808 PHYSIOLOGY 



metabolism. But it is not the amino-acids themselves that are the stimu- 

 lants. This is shown by the fact that when amino-acids are built up 

 to form new tissues, as in the baby or in the animal recovering from 

 starvation, they exert no specific dynamic action. This action only 

 occurs when the amino-acids undergo deamination, and must therefore be 

 the result of the 'products of this , deamination. Lusk suggests that in 

 the case of glycine and alanine, the stimulating substances may be 

 glycollic and lactic acids, but there is no direct proof of this suggestion. 

 We know in fact very little of the nature of the substances that are left 

 after deamination. Since they contain only the elements carbon, hydro- 

 gen, oxygen, one would expect to find that they could replace either 

 fat or carbohydrate. So far as concerns the production of energy this 

 is true. Moreover, as we shall see in dealing with the metabolism of 

 carbohydrates, we have definite evidence that part of this non-nitrogenous 

 moiety of the protein molecule may be converted into sugar or glycogen. 

 Thus, of the amino-acids formed by the digestion of proteins, glycine, alanine, 

 aspartic acid and glutamic acid can be converted quantitatively under appro- 

 priate circumstances into glucose. On the other hand, leucine, phenylala- 

 nine and tyrosine yield no glucose, even in the diabetic animal, but may 

 in the liver undergo conversion into aceto-acetic acid, which is a stage in 

 the oxidative disintegration of fats. In spite of this latter fact we have 

 no evidence that fat may be formed from this part of the protein molecule ; 

 at any rate, no- fat which can be stored in the body and give rise to the 

 production of adipose tissue. The reason why the CHO remainder of the 

 protein molecule is so prone to oxidation and does not, like an excess of 

 carbohydrates, undergo conversion into fats in the body, we shall have to 

 consider in greater detail in dealing with the fate of this latter class of sub- 

 stances. We need however considerably more evidence as to the extent 

 to which deamination occurs and as to its conditions and end-products 

 before we can hope to determine the cause for the rapid breakdown of these 

 end-products in the body. 



The Synthesis of Amino-Acids 



Many though not all of the processes in the .body are reversible. If 

 the body can effect deamination of an amino-acid, there seems no reason 

 why it should not carry out the reverse change and synthesise an amino-acid 

 from its corresponding fatty or oxy-acid and ammonia. Knoop has shown 

 that, given a right molecular grouping, the fatty acid residue may in the 

 body react with ammonia to form an amino-acid. The proof of this fact 

 was facilitated by the discovery that the next higher homologue of phenyl- 

 alanine, namely, phenyl-a-amino-butyric acid, when administered to an 

 animal, was excreted in large quantities in the urine as an ether-soluble 

 acetyl derivative, which was easily isolated in a state of purity. If then this 

 amino-acid were formed in the body, one might expect to find it without 

 difficulty in the urine. Knoop found that the administration of either 

 phenyl-a-keto-butyric acid or phenyl-a-oxybutyric acid led to the excretion 



