Determination of methylated amino acids in human serum by high-performance liquid chromatography

ESI-MS/MS analysis of protonated N-methyl amino acids and their immonium ions

Methylation is one of the important posttranslational modifications of biological systems. At the metabolite level, the methylation process is expected to convert bioactive compounds such as amino acids, fatty acids, lipids, sugars, and other organic acids into their methylated forms. A few of the methylated amino acids are identified and have been proved as potential biomarkers for several metabolic disorders by using mass spectrometry-based metabolomics workstation. As it is possible to encounter all the N-methyl forms of the proteinogenic amino acids in plant/biological systems, it is essential to have analytical data of all N-methyl amino acids for their detection and identification. In earlier studies, we have reported the ESI-MS/MS data of all methylated proteinogenic amino acids, except that of mono-N-methyl amino acids. In this study, the N-methyl amino acids of all the amino acids (1-21; including one isomeric pair) were synthesized and characterized by ESI-MS/MS, LC/MS/MS, and HRMS. These data could be useful for detection and identification of N-methyl amino acids in biological systems for future metabolomics studies. The MS/MS spectra of [M + H]⁺ ions of most N-methyl amino acids showed respective immonium ions by the loss of (H₂ O, CO). The other most common product ions detected were [MH-(NH₂ CH₃ ]⁺ , [MH-(RH)]⁺ (where R = side chain group) ions, and the selective structure indicative product ions due to side chain and N-methyl group. The isomeric/isobaric N-methyl amino acids could easily be differentiated by their distinct MS/MS spectra. Further, the MS/MS of immonium ions inferred side chain structure and methyl group on α-nitrogen of the N-methyl amino acids.

Quantification of arginine and dimethylated arginines in human plasma by field-amplified sample injection capillary electrophoresis UV detection

We report a capillary electrophoresis method by field-amplified sample injection (FASI) for the separation and detection of free plasma arginine and dimethylated arginines. Protein elimination from plasma was performed by acetonitrile/ammonia precipitation. Supernatants were dried, reswollen in water, and directly injected on capillary without complex cleanup by solid-phase extraction and/or tedious sample derivatization procedures. Analytes were baseline-separated within 22 min by using 50 mmol/L Tris phosphate pH 2.3 as running buffer. Due to the stacking effects of electrokinetic injection, it is possible to perform a considerable on-line preconcentration of the analytes before running the electrophoresis. This procedure allows to reach a detection limit in real sample of 10 nmol/L for dimethylated arginines and 20 nmol/L for arginine. Recovery of plasma ADMA was 99-104 % and inter-day CV was less than 3 %.

Improved method for plasma ADMA, SDMA, and arginine quantification by field-amplified sample injection capillary electrophoresis UV detection

Here, we describe an easy field-amplified sample injection capillary electrophoresis method with UV detection for the separation and detection of free plasma arginine and dimethylated arginines. The analytes were baseline-separated within 22 min by using 50 mmol/L Tris phosphate pH 2.3 as running buffer. The plasma samples were treated with acetonitrile/ammonia for protein elimination, the supernatants were dried, re-swollen in water and directly injected in the capillary without complex cleanup by solid phase extraction and/or tedious sample derivatization procedures. Due to the stacking effects of the electrokinetic injection, it was possible to operate a consistent on-line pre-concentration of the analytes before running the electrophoresis. This procedure allowed to reach a detection limit in the real sample of 10 nmol/L for dimethylated arginines and 20 nmol/L for arginine, thus improving about threefold our previous method, that required a more complicated pre-analytical procedure to concentrate samples. The recovery of plasma ADMA was 99-104% and inter-day CV was less than 3%. The assay performance was evaluated measuring the levels of arginine and its dimethyl derivatives in 50 subjects. The statistical tests for the methods comparison suggest that the data obtained by our new method and by our previous CE assay are similar.

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.

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.

