Detection and basic properties of carbamoyl phosphate synthetase III during teleost ontogeny: a case study in the Atlantic halibut (Hippoglossus hippoglossus L.)

Gene delivery corrects N-acetylglutamate synthase deficiency and enables insights in the physiological impact of L-arginine activation of N-acetylglutamate synthase

The urea cycle protects the central nervous system from ammonia toxicity by converting ammonia to urea. N-acetylglutamate synthase (NAGS) catalyzes formation of N-acetylglutamate, an essential allosteric activator of carbamylphosphate synthetase 1. Enzymatic activity of mammalian NAGS doubles in the presence of L-arginine, but the physiological significance of NAGS activation by L-arginine has been unknown. The NAGS knockout (Nags^−/−) mouse is an animal model of inducible hyperammonemia, which develops hyperammonemia without N-carbamylglutamate and L-citrulline supplementation (NCG + Cit). We used adeno associated virus (AAV) based gene transfer to correct NAGS deficiency in the Nags^−/− mice, established the dose of the vector needed to rescue Nags^−/− mice from hyperammonemia and measured expression levels of Nags mRNA and NAGS protein in the livers of rescued animals. This methodology was used to investigate the effect of L-arginine on ureagenesis in vivo by treating Nags^−/− mice with AAV vectors encoding either wild-type or E354A mutant mouse NAGS (mNAGS), which is not activated by L-arginine. The Nags^−/− mice expressing E354A mNAGS were viable but had elevated plasma ammonia concentration despite similar levels of the E354A and wild-type mNAGS proteins. The corresponding mutation in human NAGS (NP_694551.1:p.E360D) that abolishes binding and activation by L-arginine was identified in a patient with NAGS deficiency. Our results show that NAGS deficiency can be rescued by gene therapy, and suggest that L-arginine binding to the NAGS enzyme is essential for normal ureagenesis.

Structural insights into the substrate binding mechanism of novel ArgA from Mycobacterium tuberculosis

The Mycobacterium tuberculosis (Mtb) Rv2747 gene encodes for a functional protein known as ArgA, which plays an important role in the first step of the l-arginine biosynthesis pathway. ArgA transfers the acetyl group from the acetyl-CoA to either l-glutamate or l-glutamine, which are the known substrates. Here, we present two crystal structures of ArgA: one complexed with CoA and product bound N-acetylglutamine and the other complexed with acetyl-CoA and the inhibitor l-arginine at 2.3 and 3.0 Å resolution respectively. The Mtb ArgA protomer was found to have a "V" cleft and a "β" bulge, archetypal of a classical GCN5-related N-acetyltransferase superfamily of proteins. The product bound form implies that ArgA can also acetylate l-glutamine like l-glutamate. The active site is strongly inhibited by l-arginine resulting in a closed conformation of ArgA and both l-arginine and N-acetylglutamine were found to occupy at the same active site. Together with structural analysis, molecular docking studies, microscale thermophoresis and enzyme inhibition assays, we conclude that l-glutamine, l-glutamate and l-arginine, all occupy at the same active site of ArgA. Furthermore in case of Mtb ArgA, l-arginine does not act as an allosteric inhibitor unlike other N-acetylglutamate synthase family of proteins.

Ammonia and urea handling by early life stages of fishes

Nitrogen metabolism in fishes has been a focus of comparative physiologists for nearly a century. In this Review, we focus specifically on early life stages of fishes, which have received considerable attention in more recent work. Nitrogen metabolism and excretion in early life differs fundamentally from that of juvenile and adult fishes because of (1) the presence of a chorion capsule in embryos that imposes a limitation on effective ammonia excretion, (2) an amino acid-based metabolism that generates a substantial ammonia load, and (3) the lack of a functional gill, which is the primary site of nitrogen excretion in juvenile and adult fishes. Recent findings have shed considerable light on the mechanisms by which these constraints are overcome in early life. Perhaps most importantly, the discovery of Rhesus (Rh) glycoproteins as ammonia transporters and their expression in ion-transporting cells on the skin of larval fishes has transformed our understanding of ammonia excretion by fishes in general. The emergence of larval zebrafish as a model species, together with genetic knockdown techniques, has similarly advanced our understanding of ammonia and urea metabolism and excretion by larval fishes. It has also now been demonstrated that ammonia excretion is one of the primary functions of the developing gill in rainbow trout larvae, leading to new hypotheses regarding the physiological demands driving gill development in larval fishes. Here, we highlight and discuss the dramatic changes in nitrogen handling that occur over early life development in fishes.

