354 



3. .V-ETHYLMALEIMIDE 



half the SH groups are reacted, whereas 59-MB effectively inhibits this 

 transformation. Furthermore, iV-ethylmaleimide does not significantly alter 

 the viscosity or the Ca++-binding power of actin (Barany et al., 1962). 

 The lack of effect of iV-ethylmaleimide on the polymerization of actin was 

 confirmed by Tonomura and Yoshimura (1962) and Drabikowski and 

 Gergely (1963). There are apparently at least three classes of SH groups 

 in actin; one type is involved in polymerization, one type in ATP binding, 

 and the remainder do not function in either. It seems that only the third 

 type reacts with A^-ethylmaleimide. Katz and Mommaerts (1962) found 

 that two of the total of six SH groups of actin react readily with iV-ethyl- 

 maleimide, the other four reacting only if the protein is denatured. 



Fig. 3-3. Effects of iV-ethylnialcimide on myosin 



ATPase at pH 7.2 and after an exposure of 2 hr. 



(Modified from Kielley and Bradley, 1956.) 



Myosin SH groups react with A^-ethylmaleimide at a rate dependent on 

 the pH but only a fraction is accessible (Kielley and Bradley, 1956). In- 

 hibition of the EDTA-activated ATPase occurs readily and completely, at 

 a time when about one fourth the SH groups have reacted, but the Ca++- 

 activated ATPase activity is only augmented by iV-ethylmaleimide (Fig. 

 3-3). This situation is similar to the response to mercurials (Fig. II-7-23). 

 Blum (1962 a, b) found a pattern of activation and inhibition of the ATPase 

 activity of actomyosin, with respect both to time and to concentration 



