Non-uniform distribution of phospholipids in (Na+ + K+)-ATPase-rich membranes from Torpedo marmorata electric organ evidenced by spin-spin interactions between spin-labeled phospholipids.

(Na+ + K+)-ATPase membranes from Torpedo marmorata electric organ were labeled with high concentrations of paramagnetic phospholipid analogs (up to 5 mol of spin labels for 100 mol of endogeneous phospholipids). The mobile lipid bilayer component of the complex resonance spectra obtained showed low-field linewidth broadening due to spin-spin interactions; this was used as a measure of probe concentration in this compartment. Our results show that in the lipid shell surrounding the protein, there is a considerable enrichment in negatively charged phospholipids over neutral ones.