The role of surfactant proteins in DPPC enrichment of surface films

A pressure-driven captive bubble surfactometer was used to determine the ro le of surfactant proteins in refinement of the surface film. The advantage of this apparatus is that surface films can be spread at the interface of a n air bubble with a different lipid/protein composition than the subphase v esicles. Using different combinations of subphase vesicles and spread surfa ce films a clear correlation between dipalmitoylphosphatidylcholine (DPPC) content and minimum surface tension was observed. Spread phospholipid films containing 50% DPPC over a subphase containing 50% DPPC vesicles did not f orm stable surface films with a low minimum surface tension. Addition of su rfactant protein B (SP-B) to the surface film led to a progressive decrease in minimum surface tension toward 1 mN/m upon cycling, indicating an enric hment in DPPC. Surfactant protein C (SP-C) had no such detectable refining effect on the film. Surfactant protein A (SP-A) had a positive effect on re finement when it was present in the subphase. However, this effect was only observed when SP-A was combined with SP-B and incubated with subphase vesi cles before addition to the air bubble containing sample chamber. Compariso n of spread films with adsorbed films indicated that refinement induced by SP-B occurs by selective removal of non-DPPC lipids upon cycling. SP-A, com bined with SP-B, induces a selective adsorption of DPPC from subphase vesic les into the surface film. This is achieved by formation of large lipid str uctures which might resemble tubular myelin.