Methylated amino acid residues of proteins of brain and other organs

Functional Implications of Protein Arginine Methyltransferases (PRMTs) in Neurodegenerative Diseases

During the aging of the global population, the prevalence of neurodegenerative diseases will be continuously growing. Although each disorder is characterized by disease-specific protein accumulations, several common pathophysiological mechanisms encompassing both genetic and environmental factors have been detected. Among them, protein arginine methyltransferases (PRMTs), which catalyze the methylation of arginine of various substrates, have been revealed to regulate several cellular mechanisms, including neuronal cell survival and excitability, axonal transport, synaptic maturation, and myelination. Emerging evidence highlights their critical involvement in the pathophysiology of neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), frontotemporal dementia-amyotrophic lateral sclerosis (FTD-ALS) spectrum, Huntington's disease (HD), spinal muscular atrophy (SMA) and spinal and bulbar muscular atrophy (SBMA). Underlying mechanisms include the regulation of gene transcription and RNA splicing, as well as their implication in various signaling pathways related to oxidative stress responses, apoptosis, neuroinflammation, vacuole degeneration, abnormal protein accumulation and neurotransmission. The targeting of PRMTs is a therapeutic approach initially developed against various forms of cancer but currently presents a novel potential strategy for neurodegenerative diseases. In this review, we discuss the accumulating evidence on the role of PRMTs in the pathophysiology of neurodegenerative diseases, enlightening their pathogenesis and stimulating future research.

Protein Arginine Methyltransferases in Neuromuscular Function and Diseases

Neuromuscular diseases (NMDs) are characterized by progressive loss of muscle mass and strength that leads to impaired body movement. It not only severely diminishes the quality of life of the patients, but also subjects them to increased risk of secondary medical conditions such as fall-induced injuries and various chronic diseases. However, no effective treatment is currently available to prevent or reverse the disease progression. Protein arginine methyltransferases (PRMTs) are emerging as a potential therapeutic target for diverse diseases, such as cancer and cardiovascular diseases. Their expression levels are altered in the patients and molecular mechanisms underlying the association between PRMTs and the diseases are being investigated. PRMTs have been shown to regulate development, homeostasis, and regeneration of both muscle and neurons, and their association to NMDs are emerging as well. Through inhibition of PRMT activities, a few studies have reported suppression of cytotoxic phenotypes observed in NMDs. Here, we review our current understanding of PRMTs' involvement in the pathophysiology of NMDs and potential therapeutic strategies targeting PRMTs to address the unmet medical need.

The Role of Protein Arginine Methylation as Post-Translational Modification on Actin Cytoskeletal Components in Neuronal Structure and Function

The brain encompasses a complex network of neurons with exceptionally elaborated morphologies of their axonal (signal-sending) and dendritic (signal-receiving) parts. De novo actin filament formation is one of the major driving and steering forces for the development and plasticity of the neuronal arbor. Actin filament assembly and dynamics thus require tight temporal and spatial control. Such control is particularly effective at the level of regulating actin nucleation-promoting factors, as these are key components for filament formation. Arginine methylation represents an important post-translational regulatory mechanism that had previously been mainly associated with controlling nuclear processes. We will review and discuss emerging evidence from inhibitor studies and loss-of-function models for protein arginine methyltransferases (PRMTs), both in cells and whole organisms, that unveil that protein arginine methylation mediated by PRMTs represents an important regulatory mechanism in neuritic arbor formation, as well as in dendritic spine induction, maturation and plasticity. Recent results furthermore demonstrated that arginine methylation regulates actin cytosolic cytoskeletal components not only as indirect targets through additional signaling cascades, but can also directly control an actin nucleation-promoting factor shaping neuronal cells—a key process for the formation of neuronal networks in vertebrate brains.

A liquid chromatography tandem mass spectroscopy approach for quantification of protein methylation stoichiometry

Post-translational modifications are biologically important and wide-spread modulators of protein function. Although methods for detecting the presence of specific modifications are becoming established, approaches for quantifying their mol modification/mol protein stoichiometry are less well developed. Here we introduce a ratiometric, label-free, targeted liquid chromatography tandem mass spectroscopy-based method for estimating Lys and Arg methylation stoichiometry on post-translationally modified proteins. Methylated Lys and Arg were detected with limits of quantification at low fmol and with linearity extending from 20 to 5000 fmol. This level of sensitivity allowed estimation of methylation stoichiometry from microgram quantities of various proteins, including those derived from either recombinant or tissue sources. The method also disaggregated total methylation stoichiometry into its elementary mono-, di-, and tri-methylated residue components. In addition to being compatible with kinetic experiments of protein methylation, the approach will be especially useful for characterizing methylation states of proteins isolated from cells and tissues.

