Enzymological evidence for the indispensability of small intestine in the synthesis of arginine from glutamate. II. N-acetylglutamate synthase

Intermittent Fasting Enhanced the Cognitive Function in Older Adults with Mild Cognitive Impairment by Inducing Biochemical and Metabolic changes: A 3-Year Progressive Study

Intermittent fasting (IF) refers to various dietary regimens that cycle between a period of non-fasting and a period of total fasting. This study aimed to determine the effects of IF on cognitive function among elderly individuals who practice IF who have mild cognitive impairment (MCI). A total of 99 elderly subjects with MCI of Malay ethnicity without any terminal illness were recruited from a larger cohort study, LRGS TUA. The subjects were divided into three groups, comprising those who were regularly practicing IF (r-IF), irregularly practicing IF (i-IF), and non-fasters (n-IF). Upon 36 months of follow-up, more MCI subjects in the r-IF group reverted to successful aging with no cognitive impairment and diseases (24.3%) compared to those in i-IF (14.2%) and n-IF groups (3.7%). The r-IF group’s subjects exhibited significant increment in superoxide dismutase (SOD) activity and reduction in body weight, levels of insulin, fasting blood glucose, malondialdehyde (MDA), C-reactive protein (CRP), and DNA damage. Moreover, metabolomics analysis showed that IF may modulate cognitive function via various metabolite pathways, including the synthesis and degradation of ketone bodies, butanoate metabolism, pyruvate metabolism, and glycolysis and gluconeogenesis pathways. Overall, the MCI-afflicted older adults who practiced IF regularly had better cognitive scores and reverted to better cognitive function at 36 months follow-up.

Targeted metabolomic analysis of serum amino acids in the adult Fontan patient with a dominant left ventricle

Growing interest lies in the assessment of the metabolic status of patients with a univentricular circulation after Fontan operation, especially in changes of amino acid metabolism. Using targeted metabolomic examinations, we investigated amino acid metabolism in a homogeneous adult Fontan-patient group with a dominant left ventricle, seeking biomarker patterns that might permit better understanding of Fontan pathophysiology and early detection of subtle ventricular or circulatory dysfunction. We compared serum amino acid levels (42 analytes; AbsoluteIDQ p180 kit, Biocrates Life Sciences, Innsbruck, Austria) in 20 adult Fontan patients with a dominant left ventricle and those in age- and sex-matched biventricular controls. Serum concentrations of asymmetric dimethylarginine, methionine sulfoxide, glutamic acid, and trans-4-hydroxyproline and the methionine sulfoxide/methionine ratio (Met-SO/Met) were significantly higher and serum concentrations of asparagine, histidine, taurine, and threonine were significantly lower in patients than in controls. Met-SO/Met values exhibited a significant negative correlation with oxygen uptake during exercise. The alterations in amino acid metabolome that we found in Fontan patients suggest links between Fontan pathophysiology, altered cell energy metabolism, oxidative stress, and endothelial dysfunction like those found in biventricular patients with congestive heart failure. Studies of extended amino acid metabolism may allow better understanding of Fontan pathophysiology that will permit early detection of subtle ventricular or circulatory dysfunction in Fontan patients.

Glutamine Synthetase 1 Increases Autophagy Lysosomal Degradation of Mutant Huntingtin Aggregates in Neurons, Ameliorating Motility in a Drosophila Model for Huntington's Disease

Glutamine Synthetase 1 (GS1) is a key enzyme that catalyzes the ATP-dependent synthesis of l-glutamine from l-glutamate and is also member of the Glutamate Glutamine Cycle, a complex physiological process between glia and neurons that controls glutamate homeostasis and is often found compromised in neurodegenerative diseases including Huntington's disease (HD). Here we report that the expression of GS1 in neurons ameliorates the motility defects induced by the expression of the mutant Htt, using a Drosophila model for HD. This phenotype is associated with the ability of GS1 to favor the autophagy that we associate with the presence of reduced Htt toxic protein aggregates in neurons expressing mutant Htt. Expression of GS1 prevents the TOR activation and phosphorylation of S6K, a mechanism that we associate with the reduced levels of essential amino acids, particularly of arginine and asparagine important for TOR activation. This study reveals a novel function for GS1 to ameliorate neuronal survival by changing amino acids' levels that induce a "starvation-like" condition responsible to induce autophagy. The identification of novel targets that inhibit TOR in neurons is of particular interest for the beneficial role that autophagy has in preserving physiological neuronal health and in the mechanisms that eliminate the formation of toxic aggregates in proteinopathies.

