ADMA: a novel risk factor that explains excess cardiovascular event rate in patients with end-stage renal disease

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide (NO) synthase. By competitively displacing L-arginine from the substrate binding site of NO synthase, ADMA interferes with many of the physiological functions of NO, like endothelium-dependent vasodilation and leukocyte adhesion. ADMA, like its biologically inactive regioisomer, symmetric dimethylarginine (SDMA), can be found in human plasma and urine in low concentrations. The concentrations of both dimethylarginines are increased in patients with end-stage renal disease, which may explain at least in part endothelial dysfunction and cardiovascular complications in this patient population. In addition, the metabolism of ADMA, but not SDMA, occurs via hydrolytic degradation to citrulline and dimethylamine by the enzyme dimethylarginine dimethylaminohydrolase (DDAH). Data from experimental studies suggest that ADMA inhibits vascular NO elaboration at concentrations that can be measured in plasma of patients with renal disease. Interestingly, ADMA and SDMA are poorly eliminated during hemodialysis. This is probably due to a high level of binding of both molecules to plasma proteins. High ADMA concentrations in patients with end-stage renal disease may contribute to their excess cardiovascular event rate, as in clinical studies a relationship between ADMA and carotid artery intimal thickening was found. Moreover, in a prospective study we demonstrated recently that determination of ADMA plasma concentration is useful to predict future cardiovascular event rate and total mortality in this patient population. As other researchers reported observations that are in line with our findings, there is evidence that ADMA may be a novel cardiovascular risk factor.     

Asymmetric dimethylarginine levels in thyroid diseases

Thyroid diseases may lead to endothelial dysfunction; however, the mechanism underlying the endothelial dysfunction in thyroid disease is not clear yet. Asymmetric dimethylarginine (ADMA), a novel inhibitor of endothelial nitric oxide synthase (eNOS), blocks nitric oxide (NO) synthesis from L-arginine. Symmetric dimethylarginine (SDMA) is the structural isomer of the eNOS inhibitor ADMA. SDMA does not directly inhibit eNOS but is a competitive inhibitor of arginine transport. Increased plasma ADMA, SDMA concentrations, and low L-arginine/ADMA ratio were considered as possible contributing factors for endothelial dysfunction in hyperthyroid patients. On the other hand, plasma ADMA, SDMA levels and L-arginine/ADMA ratio in the hypothyroid group were unexpectedly found to be similar to those of the control subjects. The aim of this study is to evaluate and compare the plasma ADMA levels in hyperthyroid, hypothyroid and healthy subjects. Plasma ADMA, SDMA, and L-arginine levels were measured by high performance liquid chromatography. Plasma ADMA levels were significantly higher in both patients with hyperthyroidism and hypothyroidism than in the control group. SDMA concentrations were significantly increased in hypothyroid patients compared to control subjects. Patients with hyperthyroidism and hypothyroidism had significantly higher plasma L-arginine levels compared with healthy controls. L-arginine/ADMA ratio, which shows NO bioavailability, was significantly lower in hyperthyroid patients than in both hypothyroid and control subjects. In hyperthyroidism, plasma ADMA levels were related to age, L-arginine, and SDMA levels. SDMA was associated with age and L-arginine. L-arginine/ADMA ratio was negatively associated with freeT4 levels. There was a relationship between ADMA and L-arginine in hypothyroid patients. SDMA was significantly related to L-arginine, total cholesterol, and LDL. In conclusion, not only hyperthyroidism but also hypothyroidism was associated with alterations of ADMA and SDMA metabolism.     