Capillary electrophoretic and micellar electrokinetic separations of asymmetric dimethyl-L-arginine and structurally related amino acids: Quantitation in human plasma

We report the development of efficient electrophoretic methods for the separation and quantification of L-arginine and six naturally occurring derivatives that are structurally and functionally related. Capillary electrophoresis (CE) employing a concentrated borate buffer at pH 9.4 achieves the separation of mixtures containing dimethyl-L-arginine, NG-monomethyl-L-arginine, L-arginine, L-homoarginine, L-ornithine, and L-citrulline as 4-fluoro-7-nitrobenzofurazan derivatives. In addition, the separation of the isomeric dimethyl-L-arginine derivatives (symmetric and asymmetric) is attained with baseline resolution by micellar electrokinetic chromatography (MEKC) when a high concentration of deoxycholic acid is added as a surfactant to the same running buffer. The influence of buffer type, concentration, and pH on the separation was studied to optimize separation conditions. The limit of quantitation (LOQ) for asymmetric dimethyl-L-arginine in aqueous solution was determined to be 20 microM using UV absorption in a CE separation and 0.1 microM using laser induced fluorescence (LIF) detection in an MEKC separation. This newly developed method was successfully applied for the quantitation of asymmetric dimethyl-L-arginine and L-arginine in human plasma samples at levels that might be used as a clinical diagnostic for cardiovascular disease (0.125 microM LOQ).

Endothelium-dependent vasorelaxation and the expression of calcitonin gene-related peptide in aged rats

To explore the age-related change in endothelium-dependent vasorelaxation, plasma concentrations and the expression of calcitonin gene-related peptide (CGRP) as well as serum concentrations of asymmetry dimethylarginine (ADMA), male Sprague-Dawley rats, aged 6 and 24 months were tested. The level of ADMA and CGRP in blood, and CGRP mRNA in dorsal root ganglia (DRG) were determined by high-performance liquid chromatography (HPLC), radioimmunoassay or semi-quantitative reverse-transcription polymerase chain reaction, respectively. Vasodilator responses to cumulative concentrations of acetylcholine (ACh) (3 x 10(-9) - 3 x 10(-7)M) and CGRP (3 x 10(-10) - 3 x 10(-8)M) were tested at the plateau of contraction in aortas rings. Vasodilator responses to ACh or CGRP at 24 months of age were decreased compared with 6 months. The serum concentration of ADMA at 24 months of age was significantly increased compared with 6 months, while both plasma concentrations of CGRP and the expressions of alpha- and beta-CGRP mRNA were unchanged at two age groups. The results suggest that endothelium-dependent relaxation, but not concentrations and the expressions of CGRP, is decreased in aged rats, and the decrease in endothelium dependence of vasodilator response to CGRP may be related to the elevation of endogenous ADMA.

Carnitine biosynthesis in mammals

Carnitine is indispensable for energy metabolism, since it enables activated fatty acids to enter the mitochondria, where they are broken down via beta-oxidation. Carnitine is probably present in all animal species, and in numerous micro-organisms and plants. In mammals, carnitine homoeostasis is maintained by endogenous synthesis, absorption from dietary sources and efficient tubular reabsorption by the kidney. This review aims to cover the current knowledge of the enzymological, molecular, metabolic and regulatory aspects of mammalian carnitine biosynthesis, with an emphasis on the human and rat.

Carnitine biosynthesis in mammals

Carnitine is indispensable for energy metabolism, since it enables activated fatty acids to enter the mitochondria, where they are broken down via beta-oxidation. Carnitine is probably present in all animal species, and in numerous micro-organisms and plants. In mammals, carnitine homoeostasis is maintained by endogenous synthesis, absorption from dietary sources and efficient tubular reabsorption by the kidney. This review aims to cover the current knowledge of the enzymological, molecular, metabolic and regulatory aspects of mammalian carnitine biosynthesis, with an emphasis on the human and rat.

