Analysis of membrane halves: cholesterol.

Membrane splitting by freeze-fracture has been used as a preparative tool for chemical analysis of outer and inner "half"-membranes. In a previous report I showed that monolayers of human erythrocytes, bound to cationized glass, fracture nonrandomly, producing membrane fractions substantially enriched in outer or inner "halves." For the present study cells were used in quantities compatible with microanalysis. For quantitation the total amount of membrane present and the fractional enrichment of outer and inner "half"-membranes were determined. Cholesterol was examined by quantitative thin-layer chromatography modified to assay nanogram amounts. Comparison of lipids extracted from intact membranes with lipids from fractured membranes indicated that cholesterol was asymmetrically distributed across the plane of the membrane, more being present on the exterior side than on the interior side.