Subcellular location of glutamine synthetase and urea cycle enzymes in liver of spiny dogfish (Squalus acanthias)

Nitrogen transporters along the intestinal spiral valve of cloudy catshark (Scyliorhinus torazame): Rhp2, Rhbg, UT

As part of their osmoregulatory strategy, marine elasmobranchs retain large quantities of urea to balance the osmotic pressure of the marine environment. The main source of nitrogen used to synthesize urea comes from the digestion and absorption of food across the gastrointestinal tract. In this study we investigated possible mechanisms of nitrogen movement across the spiral valve of the cloudy catshark (Scyliorhinus torazame) through the molecular identification of two Rhesus glycoprotein ammonia transporters (Rhp2 and Rhbg) and a urea transporter (UT). We used immunohistochemistry to determine the cellular localizations of Rhp2 and UT. Within the spiral valve, Rhp2 was expressed along the apical brush-border membrane, and UT was expressed along the basolateral membrane and the blood vessels. The mRNA abundance of Rhp2 was significantly higher in all regions of the spiral valve of fasted catsharks compared to fed catsharks. The mRNA abundance of UT was significantly higher in the anterior spiral valve of fasted catsharks compared to fed. The mRNA transcript of four ornithine urea cycle (OUC) enzymes were detected along the length of the spiral valve and in the renal tissue, indicating the synthesis of urea via the OUC occurs in these tissues. The presence of Rhp2, Rhbg, and UT along the length of the spiral valve highlights the importance of ammonia and urea movement across the intestinal tissues, and increases our understanding of the mechanisms involved in maintaining whole-body nitrogen homeostasis in the cloudy catshark.

The effects of digesting a urea-rich meal on North Pacific spiny dogfish (Squalus acanthias suckleyi)

Marine elasmobranchs are nitrogen-limited owing to the requirement of nitrogen for both somatic growth and urea-based osmoregulation, and due to the loss of urea across the gills and kidney as nitrogenous waste. In this study we used in vitro stomach and intestinal gut sacs to investigate the effects of consuming a urea-rich meal (700 mM within a 2% body-mass ration of food-slurry) on nitrogen movement across the gastrointestinal (GI) tract of North Pacific spiny dogfish (Squalus acanthias suckleyi). Plasma urea concentrations did not differ between fasted (359 ± 19 mM), urea-poor fed (340 ± 16 mM), and urea-rich fed (332 ± 24 mM) dogfish. Interestingly, in vitro gut sacs of urea-rich fed dogfish showed no net urea absorption from the lumen over 3 h incubation, which contrasts previously published data on urea-poor fed dogfish that absorb urea from the lumen. In addition, ammonium (NH₄⁺) concentration within the gut sac intestinal lumen significantly increased from 0.62 to 4.35 mM over 3 h. This is likely due to a combination of tissue production and microbial urease activity in the intestine. The overall results highlight the ability of S. a. suckleyi to regulate and maintain internal nitrogen concentrations despite the addition of excess dietary urea.

Differential expression of multiple glutamine synthetase genes in air-breathing magur catfish, Clarias magur and their induction under hyper-ammonia stress

The present study demonstrates the unique presence of three different gs genes (cmgs01, cmgs02, and cmgs03) in air-breathing ureogenic magur catfish (Clarias magur), which is otherwise reported to be encoded by a single gene in higher vertebrates. Of these three genes, two (cmgs01and cmgs03) were identified as 'liver' form, predominantly expressed in liver cells, and the third one as 'brain' form (cmgs02), expressed chiefly in brain cells. Molecular characterization studies have revealed conservation of homologous active site residues in all the three gs genes. In silico analysis, accompanied by GS enzyme assay and Western blot analysis of different GS isoforms in different subcellular fractions indicated the mitochondrial localization of cmGS01 and cmGS03 in liver and kidney cells and cytosolic localization of cmGS02 in brain cells. Further, exposure of magur catfish to high external ammonia (HEA; 25 mM NH₄Cl) led to a significant induction of multiple gs genes as evidenced by higher expression of different gs mRNAs at variable levels in different tissues. The cmgs01 and cmgs03 mRNA levels elevated significantly in liver, kidney, muscle, and gills, whereas the cmgs02 mRNA level increased considerably in the brain after 14 days of exposure to HEA. These increases in mRNA levels were associated with a significant rise in cmGS01 and cmGS03 proteins in liver, kidney, muscle, and gills, and the cmGS02 protein in the brain after 14 days of exposure to HEA. Therefore, it can be concluded that the unique differential expression of three gs genes and their induction under high ammonia level probably helps in detoxification of ammonia to glutamine and further to urea via the ornithine-urea cycle in ureogenic as well as non-ureogenic tissues of these magur catfish.

