Donald M. Small

A classification of biologic lipids based upon their interaction in aqueous systems

A classification of lipids is presented, based upon their physical properties in bulk aqueous systems and at the air-water or oil-water interface. This is supported by binary-phase diagrams of the various classes of lipids in water.The interactions of the lipids of each class with a lipid of another class is illustrated by a series of different ternary-phase diagrams of two lipids in water. The various types of association and the molecular relation of one lipid to another are indicated. The interaction of three classes of lipids with water is illustrated in two examples by quaternary-phase diagrams of the three lipids in water.As an example of the application of these invitro studies, the composition of bile is correlated with a quaternary-phase diagram cholesterollecithin-bile salt-water. The correlation shows that human bile behaves as a biologic four-component system the physical state of which is entirely predictable from the quaternary-phase diagram. Although bile is a special case, it is probable that the physical arrangement of the lipids in membranes, cellular organelles, lipoproteins, and adipose tissue can be suggested by studies of the interaction of lipid classes with themselves in water.

Lyotropic Paracrystalline Phases Obtained With Ternary and Quaternary Systems of Amphiphilic Substances in Water: Studies on Agreous Systems of Lecithin, Bile Salt and Cholesterol

Abstract Lyotropic liquid crystals given by binary systems have been known for a long time to investigators in the soap and detergent field, mainly by the work of McBain and his school.1, 2 The first analysis by X-ray diffraction of soap mesophases was done by Stauff3 and by Kiessig and Philippoff.4 Liquid crystals obtained with phospholipids in the presence of water were examined by X-ray analysis already in 1941 by Schmitt et al.5 More recently Luzzati et al.6 defined the structure of the different mesophases with more precision.

Thermal Transitions of Cholesteryl Esters of C18 Aliphatic Acids

Abstract The studying of physical properties for choloesteryl esters of C18 aliphatic acids is of both fundamental and practical interest. The physical chemistry is of interest because of the mesophase or liquid crystal behavior that can be potentially exhibited, by analogy with other esters of cholesterol. Morevoer, systematic studies on these esters are rare.(1,2)) From a practical view, the cholesteryl ester content of human β-lipoprotein blood serum is ∼24.1% cholesteryl oleate (18:1) and ∼46.8% cholesteryl linoleate (18:2).(3) Recent studies at the Massachusetts Institute of Technology have indicated that measuring the amounts of cholesterol-bearing β-lipoprotein may be more important than measuring the amount of blood cholesterol itself in characterizing coronary heart disease.(4) β-Lipoprotein contains cholesterol predominantly in the esterified state, congtaining only about 11.5% free cholesterol compared with 58.2% esterified cholesterol.(3)

Cholesterol content of red blood cells and low-density lipoproteins in hypertriglyceridemia.

The red blood cells and the low-density lipoproteins in hypertriglyceridemia have a lower ratio of unesterified cholesterol to phospholipid than normal. The low-density lipoproteins are also smaller and more dense in hypertriglyceridemia, and contain only 45% of the normal unesterified cholesterol mass. The phase behavior of the lipids shows that normal red cells and low-density lipoproteins are close to saturation with cholesterol, whereas in hypertriglyceridemia less cholesterol is present. Because newly secreted triacylglycerol-rich lipoproteins are poor in cholesterol, their excess production and transport in hypertriglyceridemia may prevent maintenance of the normal cholesterol content of blood cells and low-density lipoproteins. Partitioning of cholesterol into triacylglycerol-rich lipoproteins is able to account for significant fluxes of unesterified cholesterol in the plasma compartment.

Mobilization of cholesterol from cholesterol ester-enriched tissue culture cells by phospholipid dispersions.

The accumulation of cholesterol esters in foam cells of the arterial intima is an important characteristic of fatty streak lesions of atherosclerosis. We wished to know if cholesterol ester accumulations in cells could be mobilized by altering their external milieu. Thus, phospholipid dispersions were used to remove cholesterol from a cholesterol ester-enriched cell line. Rat hepatoma cells, Fu5AH, were loaded with cholesterol esters by incubation in medium supplemented with hyperlipemic rabbit serum. After removing the loading medium, we incubated the cells in serum-free medium containing egg phosphatidylcholine dispersions. Unesterified cellular cholesterol level decreased in the first 4 h and then remained at a constant level. The cholesterol esters decreased after a lag time of about 2 h and the triacylglycerol level increased after 3 h. The decrease in cellular cholesterol ester depended on the amount of phospholipid in the medium. Cellular cholesterol ester decreased with increasing concentration of medium phospholipid to 2 mumols/ml and then plateaued. The removed cellular sterols appeared in the medium as free cholesterol. Since there was no measurable cholesterol esterase activity in the medium, the cholesterol ester in the cells was hydrolyzed before it appeared in the medium. The fatty acyl composition of the cellular cholesterol esters remained unchanged after significant reduction, suggesting that the hydrolysis of cholesterol esters was not specific for the acyl chain. Sphingomyelin and dimyristoyl phosphatidylcholine dispersions, though cytotoxic, were also effective in reducing cellular cholesterol esters. These experiments demonstrate that cholesterol ester accumulations in these cells can be reduced when phospholipid dispersions are used as cholesterol acceptors in the extracellular medium.

