Stimulatory effect of arginine on acetylglutamate synthesis in isolated mitochondria of mouse and rat liver

The N-Acetylglutamate Synthase Family: Structures, Function and Mechanisms

N-acetylglutamate synthase (NAGS) catalyzes the production of N-acetylglutamate (NAG) from acetyl-CoA and l-glutamate. In microorganisms and plants, the enzyme functions in the arginine biosynthetic pathway, while in mammals, its major role is to produce the essential co-factor of carbamoyl phosphate synthetase 1 (CPS1) in the urea cycle. Recent work has shown that several different genes encode enzymes that can catalyze NAG formation. A bifunctional enzyme was identified in certain bacteria, which catalyzes both NAGS and N-acetylglutamate kinase (NAGK) activities, the first two steps of the arginine biosynthetic pathway. Interestingly, these bifunctional enzymes have higher sequence similarity to vertebrate NAGS than those of the classical (mono-functional) bacterial NAGS. Solving the structures for both classical bacterial NAGS and bifunctional vertebrate-like NAGS/K has advanced our insight into the regulation and catalytic mechanisms of NAGS, and the evolutionary relationship between the two NAGS groups.

Citrulline and nitrogen homeostasis: an overview

Citrulline (Cit) is a non-essential amino acid whose metabolic properties were largely ignored until the last decade when it began to emerge as a highly promising nutrient with many regulatory properties, with a key role in nitrogen homeostasis. Because Cit is not taken up by the liver, its synthesis from arginine, glutamine, ornithine and proline in the intestine prevents the hepatic uptake of the two first amino acids which activate the urea cycle and so prevents amino acid catabolism. This sparing effect may have positive spin-off for muscle via increased protein synthesis, protein content and functionality. However, the mechanisms of action of Cit are not fully known, even if preliminary data suggest an implication of mTOR pathway. Further exploration is needed to gain a complete overview of the role of Cit in the control of nitrogen homeostasis.

Down-regulation of hepatic urea synthesis by oxypurines: xanthine and uric acid inhibit N-acetylglutamate synthase

We previously reported that isobutylmethylxanthine (IBMX), a derivative of oxypurine, inhibits citrulline synthesis by an as yet unknown mechanism. Here, we demonstrate that IBMX and other oxypurines containing a 2,6-dione group interfere with the binding of glutamate to the active site of N-acetylglutamate synthetase (NAGS), thereby decreasing synthesis of N-acetylglutamate, the obligatory activator of carbamoyl phosphate synthase-1 (CPS1). The result is reduction of citrulline and urea synthesis. Experiments were performed with (15)N-labeled substrates, purified hepatic CPS1, and recombinant mouse NAGS as well as isolated mitochondria. We also used isolated hepatocytes to examine the action of various oxypurines on ureagenesis and to assess the ameliorating affect of N-carbamylglutamate and/or l-arginine on NAGS inhibition. Among various oxypurines tested, only IBMX, xanthine, or uric acid significantly increased the apparent K(m) for glutamate and decreased velocity of NAGS, with little effect on CPS1. The inhibition of NAGS is time- and dose-dependent and leads to decreased formation of the CPS1-N-acetylglutamate complex and consequent inhibition of citrulline and urea synthesis. However, such inhibition was reversed by supplementation with N-carbamylglutamate. The data demonstrate that xanthine and uric acid, both physiologically occurring oxypurines, inhibit the hepatic synthesis of N-acetylglutamate. An important and novel concept emerging from this study is that xanthine and/or uric acid may have a role in the regulation of ureagenesis and, thus, nitrogen homeostasis in normal and disease states.

Glutamine: precursor or nitrogen donor for citrulline synthesis?

