Studies on membrane-bound ATPase activity from soybean cells

A membrane fraction enriched with K^+ -stimulated ATPase and glucan synthetase activity was isolated from soybean cells. This fraction is thought to be enriched with plasma membrane. The ATPase associated with this isolated membrane fraction requires 3 mM Mg^2+ and 3 mM ATP for maximal activity. Maximal K^+ -stimulation of the enzyme occurs at 50 mM KCl in the presence of 3 mM Mg^2+ and 3 mM ATP. A pH of 6.5 was found to be optimal for ATPase activity in the presence of 50 mM KCl and slightly higher without KCl. A 1:1 ratio of Mg^2+ to ATP was found to be optimal for enzyme activity. No clear patterns of monovalent cation or anion stimulation were found, nor were any synergistic monovalent ion stimulations detectable. Only slight variations exist between membrane preparations from soybean callus and root.