Effect of arginine on oligomerization and stability of N-acetylglutamate synthase

N-acetylglutamate synthase (NAGS; E.C.2.3.1.1) catalyzes the formation of N-acetylglutamate (NAG) from acetyl coenzyme A and glutamate. In microorganisms and plants, NAG is the first intermediate of the L-arginine biosynthesis; in animals, NAG is an allosteric activator of carbamylphosphate synthetase I and III. In some bacteria bifunctional N-acetylglutamate synthase-kinase (NAGS-K) catalyzes the first two steps of L-arginine biosynthesis. L-arginine inhibits NAGS in bacteria, fungi, and plants and activates NAGS in mammals. L-arginine increased thermal stability of the NAGS-K from Maricaulis maris (MmNAGS-K) while it destabilized the NAGS-K from Xanthomonas campestris (XcNAGS-K). Analytical gel chromatography and ultracentrifugation indicated tetrameric structure of the MmMNAGS-K in the presence and absence of L-arginine and a tetramer-octamer equilibrium that shifted towards tetramers upon binding of L-arginine for the XcNAGS-K. Analytical gel chromatography of mouse NAGS (mNAGS) indicated either different oligomerization states that are in moderate to slow exchange with each other or deviation from the spherical shape of the mNAGS protein. The partition coefficient of the mNAGS increased in the presence of L-arginine suggesting smaller hydrodynamic radius due to change in either conformation or oligomerization. Different effects of L-arginine on oligomerization of NAGS may have implications for efforts to determine the three-dimensional structure of mammalian NAGS.

Expression pattern and biochemical properties of zebrafish N-acetylglutamate synthase

The urea cycle converts ammonia, a waste product of protein catabolism, into urea. Because fish dispose ammonia directly into water, the role of the urea cycle in fish remains unknown. Six enzymes, N-acetylglutamate synthase (NAGS), carbamylphosphate synthetase III, ornithine transcarbamylase, argininosuccinate synthase, argininosuccinate lyase and arginase 1, and two membrane transporters, ornithine transporter and aralar, comprise the urea cycle. The genes for all six enzymes and both transporters are present in the zebrafish genome. NAGS (EC 2.3.1.1) catalyzes the formation of N-acetylglutamate from glutamate and acetyl coenzyme A and in zebrafish is partially inhibited by L-arginine. NAGS and other urea cycle genes are highly expressed during the first four days of zebrafish development. Sequence alignment of NAGS proteins from six fish species revealed three regions of sequence conservation: the mitochondrial targeting signal (MTS) at the N-terminus, followed by the variable and conserved segments. Removal of the MTS yields mature zebrafish NAGS (zfNAGS-M) while removal of the variable segment from zfNAGS-M results in conserved NAGS (zfNAGS-C). Both zfNAGS-M and zfNAGS-C are tetramers in the absence of L-arginine; addition of L-arginine decreased partition coefficients of both proteins. The zfNAGS-C unfolds over a broader temperature range and has higher specific activity than zfNAGS-M. In the presence of L-arginine the apparent Vmax of zfNAGS-M and zfNAGS-C decreased, their Km(app) for acetyl coenzyme A increased while the Km(app) for glutamate remained unchanged. The expression pattern of NAGS and other urea cycle genes in developing zebrafish suggests that they may have a role in citrulline and/or arginine biosynthesis during the first day of development and in ammonia detoxification thereafter. Biophysical and biochemical properties of zebrafish NAGS suggest that the variable segment may stabilize a tetrameric state of zfNAGS-M and that under physiological conditions zebrafish NAGS catalyzes formation of N-acetylglutamate at the maximal rate.

Waste nitrogen metabolism and excretion in zebrafish embryos: effects of light, ammonia, and nicotinamide

Bony fish primarily excrete ammonia as adults however the persistence of urea cycle genes may reflect a beneficial role for urea production during embryonic stages in protecting the embryo from toxic effects of ammonia produced from a highly nitrogenous yolk. This study aimed to examine the dynamic scope for changes in rates of urea synthesis and excretion in one such species (zebrafish, Danio rerio) by manipulating the intrinsic developmental rate (by alteration of light:dark cycles), as well as by direct chemical manipulation via ammonia injection (to potentially activate urea production) and nicotinamide exposure (to potentially inhibit urea production). Continuous dark exposure delayed development in embryos as evidenced by delayed appearance of hallmark anatomical features (heartbeat, eye pigmentation, body pigmentation, lateral line, fin buds) at 30 and 48 hr post-fertilization, as well by a lower hatching rate compared to embryos reared in continuous light. Both ammonia and urea excretion were similarly effected and were generally higher in embryos continuously exposed to light. Ammonia injection resulted in significant increases (up to fourfold) of urea N excretion and no changes to ammonia excretion rates along with modest increases in yolk ammonia content during 2-6 hr post-injection. Nicotinamide (an inhibitor of urea synthesis in mammals) reduced the ammonia-induced increase in urea excretion and led to retention of ammonia in the yolk and body of the embryo. Our results indicate that there is a relatively rapid and large scope for increases in urea production/excretion rates in developing embryos. Potential mechanisms for these increases are discussed.

