ANIMAL METABOLISM 



351 



(i)N=C(6) 



I 1(5) 



(2)C C— N 

 '')\ 



(*)C 



(9) / 



(3)N— C — N 

 (^) 



Purine skeleton 



and nitrogen 7 from glycine; carbons 2 and 8 from acetic acid; carbon 

 6 from carbon dioxide; and nitrogens 1, 3, and 9 from ammonia. In the 

 porphyrin ring system, 



A 



C(4) 



>— N 



/ (2) (1) 



(3)\ /^a\ /(4)^ 

 (2)^ ^ 



C(3) 



D 



(5), 



c. 



0) (5r% 



(2) ^ 



Porphyrin ring system^ 



carbons A-2, B-2, C-2, D-2, a, 13, y, and 8 come from CHo of glycine, 

 while A-4:, 5-4, C-4, and D-4 are derived from the CH3 of acetic acid. 

 Also the four nitrogen atoms come from the NH2 group of glycine. 

 The mechanism by which these components are put together to form 

 purine and porphyrin compounds in the animal body is still un- 

 known. 



Breakdown of amino acids in the animal body 



Deamination. In the average American diet more protein is con- 

 sumed than is needed for synthesis of tissue proteins and the other essen- 

 tial substances derived from amino acids. Consequently, the body re- 

 ceives an excess of amino acids which must be disposed of in some manner. 

 Direct excretion would be wasteful and, in fact, occurs to only a small 

 extent (in the urine). Most of the excess amino acids seem to be broken 

 down by the process of oxidative deamination. Two steps are involved ; 

 first, dehydrogenation of the amino acid forms an imino acid: 



^ The four individual pyrrole rings are sometimes designated as I, II, III, and IV, 

 respectively, rather than by A, B, C, and D (p. 137). 