Endogenous nitric oxide synthase inhibitors in sickle cell disease: abnormal levels and correlations with pulmonary hypertension, desaturation, haemolysis, organ dysfunction and death

Pulmonary hypertension (PH) in patients with sickle cell disease (SCD) is linked to intravascular haemolysis, impaired nitric oxide bioavailability, renal dysfunction, and early mortality. Asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthases (NOS), is associated with vascular disease in other populations. We determined the plasma concentrations for several key arginine metabolites and their relationships to clinical variables in 177 patients with SCD and 29 control subjects: ADMA, symmetric dimethylarginine (SDMA), NG-monomethyl L-arginine (L-NMMA), N-omega-hydroxy-L-arginine (NOHA), arginine and citrulline. The median ADMA was significantly higher in SCD than controls (0·94  μmol/l vs. 0·31 μmol/l, P  <   0·001). Patients with homozygous SCD had a remarkably lower ratio of arginine to ADMA (50 μmol/l vs. 237, P  < 0·001). ADMA correlated with markers of haemolysis, low oxygen saturation and soluble adhesion molecules. PH was associated with high levels of ADMA and related metabolites. Higher ADMA level was associated with early mortality, remaining significant in a multivariate analysis. Subjects with homozygous SCD have high systemic levels of ADMA, associated with PH and early death, implicating ADMA as a functional NOS inhibitor in these patients. These defects and others converge on the nitric oxide pathway in homozygous SCD with vasculopathy.     

Transjugular intrahepatic portosystemic shunt-placement increases arginine/asymmetric dimethylarginine ratio in cirrhotic patients

AIM：To analyze the change of dimethylarginine plasma levels in cirrhotic patients receiving transjugular intrahepatic portosystemic shunt（TIPS）.METHODS：To determine arginine,asymmetric dimethylarginine（ADMA）,symmetric dimethylarginine（SDMA）,and nitric oxide（NO） plasma levels,blood samples were collected from the superior cava,hepatic,and portal vein just before,directly after,and 3 mo after TIPS-placement.RESULTS：A significant increase in the arginine/ADMA ratio after TIPS placement was shown.Moreover,TIPS placement enhanced renal function and thereby decreased systemic SDMA levels.In patients with renal dysfunction before TIPS placement,both the arginine/ADMA ratio and creatinine clearance rate increased significantly,while this was not the case in patients with normal renal function before TIPS placement.Hepatic function did not change significantly after TIPS placement and no significant decline in ADMA plasma levels was measured.CONCLUSION：The increase of the arginine/ADMA ratio after TIPS placement suggests an increase in intracellular NO bioavailability.In addition,this study suggests that TIPS placement does not alter dimethylarginine dimethylaminohydrolase（DDAH） activity and confirms the major role of the liver as an ADMA clearing organ.     

Asymmetric dimethylarginine in adults with cystathionine β-synthase deficiency

In hyperhomocysteinemia (HHcy), an independent risk factor for cardiovascular diseases, endothelial dysfunction due to reduced bioavailability of nitric oxide is a consistent finding. However, the underlying mechanisms remain unknown. Increased levels of the nitric oxide synthase inhibitor asymmetric dimethylarginine (ADMA) have been associated with HHcy, and may contribute, at least in part, for the homocysteine-induced endothelial dysfunction, but whether cystathionine β-synthase (CBS) deficiency is associated with increased ADMA has hardly been investigated. To address this question, we measured total homocysteine (tHcy), ADMA and symmetric dimethylarginine (SDMA) in plasma of 22 adult CBS deficient patients, using established HPLC techniques. Results showed that in CBS deficient patients with elevated levels of tHcy (median (total range): 33 (14-237) μmol/L), both ADMA and SDMA levels were normal. Moreover, tHcy and ADMA concentrations were not correlated (r(s)=0.017, p=0.94). Our results favor the hypothesis that the negative vascular effects of HHcy have an ADMA-independent etiology.     

