THE NATURE OF ENZYME ACTION 299 



them, in a state of adsorption, constituents of the solutions from 

 which they are precipitated. It is not surprising, then, to find 

 that proteoclastic enzymes show certain reactions of proteins, 

 invertase apparently contains carbohydrate, and so on. But 

 when the solutions are more and more carefully purified it is 

 found that, although powerfully active preparations can be 

 obtained, these solutions show fewer and fewer characteristic 

 reactions. Pekelharing's pepsin, for instance, showed only a 

 minority of the usual protein tests, and was found to contain no 

 phosphorus. This last fact definitely excludes the view taken by 

 some that enzymes in general are nucleo-proteids. 



At the same time it seems, from evidence to be mentioned 

 immediately, that there is some very close similarity between a 

 particular enzyme and its substrate, so that, when it is found 

 that invertase cannot be prepared free from mannose, it may 

 well be that this sugar is a necessary part of its molecule. 



The facts referred to in the earlier pages of this article as to 

 the relations between certain enzymes and optically isomeric 

 substrates led Emil Fischer to put forward his famous simile of 

 lock and key relationship. 



In connection with this possible similarity of chemical 

 structure it is of interest that the statement has recently been 

 made that lipase is soluble in ether, and therefore probably of 

 a fatty nature. The lipase of the cytoplasm of castor-oil seeds 

 has also been shown by Nicloux to be destroyed by contact 

 with water. 



Mode of Action 



It is evident from what has already been said that there 

 is a very intimate association between an enzyme and its 

 appropriate substrate. A few further facts pointing to some 

 kind of combination may be mentioned here. Enzymes in 

 presence of substrate or products of reaction are considerably 

 less sensitive to the action of heat. It appears also that a 

 particular enzyme has a special affinity for certain constituents 

 of the substrate, as, for instance, invertase for fructose 

 (E. F. Armstrong), trypsin (according to Emil Fischer) for tyrosin, 

 leucin and, to a less degree, alanin. Now these bodies are 

 precisely those which exercise the greatest inhibitory power on 

 the velocity of the (hydrolytic) reaction ; in fact, glucose does not 



