THE PROTEINS 43 



out of it all the proteins of its body. The blood 

 contains a constant amount of serum-albumin and 

 serum-globulin just as it contains a constant 

 amount of sugar ; and these are constructed from 

 the free amino-acids resulting from the hydrolysis 

 of protein in the food. It is not possible to detect 

 the presence of free amino-acids, albumoses or 

 peptones in normal blood ; but the foreign proteins 

 are broken down to these forms by digestion and 

 are reconstructed apparently by the cells of the 

 alimentary canal into cellular proteins, and from 

 this source a supply of the specific proteins of the 

 blood is kept up, no matter how foreign in nature is 

 the protein substance consumed as food. 



In a similar manner the individual cells of the 

 tissues also probably build up their specific proteins. 

 Barker* says there is much evidence of a local 

 hydrolysis in the cellular tissues, by which means the 

 proteins of the serum are in turn broken down to 

 amino-acids which are transformed into the special 

 amino-acids required for constructing proteins 

 according to their own pattern. How this is done 

 is not exactly known. It is said that all proto- 

 plasmic cells contain enzymes by which the catalytic 

 process is performed. But granting that cellular 

 enzymes break down the protein molecules in con- 

 tact with them into fragments or " building-stones," 

 as the amino-acids have been called, how from such 

 fragments do the cells make up the arginin required 

 to build histone for the thymus, glycocoll to build 

 up elastin, leucin and glutaminic' acid for serum- 

 albumin, aspartic acid for keratin, etc ? What is 



* Barker's address : Brit. Med. Jour., 1906, ii., 1093-1110. 