Multi-tissue RNA-seq and transcriptome characterisation of the spiny dogfish shark (Squalus acanthias) provides a molecular tool for biological research and reveals new genes involved in osmoregulation

The spiny dogfish shark (Squalus acanthias) is one of the most commonly used cartilaginous fishes in biological research, especially in the fields of nitrogen metabolism, ion transporters and osmoregulation. Nonetheless, transcriptomic data for this organism is scarce. In the present study, a multi-tissue RNA-seq experiment and de novo transcriptome assembly was performed in four different spiny dogfish tissues (brain, liver, kidney and ovary), providing an annotated sequence resource. The characterization of the transcriptome greatly increases the scarce sequence information for shark species. Reads were assembled with the Trinity de novo assembler both within each tissue and across all tissues combined resulting in 362,690 transcripts in the combined assembly which represent 289,515 Trinity genes. BUSCO analysis determined a level of 87% completeness for the combined transcriptome. In total, 123,110 proteins were predicted of which 78,679 and 83,164 had significant hits against the SwissProt and Uniref90 protein databases, respectively. Additionally, 61,215 proteins aligned to known protein domains, 7,208 carried a signal peptide and 15,971 possessed at least one transmembrane region. Based on the annotation, 81,582 transcripts were assigned to gene ontology terms and 42,078 belong to known clusters of orthologous groups (eggNOG). To demonstrate the value of our molecular resource, we show that the improved transcriptome data enhances the current possibilities of osmoregulation research in spiny dogfish by utilizing the novel gene and protein annotations to investigate a set of genes involved in urea synthesis and urea, ammonia and water transport, all of them crucial in osmoregulation. We describe the presence of different gene copies and isoforms of key enzymes involved in this process, including arginases and transporters of urea and ammonia, for which sequence information is currently absent in the databases for this model species. The transcriptome assemblies and the derived annotations generated in this study will support the ongoing research for this particular animal model and provides a new molecular tool to assist biological research in cartilaginous fishes.

Evidence of metabolic microevolution of the limpet Nacella concinna to naturally high heavy metal levels in Antarctica

The gastropod Nacella concinna is the most conspicuous macroinvertebrate of the intertidal zone of the Antarctic Peninsula and adjacent islands. Naturally high levels of copper and cadmium in coastal marine ecosystems are accumulated in N. concinna tissues. We aimed to study the effects of metal cations on N. concinna arginase in the context of possible adaptive microevolution. Gills and muscle had the highest argininolytic activity, which was concentrated in the cytosol in both tissues. Gills had the highest levels of arginase and may be involved in the systemic control of l-arginine levels. The relatively high argininolytic activity of the N. concinna muscular foot, with K_M=25.3±3.4mmolL^-1, may be involved in the control of l-arginine levels during phosphagen breakdown. N. concinna arginases showed the following preferences for metal cations: Ni²⁺>Mn²⁺>Co²⁺>Cu²⁺ in muscle and Mn²⁺>Cu²⁺ in gills. Cu²⁺ activation is a unique characteristic of N. concinna arginases, as copper is a potent arginase inhibitor. Cu²⁺ partly neutralized N. concinna arginase inhibition by Cd²⁺, worked synergistically in muscle arginase activation by Co²⁺ and neutralized muscle arginase activation by Ni²⁺. Mn²⁺ was able to activate muscle arginase in the presence of Fe³⁺ and Pb²⁺. The selection of arginases that are activated by Cu²⁺ and resistant to inhibition by Cd²⁺ in the presence of Cu²⁺ over evolutionary timescales may have favored N. concinna occupation of copper- and cadmium-rich niches.

KCl-Dependent Release of Mitochondrial Membrane-Bound Arginase Appears to Be a Novel Variant of Arginase-II

Arginase regulates arginine metabolism, ornithine-urea cycle, and immunological surveillance. Arginase-I is predominant in cytosol, and arginase-II is localised in the mitochondria. A mitochondrial membrane-bound arginase has also been proposed to be adsorbed with outer membrane of mitochondria which gets released by 150 mM potassium chloride (KCl). It is presumed that inclusion of 150 mM KCl in the homogenization medium would not only facilitate release of arginase bound with outer membrane of mitochondria but also affect functional anatomy of mitochondria, mitochondrial enzymes, and proteins. Therefore, it has been intended to characterize KCl-dependent release of mitochondrial membrane-bound arginase from liver of mice. Results provide advancement in the area of arginase biology and suggest that fraction of mitochondrial membrane-bound arginase contains mitochondrial arginase-II and a variant of arginase-II.