Physical properties of cholesteryl esters having 20 carbons or more

By polarizing microscopy and differential scanning calorimetry we observed that the relative stability of the smectic and cholesteric mesophases of cholesteryl esters of acyl chain length of 20 carbons or more depends on the length of the acyl chain and its degree of unsaturation. Significantly, the addition of a single double bond to the acyl chain of a fully saturated cholesteryl ester which exhibits no mesophases (e.g., cholesteryl behenate (C22:0) and cholesteryl lignocerate (C24:0) yields an ester which displays an unusually stable smectic mesophase, bot no cholesteric mesophase. In fact, increasing unsaturation was found to have a destabilizing effect on the cholesteric phase. Similarly, a decrease in thermal stability of the cholesteric mesophase was observed with increasing thermal stability of the smectic mesophase increased in the same series. X-ray scattering data are presented on the smectic mesophase of cholesteryl erucate (C22:1) and cholesteryl nervonate (C24:1). Significant differences in molecular packing of these two monounsaturated omega = 9 cholesteryl esters in the crystalline state are demonstrated by preliminary X-ray scattering experiments.

The thermal transitions and structural properties of 5α-cholestan-3β-ol esters of aliphatic acids

Abstract 5α-Cholestan-3β-ol esters of aliphatic acids undergo both enantiotropic and monotropic changes of state. Ten saturated and three unsaturated esters have been examined by differential scanning calorimetry and polarizing microscopy to determine transition temperatures, enthalpies, and entropies. The results are compared with an analogous series of cholesterol esters. All esters of evennumbered n -alkanoic acids from C 2 2 to C 20 melt from a crystalline state to an isotropic liquid. The crystalline state has been studied by X-ray powder diffraction. The C 8 to C 20 esters have progressively increasing crystalline melting transition temperatures from 76 to 99°C and possess similar X-ray powder diffraction patterns, suggesting that these compounds form an isostructural series. Esters of C 2 , C 4 , and C 6 acids exhibit polymorphism. Crystalline cholestanol oleate melts to an isotropic liquid, whereas cholestanol linoleate and linolenate fail to crystallize, even after several months at −20°C. Esters of the even-numbered saturated acids from C 4 to C 14 form monotropic cholesteric liquid crystalline phases. Esters C 10 , C 12 , and C 14 form smectic liquid crystalline phases. Cholestanol oleate, linoleate, and linolenate form both cholesteric and smectic mesophases. The lower smectic to cholesteric and cholesteric to isotropic transition temperatures of the cholestanol esters compared to the corresponding transition temperatures of the analogous cholesterol esters suggest that the Δ 5 double bond in cholesterol increases the thermal stability of the mesophases of cholesterol esters.

Deposition of egg-PC to an Air/Water and Triolein/Water Interface

Phospholipid monolayers play a critical role in stabilization of biological interfaces including the alveoli of the lung, fat droplets in adipose tissue, and apolipoproteins. Behavior of phospholipids at an air-water interface is well understood. However, work at oil-water interfaces is limited due to technical challenges associated with a Langmuir trough. In this study, egg-phosphatidylcholine(PC) was deposited onto a drop of either air or triolein(TO) formed in a low salt buffer and the surface tension was measured using a drop tensiometer. The egg-PC was deposited by constituting it into SUVs and then allowing molecules to absorb to the surface. We observed that egg-PC binds irreversibly to both interfaces and at equilibrium exerts 15 and 12 mN/m at an air and triolein interface, respectively. To determine the surface concentration, which cannot be measured directly, compression isotherms from a Langmuir trough were compared to that of the drop tensiometer. The air-water interfaces had identical characteristics so the surface concentration of the drop can be determined by simply overlaying the two isotherms. Since TO is also surface active there will be triolein incorporated into the monolayer. Since TO is less surface active than PC as the pressure(Π) increases the triolein is progressively ejected. To understand the Π/area isotherm of PC on the TO drop a variety of TO-PC mixtures were spread at the air-water interface. The isotherms show an abrupt break in the curve at a specific Π caused by the ejection of TO from the monolayer into the bulk phase. A plot of these surface transition points against Π gives the monolayer surface composition at any Π. The oil drop experiment always contains bulk phase of TO, thus the 2-D phase rule predicts the monolayer composition of the droplet over a range of Π.

Body Cholesterol Removal: Role of Plasma High-Density Lipoproteins1 1Supported by National Health Service Grants HL 18673, HL 07291, and HL 22682, and a Grant-In-Aid from the American Heart Association (316–3070–2286).

Publisher Summary The human body is equipped with mechanisms for the provision and maintenance of tissue cholesterol levels. Cholesterol absorption increases with dietary intake, with no upper limit. In times of reduced intake, the liver and small intestine increase their synthesis of cholesterol. Absorbed cholesterol is transported to the liver as chylomicron remnants and hepatic cholesterol is either secreted as “very low-density lipoproteins,” which are catabolized in part to low-density lipoproteins (LDL), or secreted into bile as free cholesterol or its metabolites, the bile salts. LDL delivers cholesterol to the peripheral tissues, where specific cell surface receptors bind, internalize, and degrade the LDL particles, providing the cell with cholesterol. In the absence of an adequate supply of lipoprotein cholesterol, the cholesterol synthesis pathway is activated in peripheral tissues. This chapter discusses the high-density lipoproteins (HDL) structure and metabolism, particularly as it might be related to cholesterol homeostasis. It focuses on the composition and structure of HDL, the physical structure of the individual components of HDL, and the different HDL recombinants.