Lipid-protein interaction at solid-water interface. Adsorption of human apo-high density lipoprotein to amphiphilic...

Hydrophobic glass beads with well characterized physical properties were used as a model system to study at the amphiphilic interface the properties of apolipoproteins A-I and A-II from human serum high density lipoproteins. In this study, spherical glass beads with known diameter were coated covalently with a film of silicone to varying surface density. The decrease in surface tension induced by coating was directly related to the increase in silicone film density and likely to the hydrophobicity of the glass surface. The adsorption of apo-A-I and apo-A-II to the hydrophobic glass bead surface was determined by following the decrease of 1) the radioactivity of preparations of 125I-iodinated proteins from the solution, 2) the UV absorbance of the solution at 206 nm, and 3) the fluorescence emitted by the complex formed between free protein and Fluram II in solution. All of the three measurements gave identical results. Both proteins adsorbed rapidly and reversibly to the hydrophobic glass surface. The adsorption isotherms followed the Langmuir equation with apo-A-II showing a higher surface affinity; delta Gaff = RT ln Kd has a value of -9.1 kcal/mol and -10.5 kcal/mol for apo A-I and apo A-II, respectively. The addition of canine serum high density lipoprotein (HDL) to the above system caused a rapid desorption of apolipoproteins from the beads into the aqueous phase and adsorption onto the HDL surface with no detectable structural changes of this lipoprotein. The results indicate that apo-A-I and apo-A-II can reversibly be adsorbed at a solid hydrophobic surface and that these apoproteins are capable of moving into a HDL particle if added to the system via a solution phase. The data suggest that the rate limiting aspect of the desorption-adsorption processes is the concentration of the apoproteins in solution.