AMINO-ACIDS, PROTEINS, UREA AND URIC ACID 87 



Tyrosine, a-amino-/3-hydroxy-phenyl-propionic acid : 



CH2-CH(NH2)-COOH 



I 

 C6H4(OH) 



Tryptophane, a-amino-j8-indol-propionic acid: 



CH2 - CH(NH2) - COOH 

 I 



(CsHeN) (Indol group) 



Arginine, a-amino-5-guanidine- valeric acid: 



CH2 - CH2 - CH2 - CH(NH2) - COOH 



/NH 

 HNC< 



^NH2 



(Guanidine residue) 



PROTEINS 



The three essential organic or carbon-containing constituents 

 of animal foods are fats, e.g. lard, tallow, olive oil ; proteins, e.g. 

 egg albumin, cheese casein and wheat gluten ; and carbohydrates, 

 e.g. sugar, starch and cellulose. Fats we have already dis- 

 cussed in a preceding chapter. The consideration of proteins 

 should come in connection with the amino-acids which we have 

 just studied. The carbohydrates will be taken up a little later. 



The three food constituents above mentioned are alike in 

 that they all contain the three elements, carbon, hydrogen and 

 oxygen. When we come to study the manner in which these 

 foods are used in the animal body, we shall find that this use 

 rests fundamentally upon the property of these compounds to 

 be oxidized, and by means of this oxidation to furnish the energy 

 of the animal body. The oxidation of these substances depends 

 upon the carbon and hydrogen contained in them. As these 

 food constituents are practically the only compounds which 

 the animal body thus uses as a source of energy, they are of the 

 utmost importance, and their presence determines the value 



