Transmembrane Ca2+ gradient-mediated modulation of sarcoplasmic reticulum Ca(2+)-ATPase.

Ca(2+)-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 Ca(2+)-ATPase. A concomitant change of lipid fluidity of proteoliposomes with that of enzyme activity and Ca2+ uptake was observed. The inhibition of Ca(2+)-ATPase by the transmembrane Ca2+ gradient could be observed in the PC-PE vesicles, but not in the PS-PE or PG-PE proteoliposomes.