The Mg2+-activated ATPase activity of reconstituted actomyosin at high ionic strengths (about 0.10 M KCl) is stimulated when Na+ is substituted for K+. This phenomenon does not appear to reflect the non-specific structure-disrupting effects seen with higher concentrations of these alkali metal ions because inhibition of actomyosin ATPase followed the order Cs+ > Li+ > Rb+ = K+ > Na+. Alkali metal sensitivity was similar in the case of actomyosins reconstituted from tropomyosin-free actin preparations and from actin preparations that included the tropomyosin-containing protein complex that sensitizes actomyosin to Ca2+. Modification of the myosin, effected by blocking some of the more reactive SH groups, abolished the sensitivity of the subsequently reconstituted actomyosin to substitution of K+ by Na+. Sensitivity to replacement of K+ by Na+ was seen only during the clearing phase at higher ionic strengths, and no significant differences between the effects of these alkali metal ions were seen at lower ionic strengths or after superprecipitation. These different actions of the alkali metal ions thus appear to be upon the association of actin and myosin in the presence of Mg2+. © 1968.