PRMT3 is essential for dendritic spine maturation in rat hippocampal neurons

Protein arginine N-methyltransferase 3 (PRMT3) is a cytoplasmic enzyme that utilizes S-adenosyl-L-methionine (AdoMet) to methylate specific proteins, most of which contain GAR (glycine-arginine rich) motifs. PRMT3 has been shown to play a role in the proper maturation of the 80S ribosome by binding to and catalyzing the methylation of rpS2, a component of the 40S ribosomal subunit. However, the other roles of PRMT3 are fairly unclear, particularly in the brain, which is abundant in methylated proteins. In this study, we perturbed PRMT3 expression in cultured rat hippocampal neurons by transiently introducing siRNA oligonucleotides that were designed to hybridize with PRMT3 mRNA and then we examined the morphological and functional effects of neuronal PRMT3 depletion. PRMT3-defective neurons showed deformed spines without any change in spine number; less BDNF-induced protein translation of alphaCaMKII; and diminished rpS2 protein stability. Furthermore, overexpression of a methylation-resistant rpS2, whose methylated arginine residues were deleted, produced phenotypes that were similar to those associated with PRMT3 downregulation. These findings demonstrated that PRMT3 possibly plays a pivotal role in neuronal translation by interaction with rpS2 and that it contributes to activity-dependent changes in the dendritic spines.

2 The family of protein arginine metkyltransferases

Arginine methylation is a widespread posttranslational modification found on both nuclear and cytoplasmic proteins. The methylation of arginine residues is catalyzed by the protein arginine N-methyltransferase (PRMT) family of enzymes, of which there are at least nine members in mammals. PRMTs are evolutionarily conserved and are foundin organisms from yeast to man, but not in bacteria. Proteins that are arginine methylated are involved in a number of different cellular processes, including transcriptional regulation, RNA metabolism, and DNA damage repair. How arginine methylation impacts these cellular actions is unclear, although it is likely through the regulation of protein-protein and protein-DNA/RNA interactions. The different PRMTs display varying degrees of substrate specificity, and a certain amount of redundancy is likely to exist between different PRMT family members. Most PRMTs methylate glycine- and arginine-rich patches within their substrates. These regions have been termed GAR motifs. The complexity of the methylarginine mark is enhanced by the ability of this residue to be methylated in three different fashions on the guanidino group (with different functional consequences for each methylated state): monomethylated, symmetrically dimethylated, and asymmetrically dimethylated. This chapter outlines the biochemistry of arginine methylation, including a detailed description of the enzymes involved, the motifs methylated, and the prospects of inhibiting these enzymes with small molecules.

Protein methylation and DNA repair

DNA is under constant attack from intracellular and external mutagens. Sites of DNA damage need to be pinpointed so that the DNA repair machinery can be mobilized to the proper location. The identification of damaged sites, recruitment of repair factors, and assembly of repair "factories" is orchestrated by posttranslational modifications (PTMs). These PTMs include phosphorylation, ubiquitination, sumoylation, acetylation, and methylation. Here we discuss recent data surrounding the roles of arginine and lysine methylation in DNA repair processes.

Elevated plasma concentrations of the endogenous nitric oxide synthase inhibitor asymmetric dimethylarginine in citrullinemia

Citrullinemia is an inborn error of the urea cycle with deficiency of the argininosuccinate synthase. It is characterized by elevated concentrations of l-citrulline and decreased levels of l-arginine in body fluids. Asymmetric dimethylarginine is an endogenous inhibitor of nitric oxide synthase that converts l-arginine to l-citrulline and nitric oxide (NO). Asymmetric dimethylarginine is hydrolyzed by the enzyme dimethylarginine dimethylaminohydrolase to l-citrulline and dimethylamine. Elevation of l-citrulline in citrullinemia prompted us to study the l-arginine/NO pathway in this disorder. In 8 children with citrullinemia (3 days to 3 years of age), elevated plasma levels of asymmetric dimethylarginine (P = .028) were found compared with age-matched healthy children. We hypothesize that the l-arginine/NO pathway plays a role in the pathophysiology of citrullinemia.

