Enhancement of Adenosine Triphosphatase Activity of Purified Chloroplast Coupling Factor 1 in an Aqueous rganic Solvent1

Abstract

The ATPase activity of purified coupling factor 1 (CF₁) of spinach chloroplasts [EC 3.6.1.3] was reversibly enhanced in some aqueous organic solvents, notably methanol, ethanol, and acetone. Pretreatment of CF₁ with 20% (v/v) methanol did not affect the subsequent activity. The activity depended entirely on the final concentration of methanol in the reaction mixture. In the presence of 20% methanol, the K_m of Ca²⁺-ATPase for ATP was lowered from 0.4 mM to 0.2 mM. Not only Ca²⁺, but also Cd²⁺, Mg²⁺, Mn²⁺, and Zn²⁺ supported the ATPase activity at rates of higher than 7 μmol.mg protein⁻¹.min⁻¹. Co²⁺, Ni²⁺, and Pb²⁺ supported the activity at rates of 0.5-1.0 μ-mol-mg protein⁻¹.min⁻¹. The activities supported by the following cations, if any, were less than 0.2μmol-mg protein¹⁻-min⁻¹; Ba²⁺, Cu²⁺, Fe²⁺, Hg²⁺, Sn²⁺, and Sr²⁺. The optimum concentration of methanol for Ca²⁺-ATPase and Mg²⁺-ATPase activities was about 30% (v/v). The optimum pH values for Ca²⁺-ATPase and Mg²⁺-ATPase activities were about 8.0 and 8.8, respectively. The enhancing effect of organic solvents appears to be associated with their relative lipophilic character as defined by the octanol-water partition coefficient. The Ca²⁺-ATPase activities of the trypsin-activated and the heat-activated CF₁ were inhibited and their Mg²⁺-ATPase activities were enhanced by the presence of methanol in the reaction mixture.