Biochemical and molecular characterization of mitochondrial membrane-bound arginase in Heteropneustes fossilis

The two predominant forms of arginase, cytosolic Arginase-I and mitochondrial Arginase-II, catalyze hydrolysis of arginine into ornithine and urea. Based on presence of arginase activity in extracts using potassium chloride (KCl), mitochondrial membrane-bound arginase has also been suggested. However, the activity of arginase in fractions obtained after KCl-treatment may be either due to leakage of mitochondrial arginase or release of adhered cytosolic arginase to cell organelles having altered net charge. Therefore, it has been intended to analyse impact of KCl on ultra-structural properties of mitochondria, and biochemical analysis of mitochondrial membrane-bound proteins and arginase of Heteropneustes fossilis. Liver of H. fossilis was used for isolating mitochondria for analysis of ultrastructural properties, preparing cytosolic, mitochondrial, and mitochondrial-membrane bound extracts after treatment of KCl. Extracts were analysed for arginase activity assay, protein profiling through SDS-PAGE and MALDI MS/MS. The KCl-mediated modulation in polypeptides and arginase were also evaluated by PANTHER, MitoProt and IPSORT servers. The effects of KCl on ultra-structural integrity of mitochondria, activity of arginase, modulation on mitochondrial proteins and enzymes including arginase were observed. The 48 kDa polypeptide of mitochondrial fraction, that showed KCl-dependent alteration matched with Myb binding protein and 30 kDa bands resembles to arginase after MALDI MS/MS analysis. Results indicate KCl-dependent ultrastructural changes in mitochondria and release of mitochondrial arginase. The proposed membrane bound mitochondrial arginase could be mitochondrial arginase-II or altered form of cytosolic arginase-I contributing to KCl-induced arginase activity in H. fossilis.

Unique hepatic cytosolic arginase evolved independently in ureogenic freshwater air-breathing teleost, Heteropneustes fossilis

Hepatic cytosolic arginase (ARG I), an enzyme of the urea cycle operating in the liver of ureotelic animals, is reported to be present in an ammoniotelic freshwater air-breathing teleost, Heteropneustes fossilis which has ureogenic potential. Antibodies available against mammalian ARG I showed no cross reactivity with the H. fossilis ARG I. We purified unique ARG I from H. fossilis liver. Purified ARG I is a homotrimer with molecular mass 75 kDa and subunit molecular mass of 24 kDa. The pI value of the enzyme was 8.5. It showed maximum activity at pH 10.5 and 55°C. The Km of purified enzyme for L-arginine was 2.65±0.39 mM. L-ornithine and N(ω)-hydroxy-L-arginine showed inhibition of the ARG I activity, with Ki values 0.52±0.02mM and 0.08±0.006mM, respectively. Antibody raised against the purified fish liver ARG I showed exclusive specificity, and has no cross reactivity against fish liver ARG II and mammalian liver ARG I and ARG II. We found another isoform of arginase bound to the outer membrane of the mitochondria which was released by 150-200 mM KCl in the extraction medium. This isoform was immunologically different from the soluble cytosolic and mitochondrial arginase. The results of present study support that hepatic cytosolic arginase evolved in this ureogenic freshwater teleost, H. fossilis. Phylogenetic analysis confirms an independent evolution event that occurred much after the evolution of the cytosolic arginase of ureotelic vertebrates.

[Effect of glyphosate on the energy exchange in carp organs]

The use of glyphosate as a herbicide in agriculture can lead to the presence of its residues and metabolites (aminomethylphosphonic acid) in food for human consumption and pose a threat to health. The effect of these herbicides on the fish organism at the biochemical level has been insufficiently studied. We studied changes in the content of adenine nucleotides, enzyme activity, quantitative indexes of energy metabolism substrates in carp under the action of glyphosate. It has been found that proteins are the major energy substrate under the influence of glyphosate in the liver, brain, white muscle of carp yearlings. Glyphosphate decreases energy metabolism in the brain of carp and increases it in the white muscles. The growth of activity of catabolic enzymes in the liver under the influence of glyphosate can be attributed to the adaptive remodelling of metabolic pathways for homeostasis and enantiostasis in response to herbicides.

Nitrogen metabolism of the intestine during digestion in a teleost fish, the plainfin midshipman (Porichthys notatus)

Digestion affects nitrogen metabolism in fish, as both exogenous and endogenous proteins and amino acids are catabolized, liberating ammonia in the process. Here we present a model of local detoxification of ammonia by the intestinal tissue of the plainfin midshipman (Porichthys notatus) during digestion, resulting in an increase in urea excretion of gastrointestinal origin. Corroborating evidence indicated whole-animal ammonia and urea excretion increased following feeding, and ammonia levels within the lumen of the midshipman intestine increased to high levels (1.8±0.4 μmol N g(-1)). We propose that this ammonia entered the enterocytes and was detoxified to urea via the ornithine-urea cycle (O-UC) enzymes, as evidenced by a 1.5- to 2.9-fold post-prandial increase in glutamine synthetase activity (0.14±0.05 and 0.28±0.02 μmol min(-1) g(-1) versus 0.41±0.03 μmol min(-1) g(-1)) and an 8.7-fold increase in carbamoyl phosphate synthetase III activity (0.3±1.2 versus 2.6±0.4 nmol min(-1) g(-1)). Furthermore, digestion increased urea production by isolated gastrointestinal tissue 1.7-fold, supporting our hypothesis that intestinal tissue synthesizes urea in response to feeding. We further propose that the intestinal urea may have been excreted into the intestinal lumen via an apical urea transporter as visualized using immunohistochemistry. A portion of the urea was then excreted to the environment along with the feces, resulting in the observed increase in urea excretion, while another portion may have been used by intestinal ureolytic bacteria. Overall, we propose that P. notatus produces urea within the enterocytes via a functional O-UC, which is then excreted into the intestinal lumen. Our model of intestinal nitrogen metabolism does not appear to be universal as we were unab le to activate the O-UC in the intestine of fed rainbow trout. However, literature values suggest that multiple fish species could follow this model.

