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 ¹³N (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 ∼10-11 s, 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 s, (c) the t_1/2 for incorporation of blood-borne ammonia into glutamine in the normal rat brain is <3 s, 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.