Microphase Separation in Low Density Lipoproteins

Abstract

The structural organization of the neutral lipid core in human low density lipoproteins (LDL) was investigated in physicochemically defined, distinct human LDL subspecies in the density range of 1.0244 -1.0435 g/ml by evaluation of the core lipid transition temperature, chemical composition, and the behavior of spin-labeled core lipids. Calorimetric studies were performed on more than 60 LDL preparations, and the transition temperature, which varied between 19 and 32°C, was correlated to the chemical composition and revealed a discontinuity at a critical cholesteryl ester to triglyceride ratio of approximately 7:1. For electron spin resonance studies, several LDL preparations were probed with spin-labeled cholesteryl esters and triglycerides, respectively. In LDL with a high triglyceride content, both labels exhibited similar mobility behavior. In contrast, in LDL with only small concentrations of triglycerides, the behavior of labeled cholesteryl esters and labeled triglycerides differed distinctly. The cholesteryl esters were strongly immobilized below the transition temperature, whereas the triglycerides remained fluid throughout the measured temperatures. These results suggest that the critical cholesteryl ester to triglyceride mass ratio of 7:1 corresponds to two concentric compartments with a radial ratio of 2:1, where the liquid triglycerides occupy the core, and the cholesteryl esters form the frozen shell. At higher triglyceride contents, the triglyceride molecules insert into the cholesteryl ester shell and depress the peak transition temperature of the LDL core, whereas at lower triglyceride contents, excess cholesteryl esters are dissolved in the core. Low density lipoproteins (LDL)1 are the major carriers of cholesterol in the circulation and are intimately involved in atherogenesis (1 and references therein). These particles represent complex supramolecular assemblies of phospholipids (ϳ20% of the total mass), free (ϳ12%) and esterified (ϳ40%) cholesterol, triglycerides (ϳ5-10%), and a single copy of apolipoprotein B-100, a glycoprotein of 4,536 amino acids (2-4). In addition, LDL transports minor amounts of lipophilic vitamins and drugs (5). The structure of LDL can be described, in general terms, by a quasispherical core-shell model, in which the apolar constituents (cholesteryl esters (CE) and triglycerides (TG)) form a hydrophobic core of about 150 Å diameter, whereas the phospholipids, most of the unesterified cholesterol, and the apoprotein form an outer surface monolayer with a thickness of about 30 Å (6 -8).In this general sense, LDL would appear to fit readily into the structural core-shell scheme of circulating lipoproteins (9). However, LDL exhibits a distinct physical feature that makes it unique among all structural elements of blood: it is the only component to undergo a major structural transition just below physiological body temperature, in the range between 15 and 32°C (10, 11). The transition temperature varies among individual donors and is correlated to the core lipid compositi...

Low Density Lipoproteins as Circulating Fast Temperature Sensors

“…This lamellae-like layer organisation of LDL core lipids yields a new view of particle structure, which is consistent with low resolution X-ray diffraction patterns of LDL crystals [28], [29]. Indeed, this suggestion supports the notion that a nanophase separation occurs in LDL core lipids, in which triglyceride and cholesteryl ester molecules separate into distinct nanoenvironments [30]. However, irrespective of the internal molecular orientation of these apolar lipids, the distinct changes observed in our SAXS patterns are representative for the thermotropic transition of the LDL core.…”

“…During this time period, LDL particles might remain in their ordered state, depending on the respective lipid profile of the patient, which determines the actual core lipid transition temperature. Beyond these considerations, the “freezing-out” of the LDL core into microcompartments also implies that distinct local molecular environments may be created, in which certain active substances such as vitamins or drugs may accumulate transiently, leading to enhanced local activities [30]. Indeed, our results suggest that the core of circulating LDL particles, as a consequence of variations in blood temperature or acute hypothermia, may undergo a periodic redistribution of lipophilic constituents within its core nanodomains.…”

Structural and Biochemical Properties of Lipid Particles from the Yeast Saccharomyces cerevisiae

“…Experiments with model systems showed that addition of 5% TAG can abolish the high temperature transition (41). Moreover, Pregetter et al (42) showed that in human LDL at a CE to TAG ratio of 7:1 phase separation of a liquid TAG core and rigid shells of CE (radial ratio 2:1) occurred with an order-disorder transition temperature of 30°C. Incorporation of further TAG led to a decrease of this phase transition as also observed with yeast wild type LP (see Fig.…”

Three-dimensional structure of low density lipoproteins by electron cryomicroscopy

“…Most of the models have been built around the conceptual framework of a microemulsion model (4). A phase transition in LDL was observed by differential scanning calorimetry and x-ray and neutron solution scattering at a temperature in the range of 15 to 30°C (17)(18)(19)(20)(21)(22). The transition was associated with a liquid-crystalline, order-disorder phase change of cholesterol esters within the particles.…”