Regulation of selectin binding activity by cyclization of sialic acid moiety of carbohydrate ligands on human leukocytes

We provide here evidence that supports the occurrence of a biologically dor mant form of selectin ligand carbohydrate, the sialyl 6-sulfo Lewis X conta ining modified sialic acid, in human leukocytes, The modification of sialic acid involves first de-N-acetylation of sialic acid moiety through ubiquit ous de-N-acetylation/re-N-acetylation cycle, followed by the dehydrative cy clization of de-N-acetyl sialic acid to form "cyclic sialic acid," The enzy me involved in the dehydration of de-N-acetyl sialic acid is a calcium-depe ndent enzyme having neutral-alkaline pH optimum. De-N-acetyl sialyl 6-sulfo Lewis X retained selectin binding activity as well as parental sialyl 6-su lfo Lewis X, but cyclic sialyl 6-sulfo Lewis X was devoid of selectin bindi ng activity. Sialyl 6-sulfo Lewis X carrying the cyclic sialic acid is spec ifically recognized by the newly generated mAb, G159. The determinant was d istributed widely among normal human leukocytes, especially on monocytes an d subsets of lymphocytes including NK cells, helper memory T cells, Tcr-gam ma delta T cells, and a part of B cells. The determinant was detected also on several cultured lymphocytic leukemia cell lines and O-tetradecanoylphor bol 13-acetate-activated lymphoid cells. Cyclic sialyl 6-sulfo Lewis X is e fficiently formed by the action of the partly membrane-bound calcium-depend ent enzyme, tentatively called "sialic acid cyclase," and a possible physio logical significance of this reaction could be a rapid inactivation of sele ctin binding activity at the cell surface. Conversely, the accumulated intr acellular cyclic sialyl 6-sulfo Lewis X determinant may function as a dorma nt pool of selectin ligands, which, on appropriate stimulation, is hydrolyz ed and becomes active in selectin-dependent cell adhesion.