682 TRANf=!ACTIONS OP SECTION I. 



Another difficulty is that it is necessary to assume that the relative concentra- 

 tion of the components of the chemical system must be the same on the sur- 

 face of the enzyme as it is in the body of the solution ; in other words, the 

 adsorption of each must be the same function of its concentration. Unless this 

 were so, the equilibrium position on the enzyme surfaces, and therefore in 

 the body of the solution, would be a different one under the action of an 

 enzyme from that arrived at spontaneously or brought about by a homogeneous 

 catalyst such as an acid. This consideration was brought to my notice by Pro- 

 fessor Hopkins and requires experimental investigation. We know, indeed, 

 that in some cases there is such a difference in the position of the equilibrium 

 position, for which various explanations have been suggested. But it would be a 

 matter of some interest to know whether this difference has any relation to 

 different degrees of adsorption of the components of the system. 



At the same time, adsorption is under the control of so many factors, surface 

 tension, electrical charge, and so on, that the possibilities seem innumerable. 

 There are, moreover, two considerations to which I may be allowed to call your 

 attention. Hardy has pointed out that it is probable that the increased rate of 

 reaction at the interface between phases may be due, not merely to increased 

 concentration as such, but that in the act of concentration itself molecular 

 forces may be brought into pilay which result in a rise in chemical potential of 

 the reacting substances. In the second place, Barger has shown that the 

 adsorption of iodine by certain organic compounds is clearly related to the 

 chemical composition of the surfaces of these substances, but that this relation- 

 ship does not result in chemical combination nor in abolition of the essential 

 nature of the process as an adsorption. It would appear that those properties of 

 the surface, such as electric charge and so on, which control the degree of 

 adsorption, are dependent on the chemical nature of the surface. This 

 dependence need not cause us any surprise, since the physical properties of a 

 substance, inclusive of surface tension, are so closely related t-o its chemical 

 composition. 



There is one practical conclusion to be derived from the facts already known 

 with regard to e'uzymes. This is, that any simple application of the law of 

 mass action cannot lead to a correct mathematical expression for the rate of 

 reaction, although atteniipts of this kind have been made, as by Van Slyke. The 

 rate must be proportional to the amount of substrate adsorbed, and this, again, 

 is a function both of the concentration of the substrate and of that of the 

 products. It is, then, a continuously varying quantity. Expressed mathe- 

 matically, the differential equation for the velocity must be something of this 

 kind : 



dt 



where n itself is an unknown function of C, the concentration of the substrate 

 or products. 



The hypothesis of control by adsorjjtion gives a simple explanation of the 

 exponential ratio between the concentration of the enzyme and its activity, 

 which is found to be different numerically according to the stage of the reaction. 

 At the beginning, it may be nearly unity, in the middle it is more nearly 05, 

 as in the so-called ' square root law ' of Schiitz and Borissov, which is, however, 

 merely an approximation. Simple explanations are also given of the fact that 

 increasing the concentration of the substrate above a certain value no longer 

 causes an increased rate of reaction. This is clearly because the active surface 

 is saturated. Again, the effect of antiseptics and other substances which, by 

 their great surface activity, obtain possession of the enzyme surfaces, and thereby 

 exclude to a greater or less degree the adsorption of the substrate, receives a 

 reasonable account. In many cases, the depressant or favouring action of 

 electrolytes, including acid and alkali, is probably due to aggregation or dis- 

 persion of the colloidal particles of the enzyme, with decrease or increase of 

 their total surface. It is to be noted that such explanations are independent of 

 any possible formation of an intermediate compound between enzyme and sub- 

 strate, after adsorption has taken place. 



There is a further way in which adsorption plays a part in the chemical 

 processes of cells, including those under the influence of catalysts. It is a 



