E. J. COHN, J. GROSS, AND O. C. JOHNSON 159 



side of it. In more nearly neutral solution it is soluble and can be 

 freed from other constituents of the tomato by filtering through 

 pulp. It then migrates to the anode. The addition of acid again 

 precipitates the protein at its isoelectric point. 



SUMMARY. 



The state in which a protein substance exists depends upon the 

 nature of its combination with acids or bases and is changed by change 

 in the protein compound. 



The nature of the compound of a protein that exists at any hydrogen 

 ion concentration can be ascertained if the isoelectric point of the 

 protein is known. 



Accordingly information regarding the isoelectric points of vegeta- 

 ble proteins is of importance for operations in which it may be desir- 

 able to change the state of protein substances, as in the dehydration 

 of vegetables. 



The Protein in Potato Juice. — The hydrogen ion concentration of 

 the filtered juice of the potato is in the neighborhood of 10~^N. 

 Such juice contains the globulin tuberin to the extent of from 1 to 2 

 per cent. 



The character of the compound of tuberin that exists in nature 

 was suggested by its anodic migration in an electric field. 



The addition of acid to potato juice dissociated this compound and 

 hberated tuberin at its isoelectric point. The isoelectric point of 

 tuberin coincided with a shghtly lower hydrogen ion concentration 

 than 10~'*N. At that reaction it existed most nearly uncombined. 



The flow of current during cataphoresis was greatest in the neigh- 

 borhood of the isoelectric point. This evidence supplements that 

 of the direction of the migration of tuberin, since it also suggests the 

 existence of the greatest number of uncombined ions near this point. 



At acidities greater than the isoelectric point tuberin combined 

 with acid. The compound that was formed contained nearly three 

 times as much acid as was needed to dissociate the tuberin compound 

 that existed in nature. At such acidities tuberin migrated to the 

 cathode. 



Though never completely precipitated tuberin was least soluble in 

 the juice of the potato in the neighborhood of its isoelectric point. 



