Organization of Lipids in Sarcoplasmic Reticulum Membrane and Ca²⁺-dependent ATPase Activity

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The organization of lipids in sarcoplasmic reticulum membrane was studied with a variety of stearic spin labels and a phosphatidylcholine spin label. The ESR spectra of the spin-labeled membranes consisted of two components, one due to labels in lipid bilayer structure and the other due to more immobilized labels. The relative intensity of the immobilized component increased when the lipid content of the membrane was decreased by treatment with phospholipase A [EC 3. 1. 1. 4] and subsequent washing with bovine serum albumin. Membrane containing 30% of the intact phospholipid, i.e. 0.15mg of phospholipid per mg of protein, showed a spectrum consisting only of the immobilized component (the overall splitting ranged from 58.5G to 60.5G). The immobilized component was ascribed to lipids complexed with protein. The fraction of lipids in the two different organizations was deterined from the ESR spectrum. The activity of the Ca²⁺-Mg²⁺ dependent ATPase [ATP phosphohydrolase, EC 3. 6. 1. 3] was found to increase almost linearly with the lipid bilayer content in the membrane, whereas phosphoenzyrne formation was almost independent of the bilayer content. This indicates that the bilayer structure is necessary for the ATPase to attain its full transport activity.