The scope of the transamination reaction in animal tissues.

Publisher Summary This chapter discusses a new technique of differential centrifugation in the study of cellular organization and the theoretical basis of the technique as well as the various factors of a practical nature. The limitations of differential centrifugation become particularly severe when the technique is applied to the study of tissue enzymes. The observed partitions provide only the roughest sort of information concerning the true intracellular distribution of enzymes. They can only be considered as clues, which have to be followed by many additional experiments to arrive at their real significance. A priori assumption that specific enzymes are entirely concentrated in a given cellular site has proved extremely profitable in guiding these additional experiments, even in those cases in which it is found not to hold true. The use of enzyme determinations to ascertain the composition of isolated fractions is also of great interest and deserves more frequent application. The main object of cellular physiology is to ascertain which of the many processes discovered by biochemists actually do take place within the cell.

The Use of Differential Centrifugation in the Study of Tissue Enzymes

Distribution of enzymes between subcellular fractions in animal tissues.

Glutamate is present in the brain at an average concentration—typically 10–12 mM—far in excess of those of other amino acids. In glutamate-containing vesicles in the brain, the concentration of glutamate may even exceed 100 mM. Yet because glutamate is a major excitatory neurotransmitter, the concentration of this amino acid in the cerebral extracellular fluid must be kept low—typically µM. The remarkable gradient of glutamate in the different cerebral compartments: vesicles > cytosol/mitochondria > extracellular fluid attests to the extraordinary effectiveness of glutamate transporters and the strict control of enzymes of glutamate catabolism and synthesis in well-defined cellular and subcellular compartments in the brain. A major route for glutamate and ammonia removal is via the glutamine synthetase (glutamate ammonia ligase) reaction. Glutamate is also removed by conversion to the inhibitory neurotransmitter γ-aminobutyrate (GABA) via the action of glutamate decarboxylase. On the other hand, cerebral glutamate levels are maintained by the action of glutaminase and by various α-ketoglutarate-linked aminotransferases (especially aspartate aminotransferase and the mitochondrial and cytosolic forms of the branched-chain aminotransferases). Although the glutamate dehydrogenase reaction is freely reversible, owing to rapid removal of ammonia as glutamine amide, the direction of the glutamate dehydrogenase reaction in the brain in vivo is mainly toward glutamate catabolism rather than toward the net synthesis of glutamate, even under hyperammonemia conditions. During hyperammonemia, there is a large increase in cerebral glutamine content, but only small changes in the levels of glutamate and α-ketoglutarate. Thus, the channeling of glutamate toward glutamine during hyperammonemia results in the net synthesis of 5-carbon units. This increase in 5-carbon units is accomplished in part by the ammonia-induced stimulation of the anaplerotic enzyme pyruvate carboxylase. Here, we suggest that glutamate may constitute a buffer or bulwark against changes in cerebral amine and ammonia nitrogen. Although the glutamate transporters are briefly discussed, the major emphasis of the present review is on the enzymology contributing to the maintenance of glutamate levels under normal and hyperammonemic conditions. Emphasis will also be placed on the central role of glutamate in the glutamine-glutamate and glutamine-GABA neurotransmitter cycles between neurons and astrocytes. Finally, we provide a brief and selective discussion of neuropathology associated with altered cerebral glutamate levels.

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Central Role of Glutamate Metabolism in the Maintenance of Nitrogen Homeostasis in Normal and Hyperammonemic Brain

http://www.jbc.org/content/189/2/577.full.pdf

Enzymatic decarboxylation of aspartic acid to α-alanine.

1. Homogenat oder Suspensionen von Mitochondrien aus Rattenleber bilden aus Glutaminsaure Asparaginsaure. Die Asparaginsaure wurde durch Papierchromatographie nachgewiesen. Malonat hemmt die Reaktion.

Die Umwandlung der Glutaminsäure in Asparaginsäure in den Mitochondrien der Leber. (mit Bemerkung über das Vorkommen einer Transaminase in Clostridium Welchii)

F. Leuthardt

Papers

Die Umwandlung der Glutaminsäure in Asparaginsäure in den Mitochondrien der Leber. (mit Bemerkung über das Vorkommen einer Transaminase in Clostridium Welchii)