52 ENZYMES OR SOLUBLE FERMENTS. 



but it cannot by any means be said that their views are established. 

 Asparagin, from which aspartic acid is readily obtained, undoubtedly 

 plays an all-important part in the constructive nitrogenous meta- 

 bolism of plants ; but as yet the aldehyde of aspartic acid has not been 

 prepared by any chemical means, and Baumann 1 has cast great doubt 

 on the reliability of the methods by which the above authors have en- 

 deavoured to prove the existence of aldehydes in the protoplasm of the 

 living plant cells. And it is probably significant that the reactions by 

 which the presence of the aldehydes is supposed to be shown, are only 

 well marked in the case of the cells of the lowest plants ; in the case of 

 animal cells they are more usually wanting. 



THE ENZYMES OR SOLUBLE UNORGANISED FERMENTS 2 . 



Chemists have for a long time been familiar with an extensive, 

 and still increasing class of reactions which occur solely, or in some 

 cases most readily, in presence of minute quantities of some substance 

 which does not itself appear to enter directly into the reaction ; in 

 other words the causative agent is found to have itself undergone no 

 obvious change during the reactions which it has set up between the 

 other substances. Striking instances of such reactions are observed 

 in the preparation of ether from alcohol by means of sulphuric acid 

 and in the manufacture of sulphuric acid itself. In the former case 

 a small quantity of sulphuric acid is theoretically able to convert an 

 indefinitely large quantity of alcohol into ether, and in practice the 

 limit is determined simply by the occurrence of secondary decomposi- 

 tions between the reagents. Similarly during the manufacture of 

 sulphuric acid a minute quantity of nitric oxide suffices in the presence 

 of water to convert an indefinitely large amount of sulphurous 

 anhydride into sulphuric acid. Of late years a large number of 

 reactions have been found to depend for their occurrence upon 

 the presence of the minutest traces of water; thus dry chlorine 

 has no action on dry sodium, and dry hydrochloric acid gas and 

 oxygen do not react even when exposed to bright sunlight, neither 

 do dry oxygen and carbonic oxide explode on the passage of an electric 



1 Pfluger's Arch. Bd. xxix. (1882), S. 400. See also Hoppe-Seyler, Zt. f.physiol. 

 Chem. Bd. x. (1886), S. 39. 



2 It appears advisable to use the torm ' enzyme ' (Kiihne, Unters. a. d. physioL 

 Just. Heidelb. Bd. i. 1878, S. 293) to denote the soluble unorganised ferments 

 generally, reserving the older name of ' ferment ' for the organised agents such as 

 yeast to which it was first applied. If this be done it will be convenient to use the 

 expression zymolysis ' to denote the changes produced by the enzymes in their 

 action on other substances, and to apply the term ' fermentation ' to the action of 

 the organised ferments. In this way ' zymolysis ' corresponds to the German 

 4 Ferment- wirkung ' and ' fermentation ' to ' Gahrung.' 



