Promiscuous activity of arginine:glycine amidinotransferase is responsible for the synthesis of the novel cardiovascular risk factor homoarginine

Insufficiency of plasmatic arginine/homoarginine during the initial postoperative phase among patients with tumors affecting the medulla oblongata heightens the likelihood of neurogenic pulmonary oedema following surgery

BACKGROUND

This prospective clinical study aims to investigate the fluctuations of neurotransmitters in peripheral venous blood during the perioperative period and to identify independent predictors for postoperative neurogenic pulmonary oedema (NPE) in patients with medulla oblongata-involved tumours.

MATERIALS AND METHODS

Peripheral venous blood samples of the enroled patients at seven perioperative time points, as well as their medical records and radiologic data were collected. High-performance liquid chromatography-tandem mass spectrometry was utilized to detect the concentrations of 39 neurotransmitters in these samples. The study applied univariate and multivariate generalized estimating equation (GEE) logistic regression analyses to explore independent predictors of postoperative NPE, and one-way repeated-measures ANOVA to compare the concentrations of the same neurotransmitter at different perioperative time points.

RESULTS

The study included 36 patients with medulla oblongata-involved tumours from January to December 2019, and found that 13.9% of them experienced postoperative NPE. The absence of intraoperative use of sevoflurane ( P =0.008), decreased concentrations of arginine ( P =0.026) and homoarginine ( P =0.030), and prolonged postoperative tracheal extubation ( P <0.001) were identified as independent risk factors for postoperative NPE in medulla oblongata-involved tumour patients. Pairwise comparison analysis revealed that the perioperative decreases in arginine and homoarginine concentrations mainly occurred within the postoperative 8 h.

CONCLUSION

This study demonstrates that NPE is not uncommon in patients with medulla oblongata-involved tumours. The absence of intraoperative use of sevoflurane, decreased concentrations of plasmatic arginine and homoarginine, and prolonged postoperative tracheal extubation are independent predictors of postoperative NPE. These two neurotransmitters' concentrations dropped mainly within the early postoperative hours and could serve as potential early warning indicators of postoperative NPE in clinical practice.

Homoarginine in the cardiovascular system: Pathophysiology and recent developments

Upcoming experimental and epidemiological data have identified the endogenous non-proteinogenic amino acid L-homoarginine (L-hArg) not only as a novel biomarker for cardiovascular disease but also as being directly involved in the pathogenesis of cardiac dysfunction. The association of low L-hArg levels with adverse cardiovascular events and mortality has proposed the idea of nutritional supplementation to rescue pathways inversely associated with cardiovascular health. Subsequent clinical and experimental studies contributed significantly to our knowledge of potential effects on the cardiorenal axis, acting either as a biomarker or a cardiovascular active agent. In this review article, we provide a comprehensive summary of the L-hArg metabolism, pathophysiological aspects, and current developments in the field of experimental and clinical evidence in favor of protective cardiovascular effects. Establishing a reliable biomarker to identify patients at high risk to die of cardiovascular disease represents one of the main goals for tackling this disease and providing individual therapeutic guidance.

Identification of biomarkers associated with the feed efficiency by metabolomics profiling: results from the broiler lines divergent for high or low abdominal fat content

Background

Improving feed efficiency (FE) is one of the main objectives in broiler breeding. It is difficult to directly measure FE traits, and breeders hence have been trying to identify biomarkers for the indirect selection and improvement of FE traits. Metabolome is the "bridge" between genome and phenome. The metabolites may potentially account for more of the phenotypic variation and can suitably serve as biomarkers for selecting FE traits. This study aimed to identify plasma metabolite markers for selecting high-FE broilers. A total of 441 birds from Northeast Agricultural University broiler lines divergently selected for abdominal fat content were used to analyze plasma metabolome and estimate the genetic parameters of differentially expressed metabolites.

Results

The results identified 124 differentially expressed plasma metabolites (P < 0.05) between the lean line (high-FE birds) and the fat line (low-FE birds). Among these differentially expressed plasma metabolites, 44 were found to have higher positive or negative genetic correlations with FE traits (|r_g| ≥ 0.30). Of these 44 metabolites, 14 were found to display moderate to high heritability estimates (h² ≥ 0.20). However, among the 14 metabolites, 4 metabolites whose physiological functions have not been reported were excluded. Ultimately, 10 metabolites were suggested to serve as the potential biomarkers for breeding the high-FE broilers. Based on the physiological functions of these metabolites, reducing inflammatory and improving immunity were proposed to improve FE and increase production efficiency.

Conclusions

According to the pipeline for the selection of the metabolite markers established in this study, it was suggested that 10 metabolites including 7-ketocholesterol, dimethyl sulfone, epsilon-(gamma-glutamyl)-lysine, gamma-glutamyltyrosine, 2-oxoadipic acid, L-homoarginine, testosterone, adenosine 5'-monophosphate, adrenic acid, and calcitriol could be used as the potential biomarkers for breeding the "food-saving broilers".

The Amino Acid Homoarginine Inhibits Atherogenesis by Modulating T-Cell Function

BACKGROUND

Amino acid metabolism is crucial for inflammatory processes during atherogenesis. The endogenous amino acid homoarginine is a robust biomarker for cardiovascular outcome and mortality with high levels being protective. However, the underlying mechanisms remain elusive. We investigated the effect of homoarginine supplementation on atherosclerotic plaque development with a particular focus on inflammation.

METHODS

Female ApoE-deficient mice were supplemented with homoarginine (14 mg/L) in drinking water starting 2 weeks before and continuing throughout a 6-week period of Western-type diet feeding. Control mice received normal drinking water. Immunohistochemistry and flow cytometry were used for plaque- and immunological phenotyping. T cells were characterized using mass spectrometry-based proteomics, by functional in vitro approaches, for example, proliferation and migration/chemotaxis assays as well as by super-resolution microscopy.

RESULTS

Homoarginine supplementation led to a 2-fold increase in circulating homoarginine concentrations. Homoarginine-treated mice exhibited reduced atherosclerosis in the aortic root and brachiocephalic trunk. A substantial decrease in CD3⁺ T cells in the atherosclerotic lesions suggested a T-cell-related effect of homoarginine supplementation, which was mainly attributed to CD4⁺ T cells. Macrophages, dendritic cells, and B cells were not affected. CD4⁺ T-cell proteomics and subsequent pathway analysis together with in vitro studies demonstrated that homoarginine profoundly modulated the spatial organization of the T-cell actin cytoskeleton and increased filopodia formation via inhibition of Myh9 (myosin heavy chain 9). Further mechanistic studies revealed an inhibition of T-cell proliferation as well as a striking impairment of the migratory capacities of T cells in response to relevant chemokines by homoarginine, all of which likely contribute to its atheroprotective effects.

CONCLUSIONS

Our study unravels a novel mechanism by which the amino acid homoarginine reduces atherosclerosis, establishing that homoarginine modulates the T-cell cytoskeleton and thereby mitigates T-cell functions important during atherogenesis. These findings provide a molecular explanation for the beneficial effects of homoarginine in atherosclerotic cardiovascular disease.

