METABOLISM 31 



what the kind of protein on which he is fed may be, but 

 that certain varieties, yielding particular forms of amido- 

 acids, may be peculiarly suitable or specially harmful 

 in particular pathological states. 



Having once traversed the intestinal wall, it would 

 appear that protein is subjected to different treatment 

 according to the particular use which it is to serve. 

 That portion which is destined to serve as a source of 

 energy (or ' energy-protein,' as we may call it) appears 

 rapidly to undergo a process of ' denitrification ' (possibly 

 in the liver), by which the nitrogen-containing part 

 of the molecule is split off, leaving the carbonaceous 

 moiety, which may contain 90 per cent, of the energy of 

 the original molecule, to be utilized, like fat and carbo- 

 hydrate, as a source of work and heat. The remainder, or 

 ' repair-protein/ is conveyed to the tissues, and there enters 

 into the actual living substance of the cells, ultimately 

 replacing the molecules which are worn out in the vital 

 processes. The nitrogen-containing part of the 'energy- 

 protein ' is speedily broken down by oxidation, and 

 eliminated chiefly in the form of urea and inorganic 

 sulphates; the repair-protein, on the other hand, is 

 broken down slowly, not by oxidation, but by a process 

 which seems more to resemble ferment action, and 

 eliminated largely in the form of kreatinin, uric acid, and 

 neutral sulphur compounds. 



There are thus two main lines along which protein 

 metabolism proceeds, each with its own objects and 

 resulting in the formation of its own end-products. 



The proportion of the total intake of protein which is 

 destined to follow each of these two possible lines depends 



