The charge heterogeneity of soluble human galactosyltransferases isolated from milk, amniotic fluid and malignant ascites.

UDP-galactose: N-acetylglucosamine galactosyltransferase was isolated from pooled human milk, pooled amniotic fluid and from two different individual samples of malignant ascites. The purification procedure involving two successive affinity chromatography steps on N-acetylglucosamine--agarose and alpha-lactalbumin--agarose yielded an enzyme preparation homogeneous by size. Under non-denaturing conditions the ascites and amniotic fluid enzymes had identical electrophoretic mobility, but they moved faster than the milk enzyme. Isoelectric analysis in the presence and absence of urea resolved the milk enzyme into at least 13 different forms, nine of which had the same isoelectric points after refocusing. All enzyme forms showed similar activity when free N-acetylglucosamine, ovalbumin, sialic-acid-free ovine submaxillary mucin and glucose, in the presence of alpha-lactalbumin, were used as acceptor substrates. Comparative isoelectric focusing of the three galactosyltransferases revealed identical patterns of the amniotic and ascites enzymes, but only partial overlap with the milk enzyme, which was less negatively charged. Neuraminidase treatment of ascites and milk galactosyltransferases produced very similar focusing patterns. The possible structural basis for this charge heterogeneity is briefly discussed.

Human serum galactosyltransferase: distinction, separation and product identification of two galactosyltransferase activities.

Two different galactosyltransferase activities have been found in normal sera from A and O donors. Galactosyltransferase A incorporated galactose from UDP-Gal into sialic-acid-free ovine submaxillary mucin (asialo-mucin), whereas galactosyltransferase B transferred galactose from UDP-Gal to free N-acetylglucosamine or N-acetylglucosamine-glycoproteins. Specificity, kinetic and stability differences permitted the distinction of the activity of galactosyltransferase A from that of galactosyltransferase B; the only substrate found for galactosyltransferase A was asialo-mucin, whereas galactosyltransferase B showed only low activity towards asialo-mucin and free N-acetyl-galactosamine, but had a main specificity for either free N-acetylglucosamine or N-acetylglucosamine-protein. Galactosyltransferase B was more stable on heat inactivation than galactosyltransferase A; galactosyltransferase B could be separated from galactosyltransferase A by affinity chromatography on N-acetylglucosamine-derivatized agarose. The products of both enzyme activities have been analyzed. The galactosyltransferase A product was cleaved from asialo-mucin by alkaline-borohydride treatment. The acceptor used to identify the galactosyltransferase B product was free N-acetylglucosamine. Periodate oxidation studies performed on the reduced disaccharides indicated the linkage type of the products. The anomeric configuration of the respective galactosyltransferase products were determined with specific galactosidases. Using these methods, galactosyltransferase A was found to form a Galbeta (1 leads to 3)GalNAc-protein linkage and galactosyltransferase B was found to form a Galbeta(1 leads to 4)GlcNAc-linkage.