60 



COMPARATIVE HYDROLYSIS OF GELATIN 



accurately adjusted to pH 7.0, using neutral red as an indicator. The formal- 

 dehyde is then added and the titration carried out as usual with 0.1 N NaOH, 

 using thymol blue as indicator and an end-point of about 8.2. The titration was 

 made on 10 cc. until the titration figure became more than 5 cc, after which 5 

 cc. were analyzed. The results have been calculated on the basis of 10 cc. of 5 

 per cent gelatin. The titration value for this concentration of gelatin was found 

 to be 2.0 cc. The complete hydrolysis of gelatin increases the amino nitrogen 

 (or formol titration) about 20 times.^' * The percentage total hydrolysis may 

 therefore be found from the figures by dividing by 40.0. 



NaOH 



o 

 o 



a, 



G 

 o 



I 



-3 



e 



Days at 33 "* 



Fig. 1. Hydrolysis of gelatin by pepsin. 500 cc. of solution containing 25 

 gm. of gelatin, 20 cc. of 5 per cent dialyzed pepsin, and 40 cc. of 1.0 N HCl placed 

 at 38°C. (pH = 2.2). 50 cc. samples pipetted out after 0, 1, and 4 days, and kept 

 at 3° until all had been taken. 5 cc. of 5 per cent pepsin added to each sample 

 and the samples placed at 38°C. Formol titration was run on 10 cc. The 

 values plotted have been calculated on the basis of 10 cc. of 5 per cent gelatin. 



Hydrolysis by Pepsin Alone. 



The results of this experiment are given in Fig. 1 . The figure shows 

 that under the conditions of this experiment, the pepsin is able to 

 double the formol titration of the gelatin (an increase of 2 cc. per 

 10 cc. of 5 per cent gelatin); i.e., the number of free carboxyl groups 



Van Slyke, D. D., /. Biol. Chem., 1911-12, x, 49. 



