  Sarcoplasmic reticulum vesicles and purified Ca$^{2+}$-ATPase hydrolyze
acetyl phosphate both in the presence and absence of Ca$^{2+}$. The
Ca$^{2+}$-independent activity was fully sensitive to vanadate, insensitive to
thapsigargin, and proceeded without accumulation of phosphorylated enzyme.
Acetyl phosphate hydrolysis in the absence of Ca$^{2+}$ was activated by
dimethyl sulfoxide. The Ca$^{2+}$-dependent activity was partially sensitive to
vanadate, fully sensitive to thapsigargin, and associated with steady
phosphoenzyme accumulation. The Ca$^{2+}$/P(i) coupling ratio at neutral pH
sustained by 10 mm acetyl phosphate was 0.57. Addition of 30% dimethyl
sulfoxide completely blocked Ca$^{2+}$ transport and partially inhibited the
hydrolysis rate. Uncoupling induced by dimethyl sulfoxide included the
accumulation of vanadate-insensitive phosphorylated enzyme. When acetyl
phosphate was the substrate, the hydrolytic pathway was dependent on
experimental conditions that might or might not allow net Ca$^{2+}$ transport.
The interdependence of both Ca$^{2+}$-dependent and Ca$^{2+}$-independent
hydrolytic activities was demonstrated.
