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Effect of Polar Lipids on ATPase Activity of Membrane Preparations from Germinating Radish Seeds ¹

Centro di Studio del Consiglio Nazionale delle Ricerche per la Biologia Cellulare e Molecolare delle Piante, c/o Istituto di Scienze Botaniche, Università di Milano, 20133 Milano, Italy

Membrane preparation (sedimenting between 13,000_g and 80,000_g) of germinating radish seeds (Raphanus sativus L.) was active in hydrolyzing ATP and, to a lesser extent, a variety of other phosphorylated compounds. Dicyclohexylcarbodiimide (DCCD) and diethylstilbestrol significantly inhibited the ATPase activity (40%) while their effect on hydrolysis of other phosphorylated compounds was much less.

The sucrose density gradient analysis of the membrane preparation showed that the position of the DCCD-sensitive K⁺-dependent ATPase was similar to that found for plasma membrane of other plant material.

Cholate treatment of membrane preparation removes almost all phospholipids, and ATPase activity is barely detectable. However, the addition of polar lipids completely restores the ATPase activity but does not restore general phosphatase activity.

The ATPase of the polar lipids restored cholate preparation, showed a high sensitivity to DCCD and diethylstilbestrol (up to 90% inhibition), a complete dependence on Mg²⁺, and a strong dependence on K⁺ at low concentration; the pH optimum of ATPase was close to 6.5, and the K_m for ATP-Mg was 0.51 millimolar. ATPase activity was much greater when polar lipids from 24-hour-germinated seeds were added.