Phospholipid composition of sarcoplasmic reticulum (SR) isolated from rat hearts was changed by lipid titration method. SR was labeled with a hydrophobic fluorophore, anilinonaphtyl maleimide. The dynamic microstructure of the labeled portion of SR was studied with a time-resolved fluorophore. The lipid titration with phospholipids having shorter acyl chains increased the oscillation of the labeled portion of SR. Concurrently the Ca²⁺-ATPase activity decreased. The appropriate dynamic microstructure of SR protein which is maintained by suitable phospholipids seems to be necessary for the high activity of Ca²⁺-ATPase.

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