ANIMAL METABOLISM 



347 



Utilization of amino acids 



The main use for amino acids in the animal body is the synthesis of 

 tissue proteins. Such synthesis is not only necessary for young, growing 

 animals, but it is also essential for full-grown adults, because tissue 

 proteins are continually being broken down and resynthesized. Borsook 

 gives 10 days as the half-life (period in which one-half of a substance 

 is decomposed) of the proteins in the internal organs of man and 158 

 days for those in other tissues (mainly the muscles). 



The amount of dietary protein needed to supply the normal require- 

 ments of human beings depends on age (stage of growth) and on the 

 amino acid composition of the proteins consumed. Assuming good 

 quality food proteins, satisfactory allowances per kilogram of body 

 weight are: men, 1 g.; women, 1-1.8 g.; children, 1.5-3 g. ; infants, 3.5 g. 



Conversion of amino acids into other metabolites 



In addition to protein synthesis, amino acids are used as raw materials 

 for the synthesis of a series of essential substances by animal tissues. 

 The formation of nicotinic acid from tryptophan {p. 237) probably follows 

 the pathway* indicated below, although some details remain unproved: 



I 



H 



NH2 

 I 

 C— CH.— C— COOH 



II " H 



H H 



L- Tryptophan 



HC 



^^ 



H 



HC 



^ 



H 



NH2 



I 

 ^C-COCH^-C— COOH 

 H 



.C— NH, 



Kvnurenine 



H NH2 



/C. I 



HC^ C— CO-CH2-C-COOH 



H 



HC 



^ /C-NH2 

 C 

 I 

 OH 



3-Hydroxy kvnurenine 



HC 



^ 



H 

 C. 



HC 



*^ 



C— COOH 



II 



€— NH2 



I 

 OH 



3-Hydroxy anthranilic 

 acid 



HC^ 

 I 



H 



C. 



HC 



^ 



N' 



C— COOH 



II 



C— COOH 



Quinolinic acid 



HC^ 

 I 



H 

 C. 



HC 



^ 



C— COOH 



II 

 CH 



N 

 Nicotinic acid 



