Porcine Sugar Nucleotide: Glycoprotein Glycosyltransferases. II. Blood Serum and Liver Galactosyltransferase

Abstract

Pork serum and liver have been shown to contain UDP-galactose galactosyltransf erase activity towards both N-acetylglucosamine and sialidase- and β-galactosidase-treated α1-acid glycoprotein. The serum enzyme is soluble while the liver enzyme is membrane-bound. A soluble liver enzyme has been prepared by an organic solvent extraction method, but this preparation is unstable. Comparison of soluble and membrane-bound liver enzymes at pH 8.0–8.7 has indicated that membrane has a relatively minor modifying effect on enzyme kinetics. Comparison of serum and liver enzymes at pH 8.0–8.7 shows that both enzymes have an absolute dependence on Mn2+ ion, and have similar substrate specificities; the preparations differ in kinetic parameters to a minor extent. Thus the serum galactosyltransferase is similar but not identical to the liver enzyme; the source and function of the serum enzyme is not known. The addition of α-lactalbumin to the serum preparation and to both the soluble and membrane-bound liver enzymes results in inhibition of N-acetyllactosamine synthetase activity and the appearance of lactose synthetase activity; α-lactalbumin has no appreciable effect on the UDP-galactose:glycoprotein galactosyltransferase activity. Competition studies show that in all three preparations, N-acetylglucosamine and glycoprotein acceptor compete for a single site. It therefore appears that the same enzyme is responsible for galactosyltransferase activity towards both N-acetylglucosamine and glycoprotein, and further, that the serum and liver enzymes are similar to milk lactose synthetase A protein.