COMPARISON OF SH REAGENTS 819 



relative potency of the mercurials, the only inhibitor having a comparable 

 activity being Ag+, Ag+ and Cd++ have been included in Table 7-1 because 

 they react with SH groups (as well as exerting other actions), but will be 

 taken up in another volume with the heavy metal ions. It has been stated 

 that the arsenicals and Cd++ are often specific for vicinal dithiol groups, 

 but, as can be seen in the table, Cd++ is a much more potent inhibitor 

 than arsenite in most cases, although the difference with respect to the 

 uncoupling of oxidative phosphorylation is not so great (Fletcher et al., 

 1962). It is interesting to compare the arsenicals and the oxidants for the 

 reason that vicinal SH groups might react well with both types of inhibitor. 

 Although the oxidants are around 5 times as potent as the arsenicals 

 on 25 enzymes, there is actually some evidence that a slight correlation 

 between their actions exists. There are, of course, many other factors 

 involved. 



The table shows also the relative inactivity of the arsenicals and the 

 iodoacetate-iodoacetamide group, which is the reason they are not in gen- 

 eral useful reagents for the detection or titration of SH groups. However, 

 exactly this lack of a potent action on most enzymes makes them valuable 

 for their selectivity with certain enzymes having the types of SH group 

 with which they readily react. 



It is remarkable that some enzymes are inhibited so well by a certain 

 SH reagent and practically not at all by another substance which reacts 

 readily enough with the SH groups of other enzymes. Potency ratios of 

 several million are observed in some instances (e, g., carbonic anhydrase, 

 certain lipases, arginine kinase, /^-amylase, ATPase, and aldolase). It 

 should be pointed out that we cannot explain such results at present, al- 

 though ingenious hypotheses have been suggested. One is left with the 

 conviction that there are factors controlling SH group reactivity which 

 have not been recognized. Perhaps study of the reactions of these different 

 inhibitors with SH-containing simpler substances, such as polypeptides, 

 might help to unravel the mechanisms by which differential inhibitions 

 may be explained. 



