Human serum galactosyltransferase: distinction, separation and product identification of two galactosyltransferase activities.

Two different galactosyltransferase activities have been found in normal sera from A and O donors. Galactosyltransferase A incorporated galactose from UDP-Gal into sialic-acid-free ovine submaxillary mucin (asialo-mucin), whereas galactosyltransferase B transferred galactose from UDP-Gal to free N-acetylglucosamine or N-acetylglucosamine-glycoproteins. Specificity, kinetic and stability differences permitted the distinction of the activity of galactosyltransferase A from that of galactosyltransferase B; the only substrate found for galactosyltransferase A was asialo-mucin, whereas galactosyltransferase B showed only low activity towards asialo-mucin and free N-acetyl-galactosamine, but had a main specificity for either free N-acetylglucosamine or N-acetylglucosamine-protein. Galactosyltransferase B was more stable on heat inactivation than galactosyltransferase A; galactosyltransferase B could be separated from galactosyltransferase A by affinity chromatography on N-acetylglucosamine-derivatized agarose. The products of both enzyme activities have been analyzed. The galactosyltransferase A product was cleaved from asialo-mucin by alkaline-borohydride treatment. The acceptor used to identify the galactosyltransferase B product was free N-acetylglucosamine. Periodate oxidation studies performed on the reduced disaccharides indicated the linkage type of the products. The anomeric configuration of the respective galactosyltransferase products were determined with specific galactosidases. Using these methods, galactosyltransferase A was found to form a Galbeta (1 leads to 3)GalNAc-protein linkage and galactosyltransferase B was found to form a Galbeta(1 leads to 4)GlcNAc-linkage.