146 ISOELECTRIC POINTS OF PROTEINS 



(prolamines).^ But within each group state is affected by change 

 in concentration of these factors. Thus the solubihty of an albumin, 

 a globuHn, or a glutehn is variable with the amount of alcohol con- 

 tained in the solvent; and the solubihty of an albumin or a globuUn 

 is variable with the hydrogen ion concentration, and with the nature 

 and amount of the dissolved electrolytes. 



Recent investigations have suggested the course of certain of the 

 chemical reactions that involve changes in the solubihty of proteins. 

 It appears that, whatever the superimposed complexity resulting 

 from the colloidal nature of the systems, the behavior of protein 

 substances, whether simple or conjugated, is dependent in large part 

 upon their ionization as amphoteric electrolytes. ^'^'^ As amphoteric 

 electrolytes, proteins combine with either acids or bases, but at a 

 particular hydrogen ion concentration they exist most nearly uncom- 

 bined. The value of this singular point in ionization and in behavior 

 is characteristic of each protein and is termed its isoelectric point. 

 The isoelectric point is thus a measure of the relative strength of 

 protein as acid and base. 



The combination of the protein molecule with acid or with basic 

 radicles effects a change in its solubihty and in its hydration. The 

 compounds with simple acids or bases vary greatly in their ability 

 to absorb water and to dissolve in water, but in the neighborhood of 

 the isoelectric point protein substances are usually less soluble and 

 less swollen than elsewhere. Empirical evidence for this conclusion 

 antedates the theory for this phenomenon. Both in the laboratory 

 and in industry the preparation of proteins has depended largely 

 upon their lesser solubihty near the isoelectric point. 



The Determination of the Isoelectric Point. 



The migration of charged particles in an electric field is termed 

 cataphoresis. The charge of a protein depends upon its ionization. 



^ Recommendations of the Committee on Protein Nomenclature, Am. J. 

 Physiol., 1908, xxi, p. xxvii. 



2 Hardy, W. B., Proc. Roy. Soc. London, Series B, 1907, Ixxix, 413. Sorensen, 

 S. P. L., Conipt. rend. trav. Lab. Carlsherg, 1917, xii. 



3 Loeb, J., /. Gen. Physiol., 1918-19, i, 237. 



^Henderson, L. J., Cohn, E. J., Cathcart, P. H., Wachman, J. D., and Fenn, 

 W. O., /. Gen. Physiol., 1918-19, i, 459. 



