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J. Biol. Chem., Vol. 277, Issue 41, 38127-38132, October 11, 2002

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Dissecting the Hydrolytic Activities of Sarcoplasmic Reticulum ATPase in the Presence of Acetyl Phosphate^*

Fernando Soler, Maria-Isabel Fortea, Antonio Lax, and Francisco Fernández-Belda

From the Departamento de Bioquímica y Biología Molecular A, Edificio de Veterinaria, Universidad de Murcia, Campus de Espinardo, 30071 Murcia, Spain

Sarcoplasmic reticulum vesicles and purified Ca²⁺-ATPase hydrolyze acetyl phosphate both in the presence and absence of Ca²⁺. The Ca²⁺-independent activity was fully sensitive to vanadate, insensitive to thapsigargin, and proceeded without accumulation of phosphorylated enzyme. Acetyl phosphate hydrolysis in the absence of Ca²⁺ was activated by dimethyl sulfoxide. The Ca²⁺-dependent activity was partially sensitive to vanadate, fully sensitive to thapsigargin, and associated with steady phosphoenzyme accumulation. The Ca²⁺/P_i coupling ratio at neutral pH sustained by 10 mM acetyl phosphate was 0.57. Addition of 30% dimethyl sulfoxide completely blocked Ca²⁺ 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 Ca²⁺ transport. The interdependence of both Ca²⁺-dependent and Ca²⁺-independent hydrolytic activities was demonstrated.

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