18 . ENZYME ACTION [CH. 



point of equilibrium is reached whether hydrochloric acid is used or not, 

 it is obvious that the hydrochloric acid accelerates the reaction in both 

 directions : 



ethyl acetate 4- water ^: ethyl alcohol -I- acetic acid. 



Such a reaction is termed a reversible one. Many of the processes 

 accelerated by enzymes in the plant are reversible, and there is reason 

 to believe that the enzyme accelerates the reaction in both directions. 



The substance upon which the enzyme acts is termed the substrate, 

 and it is supposed that some kind of loose combination occurs between 

 these two substances. The enzyme is unaltered when the reaction is 

 complete, unless it is affected by the products formed. 



The enzymes are very widely distributed and form constituents of 

 all living cells, though all tissues do not necessarily contain the same 

 enzymes. 



There is no doubt that many enzymes are specific, in which case 

 an enzyme can only accelerate one reaction, or one class of reaction. 

 We cannot be sure that any enzyme is specific and different from all 

 others, until it has been proved that it accelerates one process which is 

 incapable of being accelerated by any other enzyme. It is possible 

 that some enzymes, to which separate names have been given, are really 

 identical. 



Most of the plant enzymes are soluble in water and dilute glycerol 

 and sometimes in dilute alcohol. Some can be extracted by simply 

 macerating the tissues with water; others are more intimately connected 

 with the protoplasm, and are only extracted if the protoplasm is killed 

 by certain reagents, of which those most frequently employed are toluol 

 and chloroform. These substances kill the protoplasm and do not, in 

 many cases, affect the enzyme. After the death of the protoplasm, the 

 enzymes are more readily extracted from the cell. From aqueous 

 solutions enzymes can usually be precipitated by adding strong alcohol. 



The majority of enzymes are destroyed by raising the temperature 

 above 60 C. In vitro their reactions are generally carried out most 

 rapidly between the temperatures of 35-45 C. 



In performing experiments with enzymes in vitro, it is always 

 necessary to add an antiseptic, otherwise the reaction to be studied will 

 be masked or entirely superseded by the action of bacteria unavoidably 

 present. Toluol and chloroform mentioned above, as well as thymol, 

 may be used. These reagents prevent any bacterial action from taking 

 place. Some enzymes, however, are susceptible to chloroform, as, for 

 instance, maltase. 



