Occurrence of terminal α2 →8-linked disialylated poly-N-acetyllactosamine chains with Lex and I antigenic glycotopes in tetraantennary arms of an N-linked glycoprotein isolated from rainbow trout ovarian fluid

The Pronase digestion of a 54K glycoprotein present in ovarian fluid of rainbow trout yielded a major glycopeptide. Carbohydrate compositional analysis revealed that' this glycopeptide was likely to possess a single large Nglycan chain having low molecular weight oligomers of Nacetylneuraminic acid (oligoNeu5Ac). Structural studies of this glycopeptide revealed novel ct2 —> 8-linked disialylated poIy-A'-acetyllactosamine chains with Le and I antigenic determinants on the A'-linked tetraantennary core glycan. In our recent studies (Kitazume,S., Kitajima,K., Inoue^S., Inoue,Y. and Troy,F.A. (1994) / . Biol Chem. 269, 1033010340) we presented evidence that synthesis of a2 —> 8-linked polysialic acid (polySia) chains is a two-step process in which chain initiation is catalyzed by an ct2 —• 8-sialyltransferase (at2 -» 8-ST; initiase) that catalyzes synthesis of the first Sia ot2 -* 8-linkage, forming the disialic acid (diSia) unit, Siaa2 -» 8-Siaa2 —» 6-Gal-.Chain polymerization is then postulated to be catalyzed by a second enzyme, an a2 —» 8-polyST ("polymerase") that converts the diSia units to polySia chains. The present structural studies leading to the discovery of a2 —> 8-linked disialylated units that terminate poIy-A'-acetyllactosamine chains in an A'-linked glycoprotein is further evidence in support of our hypothesis that more than one sialyltransferase activity is required for polySia chain synthesis and polymerization.