THEORIES OF PROTEIN METABOLISM 91 



Pfliiger. 



Pfltiger in 1893 subjected the view advanced by Voit to a very 

 severe criticism. He thought that the food protein must first become 

 an integral part of the living protoplasm before it could be utilized ; 

 In other words, the absorbed food protein, unlike the living tissue 

 protein, was not readily catabolized. He believed that a fundamental 

 difference in chemical constitution existed between the two forms of 

 protein and that the greater lability of the living tissue protein was 

 in all probability due to the presence of cyanogen radicles in it. 

 Verworn was inclined to agree with these views, but Vernon (406 B) 

 could obtain no experimental evidence which supported them. 



Pfliiger based his views largely on some experimental work carried 

 out in his laboratory by Schondorff. Blood was taken from a starving 

 dog and was circulated through the hind limbs and liver of a well-fed 

 dog ; the urea content of this blood was increased at the end of the 

 experiment. Blood was taken either from starved or well-fed dogs 

 and circulated in the same fashion through the hind limbs and liver of 

 a starving dog ; no increase in the urea content was detected. Folin 

 (131) severely and justly criticized these experiments, pointing out 

 that the evidence furnished by them was by no means unassailable. 

 He worked out the details of one of Schondorff's experiments, of 

 which full protocols were given, and showed that the actual amount of 

 catabolism during the four and a half hours of the experiment cor- 

 responded to 25 mg. of urea nitrogen only, instead of to the 125 

 per cent, increase calculated by Schondorff. During the four days 

 preceding the experiment the dog had catabolized about 35 grms. of 

 nitrogen per day ; this 25 mg. gain therefore amounts to less than 

 one-tenth of one per cent. As Folin says " Considering the numerous 

 sources of error and uncertainty necessarily attached to an experi- 

 ment of this kind it seems very strange that the extraction of 25 mg. 

 of urea nitrogen from the hind legs of a dog killed while engaged 

 in digesting 700 grms. of meat should be accepted as proving not only 

 that protein catabolism did occur during the experiment, but also 

 that it occurred in the bioplasm and not in the circulating protein ". 



Pfliiger's theory does not satisfactorily explain the fact that very 

 soon after the ingestion of protein there is a rapid rise in the output 

 of nitrogen in the urine. One would have to assume an extraordinarily 

 rapid synthetic process followed at once by an equally rapid catabolism. 



