Nitrogen-13 as a biochemical tracer

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

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 s) 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.

The role of glutamine synthetase and glutamate dehydrogenase in cerebral ammonia homeostasis

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 [(13)N]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 [(13)N]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 + H(2)O → Fumarate + GDP + P(i) + NH(3)) 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.

13N as a tracer for studying glutamate metabolism

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(1/2) 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(1/2) for conversion of portal vein ammonia to urea in the rat liver is ∼10-11s, 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 ≤7-8s, (c) the t(1/2) for incorporation of blood-borne ammonia into glutamine in the normal rat brain is <3s, 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.

Estimation of cytoplasmic nitrate and its electrochemical potential in barley roots using 13NO3 and compartmental analysis

13NO3 was used to determine the intracellular compartmentation of NO3 in barley roots (Hordeum vulgare cv. Klondike), followed by a thermodynamic analysis of nitrate transport. Plants were grown in one-tenth Johnson's medium with 1 mol m3 NO3 (NO3-grown plants) or 1 mol m3 NH4NO3 (NH4NO3-grown plants). The cytoplasmic concentrations of NO3 in roots were only approx. 3-6 mol m3 (half-time for exchange approx. 21 s) in both NO3 and NH4NO3 plants. These pool sizes are consistent with published nitrate microelectrode data, but not with previous compartmental analyses. The electrochemical potential gradient for nitrate across the plasmalemma was +26 +/-1 kJ mol1 in both NO3- and NH4NO3-grown plants, indicating active uptake of nitrate. At an external pH of 6, the plasmalemma electrochemical potential for protons would be approx. -29 +/- 4 kJ mol1. If the cytoplasmic pH was 7.3 +/- 0.2, then 2H+/1NO3 cotransport, or a primary ATP-driven pump (2NO3/1ATP), are both thermodynamically possible. NO3 is also actively transported across the tonoplast (approx. +6 to 7 kJ mol1).