The interaction of lanthanide and calcium salts with phospholipid bilayer vesicles: the validity of the nuclear magnetic resonance method for determination of vesicle bilayer phospholipid surface ratios.

Contrary to a recent report (B. Sears et al., Biochemistry 15 (1976) 1635), it has been determined that the ratio of the number of phospholipids on the inner and outer surfaces of phospholipid bilayer vesicles can be accurately determined by NMR paramagnetic ion shift reagent studies of vesicles. It is concluded that the metal interacts with all of the phospholipid on the exposed bilayer surface. A ratio of outer phospholipid to inner surface phospholipid of 2.1 +/- 0.1 is obtained regardless of the nucleus studied, position of the nucleus relative to the metal ion binding site, molar ratio of metal to phospholipid over three orders of magnitude, or location of the metal ion of the inside or outside of the vesicle. Additionally, P-31 NMR studies using LaC13 and CaC12 indicate that Ca2+ weakly interacts with egg PC vesicles and that the lanthanides are adequate substitutes for Ca2+ since neither metal is found to perturb measurably the average polar head group conformation.