INFLUENCE OF ACID AND ALKALI 411 



played by acids in accelerating the hydrolysis of the imino- 

 esters by increasing the active mass of the substrate through salt 

 formation (Cf. Stieglitz' experiments, cited in section 1). Loeb 

 believes that the portion of the pepsin which is active in bring- 

 ing about the hydrolysis of proteins is that portion of it which 

 is ionized, and that acids increase the active mass of the ferment 

 by forming ionized salts with it. Referring to the theory of 

 protein ionization which is developed in earlier chapters of this 

 work, and the intimate relation which, as we have seen, subsists 

 between the degree of ionization and the degree of hydration 

 of a protein (and presumably of these enzymes) this view would 

 appear very probable, especially if we accept the account of the 

 mechanism of the hydrolysis of proteins by enzymes which is 

 given in the latter part (equations (A) to (D)) of the preceding 

 section. 



As regards the influence of alkalies upon the activity of trypsin, 

 many of the facts appear, at first sight, to be more in harmony 

 with the view that the alkali acts as an accessory catalysor, 

 since it is stated that the hydroxides of the alkalies and alkaline 

 earths act in proportion to their degree of dissociation (63) 

 although divergencies from the proportionality of the action to 

 the degree of dissociation have been found (6). 



It has frequently been pointed out that the progress of hy- 

 drolysis of proteins is accompanied by marked changes in the 

 acidity or alkalinity of the solutions in which the hydrolysis 

 occurs (39) (40) (129) (107) (113). This is to be attributed to 

 the splitting of — COH.N— bonds, in hydrolysis, which do not, 

 while bound up in the protein molecule, assist in the neutra- 

 lization of bases and acids,* but which, when converted into 

 COOH— and H2N— groups may be presumed to play the part 

 which such groups ordinarily play; hence any excess of acid 

 or base tends to disappear during hydrolysis. One might expect 

 that this change in hydrogen or hydroxyl concentration would 



* We have seen that the — COH.N — groups provided by dicarboxyl- and 

 diamino-acid groups are chiefly involved in the neutrahzation of bases and 

 acids by proteins. In hydrolysis not only these bonds are spUt, but also the 

 — COH.N— bonds linking monocarboxyl and monoamino-acid groups. It 

 is for this reason that the combining capacity for bases of the products of 

 protein hydrolysis is greater, although only shghtly greater, than that of the 

 protein from which they are derived. 



