Nitrogen‐13 as a Biochemical Tracer

Cooper, Arthur J. L.; Gelbard, Alan S.; Freed, Barry R.

doi:10.1002/9780470123034.ch4

Cited by 12 publications

(7 citation statements)

References 205 publications

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“…The [ 13 N]ammonia generated in the MSKCC cyclotron was of very high specific activity, such that truly tracer quantities of labeled ammonia could be administered to the rats. (For some historical perspectives on the use of 13 N as a tracer see references [14] and [15]. )…”

Section: Brain Uptake Index (Bui) Of [13n]ammonia In the Ratmentioning

confidence: 99%

The Role of Glutamine Synthetase and Glutamate Dehydrogenase in Cerebral Ammonia Homeostasis

Cooper

2012

Neurochem Res

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In the brain, glutamine synthetase (GS), which is located predominantly in astrocytes, is largely responsible for the removal of both blood-derived and metabolically generated ammonia. Thus, studies with [13N]ammonia have shown that about 25% of blood-derived ammonia is removed in a single pass through the rat brain and that this ammonia is incorporated primarily into glutamine (amide) in astrocytes. Major pathways for cerebral ammonia generation include the glutaminase reaction and the glutamate dehydrogenase (GDH) reaction. The equilibrium position of the GDH-catalyzed reaction in vitro favors reductive amination of α-ketoglutarate at pH 7.4. Nevertheless, only a small amount of label derived from [13N]ammonia in rat brain is incorporated into glutamate and the α-amine of glutamine in vivo. Most likely the cerebral GDH reaction is drawn normally in the direction of glutamate oxidation (ammonia production) by rapid removal of ammonia as glutamine. Linkage of glutamate/α-ketoglutarate-utilizing aminotransferases with the GDH reaction channels excess amino acid nitrogen toward ammonia for glutamine synthesis. At high ammonia levels and/or when GS is inhibited the GDH reaction coupled with glutamate/α-ketoglutarate-linked aminotransferases may, however, promote the flow of ammonia nitrogen toward synthesis of amino acids. Preliminary evidence suggests an important role for the purine nucleotide cycle (PNC) as an additional source of ammonia in neurons (Net reaction: L-Aspartate + GTP + H2O → Fumarate + GDP + Pi + NH3) and in the beat cycle of ependyma cilia. The link of the PNC to aminotransferases and GDH/GS and its role in cerebral nitrogen metabolism under both normal and pathological (e.g. hyperammonemic encephalopathy) conditions should be a productive area for future research.

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Section: Brain Uptake Index (Bui) Of [13n]ammonia In the Ratmentioning

confidence: 99%

The Role of Glutamine Synthetase and Glutamate Dehydrogenase in Cerebral Ammonia Homeostasis

Cooper

2012

Neurochem Res

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“…Perhaps the first biochemical study using 13 N as a tracer was used to study N 2 fixation in plants (Ruben et al, 1940). For a historical survey of the development of the discovery of positron-emitting isotopes and the use of 13 N as a tracer see the review by Cooper et al (1985a).…”

Section: Historicalmentioning

confidence: 99%

“…Alternatively, the [ 13 N]ammonia may be incorporated into a number of L-amino acids and other nitrogenous compounds by use of appropriate enzymes and substrates. For example, we have enzymatically prepared 13 N-labeled glutamine, asparagine, glutamate, alanine, methionine, branched-chain amino acids, phenylalanine, citrulline, carbamyl 1-phosphate and N -carbamyl aspartate (Cooper and Gelbard, 1981; Gelbard et al, 1980, 1985, 1990; Cooper et al, 1985a). A particularly useful procedure involves the use of GDH immobilized onto an inert support for the synthesis of L-[ 13 N]glutamate and some other 13 N-labeled amino acids.…”

