224 HYDROGEN ION CONCENTRATION AND PEPSIN DIGESTION 



of gelatin and pepsin is combined at about pH 3.0 than in either 

 more or less acid solutions. In the case of gelatin the volume varies 

 greatly with the reaction owing to the effect of the acid on the swelUng 

 of gelatin. The swelling is greatest at about pH 3.4 {cf. Loeb^°). 

 It might be supposed therefore that more pepsin was combined with 

 the gelatin at about this degree of acidity simply because there was 



TABLE III. 



Combination of Gelatin and Pepsin. 



5 gm. of isoelectric purified gelatin (= 0.75 gm. of dry weight) suspended in 

 200 cc. of HCl of strength noted and left 16 hours at 2°C. Filtered and washed 

 twice with 100 cc. of water (5°C.) and total volume made up to 75 cc. 5 cc. of 

 2 per cent pepsin added. Allowed to stand 20 min. at 5° with occasional stirruig. 

 4 cc. of supernatant fluid pipetted off and pepsin determined* in 1 cc. of this sam- 

 ple. Gelatin filtered and volume of filtrate measured. Gelatin melted and pH 

 determined of this and of the filtrate. 



* Cf. Northrop, J. H., J. Gen. Physiol., 1919-20, ii, 113. The relative amount 

 of pepsin is taken as the reciprocal of the time in hrs. required to cause a 5 per 

 cent change in the conductivity of a 5 per cent egg albumin solution titrated to 

 pH 1.7 with hydrochloric acid when 1 cc. of pepsin solution is added to 25 cc. of 

 egg albumin at 37°C. The unit of pepsin is taken as that amount which when 

 dissolved in 1 cc. and added to 25 cc. of the egg albumin solution will cause a 

 change of 5 per cent in the conductivity in 1 hr. 



