Regulation of cytokine-induced nitric oxide synthesis by asymmetric dimethylarginine: role of dimethylarginine dimethylaminohydrolase

In response to vascular insults, inflammatory cytokines stimulate vascular smooth muscle cells (SMCs) to express an inducible isoform of nitric oxide synthase (iNOS). Asymmetric dimethylarginine (ADMA), an endogenous NO synthase inhibitor, is metabolized by dimethylarginine dimethylaminohydrolase (DDAH). To determine whether the ADMA-DDAH system regulates cytokine-induced NO production, cultured rat SMCs were exposed to interleukin-1beta (IL-1beta). IL-1beta (1 to 100 U/mL) dose-dependently stimulated not only iNOS but also DDAH expression and enzyme activity, accompanied by an increase in NO metabolite and by a decrease in ADMA content in culture media. A DDAH inhibitor (4124W, 5 mmol/L) augmented ADMA production (P<0.01) and decreased NO synthesis (P<0.01) in IL-1beta-stimulated SMCs. On the other hand, an adenovirus-mediated overexpression of DDAH reduced ADMA and enhanced NO production. Exogenous administration of NO donors (SNAP and SIN-1) dose-dependently increased NO metabolite in the culture media but had no effect on ADMA. Our results indicate two mechanisms of IL-1beta-induced NO synthesis: the direct stimulation of the expression of iNOS and the indirect stimulation of iNOS activity by upregulating DDAH and reducing ADMA. The ADMA-DDAH system may be another regulatory mechanism of inflammation-mediated NO production for human vascular diseases.     

The role of asymmetric dimethylarginine and arginine in the failing heart and its vasculature

Nitric oxide (NO) is formed from arginine by the enzyme nitric oxide synthase (NOS). Asymmetric dimethylarginine (ADMA) can inhibit NO production by competing with arginine for NOS binding. Therefore, the net amount of NO might be indicated by the arginine/ADMA ratio. In turn, arginine can be metabolized by the enzyme arginase, and ADMA by the enzyme dimethylarginine dimethylaminohydrolase (DDAH). While ADMA has been implicated as a cardiovascular risk factor, arginine supplementation has been indicated as a treatment in cardiac diseases. This review discusses the roles of ADMA and arginine in the failing heart and its vasculature. Furthermore, it proposes nutritional therapies to improve NO availability.     

all-trans-Retinoic acid increases nitric oxide synthesis by endothelial cells: a role for the induction of dimethylarginine dimethylaminohydrolase

all-trans-Retinoic acid (atRA) has important effects on the developing and mature cardiovascular system. Nitric oxide (NO) production has been associated with the atRA-induced differentiation of neuronal cells, and we hypothesized that NO may also mediate certain actions of atRA in the cardiovascular system. We studied the effects of atRA on NO production by endothelial cells and determined whether regulation of enzymes responsible for metabolism of asymmetric dimethylarginine (ADMA) contributed to the effects seen. Murine endothelioma (sEnd.1) cells were incubated with or without atRA. Nitrite production was determined using the Griess reaction. The expression of NO synthase (NOS) and dimethylarginine dimethylaminohydrolase (DDAH) genes was determined by Northern blotting. A reporter gene assay was also used to study the effect of atRA on the DDAH II promoter. atRA significantly increased nitrite production by sEnd.1 cells despite no increase in eNOS expression. atRA also increased DDAH II gene expression and promoter activity and reduced the ratio of ADMA to symmetric dimethylarginine (SDMA) in culture medium. The DDAH inhibitor 4124W significantly reduced the induction of NO synthesis by atRA. The present study demonstrates that atRA increases NO synthesis in endothelial cells without increasing eNOS expression. atRA also increases the expression of DDAH II, the predominant DDAH isoform in endothelial cells. Our data suggests that the induction of NO synthesis by atRA may be facilitated by DDAH II. This pathway may help to explain some of the effects of atRA on the cardiovascular system.     

Dimethylarginine dimethylaminohydrolase promotes endothelial repair after vascular injury.

