Binding, activation, and solubilization of the Ca2+-ATPase from sarcoplasmic reticulum by nonionic detergents

Interactions between delipidated Ca2+-ATPase from sarcoplasmic reticulum and four nonionic detergents--dodecyl octaoxyethyleneglycol monoether (C12E8), Triton X-100, Brij 58, and Brij 35--were characterized with respect to activation of ATPase activity, binding, and solubilization. C12E8 and Triton X-100 activated the delipidated ATPase to at least 80% of the original activity at the critical micelle concentrations (CMCs), whereas Brij 58 and Brij 35 activated no more than 10% of the original activity. The inability of Brij 58 and Brij 35 to activate the delipidated enzyme was probably a result of reduced binding of these detergents below the CMCs; both detergents exhibited a sixteenfold reduction in binding at the CMC compared with C12E8. The two Brij detergents were also unable to solubilize the delipidated enzyme and form monomers, as determined by sedimentation experiments. Thus the reduced binding levels of these detergents may result from an inability to overcome protein/protein interactions in the delipidated preparation. However, the Brij detergents were capable of solubilizing active enzyme from membrane vesicles, although with lower efficiency than C12E8 and Triton X-100. These results suggest that Brij 58 and 35 may be useful for solubilization of membrane proteins without disrupting protein/protein interactions, while Triton X-100 and C12E8 are more useful when bulk solubilization is the goal.