Unusual hepatic mitochondrial arginase in an Indian air-breathing teleost, Heteropneustes fossilis: purification and characterization

A functional urea cycle with both cytosolic (ARG I) and mitochondrial (ARG II) arginase activity is present in the liver of an ureogenic air-breathing teleost, Heteropneustes fossilis. Antibodies against mammalian ARG II showed no cross-reactivity with the H. fossilis ARG II. ARG II was purified to homogeneity from H. fossilis liver. Purified ARG II showed a native molecular mass of 96 kDa. SDS-PAGE showed a major band at 48 kDa. The native enzyme, therefore, appears to be a homodimer. The pI value of the enzyme was 7.5. The purified enzyme showed maximum activity at pH 10.5 and 55 °C. The K(m) of purified ARG II for l-arginine was 5.25±1.12 mM. L-Ornithine and N(ω)-hydroxy-L-arginine showed mixed inhibition with K(i) values 2.16±0.08 and 0.02±0.004 mM respectively. Mn(+2) and Co(+2) were effective activators of arginase activity. Antibody raised against purified H. fossilis ARG II did not cross-react with fish ARG I, and mammalian ARG I and ARG II. Western blot with the antibodies against purified H. fossilis hepatic ARG II showed cross reactivity with a 96 kDa band on native PAGE and a 48 kDa band on SDS-PAGE. The molecular, immunological and kinetic properties suggest uniqueness of the hepatic mitochondrial ARG II in H. fossilis.

Avian and Mammalian mitochondrial ammonia-detoxifying systems in tortoise liver

Liver mitochondria from the desert tortoise Gopherus agassizii and the Texas tortoise G. berlandieri contain glutamine synthetase, the enzyme used by birds and higher reptiles to detoxify ammonia generated intramitochondrially during amino acid catabolism. Tortoise mitochondria also contain carbamyl phosphate synthetase-I and ornithine transcarbamylase, the enzymes used by mammals for this purpose. Since chelonid reptiles have changed little since their divergence from the stem reptiles, this finding suggests that both systems were present in the latter before the emergence of the ruling reptile, avian, and mammalian lines of descent.

A single glutamine synthetase gene produces tissue-specific subcellular localization by alternative splicing

Glutamine synthetase (GS) plays a key role in two major biochemical pathways: In liver GS catalyzes ammonia detoxification, whereas in neural tissues it also functions in recycling of the neurotransmitter glutamate. In most species the GS gene gives rise to a cytoplasmic protein in both liver and neural tissues. However, in species that utilize the ureosmotic or uricotelic system for ammonia detoxification, the enzyme is cytoplasmic in neural tissues, but mitochondrial in liver cells. Since most vertebrates have a single copy of the GS gene, it is not clear how tissue-specific subcellular localization is achieved. Here we show that in the ureosmotic elasmobranch, Squalus acanthias (spiny dogfish), two different GS transcripts are generated by tissue-specific alternative splicing. The liver transcript contains an alternative exon that is not present in the neural one. This exon leads to acquisition of an upstream in-frame start codon and formation of a mitochondrial targeting signal (MTS). Therefore, the liver product is targeted to the mitochondria while the neural one is retained in the cytoplasm. These findings present a mechanism in which alternative splicing of an MTS-encoding exon is used to generate tissue-specific subcellular localization.

Evolution of glutamine synthetase in vertebrates: multiple glutamine synthetase genes expressed in rainbow trout (Oncorhynchus mykiss)

Glutamine synthetase (GSase) is a key enzyme in nitrogen metabolism and encoded by a single gene in mammals. Using PCR cloning techniques, including RT-PCR from total RNA and PCR from a cDNA library, we find evidence of four expressed GSase mRNAs for the tetraploid rainbow trout. For two of these mRNAs (Onmy-GS01, -GS02) we characterize the full-length coding regions, and for two others (Onmy-GS03, -GS04), we describe partial sequences. Northern analysis of Onmy-GS01, -GS02, -GS03 and -GS04 indicates that (1) Onmy-GS02 is expressed at higher levels relative to the other transcripts in most adult tissues, with the exception of brain and gill, where Onmy-GS01 is at the highest level, and (2) the tissue with the highest level of expression of all four transcripts is the brain, with decreasing levels in the intestine, liver, red muscle, gill/kidney, white muscle and heart. Clearly, rainbow trout possess multiple GSase genes with differing levels of tissue expression, implying manifold potential routes of regulation for this octameric enzyme. Our data also indicate that caution should be taken when interpreting mRNA expression data of a single gene, unless multiple genes have been ruled out. Consistent with a southern blot, phylogenetic and intron sequence analyses imply that the trout genes are encoded by at least four separate loci, belonging to two distinct evolutionary branches. Our data on rainbow trout, together with those from two full-length zebrafish Danio rerio GSase genes compiled from GenBank ESTs, support the idea that fish GSases are polyphyletic and that gene duplications have occurred at multiple points and in independent lineages throughout the evolution of bony fishes.

