Interaction of cholesterol with sphingomyelin in mixed membranes containing phosphatidylcholine, studied by spin-label ESR and IR spectroscopies. A possible stabilization of gel-phase sphingolipid domains by cholesterol

The ESR spectra from different positional isomers of sphingomyelin and phos phatidylcholine spin-labeled in their acyl chain have been studied in sphin gomyelin(cerebroside)-phosphatidylcholine mixed membranes that contain chol esterol, The aim was to investigate mechanisms by which cholesterol could s tabilize possible domain formation in sphingolipid-glycerolipid membranes. The outer hyperfine splittings in the ESR spectra of sphingomyelin and phos phatidylcholine spin-labeled on the 5 C atom of the acyl chain were consist ent with mixing of the components, but the perturbations on adding choleste rol were greater in the membranes containing sphingomyelin than in those co ntaining phosphatidylcholine. Infrared spectra of the amide I band of egg s phingomyelin were shifted and broadened in the presence of cholesterol to a greater extent than the carbonyl band of phosphatidylcholine, which was af fected very little by cholesterol. Two-component ESR spectra were observed from lipids spin-labeled on the 14 C atom of the acyl chain in cholesterol- containing membranes composed of sphingolipids, with or without glycerolipi ds (sphingomyelin/cerebroside and sphingomyelin/cerebroside/phosphatidylcho line mixtures). These results indicate the existence of gel-phase domains i n otherwise liquid-ordered membranes that contain cholesterol. In the gel p hase of egg sphingomyelin, the outer hyperfine splittings of sphingomyelin spin-labeled on the 14-C atom of the acyl chain are smaller than those for the corresponding spin-labeled phosphatidylcholine, In the presence of chol esterol, this situation is reversed; the outer splitting of 14-C spin-label ed sphingomyelin is then greater than that of 14-C spin-labeled phosphatidy lcholine, This result provides some support for the suggestion that transbi layer interdigitation induced by cholesterol stabilizes the coexistence of gel-phase and "liquid-ordered" domains in membranes containing sphingolipid s.