Reversible Calcium-dependent Interaction of Liposomes with Pulmonary Surfactant Protein A

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 Ca 2+ -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 K 0.5 of 20 μ m for free Ca 2+ , 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 Ca 2+ ions. However, binding of liposomes to SP-A is >10-fold faster than aggregation. Both processes are completely reversed by low Ca 2+ concentrations, but liposomes dissociate from SP-A in