Glutamine synthetase expression in liver, muscle, stomach and intestine ofBostrichthys sinensisin response to exposure to a high exogenous ammonia concentration

A previous study provided evidence that the adaptive strategy used by the teleost fish Bostrichthys sinensis (sleeper) for detoxifying ammonia during extended periods of air exposure was to synthesize and store glutamine,primarily in the muscle, accompanied by an increase in glutamine synthetase(GSase) activity in liver. The aim of the present study was to assess the effect on GSase expression in various tissues of exposure of B. sinensis to exogenous ammonia. Exogenous ammonia increases internal ammonia concentrations in fish, mimicking environmental situations such as air exposure that preclude loss of ammonia across the gills, and thus triggering alternative mechanisms for ammonia detoxification. The results reveal relatively high levels of GSase activity, not only in liver but also,unexpectedly, in muscle, and even higher levels in intestine and, in particular, stomach. Exposure to ammonia results in significant increases in GSase activity, GSase protein and GSase mRNA levels in all of these tissues except stomach. The amino acid sequences of GSases from liver and stomach deduced from the cDNA sequences are essentially identical and are >97 %identical to the amino acid sequences of GSases from Gulf toadfish(Opsanus beta) and marble goby (Oxyeleotris marmoratus).

The loach Misgurnus anguillicaudatus reduces amino acid catabolism and accumulates alanine and glutamine during aerial exposure

The loach Misgurnus anguillicaudatus inhabits rice fields in Southern China. It encounters drought during summer and ammonia loading during agricultural fertilization. In the laboratory, aerial exposure led to decreases in its ammonia and urea excretion. Ammonia accumulated to very high levels in the muscle and liver. Urea synthesis through the ornithine-urea cycle was not involved in ammonia detoxification in M. anguillicaudatus. However, M. anguillicaudatus was capable of partial amino acid catabolism leading to the accumulation of alanine in the first 24 h of aerial exposure. This was apparently coupled to a possible decrease in protein/amino acid catabolism. These are not detoxification mechanisms but mechanisms that avoid internal fouling by ammonia. Misgurnus anguillicaudatus was also capable of detoxifying internally produced ammonia in part to glutamine, which appears to be an important adaptation after 24 h of aerial exposure. However, unlike the case of the marble goby (Oxyeleotris marmoratus), there was no alteration to the kinetic properties of the hepatic glutamine synthetase. During dry seasons, M. anguillicaudatus moves actively on land until it encounters soft mud in which it can bury itself through several strong wriggling actions of the body. Hence, it is possible that M. anguillicaudatus uses partial amino acid catabolism to fuel its short period of activities on land and switches to the formation of glutamine to detoxify internally produced ammonia when it remains relatively inactive in the mud.

Characterization and sequencing of glutamine synthetase cDNA from liver of the ureotelic gulf toadfish (Opsanus beta)

The hepatic enzyme, glutamine synthetase (GSase) is a pivotal protein in the regulation of urea synthesis in fish. The sequence of the DNA encoding for GSase from liver of the ureotelic gulf toadfish (Opsanus beta) was analyzed through a suite of molecular techniques (including cDNA cloning, RACE PCR, and genomic PCR). An open reading frame (ORF) was identified in the cDNA sequence which codes for a protein of 394 amino acids with high identity (86%) to dogfish shark GSase. In the course of generating a suitable probe, a partial sequence was also obtained for horned shark GSase which also had high identity with the dogfish shark gene (93%). Like the dogfish shark GSase, the toadfish gene has two methionine translation initiation sites; the downstream site apparently codes for a cytoplasmic isozyme, while the upstream site adds an N-terminal peptide leader sequence of 23 amino acids to the 'cytoplasmic' protein. This leader sequence has characteristics consistent with a mitochondrial targeting peptide, including a cleavage recognition motif (Arg-X-Phe) and the apparent ability to form an amphiphathic helix. Northern analysis revealed that there is a single predominant transcript of approximately 2 kb in size. These results are consistent with the interpretation that in the gulf toadfish GSase cytoplasmic and mitochondrial isozymes are coded for by a single gene and mRNA transcript which is differentially translated at either initiation site. These results are discussed in the context of prior results for enzyme kinetic characteristics and urea synthesis/excretion physiology.

