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Pulmonary surfactant is a lipid-protein complex essential to stabilize alveoli, by forming surface active films able to reach and sustain very low surface tensions (<2 mN m À1 ) during the film compression that occurs at end-expiration. The particular lipid composition of surfactant, including a high proportion of dipalmitoylphosphatidylcholine (DPPC), induces segregation of fluid ordered and disordered phases in surfactant membranes and films at physiological temperatures. The segregation of DPPC-enriched ordered phase has been related with the ability of surfactant films to produce very low tensions, while the presence in surfactant of two specific hydrophobic polypeptides, SP-B and SP-C, is absolutely required to facilitate surfactant dynamics, including film formation and re-spreading during expansion at inspiration. In the present study, we have used X-ray scattering to analyze the structure of (1) whole native surfactant membranes purified from porcine lungs, (2) membranes reconstituted from the organic extract of surfactant containing the full lipid complement and the physiological proportion of SP-B and SP-C, and (3) membranes reconstituted from the lipid fraction of surfactant depleted of proteins. Small angle X-ray scattering data from whole surfactant or from membranes reconstituted from surfactant organic extract indicated the co-existence of two lamellar phases with different thicknesses. Such phase coexistence disappeared upon heating of the samples at temperatures above physiological values. When assessed in a captive bubble surfactometer, which mimics interfacial compression-expansion dynamics, the ability of surfactant films to produce very low tensions is only maintained at temperatures permitting the coexistence of the two lamellar phases. On the other hand, membranes reconstituted in the absence of proteins produced diffractograms indicative of the existence of a single dominant lamellar phase at all temperatures. These data suggest that SP-B and SP-C establish membranemembrane interactions coupling the stacks of different segregated phases. The low compressibility of surfactant films that leads to the maximal pressures

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Regions of different proton density in the low density lipoprotein (LDL) particle from human serum have been determined by neutron scattering. From measurements in various H20/D20 mixtures, the LDL particle appears to be quasi-spherical, with the centers of gravity of the hydrocarbon and polar regions coinciding. The average volume occupied by a particle was found to be 3.2 X 106 Al, with the volume fraction occupied by the hydrocarbons being 60%. The radius of gyration of the hydrocarbon region was 64 A, while that of the polar region was 100 A. Consequently, the core of LDL is predominantly occupied by the hydrocarbon chains, while the outer shell is sparsely occupied by protein emerging from the lipid core.Human serum lipoproteins are particles in which serum lipids are associated with characteristic polypeptides in discrete species generally defined by their hydrated density (1). In addition to playing a major role in lipid transport and energy metabolism (2), lipoproteins may be involved in the maintenance of cell membranes (3) and in the regulation of lipid biosynthesis (1, 4). The study of lipoproteins is of particular importance because of several diseased states associated with alterations in their concentration and/or structure (5).Low density lipoproteins (LDL) are the predominant species in the density range 1.019-1.063 g/ml in human serum (1-4).These particles, which appear to be roughly spherical in shape (6, 7) and to have an average molecular weight of 2 to 3 X 106 (8), are thought to be complex associations of 21% protein, 22% phospholipids, 8% free cholesterol, 37% cholesterol esters, and 11% triglycerides (1-4). How these lipid and protein components are arranged in LDL remains elusive, although several models have been suggested (6,7,(9)(10)(11) The wavelength spectrum was centered at 3.7 A and had a total half width of 1.7 A. The correction of the neutron scattering curves for the influence of this broad wavelength distribution turned out to be inadequate. It resolved quite clearlv the maxima of the curves (Fig. 1); however, the depth of the minima often remained quite inaccurate.In order to cover a sufficiently broad range of scattering angles we placed the detector at 0.66, 2, and 10 m from the sample. The neutron flux passing the sample area of 2 cm2 varied from 106 to 108 neutrons/'s depending on the angular resolution. 2270Abbreviation: LDL, low density lipoprotein.

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