J. H. NORTHROP 



609 



In the present work the rate of pepsin digestion of gelatin, egg 

 albumin, edestin, blood albumin, and casein in the presence of hy- 

 drochloric, nitric, acetic, sulfuric, oxalic, phosphoric, and citric 

 acids has been followed by this method. The determinations were 

 made at two ranges of hydrogen ion concentration, pH 1.0 to 1.5 and 

 pH 2.5 to 3.5. 



A summary of the results obtained with edestin at a reaction of 

 pH 2.6 is given in Table I. The results obtained with the other pro- 



TABLE I. 



Quantity of Amino Nitrogen per 10 cc. of Solution at 24°C. and 750 mm. 



Substrate, 20 cc. of edestin solution A. \ ^. , , , . _ 



-r. • \,^ r 1 ,• A r Final volume, 120 cc. 



Pepsin, 20 cc. of solution A. J 



pH of solution. 



24 



24 



2.6 



2.5 



2.6 



2.7 



2.6 

 2.6 



2.5 

 2.4 



2.4 



2.7 



2.2 

 2.4 



2.2 

 2.3 



2.5 

 2.3 



* Control with boiled pepsin solution. 



teins were practically identical with these and therefore will not be 

 given here. The experiments show that the rate of hydrolysis of all 

 the proteins studied is identical for all the acids (except acetic) 

 within the rather wide range of error of the method used (about 5 

 per cent). With gelatin acetic acid also behaves quantitatively like 

 the other acids even in concentrations as high as 25 volumes per 

 cent. With the other proteins the rate of hydrolysis in the presence 

 of acetic acid is slower than with HCl, HNO3, H2SO4, oxalic, citric, 

 or phosphoric acids. The effect therefore is evidently on the protein 



