflammatory syndrome and endothelial dysfunction. Recently, elevated ADMA levels have been demonstrated in many autoimmune diseases, suggesting that ADMA might play an important role for the associated manifestations of cardiovascular disease. In the review, we discuss the role of ADMA in the excessive cardiovascular morbidity and mortality associated with autoimmune diseases.

Plasma levels of asymmetric dimethylarginine in patients with biopsy-proven nonalcoholic fatty liver disease

Asymmetric (ADMA) and symmetric dimethylarginine (SDMA) are produced by breakdown of proteins that have been methylated posttranslationally at an arginine residue. Plasma levels of ADMA are elevated in insulin resistance states. Nonalcoholic fatty liver disease (NAFLD) is associated with insulin resistance and varying degrees of hepatic dysfunction. Because ADMA is metabolized in the liver, we hypothesized that ADMA levels will be high in patients with NAFLD as a consequence of hepatic dysfunction and insulin resistance. Plasma levels of ADMA, SDMA, total homocysteine, glucose, and insulin were measured in nondiabetic patients with biopsy-proven NAFLD (11 steatosis and 24 nonalcoholic steatohepatitis) and 25 healthy subjects. Plasma ADMA levels were significantly higher (P = .029) in patients with biopsy-proven NAFLD (0.43 ± 0.21 μmol/L) compared with controls (0.34 ± 0.10 μmol/L). However, when adjusted for insulin resistance (homeostasis model assessment), the difference between 2 groups was not evident. Plasma SDMA levels were similar in all 3 groups. Plasma levels of ADMA were positively correlated with plasma total homocysteine levels (P = .003). Plasma levels of SDMA were negatively correlated with estimated glomerular filtration rate (P = .016) and positively correlated with plasma total homocysteine levels (P = .003). The ratio of ADMA/SDMA was positively correlated with body mass index (P = .027). Elevated plasma concentrations of ADMA in biopsy-proven NAFLD were primarily related to insulin resistance. Hepatic dysfunction in NAFLD does not appear to make significant contribution to changes in plasma methylarginine levels.

The role of asymmetric and symmetric dimethylarginines in renal disease

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide synthases. By inhibiting nitric oxide formation, ADMA causes endothelial dysfunction, vasoconstriction, elevation of blood pressure, and aggravation of experimental atherosclerosis. Levels of ADMA and its isomer symmetric dimethylarginine (SDMA), which does not inhibit nitric oxide synthesis, are both elevated in patients with kidney disease. Currently available data from prospective clinical trials in patients with chronic kidney disease suggest that ADMA is an independent marker of progression of renal dysfunction, vascular complications and death. High SDMA levels also negatively affect survival in populations at increased cardiovascular risk, but the mechanisms underlying this effect are currently only partly understood. Beyond glomerular filtration, other factors influence the plasma concentrations of ADMA and SDMA. Elevated plasma concentrations of these dimethylarginines might also indirectly influence the activity of nitric oxide synthases by inhibiting the uptake of cellular L-arginine. Other mechanisms may exist by which SDMA exerts its biological activity. The biochemical pathways that regulate ADMA and SDMA, and the pathways that transduce their biological function, could be targeted to treat renal disease in the future.

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.

Oxidative stress, nitric oxide, and diabetes

In the recent decades, oxidative stress has become focus of interest in most biomedical disciplines and many types of clinical research. Increasing evidence from research on several diseases show that oxidative stress is associated with the pathogenesis of diabetes, obesity, cancer, ageing, inflammation, neurodegenerative disorders, hypertension, apoptosis, cardiovascular diseases, and heart failure. Based on this research, the emerging concept is that oxidative stress is the "final common pathway", through which risk factors of several diseases exert their deleterious effects. Oxidative stress causes a complex dysregulation of cell metabolism and cell-cell homeostasis. In this review, we discuss the role of oxidative stress in the pathogenesis of insulin resistance and beta-cell dysfunction. These are the two most relevant mechanisms in the pathophysiology of type 2 diabetes, and in the pathogenesis of diabetic vascular complications, the leading cause of death in diabetic patients.

