Diversity in sialic and polysialic acid residues and related enzymes*

The occurrence of extensive diversity in the family of sialic acids is now well recognized but biological signi®cance of this diversity has been clari®ed only partially. In 1986 we showed natural occurrence of deaminated form of neuraminic acid, 2-keto-3-deoxy-Dglycero-D-galacto-nononic acid (KDN) as a capping residue of polysialyl chain of rainbow trout egg polysialoglycoprotein. Subsequent studies of our own and by others showed ubiquitous occurrence of KDN from bacteria to human although expression of this residue in large amounts was limited to certain species of animal. KDN was different from other `families' of sialic acids usually denoted as modi®ed N-acylneuraminic acids of which O-acetyl substitution is most frequently found. The de®nition of sialic acid has to be changed by the ®nding of KDN. Prior to the ®nding of poly N-acetylneuraminic acid group in mammals, in 1978 we discovered poly N-glycolylneuraminic acid in rainbow trout eggs. This was the ®rst report of the occurrence of polysialic acid structure in animal glycoproteins. We also found the occurrence of polyKDN chains. All these ®ndings made signi®cant contribution to show that diversity of sialic acid family and especially diversity in polysialic acid structure are much more extensive and more ubiquitous than generally believed before. We showed that diversity of polysialic acid is not only originated from the diversity in building block but also the type of interresidue linkage. The diversity of sialic acid and polysialic acid is re ̄ected in diversities in molecules that recognize these structures. We studied some of these molecules that speci®cally recognize different types of sialic acids and polysialic acids, and biosynthetic mechanism of polysialic acids.