INHIBITION OF ENZYMES 



715 



genase by o-iodosobenzoate titration and 9.3 by an amperometric method, 

 and thus the value with o-iodosobenzoate is a little high in this case also. 



Kinetics of Inhibition 



The rate of oxidation of simple thiols by o-iodosobenzoate is usually quite 

 rapid, but protein or enzyme SH groups vary greatly in the rapidity with 

 which they react with this reagent. The inhibition of succinate oxidase by 

 o-iodosobenzoate at 0.2 uiM and 37° requires about 30 min to become max- 

 imal (Slater, 1949). This is shown in Fig. 1-12-12, where the maximal inhi- 

 bition of around 35% indicates that insufficient o-iodosobenzoate was pres- 

 ent for reaction with all of the enzyme and nonenzyme material. At 16° it 

 is evident that there are two phases, one complete within 10 min and 

 the other incomplete after 2 hr (Fig. 1-12-13). Although the first phase 

 undoubtedly represents oxidation of SH groups, it is not clear if the slow 

 reaction is further oxidation of other SH groups or secondary inactivation 

 of the enzyme. 



The inhibition of ribonuclease by o-iodosobenzoate also shows two phases 

 (Figs. 6-1 and 6-2), one a fairly rapid reaction inhibiting around 20% and 



Fig. 6-1. Rates of inhibition of ribo- 

 nuclease by o-iodosobenzoate at pH 7. 

 The times indicate the duration of the 

 incubation of the enzyme and inhib- 

 itor. (From Ledoux, 1954.) 



the other a much slower one that is linear at least over 1-2 hr (Ledoux, 

 1954). The rate of inhibition during the second phase is dependent on the 

 concentration of o-iodosobenzoate and, since this phase starts from essen- 

 tially the same degree of inhibition for each concentration, it seems that 



