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The effect of lysolecithin on the shape of human erythrocytes of varied cholesterol content was examined by scanning electron microscopy. Under the conditions of these experiments, all of the 14Clysolecithin incubated with cells was shown to be located in the external membrane leaflet. The membrane lysolecithin required to induce echinocytosis (spiculation) in normal cells (0.8 mol cholesterol/mol phospholipid) was approximately 0.08-0.10 mumol/10(10) cells, which contributed 1.6-2.0 micrometer 2 or 1% of the cell surface area. This value is consistent with the premise that echinocytosis was caused by a slight differential expansion of the outer surface of the bilayer. The lysolecithin required for echinocytosis decreased as the membrane cholesterol content increased; from 0.14 mumol/10(10) cells at 0.5 mol cholesterol/mol phospholipid to 0.03 mumol/10(10) cells at 1.4 mol cholesterol/mol phospholipid. These data were interpreted in terms of a bilayer couple mechanism. Assuming that the two amphipaths acted additively, the amount of lysolecithin required to induce echinocytosis was used to estimate the partition of cholesterol between the two leaflets of the red cell membrane. A value of about 51:49% in favor of the outer leaflet was found at all cholesterol levels.
Lange et al. (Mon,) studied this question.