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

Abstract

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.