502 KINETICS or TRYPSIN DIGESTION 



governed by the law of mass action and that the velocity of hydroly- 

 sis is really proportional to the concentration of free trypsin and pro- 

 tein, it is necessary to suppose that the reaction is confined to some 

 particular molecular species present in the protein solution or to 

 introduce a "catalysis" coefficient to express the ratio of actual con- 

 centration to ''active" concentration as has been done in the case of 

 hydrogen ion by Schreiner.^^ The work of Loeb, Michaelis, Soren- 

 sen, Robertson, and others has shown that proteins in solution are 

 ionized so that it would be natural to suppose that the speed of reac- 

 tion is proportional to the concentration of protein ions instead of to 

 the total concentration of protein. It has been found that in the 

 case of pepsin hydrolysis this accounted for the difficulty both as 

 regards differences in the concentration of protein and the effect of 

 the hydrogen ion concentration of the solution. In the present case, 

 however, the ionic concentration, as measured by the conductivity 

 of the solution, increases more rapidly than the rate of hydrolysis but 

 less rapidly than the concentration so that the anomaly is only partly 

 corrected for. 



It has been found by von Euler and Svanberg^' in the case of in- 

 vertase that the retardation due to inhibiting substances is independ- 

 ent of the substrate (sugar) concentration so that in the case of this 

 enzyme also the evidence is contradictory to the assumption of a 

 substrate-enzyme compound. 



Hydrolysis of Mixtures of Casein and Gelatin. 



The rate of hydrolysis of casein solutions increases less rapidly than 

 the concentration of casein, just as in the case of gelatin. This is 

 shown in Table IX. It is evident that increasing the concentration 

 of casein above 4 per cent has Httle or no effect on the rate of diges- 

 tion. According to the saturation hypothesis the tr^sin must, 

 therefore, be "saturated" with casein when the latter is at a concen- 

 tration of 4 per cent or more. It is interesting to consider the 

 digestion of a mixture of casein and gelatin from the points of view 



^^ Schreiner, E., Z. anorg. Chem., 1921, cxvi, 102. 



^^ von Euler, H., and Svanberg, , Fermentforschung, 1921, iv, 142. 



