Effects of surfactant-associated protein SP-B synthetic analogs on the structure and surface activity of model membrane bilayers

Abstract

The effect of several synthetic peptides based on the sequence of human pulmonary surfactant-associated protein B (SPB) on the molecular packing of model membrane lipids (7:1 dipalmitoyl phosphatidylcholine (DPPC)/dipalmitoyl phosphatidylglycerol (DPPG)) was studied using fluorescence anisotropy. This information was then correlated with complementary biophysical data obtained on both a modified Wilhelmy-Langmuir balance and a pulsating bubble surfactometer. The SP-B peptides examined in these studies are synthetic human SP-B Phe 1-Ser 78 (SP-B 1–78, full-length sequence), synthetic human SP-B Phe 1-Thr 60 (SP-B 1–60), synthetic human SP-B Phe 1-Ala 20 (SP-B 1–20), synthetic human SP-B Ala 20-Thr 60 (SP-B 20–60), synthetic human SP-B Leu 27-Ser 78 (SP-B 27–78), synthetic human SP-B Leu 40-Thr 60 (SP-B 40–60) and synthetic human SP-B Tyr 53-Ser 78 (SP-B 53–78). trans-parinaric acid was utilized to detect changes in ordering of lipids within the interior upon incorporation of synthetic SP-B peptide, whereas 1-hexadecanoyl-2-[ N-(7-nitro-2-benzoxa-1,3-diazol-4-yl)-aminohexanoyl] phosphatidylcholine (6-NBD-PC) and 1-acyl-2-[ N-(7-nitro-2-benzoxa-1,3-diazol-4-yl)aminohexanoyl] phosphatidylglycerol (6-NBD-PG) were utilized to determine alterations in lipid order at the surface of the model membrane bilayer. With the exception of SP-B 40–60, which corresponds to the most hydrophobic segment of the full-length SP-B, none of the other peptide significantly perturbed the interior bilayer as determined by fluorescence anisotropy of trans-parinaric acid. Incorporation of any of the peptides, with the exception of SP-B 40–60, resulted in an increase in anisotropy of NBD-PC. The most significant enhancements resulted from the addition of SP-B 1–78, SP-B 1–20, SP-B 27–78 or SP-B 53–78. The magnitude of anisotropy increase with these peptides is similar to that observed with an equivalent molar ratio of native SP-B isolated from a bovine source. These observations suggest that these four synthetic peptides have the structural and compositional characteristics required for surface ordering of the membrane bilayer in a manner similar to that observed with native SP-B, thereby facilitating the surfactant-like properties of phospholipid mixtures.