A review of the alleged health hazards of monosodium glutamate

Monosodium glutamate (MSG) is an umami substance widely used as flavor enhancer. Although it is generally recognized as being safe by food safety regulatory agencies, several studies have questioned its long-term safety. The purpose of this review was to survey the available literature on preclinical studies and clinical trials regarding the alleged adverse effects of MSG. Here, we aim to provide a comprehensive overview of the reported possible risks that may potentially arise following chronic exposure. Furthermore, we intend to critically evaluate the relevance of this data for dietary human intake. Preclinical studies have associated MSG administration with cardiotoxicity, hepatotoxicity, neurotoxicity, low-grade inflammation, metabolic disarray and premalignant alterations, along with behavioral changes. Moreover, links between MSG consumption and tumorigenesis, increased oxidative stress and apoptosis in thymocytes, as well as genotoxic effects in lymphocytes have been reported. However, in reviewing the available literature, we detected several methodological flaws, which led us to conclude that these studies have limited relevance for extrapolation to dietary human intakes of MSG risk exposure. Clinical trials have focused mainly on the effects of MSG on food intake and energy expenditure. Besides its well-known impact on food palatability, MSG enhances salivary secretion and interferes with carbohydrate metabolism, while the impact on satiety and post-meal recovery of hunger varied in relation to meal composition. Reports on MSG hypersensitivity, also known as 'Chinese restaurant syndrome', or links of its use to increased pain sensitivity and atopic dermatitis were found to have little supporting evidence. Based on the available literature, we conclude that further clinical and epidemiological studies are needed, with an appropriate design, accounting for both added and naturally occurring dietary MSG. Critical analysis of existing literature, establishes that many of the reported negative health effects of MSG have little relevance for chronic human exposure and are poorly informative as they are based on excessive dosing that does not meet with levels normally consumed in food products.

Arginine and Polyamines Fate in Leishmania Infection

Leishmania is a protozoan parasite that alternates its life cycle between the sand fly and the mammalian host macrophages, involving several environmental changes. The parasite responds to these changes by promoting a rapid metabolic adaptation through cellular signaling modifications that lead to transcriptional and post-transcriptional gene expression regulation and morphological modifications. Molecular approaches such as gene expression regulation, next-generation sequencing (NGS), microRNA (miRNA) expression profiling, in cell Western blot analyses and enzymatic activity profiling, have been used to characterize the infection of murine BALB/c and C57BL/6 macrophages, as well as the human monocytic cell-lineage THP-1, with Leishmania amazonensis wild type (La-WT) or arginase knockout (La-arg^-). These models are being used to elucidate physiological roles of arginine and polyamines pathways and the importance of arginase for the establishment of the infection. In this review, we will describe the main aspects of Leishmania-host interaction, focusing on the arginine and polyamines pathways and pointing to possible targets to be used for prognosis and/or in the control of the infection. The parasite enzymes, arginase and nitric oxide synthase-like, have essential roles in the parasite survival and in the maintenance of infection. On the other hand, in mammalian macrophages, defense mechanisms are activated inducing alterations in the mRNA, miRNA and enzymatic profiles that lead to the control of infection. Furthermore, the genetic background of both parasite and host are also important to define the fate of infection.

