Spin-label studies on the origin of the specificity of lipid-protein interactions in sodium-potassium ATPase membranes from Squalus acanthias

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Abstract
The pH dependence and salt dependence of the lipid-protein interactions of phosphatidic acid, phosphatidylserine and stearic acid with Na+, K+-ATPase membranes from S. acanthias were studied with spin-labeled electron spin resonance spectroscopy, using lipids with nitroxide labels on the 14-position C atom of the sn-2 chain. For phosphatidic acid and stearic acid, the fraction of motionally restricted spin-label increases with increasing pH, with pKa of 6.6 and 8.0, respectively. In contrast, the pKa of stearic acid in the bulk lipid environment of the membrane is estimated from spin-label spectroscopy to be .apprxeq. 6.6. The fraction of motionally restricted phosphatidylserine spin-labeled phosphatidic and stearic acids decrease with increasing salt concentration, reaching an approximately constant value at [NaCl] = 0.5-1.0 M. For stearic acid the net decrease is comparable to that obtained on protonation, but for phosphatidic acid the decrease is considerably smaller (by .apprx. 55%) than that obtained on protonating the lipid. The fraction of motionally restricted phosphatidylserine spin-label varies relatively little with salt concentration up to 1 M NaCl. Direct electrostatic effects alone cannot account for the whole of the observed specificity of interaction of the 2 phospholipids with Na+, K+-ATPase membranes.

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