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Insolubilization and activation of arterial actomyosin by bivalent cations.

1 
Under proper conditions of ionic strength, bivalent cation concentration and protein density, arterial actomyosin exhibits a contractile ATPase in vitro; it is activated by Mg2+ and trace concentrations of Ca2+ ions, and is insoluble in the presence of ATP. 
 
2 
This activation of arterial ATPase has been correlated to the physical state of the actomyosin (sol or gel); those parameters which maximally activate the enzyme promote its insolubilization in ATP. 
 
3 
At Mg2+ concentrations of 0–2 mM, natural actomyosin shows a low enzymatic activity and no sensitivity to trace Ca2+ ions, while at 10 mM Mg2+ the activity is maximal, and that in the presence of trace Ca2+ is four times that in the absence of trace Ca2+. 
 
4 
This activation of natural actomyosin by Mg2+ is a direct effect of the Mg2+ ions on the solubility of the actomyosin, and does not operate through the intermediacy of reduced free ATP. 
 
5 
Ca2+, Mn2+, and Cd2+ ions also insolubilize and activate natural actomysin, which is not a contractile ATPase in these cases. 
 
6 
At low ionic strengths, 0.05 to 0.10, natural actomyosin is activated by and sensitive to traces of Ca2+ ions, while at 0.15 it is inactive and insensitive to trace Ca2+. Low ionic strength promotes the aggregation of actin and myosin filaments, while high ionic strength has a dissociating effect. 
 
7 
High protein density promotes the interaction of actin and myosin molecules and thus has an activating effect on the specific ATPase of arterial actomyosin, and increases its sensitivity to trace C2+. Lowered protein density results in a lower specific ATPase and a loss of Ca2+ sensitivity. 
 
8 
These ionic strength and density effects must also reflect the state of activation of the enzyme by bivalent cations.