496 EFFECT OF ENZYME ON DIGESTION OF PROTEINS 



law of mass action, may be quantitatively explained by the assump- 

 tion that the enzyme in solution is in equilibrium with the products 

 of digestion of the protein, or some other substance, and that this 

 equilibrium obeys the ordinary laws of mass action. The results of 

 Peckelharing and Ringer^^ may be taken as experimental proof that 

 the enzyme is so combined. These authors found that very pure 

 solutions of pepsin showed no isoelectric point when tested between 

 two oppositely charged electrodes; but that the addition of peptone 

 caused the pepsin to change the direction of migration at a pH of 

 about 3.0; which corresponds approximately to the isoelectric point 

 of these added substances. It is difi&cult to explain this experiment 

 otherwise than to conclude that the pepsin combines with the pep- 

 tone and is carried with it to the electrode. If some of the pepsin 

 combines with peptone, therefore, and so becomes inactive the rate 

 of digestion will evidently not be directly proportional to the total 

 concentration of pepsin but to some other function of the total con- 

 centration as defined by the mass action equilibrium. This is exactly 

 analogous to the relation between the hydrogen ion concentration 

 and the total acid concentration. In this case it is only the hydro- 

 gen ion which is active in hydrolysis and the activity of the solution 

 is therefore not directly proportional to the total acid concentra- 

 tion. (In the case of acid it is known that the dissociation is electro- 

 lytic; i.e., the dissociated parts of the molecule are electrically charged. 

 Whether this is true or not in the case of the pepsin cannot be stated 

 as yet.) The rate of reaction then becomes directly proportional to 

 the active (free) enzyme concentration as demanded by the law of 

 mass action; and the apparent divergence from this law is due to 

 the fact that the total enzyme concentration and the active enzyme 

 concentration are not always directly proportional; just as the total 

 acid concentration and the active acid concentration are not always 

 directly proportional. If this hypothesis is correct, it seems probable 

 that the enzyme does not combine with the substrate for an appre- 

 ciable length of time, but that the contact of enzyme and substrate 

 molecule results in immediate decomposition of the latter into its 

 products of digestion. There is no doubt that the enzyme actually 

 does combine with the substrate when the latter is not in solution.^ 

 It is quite possible, however, that this is a case of solution of the en- 

 23 Peckelharing, C. A., and Ringer, W. E., Z. physiol. Chem., 1911, Ixxv, 282. 



