256 PRINCIPLES OF GENERAL PHYSIOLOGY 



corresponding amino-acid. It is only necessary then to supply this organ with the 

 appropriate hydroxy or ketonic acid in order that an amino-acid may be formed by 

 it. However, we know as yet of no a-hydroxy or a-ketonic acid produced by the 

 organism with the exception of lactic and pyruvic acids, both of which give alanine. 

 Dakin and Dudley (1913, 1) have shown that many amino-acids, in fact all 

 examined by them, namely glycine, alanine, valine, leucine, phenyl-alanine and 

 aspartic acid, undergo spontaneous dissociation at low temperatures into the 

 corresponding a-ketonic aldehyde and ammonia. The reaction is no doubt 

 reversible and probably catalysed by an enzyme, so that the formation of an amino- 

 acid from the hydroxy-acid seems to pass through the intermediate stage of the 

 corresponding ketonic aldehyde. Thus: 



The reader may be reminded that it is the a-amino-acids that are wanted by the 

 organism, that is, those in which the NH., group is next to the carboxyl. 



The experiments of Henriques and Hansen (1904) are of interest in this 

 connection. They found that rats could be maintained in nitrogen equilibrium, 

 that is, without loss of nitrogen, if they were fed on the mono-amino-fraction of the 

 products of digestion of proteins. Since the tissue proteins contain diamino-acids, 

 there must have been synthesis of these, if the results are correct. 



Hindhede (1914) finds that the minimum nitrogen required with a diet of 

 bread is the same as that with potatoes ; it seems that we must conclude that no 

 special kind of amino-acids is necessary, at all events for maintenance. 



On the other hand, we have to remember that it is possible for a diet to 

 contain all that is necessary for repair of adult tissue, although it may not contain 

 some constituent required for new growth. If this constituent cannot be formed 

 in the organism itself, it is clear that the diet will be inadequate for growth. 

 This fact has already been insisted upon. Osborne and Mendel (1912, 2) have 

 maintained adult rats for as long as 530 days on gliadin and protein-free milk. 

 Now gliadin is wanting in glycine and lysine, which are necessary as constituents 

 for the growth of new tissues, and, in fact, experiment showed that the diet in 

 question was actually inadequate for growth. It seems probable that the wear 

 and tear of the cells does not involve disintegration of that particular protein 

 which contains lysine, or that the group containing lysine may be left intact 

 when other parts are split off. 



There is another way in which the want of a particular constituent in diet 

 may be of importance. As we shall see in more detail later, there are numerous 

 substances produced by various organs which act upon other organs, such as 

 adrenaline. These are essential for the normal functions of the organism, so that 

 if they require for their production some particular chemical grouping, which the 

 organism itself is unable to supply, this grouping must be present in the food, 

 otherwise normal life is impossible. 



There is no doubt that Tryplophane is of especial importance. This substance 

 is found in most proteins used for food, and has the constitution : 



C CH 



COOH 

 NH 



that is, skatol-amino-acetic acid. Now the protein of maize, zein, is peculiar in 

 containing no tryptophane. Hopkins and Wilcock (1906) found that mice were 

 unable to live for more than about twenty days on a diet of zein, carbohydrate, 

 and fat. Whereas, if tryptophane were added, the animals not only lived much 

 longer, but were obviously in better condition. 



Further evidence is afforded by the experiments of Henriques (1907) in con- 

 junction with those of Abderhalden and Frank (1910). It has been mentioned 

 above that the products of complete enzymic hydrolysis of proteins are sufficient 

 as protein diet. Further, if the proteins are hydrolysed by the action of acid, 

 provided that the hydrolysis be only carried on for six hours, it was found by 

 Henriques that the products were also able to maintain nitrogen equilibrium. 



