868 



7. MERCURIALS 



burst have anything to do with the postulated structural rearrangements 

 of the enzyme following addition of ATP ? In any event, there is evidence 

 that mercurials can alter not only actomyosin stability itself (Gergely et al., 

 1959) but the structure (Kominz, 1961) or flexibility (Levy et al, 1962) of 

 the enzyme near the active center. 



The configurational changes are apparently complex inasmuch as meas- 

 urements of the optical rotatory dispersion indicate that p-MB up to 4 

 moles/ 10^ g myosin A increases the helical content several per cent, but 

 addition of 8 moles/10^ g decreases the hehcal content (Tonomura et al., 

 1963 a). It was suggested that these small changes in the helical structure 



200 



150- 



100 



- 50 



UJ < 



tr > 



02 0.04 



p-MB (/iMOLES/MG) 



006 



0.08 



0.10 



Fig. 7-33. Titration of myosin B ATPase with p-MB and 



the effects on the initial and steady-state rates at pH 6.7 



and 20°. The SH reaction measured by absorption at 255 



m/<. (From Tonomura and Kitagawa, 1960.) 



induce modifications at the active site. If the inactivated enzyme is treated 

 with /5-mercaptoethanol the mercurial is removed, the activity is restored, 

 and the rotatory dispersion returns to normal (Tonomura et al., 1963 b). 

 One might then assume that the effects of the p-MB are completely rever- 

 sible; however, it was found that the substrate inhibition at high concentra- 

 tions of ATP no longer occurs, and that EDTA now does not inhibit. It 

 may be that the treatment with ^j-MB removes divalent cations, possibly 

 the tightly bound Ca++. It was indeed shown that the progressive depression 

 of the ATPase activity is accompanied by a loss of the binding of Ca++ and 

 Mg++ (Martonosi and Meyer, 1964). 



