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Cation-stimulated Adenosine Triphosphatase Activity and Cation Transport in Corn Roots

Department of Plant Sciences, University of California, Riverside, California 92502

ATPase activity of the plasma membrane fraction from primary roots of corn (Zea mays L. WF9 x M14) was activated by Mg²⁺ and further stimulated by monovalent cations (K⁺ > Rb⁺ > Cs⁺ > Na⁺ > Li⁺). K⁺-stimulated activity required Mg²⁺ and was substrate-specific. Maximum ATPase activity in the presence of Mg²⁺ and K⁺ was at pH 6.5 and 40 C. Calcium and lanthanum (<0.5 mM) were inhibitors of ATPase, but only in the presence of Mg²⁺. Oligomycin was not an inhibitor of the plasma membrane ATPase, whereas N,N'-dicyclohexylcarbodiimide was. Activity showed a simple Michaelis-Menten saturation with increasing ATP·Mg. The major effect of K⁺ in stimulating ATPase activity was on maximum velocity. The kinetic data of K⁺ stimulation were complex, but similar to the kinetics of short term K⁺ influx in corn roots. Both K⁺-ATPase and K⁺ influx kinetics met all criteria for negative cooperativity. The results provided further support for the concept that cation transport in plants is energized by ATP, and mediated by a cation-ATPase on the plasma membrane.

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