Muscle as the primary site of urea cycle enzyme activity in an alkaline lake-adapted tilapia, Oreochromis alcalicus grahami

The tilapia fish Oreochromis alcalicus grahami from Kenya has adapted to living in waters at pH 10.5 by excreting the end product of nitrogen metabolism as urea rather than as ammonia directly across the gills as occurs in most fish. The level of activity in liver of the first enzyme in the urea cycle pathway, carbamoyl-phosphate synthetase III (CPSase III), is too low to account for the observed high rates of urea excretion. We report here the surprising finding that CPSase III and all other urea cycle enzyme activities are present in muscle of this species at levels more than sufficient to account for the rate of urea excretion; in addition, the basic kinetic properties of the CPSase III appear to be different from those of other known type III CPSases. The sequence of the CPSase III cDNA is reported as well as the finding that glutamine synthetase activity is present in liver but not in muscle. This unusual form of adaptation may have occurred because of the apparent impossibility of packaging the needed amount of urea cycle enzymes in liver.

Nitrogen excretion and expression of urea cycle enzymes in the atlantic cod (Gadus morhua l.): a comparison of early life stages with adults

For many years, the urea cycle was considered to be relatively unimportant in the life history of most teleost fishes. In previous studies, we were surprised to find that newly hatched freshwater rainbow trout embryos had relatively high activities of the key urea cycle enzyme, carbamoyl phosphate synthetase III (CPSase III), and other enzymes in the pathway, whereas adult trout had much lower or non-detectable activities. The present study tested the hypothesis that urea cycle enzyme expression is unique to early stages of rainbow trout. In marine Atlantic cod (Gadus morhua) embryos, CPSase III, ornithine transcarbamoylase (OTCase), glutamine synthetase (GSase) and arginase activities were all expressed prior to hatching. Urea excretion was detected shortly after fertilization and rates were high relative to those of ammonia excretion (50–100 % of total nitrogen excreted as urea nitrogen; total=ammonia+urea). Urea concentration was relatively constant in embryos, but ammonia concentration increased by about fourfold during embryogenesis. Two populations of cod embryos were studied (from Newfoundland and New Brunswick), and significant differences in enzyme activities and excretion rates were detected between the two populations. In adult cod, CPSase III was not detectable in liver, white muscle, intestine and kidney tissues, but OTCase, GSase and arginase were present. Adult cod excreted about 17 % of nitrogenous waste as urea. Taken together, these data indicate that early urea cycle enzyme expression is not unique to rainbow trout but is also a feature of Atlantic cod development, and possibly other teleosts. The relatively high urea excretion rates underline the importance of urea as the primary nitrogen excretory product in Atlantic cod during early embryogenesis.

The marble goby oxyeleotris marmoratus activates hepatic glutamine synthetase and detoxifies ammonia to glutamine during air exposure

Ammonia levels in various tissues of the marble goby Oxyeleotris marmoratus remained constant throughout a 72 h period of air exposure. The rate of ammonia excretion in these experimental fish decreased to approximately one-fifth of that of the submerged control. Ammonia was not converted to urea during air exposure because there were no significant increases in urea content in the tissues. Also, urea excretion rate was lowered to one-fiftieth that of the submerged fish. After 24 h of air exposure, there was a significant increase in muscle glutamine content, which peaked at 48 h. The increase in glutamine content could account for the decreases in the amounts of ammonia and urea excretion during air exposure. The specific activities of hepatic glutamate dehydrogenase (amination) and glutamine synthetase in these experimental fish increased threefold and thirtyfold, respectively, in comparison with the submerged controls. Thus, O. marmoratus appears to be the first known teleost that responds to air exposure by activating hepatic glutamine synthetase to detoxify internally produced ammonia.

Urea production and transport in teleost fishes

Teleosts appear to have retained the genes for the urea cycle enzymes. A few species express the full complement of enzymes and are ureotelic (e.g., Lake Magadi tilapia) or ammoniotelic (e.g., largemouth bass), whereas most species have low or non-detectable enzyme activities in liver tissue and excrete little urea (e.g., adult rainbow trout). It was surprising, therefore, to find the expression of four urea cycle enzymes during early life stages of rainbow trout. The urea cycle may play a role in ammonia detoxification during a critical time of development. Exposure to alkaline water (pH 9.0-9.5) or NH4Cl (0.2 mmol/l) increased urea excretion by several-fold in trout embryos, free embryos and alevin. Urea transport is either by passive simple diffusion or via carried-mediated transport proteins. Molecular studies have revealed that a specialised urea transport protein is present in kidney tissue of elasmobranchs, similar to the facilitated urea transporter found in the mammalian inner medulla of the kidney.

