JOHN H. NORTHROP 63 



A Clio n of Pepsin on Gelatin Partially Bydrolyzed by Alkali. 



The results of this experiment are given in Fig. 3 and Table II. 

 The results are very similar to those obtained with trypsin and 

 pepsin. The linkages attacked by pepsin are evidently quite rapidly 

 attacked by the alkali although some are still left after the hydrolysis 

 has reached titration value of at least 10 cc. 



Action of Pepsin on Gelatin Partially Bydrolyzed by Acid. 



These experiments are summarized in Fig, 4 and Table III. They 

 show that the linkages attacked by pepsin are among the most resistant 

 to the action of acid since the addition of the pepsin caused the normal 

 increase of 2 cc. even after the acid hydrolysis had proceeded to a 

 value of over 6.0 cc. There is no evidence of a true equilibrium in 

 the presence of pepsin since no decrease is noted at any time even 

 when the acid hydrolysis has been carried far beyond the end-point 

 reached with pepsin acting on the unhydrolyzed protein. Such a re- 

 verse or synthetic action of pepsin has occasionally been recorded 

 (the so called plastein formation^) in the case of other proteins. It is 

 apparently always connected with the formation of a precipitate. 

 This fact makes it appear possible that the decrease in the titration 

 value is due to the formation of some insoluble compound from sub- 

 stances present in the pepsin and protein solutions. This decrease has 

 not been noted in the case of gelatin solutions and these are the ones 

 in which no precipitate forms. The experiments described here show 

 that the hydrolysis by pepsin follows quite a different course from 

 that followed by acid hydrolysis. The products formed must there- 

 fore be different. It seems unlikely that the pepsin could have any 

 synthetic action on a solution containing substances which differ 

 from those formed by the action of pepsin itself. 



Hydrolysis by Trypsin. 



The extent of the hydrolysis by trypsin alone is shown in Fig. 5. 

 The hydrolysis continues until a titration value of about 15 cc. is 

 reached. The same uncertainty as to whether this is the real end-point 

 evidently exists here just as in the pepsin hydrolysis. 



'Henriques, V., and Gjaldbiik, J. K., Z. physiol. Chcm., 1911, Ixxi, 485; 1912, 

 Ixxxi, 439. 



