THE DIGESTIVE SYSTEM. 289 



37 to 40 C. ; and it is destroyed by the temperature of boiling water. 

 (5) Enzymes will act indefinitely if the products of their activity are 

 removed, that is, they are not used up and do not form permanent 

 compounds with the substances on which they exert their activity. In 

 other words, no definite proportion of enzyme to substrate, that is the 

 material acted upon, is necessary ; but the greater the proportion of 

 enzyme to substrate the more quickly is the final result attained. 



The foregoing characteristics are the principal tests by which an 

 enzyme may be distinguished. But some further points in connection 

 with the origin and action of ferments have to be noted, and help to 

 throw light on the probable mode of action of these bodies. Enzymes 

 are produced in living cells, and they either exert their action on 

 substances present in the cell body, or are turned out of the cell, as in 

 the case of the digestive juices, to exercise their function elsewhere. 

 That function is to be regarded as the acceleration of a process which 

 tends to go on, it may be with infinite slowness, in the absence of an 

 enzyme. The enzyme may be compared with the grease on the slip- 

 ways when a ship is launched, in that by its presence it facilitates a 

 process brought about by quite independent forces. The particular 

 process expedited by the presence of an enzyme is generally that of 

 hydrolysis, in which a large molecule takes up water and then the 

 resulting compound splits into smaller molecules. But in the case of 

 many, if not of all, enzymes, this process may be reversed, the smaller 

 molecules being condensed again with the loss of water. For example, 

 the enzyme maltase, if added to a solution of maltose, will convert most 

 of this sugar into dextrose. But if the same ferment is added to a 

 solution of dextrose it will convert a certain proportion of the latter 

 into maltose. The action of maltase is, in fact, to bring about a 

 certain definite proportion between the maltose and dextrose in 

 solution, the proportion, or, as it is called, the equilibrium point, 

 varying with the concentration of the solution. Thus maltase, acting 

 upon maltose, might convert it entirely into dextrose if the latter 

 substance were removed at the same rate as it was produced ; or, on 

 the other hand, the same enzyme, acting upon dextrose, might con- 

 vert the latter entirely into maltose, provided the maltose were being 

 simultaneously removed. This reversible enzyme action is of great 

 importance in the body, where, for example, dextrose is at one time 

 converted into glycogen by a synthetic process, while later the glycogen 

 is once more turned into dextrose by a process of hydrolysis. 



Enzymes, then, are bodies of unknown constitution, which facilitate 

 certain chemical reactions without providing any energy for these 

 reactions and without being used up in the process. They are, in fact, 



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