Distribution of glutamine synthetase in the snapper (Pagrus auratus) and implications for the immune system

A sensitive radiotracer method was used to detect glutamine synthetase activity in fish. The distribution of glutamine synthetase activity using this method was shown to be widespread in organs and tissues of the marine finfish, snapper,Pagrus auratus. The optimum pH and temperature for glutamine synthetase activity and Km of glutamate from the brain of the snapper are also reported. Interestingly no glutamine synthetase activity was detected in lymphoid organs, indicating, that as in the mammal, the teleostean immune system is likely to rely on glutamine in circulation for energy and nucleotide synthesis. Furthermore circulating glutamine levels fell as fish became stressed or became ill. Implications of these observations on the function of the immune system of fish are discussed.

Comparative properties of arginases

Arginase is a primordial enzyme, widely distributed in the biosphere and represented in all primary kingdoms. It plays a critical role in the hepatic metabolism of most higher organisms as a cardinal component of the urea cycle. Additionally, it occurs in numerous organisms and tissues where there is no functioning urea cycle. Many extrahepatic tissues have been shown to contain a second form of arginase, closely related to the hepatic enzyme but encoded by a distinct gene or genes and involved in a host of physiological roles. A variety of functions has been proposed for the "extrahepatic" arginases over the last three decades. In recent years, interest in arginase has been stimulated by a demonstrated involvement in the metabolism of the ubiquitous and multifaceted molecule nitric oxide. Molecular biology has begun to furnish new clues to the disparate functions of arginases in different environments and organisms. Comparative studies of arginase sequences are also beginning to elucidate the comparative evolution of arginases, their molecular structures and the nature of their catalytic mechanism. Further studies have sought to clarify the involvement of arginase in human disease. This review presents an outline of the current state of arginase research by giving a comparative overview of arginases and their associated properties.

Purification and properties of mitochondrial phosphoenolpyruvate carboxykinase from liver of Squalus acanthias

Liver from Squalus acanthias (spiny dogfish), a representative elasmobranch, contains approximately 1.4 units (mumol/min) of phosphoenolpyruvate carboxykinase activity per gram and approximately 90% of the total units of activity are localized in the mitochondria. The mitochondrial phosphoenolpyruvate carboxykinase was isolated and characterized. The purified enzyme has properties generally similar to those found in mammalian and avian species. The enzyme has a molecular weight of approximately 70,000 and exists in a functional state as a monomer. The isolated enzyme displays a dual cation requirement (e.g., 6 mM Mg2+ and 10 microM Mn2+) for maximal activity; very little activity is observed when Mg2+ is present alone, and the maximal activity attained with Mn2+ alone (millimolar concentrations required) is significantly less than that observed under optimal conditions with both cations present. When assayed in the direction of oxalacetate formation there is a lag in product formation with time; the lag can be eliminated by the presence of 50 microM GTP (product). The Km for substrates is not affected by Mn2+ concentration, suggesting that the role of Mn2+ may not be related to substrate binding. The apparent Km for phosphoenolpyruvate (approximately 1 mM) is substantially higher than that reported for phosphoenolpyruvate carboxykinase from other species. The activity of phosphoenolpyruvate carboxykinase is increased 70% by physiological concentrations of urea. Maximal velocity of the reaction in the direction of oxalacetate formation is approximately half that of the reverse reaction.

Purification and properties of ornithine carbamoyl transferase from liver of Squalus acanthias

Citrulline synthesis from ammonia by hepatic mitochondria in elasmobranchs involves intermediate formation of glutamine as the result of the presence of high levels of glutamine synthetase and a unique glutamine- and N-acetyl-glutamate-dependent carbamoyl phosphate synthetase, both of which have properties unique to the function of glutamine-dependent synthesis of urea, which is retained in the tissues of elasmobranchs at high concentrations for the purpose of osmoregulation [P.M. Anderson and C.A. Casey (1984) J. Biol. Chem. 259, 456-462; R.A. Shankar and P.M. Anderson (1985) Arch. Biochem. Biophys. 239, 248-259]. The objective of this study was to determine if ornithine carbamoyl transferase, which catalyzes the last step of mitochondrial citrulline synthesis and which has not been previously isolated from any species of fish, also has properties uniquely related to this function. Ornithine carbamoyl transferase was highly purified from isolated liver mitochondria of Squalus acanthias, a representative elasmobranch. The purified enzyme is a trimer with a subunit molecular weight of 38,000 and a native molecular weight of about 114,000. The effect of pH is significantly influenced by ornithine concentration; optimal activity is at pH 7.8 when ornithine is saturating. The apparent Km values for ornithine and carbamoyl phosphate at pH 7.8 are 0.71 and 0.05 mM, respectively. Ornithine displays considerable substrate inhibition above pH 7.8. The activity is not significantly affected by physiological concentrations of the osmolyte urea or trimethylamine-N-oxide or by a number of other metabolites. The results of kinetic studies are consistent with a steady-state ordered addition of substrates (carbamoyl phosphate binding first) and rapid equilibrium random release of products. Except for an unusually low specific activity, the properties of the purified elasmobranch enzyme are similar to the properties of ornithine carbamoyl transferase from mammalian ureotelic and other species and do not appear to be unique to its role in glutamine-dependent synthesis of urea for the purpose of osmoregulation.

