818 7. COMPARISON OF SH REAGENTS 



should be made with numerous SH reagents of different types if. any degree 

 of characterization is to be attained. 



Without doubt the mercurials are generally the most potent inhibitors 

 among the SH reagents. Of the 109 enzymes in the table for which data on 

 the mercurials are available, the mercurials are the most inhibitory in 

 70%, second most active in 12%, third most active in 9%, and of lower 

 potency in 9%. There are 17 enzymes for which Hg++ and arsenite may 

 be compared, and Hg++ is the more potent in 15, the ratio of potencies 

 being > 1000 in 10; in 25 cases comparing p-MB with arsenite, the mercurial 

 is the more potent in 22, and a ratio of > 1000 is seen in 10. It is difficult 

 to compare the various inhibitors on a quantitative basis, but a rough 

 estimate of the relative activities may be made by averaging the logarithms 

 of the potencies for those enzymes on which pairs of inhibitors have been 

 studied. The logarithm of the potency, as defined above, is really an appar- 

 ent "pX,." Some pairs of inhibitors are collected in the accompanying 

 tabulation; when the designations "mercurials," "arsenicals," or "iodo- 



acetate" are given, it refers to groups rather than individual inhibitor, 

 and in each case the most potent of the group (iodoacetate and iodoaceta- 

 mide are taken together). These figures show even more clearly the great 



