DIFFERENTIAL SCANNING CALORIMETRIC STUDY OF THE INTERACTION OF CHOLESTEROL WITH THE MAJOR LIPIDS OF THE ACHOLEPLASMA-LAIDLAWII-B MEMBRANE

It has been proposed that the lower levels of exogenous cholesterol in corporation into the membranes of the sterol-non-requiring as compared to the sterol-requiring mycoplasmas may be due to the much higher gly colipid content of the former and to the reduced ability of glycolipid s, as opposed to phospholipids, to incorporate sterols [Efrati et al. (1986) Arch. Biochem. Biophys. 248, 282-288]. In order to test this hy pothesis, we have investigated the interaction of cholesterol with the major membrane glyco- and phospholipids of the sterol-non-requiring m ycoplasma Acholeplasma laidlawii B, utilizing elaidic acid-homogenous membranes in order to obviate any differences in the nature of cholest erol-lipid interactions due to variations in the fatty acid compositio n of the different membrane components. Specifically, we have studied the effect of increasing quantities of cholesterol on the thermotropic phase behavior of aqueous dispersions of phosphatidylglycerol, digluc osyl diacylglycerol, and monoglucosyl diacylglycerol, as well as the t otal membrane polar lipids of this organism, using high-sensitivity di fferential scanning calorimetry. We find that cholesterol is highly mi scible in both the lamellar gel and liquid-crystalline states of phosp hatidylglycerol but exhibits limited miscibility in the two neutral gl ycolipids, particularly in their lamellar gel and crystalline states. We also demonstrate that cholesterol has a limited miscibility in both the lamellar gel and liquid-crystalline states of bilayers composed o f the total A. laidlawii B membrane polar lipids. These results demons trate that the nature of cholesterol-lipid interactions depends marked ly on the structure of the glycerolipid polar headgroup and suggests t hat the incorporation of lower levels of cholesterol into the membrane s of the sterol-non-requiring mycoplasmas may indeed be due, at least in part, to their high glycolipid contents. We also show that choleste rol stabilizes the lamellar liquid-crystalline phase of the monoglucos yl diacylglycerol relative to the inverted hexagonal phase at all ster ol concentrations, in contrast to the effects of cholesterol on dielai doylphosphatidylethanolamine, which destabilizes the lamellar liquid-c rystalline phase at low concentrations.