Covalent labelling of specific membrane carbohydrate residues with fluorescent probes

Two new fluorescent labelling techniques are described: one specific for sialic acid residues and a second specific for galactose and some of its derivatives. Using either technique it is possible to label covalently the desired carbohydrate residue with any one of a large variety of fluorescent probes. When the sialic acid labelling procedure is applied to human erythrocyte membranes, only the glycophorin species are labelled. However, when the galactose-directed labelling scheme is applied, fluorescence is also observed on membrane lipid components and on band 3. For each technique, the fluorescent labelling pattern is shown to reflect the distribution of the respective sugar component on the erythrocyte membrane. Thus, these techniques should provide both selectivity and versatility in the fluorescent labelling of specific carbohydrate residues in highly heterogeneous biological systems.