Single skeletal muscle cells of Balanus contain 48 ± 1 mmoles magnesium/kg dry weight. Although 28Mg can be shown either to enter the cells or to be bound to the cell surface within less than 10 min, only 2.1 ± 0.3% of cellular or cell surface Mg exchanges with this isotope even after several hours. Glycerinated cells washed out in Tris buffer at low ionic strength retain ∼70% of the Mg present in intact cells. About 85% of this Mg is removed by extraction with KCl or NaCl at concentrations of K and Na which prevail in intact cells, as well as by pyrophosphate, Tris-ATP, or reduction of the ionized Mg concentration to 1 µM. Lowering the ionized Mg concentration to 0.1 µM does not further reduce the Mg content of glycerinated cells. The pH dependence of KCl-inextractable Mg suggests that more than one class of binding sites is involved. A significant fraction of the KCl-inextractable Mg bound to glycerinated cells fails to exchange with 28Mg even after long equilibration. It is suggested that this fraction may be actin-bound Mg incorporated into the thin filaments during the polymerization of actin.

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