Relations between plasma asymmetric dimethylarginine (ADMA) and risk factors for coronary disease

BACKGROUND: Elevated plasma levels of asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide production, are reported to be associated with coronary artery disease (CAD). METHODS: We measured plasma levels of ADMA and related compounds, nitrate+nitrite (NO(x)), total homocysteine (tHCY) and assessed renal function and lipid profiles in 145 patients--75 with triple vessel coronary disease and 70 with no detectable coronary disease. RESULTS: Levels of ADMA, l-arginine, l-arginine/ADMA and plasma NO(x) were not different in the two groups but smokers with triple vessel disease had higher ADMA and lower NO(x) levels than the non-smokers, relationships also present for all smokers and non-smokers in the two groups combined. In all 145 patients ADMA, symmetric dimethylarginine (SDMA) and tHCY levels were significantly higher in patients with glomerular filtration rate (GFR) <81 mL/min/1.73 m(2) than in patients with GFR> or =81 mL/min/1.73 m(2). There was a modest positive correlation between tHCY and ADMA and both were strongly correlated with SDMA which is excreted by the kidney. ADMA, SDMA and tHCY were negatively correlated with GFR. CONCLUSIONS: We suggest that the reported ADMA increases in CAD patients are due to an associated reduction in renal function and to smoking habit.     

The emerging role of asymmetric dimethylarginine as a novel cardiovascular risk factor

There is abundant evidence that the endothelium plays a crucial role in the maintenance of vascular tone and structure. One of the major endothelium-derived vasoactive mediators is nitric oxide (NO). Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of NO synthase. ADMA inhibits vascular NO production at concentrations found in pathophysiological conditions (i.e., 3-15 micromol/l); ADMA also causes local vasoconstriction when it is infused intraarterially. The biochemical and physiological pathways related to ADMA are now well understood: dimethylarginines are the result of the degradation of methylated proteins; the methyl group is derived from S-adenosylmethionine. Both ADMA and its regioisomer, SDMA, are eliminated from the body by renal excretion, whereas only ADMA, but not SDMA, is metabolized via hydrolytic degradation to citrulline and dimethylamine by the enzyme dimethylarginine dimethylaminohydrolase (DDAH). DDAH activity and/or expression may therefore contribute to the pathogenesis of endothelial dysfunction in various diseases. ADMA is increased in the plasma of humans with hypercholesterolemia, atherosclerosis, hypertension, chronic renal failure, and chronic heart failure. Increased ADMA levels are associated with reduced NO synthesis as assessed by impaired endothelium-dependent vasodilation. In several prospective and cross-sectional studies, ADMA evolved as a marker of cardiovascular risk. With our increasing knowledge of the role of ADMA in the pathogenesis of cardiovascular disease, ADMA is becoming a goal for pharmacotherapeutic intervention. Among other treatments, the administration of L-arginine has been shown to improve endothelium-dependent vascular function in subjects with high ADMA levels.     

Asymmetric dimethylarginine, derangements of the endothelial nitric oxide synthase pathway, and cardiovascular diseases

Analogues of L-arginine that are chemically modified at the terminal guanidino nitrogen group, such as Nomega-monomethy-L-arginine (L-NMMA), have been used for nitric oxide synthase inhibition. However, L-NMMA and other methylated L-arginine analogues are also endogenously formed. Among these, asymmetric dimethylarginine (ADMA) and symmetric dimethylarginine (SDMA) have been shown to be the most abundant. Like L-NMMA, ADMA is an inhibitor of NO synthase, whereas SDMA is inactive. ADMA is synthesized by N-methyltransferases, a family of enzymes that methylate L-arginine residues within specific proteins. Free ADMA is released during proteolytic cleavage of methylated proteins; it can be detected in plasma and urine, but its intracellular concentrations appear to be much higher. ADMA is metabolized by the enzyme dimethylarginine dimethylaminohydrolase (DDAH), and inhibition of DDAH activity has been shown to lead to increased ADMA levels and endothelial dysfunction. Plasma levels of ADMA are elevated in endstage renal failure, in atherosclerosis and hypercholesterolemia, in hypertension, and in heart failure. Although the molecular cause for elevation of ADMA concentration in these diseases has not been fully elucidated, evidence is accumulating that ADMA is one cause of endothelial dysfunction in these diseases. Moreover, it may be a marker or even a risk factor for cardiovascular disease. Therefore, pharmacological modulation of ADMA concentration may be a novel therapeutic target in cardiovascular diseases.     

