In competitive binding assays, 10 mm free synthetic sialic acid (Sigma) was added 30 min before the endocytic agent and left during the assay. These were further purified by size-exclusion chromatography with elution in 50 mm KPi, 200 mm NaCl, pH 7.8, dialyzed against assay buffer (20 mm Tris, 150 mm NaCl, 2 mm tris(2-carboxyethyl)phosphine, pH 7. If you hav...