The effect of cholesterol on glycerophosphono- and glycerophosphinocholines. Permeability measurements in lipid vesicles.

The kinetics of spontaneous chloride ion efflux and valinomycin-mediated rubidium-86 efflux from vesicles prepared from synthetic phospholipids with carbon-phosphorus linkages were investigated at temperatures above the gel-to-liquid-crystalline phase transition. The rate constants for the movement of chloride and rubidium ions were reduced by incorporation of cholesterol into bilayers of phosphono- and phosphinocholines. Nonisosteric phosphonolipids in which the oxygen was removed from the glycerol side of phosphorus without substitution by a methylene group interacted less with cholesterol than the analogous isosteric derivatives, as judged from the magnitude of the decrease in the rate constants for chloride and rubidium ion efflux. The experiments reported in this study suggest that steric factors in the glycerol side of the phosphorus function are important in phosphatidylcholine-cholesterol interaction. However, the oxygen atom on the choline side of the phosphorus in the phosphatidylcholine molecule is not required for strong phosphatidylcholine-cholesterol interaction, since isosteric glycerophosphinocholines interacted as well as the corresponding isosteric glycerophosphonocholines. Furthermore, steric requirements on the choline side of phosphorus are not important in this interaction since phosphinates whose head-group structures are -P(O-)CH2CH2N+(CH3)3 and -P(O-)CH2CH2CH2N+(CH3)3 interacted equally well with cholesterol, as estimated by these permeability studies.