FLUORESCENTLY LABELED PULMONARY SURFACTANT PROTEIN-C IN SPREAD PHOSPHOLIPID MONOLAYERS

Pulmonary surfactant, a lipid-protein complex, secreted into the fluid lining of lungs prevents alveolar collapse at low lung volumes. Pulmo nary surfactant protein C (SP-C), an acylated, hydrophobic, alpha-heli cal peptide, enhances the surface activity of pulmonary surfactant lip ids. Fluorescein-labeled SP-C (F-SP-C) (3, 6, 12 wt%) in dipalmitoylph osphatidylcholine (DPPC), and DPPC:dipalmitoylphosphatidylglycerol (DP PG) [DPPC:DPPG 7:3 mol/mol] in spread monolayers was studied by epiflu orescence microscopy. Mass spectrometry of F-SP-C indicated that the p rotein is partially deacylated and labeled with 1 mol fluorescein/1 mo t protein. The protein partitioned into the fluid, or liquid expanded, phase. Increasing amounts of F-SP-G in DPPC or DPPC:DPPG monolayers d ecreased the size and total amounts of the condensed phase at all surf ace pressures. Calcium (1.6 mM) increased the amount of the condensed phase in monolayers of DPPC:DPPG but not of DPPC alone: and such monol ayers were also perturbed by F-SP-C. The study indicates that SP-C per turbs the packing of neutral and anionic phospholipid monolayers even when the latter systems are condensed by calcium, indicating that inte ractions between SP-C and the lipids are predominantly hydrophobic in nature.