Influence of fatty acid and sterol composition on the lipid phase transition and activity of membrane-bound enzymes in Acholeplasma laidlawii

Abstract

1. 1. The temperature dependency of NADH-oxidase, p- nitrophenylphosphatase and Mg 2+-dependent ATPas was studied in Acholeplasma laidlawii cell membranes with varying fatty acid and sterol composition. In these membranes the gel → liquid crystalline phase transition of the membrane lipids was measured by differential scanning calorimetry. 2. 2. The Arrhenius plots of the NADH oxidase and p- nitrophenylphosphatase activities showed no discontinuities although the membrane lipids underwent a phase transition in the temperature range studied. 3. 3. Distinct breaks in the Arrhenius plot of the ATPase activity were observed and found to be dependent upon the fatty acid composition of the membrane lipids; these breaks occurred at the lower ends of the lipid phase transitions. We conclude from these results that at these temperatures the ATPase is associated with molecular lipid species with the lowest transition temperature. 4. 4. Incorporation of cholesterol into the membrane decreased the temperature of the break in the ATPase activity and the temperature of the lower end of the lipid phase transition. This effect is due to the lipid-cholesterol interaction since (a) the effect is reversed by the polyene antibiotic filipin which complexes the cholesterol and (b) incorporation of epichoolesterol which does not interact with other lipids did not influence the temperature of the break in the Arrhenius plot.