The cytosol-membrane interface of normal and sickle erythrocytes. Effect of hemoglobin deoxygenation and sickling.

The effect of deoxygenation on the amount of hemoglobin (Hb) proximal to the membranes of intact, density-fractionated normal (AA) and sickle (SS) red cells was studied by estimating resonance energy transfer efficiencies from fluorescent probes, 12-(9-anthroyloxy)stearic acid or 2-(9-anthroyloxy)stearic acid, in the outer lipid layer to cytoplasmic hemes. For each density fraction, heme concentrations at the cytosol-membrane interface (hb) were derived from the probe decay rates for ghosts and intact cells, measured by front-face fluorometry, and compared with mean cell heme concentrations (hc). With AA cells, hb/hc varied little with cell density; a 33% drop in hb on deoxygenation is attributable to organic phosphate binding to deoxy-Hb. With oxy-SS cells, hb/hc increased with cell density to twice the values for AA cells, but SS ghosts showed no evidence of increased probe quenching by membrane-bound Hb. On deoxygenation, hb for each SS density fraction fell (reversibly) to one-third the oxy value. The finding that deoxy-HbS withdraws from the membrane bilipid layer much more than deoxy-HbA is consistent with evidence for an increased net negative charge on deoxy-HbS polymers and/or the suggestion that the cytoskeleton, readily penetrated by monomeric Hb, presents a barrier to polymeric HbS. Membrane-associated HbS is therefore thought to play an unimportant role in polymerization.