FERMENTS, OR ENZYMES 77 



object of discovering the cause of these variations in the velocity con- 

 stant in the case of enzymes, four important conditions have been recog- 

 nized: (1) reversibility; (2) gradual destruction of the enzyme; (3) com- 

 bination of the enzyme with products of the reaction; (4) autocatalysis. 



Of these four influences the only one which could be held accountable 

 for an increase in the activity of the enzyme is autocatalysis; in this 

 process the enzyme by its action produces substances which intensify 

 its own activity. In some cases at least for example, the action of 

 invertase on cane sugar these are acid bodies, a moderate increase in 

 acidity favoring the action of this enzyme. 



The other influences all tend to retard the reaction and progressively 

 lower the value of K. Negative autocatalysis occurs when the enzyme 

 produces products which interfere with its activity. Gradual destruc- 

 tion of the enzyme and its union with the products of its activity will 

 manifestly also decrease its power. There is plenty of evidence that 

 both of these processes may occur. 



Reversibility of Enzyme Action 



But the most important of all the causes of retardation of enzyme 

 activity is undoubtedly reversibility of action, which is an application of 

 the law of mass action (page 25). If we take the saponification of an 

 ester, the equation is: 



CH 3 CH 2 CH 2 COOC 2 H 5 + H 2 O => CH 3 CH 2 CH 2 COOH + C 2 H 5 OH. 

 (ethyl butyrate) (butyric acid) (ethyl alcohol) 



The equilibrium point is not so near the position of complete hydrol- 

 ysis as in the case of the inversion of cane sugar; in other words, the 

 tendency for the bodies produced by the hydrolysis to reunite and form 

 the original substances is quite marked, so that the reaction comes to an 

 end before all the ethyl butyrate has been decomposed. For some time 

 before the equilibrium point is reached, there will have existed a progres- 

 sively increasing opposition to the breakdown of the ester, as a conse- 

 quence of which, when enzymes are used to accelerate the reaction, the 

 velocity constant, as determined by the above equation, will gradually 

 fall as the reaction proceeds. Conversely, in a mixture of ethyl alcohol 

 and butyric acid there is very slow synthesis to ethyl butyrate, and here 

 again lipase accelerates the process; it induces a recognizable synthesis 

 within a short time. Ethyl butyrate is usually employed for these ex- 

 periments because, on account of its odor, the ester is readily recognized. 

 Thus, if the alcohol and acid be mixed alone, no ester will be detectable, 

 but if some lipase be added, it will soon become so. Similar synthetic 

 action of lipase has also been demonstrated for mono- and tri-olein. 



