608 EFFECT OF ACIDS ON DIGESTION OF PROTEINS 



solutions probably did not have the same hydrogen ion concentration. 

 Sorensen'' has shown that the activity of pepsin depends very largely 

 on the hydrogen ion concentration, and at the same time has de- 

 veloped accurate methods for the determination of the latter. In 

 view of his work it is obvious that the comparison of the action of 

 the different acids must be made in such a way as to keep this factor 

 constant. Failure by the earlier workers to do this probably accounts 

 in large measure for the great differences in the observed efficiency 

 of the various acids. 



More recently Michaelis and Mendelssohn^ have shown that the 

 optimum acidity for the digestion of edestin by pepsin is the same for 

 hydrochloric, nitric, tartaric, and oxalic acids. Ringer^ states that 

 the optimum reaction depends on the protein used and coincides 

 with the maximum hydration of the protein as measured by the vis- 

 cosity. A somewhat similar theory was proposed by Pfleiderer* who 

 attempted to show a relation between the. rate of digestion of fibrin 

 and the amount of swelling in various acids. 



In all these investigations the amount of digestion was followed 

 by determining (1) the amount of solution of an insoluble substrate, 

 (2) the amount of precipi table protein left in solution, or (3) the 

 rate of liberation of carmine from carmine fibrin. As has been 

 pointed out by various authors, there is considerable doubt as to 

 whether any of these methods actually follows the chemical changes 

 in the structure of the protein during hydrolysis. 



The recent improvements in the technique of the determination 

 of amino nitrogen by the Van Slyke^ method make it possible to follow 

 the increase in the number of free amino groups. From our knowledge 

 of the changes involved in the hydrolysis of proteins it would seem 

 that this increase probably follows accurately the amount of hy- 

 drolysis. The changes are small, however, and even with the greatest 

 care it is difficult to get strictly accurate figures. 



^ Sorensen, S. P. L., Biochem. Z., 1909, xxi, 131. 

 ''Michaelis, L., and Mendelssohn, A., Biochem. Z., 1914, Ixv, 1. 

 ^Ringer, W. E., Kolloid-Z., 1916, xix, 253; Arch. need, de Physiol., abstracted 

 in Physiol. Abstr., 1919, iii, 408. 



9 Van Slyke, D. D., /. Biol. Chem., 1913-14, xvi, 121. 



