Endogenous production of ammonia – charge question 1 – Arthur J

1). Endogenous production of ammonia: There are very many enzymecatalyzed reactions by which ammonia can be generated in vivo. For example, Cooper and Plum (1987) list at least seventeen enzyme-catalyzed reactions that can generate ammonia from amino acids and nucleotides in the brain. Considerable ammonia is generated in the gut from the action of bacteria on nitrogenous substance. In humans, a large portion of this ammonia is derived from the hydrolysis of urea by urease-containing bacteria in the colon (Gibson et al. 1976). Tracer studies suggest that in human volunteers 15-30% of urea synthesized in the kidneys is converted to ammonia by intestinal bacteria (Walser and Bodenlos 1959). An important source of endogenous ammonia is derived from the metabolism of amino acids. A major route for conversion of amino acid nitrogen to ammonia involves coupling of an aminotransferase (transaminase) to the glutamate dehydrogenase reaction. The amino acid is transaminated with α-ketoglutarate to the corresponding α-keto acid and glutamate. The glutamate is then converted back to α-ketoglutarate with the concomitant formation of ammonia in a reaction catalyzed by glutamate dehydrogenase. This ammonia is mainly incorporated into urea in the liver or into glutamine in extrahepatic tissues (see below). Nitrogen transferred from an amino acid to glutamate via a transaminase reaction can be further transferred to aspartate via the aspartate aminotransferase reaction. In the muscle this aspartate nitrogen is a source of ammonia via the purine nucleotide cycle. For a recent discussion of these pathways see Cooper (2012). 2). Endogenous removal of ammonia: The main route for removal of ammonia carried to the liver by the portal vein is incorporation into urea by enzymes of the urea cycle in the periportal hepatocytes. Glutamine synthetase is located in the perivenous hepatocytes downstream in the sinusoid. This enzyme acts as a backup system to remove ammonia that is not removed as urea by the periportal cells (Häussinger 1998). This two system backup arrangement is very effective. For example, Cooper et al. (1987) showed that ~93% of tracer quantities of [13N]ammonia (13N is a positron-emitting isotope with a t1/2 of 9.96 min) injected into the portal vein of anesthetized rats is removed in a single pass through the liver. Of the [13N]ammonia taken up by the liver about 93% is incorporated into urea and about 7% is incorporated into the amide position of glutamine. Despite the fact that the urea cycle consists of five enzyme steps and two mitochondrial transport processes the process is remarkably effective. It was estimated that the t1/2 for conversion of ammonia to urea in the rat liver is about 11 sec (Cooper et al. 1987). Because extrahepatic tissues do not contain a functioning urea cycle ammonia generated by the breakdown of nitrogenous substances in these tissues must be removed by another mechanism. In most tissues this removal is accomplished by incorporation of ammonia into the amide position of glutamine via a reaction catalyzed by glutamine