Asymmetric dimethylarginine causes hypertension and cardiac dysfunction in humans and is actively metabolized by dimethylarginine dimethylaminohydrolase

OBJECTIVE

Plasma levels of an endogenous nitric oxide (NO) synthase inhibitor, asymmetric dimethylarginine (ADMA), are elevated in chronic renal failure, hypertension, and chronic heart failure. In patients with renal failure, plasma ADMA levels are an independent correlate of left ventricular ejection fraction. However, the cardiovascular effects of a systemic increase in ADMA in humans are not known.

METHODS AND RESULTS

In a randomized, double-blind, placebo-controlled study in 12 healthy male volunteers, we compared the effects of intravenous low-dose ADMA and placebo on heart rate, blood pressure, cardiac output, and systemic vascular resistance at rest and during exercise. We also tested the hypothesis that ADMA is metabolized in humans in vivo by dimethylarginine dimethylaminohydrolase (DDAH) enzymes. Low-dose ADMA reduced heart rate by 9.2+/-1.4% from 58.9+/-2.0 bpm (P<0.001) and cardiac output by 14.8+/-1.2% from 4.4+/-0.3 L/min (P<0.001). ADMA also increased mean blood pressure by 6.0+/-1.2% from 88.6+/-3.4 mm Hg (P<0.005) and SVR by 23.7+/-2.1% from 1639.0+/-91.6 dyne. s. cm-5 (P<0.001). Handgrip exercise increased cardiac output in control subjects by 96.8+/-23.3%, but in subjects given ADMA, cardiac output increased by only 35.3+/-10.6% (P<0.05). DDAHs metabolize ADMA to citrulline and dimethylamine. Urinary dimethylamine to creatinine ratios significantly increased from 1.26+/-0.32 to 2.73+/-0.59 after ADMA injection (P<0.01). We estimate that humans generate approximately 300 micromol of ADMA per day, of which approximately 250 micromol is metabolized by DDAHs.

CONCLUSIONS

This study defines the cardiovascular effects of a systemic increase in ADMA in humans. These are similar to changes seen in diseases associated with ADMA accumulation. Finally, our data also indicate that ADMA is metabolized by DDAHs extensively in humans in vivo.

Homocysteine impairs the nitric oxide synthase pathway: role of asymmetric dimethylarginine

BACKGROUND

Hyperhomocysteinemia is a putative risk factor for cardiovascular disease, which also impairs endothelium-dependent vasodilatation. A number of other risk factors for cardiovascular disease may exert their adverse vascular effects in part by elevating plasma levels of asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthase. Accordingly, we determined if homocysteine could increase ADMA levels.

METHODS AND RESULTS

When endothelial or nonvascular cells were exposed to DL-homocysteine or to its precursor L-methionine, ADMA concentration in the cell culture medium increased in a dose- and time-dependent fashion. This effect was associated with the reduced activity of dimethylarginine dimethylaminohydrolase (DDAH), the enzyme that degrades ADMA. Furthermore, homocysteine-induced accumulation of ADMA was associated with reduced nitric oxide synthesis by endothelial cells and segments of pig aorta. The antioxidant pyrrollidine dithiocarbamate preserved DDAH activity and reduced ADMA accumulation. Moreover, homocysteine dose-dependently reduced the activity of recombinant human DDAH in a cell free system, an effect that was due to a direct interaction between homocysteine and DDAH.

CONCLUSION

Homocysteine post-translationally inhibits DDAH enzyme activity, causing ADMA to accumulate and inhibit nitric oxide synthesis. This may explain the known effect of homocysteine to impair endothelium-mediated nitric oxide-dependent vasodilatation.

