92 THE PHYSIOLOGY OF PROTEIN METABOLISM 



Pfliiger looked on protein as the food preferred by the organism to all 

 other foods and the one which it would assimilate rapidly. 



It seems improbable that all the nitrogen taken in is required or 

 even utilized for protein synthesis. It is extremely difficult to obtain 

 any direct evidence decisively in favour of one or the other of these 

 very divergent theories. It is indeed highly probable that the differ- 

 ences which exist between the tissue and food protein are in degree 

 and not in kind. 



Rubner. 



Rubner (343) advanced a theory of protein metabolism, in 1908, 

 in which he maintained that the study of metabolism could not be 

 divorced from the study of heat production, therefore that metabolism 

 must be considered in association with the energy exchange. He 

 referred all the metabolic changes of protein to the production of energy. 

 He believed in a "store" protein resembling Voit's "circulating" pro- 

 tein, and in a " wear and tear quota " necessary for the repair of tissue 

 waste. The greater part of the protein after absorption was rapidly 

 disintegrated into a nitrogen-free and a nitrogen-containing part. The 

 fate of the nitrogen-containing moiety, as it played but little part in 

 the energy exchange, was disregarded. The nitrogen-free part formed 

 the dynamic quota of the protein ingested. In this splitting of the 

 protein a certain liberation of energy of heat occurred which was of 

 no value as a source of energy for the cells and was therefore lost. 

 This liberation of energy was termed the specific dynamic action of 

 the protein. 



A highly speculative hypothesis explained how the various changes 

 took place. All protoplasm was not regarded as being of the same 

 type, one kind might be thermolabile, another thermostable, but all 

 varieties had in common a certain molecular grouping which acted 

 as a kind of nucleus to which other protein groups (for example 

 those which were thermostable or thermolabile) could attach themselves. 

 The mechanism of the energy exchange, which is characteristic of 

 activity, was effected by a distinct vibratory movement of the whole or 

 a definite part of the protoplasm. Owing to this specific oscillation, 

 the protoplasm had the power of bringing about the breakdown of con- 

 tiguous food stuffs. The " affinities " (specific oscillations) must be of 

 a specific nature for each tissue and were probably somewhat akin to 

 ferment action. Thus, in diabetes, the " affinities," which brought about 