Citrulline as a Biomarker for Gastrointestinal-Acute Radiation Syndrome: Species Differences and Experimental Condition Effects

Animal models of hematopoietic and gastrointestinal acute radiation syndromes (ARS) have been characterized to develop medical countermeasures. Acute radiation-induced decrease of intestinal absorptive function has been correlated to a decrease in the number of intestinal crypt cells resulting from apoptosis and enterocyte mass reduction. Citrulline, a noncoded amino acid, is produced almost exclusively by the enterocytes of the small intestine. Citrullinemia has been identified as a simple, sensitive and suitable biomarker for radiation-induced injury associated with gastrointestinal ARS (GI-ARS). Here we discuss the effect of radiation on plasma citrulline levels in three different species, C57BL/6 mice, Göttingen minipigs and rhesus nonhuman primates (NHPs), measured by liquid chromatography tandem mass spectrometry (LC-MS/MS). The effects of experimental study conditions such as feeding and anesthesia were also examined on plasma citrulline levels in the NHPs. Both the mice and Göttingen minipigs were partial-body irradiated (PBI) with doses from 13-17 Gy and 8-16 Gy, respectively, whereas NHPs were total-body irradiated (TBI) with doses from 6.72-13 Gy. Blood samples were taken at different time points and plasma citrulline levels were measured in the three species at baseline and after irradiation. Basal plasma citrulline concentrations (mean ± SEM) in mice and minipigs were 57.8 ± 2.8 μM and 63.1 ± 2.1 μM, respectively. NHPs showed a basal plasma citrulline concentration of 32.6 ± 0.7 μM, very similar to that of humans (∼40 μM). Plasma citrulline progressively decreased after irradiation, reaching nadir values between day 3.5 and 7. The onset of citrulline recovery was observed earlier at lower radiation doses, while only partial citrulline recovery was noted at higher radiation doses in minipigs and NHPs, complete recovery was noted in mice at all doses. Plasma citrulline levels in NHPs anesthetized with ketamine and acepromazine significantly decreased by 35.5% (P = 0.0017), compared to unanesthetized NHPs. In the postprandial state, citrulline concentrations in NHPs were slightly but significantly decreased by 12.2% (P = 0.0287). These results suggest that plasma citrulline is affected by experimental conditions such as anesthesia and feeding.

A pathway map of glutamate metabolism

Glutamate metabolism plays a vital role in biosynthesis of nucleic acids and proteins. It is also associated with a number of different stress responses. Deficiency of enzymes involved in glutamate metabolism is associated with various disorders including gyrate atrophy, hyperammonemia, hemolytic anemia, γ-hydoxybutyric aciduria and 5-oxoprolinuria. Here, we present a pathway map of glutamate metabolism representing metabolic intermediates in the pathway, 107 regulator molecules, 9 interactors and 3 types of post-translational modifications. This pathway map provides detailed information about enzyme regulation, protein-enzyme interactions, post-translational modifications of enzymes and disorders due to enzyme deficiency. The information included in the map was based on published experimental evidence reported from mammalian systems.

Serum citrulline as a diagnostic marker of sepsis-induced intestinal dysfunction

OBJECTIVE

To investigate the use of citrulline as an indicator for diagnosing septic acute intestinal dysfunction (SAID) in a rat model.

METHODS

SD rats were divided into three groups: a normal group (A), a model group (B), and a glutamine group (C). Group B was divided into a 36-h group (B1) and a 72-h feeding group (B2). The concentrations of serum citrulline, intestinal fatty acid-binding protein (I-FABP) and intestinal glutamine and histopathological changes were measured.

RESULTS

The lengths of the villus and thicknesses of the mucosal layer in groups B1, B2 and C were significantly different from those in group A. Citrulline concentrations in groups B1, B2 and C were lower than in group A; the serum concentrations in group C were significantly greater than in groups B1 and B2. The I-FABP levels of groups B1, B2 and C were higher than group A; I-FABP levels of groups B1 and B2 were higher than group C. Intestinal glutamine levels of groups B1 and B2 were lower than groups A and C. The serum citrulline of group C was negatively correlated with I-FABP and Chiu's score.

CONCLUSIONS

Serum citrulline could be used as the diagnostic indicator of SAID.