Influence of homoarginine on creatine accumulation and biosynthesis in the mouse

Organisms obtain creatine from their diet or by de novo synthesis via AGAT (L-arginine:glycine amidinotransferase) and GAMT (Guanidinoacetate N-methyltrasferase) in kidney and liver, respectively. AGAT also synthesizes homoarginine (hArg), low levels of which predict poor outcomes in human cardiovascular disease, while supplementation maintains contractility in murine heart failure. However, the expression pattern of AGAT has not been systematically studied in mouse tissues and nothing is known about potential feedback interactions between creatine and hArg. Herein, we show that C57BL/6J mice express AGAT and GAMT in kidney and liver respectively, whereas pancreas was the only organ to express appreciable levels of both enzymes, but no detectable transmembrane creatine transporter (Slc6A8). In contrast, kidney, left ventricle (LV), skeletal muscle and brown adipose tissue must rely on creatine transporter for uptake, since biosynthetic enzymes are not expressed. The effects of creatine and hArg supplementation were then tested in wild-type and AGAT knockout mice. Homoarginine did not alter creatine accumulation in plasma, LV or kidney, whereas in pancreas from AGAT KO, the addition of hArg resulted in higher levels of tissue creatine than creatine-supplementation alone (P < 0.05). AGAT protein expression in kidney was downregulated by creatine supplementation (P < 0.05), consistent with previous reports of end-product repression. For the first time, we show that hArg supplementation causes a similar down-regulation of AGAT protein (P < 0.05). These effects on AGAT were absent in the pancreas, suggesting organ specific mechanisms of regulation. These findings highlight the potential for interactions between creatine and hArg that may have implications for the use of dietary supplements and other therapeutic interventions.

The role of L-arginine metabolism in neurocritical care patients

Nitric oxide is an important mediator of vascular autoregulation and is involved in pathophysiological changes after acute neurological disorders. Nitric oxide is generated by nitric oxide synthases from the amino acid L-arginine. L-arginine can also serve as a substrate for arginases or lead to the generation of dimethylarginines, asymmetric dimethylarginine, and symmetric dimethylarginine, by methylation. Asymmetric dimethylarginine is an endogenous inhibitor of nitric oxide synthase and can lead to endothelial dysfunction. This review discusses the role of L-arginine metabolism in patients suffering from acute and critical neurological disorders often requiring neuro-intensive care treatment. Conditions addressed in this review include intracerebral hemorrhage, aneurysmal subarachnoid hemorrhage, and traumatic brain injury. Recent therapeutic advances in the field are described including current randomized controlled trials for traumatic brain injuries and hemorrhagic stroke.

Evaluation of the predictive values of elevated serum L-homoarginine and dimethylarginines in preeclampsia

L-homoarginine (hARG) is involved in nitric oxide biosynthesis, but its role and concentration in preeclampsia (PE) have not been fully revealed. The purpose of this study was to develop and validate a feasible clinical assay to quantify serum hARG, arginine (ARG), asymmetric (ADMA) and symmetric dimethylarginines (SDMA) levels by LC-MS/MS and investigate their differences at different stages of pregnancy with or without preeclampsia. Serum samples were collected from 84 pregnant women without complications (controls), 84 with mild preeclampsia (MPE), and 81 with severe preeclampsia (SPE) at various gestation stages (before the 20th week, during the 20th-28th week or after the 28th week of gestation). No significant difference in ARG levels was observed between PE and controls at any stage (P > 0.05). The serum hARG levels and hARG/ADMA ratios of MPE before the 20th week were higher than those of controls (P < 0.001). ADMA levels of MPE were higher than those of controls during the 20th-28th week (P < 0.01). SDMA levels of SPE were higher than those of MPE (P < 0.01) and controls (P < 0.05) after the 28th week. Elevated serum hARG before the 20th week was identified as an independent predictor for PE (OR = 1.478, 95% CI 1.120-1.950). ROC curve analysis showed serum hARG before the 20th week had a good potential to predict MPE (AUC = 0.875, 95% CI 0.759-0.948). In conclusion, our study indicated that elevated serum hARG and dimethylarginine levels detected by LC-MS/MS might serve as potential biomarkers for the early prediction of PE.

Population kinetics of homoarginine and optimized supplementation for cardiovascular risk reduction

Homoarginine is an endogenous amino acid whose levels are reduced in patients with renal, cardio- and cerebrovascular disease. Moreover, low homoarginine concentrations independently predict morbidity and mortality in these patients. Besides endogenous synthesis, homoarginine is also a constituent of the human diet. The objective of the present study was to analyze the kinetics of orally supplemented homoarginine in human plasma by means of a pharmacometric approach. We developed a pharmacometric model to evaluate different dosing regimens, especially the regimen of 125 mg once weekly, based on a previous clinical study (n = 20). The model was adapted to account for differences in baseline homoarginine plasma concentrations between healthy and diseased individuals. A novel dosing regimen of 25 mg once daily led to higher attainment of homoarginine reference concentrations using clinical trial simulations. With 25 mg/day, the trough concentration of only 6% of the older and 3.8% of the younger population was predicted to be below the target concentration of 2.0-4.1 µmol/L. In synopsis, the new dosing regimen recapitulates the kinetics of homoarginine in healthy individuals optimally.

Effects of treatment with SGLT-2 inhibitors on arginine-related cardiovascular and renal biomarkers

BACKGROUND

In patients with type 2 diabetes (T2D) sodium-glucose cotransporter 2 (SGLT-2) inhibitors improve glycaemic control as well as cardiovascular and renal outcomes. Their effects on L-arginine (Arg) related risk markers asymmetric and symmetric dimethylarginine (ADMA and SDMA) and the protective biomarker L-homoarginine (hArg) linking T2D to cardiovascular and renal disease have not yet been reported.

METHODS

Plasma and 24-h urine samples taken before and after 6 weeks of treatment were available from two prospective, randomized, double-blind, placebo-controlled, cross-over trials with empagliflozin (71 patients analyzed, NCT02471963) and dapagliflozin (59 patients analyzed, NCT02383238). In these samples, concentrations of hArg, Arg, ADMA, SDMA, and creatinine were determined by liquid-chromatography coupled to tandem mass-spectrometry. Additionally, intraindividual changes of the biomarkers in plasma were correlated with intraindividual changes of clinical parameters.

RESULTS

Treatment with empagliflozin and dapagliflozin was associated with a reduction of plasma hArg by 17.5% and 13.7% (both p < 0.001), respectively, and increase in plasma SDMA concentration of 6.7% and 3.6%, respectively (p < 0.001 and p < 0.05), while plasma Arg and ADMA concentrations were not significantly altered. 24-h urinary excretion of ADMA was reduced by 15.2% after treatment with empagliflozin (p < 0.001) but not after dapagliflozin treatment, while excretion of the other markers was not significantly altered. Renal clearance of SDMA was reduced by 9.1% and 3.9% for both drugs (both p < 0.05). A reduction in ADMA clearance was observable after empagliflozin treatment only (- 15.5%, p < 0.001), but not after dapagliflozin. Renal clearance of hArg and Arg was not significantly altered. Treatment effects on L-arginine related biomarkers were not constantly correlated with effects on glycated hemoglobin, fasting plasma glucose, body mass index, and systolic blood pressure.

CONCLUSIONS

Treatment with SGLT-2 inhibitors has divergent effects on Arg-related biomarkers and could affect risk estimates associated with these markers. The observed effects are unlikely to explain the known cardiovascular and renal benefits of treatment with empagliflozin or dapagliflozin but still may indicate new therapeutic approaches in patients treated with SGLT-2 inhibitors. Trial registration http://www.clinicaltrials.gov : NCT02471963 (registered 15th June 2015, retrospectively registered) and NCT02383238.

Homoarginine test for evaluation of metabolic renal dysfunction

Low plasma L-homoarginine (hArg) concentration is an independent predictor of adverse cardiovascular outcomes and overall mortality, as well as the progression of chronic kidney disease (CKD). The enzyme L-arginine:glycinamidinotransferase (AGAT, EC 2.1.4.1) acts in the mitochondrial membrane of the renal tubular epithelium, forming the precursor of creatine, guanidinoacetic acid, and additionnaly by-product hArg. As it was shown recently, there is a decreased level of hArg in the late stages of CKD, however, the the level of hArg in the early stages of CKD remained unexplored. The aim of this study was to determine the diagnostic threshold levels of hArg in the blood of patients with stages 1 and 2 of CKD. In patients with the initial stages of CKD (n = 44) at the age of 58 (45-67) years, compared with the group of donors of 55 (42-58) years (n = 30), a significant decrease of hArg level was found. In the subgroup with stage CKD 2, the cut-off point of 1.59 μM threshold was characterized by greater sensitivity and specificity than in the subgroup with stage CKD 1 with 1.66 μM threshold level of hArg. For the full group, the hArg cut-off threshold was 1.60 μM, which is about to 0.2 μM lower than the lower limit of the reference interval for healthy individuals. It can be assumed that even before the formation of symptoms of proteinuria and albuminuria, a significant part of individuals from population cohort develops a state of decreased AGAT activity, since the expression of this enzyme is associated with a certain regulatory feedback inhibition at the body level. As a result of the study, it can be noted that in patients with early stages of CKD in the age group 45-67 years, there is a disturbance of the kidneys metabolic function. These metabolic changes can be detected by testing the level of hArg.