Hyperhomocysteinemia impairs angiogenesis in response to hindlimb ischemia

Hyperhomocysteinemia (HH) is an independent risk factor for atherosclerosis, including peripheral arterial occlusive disease (PAOD). Because angiogenesis and collateral vessel formation are important self-salvage mechanisms for ischemic PAOD, we examined whether HH modulates angiogenesis in vivo in a rat model of hindlimb ischemia. Rats were divided into 3 groups: the control group was given tap water, the HH group was given water containing L-methionine (1 g x kg(-1) x d(-1)), and the HH+L-arg group was given water containing methionine (1 g x kg(-1) x d(-1)) and l-arginine (2.25 vol%). At day 14 of the dietary modifications, the left femoral artery and vein were excised, and the extent of angiogenesis and collateral vessels in the ischemic limb were examined for 4 weeks. Plasma homocysteine levels significantly increased (P:<0.001), and plasma and tissue contents of nitrite+nitrate as well as tissue cGMP levels significantly decreased in the HH group compared with the control group (P:<0.01). Laser Doppler blood flowmetry (LDBF) revealed a significant decrease in the ischemic/normal limb LDBF ratio in the HH group at days 7, 14, 21, and 28 (P:<0.01 versus control). Angiography revealed a significant decrease in the angiographic score in the HH group at day 14 (P:<0.001 versus control). Immunohistochemistry of ischemic tissue sections showed a significant reduction in the capillary density in the HH group (P:<0. 001 versus control). Oral l-arginine supplementation in rats with HH (HH+L-arg) restored the decreased plasma and tissue nitrite+nitrate and cGMP contents (P:<0.05) as well as angiogenesis, as assessed by LDBF (P:<0.05 versus HH), angiographic score (P:<0.01 versus HH), and capillary density (P:<0.001 versus HH). In summary, HH impaired ischemia-induced angiogenesis and collateral vessel formation in a rat model of hindlimb ischemia in vivo. The mechanism of the HH-induced impairment of angiogenesis might be mediated in part by a reduced bioactivity of endogenous NO in the HH state.

Determination of arginine and methylated arginines in human plasma by liquid chromatography-tandem mass spectrometry

Nitric oxide (NO) is synthesized from L-arginine (ARG) catalyzed by the enzyme nitric oxide synthase (NOS) and is important in the regulation of vascular tone, neurotransmission and host defense. N,N-Dimethyl L-arginine (asymmetric dimethylarginine, ADMA) and N-monomethyl L-arginine (MMA) are endogenous inhibitors of NOS. N,N'-Dimethyl L-arginine (symmetric dimethylarginine, SDMA), the inactive enantiomer of ADMA is also known to be present endogenously. A simple, sensitive and fast LC-MS-MS method was developed to extract and quantitate ADMA, SDMA, MMA and ARG from human plasma. 13C6-ARG was used as the internal standard for the assay. Protein precipitation using acetonitrile gave good recoveries of all the compounds from plasma. The compounds were separated by HPLC in less than 15 min using a silica column. The limits of detection for this method were found to be approximately 1 ng/ml for ARG, ADMA and SDMA and 2.5 ng/ml for MMA. The total LC-MS-MS analysis time is less than 15 min making this the fastest and most specific method reported to date. The use of an isocratic liquid chromatographic separation makes this method optimal for high sample throughput. The inter- and intra-day precision (% RSD) and accuracy (% error) for this assay were less than 15%. The average concentrations of ARG, ADMA, SDMA and MMA in plasma from 20 human subjects were found to be 10.9+/-4.1 microg/ml, 25.1+/-9.4 ng/ml, 33.2+/-13.1 ng/ml and 19.6+/-3.8 ng/ml, respectively.

Determination of asymmetrical dimethylarginine by capillary electrophoresis-laser-induced fluorescence

Asymmetric dimethyl-L-arginine (ADMA) is a naturally occurring analogue of L-arginine (L-Arg), the substrate of nitric oxide synthase (NOS). ADMA is a potent endogenous inhibitor of NOS and accumulates in the plasma of patients with renal failure, with peripheral arterial occlusive disease or with clinically asymptomatic hypercholesterolemia. We measured circulating concentrations of L-arginine, symmetric and asymmetric dimethylarginine (SDMA and ADMA, respectively) in human serum. We developed a new method for the rapid determination of these molecules using capillary electrophoresis and laser-induced fluorescence (CE-LIF). All methylated arginines were labeled with fluorescein isothiocyanate (FITC) prior to analysis. Under the capillary electrophoresis (CE) conditions used, methylated arginine derivatives were well separated, with a migration time of around 10 min. These migration times were smaller than the ones of other amino acids which do not have the same charge at pH 10. Consequently, such basic amino acids were well separated from most of the other amines or amino acids. Moreover, CE allowed one to separate all the analogues of fluorescein thiocarbamyl-arginine. The results indicated that CE-LIF is useful as a selective, rapid, cheap and sensitive tool for the determination of methylated arginine products. This new technology might appreciate the endogenous substrate for NO synthase and facilitate the knowledge of the physiological and pathophysiological regulation of NO synthesis.

