Fig. 1.

ST assay. The components of the ST assay include (1) a molecule or cell capable of accepting sialyl residues (sialyl acceptor). Although previous ST assays used molecules such as asialofetuin, in our assay, we used sialidase-treated RBCs or PMNs. (2) A sialic acid (or N-acetyl neuraminic acid) donor. In our assay, we use a fluorophore, CMP-5-FITC-NEU. (3) A source of ST activity. In our assay, we used exogenous ST or PMNs. Addition of CMP-5-FITC-NEU and a ST source to a sialyl acceptor mediates the transfer of sialic acid onto the acceptor (Sialylated Acceptor), which is detected by flow cytometry. This reaction is competitively blocked by CMP. Sialidase removes the sialyl residues from the acceptor cells (Desialylated Acceptor).