Stereochemical Assignment and Absolute Abundance of Nonproteinogenic Amino Acid Homoarginine in Marine Sponges

Together with arginine, the nonproteinogenic amino acid homoarginine is a substrate for the production of vasodilator nitric oxide in the human body. In marine sponges, homoarginine has been postulated to serve as a precursor for the biosynthesis of pyrrole-imidazole alkaloid and bromotyrosine alkaloid classes of natural products. The absolute abundance of homoarginine, its abundance relative to arginine, and its stereochemical assignment in marine sponges are not known. Here, using stable isotope dilution mass spectrometry, we quantify the absolute abundances of homoarginine and arginine in marine sponges. We find that the abundance of homoarginine is highly variable and can far exceed the concentration of arginine, even in sponges where incorporation of homoarginine in natural products cannot be rationalized. The [homoarginine]/[arginine] ratio in marine sponges is greater than that in human analytes. By derivatization of sponge extracts with Marfey's reagent and comparison with authentic standards, we determine the l-isomer of homoarginine to be exclusively present in sponges. Our results shed light on the presence of the high abundance of homoarginine in marine sponge metabolomes and provide the foundation to investigate the biosynthetic routes and physiological roles of this nonproteinogenic amino acid in sponge physiology.

Plasma homoarginine concentrations in ewe's pregnancy and association with the number of fetuses

A striking increase in homoarginine concentrations, about more than 100-fold that observed in humans, was recently reported during pregnancy in a nutritionally induced model of intra-uterine growth restriction in ewes. To determine whether this phenomenon is at least partially related to the nutritional regimen, estrus synchronization, or analytical method, thirty-four one-year-old primiparous, non-synchronized, and well-fed Sarda breed ewes were exposed to fertile rams allowing those who came into estrus to naturally mate. Plasma arginine, homoarginine, asymmetric dimethylarginine, symmetric dimethylarginine, mono methylarginine, and citrulline concentrations were measured in each sample using LC-MS/MS. Homoarginine concentrations showed a 44-fold variation between the highest and the lowest values while the fluctuations of arginine and its analogues and metabolites were much smaller, between 1.1 and 1.6-fold. Repeated-measures correlation analysis showed a significant negative correlation between homoarginine/arginine and arginine/asymmetric dimethylarginine ratios (Rm = -0.40; P < 0.000001). Furthermore, median homoarginine concentrations significantly increased with the number of fetuses. The marked increase in homoarginine concentrations with advancing gestational age is genuine and independent of mating, feeding, diet, and hormone treatment. The higher homoarginine concentrations found in ewes bearing multiple fetuses suggest the presence of a physiological link between this arginine analog and energy metabolism in pregnancy that warrants further investigation.

Age-Related Neurometabolomic Signature of Mouse Brain

Neurometabolites are the ultimate gene products in the brain and the most precise biomolecular indicators of brain endophenotypes. Metabolomics is the only "omics" that provides a moment-to-moment "snapshot" of brain circuits' biochemical activities in response to external stimuli within the context of specific genetic variations. Although the expression levels of neurometabolites are highly dynamic, the underlying metabolic processes are tightly regulated during brain development, maturation, and aging. Therefore, this study aimed to identify mouse brain metabolic profiles in neonatal and adult stages and reconstruct both the active metabolic network and the metabolic pathway functioning. Using high-throughput metabolomics and bioinformatics analyses, we show that the neonatal mouse brain has its distinct metabolomic signature, which differs from the adult brain. Furthermore, lipid metabolites showed the most profound changes between the neonatal and adult brain, with some lipid species reaching 1000-fold changes. There were trends of age-dependent increases and decreases among lipids and non-lipid metabolites, respectively. A few lipid metabolites such as HexCers and SHexCers were almost absent in neonatal brains, whereas other non-lipid metabolites such as homoarginine were absent in the adult brains. Several molecules that act as neurotransmitters/neuromodulators showed age-dependent levels, with adenosine and GABA exhibiting around 100- and 10-fold increases in the adult compared with the neonatal brain. Of particular interest is the observation that purine and pyrimidines nucleobases exhibited opposite age-dependent changes. Bioinformatics analysis revealed an enrichment of lipid biosynthesis pathways in metabolites, whose levels increased in adult brains. In contrast, pathways involved in the metabolism of amino acids, nucleobases, glucose (glycolysis), tricarboxylic acid cycle (TCA) were enriched in metabolites whose levels were higher in the neonatal brains. Many of these pathways are associated with pathological conditions, which can be predicted as early as the neonatal stage. Our study provides an initial age-related biochemical directory of the mouse brain and warrants further studies to identify temporal brain metabolome across the lifespan, particularly during adolescence and aging. Such neurometabolomic data may provide important insight about the onset and progression of neurological/psychiatric disorders and may ultimately lead to the development of precise diagnostic biomarkers and more effective preventive/therapeutic strategies.

Intrathecal and systemic alterations of L-arginine metabolism in patients after intracerebral hemorrhage

Alterations in the concentration of nitric oxide (NO) and L-arginine metabolites have been associated with the pathophysiology of different vascular diseases. Here, we describe striking changes in L-arginine metabolism after hemorrhagic stroke. Blood and cerebrospinal fluid (CSF) samples of patients with intracerebral hemorrhage (ICH) and/or intraventricular hemorrhage were collected over a ten-day period. Liquid chromatography-tandem mass spectrometry was used to quantify key substrates and products of L-arginine metabolizing enzymes as well as asymmetric (ADMA) and symmetric dimethylarginine (SDMA). Changes in the plasma were limited to early reductions in L-ornithine, L-lysine, and L-citrulline concentrations. Intrathecally, we observed signs of early NO synthase (NOS) upregulation followed by a decrease back to baseline accompanied by a rise in the level of its endogenous NOS-inhibitor ADMA. SDMA demonstrated increased levels throughout the observation period. For arginase, a pattern of persistently elevated activity was measured and arginine:glycine amidinotransferase (AGAT) appeared to be reduced in its activity at later time points. An early reduction in CSF L-arginine concentration was an independent risk factor for poor outcome. Together, these findings further elucidate pathophysiological mechanisms after ICH potentially involved in secondary brain injury and may reveal novel therapeutic targets.

The Pitfalls of in vivo Cardiac Physiology in Genetically Modified Mice - Lessons Learnt the Hard Way in the Creatine Kinase System

In order to fully understand gene function, at some point, it is necessary to study the effects in an intact organism. The creation of the first knockout mouse in the late 1980's gave rise to a revolution in the field of integrative physiology that continues to this day. There are many complex choices when selecting a strategy for genetic modification, some of which will be touched on in this review, but the principal focus is to highlight the potential problems and pitfalls arising from the interpretation of in vivo cardiac phenotypes. As an exemplar, we will scrutinize the field of cardiac energetics and the attempts to understand the role of the creatine kinase (CK) energy buffering and transport system in the intact organism. This story highlights the confounding effects of genetic background, sex, and age, as well as the difficulties in interpreting knockout models in light of promiscuous proteins and metabolic redundancy. It will consider the dose-dependent effects and unintended consequences of transgene overexpression, and the need for experimental rigour in the context of in vivo phenotyping techniques. It is intended that this review will not only bring clarity to the field of cardiac energetics, but also aid the non-expert in evaluating and critically assessing data arising from in vivo genetic modification.

