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Microsomes of tomato roots were prepared and the activities of microsomal ATPases were measured in order to understand the molecular mechanisms of various ion transports. The activities of plasma membrane H^+-ATPase and vacuolar H^+-ATPase were evaluated to ∼30% and ∼38% of total microsomal ATPase activity by using their specific inhibitor, vanadate and nitrate (NO₃^-), respectively. The inhibitory effects of vanadate and NO₃^- were additive and the simultaneous additions of these two inhibitors decreased the total activity up to 50∼70%. The microsomal ATPase activity was regulated key pH and the maximal activity was obtained at pH 7.4. The activity of microsomal ATPase was increased by K^+ up to ∼30% at the concentration of K^+ above 10 mM. However, the K^+- induced increase in the activity was Completely inhibited by the simultaneous addition of Na^+. To identify the ATPase activity regulated by K^+, the effects of specific inhibitors were measured. Vanadate and NO₃^- inhibited total ATPase activity by 27% and 32% in the absence, of K^+ and by 27% and 40% in the presence of 120 mM K^+, respectively. These results suggest that K^+ increases the activity of NO₃^--sensitive vacuolar H^+-ATPase but not that of vanadate-sensitive plasma membrane H^+-ATPase since vanadate has no effect on K^+-induced increase in ATPase activity. The microsomal ATPase activity was also decreased by increasing Ca^(2+) concentration. Interestingly, NO₃^- blocked the Ca^(2+)-induced inhibition of microsomal ATPase activity; however, vanadate had no effect. These results imply that vacuolar H^+-ATPase is activated by K^+ and inhibited by Ca^(2+).
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