UDP-GlcNAc:Gal beta 1-4Glc(NAc) beta 1-3N-acetylglucosaminyltransferase. Identification and characterization in human serum.

A beta 1-3-N-Acetylglucosaminyltransferase has been detected in human serum which transfers N-acetylglucosamine residues from UDP-GlcNAc to terminal Gal beta 1-4Glc(NAc) structures in oligosaccharides, glycoproteins, glycolipids, and proteoglycans. The product of the transferase reaction with lactose as acceptor was identified by methylation analysis and mass spectrometry as GlcNAc beta 1-3Gal beta 1-4Glc. The beta-linkage of the GlcNAc in the synthesized trisaccharide was confirmed by the action of the specific enzymes beta-hexosaminidase and beta-N-acetylglucosaminide beta 1-4-galactosyltransferase. Kinetic parameters were determined for UDP-GlcNAc, lactose, and N-acetyllactosamine. The enzyme requires Mn2+ ions for maximal activity and shows a pH optimum between 6 and 8. Using a wide variety of synthetic and natural oligosaccharides, the substrate specificity of the beta 1-3N-acetylglucosaminyltransferase was investigated. The enzyme was found to recognize specifically the free terminal structure Gal beta 1-4Glc(NAc). The substrate specificity was found to be equally stringent for glycoconjugates. Among the glycoproteins and glycolipids tested as acceptors, N-acetylglucosamine was incorporated only into those containing free terminal Gal beta 1-4Glc(NAc) structures. When the terminal galactose residues were partially removed, the transfer of N-acetylglucosamine was strongly reduced.