Although glutamine is considered the main precursor for citrulline synthesis, the current literature does not differentiate between the contribution of glutamine carbon skeleton vs. nonspecific nitrogen (i.e., ammonia) and carbon derived from glutamine oxidation. To elucidate the role of glutamine and nonspecific nitrogen in the synthesis of citrulline, l-[2-(15)N]- and l-[5-(15)N]glutamine and (15)N-ammonium acetate were infused intragastrically in mice. The amino group of glutamine labeled the three nitrogen groups of citrulline almost equally. The amido group and ammonium acetate labeled the ureido and amino groups of citrulline, but not the delta-nitrogen. D(5)-glutamine also infused in this arm of the study, which traces the carbon skeleton of glutamine, was utilized poorly, accounting for only 0.2-0.4% of the circulating citrulline. Dietary glutamine nitrogen (both N groups) incorporation was 25-fold higher than the incorporation of its carbon skeleton into citrulline. To investigate the relative contributions of the carbon skeleton and nonspecific carbon of glutamine, arginine, and proline to citrulline synthesis, U-(13)C(n) tracers of these amino acids were infused intragastrically. Dietary arginine was the main precursor for citrulline synthesis, accounting for approximately 40% of the circulating citrulline. Proline contribution was minor (3.4%), and glutamine was negligible (0.4%). However, the glutamine tracer resulted in a higher enrichment in the ureido group, indicating incorporation of nonspecific carbon from glutamine oxidation into carbamylphosphate used for citrulline synthesis. In conclusion, dietary glutamine is a poor carbon skeleton precursor for the synthesis of citrulline, although it contributes both nonspecific nitrogen and carbon to citrulline synthesis.

3-isobutylmethylxanthine inhibits hepatic urea synthesis: protection by agmatine

We previously showed that agmatine stimulated hepatic ureagenesis. In this study, we sought to determine whether the action of agmatine is mediated via cAMP signaling. A pilot experiment demonstrated that the phosphodiesterase inhibitor, 3-isobutylmethylxanthine (IBMX), inhibited urea synthesis albeit increased [cAMP]. Thus, we hypothesized that IBMX inhibits hepatic urea synthesis independent of [cAMP]. We further theorized that agmatine would negate the IBMX action and improve ureagenesis. Experiments were carried out with isolated mitochondria and (15)NH(4)Cl to trace [(15)N]citrulline production or [5-(15)N]glutamine and a rat liver perfusion system to trace ureagenesis. The results demonstrate that IBMX induced the following: (i) inhibition of the mitochondrial respiratory chain and diminished O(2) consumption during liver perfusion; (ii) depletion of the phosphorylation potential and overall hepatic energetic capacity; (iii) inhibition of [(15)N]citrulline synthesis; and (iv) inhibition of urea output in liver perfusion with little effect on [N-acetylglutamate]. The results indicate that IBMX directly and specifically inhibited complex I of the respiratory chain and carbamoyl-phosphate synthase-I (CPS-I), with an EC(50) about 0.6 mm despite a significant elevation of hepatic [cAMP]. Perfusion of agmatine with IBMX stimulated O(2) consumption, restored hepatic phosphorylation potential, and significantly stimulated ureagenesis. The action of agmatine may signify a cascade effect initiated by increased oxidative phosphorylation and greater ATP synthesis. In addition, agmatine may prevent IBMX from binding to one or more active site(s) of CPS-I and thus protect against inhibition of CPS-I. Together, the data may suggest a new experimental application of IBMX in studies of CPS-I malfunction and the use of agmatine as intervention therapy.

Biochemical properties of recombinant human and mouse N-acetylglutamate synthase

N-Acetylglutamate synthase (NAGS, EC 2.3.1.1) is a mitochondrial enzyme that catalyzes the formation of N-acetylglutamate (NAG) from glutamate and acetylcoenzyme A. NAG is an obligatory activator of carbamylphosphate I (CPSI), the first and a rate limiting enzyme of ureagenesis. The enzymatic activity of NAGS increases in the presence of arginine. Since the level of NAGS activity depends on the concentrations of two amino acids, glutamate and arginine, and it supplies the essential cofactor for CPSI, NAGS may play an important role in the regulation of ureagenesis. The amino acid sequences of human and mouse NAGS consist of three regions with different degrees of conservation: the mitochondrial targeting signal (MTS), the variable domain, and the conserved domain. Removal of the MTS results in mature NAGS (NAGS-M) while removal of the MTS and the variable domain results in conserved NAGS (NAGS-C). The biochemical properties of purified recombinant human and mouse NAGS-M and NAGS-C were determined in this study with the goal of better understanding the role of the variable domain in NAGS function. The activity of all four proteins doubled in the presence of arginine, while the affinities for substrates changed less than two fold. The turnover numbers of NAGS-C are double those of NAGS-M proteins. Processing of NAGS-M to form NAGS-C results in an enzyme with higher catalytic activity and could play a role in the regulation of NAG production, CPSI function, and urea synthesis.