Ontogeny of ornithine-urea cycle gene expression in zebrafish (Danio rerio)

Although the majority of adult teleosts excrete most of their nitrogenous wastes as ammonia, several fish species are capable of producing urea early in development. In zebrafish, it is unclear whether this results from a functional ornithine-urea cycle (O-UC) and, if so, how it might be regulated. This study examined the spatiotemporal patterns of gene expression of four major O-UC enzymes: carbamoyl phosphate synthase III (CPSIII), ornithine transcarboxylase, arginosuccinate synthetase, and arginosuccinate lyase, using real-time PCR and whole mount in situ hybridization. In addition, we hypothesized that CPSIII gene expression was epigenetically regulated through methylation of its promoter, a widespread mode of differential gene regulation between tissues and life stages in vertebrates. Furthermore, to assess CPSIII functionality, we used morpholinos to silence CPSIII in zebrafish embryos and assessed their nitrogenous waste handling during development, and in response to ammonia injections. Our results suggest that mRNAs of O-UC enzymes are expressed early in zebrafish development and colocalize to the embryonic endoderm. In addition, the methylation status of CPSIII promoter is not consistent with the patterns of expression observed in developing larvae or adult tissues, suggesting other means of transcriptional regulation of this enzyme. Finally, CPSIII morphants exhibited a transient reduction in CPSIII enzyme activity 24 h postfertilization, which was paralleled by reduced urea production during development and in response to an ammonia challenge. Overall, we conclude that the O-UC is functional in zebrafish embryos, providing further evidence that the capacity to produce urea via the O-UC is widespread in developing teleosts.

Metabolism, nitrogen excretion, and heat shock proteins in the central mudminnow (Umbra limi), a facultative air-breathing fish living in a variable environment

The central mudminnow ( Umbra limi (Kirtland, 1841)) is a continuous, facultative air-breathing freshwater fish found in swamps of central Canada and northeastern USA. The first goal of this field and laboratory-based study was to characterize the physicochemical conditions of mudminnow habitat during the summer. Our second goal was to determine the metabolic, stress response, and nitrogen excretion strategies of this fish following variations in water temperature, dissolved oxygen, external ammonia, and short-term periods of air exposure. We report profound diurnal fluctuations in water temperature (13–31 °C), dissolved oxygen (2%–159% air saturation), and ammonia levels (10–240 μmol·L−1) in habitat of central mudminnow measured on three dates at six different sites over 24 h. The central mudminnow does not induce urea synthesis as a mechanism of ammonia detoxification, either in response to emersion (6 or 20 h) or elevated external ammonia (10 mmol·L–1). Acute exposure to high temperature (~31 °C), aquatic hypoxia, or air resulted in significant increases in blood glucose and liver heat shock protein (Hsp) 70 and hypoxia also caused an increased reliance on anaerobic metabolism. This is the first description of the heat shock response in a facultative air-breathing fish following either hypoxia or air exposure. These metabolic and molecular responses are part of a strategy that allows the mudminnow to thrive in extremely variable freshwater environments.

A new paradigm for ammonia excretion in aquatic animals: role of Rhesus(Rh) glycoproteins

