352 ANIMAL METABOLISM 



H,N-CH-COOH -"""0 acid oxidase- ^ H N^C-COOH + 2(H) 



I (54) I 



R R 



Amino acid Corresponding imino acid 



which in the second step is hydrolyzed to the corresponding keto acid 

 and ammonia: 



HN=C-COOH + H2O -^ 0=C-COOH + NH3 



R . R 



A number of enzymes present in animal tissues catalyze the dehydrogena- 

 tion of various amino acids (see Chap. 10). The hydrogen split off in 

 the first step is transferred directly to an acceptor, which differs accord- 

 ing to the enzyme involved, but frequently is oxygen. Assuming that 

 oxygen is the acceptor, the net result of oxidative deamination may be 

 summarized by the following equation: 



HaN-CH-COOH + O2 ±^^ 0==C— COOH + NH3 + H2O2 



R R 



The hydrogen peroxide is decomposed by catalase, or used to oxidize other 

 metabolites. The other products are further metabolized as described 

 below. 



Formation of Urea. Ammonia is a toxic substance that cannot be 

 tolerated by animal tissues in large amounts and therefore must be 

 eliminated as fast as it is formed. For most higher animals it is com- 

 bined with carbon dioxide to form the waste product, urea. A summary 

 equation indicating the net result of this combination is as follows: 



2NH3 + COo -^ H2N - CO - NH2 + HoO 



Urea 



However, such a direct union does not occur in the body. Urea is formed 

 instead by a cyclic process involving several intermediate substances. 

 Almost certainly, the immediate source of the urea is arginine, which is 

 broken down into urea and ornithine by the enzyme, arginase: ' 



NH2 NH2 NH2 



C=NH (CHo)3 + CO 



I +H.0 



(CH=)a COOH 



NH ,,gi„,3e HC-NH2 NH2 



HC— NH2 



I 

 COOH 



L-Arginine L-Ornithine Urea 



