6 THE PROTEIN ELEMENT IN NUTRITION 



conjugation or condensation with one another, or with other 

 organic radicles, to form more complex bodies with high molecular 

 weights. So far, however, all attempts to build up proteins 

 from the amino-acids have failed. Emil Fischer has built up 

 bodies which he calls " polypeptides," by the condensation of 

 glycocoll, leucine, and other amino-acids. It is hoped that the 

 formation of these bodies may be the first step in the process of 

 protein-synthesis . 



Another important discovery in connection with the formation 

 of the intermediate products of protein metabolism is that the 

 cells of nearly all the organs of the body have been shown to 

 possess enzymes capable of breaking down the complex protein 

 molecules, and causing disintegration as complete as in the action 

 of the ferments of the intestinal tract. That the breaking down 

 of the nitrogenous tissues begins as a result of the action of these 

 cellular enzymes is most probable, and is supported by the fact 

 that some of the products of protein hydrolysis are found in the 

 bile and urine, having escaped further destructive changes which 

 would have terminated in them being excreted as urea or other 

 nitrogenous constituent of the urine. Glycocoll is an example 

 of one of the products which has escaped this fate ; by its union 

 with cholaic acid it is found in the bile as glycocholic acid, and 

 with benzoic acid it occurs as hippuric acid. 



Another property of the cells, again probably the work of an 

 intracellular enzyme, is their power of liberating ammonia from 

 the amino-acids. This power is most marked in the cells of the 

 intestinal mucous membrane and liver, which would explain the 

 fact that the percentage of ammonia in the portal blood is during 

 digestion several times greater than that in the systemic blood. 

 The significance of this power of the cells of the intestinal mucous 

 membrane and liver of denitrifying the hydrolyzed products of 

 protein digestion is not clear ; but it has been explained as being 

 a reaction which, while leaving the non-nitrogenous part of the 

 protein molecule untouched, facilitates the greater part of the 

 nitrogen of the hydrolyzed products of protein digestion in being 

 eliminated in the urine as urea. The speed with which a large 

 part of the nitrogen appears in the urine as urea, following on the 

 ingestion of a protein meal, has been taken as a sign that the 

 body does not require all the nitrogen, and that it must be got 

 rid of before the valuable part of the protein molecules is admitted 

 into the general circulation. This is effected by the denitrifying 