The role of mitochondrially bound arginase in the regulation of urea synthesis: studies with [U-15N4]arginine, isolated mitochondria, and perfused rat liver

The main goal of the current study was to elucidate the role of mitochondrial arginine metabolism in the regulation of N-acetylglutamate and urea synthesis. We hypothesized that arginine catabolism via mitochondrially bound arginase augments ureagenesis by supplying ornithine for net synthesis of citrulline, glutamate, N-acetylglutamate, and aspartate. [U-(15)N(4)]arginine was used as precursor and isolated mitochondria or liver perfusion as a model system to monitor arginine catabolism and the incorporation of (15)N into various intermediate metabolites of the urea cycle. The results indicate that approximately 8% of total mitochondrial arginase activity is located in the matrix, and 90% is located in the outer membrane. Experiments with isolated mitochondria showed that approximately 60-70% of external [U-(15)N(4)]arginine catabolism was recovered as (15)N-labeled ornithine, glutamate, N-acetylglutamate, citrulline, and aspartate. The production of (15)N-labeled metabolites was time- and dose-dependent. During liver perfusion, urea containing one (U(m+1)) or two (U(m+2)) (15)N was generated from perfusate [U-(15)N(4)]arginine. The output of U(m+2) was between 3 and 8% of total urea, consistent with the percentage of activity of matrix arginase. U(m+1) was formed following mitochondrial production of [(15)N]glutamate from [alpha,delta-(15)N(2)]ornithine and transamination of [(15)N]glutamate to [(15)N]aspartate. The latter is transported to cytosol and incorporated into argininosuccinate. Approximately 70, 75, 7, and 5% of hepatic ornithine, citrulline, N-acetylglutamate, and aspartate, respectively, were derived from perfusate [U-(15)N(4)]arginine. The results substantiate the hypothesis that intramitochondrial arginase, presumably the arginase-II isozyme, may play an important role in the regulation of hepatic ureagenesis by furnishing ornithine for net synthesis of N-acetylglutamate, citrulline, and aspartate.

Mitochondrial nitric oxide synthase

Nitric oxide (NO*) can bind to and inhibit the terminal enzyme of the mitochondrial respiratory chain, cytochrome c oxidase (complex IV). In vivo, NO* is made by the NO* synthase (NOS) family of enzymes, and considerable debate has recently arisen regarding a NOS inside mitochondria (termed 'mtNOS'). Such an enzyme is an intriguing proposition, since it affords unique organelle-based regulatory mechanisms for NO* synthesis, and has considerable implications for mitochondrial function. This review serves to discuss some of the current issues regarding mtNOS, such as its isoform identity, the availability of co-factors and substrates within the organelle, and potential physiological vs. pathological roles for the enzyme, all within the broader context of mitochondrial regulation by NO*.

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.

Intravenous arginine dramatically improved hyperammonemia in a patient with late-onset ornithine transcarbamylase deficiency

We describe a 12 year-old male patient with late-onset ornithine transcarbamylase deficiency, in whom infusion of arginine alone dramatically improved intercurrent hyperammonemia. The plasma glutamine level also decreased while the urea nitrogen level increased with arginine infusion, indicating that accumulated nitrogen was metabolized to urea in response to the arginine infusion.

Intestinal arginine metabolism during development. Evidence for de novo synthesis of L-arginine in newborn pig enterocytes

