No abstract

The selectins are adhesion receptors that play key roles in leukocyte trafficking. Each has an N-terminal C-type lectin domain that binds to specific carbohydrates in a calcium-dependent manner. L-selectin recognizes sulfated, sialylated ligands on lymph node high endothelial venules. This recognition is abolished by strong periodate oxidation under conditions that destroy oligosaccharides. In contrast, mild periodate oxidation, which selectively oxidizes the side chain of sialic acid residues without affecting the underlying oligosaccharide, markedly enhances this interaction. The enhancement is calcium dependent, indicating that lectin recognition is maintained. Reduction of the sialic acid aldehydes generated by mild periodate to alcohol groups abolishes this effect. Covalent cross-linking of the oxidized ligand to L-selectin can be demonstrated, suggesting Schiff base formation between lysine residues of the selectin and the newly formed aldehydes. Such selectively oxidized sialylated ligands could be used to probe the lectin domains of the selectins and to identify lysine residues near the binding site. Also, this approach could be used to design drugs for disrupting leukocyte-endothelial interactions leading to pathological inflammation.The selectins are a family of adhesion receptors involved in leukocyte migration into lymphoid tissues and areas of inflammation (1-7). Each selectin has an N-terminal C-type lectin domain that binds specific carbohydrate ligands in a calcium-dependent manner. These domains also contain 10-14 lysine residues (8), which may be involved in ligand binding. Naturally occurring selectin ligands contain sialylated fucosylated lactosamine-type oligosaccharides (9)(10)(11). Sulfation of these and/or other oligosaccharides may also be involved in binding (7,9,(12)(13)(14)(15). Two specific ligands have been more precisely identified. The L-selectin binds certain sulfated and sialylated glycoproteins from lymph node high endothelial venules (HEVs) (13, 16), and P-selectin binds a minor 120-kDa neutrophil sialoglycoprotein (17). In both instances, sialic acids are absolutely required for binding.Periodate oxidation has been used to destroy carbohydrates and to demonstrate biological functions, including lymphocyte-endothelium recognition (18). Such studies used strong periodate oxidation (e.g., 50 mM; pH 5.0), which causes cleavage between cis-diols throughout oligosaccharides and destroys them. In contrast, mild periodate oxidation (e.g., 1-2 mM; pH 7.0) selectively oxidizes only the side chain of terminal sialic acids (19)(20)(21)(22), leaving the rest of the oligosaccharide intact. Conditions can be adjusted to selectively eliminate only the C9 carbon or both the C8 and C9 carbons (Fig. 1, structures B and C). The resulting aldehyde can be reduced, leaving a truncated side chain with a primary alcohol group (Fig. 1, structures D and G). These reactions have been used to explore the importance of the sialic acid side chains in interactions involving viral hemagglutinins, lec...