The interactions ofhemoglobin S with the erythrocyte membrane were compared with the corresponding interactions of hemoglobin A by measuring in both steady-state and kinetic experiments the quenching of the fluorescence of a probe embedded in erythrocyte membranes. Whereas hemoglobin A could be dissociated from membranes, a fraction of hemoglobin S was irreversibly bound even in the oxy state. Deoxyhemoglobin S interacted much more strongly with erythrocyte membranes than did deoxyhemoglobin A: a portion of the deoxyhemoglobin S was irreversibly bound, and the reversibly bound fraction ofhemoglobin S dissociated more slowly than did deoxyhemoglobin A. It is suggested that the binding of deoxyhemoglobin S is a two-step reaction in which the first step involves electrostatic interaction with band III erythrocyte membrane protein and the second step involves a hydrophobic interaction with membrane lipids. The latter reaction reflects the greater hydrophobicity ofhemoglobin S. The unique interaction of hemoglobin S with erythrocyte membranes may be important in the formation of irreversibly siclded cells.The decreased erythrocyte flexibility observed in sickled cells reflects changes in the erythrocyte membrane (1). In some cells, irreversible sickling takes place and the erythrocyte does not resume the normal shape on exposure to oxygen. The changes in membrane structure that are responsible for irreversible sickling have not been defined, although loss of sialic acid (2) and possible alterations in membrane proteins (3) have been described, and an increase in membrane-bound hemoglobin has been noted in erythrocytes ofindividuals with sickle cell anemia (4). The erythrocyte membranes ofpatients with sickle cell anemia are initially normal, although they may undergo secondary structural changes as a result of sickle-unsickle cycles. Our observations of the effects of sickle-cell hemoglobin (Hb S) on erythrocyte membranes prepared from normal individuals were designed to ascertain whether the interactions ofHb S with normal erythrocyte membranes differed from such interactions of normal human hemoglobin A (Hb A). Shaklai and Abrahami (5) utilized similar techniques for studies of the interactions of deoxy Hb A, which showed a lower affinity of the unliganded hemoglobin for erythrocyte membranes. The mutational change (normal (36 glutamic acid is replaced by valine) that characterizes Hb S results in a greater positive charge (higher isoelectric point) and greater surface hydrophobicity (6) than in Hb A. Hb A2, the minor basic component of hemoglobin obtained from normal adults, has a higher isoelectric point than does Hb S, but does not share its greater surface hydrophobicity. In the present studies, the interactions ofthese three hemoglobins with erythrocyte membranes were compared in steady-state and kinetic observations.
MATERIALS AND METHODSAll chemicals used were reagent grade. Anthroyl stearic acid was a product of Molecular Probes (Plano, TX); glyceraldehyde-3-phosphate dehydrogenase was purchased fr...