Preweaning nutrient supply alters serum metabolomics profiles related to protein and energy metabolism and hepatic function in Holstein heifer calves

Lifting the preweaning milk restriction in dairy calves has been causally associated with beneficial effects on growth and future lactation performance. However, the biological mechanisms linking early-life nutrient supply and future performance remain insufficiently understood. Thus, the objective of this study was to characterize growth and the metabolic profiles of calves fed a restricted (RES) and an elevated (ELE) milk supply preweaning. A total of 86 female Holstein Friesian calves were blocked in pairs by maternal parity and received identical colostrum supply within block. Treatments randomized within block consisted of a milk replacer (MR; 24% crude protein, 18% crude fat, and 45% lactose) supplied at either 5.41 Mcal of ME in 8 L of MR/d (ELE) or 2.71 Mcal of ME in 4 L of MR/d (RES) from d 2 after birth until they were stepped down by 50% during wk 7 and fully weaned at wk 8. All calves had ad libitum access to pelleted calf starter (17.3% crude protein, 24.4% neutral detergent fiber, 2.0% crude fat, and 18.2% starch), chopped wheat straw, and water. At 2 and 49 d of age, blood samples were taken for metabolomics analysis. The ELE group by design consumed more milk replacer, resulting in a lower starter intake and a greater body weight and average daily gain. The ELE calves consumed 20.7% more ME and 9.7% more crude protein. However, efficiency of growth was not different between groups. Metabolomic profiling using 908 identified metabolites served to characterize treatment-dependent biochemical differences. Principal component analysis revealed clearly distinct metabolic profiles at 49 d of age in response to preweaning milk supply. Changes in energy (fatty acid metabolism and tricarboxylic acid metabolites), protein (free AA, dipeptides, and urea cycle), and liver metabolism (bile acid and heme metabolism) were the main effects associated with the dietary differences. The ELE group consumed proportionately more glucogenic nutrients via milk replacer, whereas the RES group consumed proportionately more ketogenic nutrients from the digestion of the calf starter, comprising a larger portion of total intake. Associated with the higher growth rate of the ELE group, hepatic changes were expressed as differences in bile acid and heme metabolism. Furthermore, energy metabolism differences were noted in fatty acid and AA metabolism and the urea cycle. The metabolic profile differences between the ELE and RES groups reflect the broad differences in nutrient intake and diet composition and might point to which metabolic processes are responsible for greater dairy performance for cows fed a greater milk supply preweaning.

Presence of Bromotyrosine Alkaloids in Marine Sponges Is Independent of Metabolomic and Microbiome Architectures

Marine sponge holobionts are prolific sources of natural products. One of the most geographically widespread classes of sponge-derived natural products is the bromotyrosine alkaloids. A distinguishing feature of bromotyrosine alkaloids is that they are present in phylogenetically disparate sponges. In this study, using sponge specimens collected from Guam, the Solomon Islands, the Florida Keys, and Puerto Rico, we queried whether the presence of bromotyrosine alkaloids potentiates metabolomic and microbiome conservation among geographically distant and phylogenetically different marine sponges. A multi-omic characterization of sponge holobionts revealed vastly different metabolomic and microbiome architectures among different bromotyrosine alkaloid-harboring sponges. However, we find statistically significant correlations between the microbiomes and metabolomes, signifying that the microbiome plays an important role in shaping the overall metabolome, even in low-microbial-abundance sponges. Molecules mined from the polar metabolomes of these sponges revealed conservation of biosynthetic logic between bromotyrosine alkaloids and brominated pyrrole-imidazole alkaloids, another class of marine sponge-derived natural products. In light of prior findings postulating the sponge host itself to be the biosynthetic source of bromotyrosine alkaloids, our data now set the stage for investigating the causal relationships that dictate the microbiome-metabolome interconnectedness for marine sponges in which the microbiome may not contribute to natural product biogenesis.IMPORTANCE Our work demonstrates that phylogenetically and geographically distant sponges with very different microbiomes can harbor natural product chemical classes that are united in their core chemical structures and biosynthetic logic. Furthermore, we show that independent of geographical dispersion, natural product chemistry, and microbial abundance, overall sponge metabolomes tightly correlate with their microbiomes.

Precursor-Guided Mining of Marine Sponge Metabolomes Lends Insight into Biosynthesis of Pyrrole-Imidazole Alkaloids

Pyrrole-imidazole alkaloids are natural products isolated from marine sponges, holobiont metazoans that are associated with symbiotic microbiomes. Pyrrole-imidazole alkaloids have attracted attention due to their chemical complexity and their favorable pharmacological properties. However, insights into how these molecules are biosynthesized within the sponge holobionts are scarce. Here, we provide a multiomic profiling of the microbiome and metabolomic architectures of three sponge genera that are prolific producers of pyrrole-imidazole alkaloids. Using a retrobiosynthetic scheme as a guide, we mine the metabolomes of these sponges to detect intermediates in pyrrole-imidazole alkaloid biosynthesis. Our findings reveal that the nonproteinogenic amino acid homoarginine is a critical branch point that connects primary metabolite lysine to the production of pyrrole-imidazole alkaloids. These insights are derived from the polar metabolomes of these sponges which additionally reveal the presence of zwitterionic betaines that may serve important ecological roles in marine habitats. We also establish that metabolomic richness does not correlate with microbial diversity of the sponge holobiont for neither the polar nor the nonpolar metabolomes. Our findings now provide the biochemical foundation for genomic interrogation of the sponge holobiont to establish biogenetic routes for pyrrole-imidazole alkaloid production.

Association of Lower Plasma Homoarginine Concentrations with Greater Risk of All-Cause Mortality in the Community: The Framingham Offspring Study

Lower circulating homoarginine concentrations have been associated with morbidity and mortality in patients with established cardiovascular disease (CVD). We assayed plasma homoarginine concentrations in 3331 Framingham Offspring Study participants attending examination cycle six (mean age 58.6 years, 53% women). We evaluated correlates of plasma homoarginine and related homoarginine to incident CVD and death. We also classified participants as having higher (upper quartile) versus lower (lower three quartiles) homoarginine and previously assayed asymmetric dimethylarginine (ADMA) concentrations, and created cross-classification groups. We observed 630 incident CVD events and 940 deaths during a median follow-up of 18 years. In multivariable regression analysis, homoarginine was associated positively with male sex, body mass index, anti-hypertensive medication use and systolic blood pressure, but inversely with age and smoking. Higher homoarginine levels were associated with a lower mortality risk (hazard ratio (HR) per SD increment, 0.83, 95% CI: 0.74–0.93) adjusting for standard CVD risk factors, and ADMA. Among the cross-classification groups, participants with higher homoarginine and lower ADMA had a lower mortality risk (HR, 0.81, 95% CI: 0.67–0.98) compared to those with low levels of both. Further studies are needed to dissect the mechanisms of the association of homoarginine and mortality over decades in the community.

Is Homoarginine a Protective Cardiovascular Risk Factor?

A series of recent epidemiological studies have implicated the endogenous nonproteinogenic amino acid l-homoarginine as a novel candidate cardiovascular risk factor. The association between homoarginine levels and the risk of adverse cardiovascular outcomes is inverse (ie, high cardiovascular risk is predicted by low rather than high homoarginine levels), which makes it plausible to normalize systemic homoarginine levels via oral supplementation. The emergence of homoarginine as a potentially treatable protective cardiovascular risk factor has generated a wave of hope in the field of cardiovascular prevention. Herein, we review the biochemistry, physiology, and metabolism of homoarginine, summarize the strengths and weaknesses of the epidemiological evidence linking homoarginine to cardiovascular disease and its potential protective cardiovascular effects, and identify priorities for future research needed to define the clinical utility of homoarginine as a prognostic factor and therapeutic target in cardiovascular disease.