The capacity for L-arginine metabolism was studied in villus enterocytes isolated from pigs at birth, after 2-8 days suckling and after weaning. Immediately after birth, enterocytes were able to convert 1 mM L-citrulline, 2 mM L-glutamine or 1 mM L-ornithine to L-arginine. In 2-8-day-old animals, the net production of L-arginine from L-citrulline (2.00 +/- 0.45 nmol x 10(6) cells-1 x 30 min-1), or from L-ornithine (0.29 +/- 0.06 nmol x 10(6) cells-1 x 30 min-1) was similar to the values obtained at birth. Furthermore, 40% of L-arginine synthetized de novo from L-citrulline were released into the incubation medium. In 2-8-day-old animals, the production of L-arginine from L-glutamine represented only 5% of the production at birth (the latter being 0.73 +/- 0.15 nmol x 10(6) cells-1 x 30 min-1). In enterocytes isolated from post-weaned pigs, no significant production of L-arginine from either L-glutamine or L-ornithine was detected. In contrast, although the L-arginine production from L-citrulline was very low in post-weaned animals, it was significantly enhanced in the presence of L-glutamine, representing 23% of the production measured in suckling animals. The capacity of enterocytes to cleave L-arginine to L-ornithine and urea was very limited at birth, but was increased more than threefold in 2-day-old animals. This was concomitant with a marked increase in arginase activity. In post-weaned animals, the flux through arginase in intact enterocytes, and the arginase activity were both threefold higher than in 2-8-day-old animals. It is concluded that enterocytes isolated from neonatal pigs exhibit the capacity for a net production of L-arginine since the metabolism of this amino acid is oriented to anabolism rather than catabolism. The results are discussed in relation to L-arginine metabolism in the neonatal liver.

New kinetic parameters for rat liver arginase measured at near-physiological steady-state concentrations of arginine and Mn2+

A cytosolic cell-free system from rat liver containing the last three enzymes of the urea cycle, a number of cofactors and the substrates aspartate and citrulline was shown to synthesize urea at near-physiological rates ranging between 0.40 and 1.25 mumol/min per g of liver. This system was used to determine the kinetic parameters for arginase. With saturating amounts of Mn2+ (30 microM), arginine remained at a steady-state concentration of 5-35 microM depending on the aspartate and citrulline supply. Vmax. at micromolar arginine concentrations was between 1.10 and 1.25 mumol/min per g of liver, the K0.5 (arginine) between 6.0 and 6.5 microM and positive co-operativity was observed (Hill coefficient 2). Omission of Mn2+ caused a significant accumulation of arginine during the incubation, suggesting a regulatory effect of arginase. Under these conditions, Vmax. was 1.10-1.65 mumol/min per g of liver and the Km (arginine) increased up to 14.4-21.1 microM. The apparent Ka for Mn2+ in the presence of physiological concentrations of ATP, Mg2+ and arginine was calculated to be maximally 8 microM. Initial-velocity experiments with millimolar arginine concentrations as the direct substrate gave the following results, which are in good agreement with literature data. In the absence of Mn2+, Vmax. was 71.3 mumol/min per g of liver and the Km (arginine) 1.58 mM. With 30 microM-Mn2+, Vmax. was 69.4 mumol/min per g of liver and the Km (arginine) decreased to 0.94 mM. On the basis of our results, we propose the presence of high-affinity and low-affinity sites for arginine on rat liver arginase and postulate that alterations in arginase activity arising from changes in the concentration of arginine and of the cofactor Mn2+ may contribute to the regulation of ureagenesis in vivo.

Normal N-acetylglutamate concentration measured in liver from a new patient with N-acetylglutamate synthetase deficiency: physiologic and biochemical implications

N-Acetyl-L-glutamate synthetase (NAG synthetase) is a mitochondrial matrix enzyme which catalyzes the synthesis of N-acetyl-Lglutamate (NAG), a physiologic activator of the urea cycle enzyme carbamylphosphate synthetase I. Deficiency of NAG synthetase in humans has been reported only three times previously. Two cases presented with uncontrolable neonatal hyperammonemia leading to death, while a third child presented with hyperammonemia and a neurodegenerative picture at 15 months of age after previously being healthy. We report here a new case of NAG synthetase deficiency who presented at 4 years, 10 months of age with an episode of hyperammonemia. Diagnosis was made at age 5 years, 6 months when a liver biopsy showed 9.7% of normal activity. Urine orotic acid was low, and total NAG content in liver was normal. Liver pathology revealed micro- and macrovesicular fat and mitochondria of irregular size and shape with intracristae crystallizations. NAG content in liver in patients with NAG synthetase deficiency has not previously been reported. Its normal value in the face of NAG synthetase deficiency suggests an abnormal localization of NAG to the cytoplasm and the likelihood of aberrant cytoplasmic synthesis of this compound. Additional physiologic implications of this speculative abnormal compartmentalization are discussed.

