INHIBITION OF ENZYMES 



867 



be exhibited over a wide range of mercurial concentration (Fig. 7-32). The 

 fairly sjonmetrical curves for myosin ATPase, the maximal stimulation 

 being observed at a pH around 7.5, are most likely the result of changes in 

 ionizable groups at or near the active center, whereas the more complex 

 curve for mitochondrial ATPase perhaps arises from additional factors relat- 

 ed to mitochondrial structure or the effects of intramitochondrial compo- 

 nents on ATPase. It may also be mentioned that Tonomura and Furuya 

 (1960) found essentially the same behavior for myosin B ATPase, stimul- 

 ation being maximal at pH 7.8 and absent at 5.7 and 10. 



+ 250- 



•200 



+ 100 



+ 50 



-100 



MITOCHONDRIA 



8 



Fig. 7-32. Effects of pH on the actions of p-MB on 

 ATPase. Liver mitochondrial ATPase treated with p-MB 

 at 0.1 mM (Myers and Slater, 1957 b). Myosin ATPase 

 curve # 1 treated with p-MB at 0.04 /<mole/mg (Stracher 

 and Chan, 1961), and curve #2 treated with p-MB at 

 0.0872 /<mole/mg (Blum, 1962 a). 



When ATP is added to a preparation of myosin ATPase, there is an initial 

 burst of phosphate release, followed by a steady level of hydrolysis. The ef- 

 fects of p-MB on these two phases of activity have been shown to be quite 

 different by Tonomura and Kitagawa (1957, 1960). There is a progressive 

 depression of the magnitude of the initial burst as the SH groups are ti- 

 trated, but the steady rate is accelerated until around 80% of the groups 

 have been combined (Fig. 7-33). The rate of the initial burst may be stim- 

 ulated but the amount of ATP split during this period is reduced. How 

 these observations correlate with the various theories of how mercurials 

 activate ATPase (page 819) is not known; for example, does the initial 



