The effect of different fatty acid and sterol composition on the erythritol flux through the cell membrane of Acholeplasma laidlawii

Abstract

1. 1. A technique is described whereby the [ 14C]erythritol equilibrium flux through the Acheloplasma laidlawii cell membrane is measured. Cells are separated from the surrounding medium by filtration through membrane filters. it is essential that the A. laidlawii cells are suspended in isotonic NaCl. Cells suspended in sucrose media are fragile and easily damaged by applying a mechanical force such as ultrafiltration under negative pressure. 2. 2. The [ 14C]erythritol equilibrium flux for cells grown on different fatty acids is in the order of cells grown on linoleic acid > oleic acid > elaidic acid steric acid. 3. 3. It is indicated that the permeation of glycerol through the A. laidlawii cell membrane at temperatures in the transition of the membrane lipids predominantly occurs via those parts of the membrane where the fatty acid chains of the membrane lipids are still in the liquid crystalline state. 4. 4. The a. laidlawii cells remain intact upon cooling below the temperature of the phase transition of the membrane lipids. However, at temperatures below the phase transition of the membrane lipids A. laidlawii cells are damaged when mechanical forces such as ultrafiltration or osmotic swelling are applied. 5. 5. The incorporation of cholesterol into the A. laidlawii cell membrane decreases the [ 14C]erythritol flux for cells grown on linoleic, oleic, elaidic and stearic acid. 6. 6. Cholesterol, epicholesterol, cholestanol, epicholestanol, ergosterol, stigmasterol and coprostanol, when present in the growth medium, are all incorporated in the A. laidlawii cell membrane to about the same extent. 7. 7. The 3α-hydroxy isomers epicholesterol and epicholestanol and the sterol with a cis-structured steroid nucleus, coprostanol, do not significantly alter the [ 14C]-erythritol flux through the membrane. 8. 8. The 3β-hydroxy sterols with a flat steroid nucleus cholesterol, cholestanol and ergosterol all decrease the [ 14C]erythritol flux. Stigmasterol does not influence the [ 14C]erythritol flux to a marked extend. 9. 9. For the reduction in permeability of the A. laidlawii cell membrane a 3β-hydroxyl group and a planar configuration of the sterol molecule are essential requirements. This is in good agreement with similar studies using liposomes and monolayers. 10. 10. It is demonstrated that the liquefying effect of cholesterol on the membrane lipids of A. laidlawii can be very important in maintaining proper membrane functioning in growing cells. A 3β-hydroxyl group on the sterol molecule is an absolute requirement for this liquefying effect.