Critical lipid-protein stoichiometries in erythrocyte membrane reactions governing protection and morphology switching

Echinocytosis and microvesiculation of human erythrocytes induced by insertion of merocyanine 540 into the outer membrane leaflet

Echinocytosis and release of microvesicles from human erythrocytes treated with the impermeant fluorescent dye merocyanine 540 (MC540) has been correlated with the extent of dye binding to intact cells and ghosts. At 20 degrees C binding appeared to saturate at about 9.3.10(6) molecules per cell (3.6 mol/100 mol phospholipid), equivalent to an expansion of the outer leaflet lipid area of about 2.7%. Stage 3 echinocytes were formed upon binding of (3-4).10(6) molecules of MC540/cell (about 1.3 mol/100 mol phospholipid), equivalent to an expansion of the outer leaflet lipid area of about 1.0%. Negligible release of microvesicles was observed with MC540 at 20 degrees C. Binding of MC540 to permeable ghosts was approximately twice that to cells suggesting that there was no selective binding to the unsaturated (more fluid) phospholipids which are concentrated in the inner lipid leaflet of the membrane. At 37 degrees C apparent maximal binding of MC540 was about 3.2 mol/100 mol phospholipid and correlated with the maximal release of microvesicles from the cells as measured by release of phospholipid and acetylcholinesterase. These results are discussed in relation to the bilayer couple hypothesis of Sheetz and Singer (Proc. Natl. Acad. Sci. USA 71 (1974) 4457-4461).

Cholesterol efflux from bovine sperm. I. Induction of the acrosome reaction with lysophosphatidylcholine after reducing sperm cholesterol

Methods were developed to quantitatively reduce the cholesterol (Chol)/phospholipid (PL) ratio of bovine sperm and to determine the effectiveness of this treatment in capacitating sperm. Washed sperm (2 x 10(8)) were incubated in 1.0 ml of modified Tyrode's solution (TS) containing unilamellar liposomes of phosphatidylcholine (PC), phosphatidylethanolamine (PE), and [14C]-Chol (35:35:30 molar ratio, 300 nmol total PL). [3H]-triolein was included as a nonexchangeable marker. After 90 min at 39 degrees C, a 13% net exchange of [14C]-Chol from liposomes to sperm was observed (n = 4), and sperm motility was 80%. Sperm were then washed and 50 x 10(6) sperm were incubated as before with PC/PE liposomes containing no Chol. After 90 min, sperm were separated from liposomes by centrifugation. Measurement of [14C]-Chol in the liposomes (supernatant) and parallel gas chromatographic analysis of extracted, saponified liposomes (n = 4) indicated that 30% of sperm Chol was removed by this procedure. Chol efflux decreased percent motile sperm by less than 10% but reduced sperm velocity by more than 50%. Sperm incubated with no liposomes (control), with liposomes containing Chol (+Chol), and with Chol-free liposomes (-Chol) were washed and resuspended in TS with 0.2% BSA and 30 micrograms lysophosphatidylcholine (LPC)/mg bovine serum albumin (BSA). Percent sperm undergoing the acrosome reaction (AR) upon incubation with LPC-BSA was used as a measure of sperm capacitation. After 60 min of exposure to LPC-BSA at 39 degrees C, the mean (+/- SE) percent motile sperm for control, +Chol, and -Chol treatments was 57.0 +/- 4.9, 60.0 +/- 4.7, and 57.0 +/- 6.8, respectively. Corresponding values for percent AR were 14.0 +/- 3.4, 20.3 +/- 4.4, and 39.7 +/- 1.2. These results suggest that loss of Chol from bovine sperm may be an early step in sperm capacitation in this species.