Preparation and cross-reactivity of anti-avian glutamine synthetase antibody

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.

Induction of carbamoyl phosphate synthetase III and glutamine synthetase mRNA during confinement stress in gulf toadfish (Opsanus beta)

Gulf toadfish (Opsanus β) rapidly switch to excretion of urea as their main nitrogenous waste product under several laboratory conditions, including confinement to small volumes of water. Prior evidence suggested that the activities of two key enzymes of urea synthesis exhibited potentially different modes of upregulation during this switch, with carbamoyl phosphate synthethase III (CPSase III) activated allosterically by N-acetylglutamate, and glutamine synthetase (GSase) activated by increases in the concentration of protein. The present study was undertaken to examine additional aspects of the regulation of these enzymes. The sequence for O. beta CPSase III cDNA was obtained, and it was found to be similar to that of other piscine CPSases. The sequence also allowed us to develop riboprobes for CPSase III mRNA analysis using ribonuclease protection assays (RPAs). CPSase III mRNA was expressed in liver, muscle, kidney and intestine, in agreement with prior enzymatic measurements. Levels of CPSase III mRNA increased five- to tenfold (relative to beta-actin mRNA) in liver (but not muscle) following 48 h of confinement stress. Measured by western analysis using an antibody to chicken GSase, confined O. beta GSase protein concentrations increased eightfold over control levels, in agreement with prior and present measurements of increases in GSase activity. Furthermore, RPAs of GSase mRNA levels demonstrated an increase of fivefold during confinement.

Evolution and regulation of urea synthesis and ureotely in (batrachoidid) fishes

Selected teleostean (bony) fish species of the family Batrachoididae (toadfishes and midshipmen) possess high titers of all enzymes of the ornithine-urea cycle in their livers. These species have proven valuable in understanding the short-term regulation of urea synthesis, urea permeability, and transport across epithelial tissues, and how urea synthesis and excretion have evolved among vertebrates. One species in particular, the gulf toadfish (Opsanus beta), has been shown to rapidly switch from ammonia excretion to urea synthesis and excretion during a variety of stress conditions (including confinement). The transition is accompanied by an upregulation of hepatic glutamine synthetase activity, and a switch to pulsatile urea excretion from the anterior end of the fish. In fact, a single day's excretion can be voided in a period of < 3 h. Hypotheses on the environmental significance of these patterns of urea synthesis and excretion are discussed.

Purification and immunocharacterization of human brain glutamine synthetase and its detection in cerebrospinal fluid and serum by a sandwich enzyme immunoassay

A modified procedure employing a single chromatographic step to purify glutamine synthetase from human brain is described. The enzyme was characterized by native, denaturing, and two-dimensional gel electrophoresis and immunoblotting. Yield and purity were assessed by enzyme activity assay and a newly developed sandwich enzyme immunoassay using a mouse monoclonal antibody against native sheep brain glutamine synthetase. The immunoassay detected glutamine synthetase protein in samples where the enzyme had been inactivated by repeated cycles of freezing and thawing, and in serum and cerebrospinal fluid where glutamine synthetase was undetectable by the enzyme activity assay. Native glutamine synthetase from human brain occurred as an octamer with an estimated molecular weight of 360-400 kDa. Under reducing and denaturing conditions, the enzyme dissociated into monomeric subunits with an estimated molecular weight of 44 kDa. The monomers were recognized by the monoclonal antibody on immunoblots but not in the sandwich enzyme immunoassay, suggesting that the antigenic site occurs once on each subunit. Both human and sheep brain glutamine synthetases were composed of three and four different types of subunits with isoelectric points ranging from 7.0-7.2 and 6.8-7.0, respectively.

Homologous nuclear genes encode cytoplasmic and mitochondrial glutamine synthetase in Drosophila melanogaster

We describe the cloning of the glutamine synthetase 1 (GS1) gene based on cross-homology with the glutamine synthetase 2 (GS2) gene in Drosophila melanogaster. We have determined the GS gene number in the Drosophila genome, and we describe the isolation of cDNA clones corresponding to the two isoforms, their entire sequence and their transcription pattern. We looked for subcellular localization of one enzymic isoform; in this way, we were able to locate the GS1 enzyme within the mitochondria of D. melanogaster. We have compared different GS sequences from plants and humans; emerging evolutionary implications are discussed. In addition, we have identified a certain highly stable secondary structure at the nucleotide level in the coding region of isoforms located in the organella.

