ARTICLES 415 



viscosity diminishes the activity of enzymes by decreasing the 

 dispersity. 



Concentration of Enzyme. — ^The effect of the concentration of 

 enzyme on the velocity of the reaction has already been men- 

 tioned with regard to the work of Colin and Chaudin (191 8), but 

 before that, it had been established that an increase in the 

 amount of enzyme accelerated the rate of reaction, although 

 there has been considerable divergence of opinion as to the 

 law of velocity in which the acceleration may be expressed. 

 Van Slyke (19 14) found that the activity of urease depended 

 directly on its concentration, and late in 191 7 Nelson and 

 Vosburgh came to the same conclusion regarding invertase. 

 The latter however found that this was not necessarily so with 

 very dilute solutions, and it was assumed that the lessened 

 concentration was counterbalanced by the increased dispersion, 

 while the spontaneous destruction of enzymes which tends to 

 occur in dilute solutions adds an experimental difficulty. This 

 destruction is very liable to occur when little substrate is 

 present. In 191 7 Biedermann confirmed earlier work by show- 

 ing that the rate of the formation of dextrine from starch was 

 inversely proportional to the amount of enz3^me present. 



Reference has already been made to the work of Bradley in 

 this connection, and it is obvious that the velocity of a reaction 

 will be retarded by such destruction. 



Hydrogen Ion Concentration. — Recently more attention has 

 been paid to the effect of the H* ion on the activity of enzymes. 

 It has long been known — especially as the result of the work of 

 Sorensen — that enzymes are very sensitive to changes in the 

 H* ion concentration. Certain enzymes will act only within 

 very narrow limits, some only in an acid medium, others only 

 in an alkaline medium. Van Slyke and Zacharias (19 14), 

 working on the action of urease, came to the conclusion that 

 the combining velocity of the urease with urea varied 

 inversely with the positive H' ion concentration. In the 

 estimation of the H' ion present, Falk and Nelson (1915) have 

 shown that the method of estimation is of importance. They 

 prove that it is not accurate to calculate it from the polarity of 

 the acid, but that H* ion concentration must be measured either 

 by the electromotive force or by the hydrolysis of sugar. 



State of Substrate. — ^The H* ion concentration was shown by 

 Ringer (191 6) to be related to the condition of the substrate. 

 This investigation showed that the optimum H* ion concentra- 

 tion for digestion is situated at the point of maximum swelling 

 of the substrate ; he found at the same time that the effect of 

 salts on the proteolytic activity of enzymes is parallel to the 

 effect in reducing the swelling of albumin in acid solution. 

 Ringer however found that the point of maximum swelling was 



