Ionization Behaviour of Native Apolipoproteins and of Their Complexes with Lecithin

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Abstract
The ionization behavior of native apoA-I protein [human plasma] is compared to that of its complex with synthetic dimyristoyl lecithin in studies using calorimetric, potentiometric and spectrophotometric titration. In the presence of phospholipids, 10 out of the 21 lysines together with 22 acidic residues are masked in the complex. All tyrosines remain accessible to titration below pH 13. The apparent ionization enthalpy of the 11 lysine residues is not affected by the presence of phospholipids. These data are consistent with discrete binding sites located in the apoprotein helical segments as suggested by the model of Segrest et al. A tentative localization of lysine, arginine, aspartic acid and glutamic acid residues directly involved in phospholipid binding is suggested, assuming that such helical regions are involved in apoprotein-phospholipid association.