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Membranes and Bioenergetics

Plasma Membrane Ca-ATPase of Radish Seedlings ¹

I. Biochemical Characterization Using ITP as a Substrate

Centro di Studio del Consiglio Nazionale delle Ricerche sulla Biologia Cellulare e Molecolare delle Piante, Dipartimento di Biologia, Università di Milano, Via Celoria 26, 20133 Milano, Italy, Istituto di Botanica, Università di Messina, C.P. 58, 98166 Messina-S. Agata, Italy

In this work, we exploited the capability of the plasma membrane Ca-ATPase to utilize ITP as a substrate to study its characteristics in plasma membrane vesicles purified from radish (Raphanus sativus L.) seedlings. The majority of the ITPase activity of plasma membrane was Ca²⁺-dependent. The Ca²⁺-dependent ITPase activity was Mg²⁺-dependent and was stimulated by the calcium ionophore A23187. It was inhibited by erythrosin B (concentration giving 50% inhibition, 50 nanomolar) and by vanadate (concentration giving 50% inhibition, 3 micromolar) and displayed a broad pH optimum around pH 7.2 to 7.5. Both the hydrolytic and the transport activity of the plasma membrane Ca-ATPase were half-saturated by Ca²⁺ in the micromolar concentration range. No major effect of EGTA on the saturation kinetics of the enzyme was observed. The affinity of the plasma membrane Ca-ATPase for Ca²⁺ was about fourfold higher at pH 7.5 than at pH 6.9. The Ca²⁺-dependent ITPase activity was stimulated about twofold by polyoxyethylene 20 cetyl ether, although it was inhibited by Triton X-100 and by lysolecithin.

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