GENERAL METHODS - HISTORY OF PROTEIN FOOD. 815 



from putrefactive processes, etc. It may be said, however, that 

 although these experimenters have shown that normal conditions 

 may be maintained for six months to a year or longer upon a low 

 protein diet, they do not demonstrate satisfactorily that a larger 

 protein diet is actually attended by evil consequences. 



The Intermediate Stages in Protein Metabolism. The urea 

 found in the urine and in lesser amounts in the sweat and other 

 secretions may arise in two general ways: 1. As an end-product 

 of the digestive hydrolysis of the protein food. That is to say, 

 the protein material is split by the successive actions of the pepsin, 

 trypsin, and erepsin into products similar to those obtained by 

 the action of acids at high temperatures (acid hydrolysis). As a 

 result of this process the nitrogen appears in the form of ammonia, 

 monamino-acids, and the so-called diamino-bodies, such as arginin, 

 and we may suppose that in these forms it is carried to the liver. 

 In this organ the ammonia, as ammonia salts, is transformed into 

 urea. The monamino-acids, some of them, at least, are deamidized, 

 that is, their NH 2 group is split off as ammonia which then is like- 

 wise converted to urea. The organic acid radicles left after removal 

 of the NH 2 group may subsequently be oxidized through various 

 stages to carbon dioxid and water, or they may be synthesized to 

 form a carbohydrate body or possibly a fat, and thus be kept 

 temporarily as storage, although their eventual fate is to suffer 

 oxidation to carbon dioxid and water. Regarding the diamino- 

 compounds like arginin it is known that when this substance is 

 injected subcutaneously its nitrogen is excreted for the most part 

 if not entirely as urea. Since Kossel and Dakin have shown that 

 the liver especially contains a hydrolytic enzyme, arginase, which 

 is capable of splitting off the guanidin residue of arginin to form 

 urea, we may assume that the arginin formed during protein diges- 

 tion actually undergoes this fate. The process is represented by 

 the following equation : 



NHC <NHCHCHNHCOOH + H * " CO < 2 + NH 2 (CH,),CHNH 2 COOH 



NH(CH 2 ) 3 CHNH 2 COOH 2 



Arginin or guanidin diamino-valerianic acid. Urea. Ornithin or diamino-valerianic 



acid. 



The ornithin, in turn, may suffer deamidization, its (NH,) groups 

 being converted to ammonia and urea. 



By these possible processes it is evident that much of the nitrogen 

 of the food may be excreted promptly as urea without entering at 

 any time into the formation of living protoplasmic tissue. It seems 

 clear also that so far as the protein undergoes these intermediary 

 changes its nitrogen constituent is without value to the body. 

 The body gets rid of the nitrogen and utilizes the balance of the 

 protein molecule as a source of heat energy or as material for the 



