Salt activation and inhibition of membrane ATPase from roots of the halophyte Atriplex nummularia

Abstract Salt-stimulated ATPase activity in membrane preparations obtained from roots of Atriplex nummularia Lindl. at pH 5 was not suscep-tible to inhibition by KC1 or NaCl up to 450 mol m-3 but showed a broad peak of activity between 150 and 300 mol m^?3. At pH 8 stimulation occurred at 50 mol m^?3 but concentrations above 100 mol m^?3 depressed activity below the level of the MgATPase activity. By contrast, preparations from roots of Pisum sativum L. at pH 5 showed maximal stimulation at 25 to 50 mol m^?3 of NaCl or KC1; concentrations higher than 150 mol m^?3 depressed activity below that of MgATPase activity. At pH 8 maximal stimulation was observed at 5 to 10 mol m^?3 NaCl or KC1 while the threshold for inhibition was reduced to 15 mol m^?3. With increasing salt concentrations the pH profiles for NaCl stimulation of Atriplex ATPase activity (expressed as the difference between treatment and control) showed a progressive displacement of the apparent optimum towards lower pH. The shift was not apparent when stimulation was expressed as a percentage of MgATPase activity. This shift may be accounted for if NaCl stimulated the monovalent salt-activated ATPase activity but simultaneously inhibited MgATPase activity.