120 THE PHYSIOLOGY OF PROTEIN METABOLISM 



perfused the glycogen-free liver with blood to which had been added 

 different keto- and oxy-acids and found that the corresponding amino- 

 acid was formed. In this way they demonstrated the formation of 

 tyrosine, alanine, phenylalanine and probably leucine. The first two 

 amino acids were present in their natural optically active form. They 

 also showed with a glycogen-rich liver that, if a small amount of 

 ammonium chloride be added to the perfusing fluid, there was a com- 

 paratively free synthesis of alanine. Alanine was also formed in 

 small amount even in the glycogen-poor liver, if lactic acid were 

 added to the perfusing fluid. 



It may be taken as practically proved that carbohydrate in some 

 form, or other, is absolutely essential for the synthesis of protein within 

 the tissues. 



Carbohydrates and Fats as Sparers of Protein. 



Throughout the previous pages the question of the relation of the 

 non-nitrogenous portion of the ordinary diet to the requirement and the 

 fate of protein has been dealt with indirectly (see pp. 35, 97, 113, 119). 

 It is apparent that, when fat and carbohydrate are fed at the same 

 time as protein, there is a smaller demand for that substance ; in other 

 words the size of the protein minimum is more, or less, dependent on 

 the amount of the non-protein food consumed. The same sparing 

 power, particularly of carbohydrate, is also found in early days of 

 starvation, as the many experiments of Prausnitz (334) and of Benedict 

 (58) have shown. The effect on the catabolism of protein of the 

 intake of carbohydrate in contradistinction to the fat in the food is 

 very marked as is evidenced by numerous experiments, such as 

 those of Siven (374, 375), Landergren (237), Kayser (217), Tallquist 

 (391), Folin (i 29), Cathcart (90). This influence of the intake of non- 

 nitrogenous food is very definitely shown when it is given at the con- 

 clusion of a fast. Thus, in the case of the professional faster Beaut, 

 at the conclusion of the fasting period of fourteen days the subject 

 was given a diet consisting of cream and starch with the result that 

 within three days the output of nitrogen had fallen to a third of the 

 output on the last day of the starvation period. Rubner (V) also 

 observed the same effect in the case of dogs. 



As Landergen (237), whose results were confirmed by Cathcart 

 (90), has pointed out, there is a very great difference in the power of 