The emerging role of symmetric dimethylarginine in vascular disease

Asymmetric dimethylarginine (ADMA), an endogenous methylated form of the amino acid L-arginine, inhibits the activity of the enzyme endothelial nitric oxide synthase (eNOS), with consequent reduced synthesis of nitric oxide (NO). An increased synthesis and/or a reduced catabolism of ADMA might contribute to the onset and progression of atherosclerosis and thrombosis. The detrimental effects of ADMA on endothelial function, cardiovascular homeostasis, and cardiovascular outcomes have been extensively investigated. However, little attention has been paid to another methylated form of L-arginine, symmetric dimethylarginine (SDMA), as a potential modulator of vascular homeostasis and vascular disease. The first part of this chapter discusses the synthesis, transport, and metabolism of ADMA and SDMA and summarizes the evidence linking ADMA with vascular disease and adverse cardiovascular outcomes. The second part describes the results of recent studies highlighting the important role of SDMA in modulating vascular homeostasis and vascular damage. Suggestions for future research directions on SDMA are also discussed.

Asymmetric dimethylarginine (ADMA) as a prospective marker of cardiovascular disease and mortality--an update on patient populations with a wide range of cardiovascular risk

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide (NO) synthases. By inhibiting NO formation, ADMA causes endothelial dysfunction, vasoconstriction, elevation of blood pressure, and aggravation of experimental atherosclerosis. Cross-sectional studies in humans have revealed that ADMA plasma concentration is elevated in numerous populations with vascular diseases or at high cardiovascular risk. However, the potential causal relationship between elevated ADMA and cardiovascular events and mortality in humans can only be revealed in prospective clinical studies. This review gives an overview of currently available data from prospective clinical studies in which ADMA has been measured in populations at high, intermediate, or low global vascular risk. Although the analytical methods used to quantify ADMA varied and statistical approaches to assess the association of ADMA with risk differed, these data reveal that ADMA is significantly associated with an increased risk of incident cardiovascular events and total mortality in subjects at a broad range of global risk. Hazard ratios were mostly in a range comparable to that of traditional cardiovascular risk markers even after multivariable adjustments, suggesting that ADMA may be suitable as a diagnostic marker for risk assessment, and that the biochemical pathways that regulate ADMA may be promising therapeutic approaches to treat cardiovascular disease in the future.

Plasma asymmetric dimethylarginine and incidence of cardiovascular disease and death in the community

BACKGROUND

Asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthase, induces endothelial dysfunction. Although elevated ADMA has been associated with an increased risk of cardiovascular disease events and death in referral samples, the prognostic significance of ADMA in the community has not been adequately evaluated.

METHODS AND RESULTS

We related plasma ADMA, l-arginine (Arg), and the ratio of Arg to ADMA to the incidence of cardiovascular disease (fatal or nonfatal myocardial infarction, coronary insufficiency, angina pectoris, stroke or transient ischemic attack, intermittent claudication, or heart failure) and death in 3320 Framingham Offspring Study participants (1769 women; mean age, 59 years). Over a follow-up period of 10.9 years, 281 individuals of 2956 free of cardiovascular disease at baseline developed incident cardiovascular disease, and 285 participants died. In multivariable models adjusting for established risk factors and other biomarkers (B-type natriuretic peptide, renin, homocysteine, urine albumin excretion, and C-reactive protein), ADMA and the ratio of Arg to ADMA were significantly associated with all-cause mortality (adjusted-hazard ratio [HR] per 1-SD increment, 1.21; 95% CI, 1.07 to 1.37; and HR per 1-SD increment, 0.80; 95% CI, 0.69 to 0.93, respectively), whereas Arg was not (HR per 1-SD increment, 0.89; 95% CI, 0.77 to 1.02). We noted effect modification by diabetes status; ADMA was associated with death in individuals without diabetes (adjusted HR per 1-SD increment, 1.30; 95% CI, 1.13 to 1.50) but not in individuals with diabetes (adjusted HR per 1-SD increment, 0.85; 95% CI, 0.62 to 1.16). ADMA, Arg, and the ratio of Arg to ADMA were not associated with cardiovascular disease incidence (P>0.10).

CONCLUSIONS

In our large community-based sample, ADMA was significantly associated with all-cause mortality, particularly in nondiabetic individuals.

