FUNCTIONAL-GROUPS OF SIALIC ACIDS INVOLVED IN BINDING TO SIGLECS (SIALOADHESINS) DEDUCED FROM INTERACTIONS WITH SYNTHETIC ANALOGS

The siglecs, formerly called sialoadhesins, are a family of I-type lec tins binding to sialic acids on the cell surface. Five members of this family have been identified: sialoadhesin, myelin-associated glycopro tein (MAG), Schwann cell myelin protein (SMP), CD22 and CD33. We have investigated the relevance of substituents at position C-9 and in the N-acetyl group of N-acetylneuraminic acid, using a series of synthetic sialic-acid analogues either on resialylated human erythrocytes or as free a-glycosides in hapten inhibition. All five siglecs require the hydroxy group at C-9 for binding, suggesting hydrogen bonding of this substituent with the binding site. Remarkable differences were found a mong the proteins in their specificity for modifications of the N-acet yl group. Whereas sialoadhesin, MAG and SMP do not tolerate a hydroxy group as in N-glycolylneuraminic acid, they bind to halogenated acetyl residues. In the case of MAG, N-fluoroacetylneuraminic acid is bound about 17-fold better than N-acetylneuraminic acid. In contrast, human and murine CD22 both show good affinity for N-glycolylneuraminic acid, but only human CD22 bound the halogenated compounds. In conclusion, o ur data indicate that interactions of the hydroxy group at position 9 and the N-acyl substituent contribute significantly to the binding str ength.