Metabolomic approach to the exploration of biomarkers associated with disease activity in rheumatoid arthritis

We aimed to investigate metabolites associated with the 28-joint disease activity score based on erythrocyte sedimentation rate (DAS28-ESR) in patients with rheumatoid arthritis (RA) using capillary electrophoresis quadrupole time-of-flight mass spectrometry. Plasma and urine samples were collected from 32 patients with active RA (DAS28-ESR≥3.2) and 17 with inactive RA (DAS28-ESR<3.2). We found 15 metabolites in plasma and 20 metabolites in urine which showed a significant but weak positive or negative correlation with DAS28-ESR. When metabolites between active and inactive patients were compared, 9 metabolites in plasma and 15 in urine were found to be significantly different. Consequently, we selected 11 metabolites in plasma and urine as biomarker candidates which significantly correlated positively or negatively with DAS28-ESR, and significantly differed between active and inactive patients. When a multiple logistic regression model was built to discriminate active and inactive cohorts, three variables-histidine and guanidoacetic acid from plasma and hypotaurine from urine-generated a high area under the receiver operating characteristic (ROC) curve value (AUC = 0.8934). Thus, this metabolomics approach appeared to be useful for investigating biomarkers of RA. Combination of plasma and urine analysis may lead to more precise and reliable understanding of the disease condition. We also considered the pathophysiological significance of the found biomarker candidates.

Impaired cardiac contractile function in arginine:glycine amidinotransferase knockout mice devoid of creatine is rescued by homoarginine but not creatine

Aims

Creatine buffers cellular adenosine triphosphate (ATP) via the creatine kinase reaction. Creatine levels are reduced in heart failure, but their contribution to pathophysiology is unclear. Arginine:glycine amidinotransferase (AGAT) in the kidney catalyses both the first step in creatine biosynthesis as well as homoarginine (HA) synthesis. AGAT-/- mice fed a creatine-free diet have a whole body creatine-deficiency. We hypothesized that AGAT-/- mice would develop cardiac dysfunction and rescue by dietary creatine would imply causality.

Methods and results

Withdrawal of dietary creatine in AGAT-/- mice provided an estimate of myocardial creatine efflux of ∼2.7%/day; however, in vivo cardiac function was maintained despite low levels of myocardial creatine. Using AGAT-/- mice naïve to dietary creatine we confirmed absence of phosphocreatine in the heart, but crucially, ATP levels were unchanged. Potential compensatory adaptations were absent, AMPK was not activated and respiration in isolated mitochondria was normal. AGAT-/- mice had rescuable changes in body water and organ weights suggesting a role for creatine as a compatible osmolyte. Creatine-naïve AGAT-/- mice had haemodynamic impairment with low LV systolic pressure and reduced inotropy, lusitropy, and contractile reserve. Creatine supplementation only corrected systolic pressure despite normalization of myocardial creatine. AGAT-/- mice had low plasma HA and supplementation completely rescued all other haemodynamic parameters. Contractile dysfunction in AGAT-/- was confirmed in Langendorff perfused hearts and in creatine-replete isolated cardiomyocytes, indicating that HA is necessary for normal cardiac function.

Conclusions

Our findings argue against low myocardial creatine per se as a major contributor to cardiac dysfunction. Conversely, we show that HA deficiency can impair cardiac function, which may explain why low HA is an independent risk factor for multiple cardiovascular diseases.

Amino Acids and Their Metabolism in Atherosclerosis

As a leading cause of death worldwide, cardiovascular disease is a global health concern. The development and progression of atherosclerosis, which ultimately gives rise to cardiovascular disease, has been causally linked to hypercholesterolemia. Mechanistically, the interplay between lipids and the immune system during plaque progression significantly contributes to the chronic inflammation seen in the arterial wall during atherosclerosis. Localized inflammation and increased cell-to-cell interactions may influence polarization and proliferation of immune cells via changes in amino acid metabolism. Specifically, the amino acids l -arginine (Arg), l -homoarginine (hArg) and l -tryptophan (Trp) have been widely studied in the context of cardiovascular disease, and their metabolism has been established as key regulators of vascular homeostasis, as well as immune cell function. Cyclic effects between endothelial cells, innate, and adaptive immune cells exist during Arg and hArg, as well as Trp metabolism, that may have distinct effects on the development of atherosclerosis. In this review, we describe the current knowledge surrounding the metabolism, biological function, and clinical perspective of Arg, hArg, and Trp in the context of atherosclerosis.

Phosphagen kinase function in flagellated spores of the oomycete Phytophthora infestans integrates transcriptional regulation, metabolic dynamics and protein retargeting

Flagellated spores play important roles in the infection of plants and animals by many eukaryotic microbes. The oomycete Phytophthora infestans, which causes potato blight, expresses two phosphagen kinases (PKs). These enzymes store energy in taurocyamine, and are hypothesized to resolve spatial and temporal imbalances between rates of ATP creation and use in zoospores. A dimeric PK is found at low levels in vegetative mycelia, but high levels in ungerminated sporangia and zoospores. In contrast, a monomeric PK protein is at similar levels in all tissues, although is transcribed primarily in mycelia. Subcellular localization studies indicate that the monomeric PK is mitochondrial. In contrast, the dimeric PK is cytoplasmic in mycelia and sporangia but is retargeted to flagellar axonemes during zoosporogenesis. This supports a model in which PKs shuttle energy from mitochondria to and through flagella. Metabolite analysis indicates that deployment of the flagellar PK is coordinated with a large increase in taurocyamine, synthesized by sporulation-induced enzymes that were lost during the evolution of zoospore-lacking oomycetes. Thus, PK function is enabled by coordination of the transcriptional, metabolic and protein targeting machinery during the life cycle. Since plants lack PKs, the enzymes may be useful targets for inhibitors of oomycete plant pathogens.

Low-Circulating Homoarginine is Associated with Dilatation and Decreased Function of the Left Ventricle in the General Population

Low homoarginine is an independent marker of mortality in heart failure patients and incident cardiovascular events. Whether homoarginine is related with healthier cardiac structure and function is currently unclear. We used data of the population-based "Study of Health in Pomerania" (SHIP-Trend) to assess this relation. Homoarginine was measured in serum using liquid chromatography-tandem mass spectrometry. Linear regression models assessed the relation between homoarginine and several structural as well as functional parameters and N-terminal pro B-type natriuretic peptide (NTproBNP). All models were adjusted for age, sex, body mass index, and renal function. A total of 3113 subjects (median age 48 (25th percentile 37 to 75th percentile 60) years, 46% male) were included. A standard deviation decrease in homoarginine was associated with a larger left ventricular diastolic diameter (0.3; 95%-confidence interval (CI): 0.2 to 0.5 mm; p < 0.001), left ventricular systolic diameter (0.38; 95%-CI: -0.22 to 0.54 mm; p < 0.001) as well as a less relative wall thickness (⁻0.003 95%-CI: -0.006 to -0.0008; p = 0.01), left ventricular ejection fraction (⁻0.47; 95%-CI: ⁻0.79 to -0.15%; p < 0.01) and fractional shortening (-0.35; 95%-CI: -0.62 to 0.07%; p = 0.01). Low homoarginine was also related to higher NTproBNP (-0.02 95%-CI: -0.034 to -0.009 log pg/mL; p < 0.01). Lower serum homoarginine is associated with dilatation of the heart and decreased function. Prospective clinical studies should assess if homoarginine supplementation improves cardiac health in subjects with low serum concentrations.

