The effects of lipids on the function of the Ca2+-ATPase of sarcoplasmic reticulum

Dalton, Kate Alison (1998) The effects of lipids on the function of the Ca2+-ATPase of sarcoplasmic reticulum. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

The lipid compositions of all biological membranes are complex. The effects of anionic phospholipids on the function of the Ca²⁺ -ATPase of sarcoplasmic reticulum have been studied. Reconstitution of the Ca²⁺-ATPase into bilayers of phosphatidic acid (PA) or phosphatidylserine (PS) leads to the decrease in ATPase activity. Rapid kinetic and equilibrium binding studies show that the major effect of the anionic lipids is a reduction in the maximal level of binding of MgATP. Fluorescence quenching studies show that he anionic phospholipids bind to the bulk phospholipid binding sites around the ATPase with an affinity about equal to that of phosphatidylcholine, showing no selectivity in the lipid-ATPase interaction. Although the presence of high concentrations of anionic phospholipids in the membrane around the ATPase leads to a decrease of ATPase activity, for the ATPase reconstituted into sealed vesicles their presence leads to an increase in the level of accumulation of Ca²⁺. The effects of PA and PS on accumulation of Ca²⁺ was maximal at 10 mol%. Cardiolipin was more effective than PA or PS and oleic acid was less effective. Effects of anionic phospholipids on the membrane potential and movement of H⁺generated when the vesicles accumulate Ca²⁺ was studied. It was concluded that the effects of anionic phospholipids were not on simple passive leak of Ca²⁺ across the membrane but rather on slippage, a process in which the phosphorylated ATPase releases Ca²⁺ to the cytoplasm rather than the lumen of the vesicles.

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