CHOLESTEROL MODIFIES THE PROPERTIES OF SURFACE-FILMS OF DIPALMITOYLPHOSPHATIDYLCHOLINE PLUS PULMONARY SURFACTANT-ASSOCIATED PROTEIN-B OR PROTEIN-C SPREAD OR ADSORBED AT THE AIR-WATER-INTERFACE

Cholesterol is a substantial component of pulmonary surfactant (simila r to 8 wt % or similar to 14 mol % of surfactant lipids). This study i nvestigated the effect of cholesterol on the way in which hydrophobic SP-B and SP-C modulated the adsorption of lipid into the air-water int erface and their respreading from collapsed phase produced on overcomp ression of the surface film. The properties of binary spread monolayer s of SP-B or SP-C plus cholesterol (CH) were consistent with miscibili ty between the hydrophobic proteins and the sterol. Results from surfa ce pressure versus area measurements at 23 degrees C on spread monolay ers of dipalmitoylphosphatidylcholine (DPPC) plus SP-B in the presence of 8 wt % cholesterol implied that CH did not significantly affect th e properties of the films of SP-B/(DPPC/CH) compared to those of binar y SP-B/DPPC monolayers, In contrast, CH appeared to enhance the mixing of SP-C with DPPC/CH in ternary SP-C/(DPPC/CH) films compared to the miscibility of SP-C with DPPC in the SP-C/DPPC films. It is estimated that about 10 wt % SP-C might remain in the SP-CZ(DPPC/CH) monolayers compressed to high surface pressures of about 72 mN/m, whereas SP-C at concentrations of greater than or equal to 5 wt % was squeezed out at pi approximate to 50 mN/m from SP-C/DPPC films without cholesterol. C holesterol reduced the stability of the films of SP-B/(DPPC/CH) and SP -C/(DPPC/CH) when they had been compressed to pi approximate to 72 mN/ m, in contrast to films of SP-B/DPPC and SP-C/DPPC which exhibited a r elatively slow relaxation from the collapse pressure of 72 mN/m. Dynam ic cyclic compression beyond collapse of SP-B/(DPPC/CH) and SP-C/(DPPC /CH) monolayers showed that cholesterol diminished their postcollapse respreading compared to the respreading of the protein/DPPC films with out cholesterol. Cholesterol, at 8 wt %, inhibited the rate of adsorpt ion to the air-water interface at 35 degrees C of aqueous dispersions of DPPC containing 2.5 or 5 wt % SP-B or SP-C. The results suggest tha t cholesterol has an apparent negative influence on the surfactant sur face properties, which are generally considered to be important in sur factant function, although increasing protein concentrations can count eract some of the negative influences.