74 ENZYMES OR SOLUBLE FERMENTS. 



mode of action of an enzyme and we must look for it in some other 

 direction. 



Attention has been already drawn (p. 52) to the existence of a 

 large and increasing class of chemical reactions whose occurrence is 

 determined by mere traces of some substance which does not itself at 

 the same time undergo any change during the decompositions which 

 it initiates, and the enzymes have been compared to these substances. 

 Now in the case of the reactions of which we are now speaking it is 

 known in some and probable in all that the process which takes place 

 is in general terms the following. The determinant substance interacts 

 with one of the reagents to form a compound which can now enter into 

 combination with the other; the result is the formation of a more 

 complex compound which at once decomposes, giving rise to products 

 of which one is the original determinant substance in an unaltered 

 form, the others the product characteristic of the reaction 1 . This 

 suggests at once that the enzymes may play their part in a manner 

 similar to that of the determinant in the above reactions, a view which 

 has been put forward but scarcely receives the attention that it 

 deserves 2 . 



In most cases it is known, and it is probable in all, that the soluble 

 ferments act by bringing about a union of water with the substances 

 upon which they act. This process might be supposed to take place in 

 the following way. The enzyme uniting with the substance to be de- 

 composed, the compound thus formed is now able to unite with water, 

 and this final more complex and hence less stable compound undergoes 

 a decomposition of which the original enzyme is one product, the others 

 being the hydrated and hence altered substance whose formation is 

 characteristic of the whole process. It is impossible within convenient 

 limits to bring forward here all the direct evidence in favour of the 

 above view as to the mode of action of enzymes ; it must suffice to say 

 that as regards pepsin there is some reason for thinking that it can 

 enter into combination with hydrochloric acid. Finally it may be stated 

 that the characteristic phenomena of zymolysis in connection with the 

 influence of heat, the effect of various salts and dilution, the cessation 

 of the change in presence of an excess of the products of that change 

 &c., are such as careful consideration shows might from several points 

 of view be expected on the supposition that the above theory of enzyme 



action is true. 



i 



1 Vide the reactions in the continuous etherification process and the manufacture 

 of sulphuric acid. See also Traube (Ber. d. deutsch. chem. Gesell, 1885, S. 1890), on 

 the part played by water in determining the explosion of and CO. 



2 Kiihne, Lehrb. d. physiol. Chem. 1868, S. 39. Hoppe-Seyler, Med.-cliem. 

 Unters. Hft. 4, 1871, S. 573. v. Wittich, Pfluger's Arch, Bd. v. (1872), S. 435. 

 Wurtz, Compt. Rend. T. xci. (1880), p. 787. 



