156 ISOELECTRIC POINTS OF PROTEINS 



ties of this protein. The amount of acid required to titrate the protein 

 to its isoelectric point is a measure of the amount of combined protein 

 that exists in the juice of the potato and is dissociated by the addition 

 of acid. The soluble salt of this protein that is formed on the other 

 side of its isoelectric point evidently contains three times as much 

 acid as was necessary to dissociate the compound that existed in 

 nature. The relation between this compound and those that exist 

 at other hydrogen ion concentrations and the state in which they 

 exist are suggested by these data. 



The Protein of the Carrot. 



Remarkably similar in behavior to potato juice is the juice of the 

 carrot. Existing at approximately the same hydrogen ion concen- 

 tration it combines acid and alkaH to precisely the same extent. 

 Fig. 3 illustrates the essential coincidence of their titration curves. 

 Moreover, protein separates from the juice of the carrot upon the 

 addition either of acid or of alkali. The volume of the precipitate 

 and the nitrogen in the filtrate are both recorded in Table IV. As 

 in the potato, the acid precipitate of greatest bulk appeared at a 

 hydrogen ion concentration slightly less than 10~^N. The alkaline 

 precipitate appeared near 10~^N. 



The isoelectric point of the protein in juice filtered from the acid 

 precipitate was determined. The change in direction of protein 

 migration also occurred at a hydrogen ion concentration not far from 

 10~^N. All the measurements upon carrot juice are arranged in 

 tabular form. With one exception — derived from the data upon 

 its solubiHty — ^the salient characteristics of carrot protein suggest 

 those of tuberin. The isoelectric point and amphoteric constants 

 of the protein in both vegetables are essentially identical. In nature 

 they exist at approximately the same reaction. A consideration of 

 the curves representing their solubility at different hydrogen ion con- 

 centrations suggests that they may exist as somewhat similar com- 

 pounds (Fig. 2). But whereas in the potato the alkaline precipitate 

 is slight, in the carrot it is almost as great in amount as the acid 

 precipitate. 