Ammonia excretion at the gills of fish has been studied for 80 years, but the mechanism(s) involved remain controversial. The relatively recent discovery of the ammonia-transporting function of the Rhesus (Rh) proteins, a family related to the Mep/Amt family of methyl ammonia and ammonia transporters in bacteria, yeast and plants, and the occurrence of these genes and glycosylated proteins in fish gills has opened a new paradigm. We provide background on the evolution and function of the Rh proteins, and review recent studies employing molecular physiology which demonstrate their important contribution to branchial ammonia efflux. Rhag occurs in red blood cells,whereas several isoforms of both Rhbg and Rhcg occur in many tissues. In the branchial epithelium, Rhcg appears to be localized in apical membranes and Rhbg in basolateral membranes. Their gene expression is upregulated during exposure to high environmental ammonia or internal ammonia infusion, and may be sensitive to synergistic stimulation by ammonia and cortisol. Rhcg in particular appears to be coupled to H+ excretion and Na+uptake mechanisms. We propose a new model for ammonia excretion in freshwater fish and its variable linkage to Na+ uptake and acid excretion. In this model, Rhag facilitates NH3 flux out of the erythrocyte, Rhbg moves it across the basolateral membrane of the branchial ionocyte, and an apical “Na+/NH +4 exchange complex” consisting of several membrane transporters (Rhcg, V-type H+-ATPase, Na+/H+ exchanger NHE-2 and/or NHE-3, Na+ channel) working together as a metabolon provides an acid trapping mechanism for apical excretion. Intracellular carbonic anhydrase(CA-2) and basolateral Na+/HCO –3cotransporter (NBC-1) and Na+/K+-ATPase play indirect roles. These mechanisms are normally superimposed on a substantial outward movement of NH3 by simple diffusion, which is probably dependent on acid trapping in boundary layer water by H+ ions created by the catalysed or non-catalysed hydration of expired metabolic CO2. Profitable areas for future investigation of Rh proteins in fish are highlighted: their involvement in the mechanism of ammonia excretion across the gills in seawater fish, their possible importance in ammonia excretion across the skin, their potential dual role as CO2 transporters,their responses to feeding, and their roles in early life stages prior to the full development of gills.

Inversion of allosteric effect of arginine on N-acetylglutamate synthase, a molecular marker for evolution of tetrapods

Background

The efficient conversion of ammonia, a potent neurotoxin, into non-toxic metabolites was an essential adaptation that allowed animals to move from the aquatic to terrestrial biosphere. The urea cycle converts ammonia into urea in mammals, amphibians, turtles, snails, worms and many aquatic animals and requires N-acetylglutamate (NAG), an essential allosteric activator of carbamylphosphate synthetase I (CPSI) in mammals and amphibians, and carbamylphosphate synthetase III (CPSIII) in fish and invertebrates. NAG-dependent CPSI and CPSIII catalyze the formation of carbamylphosphate in the first and rate limiting step of ureagenesis. NAG is produced enzymatically by N-acetylglutamate synthase (NAGS), which is also found in bacteria and plants as the first enzyme of arginine biosynthesis. Arginine is an allosteric inhibitor of microbial and plant NAGS, and allosteric activator of mammalian NAGS.

Results

Information from mutagenesis studies of E. coli and P. aeruginosa NAGS was combined with structural information from the related bacterial N-acetylglutamate kinases to identify four residues in mammalian NAGS that interact with arginine. Substitutions of these four residues were engineered in mouse NAGS and into the vertebrate-like N-acetylglutamate synthase-kinase (NAGS-K) of Xanthomonas campestris, which is inhibited by arginine. All mutations resulted in arginine losing the ability to activate mouse NAGS, and inhibit X. campestris NAGS-K. To examine at what point in evolution inversion of arginine effect on NAGS occur, we cloned NAGS from fish and frogs and examined the arginine response of their corresponding proteins. Fish NAGS were partially inhibited by arginine and frog NAGS were activated by arginine.

Conclusion

Difference in arginine effect on bacterial and mammalian NAGS most likely stems from the difference in the type of conformational change triggered by arginine binding to these proteins. The change from arginine inhibition of NAGS to activation was gradual, from complete inhibition of bacterial NAGS, to partial inhibition of fish NAGS, to activation of frog and mammalian NAGS. This change also coincided with the conquest of land by amphibians and mammals.

Sequence, expression and evolutionary relationships of carbamoyl phosphate synthetase I in the toadXenopus laevis

The sequence of carbamoyl phosphate synthetase I (CPSase I) cDNA and expression of the enzyme in liver of the toad Xenopus laevis are reported. CPSase I mRNA increases 6-fold when toads are exposed to high salinity for extended periods of time. The deduced 1,494-amino acid sequence of the CPSase I is homologous to other CPSases and reveals a domain structure and conserved amino acids common to other CPSases. A serine residue (S287) is present where there is a cysteine residue required for glutamine-dependent activity in CPSase Types III and II (Type I CPSases utilize only ammonia as nitrogen-donating substrate). A sequence of DNA 964 bases upstream from the ATG start codon for the CPSase I gene is also reported. Phylogenetic analysis for 30 CPSase isoforms, including X. laevis CPSase I, across a wide spectrum of phyla is reported and discussed. The results are consistent with the views that eukaryotic CPSase II as a multifunctional complex evolved from prokaryotic CPSase II and that CPSase I in terrestrial vertebrates and CPSase III in fishes arose from eukaryotic CPSase II by independent events after the divergence of plants in eukaryotic evolution.

