25-Hydroxysterols increase the permeability of liposomes to Ca2+ and other cations.

25-Hydroxycholesterol and 25-hydroxy vitamin D-3 increased the permeability of liposomes to Ca2+ measured by the arsenazo III encapsulation technique. This effect was sensitive to the lipid composition of the membrane, with changes that decreased the motional freedom of phospholipid acyl chains decreasing Ca2+ permeability. The greatest permeability was observed with the zwitter-ionic phospholipids, phosphatidylcholine and phosphatidylethanolamine, whereas the acidic phospholipids, phosphatidylinositol and phosphatidylserine, depressed Ca2+ permeability. The effect was not specific for Ca2+. Other divalent cations were translocated in the order Mn2+ greater than Mg2+ = Ca2+ much greater than Sr2+ = Ba2+. The permeability of liposomes to the monovalent cation, Na+, was also substantially increased. The effect did not appear to be due to ionophoretic properties of the sterols, and it is suggested that perturbation of the membranes by the polar 25-hydroxyl group may play a role in increasing membrane permeability.