Composition, structure and properties of POPC–triolein mixtures. Evidence of triglyceride domains in phospholipid bilayers

Abstract

We have in this study investigated the composition, structure and spectroscopical properties of multilamellar vesicles composed of a phospholipid, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), and up to 10mol% of triolein (TO), a triglyceride. We found in agreement with previous results that the mixtures with 10mol% TO spontaneously separate into two distinct phases, heavy (HF) and light (LF), with different densities and found this also to be the case for 2 and 5mol% TO. The compositions of the two phases were investigated by quantitative lipid mass spectrometric analysis, and with this method we found that TO had a solubility maximum of about 4mol% in the HF, whereas it was markedly up-concentrated in the LF. Electron paramagnetic resonance spectroscopy indicated POPC membranes of all tested concentrations of TO in both phases to be almost unperturbed by the presence of TO and to exist as vesicular structures containing entrapped water. Bilayer structure of the membranes was supported by small angle X-ray scattering that showed the membranes to form a lamellar phase. Fluorescence spectroscopy with the polarity sensitive dye Nile red revealed, that the LF samples with more than 5mol% TO contained pure TO domains. These observations are consistent with an earlier MD simulation study by us and our co-workers suggesting triglycerides to be located in lens shaped, blister-like domains between the two lipid bilayer leaflets (Khandelia et al. (2010) [26]).

Dynamics and functions of lipid droplets

“…As their concentration increases, neutral lipids eventually coalesce, forming an oil lens in a process of demixing. In model membranes, this transition occurs when the triacylglycerol concentration is in the range of 5-10 mol% (REFS 9,10 ). These and other studies indicate that simple principles of physical chemistry explain lens formation, with the demixing of neutral lipids being energetically favoured by reducing their interaction with other membrane components such as phospholipids or proteins 11 .…”

Interdigitation between Triglycerides and Lipids Modulates Surface Properties of Lipid Droplets

“…We chose a lipid composition commonly used to study LDs in vitro (42)(43)(44), namely POPC for the phospholipids and TO for the TAGs. The choice of pure TO was further motivated by two other arguments: 1) in regular LDs constituted of mixtures of TAG and sterol esters, TAGs are absolutely required to make the mixture liquid at physiological temperature (pure sterol esters have a melting temperature >37 C) (45); and 2) the TO/W interface is well characterized using biophysical experiments (42,43).…”

Interactions of Lipid Droplets with the Intracellular Transport Machinery

“…These biophysically based hypotheses made a number of predictions which have been verified experimentally. Lens formation could be visualized by electron microscopy of giant unilamellar vesicles [27] or in the ER of cells [28]. However, the fact that in cells LDs bud exclusively into the cytoplasm and not the ER, requires an explanation.…”