The physiology and evolution of urea transport in fishes

This review summarizes what is currently known about urea transporters in fishes in the context of their physiology and evolution within the vertebrates. The existence of urea transporters has been investigated in red blood cells and hepatocytes of fish as well as in renal and branchial cells. Little is known about urea transport in red blood cells and hepatocytes, in fact, urea transporters are not believed to be present in the erythrocytes of elasmobranchs nor in teleost fish. What little physiological evidence there is for urea transport across fish hepatocytes is not supported by molecular evidence and could be explained by other transporters. In contrast, early findings on elasmobranch renal urea transporters were the impetus for research in other organisms. Urea transport in both the elasmobranch kidney and gill functions to retain urea within the animal against a massive concentration gradient with the environment. Information on branchial and renal urea transporters in teleost fish is recent in comparison but in teleosts urea transporters appear to function for excretion and not retention as in elasmobranchs. The presence of urea transporters in fish that produce a copious amount of urea, such as elasmobranchs and ureotelic teleosts, is reasonable. However, the existence of urea transporters in ammoniotelic fish is curious and could likely be due to their ability to manufacture urea early in life as a means to avoid ammonia toxicity. It is believed that the facilitated diffusion urea transporter (UT) gene family has undergone major evolutionary changes, likely in association with the role of urea transport in the evolution of terrestriality in the vertebrates.

Indispensable amino acid concentrations decrease in tissues of stomachless fish, common carp in response to free amino acid- or peptide-based diets

The premise that free amino acid or dipeptide based diets will resolve the nutritional inadequacy of formulated feeds for larval and juvenile fish and improve utilization of nitrogen in comparison to protein-based diets was tested in stomachless fish, common carp (Cyprinus carpio L.) larvae. We examined the postprandial whole body free amino acid (FAA) pool in fish that were offered a FAA mixture based diet for the duration of 2 or 4 weeks. We found that the total amount and all indispensable amino acids concentrations in the whole body decreased after a meal. We then fed juvenile carp with dietary amino acids provided in the FAA, dipeptide (PP), or protein (live feed organisms; brine shrimp Artemia salina nauplii, AS) forms. Histidine concentrations in the whole fish body increased in all dietary groups after feeding whereas all other indispensable amino acids decreased in FAA and PP groups in comparison to the AS group. Taurine appears to be the major osmotic pressure balancing free amino acid in larval freshwater fish which may indicate a conditional requirement. We present the first evidence in larval fish that in response to synthetic FAA and PP diets, the whole body indispensable free AA concentrations decreased after feeding. This study shows that amino acids given entirely as FAA or PP cannot sustain stomachless larval fish growth, and may result in depletion of body indispensable AA and most of dispensable AA. The understanding of these responses will determine necessary changes in diet formulations that prevent accelerated excretion of amino acids without protein synthesis.

Complement component C3 transcription in Atlantic halibut (Hippoglossus hippoglossus L.) larvae

The complement systems of fish are well developed and play an important role in the innate immune response. Complement C3 is the central protein of all three activation pathways and is the major opsonin of the complement system and essential for the generation of the membrane attack complex. A 1548 bp part of complement component C3 was isolated from a halibut liver cDNA library by immunoscreening. The deduced amino acid sequence showed that this part of halibut C3 contained key amino acids for factor H, I and properdin binding as well as two N-glycosylation sites. Digoxigenine labelled mRNA probes were synthesised and the transcription of C3 was monitored in three larval stages at 206, 430 and 1000 degrees d (30, 50 and 99 days post hatching), by in situ hybridisation. C3 mRNA was detected in muscle, liver, brain, chondrocytes, spinal cord, eye, intestines, oesophagus and kidney. These findings are in accordance with a former immunohistochemical study on halibut C3 protein ontogeny, indicating that C3 is indeed locally expressed in many organs from the youngest stages on. Complement may thus be linked to the formation and generation of different organs during development and play an important role in the early immune response of halibut larvae.

