THEORIES OF PROTEIN METABOLISM 95 



which will provide in exact amount the different materials requisite for 

 protein tissue repair. 



Folin's view as regards the two forms of metabolism has been 

 generally accepted, although exception has been taken to some minor 

 points. He is perhaps inclined to separate his endogenous and exo- 

 genous metabolism too completely ; other workers have shown that 

 although his products of endogenous metabolism are characteristic of 

 this form, they are also found in the exogenous form. Noel Paton 

 (319) thought that some at least of Folin's results might be ex- 

 plained by variation in the activity of hepatic metabolism. Thus, on 

 a protein-poor diet the hepatic metabolism would be sluggish and must 

 therefore fail to convert a large amount of the waste nitrogen into 

 urea, while on a protein-rich diet with hepatic stimulation the con- 

 version must be much more complete. He also thought that urea 

 must be considered a definite end product of both endogenous and 

 exogenous metabolism. Folin himself, however, is prepared to allow 

 this, as he says that " the fact that the urea and inorganic sulphates 

 represent chiefly the variable catabolism does of course not preclude 

 the possibility that they also represent to some extent the constant 

 catabolism ". 



The statement that the output of creatinine is quite unaltered by 

 alterations in the amount of the intake of nitrogen may be called in 

 question. Both Noel Paton and I have found, particularly in the dog, 

 that the output of creatinine is subject to variations, which, however, 

 are usually small in amount, by altering the amount of protein ingested 

 even when this protein is creatine and creatinine free. 