Nonlinear pharmacokinetics of L-N(G)-methyl-arginine in rats: characterization by an improved HPLC assay

L-N(G)-methyl-arginine (L-NMMA) is an inhibitor of nitric oxide synthase (NOS) enzymes. We have characterized the pharmacokinetics of L-NMMA in rats using HPLC. The HPLC assay requires pre-column derivatization, gradient elution and ultraviolet detection. The limit of sensitivity in plasma was 3.0 microM (0.75 microg mL(-1)). Using this assay, the pharmacokinetics of L-NMMA were characterized following iv bolus doses of 25, 50 and 100 mg kg(-1). Compartmental and noncompartmental data analysis suggest that L-NMMA pharmacokinetics are nonlinear at these doses. From the nonlinear compartmental analysis, we estimated the K(m) and V(max) parameters of L-NMMA elimination to be 70.2 microM and 4.59 microM min(-1), respectively. This estimated K(m) value of L-NMMA elimination is consistent with its nonlinear elimination characteristics in humans and its saturable metabolism by the N(G), N(G)-dimethylarginine dimethylamino-hydrolase enzyme in isolated rat tissue.

Regulation of macrophage nitric oxide synthesis by endothelial cells: a role for NG,NG-dimethylarginine

NG,NG-dimethylarginine is an endogenous inhibitor of nitric oxide synthesis produced by endothelial cells and found in the plasma and urine of normal adults. We have examined the ability of NG, NG-dimethylarginine, produced by endothelial cells (SGHEC-7), to regulate the production of nitric oxide by lipopolysaccharide-stimulated mouse macrophage cells (J774.2). Stimulation of SGHEC-7 or J774.2 cells with lipopolysaccharide had no effect on their release of NG,NG-dimethylarginine into the culture supernatant. Stimulation of J774.2 cells with lipopolysaccharide for 24 h significantly stimulated nitric oxide production by J774.2 but not SGHEC-7 cells. When lipopolysaccharide-stimulated J774.2 cells were co-cultured with endothelial cells for 24 h, there was a significant inhibition of nitrite accumulation. The inhibition observed was dependent on the endothelial cell number (12 +/- 5% [mean +/- SEM] following incubation with 0.6 x 105 cells, up to 47 +/- 8% with 4.8 x 105 cells). The inhibitory effect of endothelial cells was prevented by incubation with increasing concentrations of L-arginine; the IC50 was 2.9 +/- 0.6 mM arginine. Western blot analysis indicated that the expression of inducible nitric oxide synthase was not inhibited by co-culture with SGHEC-7 cells. The results presented here demonstrate that NG,NG-dimethylarginine synthesized by endothelial cells may inhibit nitric oxide synthase in adjacent cells and play a role in the regulation of nitric oxide synthesis by macrophages.

Method for identification and quantitative analysis of protein lysine methylation using matrix-assisted laser desorption/ionization--time-of-flight mass spectrometry and amino acid analysis

Protein methylation is a post-translational modification that might have important functional roles in cell regulation. We present a new technique with sufficient sensitivity (sub-pmol level) for analysis of methylation of proteins in abundances typically found on proteome maps produced by two-dimensional (2-D) gel electrophoresis. The method involves the identification and quantitation of lysine (Lys) methylation using Fmoc (9-fluorenylmethyl chloroformate)-based amino acid analysis (AAA). Tri- and monomethyl-Lys were baseline-separated from other amino acids using a modified buffer system. Trimethyl-Lys was quantitatively recovered after acid hydrolysis and AAA of two known methylated proteins - yeast cytochome c and human calmodulin. The methylated peptides from tryptic digestion of those two proteins were identified by high sensitivity matrix-assisted laser desorption/ionization - time-of-flight (MALDI-TOF) mass spectrometry (MS). An automated mass-screening approach is proposed for the study of various post-translational modifications to understand the distribution of those protein isoforms separated by two-dimensional polyacrylamide gel electrophoresis. It is concluded that the combination of AAA and MALDI-TOF-MS provides a high sensitivity quantitative tool for the analysis of protein post-translational methylation in the context of proteome studies.

Pharmacokinetics of the nitric oxide synthase inhibitor L-NG-methylarginine hydrochloride in patients with septic shock. Glaxo Wellcome International Septic Shock Study Group

OBJECTIVES

To characterize the pharmacokinetics of L-NG-methylarginine in patients with septic shock.

