54 ENZYMES OR SOLUBLE FERMENTS. 



of animal and vegetable cells it is found that in many cases sub- 

 stances may be extracted from them which possess to an even 

 more striking degree the property of inducing change in an indef- 

 initely large mass of certain other substances without themselves 

 undergoing any observable alteration. These agents are known 

 as the enzymes or soluble ferments, and the essential conception 

 of an enzyme is summed up in the above statement of the most 

 remarkable characteristic of their activity. Further investigation 

 of these enzymes shows that their activity is dependent upon 

 many subsidiary factors which are more or less common to them 

 all. Thus their activity is largely dependent upon temperature, 

 being absent at sufficiently low temperatures, increasing as the 

 temperature is raised to a certain optimal point which varies 

 slightly for different enzymes, then again diminishing as the tem- 

 perature is further raised, and finally disappearing. By^the action 

 of jL-ufficiently high temperature they permanently lose their 

 characteristic powers and are now spoken of as being killed.' 

 Again the enzymes' are extremely sensitive to the reaction, 

 whether acid, alkaline, or neutral, of the solutions in which they 

 are working, also to_the presence or absence of various salts, some 

 of which merely inhibit their action while others permanently 

 destroy it ; and their activity is in all cases lessened and finally 

 stopped by the presence .of an excess of the___rjroducts to whose 

 formation they have given rise. ItTias been already said that an 

 enzyme may be killed by exposure to a high temperature, but this 

 only holds good when they are in solution, or if in the solid form 

 they are heated in a moist condition. When perfectly dry they 

 may be heated to 100 160 without any permanent loss of 

 their powers. 1 It will be seen that so far the enzymes have been 

 characterised solely with reference to the peculiarity of their 

 mode of action and to the influence of surrounding conditions 

 upon that activity, and the question of their probable chemical 

 composition has been left untouched. Notwithstanding the fre- 

 quent endeavours which have been made to prepare the enzymes 

 in a pure condition, it is unwise to lay any great stress upon the 

 results of the analysis of these so-called ' pure ferments,' bearing 

 in mind that, as in the case of the proteids, no criterion of their 

 purity exists. This much however may be said. In the major- 

 ity of cases, analysis shows that their composition approximates 

 .more nearly to that of a proteid than of any other class of syb- 

 ^stances, and this is apparently true even when they do not yield 

 to any marked degree the reactions (xanthoproteic, &c.) which 

 are characteristic of a true proteid. Ordinarily it is almost im- 

 possible to obtain an enzyme solution of any considerable activity 

 which is free from proteid reactions, and hence many authors are 



1 Hiifner, Jn.f. pralct. C/iem. Bd. v. (1872), S. 372. Al. Schmidt, Centralb. f. d. 

 med. ir?s,. 1876, 'S. 510. Salkowski, Virchow's Arch. Bd. i.xx. (1876). S. 158; i.xxxi. 

 (1880), S. 552. Hiippe, Mittheil. d. Kaiserl. Gesundheitsamtes, i. 1881. 



