1913.] Processes Operative in Solutions and Enzyme Action. 575 



residue — either PO(OH) or PO(OH) 2 . As the products of change are more 

 basic than the primary hydrolyte, in such a case the presence of acid would 

 be necessary to maintain the freedom of the enzyme. 



A similar argument may be applied to tryptic digestion in presence of 

 alkali. The fact that the action takes place in presence of alkali cannot 

 well be explained, in terms of our hypothesis, except by the assumption that 

 the active agent of change is a basic radicle — perhaps an " ammonium " 

 hydroxide. As the products of change — the glycines — are capable of 

 neutralising bases as well as acids, the presence of alkali would serve to 

 maintain the freedom of the basic radicle in the enzyme. 



The saccharoclastic enzymes, as a class, appear to be active only in almost 

 neutral solutions, a very slight degree of acidity being the most favourable 

 condition. Faint acidity rather than alkalinity appears to be the natural 

 condition of most living structures in which enzymes are present. But, as a 

 rule, under natural conditions, enzymes are not subject to control by the 

 products of their own action, as these are removed by diffusion more or less 

 rapidly and cannot accumulate as they do in laboratory experiments. 



Assuming that the enzymes are formed from internal salts (zymogens) by 

 neutralisation, it is to be supposed that the degree of acidity or alkalinity 

 which would be most favourable initially will vary according to the 

 " strength " of the salt to be neutralised ; at subsequent stages, variations 

 may arise owing to the formation of products of varying " strength." 



It has long been recognised that quantity alone is not the only factor to be 

 taken into account in using acids but that these vary greatly in " strength." 

 Thus, if cane sugar be subjected under strictly similar conditions to the 

 action of quantities of chlorhydric and acetic acids which neutralise the 

 same amount of alkali, the amount hydrolysed in a given time by the latter 

 will not be 1 per cent, of that hydrolysed by the former. It is therefore not 

 sufficient to determine the mere amount of " acid " present in a liquid by 

 determining the amount of standardised alkali required to neutralise it — the 

 apparent acidity of the solution : but the effective acidity must be ascertained. 



Two methods of determining the effective acidity or strength of dilute 

 solutions of acids are in use — the one is electrical and involves the deter- 

 mination of electromotive force, the other colorimetric ; but the two have 

 been applied in such a way that they are interdependent. A custom is 

 growing up, which is much to be deprecated, of stating the results in terms of 

 hydrogen-ion-concentration. Undoubtedly this method is not only one which 

 can be understood by those alone who are instructed in terms of a special 

 hypothesis — that of ionic dissociation — and have the necessary mathematical 

 knowledge to grasp its significance but it is misleading in more than one 



vol. lxxxvi. — B. 2 u 