Expression of ornithine-urea cycle enzymes in early life stages of air-breathing walking catfish Clarias batrachus and induction of ureogenesis under hyper-ammonia stress

The air-breathing walking catfish Clarias batrachus is a potential ureogenic teleost with having a full complement of ornithine-urea cycle (OUC) enzymes expressed in various tissues. The present study was aimed at determining the pattern of nitrogenous waste excretion in the form of ammonia-N and urea-N along with the changes of tissue ammonia and urea levels, and the expression of OUC enzymes and glutamine synthetase (GSase) in early life stages of this teleost, and further, to study the possible induction of ureogenesis in 15-day old fry under hyper-ammonia stress. The ammonia and urea excretion was visible within 12 h post-fertilization (hpf), which increased several-fold until the yolk was completely absorbed by the embryo. Although all the early developing stages were primarily ammoniotelic, they also excreted significant amount of nitrogen (N) in the form of urea-N (about 35-40% of total N). Tissue levels of ammonia and urea also increased along with subsequent developmental stages at least until the yolk absorption stage. All the OUC enzymes and GSase were expressed within 4-12 hpf showing an increasing trend of activity for all the enzymes until 350 hpf. There was a significant increase of activity of GSase, carbamyl phosphate synthetase III (CPSase III) and argininosuccinate lyase enzymes (ASL), accompanied with significant increase of enzyme protein concentration of at least two enzymes (GSase and CPSase III) in the 15-day old fry following exposure to 10 mM NH4Cl as compared to respective controls kept in water over a period of 72 h. Thus, it appears that the OUC enzymes are expressed in early life stages of walking catfish like other teleosts, but at relatively high levels and remain expressed all through the life stages with a potential of stimulation of ureogenesis throughout the life cycle as a sort of physiological adaptation to survive and breed successfully under hyper-ammonia and various other environmental-related stresses.

Expression of Four Glutamine Synthetase Genes in the Early Stages of Development of Rainbow Trout (Oncorhynchus mykiss) in Relationship to Nitrogen Excretion

The incorporation of ammonia into glutamine, catalyzed by glutamine synthetase, is thought to be important in the detoxification of ammonia in animals. During early fish development, ammonia is continuously formed as yolk proteins and amino acids are catabolized. We followed the changes in ammonia and urea-nitrogen content, ammonia and urea-nitrogen excretion, glutamine synthetase activity, and mRNA expression of four genes coding for glutamine synthetase (Onmy-GS01-GS04) over 3-80 days post fertilization and in adult liver and skeletal muscle of the rainbow trout (Oncorhynchus mykiss). Both ammonia and urea-nitrogen accumulate before hatching, although the rate of ammonia excretion is considerably higher relative to urea-nitrogen excretion. All four genes were expressed during early development, but only Onmy-GS01 and -GS02 were expressed at appreciable levels in adult liver, and expression was very low in muscle tissue. The high level of expression of Onmy-GS01 and -GS03 prior to hatching corresponded to a linear increase in glutamine synthetase activity. We propose that the induction of glutamine synthetase genes early in development and the subsequent formation of the active protein are preparatory for the increased capacity of the embryo to convert the toxic nitrogen end product, ammonia, into glutamine, which may then be utilized in the ornithine-urea cycle or other pathways.

Postprandial increases in nitrogenous excretion and urea synthesis in the giant mudskipper Periophthalmodon schlosseri

The objective of this study was to determine the effects of feeding on the excretory nitrogen (N) metabolism of the giant mudskipper, Periophthalmodon schlosseri, with special emphasis on the role of urea synthesis in ammonia detoxification. The ammonia and urea excretion rates of P. schlosseri increased 1.70- and 1.92-fold, respectively, within the first 3 h after feeding on guppies. Simultaneously, there were significant decreases in ammonia levels in the plasma and the brain, and in urea contents in the muscle and liver, of P. schlosseri at 3 h post-feeding. Thus, it can be concluded that P. schlosseri was capable of unloading ammonia originally present in some of its tissues in anticipation of ammonia released from the catabolism of excess amino acids after feeding. Subsequently, there were significant increases in urea content in the muscle, liver and plasma (1.39-, 2.17- and 1.62-fold, respectively) at 6 h post-feeding, and the rate of urea synthesis apparently increased 5.8-fold between 3 h and 6 h. Increased urea synthesis might have occurred in the liver of P. schlosseri because the greatest increase in urea content was observed therein. The excess urea accumulated in the body at 6 h was completely excreted between 6 and 12 h, and the percentage of waste-N excreted as urea-N increased significantly to 26% during this period, but never exceeded 50%, the criterion for ureotely, meaning that P. schlosseri remained ammonotelic after feeding. By 24 h, 62.7% of the N ingested by P. schlosseri was excreted, out of which 22.6% was excreted as urea-N. This is the first report on the involvement of increased urea synthesis and excretion in defense against ammonia toxicity in the giant mudskipper, and our results suggest that an ample supply of energy resources, e.g. after feeding, is a prerequisite for the induction of urea synthesis. Together, increases in nitrogenous excretion and urea synthesis after feeding effectively prevented a postprandial surge of ammonia in the plasma of P. schlosseri as reported previously for other fish species. Consequently, contrary to previous reports, there were significant decreases in the ammonia content of the brain of P. schlosseri throughout the 24 h period post-feeding, accompanied by a significant decrease in brain glutamine content between 12 h and 24 h.