METHODS

This was an international, uncontrolled, open-label study of L-NG-methylarginine (546C88) therapy given to 32 patients with septic shock. It was conducted in hospital-based intensive care units that admit general surgical and medical patients. Patient cohorts received an infusion of L-NG-methylarginine at fixed dose rates of 1, 2.5, 5, 10, and 20 mg/kg/h for up to 8 hours. The 5 dosing regimens were administered sequentially to separate groups of patients.

RESULTS

Of the 32 patients studied, 23 received complete 8-hour infusions. In the other 9 patients, the infusion was terminated prematurely within the first 1/2 to 4 hours. Median clearance of L-NG-methylarginine averaged 485 mL/h/kg for the 1 and 2.5 mg/kg/h dosing cohorts combined but decreased to 283, 181, and 98 mL/h/kg for the 5, 10, and 20 mg/kg/h dosing cohorts, respectively. Median renal clearance was similar at 9 to 26 mL/h for the 1, 2.5, and 5 mg/kg/h dosing cohorts but increased to 156 and 284 mL/h for the 10 and 20 mg/kg/h dosing cohorts, respectively. Median steady-state volume of distribution was similar in all 5 dosing cohorts, averaging 0.66 to 0.82 L/kg.

CONCLUSIONS

The 80% decrease in clearance from 485 to 98 mL/h/kg with the increase in dose suggests that a predominant metabolic pathway(s) of L-NG-methylarginine, accounting for at least 80% of clearance, is becoming progressively saturable in association with L-NG-methylarginine infusion rates > or = 5 mg/kg/h. Therefore the use of L-NG-methylarginine infusion rates > or = 5 mg/kg/h are typically expected to result in progressive inhibition of nitric oxide synthase activity. Consequently, patient hemodynamics should be monitored closely to avoid an excessive increase in vasomotor tone, which would be manifest by either an increase in mean arterial pressure or a decrease in cardiac output. The infusion rates of conventional vasopressor(s) (eg, norepinephrine [BAN, noradrenaline]) or L-NG-methylarginine or both may need to be reduced accordingly.

Nitric oxide synthase inhibitors and hypertension in children and adolescents

OBJECTIVE

To establish the role played by the circulating nitric oxide synthase inhibitors N(G)-monomethyl-L-arginine (L-NMMA), asymmetrical dimethyl arginine (ADMA) and symmetric dimethyl arginine (SDMA) and its association with hypertension of children and adolescents.

DESIGN

We measured plasma concentrations of L-NMMA, ADMA and SDMA in 38 hypertensives (median age 7.7 years) and in nine healthy normotensive controls (median age 8.2 years) using high-performance liquid chromatography. In addition, their plasma renin activity was determined. The subjects' glomerular filtration rates were calculated from plasma creatinine and height measurements. To determine the vasoactive potency of the arginine analogues, concentration-response curves were plotted for the responses in isolated endothelium-intact and endothelium-denuded mouse aortic rings that had been pre-contracted by administration of a threshold concentration of phenylephrine.

RESULTS

Plasma ADMA and SDMA concentrations in members of the hypertensive group [0.23 +/- 0.03 and 1.37 +/- 0.06 micromol/l, respectively (means +/- SEM)] were significantly higher than those in members of the control group (ADMA 0.10 +/- 0.01 micromol/l and SDMA 1.18 +/- 0.06 micromol/l). Plasma concentrations of L-NMMA were similar in members of the hypertensive (0.21 +/- 0.01 micromol/l) and control (0.18 +/- 0.02 micromol/l) groups. The glomerular filtration rate of the hypertensive group was below normal [70.4 +/- 5.4 ml/min per 1.73 m2 (mean +/- SEM)] and was significantly associated with elevated plasma concentrations of ADMA (r = -0.77, P < 0.001), SDMA (r = -0.38, P = 0.02) and L-NMMA (r = 0.35, P = 0.03). Higher plasma ADMA concentrations were associated with a lower plasma renin activity (r = -0.36, P = 0.04). The vasoactive potencies of ADMA (concentration for half-maximal effect with the endothelium intact 25.4 +/- 7.1 micromol/l) and L-NMMA (concentration for half-maximal effect with the endothelium intact 8.2 +/- 2.9 micromol/l) was significantly (P < 0.05) greater than that of SDMA. Both ADMA and L-NMMA (at 3 micromol/l concentrations) initiated a significant vasocontractile response from baseline (P = 0.03 and P < 0.001, respectively). These effects were absent after the endothelium had been removed. SDMA had no effect.

