An X-ray diffraction study of alterations in bovine lung surfactant bilayer structures induced by albumin

Lung surfactant (LS) is an extra-cellular lipid-protein system responsible for maintaining low surface tension in the lung and alveolar stability. Serum proteins cause dysfunction of this material, e.g. in adult respiratory distress syndrome (ARDS). BLES is a clinically used LS consisting of most of the lipids and associated proteins from bovine lung lavage. Aqueous phases of BLES at 30% and 70% hydration, with and without 5% by weight of bovine serum albumin (BSA), calculated on the amount of lipids, were studied using X-ray diffraction during cooling from 42 to 5 degrees C. The diffraction curves are consistent with a transition from a lamellar liquid crystalline phase to a gel phase transition at cooling in the interval 30-20 degrees C. The long-spacings correspond to a reduction of the bilayer thickness during this transition. The wide-angle region shows a peak at 4.1 A below 25 degrees C, which is characteristic of the hexagonal chain packing of the gel phase. The perturbation of the bilayers by the presence of BSA seems to induce a significant decrease of the bilayer thickness. Calculations on the observed limits of swelling (taking place in the range 50-60%) indicate that BSA is closely associated with the BLES bilayers, probably due to electrostatic interaction with the cationic surfactant proteins SP-B and SP-C. This study show that the LS lipid structural organizations are extremely susceptible to small amounts of serum albumin, which may have implications in surfactant related lung disease and clinical applications of surfactant therapy.