Cholesterol-lipid interactions in membranes. The saturation concentration of cholesterol in bilayers of various lipids.

1. The integration of cholesterol in a lipid bilayer can be visualized by changes in the fluorescence properties of the probe N-phenyl-1-naphthylamine (NPN). An increasing cholesterol content in the lipid phase corresponds to decreasing fluorescence intensity of NPN and a short wave shift of the emission spectrum. 2. Equilibrium constants for the partition of NPN between water and the various lipid phases are reported. An increasing cholesterol content in a bilayer decreases the solubility of NPN in the bilayer. 3. The saturation concentration of cholesterol in bilayers of various lipids prepared by ultrasonication is determined using the flourescence probe NPN. The maximal molar ratio of cholesterol : lipid is 2 : 1 for sphingomyelin or egg phosphatidylcholine and 1 : 1 for cerebroside, dipalmitoyl phosphatidylcholine, or dipalmitoyl phosphatidylethanolamine. 4. The comparison of the maximal molar ratio of cholesterol : lipid with the number of proton donor and proton acceptor sites in the lipid moiety is used for a discussion of the polar interactions of cholesterol within a lipid bilayer.