CONCLUSIONS

Plasma ADMA and SDMA levels are increased in hypertensive children. By inference from in-vitro data, ADMA appears to attain sufficient concentrations to produce a significant change in vascular tone and hence might play a role in the pathophysiology of childhood hypertension.

Determination of dimethylated arginines in human plasma by high-performance liquid chromatography

Using high-performance liquid chromatography (HPLC) with multigradient elution, N(G),N(G)-dimethyl-L-arginine (asymmetric-DMA, ADMA) and N(G),N'(G)-dimethyl-L-arginine (symmetric-DMA, SDMA) can be separated from human plasma samples. The dimethylarginine compounds in plasma, after extraction with a cation-exchange column, are converted to fluorescent derivatives with o-phthaldialdehyde (OPA) in an alkaline medium and the derivatives are separated simultaneously within 50 min on a reversed-phase column (Ultracarb 3 ODS(20)). The recoveries of ADMA and SDMA are over 80% and the method permits quantitative determination of dimethylated arginines at concentrations as low as 0.1 micromol/l in human plasma.

Determination of N(G),N(G)-dimethylarginine in human plasma by high-performance liquid chromatography

N(G),N(G)-Dimethylarginine (asymmetric dimethylarginine, ADMA) can be directly separated and measured from deproteinized human plasma using o-phthaldialdehyde-mercaptoethanol (OPA reagent) as a fluorogenic reagent by reversed-phase high-performance liquid chromatography. The mean recovery of ADMA was over 96% and the inter- and intra-assay coefficients of variation of amounts were lower than 3.80% and those of retention time were below 0.37% for five runs. The detection limit of the assay is 1 pmol when the signal-to-noise ratio is 3:1. It was observed that the concentration of ADMA was significantly elevated in plasma of patients with pregnancy induced hypertension (PIH) in contrast to healthy pregnant women.

High-performance liquid chromatographic determination of nitric oxide synthase-related arginine derivatives in vitro and in vivo

In this paper we present a sensitive and reproducible method for the extraction and quantification of the nitric oxide (NO) synthase (NOS)-related basic amino acids L-hydroxyarginine (L-NHA), L-arginine (L-Arg), L-monomethylarginine (L-NMA), and L-dimethylarginine (L-NDA) in human serum samples by high-performance liquid chromatography (HPLC) analysis. We demonstrate that the serum level of L-NHA can be used as a sensitive and highly specific index of a systemic increase in NOS activity in vivo whose serum concentration, unlike that of the NO degradation products nitrite and/or nitrate, is not influenced by dietary intake. First, we measured L-NHA formation by a recombinant NOS preparation and by lipopolysaccharide-stimulated alveolar macrophages to demonstrate that this amino acid is produced by NOS in vitro. HPLC determination of L-NHA in human serum, however, proved to be difficult due to the presence of amino acids interfering with its detection. Therefore, we developed a clean-up procedure for the extraction of basic amino acids from these serum samples by using a cation-exchange cartridge. The isolated amino acids were subjected to precolumn derivatization with o-pthaldialdehyde and analyzed using a short reversed-phase column which allowed the baseline separation of L-NHA, L-Arg, L-NMA, and L-NDA within 16 min. By using this technique, the average concentrations of L-NHA, L-Arg, L-NMA, and L-NDA in the serum of healthy human subjects were determined to be 9.1, 96.1, 0.1, and 0.4 microM, respectively.

