PROTEIN REGENERATION 25 



in the molecule and which are not required for the building up of 

 the tissue protein, must be eliminated. The view that the tissue 

 proteins differ from one another, that they are specific bodies of de- 

 finite constitution, and that therefore each requires a different amount 

 and supply of building material is gradually being accepted. Abder- 

 halden himself accepts this. What end then is served in having a 

 single uniform pabulum formed when the demand is so varied ? This 

 is all the more questionable when it is remembered that there is 

 no indubitable evidence which shows that one amino acid can be 

 converted into another. Further, the belief is gradually gaining 

 ground, as regards the protein requirements of the organism, that 

 it is not so much the actual quantity as the quality of the protein 

 supplied in the food, which is of importance. If the material supplied 

 be uniform it necessitates a fresh breakdown by each tissue, perhaps by 

 each individual cell. Although the tissues all probably possess this 

 power of breaking down protein material by means of their intracellular 

 proteolytic enzymes, still the extra work involved seems to negative the 

 immediate resynthesis hypothesis, especially when the hypothesis of the 

 circulating digestion product postulates the presence of the individual 

 food material in the blood. As already remarked, the mere failure to 

 detect these products in the blood does not give adequate reason for 

 concluding that they are not present. The tissues certainly do not 

 break down in regular sequence, nor are they left to fall to pieces for 

 lack of repair material. Repair is among the most active functions 

 of all tissues. Must, then, a tissue of highly complex structure keep 

 destroying and digesting plasma, picking out from the debris the nuclei 

 which it requires and letting the rest go ? (Why, and this destruction 

 is admitted by Abderhalden, are the superfluous amino acids not found 

 in the blood ?) What happens, for instance, in the case of the connec- 

 tive tissues with their demand for, say, glycine, where the food supply 

 is not over-abundant as the circulation is poor, and the tissue not very 

 suited for lymph perfusion? It will not do merely to say that there 

 is no great breakdown of material here. If the demand for food exist 

 how is it satisfied? Pfluger (331), in an interesting paper in which 

 he combated this immediate resynthesis hypothesis, ascribed to the 

 cells of the intestinal wall, with regard to the protein synthesis, the 

 same capacity as the cells of all tissues, but denied that the syn- 

 thesis of protein for the whole organism was carried out there. He 

 held that such a hypothesis was contrary to all existent knowledge of 

 physiological assimilation. 



If immediate resynthesis take place, to what extent does it occur? 