N-acetylglutamate synthase deficiency: an insight into the genetics, epidemiology, pathophysiology, and treatment

The conversion of ammonia into urea by the human liver requires the coordinated function of the 6 enzymes and 2 transporters of the urea cycle. The initial and rate-limiting enzyme of the urea cycle, carbamylphosphate synthetase 1 (CPS1), requires an allosteric activator, N-acetylglutamate (NAG). The formation of this unique cofactor from glutamate and acetyl Coenzyme-A is catalyzed by N-acetylglutamate synthase (NAGS). An absence of NAG as a consequence of NAGS deficiency may compromise flux through CPS1 and result in hyperammonemia. The NAGS gene encodes a 528-amino acid protein, consisting of a C-terminal catalytic domain, a variable segment, and an N-terminal mitochondrial targeting signal. Only 22 mutations in the NAGS gene have been reported to date, mostly in the catalytic domain. NAGS is primarily expressed in the liver and intestine. However, it is also surprisingly expressed in testis, stomach and spleen, and during early embryonic development at levels not concordant with the expression of other urea cycle enzymes, CPS1, or ornithine transcarbamylase. The purpose of NAGS expression in these tissues, and its significance to NAGS deficiency is as yet unknown. Inherited NAGS deficiency is the rarest of the urea cycle disorders, and we review the currently reported 34 cases. Treatment of NAGS deficiency with N-carbamyglutamate, a stable analog of NAG, can restore deficient urea cycle function and normalize blood ammonia in affected patients.

A new method for accurately measuring Delta(1)-pyrroline-5-carboxylate synthetase activity

Proline is a key factor in plant adaptation to environmental stresses. The Delta(1)-pyrroline-5-carboxylate synthetase catalyzes the first committed step and the rate-limiting step for proline biosynthesis in both plants and mammals. This enzyme catalyzes the reduction of glutamate to pyrroline-5-carboxylate in two sequential steps including the phosphorylation and the reduction of its precursor. Several methods were established to assay P5CS activity but however none of them are fully reliable. Therefore, we developed a new simple and reliable assay which is based on the quantification of Pi. This assay allowed us to determine the optimal pH, the apparent K(m) and V(m) of P5CS with regard to ATP and glutamate.

Metabolism and functions of L-glutamate in the epithelial cells of the small and large intestines

l-Glutamate is one of the most abundant amino acids in alimentary proteins, but its concentration in blood is among the lowest. This is largely because l-glutamate is extensively oxidized in small intestine epithelial cells during its transcellular journey from the lumen to the bloodstream and after its uptake from the bloodstream. This oxidative capacity coincides with a high energy demand of the epithelium, which is in rapid renewal and responsible for the nutrient absorption process. l-Glutamate is a precursor for glutathione and N-acetylglutamate in enterocytes. Glutathione is involved in the enterocyte redox state and in the detoxication process. N-acetylglutamate is an activator of carbamoylphosphate synthetase 1, which is implicated in l-citrulline production by enterocytes. Furthermore, l-glutamate is a precursor in enterocytes for several other amino acids, including l-alanine, l-aspartate, l-ornithine, and l-proline. Thus, l-glutamate can serve both locally inside enterocytes and through the production of other amino acids in an interorgan metabolic perspective. Intestinal epithelial cell capacity to oxidize l-glutamine and l-glutamate is already high in piglets at birth and during the suckling period. In colonocytes, l-glutamate also serves as a fuel but is provided from the bloodstream. Alimentary and endogenous proteins that escape digestion enter the large intestine and are broken down by colonic bacterial flora, which then release l-glutamate into the lumen. l-Glutamate can then serve in the colon lumen as a precursor for butyrate and acetate in bacteria. l-Glutamate, in addition to fiber and digestion-resistant starch, can thus serve as a luminally derived fuel precursor for colonocytes.