Unchanged plasma levels of dimethylarginines and nitric oxide in chronic hepatitis C

Objective. Previous studies have shown that asymmetric dimethylarginine (ADMA), symmetric dimethylarginine (SDMA) and nitric oxide (NO) play a prominent role in liver dysfunction. The objective of this study was to determine whether plasma levels of ADMA, SDMA and NO are altered in patients with chronic hepatitis C. Material and methods. Plasma levels of ADMA, SDMA and NO (nitrite plus nitrate) were measured in 22 patients with chronic hepatitis C and 24 patients with sustained virologic response after treatment with peginterferon plus ribavirin. Seven healthy volunteers served as controls. Results. Plasma levels of ADMA, SDMA and NO were not significantly different between groups: chronic hepatitis C, ADMA 0.55±0.06, SDMA 0.22±0.03, NO 36.3±5.9 µmol/l; treated patients, ADMA 0.60±0.15, SDMA 0.31±0.05, NO 36.1±5.5 µmol/l; controls, ADMA 0.65±0.08, SDMA 0.28±0.05, NO 40.7±8.9 µmol/l). Conclusions. Our results show that plasma NO, ADMA and SDMA concentrations are not changed in patients with chronic hepatitis C without superimposed signs of acute inflammatory activity. Furthermore, no significant differences in plasma values of NO and dimethylarginines were observed between the group of untreated patients and the group of patients treated with interferon plus ribavirin     

Asymmetric dimethylarginine and impaired cardiovascular healing

Asymmetric dimethylarginine (ADMA) typically accumulates in the plasma of patients with chronic renal failure. Moreover, its plasma levels are raised in the presence of virtually all of the traditional cardiovascular risk factors. ADMA inhibits the three isoforms of nitric oxide (NO) synthase, thereby blunting the known cardioprotective effects of NO. Through its NO inhibitor actions, ADMA also exerts pro-apoptotic effects and suppresses progenitor cell mobilization, differentiation and function. Among patients with ischemic heart disease, low progenitor cell bioavailability and kidney dysfunction are emerging as strong predictors of death and recurrent cardiovascular events. We propose that patients with ischemic heart disease, kidney dysfunction, and high risk factor burden exhibit adverse cardiovascular outcomes, at least in part, through ADMA-mediated NO depression, enhanced apoptotic signalling, and reduced progenitor cell bioavailability, with consequent blunting of cardiovascular healing. Further research into the mechanisms that regulate the NO/ADMA balance may advance our understanding of cardiovascular diseases.     

Asymmetrical dimethylarginine is related to renal function, chronic inflammation and macroangiopathy in patients with Type 2 diabetes and albuminuria.

AIMS: Patients with Type 2 diabetes mellitus (T2DM) and micro- and macroalbuminuria are at increased cardiovascular risk. The endogenous nitric oxide synthase inhibitor asymmetrical dimethylarginine (ADMA) is increased in renal failure and could promote atherosclerosis. To determine the relationship between ADMA, renal albumin excretion rate (AER) and cardiovascular risk, we studied 103 T2DM patients. METHODS: ADMA, symmetrical dimethylarginine (SDMA) and L-arginine were determined by high-performance liquid chromatography in plasma from 36 normo-, 40 micro- and 27 macroalbuminuric patients with T2DM (age 64 +/- 11 years; 38 women) who had comparable age, sex and metabolic parameters. Forty-six patients had macrovascular disease (MVD). RESULTS: ADMA was significantly increased in patients with micro- and macroalbuminuria [median 0.61 (interquartile range 0.55-0.70) micromol/l and 0.62 (0.50-0.79) micromol/l, respectively] compared with those with normoalbuminuria [0.55 (0.48-0.63) micromol/l; both P < 0.05]. SDMA was elevated in micro- and macroalbuminuria [0.57 (0.42-0.80) micromol/l and 0.64 (0.50-0.96) micromol/l] compared with normoalbuminuric subjects [0.44 (0.37-0.53) micromol/l; both P < 0.01]. Patients with increased AER and MVD had higher ADMA and SDMA compared with those without MVD (both P < 0.001). L-arginine was comparable between all groups. ADMA correlated significantly with high-sensitivity C-reactive protein (hsCRP) and glomerular filtration rate (GFR) but not with the extent of albumin excretion, body mass index, fasting glucose, HbA(1c) or plasma lipids. CONCLUSIONS: Increased ADMA in T2DM patients with albuminuria is linked to cardiovascular disease and is associated with renal dysfunction and subclinical inflammation.     

