Role of galactosyl-transferases in rat gastric epithelial glycoprotein synthesis

Two galactosyl-transferases have been found in the Golgi-enriched subcellular fractions derived from rat gastric mucosa. One incorporates galactose into ovomucoid at optimal pH 6.8. The reaction can be completely inhibited by acetylglucosamine. The apparent Km for UDPgalactose is 0.024 mM. The other galactosyl-transferase incorporates galactose into desialated ovine submaxillary mucin at optimal pH 7.5 and the transfer cannot be inhibited by acetylglucosamine. The apparent Km for UDPgalactose is 0.191 mM. Both enzymes require Mn2+ and Triton X-100 for optimal galactose incorporation. The enzymes could be separated by polyacrylamide gel electrophoresis. Incorporation into endogenous glycoprotein was studied in conditions optimal for the two galactosyl-transferases: (1) at pH 6.8, using Mes as buffer system, and (2) at pH 7.5, using Tris-HCl in the presence of an inhibitory excess of acetylglucosamine. In both cases, most radioactive galactose is incorporated into macromolecules, which could be identified as epithelial glycoprotein. Endogenous incorporation in the presence of excess acetylglucosamine results in the formation of a substantial amount of a disaccharide (probably galactose-beta-(1-3)acetylgalactosamine), whereas upon incorporation at pH 6.8 almost no disaccharide is formed. Quantitative immunoprecipitation experiments with specific antibodies to the endogenous product, labelled by [3H]galactose in the presence of varying amounts of desialated ovine submaxillary mucin and/or acetylglucosamine, indicated that other galactosyl-transferases are involved in the biosynthesis of epithelial glycoprotein.