L-Homoarginine and its AGXT2-metabolite GOCA in chronic kidney disease as markers for clinical status and prognosis

Plasma concentrations of L-homoarginine (hArg) are an emerging marker for clinical status and prognosis in renal and cardiovascular disease. Lowered hArg concentrations are associated with higher risk for these conditions, although a clear pathophysiological explanation for this association has not been established. Baseline plasma samples of patients with different stages of chronic kidney disease (CKD) (n = 527) were obtained from the CARE FOR HOMe study and were analyzed for hArg and, for the first time, its metabolite 6-guanidino-2-oxocaproic acid (GOCA) by isotope dilution LC-MS/MS methods. GOCA is converted from hArg by the enzyme alanine:glyoxylate aminotransferase 2 (AGXT2), which is also in the focus of current cardiovascular research. hArg levels ranged from 0.20-4.01 µmol/L with a median of 1.42 µmol/L, whereas GOCA levels were 0.08-25.82 nmol/L with a median of 1.45 nmol/L. hArg levels in the highest tertile (≥ 1.71 µmol/L) were associated with significantly lower risk for reaching the renal (hazard ratio 0.369, 95% confidence interval 0.028-0.655) or cardiovascular (HR 0.458, CI 0.295-0.712) endpoints in univariate Cox regression analysis. Inversely, GOCA levels in the highest tertile (≥ 2.13 nmol/L) were associated with increased renal (HR 3.807, CI 1.963-7.381) and cardiovascular (HR 1.611, CI 1.041-2.495) risk. A decreased ratio between hArg and GOCA predicted even more pronounced the risks for renal (HR 0.178, CI 0.087-0.363) and cardiovascular (HR 0.447, CI 0.281-0.709) events. However, adjustment for the confounders eGFR and albuminuria attenuated these findings. A pathophysiological role of an increased activity of AGXT2 in CKD should be evaluated in future clinical studies.

Delayed Influence of Spinal Cord Injury on the Amino Acids of NO• Metabolism in Rat Cerebral Cortex Is Attenuated by Thiamine

Severe spinal cord injuries (SCIs) result in chronic neuroinflammation in the brain, associated with the development of cognitive and behavioral impairments. Nitric oxide (NO•) is a gaseous messenger involved in neuronal signaling and inflammation, contributing to nitrosative stress under dysregulated production of reactive nitrogen species. In this work, biochemical changes induced in the cerebral cortex of rats 8 weeks after SCI are assessed by quantification of the levels of amino acids participating in the NO• and glutathione metabolism. The contribution of the injury-induced neurodegeneration is revealed by comparison of the SCI- and laminectomy (LE)-subjected animals. Effects of the operative interventions are assessed by comparison of the operated (LE/SCI) and non-operated animals. Lower ratios of citrulline (Cit) to arginine (Arg) or Cit to ornithine and a more profound decrease in the ratio of lysine to glycine distinguish SCI animals from those after LE. The data suggest decreased NO• production from both Arg and homoarginine in the cortex 8 weeks after SCI. Both LE and SCI groups show a strong decrease in the level of cortex glutathione. The neurotropic, anti-inflammatory, and antioxidant actions of thiamine (vitamin B1) prompted us to study the thiamine effects on the SCI-induced changes in the NO• and glutathione metabolism. A thiamine injection (400 mg/kg intraperitoneally) within 24 h after SCI abrogates the changes in the cerebral cortex amino acids related to NO•. Thiamine-induced normalization of the brain glutathione levels after LE and SCI may involve increased supply of glutamate for glutathione biosynthesis. Thus, thiamine protects from sequelae of SCI on NO•-related amino acids and glutathione in cerebral cortex.

Sex Differences in the Associations between L-Arginine Pathway Metabolites, Skeletal Muscle Mass and Function, and their Responses to Resistance Exercise, in Old Age

OBJECTIVES

The current study was designed to explore the associations between L-arginine metabolites and muscle mass and function in old age, which are largely unknown.

DESIGN

The study used a randomised, double-blind, placebo-controlled design.

SETTING

The study was carried out in a laboratory setting.

PARTICIPANTS

50 healthy older adults [median age 70 years (IQR 67-73); 27 males].

INTERVENTION

Participants undertook an 18-week resistance exercise program, and a nutritional intervention (fish oil vs. placebo).

MEASUREMENTS

Serum homoarginine, ornithine, citrulline, asymmetric dimethylarginine (ADMA), NG-monomethyl-L-arginine (L-NMMA), and symmetric dimethylarginine (SDMA), maximal voluntary contraction (MVC) and isokinetic torque of the knee extensors at 30° s-1 (MIT), muscle cross sectional area (MCSA) and quality (MQ) were measured at baseline and after the intervention.

RESULTS

No significant exercise-induced changes were observed in metabolite concentrations. There were significant sex differences in the associations between metabolites and muscle parameters. After adjusting for age, glomerular filtration rate and fish oil intervention, citrulline (P=0.002) and ornithine (P=0.022) were negatively associated with MCSA at baseline in males but not females. However, baseline citrulline was negatively correlated with exercise-induced changes in MVC (P=0.043) and MQ (P=0.026) amongst females. Furthermore, amongst males, baseline homoarginine was positively associated with exercise-induced changes in MVC (P=0.026), ADMA was negatively associated with changes in MIT (P=0.026), L-NMMA (p=0.048) and ornithine (P<0.001) were both positively associated with changes in MCSA, and ornithine was negatively associated with changes in MQ (P=0.039).

CONCLUSION

Therefore, barring citrulline, there are significant sex differences in the associations between L-arginine metabolites and muscle mass and function in healthy older adults. These metabolites might enhance sarcopenia risk stratification, and the success of exercise programs, in old age.

The role of L-arginine/L-homoarginine/nitric oxide pathway for aortic distensibility and intima-media thickness in stroke patients

Asymmetric dimethylarginine (ADMA) and L-homoarginine (hArg) are L-arginine (Arg) metabolites derived from different pathways. Protein arginine N-methyltransferase (PRMT) and subsequent proteolysis of proteins containing methylarginine residues release ADMA. Arginine:glycine amidinotransferase (AGAT) converts Arg to hArg and guanidinoacetate (GAA). While high concentrations of ADMA and low concentrations of hArg in the blood have been established as cardiovascular risk markers, the cardiovascular relevance of GAA is still unexplored. Arg and hArg are substrates and ADMA is an inhibitor of nitric oxide (NO) synthase (NOS). The cardiovascular effects of ADMA and hArg have been related to NO, a potent endogenous vasodilator. ADMA and hArg are considered to exert additional, not yet explored, presumably NO-unrelated effects and to act antagonistically in the renal and cardiovascular systems. Although the physiological role of Arg, ADMA, hArg and NO for endothelial function in small- and medium-sized arteries has been intensively studied in the past, the clinical relevance of aortic wall remodeling still remains unclear. Here, we evaluated potential relation between aortic distensibility (AD) or aortic intima-media thickness (aIMT) and circulating ADMA, hArg, GAA, and the NO metabolites nitrite and nitrate in the plasma of 78 patients (24 females, 54 males; aged 59 ± 14 years) with recent ischemic stroke or transient ischemic attack (TIA). All biochemical parameters were determined by stable-isotope dilution gas chromatography-mass spectrometry. AD and aIMT were measured by transesophageal echocardiography. Arg, hArg, ADMA and GAA median plasma concentrations (µM) were determined to be 61, 1.43, 0.50 and 2.16, respectively. hArg, ADMA and GAA correlated closely with Arg. Nitrite, nitrate and creatinine median plasma concentrations (µM) were 2.49, 48.7, and 84.1, respectively. Neither AD (2.61 vs. 1.85 10^-6 × cm² × dyn^-1, P = 0.064) nor aIMT (1.25 vs. 1.13 mm, P = 0.596) differed between females and males. The hArg/ADMA molar ratio (r = -0.351, P = 0.009), nitrate (r = 0.364, P = 0.007) and nitrite (r = 0.329, P = 0.015) correlated with aIMT but not with AD. Arg, hArg, ADMA and GAA correlated with aIMT but not with AD. The results demonstrate a strong relation between the Arg/NO pathway and aortic atherosclerosis but not with AD suggesting different mechanisms underlying the two aspects of aortic wall remodeling.