OBJECTIVES: We sought to determine if a reduction in asymmetric dimethylarginine (ADMA) enhances endothelial regeneration. BACKGROUND: Asymmetric dimethylarginine is an endogenous inhibitor of nitric oxide synthase (NOS). Increased plasma levels of ADMA are associated with endothelial vasodilator dysfunction in patients with vascular disease or risk factors. Asymmetric dimethylarginine is eliminated largely by the action of dimethylarginine dimethylaminohydrolase (DDAH), which exists in 2 isoforms. Dimethylarginine dimethylaminohydrolase-1 transgenic (TG) mice manifest increased DDAH activity, reduced plasma and tissue ADMA levels, increased nitric oxide synthesis, and reduced systemic vascular resistance. METHODS: The left femoral arteries of DDAH1 TG mice and wild-type (WT) mice were injured by a straight spring wire, and regeneration of the endothelial cell (EC) monolayer was assessed. Endothelial sprouting was assayed with growth factor-reduced Matrigel. RESULTS: Regeneration of the EC monolayer was more complete 1 week after injury in TG mice (WT vs. TG: 40.0 +/- 6.5% vs. 61.2 +/- 6.4%, p < 0.05). The number of CD45 positive cells at the injured sites was reduced by 62% in DDAH TG mice (p < 0.05). Four weeks after injury, the neointima area and intima/media ratio were attenuated in DDAH TG mice (WT vs. TG: 0.049 +/- 0.050 mm2 vs. 0.031 +/- 0.060 mm2, 3.1 +/- 0.5 vs. 1.7 +/- 0.2, respectively, p < 0.05). Endothelial cell sprouting from vascular segments increased in TG mice (WT vs. TG: 24.3 +/- 3.9 vs. 39.0 +/- 2.2, p < 0.05). CONCLUSIONS: We find for the first time an important role for DDAH in EC regeneration and in neointima formation. Strategies to enhance DDAH expression or activity might be useful in restoring the endothelial monolayer and in treating vascular disease.     

Accumulated endogenous nitric oxide synthase inhibitors, enhanced arginase activity, attenuated dimethylarginine dimethylaminohydrolase activity and intimal hyperplasia in premenopausal human uterine arteries

The present study was designed to investigate the involvement of nitric oxide synthase (NOS), endogenous NOS inhibitors, arginase, which shares L-arginine as a common substrate with NOS, and dimethylarginine dimethylaminohydrolase (DDAH) as a metabolizing enzyme of NOS inhibitors in the occurrence of intimal hyperplasia in premenopausal human uterine arteries. Fifty-two uterine arteries were obtained from 52 patients undergoing total hysterectomy with an informed consent for the present study. All specimens were assessed histologically and the intima:media ratio (%) was evaluated as an index of intimal hyperplasia. Nineteen specimens were found to be histologically normal (intima:media ratio=16.1+/-0.8%), whereas remaining 33 specimens were categorized as intimal hyperplasia (intima:media ratio=34.4+/-1.5%). The intimal hyperplasia was associated with the impaired cyclic GMP production without change in endothelial NOS activity per se, accumulation of endogenous NOS inhibitors in endothelial cells, attenuated DDAH activity in endothelial cells and enhanced arginase activity in endothelial cells and smooth muscle layer. These findings suggest that the impaired cyclic GMP production as a marker of NO production is possibly due to the accumulated endogenous NOS inhibitors and enhanced arginase activity, which, in turn, closely relates to the occurrence of intimal hyperplasia, and that the impaired DDAH activity would result in the accumulation of endogenous NOS inhibitors in endothelial cells. Because of the enhanced arginase activity in endothelial cells and smooth muscle layer, the accelerated polyamine biosynthetic pathway may be implicated in the occurrence of intimal hyperplasia in premenopausal human uterine arteries.     

