698 PHYSICAL CHEMISTRY OF THE PROTEINS. I 



The isoelectric points of many proteins have been inferred from the 

 change in the direction of their migration in an electric field. The 

 method of cataphoresis has, however, not been found universally 

 applicable. Certain proteins, notably glutenin, casein, and the pro- 

 lamines are only very slightly soluble at certain hydrogen ion con- 

 centrations. If acid, or, under other circumstances, alkali, be added 

 to their solutions a precipitate appears, increases in amount, and 

 finally disappears if the hydrogen ion concentration is sufficiently 

 altered. Michaelis observed (6) that such proteins, like the denatured 

 albumin that Hardy first studied (1), migrated to the cathode on the 

 alkaline side of the precipitation zone, and to the anode on the acid 

 side. Accordingly he concluded, in part from theoretical considera- 

 tions to be discussed later, that the protein precipitated in the isoelec- 

 tric condition, and that the maximum precipitation coincided with the 

 isoelectric point. The phenomenon of precipitation has therefore 

 generally been substituted for the phenomenon of cataphoresis in 

 determining the isoelectric point of this class of substances; the iso- 

 electric point being considered coincident with the point of maximum 

 precipitation or flocculation. 



Still a third class of proteins, the globulins, although like the last 

 group they are also precipitated at certain hydrogen ion concentrations 

 in the absence of any appreciable concentration of electrolytes, are 

 readily soluble in solutions of neutral salts. In this class also the pre- 

 cipitation zone has been found to correspond to the isoelectric zone. 

 That is to say, isoelectric globulin was found to migrate toward the 

 cathode when dissolved by acid, toward the anode when dissolved by 

 base. When serum globulin was dissolved in a salt solution, however, 

 neither Michaelis (6) nor Chick (7), who confirmed his observations, 

 was able to detect a charge on the protein molecules by cataphoresis. 



Since globulins are dissolved by neutral salts over a considerable 

 range of hydrogen ion concentrations — and the nature of this phe- 

 nomenon and its relationship to the isoelectric point of the globulins 

 we shall reserve for a subsequent communication — practise has con- 

 sisted in reducing this range by the removal of salt, and then either 

 in noting the point of maximum precipitation of the globulin, or the 

 limiting hydrogen ion concentrations at which migration occurred 

 in an electric field. 



