Pulmonary surfactant protein SP-C and phosphatidylcholines in bilayers and monolayers

Abstract

Pulmonary surfactant lowers surface tension in the lung alveoli, reduces the work of breathing and prevents alveolar collapse at low lung volumes. The material consists of about 90% lipids, of which dipalmitoylphosphatidylcholine (DPPC) is the major component, and at least four proteins, SP-A, SP-B, SP-C, and SP-D. SP-B and SP-C, are very hydrophobic and are considered to help in the process of transferring the lipid from bilayers, the form in which it is secreted into a monolayer at the air-water interface. SP-C in saturated phosphatidylcholines in bilayers and monolayers has been studied by 2H nuclear magnetic resonance spectroscopy, high sensitivity differential scanning calorimetry, and epifluorescence of monolayers under compression. The gel to liquid-crystalline phase transition was broadened by SP-C. The T 1 relaxation times of the acyl chains were essentially unaffected by the protein. The T 2e of the chains were decreased in the liquid crystal and their temperature dependence was altered in the gel. Initial studies indicate that the first moments of the 2H NMR spectra of the head group of DPPC were only slightly affected by SP-C. SP-C affected the distribution of condensed and fluid domains in monolayers in the liquid expanded/liquid condensed region by producing more, smaller, condensed domains in the two phase region than seen for pure lipid monolayers. The results are consistent with SP-C being embedded, at least in part, in the acyl chains of the lipid, and that severe packing dislocations in PC bilayers are not produced by the protein.