Dimethylarginine dimethylaminohydrolase-1 (DDAH1) is frequently upregulated in prostate cancer,and its overexpression conveys tumor growth and angiogenesis by metabolizing asymmetric dimethylarginine (ADMA)

Tissue microarray analysis confirmed higher dimethylarginine dimethylaminohydrolase-1 (DDAH1) expression in prostate cancer (PCa) compared to benign and normal prostate tissues. DDAH1 regulates nitric oxide (NO) production by degrading endogenous nitric oxide synthase (NOS) inhibitor, asymmetric dimethylarginine (ADMA). This study examined whether DDAH1 has any physiological role in PCa progression. Using overexpression of DDAH1 in PCa (PC3 and LNCaP) cell lines, we found that DDAH1 promotes cell proliferation, migration and invasion by lowering ADMA levels, as well as increasing NO production. VEGF, HIF-1α and iNOS were upregulated in DDAH1 expressing cells as result of elevated NO. DDAH1 increased secretion of pro-angiogenic signals bFGF and IL-8, into conditioned media. Treatment of DDAH1-positive PCa cells with NOS inhibitors (L-NAME and 1400 W) attenuated DDAH1 activity to promote cell growth. Xenografts derived from these cells grew significantly faster (> twofold) than those derived from control cells. Proliferation rate of cells stably expressing mutant DDAH1 was same as control cells unlike wild-type DDAH1-positive PCa cells. Xenograft tumors derived from mutant-positive cells did not differ from control tumors. VEGF, HIF-1α and iNOS expression did not differ in DDAH1 mutant-positive tumors compared to control tumors, but was upregulated in wild-type DDAH1 overexpressing tumors. Furthermore, CD31 immunostaining on xenograft tissues demonstrated that DDAH1 tumors had high endothelial content than mutant DDAH1 tumors. These data suggest that DDAH1 is an important mediator of PCa progression and NO/DDAH pathway needs to be considered in developing therapeutic strategies targeted at PCa.     

Estradiol,acting through estrogen receptor alpha,restores dimethylarginine dimethylaminohydrolase activity and nitric oxide production in oxLDL-treated human arterial endothelial cells

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide (NO) synthase. ADMA accumulation, mainly due to a decreased dimethylarginine dimethylaminohydrolase (DDAH) activity, has been related to the development of cardiovascular diseases. We investigate whether estradiol prevents the changes induced by oxidized low density lipoprotein (oxLDL) on the DDAH/ADMA/NO pathway in human umbilical artery endothelial cells (HUAEC). HUAEC were exposed to estradiol, native LDL (nLDL), oxLDL and their combinations for 24 h. In some experiments, cells were also exposed to the unspecific estrogen receptor (ER) antagonist ICI 182780, the specific ERα antagonist MPP or specific agonists for ERα, ERβ and GPER. ADMA concentration was measured by HPLC and concentration of NO by amperometry. Protein expression and DDAH activity were measured by immunoblotting and an enzymatic method, respectively. oxLDL, but not nLDL, increased ADMA concentration with a concomitant decrease on DDAH activity. oxLDL reduced eNOS protein and NO production. Estradiol alone had no effects on DDAH/ADMA/NO pathway, but increased the attenuated endothelial NO production induced by oxLDL by reduction in ADMA and preventing loss of eNOS protein levels. ICI 182780 and MPP completely abolished these effects of estradiol on oxLDL-exposed cells. ERα agonist, but not ERβ and GPER agonists, mirrored estradiol effects on NO production. In conclusion, estradiol restores (1) DDAH activity, and therefore ADMA levels, and (2) NO production impaired by oxLDL in HUAEC acting through ERα.     

Dimethylarginine dimethylaminohydrolase inhibition and asymmetric dimethylarginine accumulation contribute to endothelial dysfunction in rats exposed to glycosylated protein: effects of aminoguanidine

