Characterization of the Na + ‐stimulated ATPase of Propionigenium modestum as an enzyme of the F 1 F 0 type

The ATP‐hydrolyzing activity of Propionigenium modestum was extracted from the membranes with Triton X‐100 or by incubation with EDTA at low ionic strength. The ATPase in the Triton extract was highly sensitive to dicyclohexylcarbodiimide but not to vanadate. These properties are characteristic for enzymes of the F 1 F 0 type. The ATPase was specifically activated by Na + ions yielding a 15‐fold increase in catalytic activity at 5 mM Na + concentration. The additional presence of 1% Triton X‐100 caused a further 1.5–fold activation. In the absence of Na + Triton stimulated the ATPase about 13‐fold. The Triton‐stimulated ATPase was further activated about 1.5–2‐fold by Na + addition. The ATPase extracted by the low‐ionic‐strength treatment was purified to homogeneity by fractionation with poly(ethylene glycol) and gel chromatography. The enzyme had the characteristic F 1 ‐ATPase subunit structure with M r values of 58000 (α), 56000 (β), 37600 (γ), 22700 (δ), and 14000 (ɛ). The F 1 ‐ATPase was not stimulated by Na + ions. The membrane‐bound ATPase was reconstituted from the purified F 1 part and F 1 ‐depleted membranes, thus further indicating an F 1 F 0 structure for the ATPase of P. modestum . Upon reconstitution the ATPase recovered its stimulation by Na + ions, suggesting that the binding site for Na + is localized on the membrane‐bound F 0 part of the enzyme complex.