Increasing dimethylarginine levels are associated with adverse clinical outcome in severe alcoholic hepatitis

Previous studies suggest reduced hepatic endothelial nitric oxide synthase activity contributes to increased intrahepatic resistance. Asymmetric dimethylarginine (ADMA), an endogenous nitric oxide synthase inhibitor, undergoes hepatic metabolism via dimethylarginine-dimethylamino-hydrolase, and is derived by the action of protein-arginine-methyltransferases. Our study assessed whether ADMA, and its stereo-isomer symmetric dimethylarginine (SDMA), are increased in alcoholic hepatitis patients, and determined any relationship with severity of portal hypertension (hepatic venous pressure gradient measurement) and outcome. Fifty-two patients with decompensated alcoholic cirrhosis were studied, 27 with acute alcoholic hepatitis and cirrhosis, in whom hepatic venous pressure gradient was higher (P = 0.001) than cirrhosis alone, and correlated with ADMA measurement. Plasma ADMA and SDMA were significantly higher in alcoholic hepatitis patients and in nonsurvivors. Dimethylarginine-dimethylamino-hydrolase protein expression was reduced and protein-arginine-methyltransferase-1 increased in alcoholic hepatitis livers. ADMA, SDMA and their combined sum, which we termed a dimethylarginine score, were better predictors of outcome compared with Pugh score, MELD and Maddrey's discriminant-function. CONCLUSION: Alcoholic hepatitis patients have higher portal pressures associated with increased ADMA, which may result from both decreased breakdown (decreased hepatic dimethylarginine-dimethylamino-hydrolase) and/or increased production. Elevated dimethylarginines may serve as important biological markers of deleterious outcome in alcoholic hepatitis.     

ADMA and oxidative stress

Elevated plasma concentrations of the endogenous nitric oxide synthase (eNOS) inhibitor asymmetric dimethylarginine (ADMA) represent a novel risk factor for the development of endothelial dysfunction and a predictor for all-cause and cardiovascular mortality. However, it is unknown whether elevated ADMA plasma concentrations may be considered simply as a marker for cardiovascular disease or whether increased ADMA levels per se may predispose to the development of vascular disease. There is experimental and clinical evidence linking endothelial dysfunction to increased production of oxygen-derived free radicals such as superoxide anion. Oxidative stress has been shown to increase the activity of arginine methylating and ADMA degrading enzymes leading to increased ADMA concentrations. Interestingly, the endothelial nitric oxide synthase may become uncoupled in the presence of high ADMA levels further contributing to the vascular oxidative stress burden. It remains to be established to what extent ADMA is able to interact with eNOS in vivo. Possible mechanisms underlying increased oxidative stress in the setting of elevated ADMA concentrations and therapeutic implications will be discussed.     

