Effect of pulmonary surfactant protein

The effects of surfactant-associated protein A (SPA) on lipid adsorption to the air-water interface and accumulation of dipalmitoylphosphatidylcholine (DPPC) in the surface region were investigated at 37%. Dispersions used were bovine pulmonary lipid extract surfactant with or without neutral lipid (NL). Lipid adsorption was examined with the Wilhelmy plate technique and DPPC accumulation by monitoring surface radioactivity from [14C]DPPC with a scintillation probe. SP-A enhanced the rate of lipid adsorption, while both SP-A and NL increased the extent of DPPC accumulation. At the specific radioactivity used [I4C]DPPC monolayers were undetectable: the surface radioactivity arose from surface-associated DPPC beneath the monolayer. At the highest concentration studied (0.3 mg lipid/ml), NL greatly enhanced DPPC accumulation and SP-A attenuated this effect. Langmuir-Blodgett (L-B) films were prepared from [ *4C]DPPC-labeled dispersions (0.3 mg lipid/ml) at equilibrium surface tension. Autoradiographs of L-B films from lipid extract surfactant exhibited a diffuse misty appearance, while NL promoted formation of heterogeneous DPPC aggregates. Addition of SP-A to lipid extracts without NL generated DPPC aggregates; more uniform larger aggregates appeared in the presence of SP-A and NL. Radiation measurements confirmed that the L-B films were composed of more than monolayers. SP-A did not increase DPPC levels in films deposited from lipid extracts unless NL was present. P These results indicate that neutral lipid cooperates with surfactantassociated protein A to organize dipalmitoylphosphatidylcholine in the surface films and enhance formation of a DPPC-rich reservoir below the air-water interface.-Yu, S-H., and F. Possmayer. Effect of pulmonary surfactant protein A and neutral lipid on accretion and organization of dipalmitoylphosphatidylcholine in surface films. J. Lipid Res. 1996. 37: 1278-1288. Supplementary key words terol adsorption surface tension Langmuir-Blodgett film surfactant-associated protein A cholesmaterial responsible for reducing the surface tension of the alveolus to very low values. At physiological temperatures, DPPC alone adsorbs and spreads very slowly to form a surface film. However, pulmonary surfactant does so readily. It is believed that the main function of non-DPPC components in pulmonary surfactant is to fluidize DPPC, thereby assisting DPPC adsorption, spreading and respreading ( 1-5). Nonetheless, their exact functional relationship in pulmonary surfactant is not clear. Pulmonary natural surfactant consists of approximately 90% lipids and 10% surfactant-associated proteins, namely SP-A, SP-B, and SP-C (6). Phospholipid (PL) accounts for approximately 96% of the total lipids in bovine pulmonary surfactant (7). Of the total PL, 40% is DPPC, 3'7% unsaturated phosphatidylcholine (PC), 3% dimyristoylphosphatidylcholine (DMPC), 12% phosphatidylglycerol (PG), 2-3% each of phosphatidylethanolamine (PE) and sphingomyelin (7). The amount of neutral lipid in pulmonary surfactants varies from 4% (w/w) in bovine (7) to 7% in canine surfactant (5). Cholesterol is the major component of the neutral lipid (approximately 90% in bovine). This sterol could act as a fluidizer to enhance the adsorption and respreading of DPPC (8, 9). However, cholesterol destabilizes surface films under compression and impairs the surface tension lowering ability of surfactants in the absence of SP-A (10-14). Approximately 80-90% of the total surfactant protein Pulmonary surfactant is essential for normal lung function. surfactant stabilizes the lung by reducing the surface tension Of the Dipalmitoylphosphatidylcholine (DPPC) is the principal Abbreviations: DPPC, dipalmitoylphosphatidylcholine; L-B film, Langmuir-Blodgett film; NL, neutral lipid; PL, phospholipid; bLES, bovine lipid extract surfactant; SP-, surfactant-associated protein. during expiration ( 'To whom correspondence should be addressed. 1278 Journal of Lipid Research Volume 37, 1996 by gest, on N ovem er 6, 2017 w w w .j.org D ow nladed fom is SP-A. Investigations into the biophysical functions of surfactant proteins have demonstrated that the low molecular weight hydrophobic proteins, SP-B and SP-C, significantly improve the surface activities of surfactants (15-17). SP-A enhances the ability of SP-B to promote PL adsorption and surface tension lowering in the presence of calcium (16-20). Our previous studies showed that SP-A suppressed the appearance of radioactive cholesterol at the surface and stabilized surfactant films containing cholesterol at surface tensions compressed to below equilibrium (1 0). These results indicated that SP-A may be able to influence the composition of the lipids in the surface films. Moreover, studies conducted with the captive bubble tensiometer revealed that addition of SP-A to bovine lipid extract surfactant (without neutral lipid) decreased the extent of surface area reduction required to achieve near zero surface tension. This suggested that SP-A either promoted the selective transport of DPPC to the surface monolayer or generated a very effective mechanism for the removal of non-DPPC lipids during lateral compression (21). We have attempted to clarify these suggestions by labeling surfactant preparations with [ 14C]DPPC and monitoring surface radioactivities. We have also investigated the effects of neutral lipid on the transport of DPPC to the surface region in the presence and absence of SP-A. For clarification it should be noted that in this paper “adsorption” will be used to indicate monolayer formation and ”surface films” will refer to the monolayer plus monolayer-associated surfactant material. Material detected by the surface radioactivity probe will be referred to as being in the surface region. MATERIALS AND METHODS