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In order to characterize the effects of heavy metal ions on the microsomal ATPase activities, microsomes were prepared from the roots of tomato plant and the activity of microsomal ATPase was measured by an enzyme-coupled assay. Hg^(2+) inhibited the activity of microsomal ATPase in a dose-dependent manner, while Gd^(3+), Fe^(3+), La^(3+), Zn^(2+), and Pb^(2+) inhibited not only the ATPase activity but also the activities of enzymes used in the assay. However, Cs^+ and Ba^(2+) showed no significant effect. Hg^(2+) inhibited the activities of both plasma membrane and vacuolar membrane H^+-ATPases. In the dose-response to Hg^(2+), the activities of both microsomal H^+-ATPases were severely inhibited at the concentration of Hg^(2+) above 10 μM and were completely inhibited at 1 mM Hg^(2+). Apparent Ki values of Hg^(2+) on the inhibitions of plasma membrane and vacuolar membrane H^+-ATPases were 8 μM and 58 μM, respectively. The Hg^(2+)+-induced inhibitions were reversible since the addition of dithiothreitol completely reversed the inhibitory effects of Hg^(2+). These results suggest that the inhibitory effects of Hg^(2+) on both plasma, membrane and vacuolar membrane H^+-ATPases are nonselective and reversible.
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