IDENTIFICATION AND CHARACTERIZATION OF A UDP-GALNAC-GLCNAC-BETA-R BETA-1-]4-N-ACETYLGALACTOSAMINYLTRANSFERASE FROM CERCARIAE OF THE SCHISTOSOME TRICHOBILHARZIA-OCELLATA - CATALYSIS OF A KEY STEP IN THE SYNTHESIS OF N,N'-DIACETYLLACTOSEDIAMINO (LACDINAC)-TYPE GLYCANS

Three different stages of the avian schistosome Trichobilharzia ocella ta appeared to contain a novel N-acetylgalactosaminyltransferase activ ity. To investigate its function in the biosynthesis of schistosome gl ycoconjugates, the enzyme was partially purified from cercariae, a fre e-living stage of the parasite, by affinity chromatography on UDP-Seph arose. Acceptor specificity studies showed that the enzyme catalyses t he transfer of N-acetylgalactosamine (GalNAc) from UDP-GalNAc to oligo saccharides, glycopeptides and glycoproteins carrying a terminally bet a-linked N-acetylglucosamine (GlcNAc) residue, regardless of the under lying structure. Analysis of the products obtained with GlcNAc and a d esialylated and degalactosylated diantennary glycopeptide by 400 MHz H -1-NMR spectroscopy revealed that a GalNAc beta 1-->4GlcNAc (N,N'-diac etyllactosediamine, lacdiNAc) unit was formed. The enzyme can therefor e be described as a UDP-GalNAc:GlcNAc beta-R beta 1-->4-N-acetylgalact osaminytransferase (beta 4-GalNAcT). Using specific accepters, the enz yme could be distinguished from all other beta 4-GalNAcTs described to date, including the one from pituitary gland that is involved in the specific glycosylation of pituitary glycohormones. By contrast, the en zymatic properties of the schistosome beta 4-GalNAcT (except for the s ugar-donor specificity) strongly resemble those of the beta 4-galactos yltransferase of higher animals, an enzyme which is known to control t he synthesis of Gal1-->4GlcNAc (lacNAc)-type oligosaccharide chains. B y analogy, the beta 4-GalNAcT is concluded to control the key step in the synthesis of lacdiNAc-type chains. LacdiNAc-type glycans are also common to the mollusc Lymnaea stagnalis, which is the intermediate hos t of T.ocellata. It is proposed that the schistosome beta 4-GalNAcT fu nctions in the expression of specific carbohydrate structures that con tribute to a molecular mimicry, enabling the schistosome to evade the defence system of the snail host.