title: Plasma Membrane Ca-ATPase of Radish Seedlings : I. Biochemical Characterization Using ITP as a Substrate.
abstract: 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 Ca(2+)-dependent. The Ca(2+)-dependent ITPase activity was Mg(2+)-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 Ca(2+) 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 Ca(2+) was about fourfold higher at pH 7.5 than at pH 6.9. The Ca(2+)-dependent ITPase activity was stimulated about twofold by polyoxyethylene 20 cetyl ether, although it was inhibited by Triton X-100 and by lysolecithin.
authors: 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.
authors: A
authors: A
authors: Carnelli
authors: M I
authors: MI
authors: De Michelis
authors: F
authors: F
authors: Rasi-Caldogno