Section: Historicalmentioning

confidence: 99%

13N as a tracer for studying glutamate metabolism

Cooper

2011

Neurochemistry International

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This mini-review summarizes studies my associates and I carried out that are relevant to the topic of the present volume [i.e. glutamate dehydrogenase (GDH)] using radioactive 13 N (t ½ 9.96 min) as a biological tracer. These studies revealed the previously unrecognized rapidity with which nitrogen is exchanged among certain metabolites in vivo. For example, our work demonstrated that a) the t ½ for conversion of portal vein ammonia to urea in the rat liver is ~10-11 sec, despite the need for five enzyme-catalyzed steps and two mitochondrial transport steps, b) the residence time for ammonia in the blood of anesthetized rats is ≤7-8 sec, c) the t ½ for incorporation of bloodborne ammonia into glutamine in the normal rat brain is <3 sec, and d) equilibration between glutamate and aspartate nitrogen in rat liver is extremely rapid (seconds), a reflection of the fact that the components of the hepatic aspartate aminotransferase reaction are in thermodynamic equilibrium. Our work emphasizes the importance of the GDH reaction in rat liver as a conduit for dissimilating or assimilating ammonia as needed. In contrast, our work shows that the GDH reaction in rat brain appears to operate mostly in the direction of ammonia production (dissimilation). The importance of the GDH reaction as an endogenous source of ammonia in the brain and the relation of GDH to the brain glutamine cycle is discussed. Finally, our work integrates with the increasing use of positron emission tomography (PET) and nuclear magnetic resonance (NMR) to study brain ammonia uptake and brain glutamine, respectively, in normal individuals and in patients with liver disease or other diseases associated with hyperammonemia. Keywords[ 13 N]Ammonia; 13 N-labeled amino acids; alanine aminotransferase; aspartate aminotransferase; branched chain amino acid aminotransferase; glutamate dehydrogenase; glutamine cycle; nitrogen flux; transreamination; transdeamination; urea cycle Historical13 N is a positron emitting isotope. In positron (or β + ) emission, the particle ejected from the nucleus of the decaying nuclide has the same mass, but opposite charge, to that of the electron. As with the more familiar β − (electron) emission, the ejected positrons are emitted with a spectrum of kinetic energies with characteristic E average and E max values. In the case of 13 N, the E max value is 1.2 MeV. By comparison, the E max values of the more familiar β − emissions of 14 C and 32 P are 0.16 and 1.7 MeV, respectively. After rapidly losing most of its kinetic energy, the ejected positron combines with a nearby electron to form a * Tel.: +1-914-597-3330; fax: +1-914-594-4058, Arthur_cooper@nymc.edu.Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production...

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“…The emitted positron annihilates a nearby electron giving rise to two coincidental (or almost coincidental) γ-rays that are easily detected and quantified by an external device. Two advantages of 13 N over 15 N (a non-radioactive isotope of nitrogen with a natural abundance of ~0.4%) to assess ammonia metabolism in short-term studies are that 13 N is radioactive so that the tissue distribution of label is relatively easy to measure and it is used in tracer amounts that do not alter blood and tissue ammonia levels and ammonia homeostasis (reviewed in [12]). 13 N has been useful for elucidation of ammonia metabolism in normal [13, 14] and hyperammonemic [10] rat brain, rat liver [15], and rat lung [16], as well as in positron-emission tomographic (PET) studies of cerebral ammonia uptake in patients with hepatic encephalopathy (HE) (e.g.…”

Section: Introductionmentioning

confidence: 99%

“…Freed and Gelbard [18] determined the organ distribution of label after a bolus intravenous injection of [ 13 N]ammonia into normal, anesthetized adult male rats, and reported that most of the administered 13 N is cleared from the circulation within seconds, with a high extraction efficiency for most organs in the range of 60–100% in a single pass compared with brain (~40%) and lungs (~25%). Ammonia ingress for many tissues is via transport of NH 4 + , whereas its entry into brain is largely by diffusion of NH 3 (and possibly some NH 4 + ) across the blood-brain barrier (BBB) [2, 10, 12, 14, 19–22]. Recently, the water channel aquaporin 4 (AQP4) that is highly expressed in astrocytic endfeet [23] and is found in many body organs, including kidney, lung, muscle, and testes [24, 25], was identified as an NH 3 -permeable channel [26].…”

Section: Introductionmentioning

confidence: 99%

Organ Distribution of 13N Following Intravenous Injection of [13N]Ammonia into Portacaval-Shunted Rats

et al. 2016

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Ammonia is neurotoxic, and chronic hyperammonemia is thought to be a major contributing factor to hepatic encephalopathy in patients with liver disease. Portacaval shunting of rats is used as an animal model to study the detrimental metabolic effects of elevated ammonia levels on body tissues, particularly brain and testes that are deleteriously targeted by high blood ammonia. In normal adult rats, the initial uptake of label (expressed as relative concentration) in these organs was relatively low following a bolus intravenous injection of [13N]ammonia compared with lungs, kidneys, liver, and some other organs. The objective of the present study was to determine the distribution of label following intravenous administration of [13N]ammonia among 14 organs in portacaval-shunted rats at 12 weeks after shunt construction. At an early time point (12 sec) following administration of [13N]ammonia the relative concentration of label was highest in lung with lower, but still appreciable relative concentrations in kidney and heart. Clearance of 13N from blood and kidney tended to be slower in portacaval-shunted rats versus normal rats during the 2–10 min interval after the injection. At later times post injection, brain and testes tended to have higher-than-normal 13N levels, whereas many other tissues had similar levels in both groups. Thus, reduced removal of ammonia from circulating blood by the liver diverts more ammonia to extrahepatic tissues, including brain and testes, and alters the nitrogen homeostasis in these tissues. These results emphasize the importance of treatment paradigms designed to reduce blood ammonia levels in patients with liver disease.

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Nitrogen‐13 as a Biochemical Tracer

Cited by 12 publications

References 205 publications

The Role of Glutamine Synthetase and Glutamate Dehydrogenase in Cerebral Ammonia Homeostasis

The Role of Glutamine Synthetase and Glutamate Dehydrogenase in Cerebral Ammonia Homeostasis

13N as a tracer for studying glutamate metabolism

Organ Distribution of 13N Following Intravenous Injection of [13N]Ammonia into Portacaval-Shunted Rats

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