The surface properties of apolipoproteins A-I and A-II at the lipid/water interface

Abstract

The monolayer system was employed to investigate the relative affinities of apolipoproteins A-I and A-II for the lipid/water interface. The adsorption of reductively 14C-methylated apolipoproteins to phospholipid monolayers spread at the air/water interface was determined by monitoring the surface pressure of the mixed monolayer and the surface concentration of the apoprotein. ApoA-II has a higher affinity than apoA-I for lipid monolayers; for a given initial surface pressure, apoA-II adsorbs more than apoA-I to monolayers of egg phosphatidylcholine (PC), distearoyl-PC and human high-density lipoprotein (HDL 3) surface lipids. Comparison of the molecular packing of apolipoproteins A-I and A-II suggests that apoA-II adopts a more condensed conformation at the lipid/water interface compared to apoA-I. The ability of apoA-II to displace apoA-I from egg PC and HDL 3 surface lipid monolayers was studied by following the adsorption and desorption of the reductively 14C-methylated apolipoproteins. At saturating subphase concentrations of the apoproteins (3·10 −5 g/100 ml), two molecules of apoA-II adsorbed for each molecule of apoA-I displaced. This displacement was accompanied by an increase in surface pressure. An identical stoichiometry for the displacement of apoA-I from HDL particles by apoA-II has been reported by others. At low subphase concentrations of apoproteins (5·10 −6 g/100 ml), the apoA-I/lipid monolayer was not fully compressed and could accomodate the adsorbing apoA-II molecules without displacement of apoA-I molecules. ApoA-I molecules were unable to displace apoA-II from the lipid/water interface. The average residue hydrophobicity of apoA-II is higher than that of apoA-I; this may contribute to the higher affinity of apoA-II for lipids compared to apoA-I. The probable helical regions in apolipoproteins A-I and A-II were located using a secondary structure prediction algorithm. The analysis suggests that the amphiphilic properties of the α-helical regions of apoA-I and apoA-II are probably not significantly different. Further understanding of the differences in surface activity of these apolipoproteins will require more knowledge of their secondary and tertiary structures.