Evidence for restricted oligosaccharide mobility at the erythrocyte membrane surface: A fluorescence study

The glycophorins of whole, human erythrocytes were labeled at their sialic acid residues with one of three fluorescent probes. After preparation of the erythrocyte ghosts, the mobility of each fluorescent probe on the intact membrane was compared with its mobility on the isolated, labeled glycopeptides dissolved in aqueous buffer. A four- to ninefold decrease in the rotational relaxation time, as defined by the Perrin equation, accompanied the proteolytic removal of the labeled glycopeptides from the membrane. This suggests that the fluorescent probes, and by extrapolation, the sugars to which they are immediately attached, are restricted in their mobility at the membrane surface. A crude model of the carbohydrate layer of the erythrocyte surface was constructed by incorporating the labeled, tryptic glycopeptides into agarose gels of different agarose content. A decrease in the probe's mobility was observed as agarose content was raised. This indicates that the high oligosaccharide density at the erythrocyte membrane surface may contribute to the observed immobilization of the fluorescent probes in situ.