Lipid binding properties of the Tangier apolipoprotein A-I and its isoproteins

The apolipoprotein A-I was isolated from the plasma of normal individuals and of three homozygous patients with Tangier disease by immunoprecipitation. The apoA-I isoforms were further fractionated by isofocusing on polyacrylamide gels. The physicochemical behavior of normal and Tangier apoA-I and of the isoproteins-2 and -4 was studied by monitoring the tryptophanyl fluorescence emission as a function of temperature, pH, and under exposure to guanidinium (guanidine) hydrochloride (GdmCl). Lipid-apoprotein complexes were generated by incubation with dimyristoylphosphatidylcholine and isolated by density gradient ultracentrifugation. Our results show that normal apoA-I and its isoprotein-4 associate with lipids to yield a complex containing 150-200 mol lecithin/mol apoA-I. The isoprotein-2 of normal apoA-I and the isoprotein-4 of Tangier apoA-I generate lipid-rich complexes with lecithin, while the isoprotein-2 of Tangier apoA-I shows only a limited association with lipids. ApoA-I normal and Tangier and their isoproteins-4 undergo a structural transition around 45 degrees C, which is not observed in the lecithin-apoA-I complexes. This transition is accompanied by an increased exposure of the tryptophanyl residues to the solvent. This transition was observed for the isoprotein-2 of apoA-I Tangier both in its lipid-free form and in the presence of lecithin. The pH denaturation of apoA-I and of the isoprotein-4 between pH 9 and 13 and between pH 7 and 2 is accompanied by a similar conformational transition. The transition occurs around pH 10.8 for the native apoproteins and is shifted towards respectively higher and lower pH's as result of the protective action of lipid binding on the protein conformation. Such an effect was not observed with the isoprotein-2 of apoA-I Tangier which is denatured at lower pH's both in its native form and in a lipid-protein mixture. Finally the denaturation of apoA-I by GdmCl indicates that apoA-I normal and Tangier undergo structural changes around 1 M GdmCl, whereas the apoA-I-Tangier-lecithin complex is more susceptible to denaturation than the complex with apoA-I normal. These data suggest that the apoA-I normal and Tangier and their isoproteins-4 are able to associate with lipids although the association between apoA-I Tangier with lecithin is weaker than that of apoA-I normal. The isoprotein-2 of normal apoA-I associates to a greater extent with lipids than the isoprotein-2 of Tangier apoA-I, whose structure differs from that of the isoprotein-4.(ABSTRACT TRUNCATED AT 400 WORDS)