812 



7. MERCURIALS 



Progressive inhibition or inactivation of enzymes by mercurials is very 

 common and takes a variety of forms. Epididymal a-mannosidase is inhi- 

 bited 62% by 0.01 mM Hg++ without preincubation with the inhibitor; 

 the inhibition is 67% at 30 min, 77% at 60 min, and 88% at 120 min 

 (Conchie and Hay, 1959). This is one of the numerous examples in which 

 an enzyme is rapidly inhibited to a certain level, further increase in inhibi- 



FiG. 7-28. Logarithmic plots of if— i for the en- 

 zymes in Figs. 7-26 and 7-27, either loss of ac- 

 tivity or reaction of SH groups being used as a 

 measure of the reaction with p-MB. These curves 

 have been estimated from the published curves 

 and hence are not strictly accurate, but indicate 

 the relative rates for the more slowly reacting 

 SH groups. 1, Alcohol dehydrogenase (35°); 

 2, Alcohol dehydrogenase (20"); 3, phosphorylase; 

 4, lactate dehydrogenase; 5, glutamate decar- 

 boxylase (25°); 6, glutamate decarboxylase (0"); 

 7, ^-fructofuranosidase (0.1 mM); 8, ^-fructo- 

 furanosidase (0.02 raM); 9, j3-fructofuranosidase 

 (0.01 mM); 10, /3-fructofuranosidase (0.04 mM.) 



tion being slow. Such behavior is not surprising when one measures SH 

 reaction, since one assumes generally the occurrence of SH groups of dif- 

 ferent reactivities; thus when 3-phosphoglyceraldehyde dehydrogenase is 

 titrated with p-MB, 11 SH groups react immediately, but 3 more require 

 at least 40 min (Koeppe et al., 1956), and aldolase behaves very similarly, 

 7 SH groups being blocked rapidly and 3-4 more groups taking 40 min for 

 reaction (Szabolcsi and Biszku, 1961). But the interpretation of inhibition 

 following such a time course is not so clear. If a certain level of inhibition 

 is reached rapidly and then the rate falls off markedly, one must assume 

 that the enzyme is not completely mhibited when its rapidly reacting SH 



