THE SOLUBILITY OF PROTEINS AND THEIR 



SEPARATION FROM MIXTURES WITH 



SPECIAL REFERENCE TO SERUM 



THE PROCEDURES Ordinarily used for the isolation of individual proteins from 

 mixtures such are are found in plasma or in extracts of muscle, placenta, 

 liver, kidney, or endocrine organs are relatively simple. Isolation depends 

 upon solubility in a given medium under given conditions, and the process 

 of finding the optimum conditions is too often likely to be a matter of art or 

 of luck. The physicochemical characteristics of the molecule undoubtedly 

 determine its solubility but the protein must be isolated before such charac- 

 teristics can be determined. There are, however, certain general, more or less 

 theoretical considerations governing the solubility of proteins which will be 

 summarized here briefly. 



In general, solubility depends upon the type of solvent, the presence of 

 electrolytes, the kind and concentration of such electrolytes, the pH, the tem- 

 perature, and the presence of other organic materials, especially other proteins. 

 These factors must all be considered in the extraction as well as in the puri- 

 fication of proteins. 



To avoid conftision it may be well to define the ordinary classes of protein. 

 Albumins are water-soluble proteins precipitated only in relatively high con- 

 centrations of neutral salts. Globulins are precipitated in lower concentrations 

 of neutral salts and may be further divided into (i) water-insoluble globulins, 

 true globulins or etiglobulins, and (2) water-soluble or pseudoglobulins. The 

 original definition of a globulin as a protein insoluble in water and soluble in 

 low concentrations of salts is still applicable. The fraction of serum precipita- 

 ble in one-third saturation -with ammonium sulfate can no longer be termed 

 "euglobulin" since it contains much water-soluble globulin and since water- 

 insoluble globulins are found also in the "albumin" fraction. 



Separation by Isoelectric Precipitation 



Separation of proteins from aqueous solutions of low ionic strength depends 

 upon isoelectric precipitation. Etiglobulins exhibit minimum solubility in 

 the neighborhood of the isoelectric point and increase in solubility at either 

 a lower or a higher pH, where the molecule has a higher positive or nega- 

 tive charge. Horse hemoglobin, serum euglobulins, fibrinogen, prothrombin, 

 pneumococcus and meningococcus antibodies, the coagulant from the pla- 

 centa or limg, myosin from muscle, and certain hormones from the anterior 

 pituitary may all be purified to varying degrees by this means. If more than 

 one euglobulin is present in a solution, advantage may be taken of differences 

 in isoelectric points or differences in solubility in neutral salt solutions. It 

 should be noted that some proteins are more nearly insoluble than others at 