Reference intervals of plasma homoarginine from the German Gutenberg Health Study

BACKGROUND

Low circulating homoarginine has been associated with adverse cardiovascular (CV) outcome and mortality in patients at risk and in the general population. The present study aimed to define plasma homoarginine reference intervals from a representative population sample to improve risk stratification between healthy individuals and individuals at risk.

METHODS

We determined age- and sex-specific reference intervals for circulating plasma homoarginine in a subgroup of 786 healthy participants (no CV disease or risk factors) of the Gutenberg Health Study. Homoarginine concentrations were measured using a validated liquid chromatography-tandem mass spectrometry method.

RESULTS

Median EDTA plasma homoarginine concentration was 1.88 [25th; 75th percentile, 1.47; 2.41] μmol/L, with lower concentrations in women (1.77 [1.38; 2.26] μmol/L) than in men (2.01 [1.61; 2.56] μmol/L; p<0.001). Sex-specific 2.5th and 97.5th percentiles of reference intervals were 0.84 and 3.89 μmol/L in women and 0.98 and 4.10 μmol/L in men, respectively. Homoarginine concentrations also depended on age and single nucleotide polymorphisms related to the L-arginine:glycine amidinotransferase gene.

CONCLUSIONS

We provide plasma homoarginine reference intervals in men and women of the general population. The determination of homoarginine levels might be favorable for individual risk stratification.

Arginine Metabolism Revisited

Mammalian arginine metabolism is complex due to the expression of multiple enzymes that utilize arginine as substrate and to interactions or competition between specific enzymes involved in arginine metabolism. Moreover, cells may contain multiple intracellular arginine pools that are not equally accessible to all arginine metabolic enzymes, thus presenting additional challenges to more fully understanding arginine metabolism. At the whole-body level, arginine metabolism ultimately results in the production of a biochemically diverse range of products, including nitric oxide, urea, creatine, polyamines, proline, glutamate, agmatine, and homoarginine. Included in this group of compounds are the methylated arginines (e.g., asymmetric dimethylarginine), which are released upon degradation of proteins containing methylated arginine residues. Changes in arginine concentration also can regulate cellular metabolism and function via a variety of arginine sensors. Although much is known about arginine metabolism, elucidation of the physiologic or pathophysiologic roles for all of the pathways and their metabolites remains an active area of investigation, as exemplified by current findings highlighted in this review.

A Novel Pathway for Metabolism of the Cardiovascular Risk Factor Homoarginine by alanine:glyoxylate aminotransferase 2

Low plasma concentrations of L-homoarginine are associated with an increased risk of cardiovascular events, while homoarginine supplementation is protective in animal models of metabolic syndrome and stroke. Catabolism of homoarginine is still poorly understood. Based on the recent findings from a Genome Wide Association Study we hypothesized that homoarginine can be metabolized by alanine:glyoxylate aminotransferase 2 (AGXT2). We purified human AGXT2 from tissues of AGXT2 transgenic mice and demonstrated its ability to metabolize homoarginine to 6-guanidino-2-oxocaproic acid (GOCA). After incubation of HepG2 cells overexpressing AGXT2 with isotope-labeled homoarginine-d4 we were able to detect labeled GOCA in the medium. We injected wild type mice with labeled homoarginine and detected labeled GOCA in the plasma. We found that AGXT2 knockout (KO) mice have higher homoarginine and lower GOCA plasma levels as compared to wild type mice, while the reverse was true for AGXT2 transgenic (Tg) mice. In summary, we experimentally proved the presence of a new pathway of homoarginine catabolism – its transamination by AGXT2 with formation of GOCA and demonstrated that endogenous AGXT2 is required for maintenance of homoarginine levels in mice. Our findings may lead to development of novel therapeutic approaches for cardiovascular pathologies associated with homoarginine deficiency.

Oral supplementation with L-homoarginine in young volunteers

AIMS

Low blood concentrations of the naturally occurring amino acid L-homoarginine (L-hArg) are related to impaired cardiovascular outcome and mortality in humans and animals. L-hArg is a weak substrate of nitric oxide synthase and an inhibitor of arginases in vitro. The aim of our study was to obtain kinetic and dynamic data after oral L-hArg supplementation.

METHODS

In a double-blind, randomized, placebo-controlled crossover study, 20 young volunteers received 125 mg L-hArg once daily for 4 weeks. Kinetic parameters (C_max , T_max and AUC_0-24h ) were calculated after ingestion of single and multiple doses of oral supplementation as primary endpoint. Secondary endpoints that were evaluated were routine laboratory, L-arginine, asymmetric dimethylarginine (ADMA), pulse wave velocity (PWV), augmentation index (AIx), flow-mediated vasodilatation (FMD), corticospinal excitability, i.e. motor threshold (MT), and cortical excitability, i.e. intracortical inhibition (ICI) and facilitation (ICF).

RESULTS

One hour after ingestion (T_max ), L-hArg increased the baseline L-hArg plasma concentration (2.87 ± 0.91 μmol l^-1 , mean ± SD) by 8.74 ± 4.46 [95% confidence intervals 6.65; 10.9] and 17.3 ± 4.97 [14.9; 19.6] μmol l^-1 (C_max ), after single and multiple doses, respectively. Once-only and 4 weeks of supplementation resulted in AUCs_0-24h of 63.5 ± 28.8 [50.0; 76.9] and 225 ± 78.5 [188; 2624] μmol l^-1 *h, for single and multiple doses, respectively. Routine laboratory parameters, L-arginine, ADMA, PWV, AIx, FMD, MT, ICI and ICF did not change by L-hArg supplementation compared to baseline.

CONCLUSION

Once daily orally applied 125 mg L-hArg raises plasma L-hArg four- and sevenfold after single dose and 4 weeks of supplementation, respectively, and is safe and well tolerated in young volunteers.

L-homoarginine administration reduces neointimal hyperplasia in balloon-injured rat carotids

Supplementary Material to this article is available at www.thrombosis-online.com.

Characterization of AmtA, an amidinotransferase involved in the biosynthesis of phaseolotoxins

Phaseolotoxins (PHTs), which are produced by Pseudomonas, belong to a family of phosphoramidate natural products. Two nonproteinogenic amino acid precursors, N^δ(N′‐sulfo‐diaminophosphinyl)‐ornithine (PSOrn) and homoarginine (hArg), are involved in biosynthesis of PHTs. Amidinotransferase AmtA catalyses the formation of hArg, with arginine and lysine as substrates. AmtA was overexpressed and purified in an Escherichia coli system. An in vitro enzyme assay showed that it has stricter substrate specificity than certain other amidinotransferases. Site‐directed mutagenesis experiments showed that the mutation AmtA Met243His244 is an alternative while Met246 is essential for the transamidination activity.

Whole-body synthesis of L-homoarginine in pigs and rats supplemented with L-arginine

Recent studies suggest an important role for L-homoarginine in cardiovascular, hepatic and neurological functions, as well as the regulation of glucose metabolism. However, little is known about whole-body L-homoarginine synthesis or its response to dietary L-arginine intake in animals. Four series of experiments were conducted to determine L-homoarginine synthesis and catabolism in pigs and rats. In Experiment 1, male and female pigs were fed a corn- and soybean meal-based diet supplemented with 0.0-2.42 % L-arginine-HCl. In Experiment 2, male and female rats were fed a casein-based diet, while receiving drinking water containing supplemental L-arginine-HCl to provide 0.0-3.6 g L-arginine/kg body-weight/day. In both experiments, urine collected from the animals for 24 h was analyzed for L-homoarginine and related metabolites. In Experiment 3, pigs and rats received a single oral dose of 1 or 10 mg L-homoarginine/kg body-weight, respectively, and their urine was collected for 24 h for analyses of L-homoarginine and related substances. In Experiment 4, slices of pig and rat tissues (including liver, brain, kidney, heart, and skeletal-muscle) were incubated for 1 h in Krebs-bicarbonate buffer containing 5 or 50 µM L-homoarginine. Our results indicated that: (a) animal tissues did not degrade L-homoarginine in the presence of physiological concentrations of other amino-acids; (b) 95-96 % of orally administered L-homoarginine was recovered in urine; (c) L-homoarginine was quantitatively a minor product of L-arginineg catabolism in the body; and (d) dietary L-arginine supplementation dose-dependently increased whole-body L-homoarginine synthesis. These novel findings provide a new framework for future studies of L-homoarginine metabolism and physiology in animals and humans.

