BOVINE COLOSTRUM CMP-NEUAC-GAL-BETA(1-]4)GLCNAC-R ALPHA(2-]6)-SIALYLTRANSFERASE IS INVOLVED IN THE SYNTHESIS OF THE TERMINAL NEUAC-ALPHA(2-]6)GALNAC-BETA(1-]4)GLCNAC SEQUENCE OCCURRING ON N-LINKED GLYCANS OF BOVINE-MILK GLYCOPROTEINS

Bovine colostrum CMP-NeuAc:Galbeta(1-->4)GlcNAc-R alpha(2-->6)-sialyltransferase (alpha6-sialyltransferase) appears to be capable of catalysing alpha6-sialylation of the disaccharide GalNAcbeta(1-->4)GlcNAc to yield the trisaccharide NeuAcalpha(2-->6)GalNAcbeta(1-->4)GlcNAc. This provides an enzymic basis for the occurrence of this sialylated structure on the N-linked glycans of a number of bovine milk glycoproteins. Competition experiments using Galbeta(1-->4)GlcNAc and GalNAcbeta(1-->4)GlcNAc as acceptors indicate that both substrates are recognized by a single active site on the alpha6-sialyltransferase. Extrapolation of these results suggests that the NeuAcalpha(2-->6)GalNAcbeta(1-->4)GlcNAc structural element occurring on the N-linked glycans of several human glycoproteins are similarly synthesized by the action of a Galbeta(1-->4)GlcNAc-R alpha(2-->6)-sialyltransferase.