On the Origin of Sphingolipid/Cholesterol-Rich Detergent-Insoluble Cell Membranes: Physiological Concentrations of Cholesterol and Sphingolipid Induce Formation of a Detergent-Insoluble, Liquid-Ordered Lipid Phase in Model Membranes

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Abbreviations CB, bovine cerebrosides

Gel phase is also resistant to detergent solubilization (Ribiero & Dennis, 1973) but is unlikely to occur in the cholesterol-rich plasma membrane. Interestingly, we found that SM alone was detergent-soluble at 23 °C despite being in the gel phase. This solubility is not a surprise, as insolubility of gel phases can require temperatures well belowTm(Ribiero & Dennis, 1973). We found that pure SM was insoluble at 4 °C (data not shown).

The absolute decrease in light scattering when detergent was added was slightly variable, within the range 0−20% for DPPC/cholesterol and SM/cholesterol and 80−100% for DOPC/cholesterol and 12SLPC/cholesterol. This variation appears to reflect an artifact in light scattering measurements. Centrifugation experiments previously demonstrated the total insolubility of 2:1 (mol/mol) DPPC/cholesterol and total solubility of 2:1 DOPC/cholesterol (Schroeder et al., 1994). In addition, centrifugation and filtration experiments showed that 12SLPC/cholesterol and DOPC/cholesterol were equally detergent-soluble (unpublished observations).