Arginine:glycine amidinotransferase (AGAT) deficiency: Clinical features and long term outcomes in 16 patients diagnosed worldwide

BACKGROUND

Arginine:glycine aminotransferase (AGAT) (GATM) deficiency is an autosomal recessive inborn error of creative synthesis.

OBJECTIVE

We performed an international survey among physicians known to treat patients with AGAT deficiency, to assess clinical characteristics and long-term outcomes of this ultra-rare condition.

RESULTS

16 patients from 8 families of 8 different ethnic backgrounds were included. 1 patient was asymptomatic when diagnosed at age 3 weeks. 15 patients diagnosed between 16 months and 25 years of life had intellectual disability/developmental delay (IDD). 8 patients also had myopathy/proximal muscle weakness. Common biochemical denominators were low/undetectable guanidinoacetate (GAA) concentrations in urine and plasma, and low/undetectable cerebral creatine levels. 3 families had protein truncation/null mutations. The rest had missense and splice mutations. Treatment with creatine monohydrate (100-800 mg/kg/day) resulted in almost complete restoration of brain creatine levels and significant improvement of myopathy. The 2 patients treated since age 4 and 16 months had normal cognitive and behavioral development at age 10 and 11 years. Late treated patients had limited improvement of cognitive functions.

CONCLUSION

AGAT deficiency is a treatable intellectual disability. Early diagnosis may prevent IDD and myopathy. Patients with unexplained IDD with and without myopathy should be assessed for AGAT deficiency by determination of urine/plasma GAA and cerebral creatine levels (via brain MRS), and by GATM gene sequencing.

Homoarginine, arginine, and relatives: analysis, metabolism, transport, physiology, and pathology

The year 2008 witnessed the first report on the increase in the concentration of L-homoarginine (hArg) in the maternal plasma during human pregnancy. This observation, along with a well-known function of hArg, the methylene homologue of L-arginine (Arg), as a substrate for nitric oxide (NO) synthase, was the ignition for the start of intense research on the physiology and pathology of hArg. The circulating concentration of hArg was found to be lower in patients suffering from various diseases, and hArg emerged within only very few years as a novel cardiovascular risk factor. The compendium in hand comprises original and review articles covering several aspects of hArg, Arg and its symmetrically and asymmetrically guanidine (N (G))-dimethylated derivatives SDMA and ADMA, respectively. In contrast to ADMA and SDMA, low hArg concentrations in plasma or serum and in urine are associated with high risks for morbidity and mortality, notably in the renal and cardiovascular systems. Acutely and chronically administered Arg as a nutritional supplement or in the form of dietary proteins is safe in animals and humans and leads to concomitant formation of hArg and ADMA, albeit in a different hArg/ADMA ratio. Despite the close but opposite associations of hArg and ADMA with disease in adults, children and adolescents, the underlying biochemical processes are largely unknown, presumably not restricted to NO, and warrant deeper investigation. As the common substrate for hArg and ADMA, Arg may play a key role in the biosynthesis and homeostasis of hArg and ADMA, two putative antagonists. In animal models of stroke and obesity, hArg has beneficial effects. The potential utility of hArg as a therapeutic drug or nutritional supplement in humans and animals remains to be elaborated.

Biosynthesis of homoarginine (hArg) and asymmetric dimethylarginine (ADMA) from acutely and chronically administered free L-arginine in humans

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide (NO) synthesis, whereas L-arginine (Arg) and L-homoarginine (hArg) serve as substrates for NO synthesis. ADMA and other methylated arginines are generally believed to exclusively derive from guanidine (N (G))-methylated arginine residues in proteins by protein arginine methyltransferases (PRMTs) that use S-adenosylmethionine (SAM) as the methyl donor. L-Lysine is known for decades as a precursor for hArg, but only recent studies indicate that arginine:glycine amidinotransferase (AGAT) is responsible for the synthesis of hArg. AGAT catalyzes the formation of guanidinoacetate (GAA) that is methylated to creatine by guanidinoacetate methyltransferase (GAMT) which also uses SAM. The aim of the present study was to learn more about the mechanisms of ADMA and hArg formation in humans. Especially, we hypothesized that ADMA is produced by N (G)-methylation of free Arg in addition to the known PRMTs-involving mechanism. In knockout mouse models of AGAT- and GAMT-deficiency, we investigated the contribution of these enzymes to hArg synthesis. Arg infusion (0.5 g/kg, 30 min) in children (n = 11) and ingestion of high-fat protein meals by overweight men (n = 10) were used to study acute effects on ADMA and hArg synthesis. Daily Arg ingestion (10 g) or placebo for 3 or 6 months by patients suffering from peripheral arterial occlusive disease (PAOD, n = 20) or coronary artery disease (CAD, n = 30) was used to study chronic effects of Arg on ADMA synthesis. Mass spectrometric methods were used to measure all biochemical parameters in plasma and urine samples. In mice, AGAT but not GAMT was found to contribute to plasma hArg, while ADMA synthesis was independent of AGAT and GAMT. Arg infusion acutely increased plasma Arg, hArg and ADMA concentrations, but decreased the plasma hArg/ADMA ratio. High-fat protein meals acutely increased plasma Arg, hArg, ADMA concentrations, as well as the plasma hArg/ADMA ratio. In the PAOD and CAD studies, plasma Arg concentration increased in the verum compared to the placebo groups. Plasma ADMA concentration increased only in the PAOD patients who received Arg. Our study suggests that in humans a minor fraction of free Arg is rapidly metabolized to ADMA and hArg. In mice, GAMT and N (G)-methyltransferases contribute to ADMA and hArg synthesis from Arg, whereas AGAT is involved in the synthesis of hArg but not of ADMA. The underlying biochemical mechanisms remain still elusive.

Safety of long-term dietary supplementation with L-arginine in rats

This study was conducted with rats to determine the safety of long-term dietary supplementation with L-arginine. Beginning at 6 weeks of age, male and female rats were fed a casein-based semi-purified diet containing 0.61 % L-arginine and received drinking water containing L-arginine-HCl (0, 1.8, or 3.6 g L-arginine/kg body-weight/day; n = 10/group). These supplemental doses of L-arginine were equivalent to 0, 286, and 573 mg L-arginine/kg body-weight/day, respectively, in humans. After a 13-week supplementation period, blood samples were obtained from rats for biochemical analyses. Supplementation with L-arginine increased plasma concentrations of arginine, ornithine, proline, homoarginine, urea, and nitric oxide metabolites without affecting those for lysine, histidine, or methylarginines, while reducing plasma concentrations of ammonia, glutamine, free fatty acids, and triglycerides. L-Arginine supplementation enhanced protein gain and reduced white-fat deposition in the body. Based on general appearance, feeding behavior, and physiological parameters, all animals showed good health during the entire experimental period; Plasma concentrations of all measured hormones (except leptin) did not differ between control and arginine-supplemented rats. L-Arginine supplementation reduced plasma levels of leptin. Additionally, L-arginine supplementation increased L-arginine:glycine amidinotransferase activity in kidneys but not in the liver or small intestine, suggesting tissue-specific regulation of enzyme expression by L-arginine. Collectively, these results indicate that dietary supplementation with L-arginine (e.g., 3.6 g/kg body-weight/day) is safe in rats for at least 91 days. This dose is equivalent to 40 g L-arginine/kg body-weight/day for a 70-kg person. Our findings help guide clinical studies to determine the safety of long-term oral administration of L-arginine to humans.