Dietary arginine degradation is a major pathway in ureagenesis in juvenile turbot (Psetta maxima)

Recent studies indicate that urea excretion is responsive to protein intake and that turbot, Psetta maxima, appear to differ from other species by their urea excretion pattern and levels. This study was undertaken to evaluate the influence of dietary nitrogen and arginine on ureagenesis and excretion in turbot. Juvenile turbot (29 g) were fed semi-purified diets containing graded levels of nitrogen (0-8% dry matter) and arginine (0-3% dry matter) for 6 weeks. Growth data showed that turbot have high dietary nitrogen (123 mg/kg metabolic body weight/day) and very low dietary arginine (9.3 mg/kg metabolic body weight/day) requirements for maintenance. Requirements for unit body protein accretion were 0.31 g and 0.15 g for nitrogen and arginine respectively. Post-prandial plasma urea levels and urea excretion rates showed that urea production was significantly (P<0.05) influenced by dietary arginine levels. While hepatic arginase (EC 3.5.3.1) activity increased significantly (P<0.05) with increasing dietary arginine levels, activities of other enzymes of the ornithine urea cycle were very low. Our data strongly suggest that the ornithine urea cycle is not active in the turbot liver and that dietary arginine degradation is a major pathway of ureagenesis in turbot.

Kinetics and fates of ammonia, urea, and uric acid during oocyte maturation and ontogeny of the Atlantic halibut (Hippoglossus hippoglossus L.)

Considering that amino acids constitute an important energy fuel during early life of the Atlantic halibut (Hippoglossus hippoglossus L.), it is of interest to understand how the nitrogenous end products are handled. In this study we focused on the kinetics and fates of ammonia, urea and uric acid. The results showed that ammonia (T(Amm): NH(3)+NH(4)(+)), and urea-N contents increased during final oocyte maturation. Urea-N excretion dominated the total nitrogenous end product formation in early embryos. Later, yolk T(Amm) levels increased in embryos and ammonia excretion was low. In the last part of the embryonic stage T(Amm) accumulation dominated, and was apparently due to yolk storage. Around hatching, the larval body tissues (larva with yolk-sac removed) accounted for 68% of whole animal urea-N accumulation, while T(Amm) levels increased predominately by yolk accumulation. Afterwards, ammonia excretion dominated and uric acid accumulation accounted for less than 1%. Urea, synthesised either through the ornithine-urea cycle, argininolysis or uricolysis, accounted for approximately 8% of total nitrogenous end product formation in yolk-sac larvae. The results suggested that a sequence occurred regarding which nitrogenous end products dominated and how they were handled. Urea excretion dominated in early embryos (<7 dPF), followed by yolk ammonia accumulation (7-12 dPF), and finally, ammonia excretion dominated in later embryonic and yolk-sac larval stages (>12 dPF).

Nitrogen metabolism and excretion in the mangrove killifish Rivulus marmoratus II. Significant ammonia volatilization in a teleost during air-exposure

The mangrove killifish Rivulus marmoratus can tolerate prolonged periods of air-exposure (&gt;1 month). During these periods of emersion, we hypothesized that R. marmoratus would convert potentially toxic ammonia into urea and free amino acids (FAAs). In air-exposed fish, both ammonia (JAmm) and urea (JUrea) excretion continued at approximately 57 % and 39 %, respectively, of submerged rates. Remarkably, approximately 42 % of the total ammonia excreted during air-exposure was through NH3 volatilization. Ammonia did not accumulate in whole-body tissues of air-exposed fish, but levels of both urea and some FAAs (primarily alanine and glutamine) were up to twofold higher after 10 days. The activities of the ornithine–urea cycle enzymes carbamoyl phosphate synthetase III and ornithine transcarbamylase increased (by approximately 30 % and 36 %, respectively) in whole-body tissues of air-exposed fish, while levels of arginase remained unchanged. The activities of enzymes involved in amino acid and oxidative metabolism were not significantly different between control and air-exposed fish. Partitioning of the anterior and posterior ends of immersed fish revealed that just over half (57 %) of the total nitrogen (ammonia+urea) was excreted through the anterior end of the fish, presumably via the branchial tissues, while emersed fish increased excretion via the posterior end (kidney+skin). R. marmoratus do not undergo a shift towards ureotelism during air-exposure. Rather, we propose that R. marmoratus are able to survive on land for extended periods without significant ammonia accumulation because they continuously release ammonia, partially by NH3 volatilization.

