Biophysical characterization of interaction between apolipoprotein A-I and bacterial lipopolysaccharide

We studied the effect of bacterial lipopolysaccharide (LPS)-apolipoprotein A-I (apo A-I) interaction on the structure and function of this protein. The micellization process of dimirystoil phosphatidylcholine liposomes (MLV-DMPC) by apo A-I in the presence of LPS was characterized. Apo A-I may interact with MLV-DMPC at the lipid transition temperature, forming micellar complexes. The kinetics of MLV-DMPC micellization was studied by turbidimetry. In the absence of LPS, a monoexponential decrease in turbidity is observed. Preincubation of apo A-I with LPS impairs the micellization reaction, resulting in biphasic kinetics. The amplitude of the fast phase decreases with increasing concentrations of LPS. In the absence or in the presence of low amounts of LPS (1∶0.1 protein:LPS weight ratio), two major micellization products-containing two and three apo A-I molecules per particle-were observed. However, in the presence of higher amounts of LPS (1∶1 protein:LPS weight ratio), particles mainly contained two apo A-I molecules. In contrast, a decrease in intrinsic fluorescence intensity of the protein was observed in the presence of an increasing LPS concentration. Finally, we studied the effect of LPS on the transition temperature (Tt) of MLV-DMPC without detecting changes in Tt. In conclusion, the changes found in the micellization process are likely to be mainly caused by changes in the apo A-I conformation by LPS interaction in solution.