Transmembrane Ca²⁺ Gradient-Mediated Modulation of Sarcoplasmic Reticulum Ca²⁺-ATPase

Ca²⁺-ATPase from skeletal muscle sarcoplasmic reticulum was reconstituted into liposomes with (100-1000 fold) or without transmembrane Ca²⁺ gradient. The highest enzyme activity and Ca²⁺ uptake were observed in the vesicles without transmembrane Ca²⁺ gradient. If there existed transmembrane Ca²⁺ gradient, no matter what the direction was, a lower activity would appear. Dissipation of transmembrane Ca²⁺ gradient by A23187 could lead to a change in enzyme activity of incorporated Ca²⁺-ATPase. A concomitant change of lipid fluidity of proteoliposomes with that of enzyme activity and Ca²⁺ uptake was observed. The inhibition of Ca²⁺-ATPase by the transmembrane Ca²⁺ gradient could be observed in the PC-PE vesicles, but not in the PS-PE or PG-PE proteoliposomes.