Amino Acids Linked with the Citric Acid Cycle

AMINO ACIDS LINKED WITH THE CITRIC ACID CYCLE 

The catabolic pathways of the carbon chains of the amino acids, alanine, glutamic, and aspartic acids, appear to be readily apparent once these amino acids lose their amino groups. When this occurs, alanine is converted to pyruvic acid, glutamic acid to a-ketoglutaric acid, and aspartic acid to either oxalacetic or fumaric acid. All of the above acids are integral members of the citric acid cycle, and the subsequent degradation of each one has been adequately explained in terms of the operation of the citric acid cycle (see the chapter. The Tricarboxylic Acid Cycle). 

Glutamic acid is oxidized to completion by the kidney and liver cyclophorase suspensions of Green and co-workers. The oxidation proceeds by way of a-ketoglutarate, and the cyclophorase complex contains the enzymes necessary for the aerobic and anaerobic synthesis of glutamate from a-ketoglutarate and ammonia. The enzyme preparation is specific for L-glutamate D-glutamate appears to exert an inhibitory effect on the oxidation of the L-isomer. Phosphate esterification accompanies the oxidation. 

The oxidation of n-glutamate by the cyclophorase system requires the presence of AMP, Mg++, and Pi. Only with a well-aged cyclophorase preparation is there a stimulation of glutamate oxidation by added DPN. The authors state that the n- lutamic acid oxidase of Euler et al. and Dewan is involved, but that there is a difference in the state of the oxidase in the two systems. In the cyclophorase preparation the enzyme is fully conjugated, whereas in the isolated enzyme the protein is dissociated from the pyridine nucleotide. 

Amino Acids Linked with the Citric Acid Cycle 

L-Glutamic acid is oxidized by a specific enzyme which has been found to be widely distributed in bacteria, yeast, plants, and mammalian tissues. This enzjrme is unique among the enzymes acting on amino acids in that dipho hop3rridine nucleotide (DPN) serves as its coenz3rme. The equation for the reaction is  

L-Glutamic acid dehydrogenase was discovered independently by von Euler et al. (4) and Dewan (5). Crystallization of this enzyme was achieved by Olson and Anfinsen (d) from beef liver and by Snoke (7) from chicken liver. 

The procedure of Snoke is exceedingly simple. An acetone powder of the liver is extracted with phosphate buffer, and inert protein precipitated by a mixture of ribonucleic acid (RNA) and 20% by volume of ethanol. Upon adjusting the supernatant fluid to pH 6.25 at —5° the enzyme comes down in the precipitate formed. This is dissolved in water and 0.6 volume of 

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II. AMINO ACIDS LINKED WITH THE CITRIC ACID CYCLE... 

II. Amino Acids Linked with the Citric Acid Cycle... 

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