De novo synthesis of trideuteromethyl esters of amino acids for use in GC-MS and GC-tandem MS exemplified for ADMA in human plasma and urine: standardization, validation, comparison and proof of evidence for their aptitude as internal standards

Asymmetric dimethylarginine (ADMA, N(G),N(G)-dimethyl-L-arginine) is an endogenous inhibitor of nitric oxide (NO) synthesis, a potential risk factor for cardiovascular diseases and a powerful biochemical parameter in clinical studies. In our previous work we have reported on a GC-tandem MS method for the accurate and precise quantification of ADMA in biological fluids using de novo synthesized [(2)H(3)]-methyl ester ADMA (d(3)Me-ADMA) as internal standard (IS). This method provides basal ADMA concentrations in biological fluids that agree with those obtained by other groups using other validated methods for ADMA. Unanimously, de novo synthesized stable-isotope labeled analogues are considered not ideal IS, because they must be prepared in a matrix different from the biological sample. Recently, [2,3,3,4,4,5,5-(2)H(7)]-ADMA (d(7)-ADMA) has become commercially available and we took this opportunity to test the reliability of the de novo synthesized d(3)Me-ADMA as an IS for ADMA in GC-tandem MS. In this article, we report on the re-validation of the previously reported GC-tandem MS method for ADMA in human plasma and urine using d(7)-ADMA as IS, and on comparative quantitative analyses of ADMA by GC-tandem MS using d(7)-ADMA and d(3)Me-ADMA. After thorough standardization of d(7)-ADMA and methods validation, we obtained by GC-tandem MS very similar ADMA concentrations in plasma and urine using d(7)-ADMA and d(3)Me-ADMA. The present study gives a proof of evidence for the aptitude of (2)H(3)-ADMA as IS in GC-tandem MS and suggests that de novo synthesis of stable-isotope labeled alkyl esters of amino acids and amino acid derivates may be a generally applicable method in mass spectrometry-based methods for amino acids. This approach is especially useful for amino acids for which no stable-isotope labeled analogues are commercially available.

Determination of asymmetric dimethylarginine in biological fluids: a paradigm for a successful analytical story

PURPOSE OF REVIEW

To review recently reported analytical methods for the quantification in biological fluids of asymmetric dimethylarginine, an endogenous inhibitor of nitric oxide synthesis, and to evaluate their impact on clinical research.

RECENT FINDINGS

Recently developed and increasingly used analytical methods in this area are based on mass spectrometry coupled with gas chromatography (i.e., gas chromatography-mass spectrometry and gas chromatography-mass spectrometry/mass spectrometry) or liquid chromatography (i.e., liquid chromatography-mass spectrometry and liquid chromatography-mass spectrometry/mass spectrometry). These approaches revealed asymmetric dimethylarginine concentrations in plasma and serum of healthy adults in the range 400-500 nmol/l. High-performance liquid chromatography methods with fluorescence detection provide asymmetric dimethylarginine plasma/serum concentrations comparable to those of mass spectrometry-based methods. This interval for circulating asymmetric dimethylarginine and the mass spectrometry/mass spectrometry-based methods have the potential to serve as reference values and analytical methods, respectively. An enzyme-linked immunosorbent assay method for asymmetric dimethylarginine has become commercially available and is increasingly used in clinical studies. Comparative studies revealed that the enzyme-linked immunosorbent assay method produces considerably higher asymmetric dimethylarginine concentrations in plasma or serum in healthy humans in the basal state than mass spectrometry and high-performance liquid chromatography methods and runs varyingly in different laboratories.

SUMMARY

At present, many analytical methods allow for the accurate and precise quantification of asymmetric dimethylarginine in biological fluids. However, reliable quantification of biological asymmetric dimethylarginine remains an analytical challenge in basic and clinical research.

