1601504

11727

Recent work on surfactant protein A (SP-A) has shown that Ca2+ induces an active conformation, SP-Â, which binds rapidly to liposomes and mediates their aggregation. Employing sensitive real time assays, we have now studied the lipid binding characteristics of the SP-Â liposome interaction. From the final equilibrium level of the resonant mirror binding signal, an apparent dissociation constant of ca. Kd=5 μM is obtained for the complex between SP-A and dipalmitoylphosphatidylcholine (DPPC) liposomes. At nanomolar SP-A concentrations, this complex is formed with a subsecond (0.3 s) reaction time, as measured by light-scattering signals evoked by photolysis of caged Ca2+. With palmitoyloleoylphosphatidylcholine (POPC), the complex formation proceeds at half the rate, compared to DPPC, leading to a lower final equilibrium level of SP-A lipid interaction. Distearoylphosphatidylcholine (DSPC) shows a stronger interaction than DPPC. Regarding the phospholipid headgroups, phosphatidylinositol (PI) and sphingomyelin (SM) interact comparable to DPPC, while less interaction is seen with phosphatidylethanolamine (PE) or with phosphatidylglycerol (PG). Thus both headgroup and fatty acid composition determine SP-A phospholipid interaction. However, the protein does not exhibit high specificity for either the polar or the apolar moiety of phospholipids.

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Surfactant protein A , Real time kinetics , Caged Ca2? , Lipid speci¢city , light scattering , Resonant mirror spectroscopy

Studia Iranica

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