INHIBITION OF ENZYMES 



39 



1.5 mM iodoacetate at pH 8.8 and 0° for 30 min leads to some alkylation 

 of methionine, histidine, and lysine. However, 2 SH groups are also reacted 

 and 5.7 half-cystine residues remain. The susceptible SH group may be 

 linked under certain conditions to an imidazole ring through a hydrogen 

 bond, and this may alter the reactivity of the SH group with alkylating 

 agents (Rabin and Watts, 1960). 



Fig. 1-3. Relationship between the reaction of 

 SH groups (determined by iodide release) and 

 inactivation of creatine kinase by 0.5 mikf iodo- 

 acetate at pH 9 and ionic strength of 0.1. The 

 curve gives the iodide release and the points 

 the inactivation of the enzyme. (From Watts 

 et al, 1961.) 



Proteolytic Enzymes and Peptidases 



Enzymes hydrolyzing proteins and peptides have been frequently studied 

 with respect to their SH group requirements, because some of the earliest 

 isolated enzymes of this type were found to be readily inhibited by SH 

 reagents. It now appears after numerous proteinases have been tested that 

 in general they may be divided into two classes, those which are quite 

 resistant and do not possess reactive SH groups at the active center, and 

 those which are inhibited strongly, often by low concentrations of SH re- 

 agents, as may be seen by inspection of Table 1-8. These enzymes do not 

 constitute a homogeneous group and it is not surprising that the suscepti- 



