168 INTERMEDIATE METABOLISM 



protein requirement in diet. If the tissues cannot make 

 amino-acids, but can only utilise for tissue-building pur- 

 poses such amino-acids as are presented to them, then the 

 form as well as the quantity of the protein in the food 

 must be taken into account. But if the body can convert 

 the nitrogen compounds presented to it into the amino- 

 acids required for the specific structure of its tissues, then 

 the quantity of protein is the sole consideration. 



There is some indirect evidence that the body has the 

 power of manufacturing amino-acids. 



Synthesis of Alanine. — When the liver is perfused with 

 pyruvic acid alanine is formed — 



Alanine is also formed on perfusion of the hver with 

 ammonia, provided that the Uver is rich in glycogen. 

 These facts point to a synthesis of alanine from ammonia 

 and non-nitrogenous compounds. 



Fornuition of Glycine. — Herbivorous animals daily excrete 

 considerable quantities of hippuric acid. This is formed 

 in the kidney by synthesis of the benzoic acid from the 

 food with glycine. 



CeH^COOH + NH2CH2COOH= C6H5CO.NH.CH2COOH 



Hippuric acid. 



Now the amount of glycine thus used is far greater than 

 the amount which exists in the tissues and food. Glycine 

 is therefore being formed in the body from more complex 

 amino-acids. 



There is also evidence that the body can effect the 

 interconversion of histidine and arginine, and of tyrosine 

 and phenylalanine. 



But the positive evidence for the synthesis of amino- 



