810 



7. MERCUKIALS 



not surprising that one finds a great deal of variation in the rates at which 

 enzyme SH groups react and at which inhibition occurs. In some cases the 

 inhibition has been said to appear instantaneously, or to reach full magni- 

 tude within 1-2 min; such is the inhibition of succinate dehydrogenase 

 (Fig. 1-12-12) (Slater, 1949), bromelain (Murachi and Neurath, 1960), pyru- 

 vate decarboxylase (Stoppani et al., 1953), and leucine decarboxylase (Sut- 

 ton and King, 1962). Then there are enzymes which require about 5 min 

 for maximal inhibition to develop; examples are catalase (Cook et ah, 1946), 

 transaminases (Grein and Pfleiderer, 1958; Segal et al., 1962) and phospho- 

 glucomutase (Milstein, 1961), although in the last instance only 2 of the 

 3 SH groups react so rapidly. It is interesting to note that Nygaard (1955) 

 has reported marked differences in rates between the mercurials, lactate 

 dehydrogenase being very rapidly inhibited by Hg++ but only slowly by 

 p-MB. The next group of enzymes seems to require about 15-20 min for 

 complete inhibition: 3-phosphoglyceraldehyde dehydrogenase (Boyer and 

 Segal, 1954) and enolase (Malmstrom, 1962) may be cited. These are, of 

 course, arbitrary categories and if one knew the rates of reaction for many 



100 

 80 

 60 

 40 

 20 

 



20 



TIME » 



40 



60 



80 



MIN 



Fig. 7-26. Rates of inhibition of liver alcohol 

 dehydrogenase by p-MB, at pH 7.6 and two 

 different temperatures. ADH = 1.78x IQ-s M, 

 p-MB = 10-« M, and NAD = 3x10"* M. 

 (From WaUenfels et al, 1959.) 



enzymes, there would be a continuous distribution, and furthermore the 

 rate in any particular case will depend on a number of factors, so that the 

 values given above and below must be taken as applying only to the ex- 

 perimental conditions imposed on each enzyme. 



More interesting are those enzymes which react slowly enough with mer- 

 curials for the kinetics to be investigated. Some typical curves for p-MB are 

 given in Figs. 7-26 and 7-27, and similar rate curves have been previously 

 presented for cholinesterase (Fig. 1-12-8) and lactate dehydrogenase (Fig. 

 1-12-11). The results in Figs. 7-26 and 7-27 have been exponentially plotted 



