Characterization of UDP-galactosyl:asialo-mucin transferase activity in the Golgi system of rat liver.

UDPgalactosyltransferase activity (UDPgalactose:mucopolysaccharide galactosyltransferase, EC 2.4.1.74) was measured in a well-characterized fraction of Golgi membranes in the presence of UDPgalactose and exogenous acceptor sites. Substrate saturation for 0.05 mg Golgi protein was achieved at a concentration of 4.6 mM UDPgalactose. Desialylated mucin proved to be the most suitable acceptor protein. Access to galactose acceptor sites was not rate limiting for the reaction when 20 mg of asialo-mucin/ml of incubation mixture was used. With these concentrations of substrates the use of nucleotides to inhibit pyrophosphatases and of detergents to perturb the membrane structure was not necessary and proved, in fact, to be inhibitory to galactose transfer. UDPgalactosyl:asialo-mucin transferase activity in Golgi membranes was 230 nmol galactose transferred/mg Golgi protein per 30 min.