508 KINETICS OF TRYPSIN DIGESTION 



termined at the same stage of the reaction by interpolation from the 

 time curves, it is found that the rate of hydrolysis increases in direct 

 proportion to the casein concentration. This is shown by the last 

 line of Table XII. The same result is shown by comparing 

 the monomolecular constants at corresponding values of x. This 

 experiment shows that when the substrate concentration is meas- 

 ured directly the reaction proceeds according to the law of mass 

 action both as regards the concentration of enzjrme and of sub- 

 strate and that the only divergence from the simple monomolecular 

 formula is due to the fact that the enzyme concentration also 

 changes during the course of the reaction, owing to the effect of the 

 products of the reaction, an effect which can easily be demonstrated 

 directly. The experiment described was repeated several times with 

 the same result. It was also found that if the rate of hydrolysis of 

 gelatin was followed by means of the change in viscosity (which is 

 roughly proportional to the gelatin concentration) , the same result 

 was obtained. This indicates that in the case of gelatin as well, 

 the apparent discrepancy from the law of mass action is due to the 

 fact that the reaction is really a series of consecutive reactions and 

 that the change in concentration of the original substance cannot be 

 determined from the increase in the total products of reaction. It 

 will be seen that this mechanism will account also for the fact that 

 the increase in the products of reaction as plotted against the time 

 is occasionally a straight line. Assume, for instance, that the reac- 

 tion may be written A-^B^^C and that C is what is determined ex- 

 perimentally. The rate of formation of C will evidently be propor- 

 tional to the concentration of B at any instant, and this in turn will 

 depend on the relation between its rate of formation and of decomposi- 

 tion. It is possible therefore for the rate of formation of C to remain 

 constant, increase or decrease with time, and there is not neces- 

 sarily any simple relation between the concentration of A and the 

 rate of formation of C. 



SUMMARY. 



1 . The velocity of hydrolysis of gelatin by tr3^sin increases more 

 slowly than the gelatin concentration and finally becomes nearly 

 independent of the gelatin concentration. The relative velocity of 