Ammonia detoxification and localization of urea cycle enzyme activity in embryos of the rainbow trout (Oncorhynchus mykiss) in relation to early tolerance to high environmental ammonia levels

The present study investigated the role of ammonia as a trigger for hatching, mechanisms of ammonia detoxification and the localization of urea cycle enzymes in the early life stages of freshwater rainbow trout (Oncorhynchus mykiss). The key urea cycle enzyme carbamoyl phosphate synthetase III was found exclusively in the embryonic body (non-hepatic tissues); related enzymes were distributed between the liver and embryonic body. ‘Eyed-up’ trout embryos were exposed either acutely (2h) to 10mmoll−1 NH4Cl or chronically (4 days) to 0.2mmoll−1 NH4Cl. Time to hatching was not affected by either acute or chronic NH4Cl exposure. Urea levels, but not ammonia levels in the embryonic tissues, were significantly higher than in controls after both acute and chronic NH4Cl exposure, whereas there were no significant changes in urea cycle enzyme activities. Total amino acid levels in the embryonic tissues were unaltered by chronic ammonia exposure, but levels of most individual amino acids and total amino acid levels in the yolk were significantly lower (by 34–58%) than in non-exposed controls. The data indicate that trout embryos have an efficient system to prevent ammonia accumulation in embryonic tissue, by conversion of ammonia to urea in embryonic tissues and through elevation of ammonia levels in the yolk.

Pathways for urea production during early life of an air-breathing teleost, the African catfish Clarias gariepinus Burchell

Embryos and larvae of the African catfish Clarias gariepinus excrete significant quantities of urea. The present study focused on the potential urea-generating pathways during early development of this teleost; uricolysis, argininolysis and the ornithine–urea cycle (OUC). Uricase, allantoinase, allantoicase and ureidoglycollate lyase of the uricolytic pathway were expressed in all early life stages and in adult liver of C. gariepinus. Uricase activity increased in starved larvae compared with yolk-sac larvae. The key regulatory enzyme of the teleost OUC, carbamoyl phosphate synthetase III (CPSase III), was expressed predominantly in muscle of developing C. gariepinus larvae and showed negligible activity in the absence of its allosteric effector N-acetyl-l-glutamate. CPSase III and ornithine carbamoyl transferase activities increased in fed larvae compared with starved larvae. In contrast to the early developmental stages, adult C. gariepinus expressed only low and variable levels of CPSase III, suggesting that, under the experimental conditions employed, OUC expression is influenced by developmental stage in this species. The data indicate that early C. gariepinus life stages express the enzymes necessary for urea production by uricolysis, argininolysis and the OUC, and this may explain why urea tissue levels and urea excretion rates are substantial during the early development of this air-breathing teleost.

Functional ureogenesis in the gobiid fish Mugilogobius abei

To examine the transition to ureogenesis, the gobiid fish Mugilogobius abei was immersed in 2 mmol l(−)(1) NH(4)HCO(3) or a (15)N-labelled ammonia solution [1 mmol l(−)(1) ((15)NH(4))(2)SO(4), pH 8.0] for 4–8 days. When exposed to 2 mmol l(−)(1) NH(4)HCO(3) or (15)N-labelled ammonia solution for 4 days, the rate of urea excretion increased to seven times that of the control (in 20 % synthetic sea water) and remained at this level for 4 days. The proportion of nitrogen excreted as urea reached 62 % of total nitrogen excretion (ammonia-N + urea-N). (15)N-enrichment of the amide-N in glutamine in the tissues of fish exposed to (15)N-labelled ammonia was virtually the same as that of ammonia-N: i.e. approximately twice that of urea-N in the excreta and the tissues. Glutamine contents and glutamine synthetase activities in the liver and muscle increased greatly following exposure to ammonia. Urea and citrulline contents in the muscle and whole body of the exposed fish increased significantly, whereas uric acid contents remained unchanged. Carbamoyl phosphate synthetase III (CPSase III) mRNA expression and CPSase III activity were detected in the muscle, skin and gill, but levels were negligible in the liver. Furthermore, all other ornithine-urea cycle (O-UC) enzymes were also detected in muscle, skin and gill. Thus, M. abei clearly shows the transition from ammoniotely to ureotely under ammonia-loading condition and is able to produce urea mainly via the O-UC operating in multiple non-hepatic tissues as a means for ammonia detoxification.