562 12. RATES OF INHIBITION 



account such i^rocesses of inactivation because the fall in enzymic activity 

 will be due to both the direct action of the inhibitor and the spontaneous 

 inactivation. The presence of the inhibitor on the enzyme may alter the 

 rate of inactivation. A generalized scheme may be written as: 



E+I^ EI 



X 



where the free enzyme is inactivated to X with a rate constant A^g and EI 

 is inactivated to X (or a similar derivative) with a rate constant ^3. We 

 may now outline the various possible situations. 



I. Free enzyme is stable: the presence of the inhibitor promotes the irre- 

 versible inactivation of the enzyme {k.^ = 0). 



II. Free enzyme is unstable: the free enzyme is inactivated at a certain rate 

 and the inhibitor affects this in the following ways. 



A. Inhibitor has no effect on inactiimtion: E and EI break down at the 

 same rate [k-i = A'g). 



B. Inhibitor increases instability: EI is more rapidly inactivated than 

 E {k^ > k^). 



C. Inhibitor decreases instability: EI is more stable than E {k2 > k^,). 



In the situation IIC, it may be noted that ^3 could be zero, in which case 

 the inhibitor is exerting a completely protective action. Such a protective 

 effect is quite possible inasmuch as substrate bound to certain enzymes in- 

 creases the stability markedly. The term ^'inactivation" will be used here 

 to designate those processes, usually irreversible, that lead to a loss of en- 

 zyme activity and are not the direct result of the binding of the inhibitor 

 at the region of the active site. 



Detection of inactivation in the presence of an inhibitor is often diffi- 

 cult. Inactivation may be suspected in the case of certain inhibitors when 

 the initial loss of enzyme activity is followed by a progressive fall of ac- 

 tivity. Somewhat better evidence for inactivation is a continuing decrease 

 in activity after removal of the inhibitor from the medium surrounding 

 the enzyme molecules, since this cannot now be attributed to a slow phase 

 in the reaction of the inhibitor with the enzyme. In those cases in which 

 complete enzyme activity is not restored upon removal of the inhibitor from 

 the enzyme, assuming that this removal can be experimentally demons- 

 trated, there is justification for suspecting inactivation. It is very impor- 

 tant to recognize inactivation when quantitative studies are being x)ursued 



