Hemoglobin (Hb) was incubated with small unilamellar vesicles of different lipid compositions in physiological or low pH-low ionic strength media, and the alterations in the protein and bilayer structure were analyzed by measuring (i) the sedimentation properties and elution pattern of the vesicles upon gel filtration, (ii) the quenching effect of Hb on the fluorescence of membrane-embedded probes, (iii) the intrinsic fluorescence of Hb, and (iv) the Soret spectra of Hb. The results indicate complex formation between Hb and the membrane, followed by intercalation of the protein into the bilayer. These processes can lead to secondary alterations, including aggregation, peroxidative decomposition of unsaturated fatty acids, unfolding of Hb, oxidation of the heme iron, displacement of the heme relative to globin, and oxidative or nonoxidative deconjugation of the porphyrin ring. Complex formation and intercalation of Hb into the bilayer are primarily due to hydrophobic interaction between the protein and the membrane, whereas displacement of the heme and its nonoxidative deconjugation are elicited by ionic interaction between the heme and negative membrane surface charges. These charges, on the other hand, inhibit the oxidative processes in unsaturated lipid vesicles. The rate and extent of alterations both in the protein and in the membrane show inverse correlation with the stability of the heme-globin linkage. The liquid-crystalline phase state promotes the penetration of Hb into the bilayer, whereas displacement of the heme is most expressed in the case of saturated bilayers in the gel state. Cholesterol inhibits both the hydrophobic and ionic interactions between the protein and bilayer, most remarkably the partition of the heme into negatively charged saturated bilayers. It is suggested that the effects of cholesterol are mediated through changes in membrane fluidity.
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