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

We describe here a concise assay procedure for N-acetylglutamate (AGA) synthase (AGAS) and its application to an extensive study of tissue distribution of AGAS activity. Crude mitochondria from several tissues were incubated in a pair of assay mixtures with [14C]glutamate in the absence and presence of acetyl-CoA at 15 degrees C for 10 min. Anionic components including [14C]AGA were first isolated from glutamate by a cation exchanger column. In order to remove anionic contaminants such as succinate, the AGA was converted to glutamate enzymatically by aminoacylase, and then the glutamate was isolated by cation exchange chromatography and counted. Recoveries were corrected individually. The difference between the pair incubations was taken as the activity. An extensive survey of AGAS activity in rats showed that, although the liver expressed the highest activity, the small intestine, testis, lung and submaxillary gland also exhibited considerable activity. Sexual differences in activity were not found in the liver and small intestine. We also detected activity in the human small intestine for the first time. Optimization of incubation temperature and time and the presence of arginine in an assay mixture was essential and we demonstrated that the AGAS reaction with crude mitochondria as an enzyme source was unstable without arginine and at higher temperatures. This procedure appears suitable for studying the physiological and nutritional role of AGAS in non-hepatic tissues. In the accompanying paper we applied this procedure to study the ontogeny of AGAS in developing rat tissues.

Effect of level of dietary protein on arginine-stimulated citrulline synthesis. Correlation with mitochondrial N-acetylglutamate concentrations

Increases in dietary protein have been reported to increase the rate of citrulline synthesis and the level of N-acetylglutamate in liver. We have confirmed this effect of diet on citrulline synthesis in rat liver mitochondria and show parallel increases in N-acetylglutamate concentration. The magnitude of the effect of arginine in the suspending medium on citrulline synthesis was also dependent on dietary protein content. Mitochondria from rats fed on a protein-free diet initially contained low levels of N-acetylglutamate, and addition of arginine increased the rate of its synthesis. Citrulline synthesis and acetylglutamate content in these mitochondria increased more than 5-fold when 1 mM-arginine was added. A diet high in protein results in mitochondria with increased N-acetylglutamate and a high rate of citrulline synthesis; 1 mM-arginine increased citrulline synthesis in such mitochondria by only 36%. The concentration of arginine in portal blood was 47 microM in rats fed on a diet lacking protein, and 182 microM in rats fed on a diet containing 60% protein, suggesting that arginine may be a regulatory signal to the liver concerning the dietary protein intake. The rates of citrulline synthesis were proportional to the mitochondrial content of acetylglutamate in mitochondria obtained from rats fed on diets containing 0, 24, or 60% protein, whether incubated in the absence or presence of arginine. Although the effector concentrations are higher than the Ka for the enzymes, these results support the view that concentrations of both arginine and acetylglutamate are important in the regulation of synthesis of citrulline and urea. Additionally, the effects of dietary protein level (and of arginine) are exerted in large part by way of modulation of the concentration of acetylglutamate.

Human hepatic N-acetylglutamate content and N-acetylglutamate synthase activity. Determination by stable isotope dilution

N-Acetyl-L-glutamate (N-acetylglutamate) content and N-acetylglutamate synthase activity ranges were established in human liver tissue homogenates by stable isotope dilution. The methods employ N-[methyl-2H3]acetyl[15N]glutamate as internal standard, extraction of N-acetylglutamate by anion-exchange technique and its determination by g.l.c.-mass spectrometry by using selected ion monitoring. Hepatic N-acetylglutamate content in 16 different human livers, normal in structure and function, ranged from 6.8 to 59.7 nmol/g wet wt. (25.0 +/- 13.4 mean +/- S.D.) or from 64.6 to 497.6 nmol/g of protein (223.2 +/- 104.2 mean +/- S.D.). In vitro, N-acetylglutamate synthase activity in liver tissue homogenate ranged from 44.5 to 374.5 (132.0 +/- 90.6 mean +/- S.D.) nmol/min per g wet wt. or from 491.7 to 3416.9 (1159.6 +/- 751.1 mean +/- S.D.) nmol/min per g of protein. No correlation was found between hepatic N-acetylglutamate concentrations and the respective maximal enzymic activities in vitro of N-acetylglutamate synthase. The marked variability in this system among individual livers may reflect its regulatory role in ureagenesis.