Effects of a surfactant-associated protein and calcium ions on the structure and surface activity of lung surfactant lipids

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Abstract
Previous studies have demonstrated that lung-specific proteins are associated with surfactant lipids, particularly the highly surface active subfraction known as tubular myelin. A surfactant-associated protein complex with MW components of 36,000, 32,000 and 28,000 was isolated and reassembled with protein-free lung surfactant lipids prepared as small unilamellar liposomes. The effects of divalent cations on the structure and surface activity of this protein-lipid mixture were investigated by following the state of lipid dispersion by changes in turbidity and by EM and following the ability of the surfactant lipids to form a surface film from an aqueous subphase at 37.degree. C. The protein complex markedly increased the rate of Ca2+-induced surfactant-lipid aggregation. EM demonstrated transformation of the small unilamellar liposomes (median diameter 440 .ANG.) into large aggregates. The threshold Ca2+ concentration required for rapid lipid aggregation was reduced from 13 to 0.5 mM by the protein complex. This protein-facilitated lipid aggregation did not occur if Mg2+ was the only divalent cation present. Similary, 5 mM Ca2+ but not 5 mM Mg2+ improved the ability of the protein-lipid mixture to form a surface film at 37.degree. C. Extensive aggregation of the surfactant lipids without protein by 20 mM Ca2+ or 20 mM Mg2+ did not promote rapid surface film formation. These results add to the growing evidence that specific Ca2+-protein-lipid interactions are important in determining the structure and function of extracellular lung surfactant fractions.