OBJECTIVES: To determine whether alterations of endogenous asymmetric dimethylarginine (ADMA) concentration and dimethylarginine dimethylaminohydrolase (DDAH) activity are involved in endothelial dysfunction induced by glycosylated bovine serum albumin (GBSA) in rats and effects of aminoguanidine on them. METHODS: Endothelium-dependent relaxation of aortic rings from Sprague-Dawley rats after treatment with GBSA in vitro and in vivo was tested. Serum concentrations of ADMA, nitrite/nitrate, and activities of aortic DDAH, nitric oxide synthase (NOS) and superoxide dismutase were measured in GBSA-treated rats. Moreover, serum contents of glycosylated serum protein, and malondialdehyde were also assayed. RESULTS: Endothelium-dependent relaxation was significantly impaired either by incubation of aortic rings with GBSA (1.70mmol/l) in vitro for 60min or by injection of GBSA (35mg/kg/d, i.v.) to normal rats for 4 weeks, and serum ADMA levels were remarkably elevated in GBSA-treated rats, which was accompanied by decreases of nitrite/nitrate concentrations, NOS and DDAH activities. Furthermore, elevated glycosylated serum protein, malondialdehyde levels, and reduced superoxide dismutase activity were also observed in GBSA-treated rats. Treatment with aminoguanidine not only improved impairment of endothelium-dependent relaxation but also prevented elevation of endogenous ADMA, which were concomitant with increases of nitrite/nitrate concentration, NOS and DDAH activity. Serum levels of glycosylated serum protein, malondialdehyde, and vascular superoxide dismutase activity were also normalized after aminoguanidine treatment. CONCLUSIONS: Decreased DDAH activity and elevated endogenous ADMA is implicated in endothelial dysfunction of rats exposed to GBSA. Aminoguanidine can protect endothelium of rat aorta against injury induced by GBSA both in vitro and in vivo.     

Role of asymmetric dimethylarginine in vascular injury in transgenic mice overexpressing dimethylarginie dimethylaminohydrolase 2

Dimethylarginie dimethylaminohydrolase (DDAH) degrades asymmetric dimethylarginine (ADMA), an endogenous nitric oxide (NO) synthase inhibitor, and comprises 2 isoforms, DDAH1 and DDAH2. To investigate the in vivo role of DDAH2, we generated transgenic mice overexpressing DDAH2. The transgenic mice manifested reductions in plasma ADMA and elevations in cardiac NO levels but no changes in systemic blood pressure (SBP), compared with the wild-type mice. When infused into wild-type mice for 4 weeks, ADMA elevated SBP and caused marked medial thickening and perivascular fibrosis in coronary microvessels, which were accompanied by ACE protein upregulation and cardiac oxidative stress. The treatment with amlodipine reduced SBP but failed to ameliorate the ADMA-induced histological changes. In contrast, these changes were abolished in transgenic mice, with a reduction in plasma ADMA. In coronary artery endothelial cells, ADMA activated p38 MAP kinase and the ADMA-induced ACE upregulation was suppressed by p38 MAP kinase inhibition by SB203580. In wild-type mice, long-term treatment with angiotensin II increased plasma ADMA and cardiac oxidative stress and caused similar vascular injury. In transgenic mice, these changes were attenuated. The present study suggests that DDAH2/ADMA regulates cardiac NO levels but has modest effect on SBP in normal conditions. Under the circumstances where plasma ADMA are elevated, including angiotensin II-activated conditions, ADMA serves to contribute to the development of vascular injury and increased cardiac oxidative stress, and the overexpression of DDAH2 attenuates these abnormalities. Collectively, the DDAH2/ADMA pathway can be a novel therapeutic target for vasculopathy in the ADMA or angiotensin II-induced pathophysiological conditions.     

Role of dimethylarginine dimethylaminohydrolase activity in regulation of tissue and plasma concentrations of asymmetric dimethylarginine in an animal model of prolonged critical illness

