Frequent occurrence of pre-existing α2→8-linked disialic and oligosialic acids with chain lengths up to 7 Sia residues in mammalian brain glycoproteins -: Prevalence revealed by highly sensitive chemical methods and anti-di-, oligo-, and poly-Sia antibodies specific for defined chain lengths

The pre-existence of alpha 2-->8-linked disialic acid (di-Sia) and oligosialic acid (oligo-Sia) structures with up to 7 Sia residues was shown to occur on a large number of brain glycoproteins, including neural cell adhesion molecules (N-CAMs), by two highly sensitive chemical methods (Sato, C., Inoue, S., Matsuda, T., and Kitajima, K. (1998) Anal. Biochem. 261, 191-197; Sate, C., Inoue, S,, Matsuda, T., and Kitajima, K, (1999) Anal Biochem, 266, 102-109). This unexpected finding was also confirmed using a newly developed antibody prepared using a copolymer of alpha 2-->8-linked N-acetylneuraminyl p-vinylbenzylamide and acrylamide as an immunogen and known antibodies whose immunospecificities were determined to be di- and oligo-Sia residues with defined chain lengths. The major significance of the new finding that di- and oligo-Sia chains exist on a large number of brain glycoproteins is 2-fold. First, it reveals a surprising diversity in the number and M-r of proteins distinct from N-CAM that are covalently modified by these short sialyl glycotopes. Second, it suggests that synthesis of di- and/or oligo-Sia units may be catalyzed by alpha 2-->8-sialyltransferase(s) that are distinct from the known polysialyltransferases, STX and PST, which are partially responsible for polysialylation of N-CAM.