Changes of membrane-associated Mg2+-activated ATPase of cucumber roots during calcium starvation

The properties of membrane-associated ATPase of cucumber (Cucumis sativus cv. Seiriki No. 2) roots cultured in a complete medium (complete enzyme) and in a medium lacking Ca²⁺ (Ca²⁺-deficient enzyme) were investigated. The basal activity of membrane-associated ATPase increased during Ca²⁺ starvation, while Mg²⁺-activation of the enzyme decreased and even resulted in inhibition by high Mg²⁺ concentration at the late stage of the Ca²⁺ starvation. The complete enzyme had low basal activity and showed a Mg²⁺-activated hyperbolic reaction curve in relation to ATP concentration. Ca²⁺-deficient enzyme with high basal activity showed a biphasic reaction curve and Mg²⁺-activation was seen only at high ATP concentrations. Activation of membrane-associated ATPase by various cations was decreased or lost during Ca²⁺ starvation. The basal ATPase activity of Ca²⁺-deficient enzyme increased for various substrates including pyrophosphate, p-nitrophenyl phosphate, glucose-6 phosphate, β-glycerophosphate, AMP, ADP and ATP. Mg²⁺-activation was found only for ADP and ATP in both the complete and Ca²⁺-deficient enzymes, but the activation for ATP was greatly reduced by Ca²⁺ starvation. The heat inactivation curves for basal and Mg²⁺-activated ATPase did not differ much between the complete and Ca²⁺-deficient enzyme. The delipidation of membrane-associated enzyme by acetone affected the protein content and the basal activity slightly, but inhibited the Mg²⁺-activated ATPase activity clearly with somewhat different behaviour between the complete and Ca²⁺-deficient enzyme.