Ionization Behaviour of Native Apolipoproteins and of Their Complexes with Lecithin

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Abstract
A comparison of the ionization behavior of the human [plasma] apoA-II, apoC-I, apoC-III proteins and of their complexes with dimyristoyl lecithin is based on potentiometric titration of the basic and acidic residues and spectrophotometric titration of the phenolic groups. Experimental data suggest that a number of lysine, arginine, aspartic acid and glutamic acid residues are masked in the complexes. For each of these amino acids and in all 3 proteins the number of masked residues is consistent with the content of those regions predicted to be involved in lipid binding by the model of Segrest et al. These data taken together with the results of calorimetric and titration experiments with the apoA-I protein reported in the accompanying article strongly support the general nature of the proposed model and further suggest that ionic interactions have some role in the formation of the dimyristoyl lecithin/apolipoprotein complexes.