MODE OF A CTION OF ENZ YMES. 3 1 7 



in glycerin, but it has not been shown that the dried enzymes are 

 soluble in anhydrous glycerin; on the contrary, Kiihne 1 states that 

 pure trypsin is not soluble in strong glycerin ; and it is well known 

 that, after precipitation by alcohol from glycerin extracts, the enzymes 

 are afterwards much less soluble in glycerin. 2 



An elevated temperature rapidly destroys all enzymes when in 

 solution, and it is of some importance that the temperature at which 

 they are rapidly destroyed, although it varies considerably with the 

 reaction of the solution, lies just a little below the range at which the 

 bulk of the proteids coagulate. In the dried condition the enzymes are 

 much more resistant to increased temperature, and can be heated to over 

 100 C. for some time without losing their digestive properties on cooling 

 and dissolving in water. 



The digestive action of the enzymes is not stopped by the presence 

 of disinfectants, such as thymol, chloroform, or salicylic acid, in quantity 

 sufficient to stop completely the action of organised ferments, particu- 

 larly that of the putrefactive bacteria. 3 This fact has been turned to 

 account practically in conducting prolonged digestion experiments, 

 especially when the digestive action must be allowed to proceed in 

 alkaline solution. 



Mode of action of enzymes. The manner in which ferments bring 

 about the changes characteristic of them is very puzzling. The enzymes 

 are altogether unaffected by the changes which they occasion, and, pro- 

 vided the products of the action are not allowed to become concen- 

 trated in solution, the ferment can work on indefinitely, and a finite 

 amount of ferment can convert an infinite amount of material. The 

 ferment may become by dilution, or unavoidable loss in manipulation, 

 so weak that finally its action becomes inappreciable ; but before this 

 happens it can be shown that it has converted a mass of material so 

 many times greater than its own, that the idea that it undergoes any 

 permanent alteration in the reaction which it induces must be abandoned. 

 Thus, according to Hammarsten, 4 one part of rennin will curdle 400,000 

 to 800,000 parts of milk ; while Petit 5 prepared a pepsin powder which 

 in seven hours dissolved 500,000 times its weight of fibrin. 



There are numberless examples of chemical reactions, in which only 

 well-known and much simpler compounds take a part, of a substance 

 inducing a chemical reaction without itself becoming altered thereby. 

 Such a substance is called a catalytic agent, and the reaction a catalysis 

 or catalytic reaction. Ferment actions are such catalytic reactions, but 

 when we say that ferments act catalytically the problem of how they 

 act is not by any means solved ; we have merely found a name for it. 



In some cases, in which the presence of a substance is essential to a 

 certain reaction, although this substance is not finally altered thereby, 

 there is evidence that it is altered intermediately and rechanged again 

 back to its initial condition during the reaction. 



Such a case is to be found in the action of sulphuric acid in the con- 

 tinuous etherification process for producing ether from alcohol. It can 

 be shown that the sulphuric acid first combines with part of the alcohol 



1 Verhandl. d. naturh.-med. Vcr. zu Heidelberg, 1876, N. F., Bd. i. S. 196. 



2 v. Wittich, Arch.f. d. ges. PhysioL, Bonn, 1869, Bd. ii. S. 193. 



3 Klihne, Verhtmdl. d. naturh.-med. Ver. zu Heidelberg, 1876, N. F., Bd. i. S. 190. 



4 Jahresb. ii. d. Fortschr. d. Thier-Chem., Wiesbaden, 1877, Bd, vii. S. 166. 



5 Journ. de tMrap., Paris, 1880. 



