Incorporation of N-Acetyl-d-glucosamine into Glycoproteins

Fractionation of goat colostrum gave a 100- to 200-fold purified enzyme that catalyzed the transfer of N-acetylglucosamine from uridine diphosphate N-acetylglucosamine to certain glycoprotein acceptors. The latter were prepared from α1-acid glycoprotein (orosomucoid) and fetuin by treatment with purified sialidase, β-galactosidase, and hexosaminidase. Ribonuclease B and ovalbumin were active as N-acetylglucosamine acceptors without pretreatment with glycosidases. Based on the structures of the glycoprotein acceptors, it is tentatively concluded that the N-acetylglucosamine residue is transferred to terminal mannose units in the glycoproteins. This reaction is suggested to be an important step in the biosynthesis of glycoproteins.