CHAPTER 7 



COMPARISON OF SH REAGENTS 



The SH reagents are generally used to detect and characterize the 

 SH groups of enzymes. A number of problems involved in such studies 

 were discussed in Chapter II-4 and have been considered in connection with 

 the individual inhibitors. A brief comparison of some of these SH reagents 

 with respect to their actions on enzymes will now be presented. Inasmuch 

 as it is difficult to compare the relative potencies of inhibition from the 

 numerous tables given in the preceding chapters, a few results have been 

 collected in Table 7-1 to give a clearer picture of the great variability 

 in the susceptibilities of enzymes to the different types of SH reagent. 



Investigations have been included in the table with the following criteria 

 in mind: (1) three or more SH reagents are examined; (2) the concentrations 

 of the inhibitors used should provide an adequate basis for comparison; 

 (3) all of the inhibitors are tested under the same conditions; and (4) the 

 enzymes should be fairly well inhibited so that the likelihood of an SH 

 mechanism is great. It is obvious, however, that one cannot be certain 

 that all the inhibitions recorded are actually due to reactions with enzyme 

 SH groups. In addition to the ordinary inhibition data, a potency factor 

 has been calculated for more ready comparison, since in most studies the 

 concentrations or degrees of inhibition vary a good deal. These potencies 

 should be taken only as rough estimates for comparative purposes. If the 

 inhibition is assumed to be given by a relationship of the type: 



(I) 



(I) + rK^ 



where r can represent any factor by which the inhibition deviates from the 

 purely noncompetitive type, one can manipulate this to write: 



Potency 



Ki {\-i){l) 



so that the potency values given are really IjK^ if the inhibition is non- 

 competitive, or presumably some constant factors of this if there is a com- 



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