Biosynthesis of a disialylated sequence in N-linked oligosaccharides: identification of an N-acetylglucosaminide (alpha26)-sialyltransferase in Golgi apparatus from rat liver

Abstract

Rat liver Golgi apparatus have a CMP-N-acetylneuraminate: N-acetylglucosaminide (.alpha.2 .fwdarw. 6)-sialyltransferase which catalyzes the conversion of the human milk oligosaccharide LS-tetrasaccharide-a(NeuAc.alpha.2 .fwdarw. 3Gal.beta.1 .fwdarw. 3GlcNAc.beta.1 .fwdarw. 3Gal.beta.1 .fwdarw. 4Glc) to disialyllacto-N-tetraose containing the terminal sequence found in N-linked oligosaccharides of glycoproteins. The N-acetylglucosaminide (.alpha.2 .fwdarw. 6)-sialyltranferase has a marked preference for the sequence NeuAc.alpha.2 .fwdarw. 3-Gal.beta.1 .fwdarw. 3GlcNAc as an acceptor substrate. Thus, the order of addition of the 2 sialic acids in the disialylated structure is proposed to be first the terminal sialic acid in the Neu-Ac.alpha.2 .fwdarw. 3Gal linkage followed by the internal sialic acid in the NeuAc.alpha.2 .fwdarw. 6GlcNAc linkage. Sialylation in vitro of the type 1 branches (Gal.beta.1 .fwdarw. 3GlcNAc-) of the N-linked oligosaccharides of asialo prothrombin to produce the same disialylated sequence is also demonstrated.