Characterization of Plasma Membrane-associated Adenosine Triphosphase Activity of Oat Roots

ATPase activity of plasma membranes isolated from oat (Avena sativa L. cv. Goodfield) roots was activated by divalent cations (Mg(2+) = Mn(2+) > Zn(2+) > Fe(2+) > Ca(2+)) and further stimulated by KCl and a variety of monovalent salts, both inorganic and organic. The enzyme exhibited greater specificity for cations than anions. The presence of Mg(2+) was necessary for KCl stimulation. Ca(2+) was ineffective in replacing Mg(2+) for activation of plasma membrane ATPase, but it did activate other membrane-bound ATPases. The pH optima for Mg(2+) activation and KCl stimulation of the plasma membrane ATPase were 7.5 and 6.5, respectively.The plasma membrane ATPase showed little synergistic effects of K(+) and Na(+), and it was only slightly sensitive to ouabain. Oligomycin did not inhibit the ATPase, while N,N'-dicyclohexylcarbodiimide was a potent inhibitor of the enzyme.The apparent Km for Mg(2+) activation (0.84 mm) of the ATPase was about twice that of the apparent Km for ATP (0.38 mm). The effect of KCl in stimulating the enzyme was not on the apparent Km values for ATP and Mg(2+) but rather on maximum velocity. The kinetics of KCl stimulation of the plasma membrane ATPase were similar to the kinetics of (42)K(+) influx into oat roots and neither followed the Michaelis-Menten equation but rather were best described by a single activity curve with continually changing kinetic parameters. These results support the concept that cation transport at the plasma membrane of root cells is coupled to a cation-activated ATPase which is functional from low (0.01 mm) to high (50.0 mm) concentrations of KCl.