Asymmetrical dimethylarginine level in atrial fibrillation

OBJECTIVE: Atrial fibrillation (AF) is known to be related with increased risk of thromboembolic events. Asymmetrical dimethylarginine (ADMA), which is an endogenous inhibitor of nitric oxide synthase (NOS), can cause endothelial dysfunction by decreasing nitric oxide (NO) and lead to increased risk of thrombosis. In the present study our aim was to compare plasma levels of ADMA in patients with acute onset (< 24 hours) and chronic AF (> 1 year) to determine the risk of thrombosis. METHOD: 17 patients with the first detected attack of AF within the first 24 hours of presentation (group 1), 25 patients who had permanent chronic AF lasting at least 1 year or more (group II) and 18 healthy persons as the control group (group III) were included in the study. For each patient the plasma ADMA, L-arginine, symmetrical dimethylarginine (SDMA) concentrations were measured by high-performance liquid chromatography in venous blood samples collected before cardioversion. We compared the plasma ADMA, L-arginine and SDMA concentrations between the groups. RESULTS: Plasma L-arginine (78.18 +/- 28.29 vs. 73.14 +/- 14.11 vs. 71.03 +/- 21.31, P = 0.549) and plasma SDMA concentrations (0.38 +/-0.18 vs. 0.42 +/- 0.21 vs. 0.32 +/- 0.24, P = 0.224) were similar in all groups. There was a significant difference between plasma ADMA concentrations (0.76 +/- 0.27 vs. 0.50 +/- 0.26 vs. 0.36 +/- 0.20, P < 0.001) among the groups. When we compared plasma ADMA levels between the subgroups, we also found a significant difference (P = 0.002 when comparing group I and group II, P < 0.001 when comparing of group I and group III, P = 0.042 when compareng of group II and group III). CONCLUSION: ADMA levels in patients with acute onset AF were significantly increased when compared with patients with chronic AF and the healthy control group indicating the presence of endothelial dysfunction and a prothrombotic state even in a very early phase of AF.     

Does ADMA cause endothelial dysfunction?

Asymmetric dimethylarginine (ADMA) is an endogenous and competitive inhibitor of nitric oxide synthase. Plasma levels of this inhibitor are elevated in patients with atherosclerosis and in those with risk factors for atherosclerosis. In these patients, plasma ADMA levels are correlated with the severity of endothelial dysfunction and atherosclerosis. By inhibiting the production of nitric oxide, ADMA may impair blood flow, accelerate atherogenesis, and interfere with angiogenesis. ADMA may be a novel risk factor for vascular disease.     

ADMA and hyperhomocysteinemia

Hyperhomocysteinemia is a risk factor for cardiovascular disease and stroke. Like many other cardiovascular risk factors, hyperhomocysteinemia produces endothelial dysfunction due to impaired bioavailability of endothelium-derived nitric oxide (NO). The molecular mechanisms responsible for decreased NO bioavailability in hyperhomocysteinemia are incompletely understood, but emerging evidence suggests that asymmetric dimethylarginine (ADMA), an endogenous inhibitor of NO synthase, may be a key mediator. Homocysteine is produced during the synthesis of ADMA and can alter ADMA metabolism by inhibiting dimethylarginine dimethylaminohydrolase (DDAH). Several animal and clinical studies have demonstrated a strong association between plasma total homocysteine, plasma ADMA, and endothelial dysfunction. These observations suggest a model in which elevation of ADMA may be a unifying mechanism for endothelial dysfunction during hyperhomocysteinemia. The recent development of transgenic mice with altered ADMA metabolism should provide further mechanistic insights into the role of ADMA in hyperhomocysteinemia.     

Associations of Methylarginines and Homoarginine With Diastolic Dysfunction and Cardiovascular Risk Factors in Patients With Preserved Left Ventricular Ejection Fraction

BackgroundAsymmetric dimethylarginine (ADMA), symmetric dimethylarginine (SDMA), and homoarginine are considered to modulate nitric oxide synthesis. We evaluated whether ADMA, SDMA, and homoarginine are associated with diastolic dysfunction.Methods and ResultsWe investigated primary care patients at cardiovascular risk with preserved left ventricular ejection fraction from the multicenter DIAST-CHF study. We measured serum concentrations of ADMA, SDMA, and homoarginine and performed standardized echocardiographic examinations. Among 1,396 patients (mean age 65.3 ± 8.3 y, 54.6% women), diastolic dysfunction was ruled out in 261 patients (18.7%). Mild and moderate/severe grades of diastolic dysfunction were present in 900 (64.5%) and 235 (16.8%) study participants, respectively. After adjustments for cardiovascular risk factors, ADMA and SDMA were positively and homoarginine negatively associated with N-terminal pro–B-type natriuretic peptide and midregional pro-adrenomedullin (P < .05 for all). Lower homoarginine levels were associated with diastolic dysfunction, and higher ADMA and SDMA levels were associated with the severity of diastolic dysfunction (P < .05 for all).ConclusionsHigher levels of ADMA and SDMA and lower levels of homoarginine are associated with an adverse cardiovascular risk profile and diastolic dysfunction. Further studies should clarify the potential of these amino acid derivatives for the therapy of cardiovascular diseases.     

