STRUCTURAL DIVERSITY IN THE ALPHA-2-]8-LINKED POLYSIALIC ACID CHAINS IN SALMONID FISH EGG GLYCOPROTEINS - OCCURRENCE OF POLY(NEU5AC), POLY(NEU5GC), POLY(NEU5AC, NEU5GC), POLY(KDN), AND THEIR PARTIALLY ACETYLATED FORMS

Alpha2-->8-Linked polysialic acid (polySia) chains terminate O-linked oligosaccharide chains on Salmonidae fish egg polysialoglycoproteins ( PSGPs). Expression of these surface PSGPs are developmentally regulate d and the polySia epitope is functionally implicated in a number of di stinct species-specific cell-cell recognition events during fertilizat ion and early embryogenesis. To better understand the functional diver sity of these PSGPs, structural studies of the polySia chains isolated from three genera and eight species of Salmonidae fish eggs were carr ied out by chemical, immunochemical, enzymatic, and H-1 NMR methods. A remarkable degree of structural diversity was found, including differ ences in the N-acyl groups, i.e. N-acetylneuraminic acid (Neu5Ac) or N -glycolylneuraminic acid (Neu5Gc), and in the presence of either O-ace tyl substitution at C4, C7, or C9 or O-lactyl substitution at C9. The presence of heteropolymers containing both Neu5Ac and Neu5Gc residues was also an unexpected finding. Accordingly, the different forms of al pha2-->8-linked homo- and heteropolymers of these polySia structures i nclude: poly(Neu5Ac), poly(Neu5Gc), poly(Neu5,xAc2), poly(Neu5GcxAc), poly(Neu5Ac, Neu5Gc), poly(Neu5Ac, Neu5,xAc2), Poly(Neu5Ac, Neu5GcxAc) , poly(Neu5Gc,Neu5,xAc2), and poly(Neu5Gc, Neu5GcxAc), where x represe nts the site of acetylation at carbon atom 4, 7, or 9. The significanc e of this new structural information, together with our recent finding of alpha2-->8-linked polydeaminoneuraminic acid, poly(KDN), in the ra inbow trout egg vitelline envelope, is that it demonstrates the natura l occurrence of multiple forms of alpha2-->8-linked polySia chains in Salmonidae fish glycoproteins that have not been previously described. The results also predict that a remarkable array of polysialylated gl ycoconjugates is yet to be discovered in animals other than teleost fi shes.