Recent insights into the regulation of cerebral circulation

1. Mechanisms that regulate the cerebral circulation have been intensively investigated in recent years. The role of several vasodilator mechanisms has been examined in the cerebral circulation, including nitric oxide (NO), trigeminal peptides and potassium channels, as well as the potent vasoconstrictor endothelin. These mediators appear to play a role in physiological and pathophysiological responses of the cerebral circulation. In the present review, we will focus on some recent developments in each of these areas. 2. Nitric oxide is an important regulator of cerebral vascular tone. Tonic production of NO maintains the cerebral vasculature in a dilated state. NO appears to be an important vasodilator during activation of neurons by excitatory amino acids, somatosensory stimulation and cortical spreading depression. Tonic production of NO appears to be critical in vasodilatation during hypercapnia, although NO may not directly mediate vasodilatation. NO produced by immunological NO-synthase appears to be important in dilatation following exposure to bacterial endotoxin. 3. Calcitonin gene-related peptide (CGRP), released from trigeminal perivascular sensory nerves in the brain, is an extremely potent dilator of brain vessels. CGRP may limit noradrenaline-induced constriction of cerebral vessels and contribute to dilatation during hypotension (autoregulation), reactive hyperaemia, seizures and cortical spreading depression. 4. Activation of potassium channels leads to hyperpolarization of cerebral vascular smooth muscle and appears to be a major mechanism for dilatation of cerebral arteries. Agents that increase the intracellular concentration of cyclic 3' 5'-adenosine monophosphate (cAMP) produce vasodilatation in part by activation of large conductance calcium-activated potassium channels (BKCa) and ATP-sensitive potassium channels (KATP). Activation of both KATP and BKCa channels also appears to contribute to vasodilatation during hypoxia. In contrast to KATP channels, BKCa channels appears to be active under basal conditions, contributing to tonic dilatation of cerebral blood vessels. 5. Endothelin is produced in the brain, but its role in the physiological regulation of cerebral blood flow is not known. Endothelin may contribute to the spasm of cerebral arteries following subarachnoid haemorrhage.

Determination of NG-nitro-L-arginine and NG-nitro-L-arginine methyl ester in plasma by high-performance liquid chromatography

An HPLC method has been developed for the measurement of the nitric oxide synthase inhibitors, NG-nitro-L-arginine (L-NOLA) and NG-nitro-L-arginine methyl ester (L-NAME), in sheep plasma. Using an ion-exchange HPLC column (JWAS 150, 100 x 3.9 mm I.D., Millipore-Waters, Australia) and post-column ninhydrin detection, L-NOLA was separated from valine and other plasma amino acids. When added to sheep plasma, good recovery (mean 102%) and precision (mean coefficient of variation 2.7%) in the measurement of L-NOLA was obtained over the range 2-50 mg/l. L-NAME was unstable in sheep plasma at 37 degrees C, and was converted to L-NOLA with a half-life of 250 min. This method will permit pharmacokinetic parameters to be determined for these potential drugs, and will allow plasma drug concentrations to be correlated with the pharmacodynamic effects of these compounds.

Metabolism of methylarginines by human vasculature; implications for the regulation of nitric oxide synthesis

1. The metabolism of methylarginines by human cultured endothelial cells and human saphenous vein was studied in vitro. The human endothelial cell line (SGHEC-7), primary cultures of human umbilical vein endothelial cells (HUVEC) and human saphenous vein were incubated with [14C]-monomethyl-L-arginine ([14C]-L-NMMA) and the cytosolic extract analysed by high performance liquid chromatography (h.p.l.c.) with on-line radioisotope detection. 2. SGHEC-7, HUVEC and human saphenous vein metabolized [14C]-L-NMMA to a compound which co-eluted with [14C]-citrulline. A second metabolite which co-eluted with [14C]-arginine was evident on the radiochromatograms of HUVEC cytosol and saphenous vein extracts. 3. The intracellular levels of [14C]-L-NMMA and [14C]-citrulline in SGHEC-7 cells incubated with [14C]-L-NMMA (0.5 microCi ml-1: 8.9 microM) for 1 h were 113 +/- 22 and 67.6 +/- 6.2 pmol mg-1 cell protein respectively (n = 7). Co-incubation with NGNGdimethyl-L-arginine (ADMA; 100 microM) but not NGNGdimethyl-L-arginine (SDMA; 100 microM) reduced the intracellular level of [14C]-citrulline to 26.3 +/- 3.7 pmol mg-1 cell protein (P < 0.01; n = 3) without reducing the intracellular level of [14C]-L-NMMA. 4. The intracellular levels of [14C]-citrulline in SGHEC-7 cells incubated with [14C]-L-NMMA for 1 h were reduced following co-incubation with NGnitro-L-arginine methylester (L-NAME; 1 mM), NGnitro-L-arginine (L-NOARG; 1 mM) and L-canavanine (1 mM) to 47.1 +/- 6.2, 24.7 +/- 3.6 and 12.5 +/- 2.8% of control levels (P < 0.001; n = 9). ADMA (1 mM; n = 3) reduced intracellular [14C]-citrulline levels to4 +/- 4% of control (P<0.01) but SDMA (1 mM; n = 3) had no effect.5. The accumulation of endogenously synthesized ADMA in the culture supernatant of SGHEC-7 cells was increased by co-incubation with L-NMMA (1 mM) from 1.98 +/- 0.08 to 2.74 +/- 0.36 nmol mg- cell protein, an increase of 40%.6. These results demonstrate that human vasculature possesses an enzyme which has similar properties to dimethylarginase; human endothelial cells and human saphenous vein metabolize L-NMMA to citrulline via a process inhibited by ADMA but not SDMA. The increase in endothelium-derivedADMA following co-incubation with L-NMMA is consistent with competition between ADMA and L-NMMA for dimethylarginase. Inhibition of this enzyme might increase the intracellular concentration of ADMA, an endogenously produced compound that inhibits nitric oxide synthesis.