Subcellular localization and kinetic properties of arginase from the liver of Genypterus maculatus

1. From the liver of the teleost fish Genypterus maculatus, a partially purified preparation of arginase was obtained and characterized. 2. The Km value for arginine was found to be 9.1 mM at pH 7.5 and 11.5 mM at the optimum pH of 9.5. At both pH values, competitive inhibition was caused by ornithine and lysine, whereas proline, leucine, valine and isoleucine caused a non-competitive inhibitory effect. Branched chain amino acids were more inhibitory than proline. 3. The enzyme was found localized in the mitochondrial matrix of the liver of Genypterus maculatus. It is suggested that this localization would be of importance in the use of arginine as an energy source.

Effects of urea, trimethylamine oxide, and osmolality on respiration and citrulline synthesis by isolated hepatic mitochondria from Squalus acanthias

The effects of osmolality and of urea and trimethylamine oxide on citrulline synthesis and respiration by isolated hepatic mitochondria from Squalus acanthias (spiny dogfish) were determined. Optimal rates of citrulline synthesis and of respiration, as well as maximal respiratory control ratios, were obtained when the osmolality was about 300 mOsm when physiological concentrations of urea and trimethylamine oxide were present, analogous to mammalian mitochondria. Urea and trimethylamine oxide do not act as osmolytes and do not significantly affect these rates. When glutamate is not saturating, increasing concentrations of urea over the range of concentrations of physiological significance (e.g. 0.1-0.5 M) significantly inhibit citrulline synthesis, but not respiration. This response to urea concentration may reflect a specific homeostatic mechanism for maintaining physiological concentrations of urea.

Purification and properties of glutamine synthetase from liver of Squalus acanthias

Ammonia assimilation for urea synthesis by liver mitochondria in marine elasmobranchs involves, initially, formation of glutamine which is subsequently utilized for mitochondrial carbamoyl phosphate synthesis [P. M. Anderson and C. A. Casey (1984) J. Biol. Chem. 259, 456-462]. The purpose of this study was to determine if the glutamine synthetase catalyzing this first step in urea synthesis has properties uniquely related to this function. Glutamine synthetase has been highly purified from isolated liver mitochondria of Squalus acanthias, a representative elasmobranch. The purified enzyme has a molecular weight of approximately 400,000 in the presence of Mg2+, MgATP, and L-glutamate, but dissociates reversibly to a species with a molecular weight of approximately 200,000 in the absence of MgATP and L-glutamate. Association with the glutamine- and acetylglutamate-dependent carbamoyl phosphate synthetase, also located in the mitochondria, could not be demonstrated. The subunit molecular weight is approximately 46,000. The pH optimum of the biosynthesis reaction is 7.1-7.4. The purified enzyme is stabilized by MgATP and glutamate and by ethylene glycol, and is activated by 5-10% ethylene glycol. The apparent Km values for MgATP, L-glutamate, and ammonia (NH4+-NH3) are 0.7, 11.0, and 0.015 mM, respectively. Mg2+ in excess of that required to complex ATP as MgATP is required for maximal activity; Mn2+ cannot replace Mg2+. The enzyme is activated by low concentrations of chloride, bromide, or iodide; this effect appears to be related to decreases in the apparent Km for glutamate. The enzyme is inhibited by physiological concentrations of urea, but is not significantly affected by physiological concentrations of trimethylamine-N-oxide. Except for activation by halogen anions and the very low apparent Km for ammonia, this elasmobranch glutamine synthetase has properties similar to those reported for mammalian and avian glutamine synthetases. The very low apparent Km for ammonia may be specifically related to the unique role of this glutamine synthetase in mitochondrial assimilation of ammonia for urea synthesis.

Submitochondrial localization of arginase and other enzymes associated with urea synthesis and nitrogen metabolism, in liver of Squalus acanthias

The submitochondrial localization of the four mitochondrial enzymes associated with urea synthesis in liver of Squalus acanthias (spiny dogfish), a representative elasmobranch, was determined. Glutamine- and acetylglutamate-dependent carbamoyl-phosphate synthetase, ornithine carbamoyltransferase, glutamine synthetase, and arginase were all localized within the matrix of liver mitochondria. The subcellular and submitochondrial localization and activities of several related enzymes involved in nitrogen metabolism and gluconeogenesis in liver and dogfish are also reported. Pyruvate carboxylase and phosphoenolpyruvate carboxykinase were localized in the mitochondrial matrix. Synthesis of citrulline by isolated mitochondria from ornithine proceeds at a near optimal rate at ornithine concentrations as low as 0.08 mM. The same stoichiometry and rates of citrulline synthesis are observed when ornithine is replaced by arginine. The mitochondrial location of arginase does not appear to reflect a mechanism for regulating ornithine availability.