The effect of alterations in fatty acid composition and cholesterol content on the nonelectrolyte permeability of Acholeplasma laidlawii B cells and derived liposomes

Abstract

1. 1. The fatty acid composition and cholesterol content of the membrane lipids of Acholeplasma laidlawii B were systematically altered and the rates at which glycerol and erythritol passively diffuse into intact cells and into liposomes prepared from the total membrane lipid were measured at a variety of temperature. 2. 2. The permeability of intact cells and derived liposomes is markedly dependent on the chemical structure and chain length of the fatty acids incorporated into the membrane lipids. Incorporation of branched-chain or unsaturated fatty acids, or fatty acids of reduced chain length, increases nonelectrolyte permeability to a similar extent in both cells and liposomes. 3. 3. The nonelectrolyte permeability of both the plasma and liposomal membrane is reduced by the incorporation of cholesterol. 4. 4. The mean activation energy values calculated for the permeation of glycerol and erythritol into intact cells are 18.2 and 21.3 kcal/mole, respectively. These values are the same, within experimental error, as those calculated for the liposomal system and suggest that glycerol and erythritol permeate both the biological and artificial membrane systems as single, fully dehydrate molecules. 5. 5. In contrast to permeation rates, which are dependent on both permeant structure and membrane lipid composition, activation energy values for the overall permeation process are dependent only on permeant structure and are not sgnificantly affected by variations in fatty acid composition or cholesterol content. 6. 6. The permeability of intact cells and derived liposomes is a function of the fluidity of the membrane lipids are measured by their reversible, thermotropic gel to liquid-crystalline phase transition temperatures. Cells or liposomes placed in isotonic permeant solutons undergo spontaneous lysis when the temperature is reduced to the point where most of the membrane lipids exist in the gel state.