Increase of an endogenous inhibitor of nitric oxide synthesis in serum of high cholesterol fed rabbits

In the present study, the concentration of NG,NG-dimethylarginine (DMA), an endogenous inhibitor of nitric oxide synthesis, was measured with high-performance liquid chromatography, and the effect of hypercholesterolemia on DMA level was investigated in the high fat, high cholesterol fed rabbit. After 6 weeks on a high fat, high cholesterol diet, serum total cholesterol, triglycerides, and lipid peroxides were increased, and atherosclerosis was shown by means of morphological examination. In these rabbits with atherosclerosis, serum level of DMA was significantly raised, while serum creatine level remained normal. This study suggests that chronic hypercholesteremia may stimulate DMA production through elevation of lipid peroxides. The present results implicate endogenous DMA as a contributor to the development of atherosclerosis.

Accumulation of an endogenous inhibitor of nitric oxide synthesis in chronic renal failure

Nitric oxide (NO), synthesised from L-arginine, contributes to the regulation of blood pressure and to host defence. We describe in-vitro and in-vivo evidence that NO synthesis can be inhibited by an endogenous compound, NG,NG-dimethylarginine (asymmetrical dimethylarginine, ADMA). In man, this inhibitor is found in plasma and more than 10 mg is excreted in urine over 24 h. However, in patients with end-stage chronic renal failure, who have little or no urine output, elimination is blocked and circulating concentrations of the inhibitor rise sufficiently to inhibit NO synthesis. Accumulation of endogenous ADMA, leading to impaired NO synthesis, might contribute to the hypertension and immune dysfunction associated with chronic renal failure.

Substrate specificity for myelin basic protein-specific protein methylase I

The substrate specificity of bovine brain myelin basic protein (MBP)-specific protein methylase I (S-adenosyl-L-methionine:protein-L-arginine N-methyltransferase, EC 2.1.1.23), which methylates arginine residues of protein, has been studied using various MBPs, several synthetic peptides and heterogeneous nuclear ribonucleoprotein complex protein (hnRNP). (1) Among MBPs from different species of brain, the carp MBP was found to be the best substrate for MBP-specific protein methylase I. This high degree of methyl acceptability is most likely due to the fact that carp MBP is not in vivo methylated at the arginine residue (Deibler, G.E. and Martenson, R.E. (1973) J. Biol. Chem. 248, 2387-2391) and that the methylatable amino acid sequence is present in this protein. (2) In order to study the minimum chain length of MBP polypeptide which functions as the methyl acceptor, several synthetic polypeptides whose sequences are identical to the region surrounding the residue 107 of bovine MBP (the in vivo methylation site) were synthesized. It was found that the hexapeptide, Gly-Lys-Gly-Arg-Gly-Leu (corresponding to residues 104-109 of bovine MBP), was the shortest methyl accepting peptide, while the tetrapeptide, Gly-Arg-Gly-Leu (corresponding to residues 106-109) was inactive as a substrate. (3) hnRNP protein is known to contain methylarginine at residue 193 (Williams, K.R., Stone, K.L., LoPresti, M.B., Merrill, B.M. and Plank, S.R. (1985) Proc. Natl. Acad. Sci. USA 82, 5666-5670) which is post-translationally modified. Thus, the RNP protein overproduced in Escherichia coli and therefore did not contain methylarginine was examined for its methyl acceptability. It was found that neither MBP-specific nor histone-specific protein methylase I could methylate this methylarginine-less RNP protein. This suggests a possible existence of a distinct protein methylase I specific for this nuclear protein.