814 



7. MERCURIALS 



than at 10 min. Other more recently observed examples of this include 

 epididymal /5-galactosidase with low concentrations (0.0002 milf ) of Hg++ 

 (Conchie and Hay, 1959), xanthine oxidase with 0.44 mM p-MB, which 

 inhibits NADH oxidation 50% initially but less and less as the reaction 

 proceeds (Westerfeld et at., 1959), and leucine decarboxylase with 0.005 mM 

 p-MB (Sutton and King, 1962). The most marked spontaneous recovery is 

 seen with pig heart lactate dehydrogenase, the rate and degree of reactiva- 

 tion being dependent on the molar ratio of p-MB to enzyme (Fig. 7-29) 



50 



HOURS 



Fig. 7-29. Effects of p-MB on pig heart 

 lactate dehydrogenase, showing the ini- 

 tial inhibition and the spontaneous reac- 

 tivation. The numbers on the curves 

 are the molar ratios of p-MB to LDH. 

 (From Gruber et al., 1962.) 



(Gruber et al., 1962). The most common explanation for such recovery is 

 a slow migration of the mercurial from those groups initially attacked to 

 other groups not involved in the enzyme activity. The rates at which var- 

 ious SH groups react with a mercurial are not necessarily related to the 

 affinities of the groups for the mercurial. Groups which bind the mercurial 

 very tightly may be masked and react very slowly, as fairly conclusively 

 demonstrated for myosin ATPase by Gilmour and Gellert (1961). Another 

 factor which may be of importance when the inhibition decreases during 

 the period when the enzyme activity is measured, as was the case with 

 leucine decarboxylase, is the displacement of the mercurial by the substrate. 

 Leucine was shown to protect the enzyme against 2)-MB and it could even- 

 tually overcome the inhibition somewhat, especially since its concentration 

 was some 1000 times greater than the mercurial. A substrate might also be 

 able to restore toward a normal configuration a slightly luxated active cen- 

 ter, substrates being known to stabilize the active forms of certain enzymes. 