The hypothalamus and hypertension

Most forms of hypertension are associated with a wide variety of functional changes in the hypothalamus. Alterations in the following substances are discussed: catecholamines, acetylcholine, angiotensin II, natriuretic peptides, vasopressin, nitric oxide, serotonin, GABA, ouabain, neuropeptide Y, opioids, bradykinin, thyrotropin-releasing factor, vasoactive intestinal polypeptide, tachykinins, histamine, and corticotropin-releasing factor. Functional changes in these substances occur throughout the hypothalamus but are particularly prominent rostrally; most lead to an increase in sympathetic nervous activity which is responsible for the rise in arterial pressure. A few appear to be depressor compensatory changes. The majority of the hypothalamic changes begin as the pressure rises and are particularly prominent in the young rat; subsequently they tend to fluctuate and overall to diminish with age. It is proposed that, with the possible exception of the Dahl salt-sensitive rat, the hypothalamic changes associated with hypertension are caused by renal and intrathoracic cardiopulmonary afferent stimulation. Renal afferent stimulation occurs as a result of renal ischemia and trauma as in the reduced renal mass rat. It is suggested that afferents from the chest arise, at least in part, from the observed increase in left auricular pressure which, it is submitted, is due to the associated documented impaired ability to excrete sodium. It is proposed, therefore, that the hypothalamic changes in hypertension are a link in an integrated compensatory natriuretic response to the kidney's impaired ability to excrete sodium.

Endogenous nitric oxide synthase inhibitors are responsible for the L-arginine paradox

L-Arginine, the substrate of nitric oxide (NO) synthases (NOSs), is found in the mammalian organism at concentrations by far exceeding K(M) values of these enzymes. Therefore, additional L-arginine should not enhance NO formation. In vivo, however, increasing L-arginine concentration in plasma has been shown repeatedly to increase NO production. This phenomenon has been named the L-arginine paradox; it has found no satisfactory explanation so far. In the present work, evidence for the hypothesis that the endogenous NOS inhibitors methylarginines, asymmetric dimethylarginine being the most powerful (IC(50) 1.5 microM), are responsible for the L-arginine paradox is presented.

RNA and protein interactions modulated by protein arginine methylation

This review summarizes the current status of protein arginine N-methylation reactions. These covalent modifications of proteins are now recognized in a number of eukaryotic proteins and their functional significance is beginning to be understood. Genes that encode those methyltransferases specific for catalyzing the formation of asymmetric dimethylarginine have been identified. The enzyme modifies a number of generally nuclear or nucleolar proteins that interact with nucleic acids, particularly RNA. Postulated roles for these reactions include signal transduction, nuclear transport, or a direct modulation of nucleic acid interactions. A second methyltransferase activity that symmetrically dimethylates an arginine residue in myelin basic protein, a major component of the axon sheath, has also been characterized. However, a gene encoding this activity has not been identified to date and the cellular function for this methylation reaction has not been clearly established. From the analysis of the sequences surrounding known arginine methylation sites, we have determined consensus methyl-accepting sequences that may be useful in identifying novel substrates for these enzymes and may shed further light on their physiological role.

Characterization of dimethylargininase from bovine brain: evidence for a zinc binding site

Dimethylargininase (EC 3.5.3.18) is involved in the regulation of the levels of the natural occurring free arginine derivatives L-Nomega,Nomega-dimethylarginine and L-Nomega-methylarginine, which are reversible inhibitors of nitric oxide synthase. A dimethylargininase has been isolated from bovine brain tissue and was characterized by using immunological, kinetic, and spectroscopic techniques. Western blot analysis using polyclonal antibodies revealed that the enzyme is widely distributed in bovine with the highest relative concentrations found in brain and kidney tissue. A similar tissue distribution has also been reported for the other so far isolated dimethylargininase from rat kidney [Ogawa, T., Kimoto, M., and Sasaoka, K. (1989) J. Biol. Chem. 264, 10205-10209]. The bovine enzyme is a monomeric, globular protein (molecular mass approximately 31.2 kDa) containing one tightly bound Zn2+ ion, which can be removed by dialysis against 1,10-phenanthroline. The determination of kinetic constants for both the native (holo-protein) and the zinc-depleted (apo-protein) enzyme at 37 degrees ¿C established that the dimethylargininase is not a zinc hydrolase. The specific activity was 0.66 unit/mg for the holo-protein and 0.19 unit/mg for the apo-protein. The secondary structure determination of the native enzyme by circular dichroism revealed 41% alpha-helix and 32% beta-sheet and beta-turn structure. In the apo-enzyme, a small, but significant decrease in the alpha-helical content (5%) was observed, consistent with a marked decrease in enzymatic activity to 30%. Upon preincubation of both enzyme forms at 50 degrees C, only the holo-enzyme showed a residual enzymatic activity. In thermostability studies, a 7 degrees C lower apparent Tm value was observed for the apo-enzyme compared to the 66 degrees C for the holo-enzyme, suggesting that the zinc ion has a structure-stabilizing role. Besides the tightly bound zinc, additional Zn2+ ions inhibit the enzyme competitively with a Ki value of 2.0 microM. A possible interrelationship between dimethylargininase and nitric oxide synthase is discussed.

