The apolipoprotein A-I(Milano) (apoA-I(M)) is a molecular variant of apoA-I characterized by the Arg173→Cys substitution, resulting in the formation of homodimers (A-I(M)/A-I(M)) and heterodimers with apoA-II. In order to examine the effects of the introduction of an interchain disulfide bridge on the lipid-binding properties of apoA-I, the present studies compare the kinetics of association of A-I(M)/A-I(M) and apoA-I with dimyristoylphosphatidylcholine (DMPC), and the structure and properties of reconstituted HDL containing palmitoyloleoylphosphatidylcholine (POPC) and either A-I(M)/A-I(M) or apoA-I. The results show that apoA-I dimerization does not affect the rate of association with DMPC. Apolipoprotein-POPC complexes instead, when analyzed by nondenaturing gradient gel electrophoresis, demonstrate that, differently from apoA-I, A-I(M)/A-I(M) forms only two species of rHDL particles despite a wide range of initial lipid to protein ratios. These two rHDL species contain one or two A-I(M)/A- I(M) molecules and have a diameter of 7.8 nm and 12.5 nm. Investigations of the A-I(M)/A-I(M) structure in these two rHDL, by circular dichroism, fluorescence, and second-derivative UV spectroscopy, suggest that the secondary and tertiary structures of A-I(M)/A-I(M) re remarkably similar in both small and large particles. These results suggest that the introduction of an interchain disulfide bridge does not affect the association of apoA-I with lipids but restricts HDL particle size heterogeneity, thus possibly affecting HDL function in lipid metabolism and atherosclerosis protection.
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