Palmitoylation of a pulmonary surfactant protein C analogue affects the surface associated lipid reservoir and film stability

Surfactant protein C (SP-C) is a lipopeptide that contains two thioester-li nked palmitoyl groups and is considered to be important for formation of th e alveolar surface active lipid film. Here, a non- or dipalmitoylated SP-C analogue (SP-C(Leu)), in which all helical Val residues were replaced with Leu and Cys-5 and Cys-6 were replaced with Ser, was tested for surface acti vity in a captive bubble system (CBS). SP-C(Leu), either palmitoylated at S er-5 and Ser-B or non-palmitoylated, was added to mixtures of 1,2-dipalmito yl-sn-glycero-3-phosphocholine (DPPC)/phosphatidyl glycerol (PG)/palmitic a cid (PA), 68:22:9, (by mass) at a concentration of 2 and 5%. With 2% peptid e, surface film formation was rapid, reaching a surface tension below 25 mN /m within 5 s, but the samples with 5% SP-C(Leu) required more than 20 s to reach values below 25 mN/m. Minimum surface tension for the samples with d ipalmitoylated SP-C(Leu) was below 1.5 mN/m and very stable, as the surface tension increased by less than 0.5 mN/m within 10 min at constant bubble v olume. Minimum surface tension for the non-palmitoylated SP-C(Leu) was appr oximately 2 and 5 mN/m for 2 and 5% peptide, respectively, but the films we re less stable as seen by frequent bubble clicking at low surface tensions. Films with dipalmitoylated SP-C(Leu) that were dynamically cycled at 20-30 cycles/min were substantially less compressible at a surface tension of 20 mN/m (0.007 m/mN) than those that contained the non-palmitoylated peptide (0.02 m/mN). After subphase depletion, the incorporation of lipids into the surface active film during initial bubble expansion occurred at a relative ly low surface tension (about 35 mN/m) for the samples with dipalmitoylated SP-C(Leu) compared to approximately 45 mN/m for those containing the non-p almitoylated peptide. Furthermore, for samples that contained non-palmitoyl ated SP-C(Leu), the ability to reach near zero stable surface tension was l ost after a few adsorption steps, whereas with the dipalmitoylated peptide the film quality did not deteriorate even after more than 10 expansion step s and the incorporation of reservoir material equivalent to more than two m onolayers. It appears that the covalently linked palmitoyl groups of the SP -C analogue studied are important for the mechanical stability of the lipid film, for the capacity to incorporate material from the reservoir into the surface active film upon area expansion, and for the low film compressibil ity of dynamically cycled films. (C) 2000 Elsevier Science B.V. All rights reserved.