4i6 SCIENCE PROGRESS 



not necessarily at the same H* ion concentration for different 

 acids, but depended on the nature of the anion. This has not 

 been corroborated by Northrop, who in 191 9 showed that at 

 equal H* ion concentration of hydrochloric, nitric, sulphuric, 

 oxalic, citric and phosphoric, the rate of digestion was the same, 

 although for some unexplained reason acetic acid diminished 

 the action. An interesting point in support of the importance 

 of the state of a substrate is that of the digestibility of the 

 elastic and white fibres of connective tissue. The white fibres 

 are easily digested by pepsin, and it is noted that they readily 

 swell if treated with acetic acid ; but elastic fibres, on the other 

 hand, are not affected by dilute acids such as acetic, and are 

 only digested by pepsin after boiling for some time. Further 

 facts in relation to this point are referred to under the heading 

 of " Electrolytes," the action of which is more and more being 

 shown to depend considerably on their effect on imbibition or 

 swelling of the substrate, i.e. factors affecting the permeability 

 of the substrate. 



As evidence accumulates it appears increasingly probable 

 that with a clearer conception of the physical processes at 

 work, the importance of the H* ions and of their local concentra- 

 tion as a result of adsorption will be found to play a very large 

 part in enzyme action. 



Electrolytes. — In many cases the presence of an electrolyte 

 appears to be essential for enzyme activity : indeed a par- 

 ticular electrolyte may be necessary. For example, pepsin is 

 practically inactive without hydrion and trypsin without 

 hydroxidion. All degrees of similar requirement of electro- 

 lytes exist. Thus ptyalin was shown by Bang (191 1) to be 

 almost inert in the absence of sodium chloride, while— as will be 

 seen below — the activities of enzymes may be greatly modified 

 by salts, even in dilute solution. An instance of the great 

 sensitiveness of enzymes to electrolytes was given by Pavy and 

 Bajrwaters (191 1) who, when investigating the environment of 

 enzymes — especially diastase and invertase — showed that acid 

 hastened action while alkali neutralised, and that even the 

 basic substances in tap water caused a marked retardation. 



Much of the early literature deals with work the object of 

 which has been to determine the relation of electric charges to 

 the effect of the electrolytes and adsorption ; while later fuller 

 knowledge of colloid chemistry has directed attention to 

 the effect of electrolytes on colloids— that is, either precipita- 

 tion or an effect on the dispersion of the colloid ; in which 

 connection Bayliss had suggested that the phenomenon of an 

 optimal H* ion concentration for each enzyme may be 

 connected with the maximum state of dispersion of the 

 enzyme. 