Hypothalamic Hypertensive Factor

Abstract Human and rat plasma and rat hypothalamus contain a cytochemically detectable substance, the concentration of which rises with an increase in salt intake. The plasma concentration of this material is also raised in essential hypertension and in the spontaneously hypertensive rat (SHR), the Milan hypertensive rat, and the reduced renal mass (RRM) hypertensive rat. In the normal rat, the greatest concentration is found in the hypothalamus of the SHR and the RRM hypertensive rat. The physicochemical characteristics of this cytochemically detectable hypothalamic hypertensive factor (HHF), including chromatographic behavior and molecular weight range, suggest that it may share features common to a substituted guanidine that is present in established nitric oxide synthase (NOS) inhibitors. It was therefore decided to determine the effect on NOS activity of the HHF obtained from mature SHR. The ability of HHF to inhibit NOS activity was studied on (1) NOS extracted from bovine aorta, rat brain, and human platelets by measuring the conversion of radiolabeled l- arginine to l -citrulline and (2) rat liver NOS measured indirectly with a cytochemical technique based on the stimulation of soluble guanylate cyclase activity in hepatocytes by NO. HHF showed a biphasic inhibitory action on platelet NOS activity that was greater with HHF obtained from SHR than from Wistar-Kyoto rats. HHF also had a biphasic inhibitory effect on hepatocyte NOS activity that was more potent when obtained from SHR. It is proposed that the increase in HHF, a novel form of NOS inhibitor that is elevated in SHR, may be involved in the rise in arterial pressure.

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.

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.

Distribution of free methylarginines in rat tissues and in the bovine brain

A sensitive and specific method for determining three forms of methylarginine, i.e., NG-monomethylarginine, NG,NG-dimethylarginine, and NG,N'G-dimethylarginine, in mammalian tissues was developed. After partial purification by ion-exchange chromatography, the methylarginines were derivatized to phenylthiocarbamyl compounds and quantitatively determined using HPLC with a reverse-phase C18 column. In rat organs, the highest concentrations of methylarginines were observed in the spleen. In rat brain, cerebellum and olfactory bulb contained large amounts of NG-monomethylarginine and NG,NG-dimethylarginine. A detailed study of the distribution of methylarginines in the bovine brain was also made, and the concentration of NG,N'G-dimethylarginine was almost the same in all regions. The cerebellar gray matter, hippocampus, and hypothalamus contained large amounts of methylarginines. The distribution of methylarginines seems to parallel the distribution of nitric oxide synthase, which is known to be inhibited by NG-monomethylarginine. This may indicate that methylarginines play some role in controlling nitric oxide synthase activity.

Isolation and identification of methylarginines from bovine brain

Methylarginines in free form were identified in bovine brain. Three compounds were isolated from the basic aliphatic amino acid fraction of bovine brain with several ion-exchange chromatographies. They showed the same Rf values in paper and thin-layer chromatographies as those of authentic NG-monomethylarginine, NG,NG-dimethylarginine, and NG,N'G-dimethylarginine. The migration distance of the isolated compounds in high-voltage paper electrophoresis and the retention times in ion-exchange HPLC were also identical to those of the above authentic methylarginines. We concluded that these three compounds are the methyl derivatives of arginine described above. The amount of these three compounds isolated from 1,090 g of bovine brain was 0.3 mumol of NG-monomethylarginine, 0.1 mumol of NG,NG-dimethylarginine, and 0.5 mumol of NG,N'G-dimethylarginine. The occurrence of these free methylarginines may have an important role in regulating the signal transduction through the nitric oxide system.

