A (2)H-NMR study of interactions in model membranes containing pulmonary surfactant proteins SP-B and SP-C

Pulmonary surfactant is a lipid/protein mixture which reduces the work of breathing by rapidly spreading into a monolayer at the alveolar air/water interface. 2H NMR was used to examine the effect of porcine pulmonary surfactant protein SP-B and SP-C on lipid phase behaviour, chain order, and dynamics in bilayers of chain-perdeuterated dipalmitoylphosphatidylglycerol (DPPG-d62) or mixed bilayers containing 70 mol % dipalmitoylphosphatidylcholine (DPPC) and 30 mol % DPPG with one or the other lipid labeled. While SP-B was found to have little effect on chain order, deuteron transverse relaxation was strongly affected by the presence of the protein in both the liquid crystal and gel phases of each lipid system. Perturbation of the bilayer by SP-B was insensitive to the relative amounts of DPPC and DPPG present. There was no indication of a preferential interaction of SP-B with either lipid component. These observation may constrain possible models for SP-B/phospholipid interaction in bilayer precursors of the pulmonary surfactant monolayer. SP-C was found to have a similar effect on the chain order and phase behaviour of DPPC and DPPG in bilayers with a single lipid component. In gel phase DPPC/DPPG (70:30 w/w) bilayers with one or the other lipid component chain-perdeuterated, SP-C was found to affect the first spectral moment more strongly for DPPG than for DPPC. This may indicate that SP-C induced a non-random lateral distribution in the mixed lipid bilayer. SP-C was also found to influence motions responsible for deuteron transverse relaxation in both the gel and liquid crystalline phases. The presence of 5 mM Ca2+ in the aqueous phase substantially altered the effect of SP-C on the acyl chains' orientational order and transverse relaxation in the bilayer. In the presence of Ca2+, SP-C was found to have no or little effect on the transverse relaxation and chain order in the gel phase of DPPC/DPPG (70:30 w/w) mixed bilayers. This removal of the effect of SP-C on gel phase may reflect a Ca2+-induced partial separation of bilayer components at the phase transition. The effect of SP-B on the DPPC head group was also studied. It was found that the effect of SP-B on the head group region is concentration dependent. Increasing SP-B concentration induced motional asymmetry in both head group and acyl chains region. This was indicated by the highly asymmetric 2H-NMR spectra observed for both head group deuterated DPPC (DPPC- d4) and chain perdeuterated DPPC (DPPC-d 62). The results suggest that SP-B interacts with the bilayer in two different ways. One mode of interaction is primarily at the head group region while the other affects motions of whole molecule and is seen in our experiments only at high SP-B concentration.