Determination of asymmetric dimethylarginine, an endogenous nitric oxide synthase inhibitor, in umbilical blood

Endothelial cells produce nitric oxide (NO), a potent vasodilator. Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of NO synthase. Little is known about the potential physiological roles of ADMA in a perinatal setting. This study measures concentrations of ADMA in umbilical blood using enzyme-linked immunosorbent assay and those of NO as nitrite/nitrate (NOx(-)) using the Griess assay. Their relationship to the degree of prematurity and maternal clinical condition is examined. Results show that ADMA concentrations in umbilical blood from control newborns were about twice as high as those of lactating women, healthy children, and healthy adults. Umbilical blood NOx(-) concentrations from control newborns were about half of those of lactating women, healthy children, and healthy adults. Consequently, the levels of ADMA relative to NOx(-) were about 4-fold higher in umbilical blood from control newborns than in blood from lactating women, healthy children, and healthy adults. Furthermore, the umbilical blood ADMA concentrations and the ratios of ADMA to NOx(-) in newborns were higher according to their birth prematurity and lower birth weight. The umbilical ADMA concentrations were independent of the delivery mode and maternal preeclampsia. We infer that the high ADMA levels play physiological roles in maintaining vascular tone and blood redistribution to vital organs during birth, thereby favoring the circulatory transition from fetal to neonatal life.

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.

High-throughput CZE-UV determination of arginine and dimethylated arginines in human plasma

Experimental studies document that increased asymmetric dimethylarginine (ADMA) blood levels inhibit NOS significantly, reducing NO generation. ADMA measurement often needs sample cleanup by SPE prior to chromatography and precolumn derivatization that cannot be easily employed in a routine clinical setting. We set up a new reliable CE method to measure ADMA, symmetric dimethylarginine (SDMA), and arginine without sample extraction or precolumn derivatization in order to examine their concentrations in human plasma. Sample was concentrated prior to CE injection and analytes were monitored by UV detection. CE analysis was performed in an uncoated fused-silica capillary, 75 microm id and 60.2 cm length (50 cm to the detection window), injecting 1 s water plug (0.5 psi) followed by 10 s of the sample (0.5 psi). Separation was carried out in a 50 mmol/L Tris-phosphate run buffer at pH 2.30, 15 degrees C and 15 kV (75 microA) at normal polarity. Recovery of plasma ADMA was 101-104% and inter-day CV was less than 3%. Assay performance was evaluated measuring the levels of arginine and its dimethyl derivatives in 77 subjects. Passing-Bablok regression and Bland-Altman test for methods comparison suggest that the data obtained by our method and by a reference CE-LIF assay are similar.

Bioanalytical profile of the L-arginine/nitric oxide pathway and its evaluation by capillary electrophoresis

This review briefly summarizes recent progress in fundamental understanding and analytical profiling of the L-arginine/nitric oxide (NO) pathway. It focuses on key analytical references of NO actions and the experimental acquisition of these references in vivo, with capillary electrophoresis (CE) and high-performance capillary electrophoresis (HPCE) comprising one of the most flexible and technologically promising analytical platform for comprehensive high-resolution profiling of NO-related metabolites. Another aim of this review is to express demands and bridge efforts of experimental biologists, medical professionals and chemical analysis-oriented scientists who strive to understand evolution and physiological roles of NO and to develop analytical methods for use in biology and medicine.

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.

HPLC analysis of ADMA and other methylated l-arginine analogs in biological fluids

Post-translational methylation of arginine residues in proteins leads to generation of N(G)-monomethylarginine (MMA) and both symmetric and asymmetric dimethylarginine (SDMA and ADMA), that are released into the cytosol upon proteolysis. Both MMA and ADMA are inhibitors of nitric oxide synthase and especially elevated levels of ADMA are associated with endothelial dysfunction and cardiovascular disease. Plasma concentrations of ADMA and SDMA are very low, typically between 0.3 and 0.8 microM, making their quantification by HPLC an analytical challenge. Sample preparation usually involves a cleanup step by solid-phase extraction on cation-exchange columns followed by derivatization of amino acids into fluorescent adducts. Because ADMA and SDMA concentrations in healthy subjects show a very narrow distribution, with a between-subject variability of 13% for ADMA and 19% for SDMA, very low imprecision is an essential assay feature. Procedures for sample cleanup, derivatization, and chromatographic separation of arginine and its methylated analogs are the main topics of this review. In addition, important aspects of method validation, pre-analytical factors, and reference values are discussed.

Asymmetrical dimethylarginine (ADMA) and coronary endothelial function in patients with coronary artery disease and mild hypercholesterolemia

OBJECTIVE

To investigate the association of the endogenous nitric oxide synthase inhibitor asymmetrical dimethylarginine (ADMA) and coronary endothelial function.

