ACTIVATORS 355 



cases the substances are quite simple chemical individuals, such 

 as acids, alkalis, or salts, and in others they may be complex 

 and of unknown constitution, as in the case of the co-enzyme 

 of zymase(but see p. 378). They have, however, properties in 

 common, namely that they can be separated from the enzymes 

 by dialysis, and are not destroyed by heating. Moreover, an 

 enzyme rendered inactive by removal of its co-enzyme can 

 be restored to its original activity by mixing again with this 

 substance. 



This latter effect has been shown especially for dialysed 

 liver extract which has no lipolytic action ; if, however, this 

 extract be mixed with some of the solution which had been 

 dialysed out, or with boiled liver extract, the characteristic 

 lipolytic action is regained. In this case it can be shown that 

 the bile salts are the active co-enzyme. Similarly, the activity 

 of zymase, the enzyme of yeast cells, is dependent on the 

 presence of certain complex phosphoric esters, which, likewise, 

 can be separated from zymase by dialysis and are not destroyed 

 by boiling water. 



Other examples of the dependence of the enzyme upon 

 activators are the necessity for the presence of a small quantity 

 of acid in order that pepsin and the lipase of castor-oil seeds, 

 for instance, may exert their respective actions. 



Similarly trypsin requires a faintly alkaline medium to 

 exert its proteoclastic action ; in many cases the presence of 

 calcium salts is essential, as, for example, in the clotting of 

 milk by rennin, the clotting of blood by thrombin, and the 

 gelatinization of pectin by pectase. 



In the case of some enzymes the substance at its seat of 

 origin is not a true enzyme but a so-called preferment or 

 zymogen which is not itself active but only becomes so on 

 being brought into contact with another more or less complex 

 substance known as a kinase. Thus, for example, trypsinogen, 

 which is contained in pancreatic juice, has very little action 

 on proteins but is converted into the true proteolytic enzyme 

 trypsin on coming in contact with the kinase entero-kinase 



exert its activity, and the latter for substances which stimulate or accelerate a 

 reaction without being absolutely essential to its taking place ; an example of 

 the latter class is furnished by traces of manganese salts which greatly increase 

 the oxidizing power of the enzyme laccase, though it has yet to be proved that 

 the laccase is unable to act in their absence. 



23* 