Rabbit brain contains an endogenous inhibitor of endothelium-dependent relaxation

1. Supernatants prepared from the rabbit brain, lung and liver caused an endothelium-dependent and volume-related contraction of the phenylephrine-pretreated rabbit aorta and inhibited relaxation due to acetylcholine (ACh). 2. Perfusion in situ of the rabbit lung or liver with Krebs solution substantially reduced or removed the endothelium-dependent inhibitor. Spectrophotometric analysis revealed the presence of substantial amounts of haemoglobin (1.8-2.1 microM) in these organ supernatants. 3. Supernatants prepared from the Krebs-perfused rabbit brain retained the ability to contract the phenylephrine-pretreated rabbit aorta and to inhibit relaxation due to ACh and substance P (SP). Rabbit brain supernatant did not reduce the vasodilator effect of sodium nitroprusside (NP) or nitric oxide (NO). 4. Rabbit brain supernatant contained low (less than 0.35 microM) concentrations of haemoglobin. 5. The inhibitory effect of rabbit brain supernatant was reversed by L-arginine (500 microM) but not D-arginine (500 microM). 6. The inhibitor of endothelium-dependent vasodilatation present in rabbit brain was not removed by dialysis (24 h, 4 degrees C) but was partially precipitated by ammonium sulphate (30% w/v). 7. Rabbit brain contains an endogenous inhibitor of vascular NO biosynthesis. The identity of this inhibitor is not known although it seems likely to be a large peptide or protein.

The metabolism of L-arginine and its significance for the biosynthesis of endothelium-derived relaxing factor: cultured endothelial cells recycle L-citrulline to L-arginine

We have investigated the mechanism by which cultured endothelial cells generate L-arginine (L-Arg), the substrate for the biosynthesis of endothelium-derived relaxing factor. When Arg-depleted endothelial cells were incubated in Krebs' solution for 60 min, L-Arg levels were significantly (9.7-fold) elevated. The generation of L-Arg coincided with a substantial decrease (90%) in intracellular L-glutamine (L-Gln), whereas all other amino acids were virtually unaffected. Changes in calcium, pH, or oxygen tension had no effect on L-Arg generation, which was, however, prevented when the cells were incubated in culture medium containing L-Gln. L-Arg generated by endothelial cells labeled with L-[14C]Arg was derived from an unlabeled intracellular source, for the specific activity of the intracellular L-Arg pool decreased substantially (8.8-fold) over 60 min. Arg-depleted endothelial cells did not form urea or metabolize L-ornithine but converted L-citrulline (L-Cit) to L-Arg possibly via formation of L-argininosuccinic acid. Nondepleted cells stimulated with the calcium ionophore A23187 showed only a transient accumulation of L-Cit, indicating that L-Cit is recycled to L-Arg during the biosynthesis of endothelium-derived relaxing factor. The generation of L-Arg by Arg-depleted endothelial cells was partially (45%) blocked by protease inhibitors, and various Arg-containing dipeptides were rapidly cleaved to yield L-Arg. Thus, cultured endothelial cells recycle L-Cit to L-Arg and possibly liberate peptidyl L-Arg. The Arg-Cit cycle appears to be the equivalent in the endothelial cell to the formation of urea by the liver. The biosynthesis of endothelium-derived relaxing factor may, therefore, not only produce a powerful vasodilator but also relieve the endothelial cell of excess nitrogen.

Purification and properties of calmodulin-lysine N-methyltransferase from rat brain cytosol

A S-adenosylmethionine:protein-lysine N-methyltransferase (EC 2.1.1.43) has been purified from rat brain cytosol 7,080-fold with a yield of 8%, using octopus calmodulin as a substrate. It contains a lysine residue that is not fully methylated. The enzyme was purified by ammonium sulfate fractionation, Sephacryl S-200 gel filtration, and phosphocellulose and octopus calmodulin-Sepharose affinity chromatographies. Among protein substrates, it was highly specific toward octupus calmodulin. The Km values for octopus calmodulin and S-adenosyl-L-methionine were found to be 2.2 X 10(-8) M and 0.8 X 10(-6) M, respectively. The molecular weight was estimated to be 57,000 by gel filtration and the pH optimum was between 7.5 and 8.5. The enzyme was stimulated in the presence of 10(-7) M Mn2+ and 10(-4) M Ca2+. HPLC of the acid hydrolysate of methyl-3H-labeled calmodulin showed the formation of epsilon-N-mono, epsilon-N-di, and epsilon-N-trimethyllysine. Reverse-phase HPLC of tryptic peptides of the methyl-3H-labeled calmodulin demonstrated that the labeled N-methyllysine lies in the 107-126 peptide. These findings suggest that this enzyme methylated a specific lysine residue of octopus calmodulin.

