Stability differences of muscle F-actin in formamide in the presence of Mg2+ and Ca2+.

Muscle G-actin was polymerized by addition of 2 mM Mg2+ or 2 mM Ca2+. Subsequent addition of formamide reduced the specific viscosity of the polymer solution. However, kinetic analysis of this reduction in the presence or absence of 0.1 M KCl revealed differences between F-actin formed in the presence of Mg2+ and F-actin formed in the presence of Ca2+. In the presence of Mg2+ the viscosity dropped instantaneously, reaching within minutes a steady-state level that was constant for many hours. In contrast, in the presence of Ca2+ the high-shear viscosity continued to decrease slowly after an initial drop, and it could take hours until a quasi-equilibrium was obtained. The time was dependent on both formamide and protein concentration. Addition of formamide increased the critical actin concentration in the presence of Ca2+, but not in the presence of Mg2+. This is taken as evidence that in the presence of Ca2+, but not in the presence of Mg2+, formamide causes partial depolymerization of F-actin.