I 



PROTEIN METABOLISM 807 



for this view, but is rather a sign that the body, after satisfying its modest 

 needs for the repair of its tissue waste, has no need for the rest of the nitro- 

 genous content in its food, and that this must be got rid of before the really 

 valuable part, the energy-giving part of the protein molecule, is admitted 

 into the metabolic cycle of the cells. 



The important problem in the energy metabolism of protein is thus not 

 the origin of the urea, but the fate and nature of the substances that are 

 left after deamination. We have seen that the protein when taken as a 

 food, more than either of the other two foodstuffs, causes a direct augmenta- 

 tion of the respiratory exchanges of the body. Thus in one experiment by 

 Rubner, an animal previously starved received on one day 574 Calories 

 protein, on another day 54*2 Calories fat, and on the third day 57 Calories 

 carbohydrates per kilo, body weight. During hunger the total metabolism 

 per kilo, body weight amounted to 37-5 Calories ; with meat, to 46 Calories ; 

 with fat, to 394 Calories ; with carbohydrates, to 394 Calories. Compared 

 with the metabolism during starvation the rise per cent, with protein was 

 24-3, and with fat and carbohydrates 5-L This surplus output of energy 

 resulting from the administration of protein cannot be ascribed to increased 

 work thrown on the digestive organs. There is no evidence that this is 

 greater in the case of proteins than it would be with carbohydrates or fats ; 

 and even if the capacity of these organs be strained to their utmost by 

 administration of large quantities of bones, the increase in the C0 2 output 

 which results is not so great as that following a large protein meal. It 

 might be concluded that the CHO moiety of the protein undergoes oxidation 

 more rapidly than either glucose or the ordinary fats of the diet, and that 

 its metabolism is dependent rather on the quantity presented to the organism 

 than on the actual needs of the cells of the body. The work of Lusk points 

 however to the earlier view of Voit as being correct, according to which 

 protein food acts as a stimulant to all the metabolic processes of the body. 

 Lusk has shown that this specific dynamic action of protein is possessed 

 also by certain of the amino-acids resulting from its decomposition, but not 

 by all. Thus while glycine and alanihe exert a well marked specific dynamic 

 action, glutamic acid, leucine and tyrosine exert little or no effect upon 

 heat production. The question then arises whether this increased heat 

 production resulting from the ingestion of glycine is due to the rapid dis- 

 integration and oxidation of the glycine molecule itself, or is due to a direct 

 stimulating action upon the body cells. The question was decided by giving 

 glycine to an animal which had been rendered diabetic by the injection of 

 the phlorhizine. Under these circumstances glycine is converted quanti- 

 tatively into glucose, which is excreted in the urine, so that the CHO moiety 

 of the glycine molecule undergoes no oxidation in the body. Notwith- 

 standing this fact glycine produces the same augmentation of metabolism in 

 the phlorhizinised animal as it would in a normal animal. The same results 

 were obtained with alanine, so that it must be concluded that the specific 

 dynamic action of protein is due to the quality possessed by certain of the 

 amino-acids of stimulating the cells of the body and raising their rate of 



