354 ANIMAL METABOLISM 



needed for reaction 57 does not react directly with ornithine, but first 

 combines with ghitamic acid to form an intermediate substance, which 

 then transfers its carbon dioxide to ornithine. Glutamine probably is 

 not involved in urea formation ; but it does serve as the source of urinary 

 ammonia (i.e., ammonium salts) in mammals. 



Metabolism of deaminated amino acid residues 



The products resulting from deamination of amino acids (usually a-keto 

 acids) are in most cases utilized as a source of energy by the animal 

 body, that is, oxidized to carbon dioxide and water. The metabolic 

 pathways by which this oxidation occurs are not fully known, but are 

 certainly different for each amino acid. 



The keto acids from the deamination of alanine, aspartic acid, and 

 glutamic acid are normal intermediates in carbohydrate metabohsm and 

 can be either broken down to carbon dioxide and water or built up into 

 glycogen or glucose by the reactions already considered (Figs. 13-1, 

 13-3, and 13-^). In diabetic dogs several other amino acids also are 

 convertible, wholly or in part, into glucose. Included in this group are 

 glycine, serine, arginine, and proline. 



Three of the amino acids, phenylalanine, tyrosine, and leucine, produce 

 acetoacetic acid when fed to diabetic dogs. This substance, it will be 

 recalled, is one of the compounds resulting from oxidation of fatty acids 

 in the animal body. These amino acids therefore appear to be oxidized, 

 after being deaminated, in the manner of fatty acids. Amino acids which 

 give rise to acetoacetic acid in the body are said to be ketogenic (form 

 ketone bodies), whereas those convertible into carbohydrate are anti- 

 ketogenic. 



In some cases ring structures of the more complex amino acids are 

 nonutilizable. Tryptophan, for example, is partly degraded into indole 

 and skatole, which are excreted in the feces. 



H H 



C C 



HC^ ^C CH HC^ ""C C-CHa 



HC^ /C. XH HC^ ^C^ ^CH 

 ^C N'^ C N 



H H H H 



Indole Skatole 



REVIEW QUESTIONS ON ANIMAL METABOLISM 



L Define metabolism, catabolism, anabolism. 



2. What is the main purpose of carbohydrate metabolism in the animal body? In 

 which respects does this process resemble actual burning of carbohydrate in a flame? 



