E. J. COHN, j. GROSS, AND O. C. JOHNSON 149 



solved by the high concentration of electrolytes in the juice that can 

 be squeezed from the potato. With the exception of a small amount 

 of proteose only one well defined protein, the globulin tuberin,^^ 

 has been isolated from the potato. It is present to the extent 

 of from about 1 to 2 per cent in the juice, and was isolated and 

 prepared from this and from the sodium chloride extract of the whole 

 potato. 



The Hydrogen Ion Concentration of Potato Juice. — The hydrogen 

 ion concentration of the juice of the potato was sHghtly less than 10~^N. 

 When the juice was freed from suspended material — mostly starch — by 

 filtration through pulp, the apparent hydrogen ion concentration was 

 further reduced to about 10~''n. It was then perfectly clear but 

 darkened by the action of an oxidase intimately related to the globulin 

 in location and in behavior. The oxidase was more active the lower 

 the hydrogen ion concentration. A study of the action of the oxidase 

 has been reported from this laboratory.^^ 



The Precipitation of Tuberin — A precipitate separated upon the 

 addition to the juice of the potato o't either acid or alkaH. The 

 alkaline precipitate was less voluminous, more gelatinous, and more 

 variable in amount than the acid precipitate. It was at first soluble 

 in excess alkali, but later was denatured. Upon the addition of 

 acid to potato juice a white, flocculent precipitate^* appeared at 

 about pH 6. The precipitate increased in amount and then gradually 

 redissolved upon the addition of more acid. At acidities greater than 

 those reported a protein precipitate gradually reappeared. This pre- 

 cipitate increased with increase in acidity and time and did not 

 resemble a globulin in behavior. 



The volumes of the precipitates that settled when different amounts 

 of acid were added to potato juice in cyhndrical vessels are recorded 

 in Table I. In every instance the greatest precipitation appeared 

 at a hydrogen ion concentration near 10~^N. In bulk, however, and 

 rate of settling, the precipitates varied to an extent which could not 



^^ Osborne, T. B., and Campbell, G. F., J. Am. Chem. Soc, 1896, xviii, 575. 

 13 Falk, K. G., McGuire, G., and Blount, E., /. Biol. Chem., 1919. xxxviii, 229. 

 1'* Upon standing, the protein frequently separated as a result of acid produced 

 by bacteria. 



