358 ENZYMES 



which is secreted by the mucous membrane of the duo- 

 denum. 



The relation between preferment and kinase is different 

 from that existing between enzyme and co-enzyme inasmuch 

 as the two latter can be alternately mixed and separated ; on 

 the other hand, the reaction between preferment and kinase 

 is not reversible ; furthermore, the preferment is not really 

 regarded as a complete ferment, while the true ferment pro- 

 duced from it, by combination with the kinase, may still be 

 dependent upon an activator for its activity. 



The following example of the dependence of thrombin 

 upon calcium salts will illustrate this ; the coagulation of blood 

 by thrombin consists in the conversion of the soluble sub- 

 stance fibrinogen into the insoluble substance fibrin. The 

 blood plasma contains a preferment thrombogen and also 

 calcium salts, but these two substances alone are unable to 

 coagulate fibrinogen. . When, however, the blood is drawn, 

 a kinase, known as thrombokinase, which is secreted by the 

 blood corpuscles, combines with the preferment thrombogen 

 forming the true coagulating ferment thrombase. 



Both enterokinase and thrombokinase are destroyed by 

 heat. 



PARALYSERS. 



Paralysers are substances which reduce or destroy the 

 activity of enzymes. These may be either the products of the 

 activity of the enzyme or of foreign substances. Examples 

 of the first class are the acetic or lactic acids which, unless 

 neutralized, destroy the ferments producing them ; similarly 

 the alcohol produced by the fermentation of sugar ultimately 

 stops the fermentation. Also, Tammann has shown that 

 the hydrolysis of amygdalin by emulsin was retarded by the 

 addition of any one of the products of hydrolysis, namely 

 glucose, benzaldehyde or hydrocyanic acid, but most markedly 

 by the latter. Similarly Croft-Hill found that glucose interfered 

 with the action of maltase, and the Armstrongs * likewise have 

 pointed out a number of examples of the inhibiting action of 

 the reaction-products upon the enzyme. 



Amongst foreign substances having a retarding effect on 



* Armstrong : " Proc. Roy. Soc., Lond.," B., 1907, 79, 360. 



