718 PHYSICAL CHEMISTRY OF THE PROTEINS. I 



They were not correctly interpreted, however, since the isoelectric 

 point was inferred to occur at a more acid reaction than now seems 

 probable from observations of cataphoresis (46). The inapplicability 

 of cataphoresis in determining the isoelectric point of a globulin has 

 also been discussed. 



The same methods were used in purifying tuberin after it had been 

 extracted from the potato, and in detemiining its solubility, as were 

 used with serum globulin. Tuberin has been found to be only slightly 

 more soluble than the latter. The measurements in the three experi- 

 ments recorded in Table V indicate a solubility in water of approxi- 

 mately 0.1 gm. in 1 liter at 25°C. Had a larger amount of material 

 been available, and a more protracted experiment been possible, a 

 slightly smaller value might have been obtained. 



The Solubility of Casein. 



Casein has previously been prepared in a state of great purity by a 

 method very similar to the one that we have described (37) . Baker and 

 Van Slyke were able to show that if casein was very carefully precipitated 

 at its isoelectric point, and was then triturated with distilled water, 

 a product of very low ash could be obtained. We have used a modi- 

 fication of this method in the early stages of the purification. This 

 modification depends upon the observation of Loeb (47) that if a 

 divalent rather than a monovalent base is used to dissolve isoelectric 

 casein, twice the normal concentration is required. As a result the 

 hydroxyl ion concentration of casein dissolved by calcium hydroxide 

 is very much greater than that dissolved by sodium hydroxide. Ac- 

 cording to Loeb the pH of the soluble sodium compound of casein is 

 7.02 and of the soluble calcium compound 10.53. Presumably diva- 

 lent bases of the type of calcium hydroxide form insoluble acid salts 

 with casein at neutral reactions. 



Our practise has rested upon this phenomenon. After the casein 

 has been precipitated at its isoelectric point and washed with distilled 

 water according to the method of Baker and Van Slyke, only enough 

 sodium hydroxide was added to bring the casein to a neutral reaction. 

 In this our method differed from that of Hammarsten (48) in which 

 enough sodium hydroxide is added to dissolve completely the casein. 



