2 24 Solubility of Proteins 



After one has succeeded in obtaining homogeneous fractions of constant 

 chemical composition and constant physicochemical characteristics it is dis- 

 heartening to remember that not only will similar fractions from another 

 species be different, but probably fractions obtained in the same manner from 

 serum of another animal of the same species or even from serum of the 

 same animal under different conditions will vary considerably. 



The recent work of Schoenheimer and co-workers^" on the heavy nitrogen 

 containing amino-acid content of antibody-protein upon injection of the 

 heavy nitrogen after the antibodies have been completely formed demon- 

 strates the constant fluidity of the protein molecule even in regard to its 

 amino-acid composition. In fact, it is surprising that the composite picture 

 of serum proteins has such fundamentally stable and definite outlines that 

 reproducible fractions can be obtained. 



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