Interaction of cholesterol with sphingomyelin in monolayers and vesicles.

To understand the structural basis for the apparent strong interaction between cholesterol and sphingomyelin (SPM), we have synthesized an analogue of SPM, 3-deoxy-2-O-stearoyl-SPM, in which an ester-linked acyl chain replaces the amide-linked acyl chain at C-2 and a hydrogen replaces the hydroxy group at C-3. We have compared the behavior of this analogue with that of 3-deoxy-N-stearoyl-SPM in monolayers and vesicles, both as pure phospholipids and in mixtures with cholesterol. The force-area isotherm of 3-deoxy-2-O-stearoyl-SPM was similar to that of 3-deoxy-N-stearoyl-SPM. The surface potential across the pure SPM monolayer at the air-water interface was larger for 3-deoxy-2-O-stearoyl-SPM than for 3-deoxy-N-stearoyl-SPM (about 430 mV and 330 mV, respectively, at 50 A2). The overall dipole moment of 3-deoxy-2-O-stearoyl-SPM was almost constant at 570 mD (between a mean molecular area range of 45-85 A2), whereas that of 3-deoxy-N-stearoyl-SPM was about 420 mD. Cholesterol appeared to be equally miscible in both SPM monolayers, as determined from the condensing effect cholesterol had on the lateral packing of the two SPMs. The oxidation of monolayer cholesterol by cholesterol oxidase was also determined using both SPMs. The stoichiometry at which free cholesterol clusters disappeared in monolayers, when going from high to low cholesterol content, was 2:1 (mol sterol/mol SPM) for both SPMs.(ABSTRACT TRUNCATED AT 250 WORDS)