58 COMPARATIVE HYDROLYSIS OF GELATIN 



number of linkages split was the same in all the solutions, different 

 ones had been split in each case. They found that the course of the 

 reaction was similar with pepsin and acid, and with trypsin and 

 alkali. The study of the hydrolytic products of trypsin and pepsin 

 leads to the same conclusion.' It is usually stated that trypsin 

 hydrolysis leads to more amino-acid formation. 



More quantitative evidence may be obtained, however, by a slightly 

 different mode of procedure; namely, by adding the enzyme in question 

 to a solution of the protein which has already been partially hydrolyzed 

 by some other means and noting the subsequent increase in hydrolysis. 

 If the two reactions follow the same course, i.e., if the same linkages 

 are split in both cases, the final amount of hydrolysis will be the 

 same irrespective of the stage at which the enzyme is added. If, 

 however, the preceding hydrolysis has split the protein at a linkage 

 which is not attacked by the enzyme, then the total amount of hydrol- 

 ysis will be the greater the farther the hydrolysis had proceeded before 

 the enzyme was added. That is, the total hydrolysis will be equal to 

 the sum of the enzyme hydrolysis (on the unhydrolyzed protein) 

 plus that which had taken place due to the other hydrolytic agent. 

 Since it is known that acids and alkali are capable of hydrolyzing 

 the proteins to their constituent amino-acids, it is obvious that they 

 must be able to split all the linkages so that a stage must eventually 

 be reached in acid or alkali hydrolysis at which the addition of the 

 enzymes will cause no further hydrolysis. The stage in the hydrolysis 

 at which this occurs will evidently be a measure of the difference in 

 the course of the two reactions. If the linkages which are most 

 easily split by pepsin, for instance, are the ones which are most 

 slowly attacked by acid, it will evidently be necessary to almost com- 

 pletely hydrolyze with acid the protein before reaching a stage at 

 which the addition of pepsin will cause no further increase. As will 

 be seen below, this is really the case. 



The results are complicated by the fact that the ease of hydrolysis 

 depends on other factors as well as on the particular linkage which 

 is hydrolyzed. This was shown by Fischer^ who found that tetraglycyl 

 glycine may be hydrolyzed by trypsin whereas triglycyl glycine is not 



^ Cf. Fischer,^ 