Time dependence of the methylation of myelin basic protein from bovine brain; evidence for protein-methylarginine demethylation

In the presence of excess S-adenosylmethionine (AdoMet), the extent of methylation of myelin basic protein (MBP) by partially purified AdoMet:MBP methyltransferase is a non-linear function of time, reaching a limiting value as available MBP is depleted and then decreasing monotonically. This decrease is not caused by proteolytic cleavage of MBP nor by effects related to substrate or product instability under the incubation conditions and is not observed in heat-inactivated samples. S-Adenosylhomocysteine is not required for the demethylation to occur, and with purified enzyme, the decrease is not observed. The data strongly suggest that the decrease in methyl content represents an enzyme-catalyzed demethylation reaction. This would represent the first report of an enzyme which catalyzes protein-methylarginine demethylation.

Accelerated protein turnover in the skeletal muscle of dystrophic mice

The excretion of 3-methylhistidine increased in the urine of dystrophic mice C57BL/6J. The content of 3-methylhistidine residue decreased in the muscle proteins of dystrophic mice, but not in other organs. Methylated proteins in the skeletal muscle, actin and myosin, were partially purified from the dystrophic and control muscles. The amount of 3-methylhistidine residue in unit weight of the actin and myosin preparations was normal in dystrophic muscle. These three facts indicate that the turnover rates of actin and myosin are increased in the muscle of the dystrophic mice.

Urinary excretion of dimethylarginines in premature infants

Urinary excretion of NG,N'G-dimethylarginine (NG,N'G-Me2Arg) and NG,NG-dimethylarginine (NG,NG-Me2Arg) was measured in premature infants. The NG,N'G-Me2Arg/NG,NG-Me2Arg ratio was much higher in newborn infants than in older children or adults. Linear regression analysis showed a significant negative correlation between the degree of maturity and the excretion of NG,N'G-Me2Arg. A significant direct linear relationship also was found between the excretion of NG,N'G-Me2Arg and the rate of whole body nitrogen flux and of protein synthesis and catabolism. No correlation was found between the excretion of the dimethylarginines and 3-methylhistidine, but the dimethylarginine/3-methylhistidine ratio declined with advancing conceptual age. A direct linear relationship was found between excretion of NG,N'G-Me2Arg and NG,NG-Me2Arg and whole body nonskeletal muscle protein breakdown. No correlation was found between nonskeletal muscle protein catabolism and 3-methylhistidine excretion. We estimate that approximately 0.34 mumole of dimethylarginine are excreted per gram of nonskeletal muscle protein catabolized. Dietary intake did not affect the excretion of either NG,N'G-Me2Arg or NG,NG-Me2Arg. The data suggest that measurement of urinary dimethylarginines might be useful in the nutritional assessment of premature infants.

The relationship of myelin basic protein (arginine) methyltransferase to myelination in mouse spinal cord

The relationship between the activity of myelin basic protein (arginine) methyltransferase and myelination in the mouse spinal cord has been examined. The activity of this methylase increases between 8 and 45 days postnatal age and correlates well with other parameters of myelination. A comparison of myelin basic protein methylase with histone methylase activity during development indicates that each is a distinct, specific enzyme activity. Together, these results are considered to establish myelin based protein methylase as a myelination-related enzyme.

Methylation of elongation factor 1 alpha from the fungus Mucor

A basic protein from the dimorphic fungus Mucor racemosus, found to be highly methylated, is shown to be protein synthesis elongation factor 1 alpha. This protein is the most abundant protein in hyphal cells but is less abundant in yeast cells. It is post-translationally methylated with the formation of mono-, di-, and trimethyllysine at as many as 16 sites. Nearly 20% of the 44 lysine residues of elongation factor 1 alpha from mycelia are modified while those from sporangiospores are virtually unmethylated.

In vivo methylation of an arginine in chicken myelin basic protein

The amino acid sequence around the sole methylarginine residue in chicken myelin basic protein was determined and was found to be similar to that previously reported for mammalian myelin basic protein. The ratio NG, N'G-dimethylarginine: NG-monomethylarginine:arginine was approximately 1.3:0.9:1.0. No NG, NG-dimethylarginine was detected in the protein. The in vivo incorporation of methyl groups from [methyl-3H]methionine into methylarginines in myelin was found to occur readily in 2-day-old chickens. Radioactively labelled NG,N'G-dimethylarginine and NG-monomethylarginine in myelin were derived solely from myelin basic protein. Radioactivity was also incorporated into NG,NG-dimethylargnine, although this was not derived from myelin basic protein. As NG-monomethylarginine was easily separated from the dimethylarginines, and as it was derived from myelin basic protein, it may be a good marker for myelin basic protein turnover in vivo. A time course study of the incorporation showed that radioactivity was incorporated into NG-monomethylarginine up to 6 h after injection, and decayed slowly, with an apparent half-life of about 40 days.

