Interactions between DPPC as a component of lung surfactant and amorphous silica nanoparticles investigated by HILIC-ESI-MS.

This paper reports a rapid HILIC-ESI-MS assay to quantify dipalmitoylphosphatidylcholine (DPPC) as component of lung surfactant for nanosafety studies. The technique was used to investigate the concentration-dependent sorption of DPPC to two-sizes of amorphous SiO2 nanoparticles (SiO2-NPs) in a MeOH:H2O (50/50v/v) mixture and in cell culture medium. In MeOH:H2O (50/50v/v), the sorption of DPPC was positively correlated with the nanoparticles concentration. A substantial affinity of small amorphous SiO2-NPs (25nm) to DPPC standard solution compared to bigger SiO2-NPs (75nm) was not confirmed for biological specimens. After dispersion of SiO2-NPs in DPPC containing cell culture medium, the capacity of the SiO2-NPs to bind DPPC was reduced in comparison to a mixture of MeOH:H2O (50/50v/v) regardless from the nanoparticles size. Furthermore, HILIC-ESI-MS revealed that A549 cells internalized DPPC during growth in serum containing medium complemented with DPPC. This finding was in a good agreement with the potential of alveolar type II cells to recycle surfactant components. Binding of lipids present in the cell culture medium to amorphous SiO2-NPs was supported by means of HILIC-ESI-MS, TEM and ICP-MS independently.

Apolipoprotein E*dipalmitoylphosphatidylcholine particles are ellipsoidal in solution.

Apolipoprotein E (apoE) is a major protein component of cholesterol-transporting lipoprotein particles in the central nervous system and in plasma. Polymorphisms of apoE are associated with cardiovascular disease and with a predisposition to Alzheimer's disease and other forms of neurodegeneration. For full biological activity, apoE must be bound to a lipoprotein particle. Complexes of apoE and phospholipid mimic many of these activities. In contrast to a widely accepted discoidal model of apoA-I bound to dimyristoylphosphatidylcholine, which is based on solution studies, an X-ray diffraction study of apoE bound to dipalmitoylphosphatidylcholine (DPPC) indicated that apoE*DPPC particles are quasi-spheroidal and that the packing of the phospholipid core is similar to a micelle. Using small-angle X-ray scattering, we show that apoE*DPPC particles in solution are ellipsoidal and that the shape of the phospholipid core is compatible with a twisted-bilayer model. The proposed model is consistent with the results of mass spectrometric analysis of products of limited proteolysis. These revealed that the nonlipid-bound regions of apoE in the particle are consistent with an alpha-helical hairpin.

Participation of carnitine palmitoyltransferase in the synthesis of dipalmitoylphosphatidylcholine in rat alveolar type II cells.

We have investigated the role of carnitine palmitoyltransferase (EC 2.3.1.21) in pulmonar type II pneumocyte, a lung cell responsible for the synthesis of surface active lipids. Adult type II pneumocytes were isolated from rat lung and purified by differential adherence. When these lung cells were incubated with radioactive palmitate, the percentage of radioactivity recovered into dipalmitoylphosphatidylcholine (DPPC), a major surface active lipid, was almost 60% with respect to total phosphatidylcholine (PC) molecular species. Cellular lysates from type II pneumocytes contained detectable amount of carnitine palmitoyltransferase (CPT) activity (1 nmol/min/mg). Most of the CPT activity found in these cells could be inhibited by incubating them for 60 min with 5 microM tetradecylglycidic acid (TDGA), a specific and irreversible CPT inhibitor of the malonyl-CoA sensitive CPT isoform (CPT I). TDGA treatment of adult type II pneumocytes caused a significant reduction in the incorporation of radioactive palmitate into PC, though this effect did not seem to be specific for DPPC. TDGA affected the incorporation of radioactive palmitate at the sn2 rather than the sn1 position of the glycerol backbone of PC. The incorporation of radioactive palmitate into DPPC was also observed when these lung cells were incubated with palmitate-labeled palmitoyl-L-carnitine. Our data suggest that type II pneumocyte CPT may play an important role in remodelling PC fatty acid composition and hence DPPC synthesis.

Ca2+-induced lateral phase separation in ternary mixtures of phosphatidic acid, phosphatidylcholine, and phosphatidylethanolamine inferred by calorimetry.

Phase transition characteristics of ternary mixtures of dipalmitoylphosphatidic acid, dipalmitoylphosphatidylcholine, and phosphatidylethanolamine (dilauroyl-, dimyristoyl-, or dipalmitoyl-phosphatidylethanolamine) were examined by differential scanning calorimetry at various concentrations of calcium ions. In the absence of calcium ion, these ternary mixtures showed a broad phase transition, which suggested a high miscibility of these components. Addition of a low concentration of calcium ions showed a tendency to induce separation of the transition into a major one and a small one. As the concentration of calcium ions increased, the separation became more distinct and the transition enthalpy of the major transition decreased. At a Ca2+/dipalmitoylphosphatidic acid ratio (mol/mol) of 1.5, the major transition became similar to the transition of dipalmitoylphosphatidylcholine and the phosphatidylethanolamine binary mixture. On the other hand, in a binary mixture dipalmitoylphosphatidic acid and dipalmitoylphosphatidylcholine, the Ca2+-induced phase separation was distinct even at the lowest concentration of calcium ions used in the present experiment. The results indicate that a high concentration of calcium ion is required for inducing complete phase separation of the transition event in the ternary mixture because of its high miscibility. It is suggested that the phase separation revealed by spin-labeled phospholipid in ternary mixtures at a low Ca2+ concentration might be a phase separation in a local domain.