PROTEINS AS CHEMICAL INDIVIDUALS 35 



usually be brought about by inducing a certain reaction (i.e., 

 hydrogen or hydroxyl ion concentration) in their solution, through 

 the addition of acids or of bases, the hydrogen- or hydroxyl-ion 

 concentration being sufficient to set the protein free from its 

 combination with bases or acids respectively and insufficient to 

 lead to the formation of a new salt with the acid or base employed 

 for its precipitation. Thus, for example, casein is precipitated 

 from milk by the addition of acetic acid, "insoluble" serum globulin 

 from serum by dilution and the passage of C0 2 , histones from cell- 

 extracts by the addition of ammonia. 



The isolation of proteins of this class is usually most conven- 

 iently carried out by employing for the precipitation an acid or 

 base which is insufficiently dissociated to transform the protein 

 into a base, if an acid is employed, or into an acid if a base is 

 employed for the precipitation, so that the protein which is at 

 first precipitated does not, on adding an excess of the acid or base, 

 form a salt and pass into solution again. For instance, if a 

 " strong" acid such as hydrochloric acid be added to milk, or to 

 any solution of a caseinate of an alkali or an alkaline earth, free 

 casein is at first precipitated. But if the addition of the hydro- 

 chloric acid be continued the casein passes into solution again 

 and it is now found that the casein is behaving as a base and that 

 a certain proportion of the acid is neutralized by it. If a "weak" 

 acid such as acetic be employed, however, the acidity of the mix- 

 ture is insufficient to transform the casein into a base, and a 

 considerable excess of acetic acid may be added to the mixture 

 without danger of loss of material owing to resolution of the protein. 



The acid or base which is employed in setting free the insoluble 

 uncombined proteins of this class must, however, be sufficiently 

 strong to set free the protein from its salts; for example, C0 2 

 cannot be employed to precipitate casein because casein is a 

 stronger acid than C0 2 and displaces it from carbonates. But 

 C0 2 can be and is successfully employed to precipitate "insoluble" 

 serum- -globulin because this protein is a weaker acid than casein 

 and can be displaced from its salts by so feeble an acid as car- 

 bonic acid. Undoubtedly correlated with this is the fact that 

 serum globulin is more readily transformed into a base than 

 casein, so that acetic acid redissolves and is partially neutralized 

 by serum globulin, although it is insufficiently strong except in 

 very high concentrations to redissolve casein. 



