Reversible calcium-dependent interaction of liposomes with pulmonary surfactant protein A. Analysis by resonant mirror technique and near-infrared light scattering.
Surfactant protein A (SP-A) is crucial for lung function, including tubular myelin formation and lipid uptake by type II pneumocytes. Known properties of SP-A in vitro are its Ca2+-dependent interaction with phospholipids and its role in the aggregation of liposomes. To dissect and to analyze these processes, we have immobilized SP-A and measured binding of liposomes by the resonant mirror technique. Liposome aggregation was followed separately by kinetic light scattering in suspensions. It was found that SP-A-mediated binding and aggregation of liposomes have a common K0.5 of 20 microM for free Ca2+, independent of the species (sheep, rat, or cow) and of the phospholipid composition, and that both reactions exhibit the same high cooperativity (Hill coefficients of 6-9) for Ca2+ ions. However, binding of liposomes to SP-A is >10-fold faster than aggregation. Both processes are completely reversed by low Ca2+ concentrations, but liposomes dissociate from SP-A in <0.3 s, whereas disaggregation of the liposomes takes approximately 30 s. At equilibrium, the level of aggregation depends on the concentration of free SP-A. We interpret these results to be a rapid and reversible sequence of three reactions: (i) a cooperative Ca2+-dependent conformational change in SP-A, (ii) binding of Ca2+-bound SP-A to liposomes, and (iii) aggregation of the Ca2+/SP-A-bound liposomes.
Meyboom
A
A
Institut für Medizinische Physik und Biophysik, Universitätsklinikum Charité, Humboldt Universität, D-10098 Berlin, Federal Republic of Germany.
Maretzki
D
D
Stevens
P A
PA
Hofmann
K P
KP
D000818:Animals; D015374:Biosensing Techniques; D002118:Calcium / Q000494:pharmacology*; D002417:Cattle; D004492:Edetic Acid / Q000494:pharmacology; D006023:Glycoproteins / Q000378:metabolism; D007259:Infrared Rays; D007700:Kinetics; D008081:Liposomes / Q000378:metabolism*; D011510:Proteolipids / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism*; D037662:Pulmonary Surfactant-Associated Protein A; D037661:Pulmonary Surfactant-Associated Proteins; D011663:Pulmonary Surfactants / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D012542:Scattering, Radiation; D012756:Sheep; D013053:Spectrophotometry
D006023:Glycoproteins; D008081:Liposomes; D011510:Proteolipids; D037662:Pulmonary Surfactant-Associated Protein A; D037661:Pulmonary Surfactant-Associated Proteins; D011663:Pulmonary Surfactants; D004492:Edetic Acid; D002118:Calcium