A ligand binding assay for E-selectin

We describe here a cell-free ligand binding assay for E-selectin. The assay involves immobilizing soluble E-selection onto microtiter plates and incubating with 125I-labeled carcinoembryonic antigen (CEA) which carries the tetrasaccharide sialyl Lewis x (sLex). The bound CEA is eluted by ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N',-tetraacetic acid and monitored by radioactivity. The binding is abolished by preincubation of either CEA with an anti-sLex antibody or E-selectin with a neutralizing anti-E-selectin antibody, indicating that this is an E-selectin/sLex-specific interaction. The binding is Ca2+ and pH dependent (the optimal pH at 7.0) and also requires sialic acid. Removal of sialic acid from CEA by neuraminidase digestion abrogates the binding. When the protein but not carbohydrate constituent of CEA was denatured by reduction and alkylation, the binding was partially eliminated, indicating that the interaction may also be dependent on either protein conformation or carbohydrate orientation. The assay is simple, sensitive, and comparable to other methods as indicated by an IC50 of 1 mM for sLex. Thus, the assay should be useful for screening antagonists and studying E-selectin structure/function.