METABOLISM 175 



the blood, enzymes appear in the blood stream capable of break- 

 ing them down. No such enzymes occur in the blood normally. 

 Obviously then, proteoses and peptones normally are not present 

 in the blood. The chief difficulty in showing that free amino 

 acids are present in the blood lay in the complex character of 

 that liquid, and its high content of protein. During the diges- 

 tion of a protein meal the amount of amino acids absorbed at 

 any one time would be very small. Also the flow of the blood is 

 rapid, so that any increase in the total nitrogen of the blood would 

 be slight and thus difficult to detect in the presence of so much 

 other nitrogenous material. Folin and his co-workers showed an 

 increase in the non-protein nitrogen of the blood after a pro- 

 tein meal. Abderhalden demonstrated that amino acids are 

 present in the blood, by using very large volumes of blood. Van 

 Slyke, by his now well known method for determining small 

 amounts of oc amino groups, showed a marked increase after 

 a protein meal, and Abel and his co-workers by passing the cir- 

 culating blood of a living animal through a diffusion device suc- 

 ceeded in isolating considerable quantities of amino acids from 

 the blood. The problem was thus solved. The proteins of the 

 food are reduced to amino acids in the digestive tract, are 

 absorbed as such, and transported to the tissues. It has been re- 

 ported that a portion of the amino acids lose their amino groups 

 in the intestinal wall. Possibly this is the case. This statement 

 also has been contradicted, however. 



The further fate of the amino acids still is very obscure. 

 They are taken up by the tissues from the blood. Probably cer- 

 tain of the amino acids are used for the repair of cell proteins, 

 or in the manufacture of cell products. There appears to be 

 little or no storage of protein material, however, for the nitrogen 

 from ingested protein reappears in the urine within twenty-four 

 hours or so after taking. The protein structures of the body ap- 

 pear to increase only in growth, in recovery after a wasting dis- 

 ease, or in connection with increased activity, and not as the re- 

 sult of taking an excessive supply of this material. 



Various modes of decomposition are possible in the destruc- 



