SURFACE-AREA CYCLING OF DIFFERENT SURFACTANT PREPARATIONS - SP-A AND SP-B ARE ESSENTIAL FOR LARGE-AGGREGATE INTEGRITY

Surface-area cycling is an in vitro procedure for the conversion of la rge into small surfactant aggregates. In this procedure a tube contain ing a surfactant suspension is rotated end-over-end at 37 degrees C so that the surface area of the suspension changes twice each cycle. We have utilized this method to study the mechanisms involved in aggregat e conversion. Several different surfactant preparations were analysed: (1) bovine natural surfactant, a sucrose-gradient-purified material c ontaining surfactant phospholipid and surfactant-associated proteins ( SP-) SP-A, SP-B and SP-C; (2) bovine lipid-extract surfactant, which c ontains the surfactant phospholipids and SP-B and SP-C; (3) mixtures o f dipalmitoyl phosphatidylcholine and phosphatidylglycerol (7:3, w/w) reconstituted with one or more surfactant proteins. Aggregate conversi on was measured by phosphorus analysis of a 40000 g supernatant (small aggregate) and pellet (large aggregates) before and after surface-are a cycling. Surface-area cycling of lipid extract surfactant or lipids plus SP-B or SP-C resulted in rapid aggregate conversion. Lipids alone were not converted. Only a small percentage of purified natural surfa ctant was converted into small aggregates. Addition of SPA to lipid ex tract surfactant could inhibit aggregate conversion of this material, but this was only observed when an additional 1% (w/w) of SP-B was add ed to the lipid extract. It is concluded that SP-A is important for la rge-aggregate integrity. It appears that SP-A acts in conjunction with SP-B. The presence of SP-B and/or SP-C is required for aggregate conv ersion; it is proposed that this reflects the necessity for lipid adso rption in aggregate conversion.