AbstractA nitrate‐sensitive, azide‐insensitive ATPase isolated fromM. crystallinumin the C3and in the CAM state has been solubilized in active form using octylglucoside and Zwittergent 3–14. Like the membrane‐bound tonoplast ATPase, the solubilized ATPase showed an increase in ATP‐hydrolysis activity after transition from the C3to the CAM mode of photosynthesis. The characteristics of the membrane‐bound and the solubilized tonoplast ATPase were comparable with respect to salt stimulation, inhibitor effects, and MgATP2–‐concentration dependence. Differing from the membrane‐bound ATPases, the solubilized ATPase from C3‐ and CAM‐M. crystallinumshowed a pH optimum between pH 6.5 and 7.0.In order to compare the solubilized ATPases immunologically, antibodies were prepared against the tonoplast fraction of C3‐ and CAM‐M. crystallinum. A cross‐reaction was observed between antibodies against the tonoplast ATPase from C3‐ and CAM‐M. crystallinumand the solubilized ATPase from C3‐ and CAM‐M. crystallinum. A cross‐reaction was also observed between antibodies against the tonoplast ATPase from C3‐ and CAM‐M. crystallinumand the solubilized tonoplast ATPase fromKalanchoë daigremontiana. However, there was no cross‐reaction with the solu