HISTORY OF DIFFERENT FOODSTUFFS IN BODY 1049 



The proportion of each must, of course, differ with the condition of the 

 body. If the latter is in nitrogen-equilibrium, a more considerable 

 proportion of it will be oxidized directly to yield energy than when 

 the body is in need of this substance to make good a previous loss. 

 Quite similarly, the greater the amount of protein taken in under nor- 

 mal conditions, the greater must be that amount of it which is directly 

 converted into energy. 



This division of the absorbed proteins into tissue and circulating 

 proteins shows that their catabolism is not uniform, but consists es- 

 sentially of two separate processes (Liebig). Obviously, the tissue 

 catabolism must remain rather constant under normal conditions, 

 while the catabolism of the circulating proteins must differ more di- 

 rectly with the amount of proteins ingested. Consequently, any 

 attempt made to determine the metabolism of the proteins requires 

 the reduction of the circulating proteins to a minimum. Only when 

 this end has been accomplished are we in a position to obtain a fair 

 insight into the protein metabolism of the tissues. Any analysis of 

 this kind, therefore, must take into account first the so-called exogenous 

 protein, namely, that portion of it which is derived directly from the 

 food, and secondly, the endogenous protein, which is the result of the 

 catabolism of the substance of the tissue cells. Clearly, the first has 

 really little to do with the life of the cells, while the latter actually 

 serves as a measure of the waste of the tissues. 



The Utilization of the Proteins. — The amino-acids appear in the 

 blood in amounts scarcely sufficient for a quantitative analysis. Van- 

 Slyke,V however, states that their amount is fairly constant and that the 

 fasting animal contains from 3 to 5 mgr. in each 100 c.c. of blood. 

 After meals, when an active absorption of proteins is going on, their 

 amount may be doubled and similar increases may be obtained by the 

 injection of amino-acids into the intestine. Thus, 10 grm. of alanin 

 administered in this way yielded as much as 6.3 mgr. in each 100 c.c. 

 of mesenteric blood. A method by means of which such substances 

 may be withdrawn from the circulating blood has been described by 

 Abel. 2 It is known as vividiffusion. The apparatus consists of a long 

 tube of collodion coiled upon itself and immersed in a solution contain- 

 ing approximately the saitie content in salts as blood plasma. The 

 ends of the collodion tube are connected with the central and distal 

 ends of an artery. As the blood circulates through it, its diffusible 

 constituents dialyze into the saline solution and may be recovered 

 from the latter. In this way it has been possible to isolate alamine 

 and valine in crystalline form, and also to detect the presence in the 

 blood of histidine and creatine. 



It is a well-known fact that a large meal of protein gives rise to a 

 rapid increase of the urea in the urine until about the fifth hour, 

 when at least 50 per cent, of the total nitrogen of the food will have 

 passed into the urine. If we consider that digestion is going on mean- 



1 Harvey Lectures, New York, Lippincott and Co., 1916. 



2 The Mellon Lecture, Science, xlii, 1915, 13§. 



