Studies on Phospholipid Mono- and Bilayers

Abstract Lipids play a fundamental part in the lipid-protein network of biological membranes. Some of the variations in biological membranes may be brought about by differences in the chemical structure of the lipid constituents. A study was made by the interfacial force-area characteristics of saturated and highly unsaturated phospho-lipids and lyso phosphatidyl compounds. The action of some of these phospholipids on lipid bilayers has also been studied. Comparisons were made between the interfacial behavior of in-dividual phospholipid species which were chem-ically synthesized and phospholipids from natural sources. The influence of diets on the force-area characteristics of liver lecithins has been studied. From studies of mixed monolayers of cholesterol and phospholipids it was found that the mean area per molecule in mixed films of cholesterol with (l,2-distearoyl)-3-lecithin and (1,2-dideca-noyl)-3-lecithin at 22C followed practically the additivity rule. A condensing effect of cholesterol was evident with films of (1-stearoy1-2-1auroy1)-3-1ecithin, (l,2-ditetradecanoyl)-3-lecithin, (1-stearoyl-2-oleoyl)-3-lecithin, and the correspond-ing ethanolamine analogue as well as with (l,2-dioleoyl)-3-lecithin at 22C. At 5C the con-densation effect with (l,2-ditetradecanoyl)-3-lecithin was much reduced. The expanded films of synthetic lecithins and phosphatidylethanolamines containing linoleic or linolenic acid showed no appreciable condensation effects with cholesterol. The behavior of the mixed-phospholipid films is governed by a number of factors, including van der Waals interactions, configurational entropy effects, and alterations in the structure of water adjacent to the monolayers. These factors depend on chain length and degree of unsaturation. Polyene antibiotics are found to lyse fungi, protozoa, and erythrocytes, but bacteria proto-plasts and blue-green algae are not. Cholesterol as well as lecithin were found by some authors to re-duce the effective polyene antibiotic concentration. It therefore seemed desirable to determine whether polyene antibiotics can interact with lipids other than sterols. Filipin, nystatin, amphotericin B, etruscomycin, and pimaricin readily penetrate monolayers of cholesterol and ergosterol at initial surface pressures greater than the collapse pres-sure of the antibiotics. Under the same conditions there was essentially no interaction with a variety of pure synthetic phospholipids unless sterol was present. Filipin did not penetrate monolayers prepared from polyene-insensitive bacteria. The increase in surface pressure of mixed films of phospholipid and cholesterol after the injection of filipin was highly dependent on the relative quantity of sterol as well as on the molar ratio lipid-polyene antibiotic. From the results of mono-and bilayer experiments which are in good agree-ment with the physiological experiments, it is concluded that cholesterol is a necessary mem-brane constituent for the polyene antibiotic action.