Distribution of glutamine synthetase and carbamoyl-phosphate synthetase I in vertebrate liver

Mitochondrial glutamine synthetase (EC 6.3.1.2) is the primary ammonia-detoxifying enzyme in avian liver and is therefore analogous in function to carbamoyl-phosphate synthetase I (ammonia) (EC 6.3.4.16) in mammalian liver. In mammalian liver, glutamine synthetase is cytosolic and its distribution is restricted to a few hepatocytes around the terminal venules. These cells do not express carbamoyl-phosphate synthetase I. Using immunocytochemistry, we show here that there is little or no zonation of glutamine synthetase in avian liver. Rather, it is broadly distributed to most hepatocytes, much like carbamoyl-phosphate synthetase I in mammalian liver. In situ hybridization with a cloned glutamine synthetase cDNA probe showed the distribution of glutamine synthetase mRNA in both mammalian and avian liver to correspond to the distribution of immunoreactive protein. Neither glutamine synthetase nor carbamoyl-phosphate synthetase I and ornithine transcarbamoylase (EC 2.1.3.3) are strictly zoned in liver of the Texas tortoise or of an Argentine tree frog, both of which possess a complete urea cycle but which may also rely on glutamine synthetase for ammonia detoxication. These latter results suggest that the mutually exclusive expression of either carbamoyl-phosphate synthetase I or glutamine synthetase may be unique to mammalian liver.

Uricoteley:its nature and origin during the evolution of tetrapod vertebrates

The hepatic mechanism for detoxication of ammonia formed during amino acid gluconeogenesis in uricotelic vertebrates requires the intramitochondrial synthesis of glutamine by glutamine synthetase. This glutamine then serves as a precursor of uric acid in the cytosol. The evolutionary development of uricoteley thus required the localization of glutamine synthetase in liver mitochondria. The mechanism for the mitochondrial import of glutamine synthetase in uricotelic vertebrate liver is not yet known. Tortoises, extant relatives of the stem reptiles, possess both the ureotelic and uricotelic hepatic systems. It therefore seems likely that the genetic events allowing the mitochondrial localization of glutamine synthetase in liver occurred in the amniote amphibian ancestors of the stem reptiles. The selection of ureoteley by the theropsids and of uricoteley by the sauropsids were major events in the divergence and subsequent evolution of these two lines. Once established in the sauropsid line, uricoteley has persisted through to the higher reptiles, crocodilians, and birds. Uricoteley was in part responsible for the radiation of the archosaurs during the Triassic as a water-conserving mechanism in the adult, thereby allowing them to invade the arid environments of that period. Contrary to dogma, uricoteley was probably of minor significance in the development of the cleidoic egg. Neither mammalian nor avian embryonic liver tissues catabolize amino acids to any great extent, so it is inappropriate to attribute to them a kind of "waste" nitrogen metabolism.

Reciprocal regulation of glutamine synthetase and carbamoylphosphate synthetase levels in rat liver

In glucocorticosteroid-treated diabetic rats, glutamine synthetase enzyme levels in the liver are decreased 3-fold, whereas carbamoylphosphate synthetase enzyme levels are increased 2.3-fold. In addition, immunohistochemistry shows that under these conditions the distribution of carbamoylphosphate synthetase is expanded over the entire liver acinus, whereas that of glutamine synthetase is reduced to very few cells bordering the central (terminal hepatic) veins. Using a newly isolated cDNA complementary to rat liver glutamine synthetase mRNA, we show that this regulation is primarily effected at a pretranslational level. (For data on carbamoylphosphate synthetase mRNA levels, see De Groot et al. (1986) Biochim. Biophys. Acta 866, 61-67). Furthermore, hybridization studies show stimulatory effects of both glucocorticosteroids and thyroid hormone on the glutamine synthetase mRNA level. Attempts to localize glutamine synthetase mRNA within the liver acinus by selective destruction of the pericentral zone failed because of generally low levels of liver mRNAs after CCl4 poisoning. In contrast to the situation after birth, significantly higher glutamine synthetase mRNA/enzyme activity ratios in fetal rat liver point to the presence of additional post-transcriptional control mechanisms before birth. These findings complement similar observations on carbamoylphosphate synthetase gene expression (De Groot et al. (1986) Biochim. Biophys. Acta 866, 61-67).