Association and interactions of pulmonary surfactant lipids and proteins in model membranes at the air - water interface

Nag, Kaushik (1996) Association and interactions of pulmonary surfactant lipids and proteins in model membranes at the air - water interface. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Pulmonary surfactant (PS) a lipid-protein complex secreted by type-II pneumocytes is responsible for alveolar stability and prevents lung collapse at low volumes. PS lines the air-alveolar fluid interface with a putative ultra-thin monomolecular film, which by reducing the surface tension of that interface counteracts the contractile forces of lung collapse at low volumes. Monomolecular films (monolayers) are appropriate models for studying biological membranes, in which the lipids and proteins are organized in bilayers, and for pulmonary surfactant films. -- Interactions of the main phospholipid component of PS, dipalmitoylphosph-atidylcholine (DPPC) were studied with the other component lipids and proteins of PS at the air-water interface in monolayers. Over the last decade, epifluorescence microscopy has become a novel and powerful tool to study the organization of molecules in films. Epifluorescence microscopy of films formed in a surface balance of DPPC in combination with unsaturated phosphatidylcholine (dioleyl-PC or DOPC); dipalmitoylphosphatidylglycerol (DPPG); cholesterol; and fluorescently labelled surfactant proteins SP-A, SP-B and SP-C were performed. Visual observation of such lipid-lipid and lipid-protein films using epifluorescence microscopy allowed for the semi-quantitative understanding of the surface chemistry, phase transition, association and interactions of such components of PS with each other, at the air-water interface. -- The unsaturated phosphatidylcholine, DOPC, fluidized films of DPPC and were squeezed out of such films upon dynamic cycling (Chapter 3). Phosphatidylglycerol (DPPG) condensed films of DPPC under the influence of calcium (Chapter 4). Cholesterol drastically altered the DPPC condensed phase structures and fluidized such films (Chapter 5). Hydrophobic surfactant protein - C (SP-C) perturbed the packing of DPPC and DPPC:DPPG films, occupied the fluid phase and was present in the films up to high packing states (Chapter 6). Films adsorbed from liposomes and solvent-spread ones of some PS components were found to display similar micro-architecture and phase properties in equivalent packing states, and SP-C enhanced the adsorption of DPPC vesicles (Chapter 7). Hydrophobic surfactant protein - B (SP-B) also perturbed the packing and associated with the fluid phase of DPPC, albeit differently than did SP-C (Chapter 8 and 9). Hydrophilic surfactant protein - A (SP-A) adsorbed to DPPC films, and was associated with the condensed-fluid phase boundaries of the lipid, and perturbed the packing of such films (Chapter 10). Porcine lipid surfactant extract (LSE) films showed fluid to condensed phase transition upon compression, and probably other complex transitions which was indicated by a decrease in the amount of condensed domains with increasing packing states (Chapter 11). The condensed phase of LSE films were increased under the influence of millimolar calcium dissolved in the subphase compared to the ones without the cation. Dissolved SP-A in the subphase, adsorbed on to solvent-spread LSE films and altered the distribution of condensed domains (Chapter 11). Some of the properties of LSE films (Chapter 11) were correlated to those exhibited by its component combinations in films (Chapters 3-10). -- These studies, using a novel technique, elucidate some of the possible modes of association and interactions between the major pulmonary surfactant components at the air-water interface. This study may also indicate feasible lipid-lipid and lipid-protein associations and micro-organisation in model of biological membranes.

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