512 



SCIENCE 



[N. S. Vol. XLH. No. 1085 



be expressed by a simple application of the 

 law of mass action, since the active masses 

 are not functions of the molecular concen- 

 trations, but of the surface of the phase 

 boundaries. The application of these con- 

 siderations to the problem of the action of 

 enzymes and of heterogeneous catalysis in 

 general will be apparent. That the action 

 of enzymes is exerted by their surfaces is 

 shown, apart from the fact that they are in 

 colloidal solution, by the results of experi- 

 ments made in liquids in which the enzymes 

 themselves are insoluble in the usual sense, 

 so that they can be filtered off by ordinary 

 filter paper and the filtrate found to be free 

 from enzyme. Notwithstanding this in- 

 solubility, enzymes are still active in these 

 liquids. The statement has been found, up 

 to the present, to apply to lipase, emulsin 

 and urease, probably to trypsin, and the 

 only difficulty in extending it to all enzymes 

 is that of finding a substrate soluble in some 

 liquid in which the enzyme itself is not. 

 That adsorption is a controlling factor in 

 the velocity of enzyme action has been ad- 

 vocated by myself for some years, but it is 

 not to be understood as implying that the 

 whole action of enzymes is an "adsorption 

 phenomenon," whatever may be the mean- 

 ing of this statement. The rate at which 

 the chemical reaction proceeds is controlled 

 by the mass of the reagents concentrated on 

 the surface of the enzyme phase at any 

 given moment, but the temperature coeffi- 

 cient will, oi^ course, be that of a chemical 

 reaction. 



The thought naturally suggests itself, 

 may not the adsorption of the reacting sub- 

 stances on the surface of the enzyme suffice 

 in itself to bring about the equilibrium at 

 a greater rate, so that the assumption of a 

 secondary chemical combination of a chem- 

 ical nature between enzyme and substrate 

 may be superfluous? I should hesitate 

 somewhat to propose this hypothesis for 



serious consideration were it not that it was 

 given by Faraday as the explanation of 

 one of the most familiar cases of hetero- 

 geneous catalysis, namely, the union of 

 oxygen and hydrogen gases by means of the 

 surfaces of platinum and other substances. 

 The insight shown by Faraday into the 

 nature of the phenomena with which he was 

 concerned is well known and has often 

 caused astonishment. Now, this case of 

 oxygen and hydrogen gases is clearly one 

 of those called "catalytic" by Berzelius. 

 The fact that the agent responsible for the 

 effect did not itself suffer change was clear 

 to Faraday. I would also, in parenthesis, 

 direct attention to the fact that he correctly 

 recognized the gold solutions which he pre- 

 pared as suspensions of metallic particles 

 — that is, as what we now call colloidal solu- 

 tions. Although the systematic investiga- 

 tion of colloids, and the name itself, were 

 due to Graham, some of the credit of the 

 discovery should be given to the man who 

 first saw what was their nature. Adsorp- 

 tion, again, was accurately described by 

 Faraday, but without giving it a name. 



I confess that there are, at present, diffi- 

 culties in the way of accepting concentra- 

 tion by adsorption as a complete explana- 

 tion of the catalytic activities of enzymes. 

 It is not obvious, for example, why the same 

 enzyme should not be able to hydrolyze both 

 maltose and saccharose, as it is usually ex- 

 pressed- Another difficulty is that it is 

 necessary to assume that the relative con- 

 centration of the components of the chem- 

 ical system must be the same on the surface 

 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 con- 

 centration. Unless this were so, the equilib- 

 rium 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 



