348 



ANIMAL METABOLISM 



It has been established that nicotinic acid can be formed from trypto- 

 phan in the actual tissues of the animal body, although the same con- 

 version can also be brought about by intestinal microorganisms. 



H2N— CH— COOH 

 I 



CH2 

 I 



I 

 CONH2 



L-Glutamine 



Glutamic acid is extensively converted into glutamine in animal tissues. 

 In fact, a large part of the glutamic acid in the body, both free and 

 combined, probably exists in the form of glutamine. 



Creatine and creatinine are produced from three amino acids, glycine, 

 arginine, and methionine. The guanidine group of arginine first com- 

 bines with glycine to form guanidoacetic acid, which then is methylated 

 by methionine : 



H^NCH^COOH + I 



NH2 



I 

 C=NH 



I 

 NH 



I 

 (CH,)3 



I 

 HCNH2 



Glycine 



COOH 



L-Arginine 



(51) 



NH2 



I 

 C=NH 



NH 



I 

 CH., 



I 

 COOH 



+ 



NH, 



I 

 (CH,)3 



HCNH2 



I 

 COOH 



Guanidoacetic acid Ornithine 



NH, 



CH3 



NH, 



H 



The formation of ornithine in the first reaction has not been estab- 

 lished, but would certainly be expected. Choline can also furnish the 

 methyl group for the second reaction, but only indirectly by first trans- 

 ferring it to methionine. The creatine so formed is converted into the 