High plasma concentrations of asymmetric dimethylarginine (ADMA), an endogenous nitric oxide synthase inhibitor, are associated with adverse outcome in critically ill patients. Asymmetric dimethylarginine is released within cells during proteolysis of methylated proteins and is either degraded by dimethylarginine dimethylaminohydrolase (DDAH) or exported to the circulation via cationic amino acid transporters. We aimed to establish the role of DDAH activity in the regulation of tissue and plasma concentrations of ADMA. In 33 critically ill rabbits, we measured DDAH activity in kidney, liver, heart, and skeletal muscle and related these values to concentrations of ADMA in these tissues and in the circulation. Both DDAH activity and ADMA concentration were highest in kidney and lowest in skeletal muscle, with intermediate values for liver and heart. Whereas ADMA content was significantly correlated between tissues (r = 0.40-0.78), DDAH activity was not. Significant inverse associations between DDAH activity and ADMA content were only observed in heart and liver. Plasma ADMA was significantly associated with ADMA in the liver (r = 0.41), but not in the other tissues. In a multivariable regression model, DDAH activities in muscle, kidney, and liver, but not in heart, were negatively associated with plasma ADMA concentration, together explaining approximately 50% of its variation. In critical illness, plasma ADMA poorly reflects intracellular ADMA. Furthermore, tissue DDAH activity is a stronger predictor of plasma ADMA than of intracellular ADMA, indicating that, compared with DDAH activity, generation of ADMA and cationic amino acid transporter-mediated exchange may be more important regulators of intracellular ADMA.     

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.     

The DDAH/ADMA/NOS pathway

An increasing number of reports in the literature indicate that endogenously produced inhibitors of nitric oxide synthase (NOS), particularly asymmetric dimethylarginine (ADMA) regulate nitric oxide generation in numerous disease states. Two dimethylarginine dimethylaminohydrolase (DDAH) enzymes metabolise ADMA. We and others have postulated that activity of DDAH is a key determinant of ADMA levels in vivo. This review summarises recent advances in the regulation and function of DDAH enzymes and its role in the regulation of nitric oxide generation.     

ADMA regulates angiogenesis: genetic and metabolic evidence

Endothelium-derived nitric oxide (NO) plays an important role in transducing the effects of angiogenic factors. Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of NO synthase (NOS). We used a murine model of hindlimb ischemia to investigate whether genetic or metabolic changes in ADMA levels could impair angiogenic response in vivo. Hindlimb ischemia was surgically induced in C57BL/6J mice, apo E-deficient mice, or transgenic mice overexpressing dimethylarginine dimethylaminohydrolase (DDAH). Some animals were also treated with the NOS antagonist L-nitro-arginine, or the NO precursor L-arginine. Angiogenesis was quantified in the hindlimb skeletal muscle by capillary/myocyte ratio. Plasma or tissue ADMA levels were measured by HPLC. In normal mice, hindlimb ischemia increased tissue ADMA twofold, and reduced DDAH and NOS expression. This was associated with a reduced NOS activity (by over 80%) three days following surgery. On day seven, a threefold increase in DDAH expression and a fall in tissue ADMA levels were associated with a sevenfold increase in NOS activity, whereas NOS expression did not increase above baseline. In DDAH transgenic mice, the elevation of ADMA and decrement in NOS activity was blunted during hindlimb ischemia. Plasma ADMA levels were increased in apo E-mice (1.79 +/- 0.45 versus 1.07 +/- 0.08 pmol/l; p = 0.008). Capillary index was significantly reduced in apo E-mice up to seven weeks after surgery (0.25 +/- 0.05 versus 0.62 +/- 0.08; p < 0.001). The effect of hypercholesterolemia on capillary index was reversed by L-arginine, and (in wild-type mice) mimicked by administration of the NOS antagonist L-nitro-arginine. In conclusion, metabolic or genetic changes in plasma and tissue ADMA levels affect tissue NO production and angiogenic response to ischemia.     

非对称性二甲基精氨酸与糖尿病脑血管病变

一氧化氮(nitric oxide,NO)是已知最重要的内源性血管舒张因子.内皮型一氧化氮合酶(nitric oxide and enzyme,NOS)的竞争性抑制剂非对称性二甲基精氨酸(asymmetric dimethylarginine,ADMA)可抑制NO的合成,使NO/NOS通路发生障碍,NO合成减少.近年来的研究表明,ADMA与糖尿病脑血管病变的发生和发展有关,为进一步明确该病的发生机制提供了新的见解.     

