2l6 



Solubility o£ Proteins 



their isoelectric points, that small changes in electrolyte concentration may 

 have a great effect on the pH of minimum solubility, and that solubility may 

 be greatly influenced by temperature. 



Solubility in Concentrated Salt Solutions 



In concentrated solutions of neutral salts proteins are precipitated. The solu- 

 bility of pure proteins may be described by the linear equation 



log ^ = iS - KJtx (i) 



where 5 is the solubility in grams per liter, ^i is the ionic strength per 1,000 gm. 



1.0 



-J 



o 

 (/I 



u. 



< 



a 



o 

 -I 



T.0 



Mf1,1^0. a 

 rijsq, A 



Kj-ipq, 



\ 



\. 



2 4 6 8 



IONIC STRENGTH 



Fig. 1. The solubility of carboxyhemoglobin at 25° and pH 6.6 in 

 concentrated solutions of various electrolytes. 



(From Green, A. A.: Jl. Biol. Chem. 93:512, 1931.) 



water, ^ the intercept constant, and K/ the "salting out" constant. If the 

 logarithm of the solubility be plotted against the concentration of salt or 

 against the ionic strength, a straight line results. If the quantities are expressed 

 in terms of units per liter or in terms of mol fractions, the straight line still 

 holds but the values of the constants are different. This form of equation was 

 first applied to proteins by Cohn^ using the data of S0rensen and H0yrup^ and 

 of Chick and Martin^ for the solubility of egg albumin in ammonium sulfate, 

 and of S0rensen* for the solubility of pseudoglobulin in ammonium sulfate. 



a. Effect, of Type of Electrolyte on Solubility 



Uni-univalent salts are relatively ineffective in precipitating proteins. Only 

 the more insoluble proteins such as fibrinogen are precipitated by these salts 

 even when they are used in high concentrations. Higher valent salts produce 

 much higher ionic strengths and are much more effective as precipitating 

 agents. 