Pharmacotherapies and their influence on asymmetric dimethylargine (ADMA)

Elevated plasma concentrations of the endogenous nitric oxide synthase inhibitor asymmetric dimethylarginine (ADMA) are found in various clinical settings, including renal failure, coronary heart disease, hypertension, diabetes and pre-eclampsia. In healthy people acute infusion of ADMA promotes vascular dysfunction, and in mice chronic infusion of ADMA promotes progression of atherosclerosis. Thus, ADMA may not only be a marker but also an active player in cardiovascular disease, which makes it a potential target for therapeutic interventions. This review provides a summary and critical discussion of the presently available data concerning the effects on plasma ADMA levels of cardiovascular drugs, hypoglycemic agents, hormone replacement therapy, antioxidants, and vitamin supplementation. We assess the evidence that the beneficial effects of drug therapies on vascular function can be attributed to modification of ADMA levels. To develop more specific ADMA-lowering therapies, mechanisms leading to elevation of plasma ADMA concentrations in cardiovascular disease need to be better understood. ADMA is formed endogenously by degradation of proteins containing arginine residues that have been methylated by S-adenosylmethionine-dependent methyltransferases (PRMTs). There are two major routes of elimination: renal excretion and enzymatic degradation by the dimethylarginine dimethylaminohydrolases (DDAH-1 and -2). Oxidative stress causing upregulation of PRMT expression and/or attenuation of DDAH activity has been suggested as a mechanism and possible drug target in clinical conditions associated with elevation of ADMA. As impairment of DDAH activity or capacity is associated with substantial increases in plasma ADMA concentrations, DDAH is likely to emerge as a prime target for specific therapeutic interventions.     

Plasma concentrations of arginine and asymmetric dimethylarginine do not reflect their intracellular concentrations in peripheral blood mononuclear cells

Objective

Production of nitric oxide (NO) from arginine is inhibited by endogenously produced monomethylarginine (MMA) and asymmetric dimethylarginine (ADMA). Elevated levels of ADMA, by limiting NO production, may lead to endothelial dysfunction and cardiovascular disease. Symmetric dimethylarginine (SDMA) and the arginine homolog homoarginine have also been associated with cardiovascular disease. Although NO synthesis, as well as generation of MMA, ADMA, SDMA and homoarginine, occurs intracellularly, these biomarkers are usually measured in plasma. Despite extensive transmembrane transport, it is not clear whether plasma levels of these biomarkers are a valid proxy for their intracellular levels in the cardiovascular system. Since it is difficult to obtain vascular tissue from healthy humans, we explored the relations between concentrations of these biomarkers in plasma and intracellular concentrations in peripheral blood mononuclear cells (PBMC).

Methods

In PBMC and plasma of 27 healthy subjects, concentrations of arginine, MMA, ADMA, SDMA, and homoarginine were determined using stable isotope dilution liquid chromatography tandem mass spectrometry.

Results

In PBMC, significant positive correlations were observed among arginine and its methylated forms (ρ = 0.43 to 0.81) and these correlations were slightly less pronounced in plasma. Homoarginine was not significantly correlated with (methylated) arginine in either PBMC or plasma. Plasma concentrations of arginine and its methylated forms showed non-significant inverse associations with their respective intracellular concentrations in PBMC and only for homoarginine was a weak positive association observed (ρ = 0.37).

Conclusion

In healthy individuals, plasma levels of arginine, MMA, ADMA, and SDMA poorly reflect their intracellular levels in PBMC.