Probucol Decreases Asymmetrical Dimethylarginine Level by Alternation of Protein Arginine Methyltransferase I and Dimethylarginine Dimethylaminohydrolase Activity

Hypothesis Asymmetric dimethylarginine (ADMA), an endogenous nitric oxide synthase inhibitor (NOS), may play an important role in endothelium dysfunction. Probucol, a potent antioxidant drug, may improve endothelium function via reduction of NOS inhibitor level. The present study examined whether the decreased level of ADMA by probucol is related to enhancement of protein arginine methyltransferase I (PRMT I) expression and reduction of dimethylarginine dimethylaminohydrolase (DDAH) activity. Methods Endothelial cells were cultured and used for all these studies. ADMA concentration and DDAH activity were determined by HPLC. Expression of PRMT I and eNOS were characterized by western blot. Results Pretreatment with oxidized-low density lipoprotein (ox-LDL) (10, 30 or 100 μg/ml) or lysophosphatidylcholine (LPC) (1.0, 2.5 or 5.0 μg/ml) for 12, 24 or 48 h markedly increased the activity of lactate dehydrogenase (LDH) in cultured endothelial cell. Incubation ofendothelial cells with ox-LDL (100 μg/ml) or LPC (5.0 μg/ml) for 48 h significantly increased the expression of PRMT I, and levels of MDA and ADMA, and decreased the concentration of nitrite/nitrate, the expression of eNOS and the activity of DDAH. Probucol significantly decreased the level of ADMA, concomitantly with reduction of PRMT I expression and elevation of DDAH activity and up-regulation of eNOS expression. Conclusion In summary, the present results suggest that the protective effect of probucol on endothelium is related to reduction of ADMA concentration by inhibition of PRMT I expression and enhancement of DDAH activity.     

Biological significance of endogenous methylarginines that inhibit nitric oxide synthases

The guanidino-methylated arginine analogue NG monomethyl-L-arginine (L-NMMA) has been the standard nitric oxide synthase inhibitor used to evaluate the role of the L-arginine:nitric oxide pathway. However, L-NMMA and other methylated arginine residues are also synthesised in vivo by the action of a family of enzymes known as protein arginine methyltransferases. Proteolysis of proteins containing methylated arginine residues releases free methylarginine residues into the cytosol from where they may pass out of the cell into plasma. Of the three known methylarginine residues produced in mammals only asymmetrically methylated forms (L-NMMA and asymmetric dimethylarginine (ADMA)) but not symmetrically methylated arginine (symmetric dimethylarginine (SDMA)) inhibit nitric oxide synthase (NOS). We and others have proposed that endogenously produced asymmetrically methylated arginines may modulate NO production and that the accumulation of these residues in disease states may contribute to pathology. The activity of the enzyme dimethylarginine dimethylaminohydrolase that metabolises asymmetric methylarginines may be of critical importance in affecting NO pathways in health or disease.     

ADMA/DDAH系统与脑白质疏松症的关系

脑白质疏松症(LA)是深部脑小血管病变(SVD)引起的弥漫性脑缺血,可导致脑白质区神经传导纤维脱髓鞘疾病,是脑损害的一个早期标志。一氧化氮(NO)是血管内皮舒张因子,由体内的一氧化氮合酶(NOS)催化产生。非对称性二甲基精氨酸(ADMA)是NOS的内源性竞争抑制物。ADMA增加使NO生成减少,导致血管内皮功能障碍。二甲基精氨酸二甲胺水解酶(DDAH)是内源性ADMA的主要代谢酶,是决定血浆ADMA浓度的关键因素。ADMA/DDAH通路可能通过血管内皮损伤机制影响LA。     

Cardiovascular biology of the asymmetric dimethylarginine:dimethylarginine dimethylaminohydrolase pathway

An increasing number of reports indicate that endogenously produced inhibitors of nitric oxide synthase, particularly asymmetric dimethylarginine (ADMA), regulate nitric oxide generation in disease states. This article describes the biology of ADMA and the implications for cardiovascular physiology and pathophysiology.     

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.