Effects of formamide on the polymerization and depolymerization of muscle actin

Formamide was found to interfere with the polymerization of electrophoretically pure rabbit skeletal muscle G-actin to F-actin in vitro. It decreased the rate as well as the extent of polymerization. However, this influence was dependent on the way the polymerization reaction was initiated. If polymerization of G-actin was induced by 2 mM MgCl2, formamide inhibited the rate and extent of the polymerization much less than if the polymerization was induced by either 2 mM CaCl2 or 0.1 M KCl. The critical protein concentration was increased when formamide was present. This effect was small in the presence of MgCl2 but an approximately tenfold increase in the critical value was observed for the CaCl2-induced or KCl-induced polymerization. Depolymerization of F-actin by molar amounts of formamide was faster and proceeded further when the polymer had been formed in the presence of KCl or CaCl2 than when it been formed in the presence of MgCl2. It is concluded that Mg2+ stabilizes the F-actin structure rendering it more resistant than either Ca2+ or K+ against the destabilizing action of formamide.