Decrease of 3-methylhistidine and increase of NG,NG-dimethylarginine in the urine of patients with muscular dystrophy

The amounts of 3-methylhistidine, N epsilon,N epsilon-dimethyllysine, N epsilon, N epsilon, N epsilon-trimethyllysine, NG,NG-dimethylarginine, and NG,N'G-dimethylarginine were determined in the urine specimens of healthy subjects and patients of corresponding ages with Duchenne, limb-girdle, and congenital types of muscular dystrophy, and motor neuron diseases. The amount of excretion of 3-methylhistidine decreased and that of NG,NG-dimethylarginine increased significantly in Duchenne and limb-girdle types of muscular dystrophy, but not in diseases with neurogenic muscular atrophy. The decrease of 3-methylhistidine was observed consistently throughout the course of the Duchenne type of muscular dystrophy. The amounts of the other methylamino acids both in myogenic and neurogenic myopathies were not different from those in healthy subjects.

Human muscular dystrophy: elevation of urinary dimethylarginines

The amounts of the dimethylarginines NG,NG-dimethylarginine (DMA) and NG,N'G-dimethylarginine (DM'A) excreted in the urine of muscular dystrophic patients were examined and compared with the amounts excreted by normal controls, patients with other types of neuromuscular diseases, and patients with disuse muscle atrophy resulting from traumatic paralysis. The patients with muscular dystrophy excreted high concentrations of DMA and this urine showed high ratios of DMA to DM'A. This finding indicates a relation between protein methylation processes and muscular dystrophy.

Urinary excretion of methylarginine in human disease

Methylated amino acids are excreted in urine upon degradation of some tissue proteins. The urinary excretion ratios of NG,N'G-dimethylarginine (syn-DMA) and NG,NG-dimethylarginine (unsym-DMA) were studied in healthy adults and in patients with various diseases. The normal ratio of sym- to unsym-DMA in urine was 0.98 and ranged from 0.71 to 1.33; ratios were not significantly different in multiple sclerosis, cerebrovascular accident, cancer, and systemic lupus erythematosus. However, patients with liver, disease, including chronic active hepatitis, were found on average to have a significantly altered ratio of 0.79, range 0.49-1.30, owing to an increase in the excretion of unsym-DMA. Hence measurements of the urinary excretion of dimethylarginine could become a useful aid in assessing recovery of liver cells in patients with chronic liver disease.

Synthesis and degradation of methylated proteins of mouse organs: correlation with protein synthesis and degradation

L-(Methyl-14C)-methionine was administered i.p. to mice, and the incorporation of radioactive methionine into proteins and methyllysine and methylarginine residues formed by the transfer of the methyl-14C group of methionine were measured. Tissue protein was actively methylated in organs having a high activity of protein synthesis, and the in vivo methylating activity in organs was not correlated with theprotein methylating activity of the organs determined in vitro. Puromycin inhibited both protein synthesis and protein methylation in mouse organs to a similar degree. Neither the formation of S-adenosyl-(methyl-14C)-methionine nor protein methylase was inhibited by puromycin. The data suggests that proteins are methylated immediately after protein synthesis, that is, newly synthesized proteins are the substrates of protein methylation. Radioactive methionine and the [C14] methyl groups of methyllysine and methylarginine residues of tissue proteins are degraded in parallel over a period of 3 wk, suggesting that protein methylation is an irreversible type of protein modification.

Studies on the catabolism of Ng-methylarginine, Ng, Ng-dimethylarginine and Ng, Ng-dimethylarginine in the rabbit

1. The routes of elimination of Ng-methylarginine, Ng, Ng-dimethylarginine and Ng, Ng-dimethylarginine were investigated in the rabbit. 2. Analyses showed low plasma concentrations of these amino acids (around 1 nmol/ml) and ratios similar to those found in tissue proteins. The concentrations of these amino acids in extracts of brain, kidney, liver and spleen were similar except that liver had a lower concentration of Ng-methylarginine and Ng, Ng-dimethylarginine. Cerebrospinal fluid contained traces of each amino acid.