Plasma citrulline concentration reflects enterocyte mass in children with short bowel syndrome

Plasma citrulline was recently shown to reflect the residual functional enterocyte mass in various situations characterized by intestinal failure. However, few data are available in children with short bowel syndrome. The objective of this study was to assess the value of citrulline assays in this situation. Prospective plasma citrulline assays were performed in 31 children with short bowel syndrome. Median age was 16 mo (range, 1 mo to 15 y), and median follow-up was 14 mo (6-40 mo). The energy supplied by parenteral nutrition (PN), served to assess intestinal failure severity. Plasma citrulline at inclusion showed a positive correlation with residual short bowel length. Subsequent values correlated negatively with intestinal failure severity. Plasma citrulline increased over time during or after weaning from PN (from 15.8 +/- 11.5 microM to 19.3 +/- 3.8 microM) but remained stable and low in patients who continued to need PN (6.5 +/- 3.0 microM at inclusion and 7.7 +/- 6.0 microM at last follow-up). No weaned patients had a residual short bowel length less than 40 cm and plasma citrulline less than 11 microM. Our findings constitute the first evidence that serial plasma citrulline assays help to monitor residual small bowel adaptation in children.

N-acetylglutamate and its changing role through evolution

N -Acetylglutamate (NAG) fulfils distinct biological roles in lower and higher organisms. In prokaryotes, lower eukaryotes and plants it is the first intermediate in the biosynthesis of arginine, whereas in ureotelic (excreting nitrogen mostly in the form of urea) vertebrates, it is an essential allosteric cofactor for carbamyl phosphate synthetase I (CPSI), the first enzyme of the urea cycle. The pathway that leads from glutamate to arginine in lower organisms employs eight steps, starting with the acetylation of glutamate to form NAG. In these species, NAG can be produced by two enzymic reactions: one catalysed by NAG synthase (NAGS) and the other by ornithine acetyltransferase (OAT). In ureotelic species, NAG is produced exclusively by NAGS. In lower organisms, NAGS is feedback-inhibited by L-arginine, whereas mammalian NAGS activity is significantly enhanced by this amino acid. The NAGS genes of bacteria, fungi and mammals are more diverse than other arginine-biosynthesis and urea-cycle genes. The evolutionary relationship between the distinctly different roles of NAG and its metabolism in lower and higher organisms remains to be determined. In humans, inherited NAGS deficiency is an autosomal recessive disorder causing hyperammonaemia and a phenotype similar to CPSI deficiency. Several mutations have been recently identified in the NAGS genes of families affected with this disorder.

Effect of an elemental vs a complex diet on L-citrulline production from L-arginine in rat isolated enterocytes

BACKGROUND

L-Arginine and L-glutamine are highly metabolized by intestinal cells, leading to various metabolites, including L-citrulline, which is required for optimal growth. Elemental diets, used in clinical practice to treat growth failure and malnutrition, are very different from complex diets normally consumed. The aim of the present study was to assess the effects of an elemental diet compared with a complex diet on L-arginine metabolism in rat isolated enterocytes and its modulation by L-glutamine.

METHODS

Rats were fed the elemental diet (group ED) or the control diet (group C) for 14 days. Villus enterocytes then were isolated, and metabolic capacities or enzyme activities were assessed.

RESULTS

The incubation of enterocytes isolated from group C with 0.1 mmol/L L-[U-14C]-arginine led to the production of 125 +/- 25 pmol L-citrulline/10(6) cells per 30 minutes. This production showed a twofold increase in the presence of 2 mmol/L L-glutamine. In group ED, L-citrulline synthesis from L-arginine was markedly lower in the absence or in the presence of L-glutamine. This coincided with lower carbamoylphosphate synthase I activity and carbamoylphosphate (CP) content of enterocytes. Other L-arginine and L-glutamine metabolic pathways were not affected. Similar results were obtained when the elemental diet was administered continuously through a gastric catheter or fed by mouth.

CONCLUSIONS

L-Glutamine favors the synthesis of L-citrulline from L-arginine in isolated enterocytes, probably via an increase in CP production. Changing the diet composition, from a complex to an elemental diet, results in an alteration of the enterocyte capacity to synthesize L-citrulline from L-arginine, irrespective of the rhythm of delivery.