METHODS AND RESULTS

In 289 patients with coronary artery disease we assessed coronary endothelium-dependent and -independent vascular responses to intracoronary infusion of acetylcholine, adenosine, and nitroglycerin, respectively, and determined plasma ADMA and l-arginine concentrations by HPLC. After 6 months of treatment with either cerivastatin, nifedipine, cerivastatin+nifedipine, or placebo, coronary vascular function testing as well as ADMA and l-arginine determinations were repeated. We observed no correlation of plasma ADMA or l-arginine concentration and coronary response to acetylcholine, adenosine or nitroglycerin baseline, and no correlation of changes of ADMA or l-arginine plasma concentration with changes in coronary function (all r and rho<0.3, all p>0.05).

CONCLUSION

At physiological plasma concentrations ADMA appears to have only little impact on coronary endothelial function.

A stable-isotope based technique for the determination of dimethylarginine dimethylaminohydrolase (DDAH) activity in mouse tissue

The enzyme dimethylarginine dimethylaminohydrolase (DDAH) is responsible for the hydrolysis of asymmetric dimethylarginine (ADMA) to L-citrulline and dimethylamine. DDAH is currently investigated as a promising target for therapeutic interventions, as ADMA has been found to be elevated in cardiovascular disease. In many tissues continuous endogenous formation of ADMA and L-citrulline poses considerable limitations to the presently used assays for DDAH activity, which are commonly based on the measurement of ADMA or L-citrulline. We therefore developed a stable-isotope-based assay suitable for 96-well plates to determine DDAH activity. Using deuterium-labeled ADMA ([(2)H(6)]-ADMA) as substrate and double stable-isotope labeled ADMA ([(13)C(5)-(2)H(6)]-ADMA) as internal standard we were able to simultaneously determine formation and metabolism of ADMA in renal and liver tissue of mice by LC-tandem MS. Endogenous formation of ADMA could largely be abolished by addition of protease inhibitors, while metabolism of [(2)H(6)]-ADMA was not significantly altered. The intra-assay coefficient of variation for the determination of endogenous ADMA and [(2)H(6)]-ADMA was 2.4% and 4.8% in renal and liver tissue, respectively. The inter-assay coefficient of variation for DDAH activity based on degradation of [(2)H(6)]-ADMA determined in separate samples from the same organs was determined to be 8.9% and 10% for mouse kidney and liver, respectively. The present DDAH activity assay allows for the first time to simultaneously determine DDAH activity and endogenous formation of ADMA, SDMA, and L-arginine in tissue.

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.

Asymmetric dimethylarginine is an independent risk factor for coronary heart disease: results from the multicenter Coronary Artery Risk Determination investigating the Influence of ADMA Concentration (CARDIAC) study

BACKGROUND

Asymmetric dimethylarginine (ADMA) plasma levels have been shown to be elevated in diseases related to endothelial dysfunction such as hypertension, hyperlipidemia, diabetes mellitus, and others. It has been shown that ADMA predicts cardiovascular mortality in patients who have coronary heart disease (CHD). However, the question whether ADMA is an independent risk factor for CHD still remains unresolved.

METHODS

The CARDIAC study is a multicenter case-control study, designed to detect differences in ADMA plasma levels between patients with CHD and controls from the general population. We included in our analysis 131 cases and 131 controls, matched for age, sex, and body mass index.

RESULTS

We found that cases had higher ADMA plasma levels than controls (0.70 micromol/L [0.59-0.87 micromol/L] vs 0.60 micromol/L [0.54-0.69 micromol/L], P < .001). To evaluate the predictive power of ADMA regarding CHD, we calculated 2 multivariate logistic regression models including laboratory parameters and traditional risk factors. The odds ratio for ADMA in the multivariate model including the laboratory characteristics was 2.59 (1.61-4.17; P < .001); the odds ratio for the multivariate model including other risk factors was 6.04 (2.56-14.25; P < .001) for the third tertile (>0.72 micromol/L) versus the first (<0.58 micromol/L).

CONCLUSIONS

We conclude from the results of our study that ADMA is an independent risk factor for CHD.