Quantitative differences in GlcNAc:beta1-->3 and GlcNAc:beta1-->4 galactosyltransferase activities between human...

The activities of GlcNAc:beta1-->3 and GlcNAc:beta1->4 galactosyltransferases in normal human colonic mucosa and well or moderately differentiated colonic adenocarcinomas and their enzyme-kinetic characteristics were investigated. After UDP-[3H]galactose and N-linked type monoantennary oligosaccharides GlcNAc beta1-->2Man alpha1-->3(6)Man beta1-->4GlcNAc) had been incubated with microsome fractions prepared from these tissues, the synthesized [3H]galactose-labeled oligosaccharides were analyzed by Ricinus communis agglutinin-I agarose chromatography, Streptococcus 6646K beta-galactosidase, Gal beta1-->4-specific diplococcal beta-galactosidase, and Gal beta1-->3GlcNAc-specific lacto-N-biosidase digestion. The beta-galactosyltransferases from normal mucosa synthesized both type 1 and type 2 chains at comparable levels, whereas those from adenocarcinomas predominantly synthesized type 2 chains. To our knowledge, this is the first quantitative estimation of GlcNAc:beta1-->3 galactosyltransferase activity toward N-linked sugar chains. Furthermore, we compared the two galactosyltransferase activities in 10 normal mucosa and adenocarcinoma samples and found that while there existed similar levels of GlcNAc:beta1-->4 galactosyltransferase activity in normal mucosa and adenocarcinomas, GlcNAc:beta1-->3 galactosyltransferase activity apparently decreased from 0.67 +/- 0.26 (normal mucosa) to 0.18 +/- 0.11 nmol/min/mg of protein (adenocarcinomas). These results are consistent with those of comparative structural studies on N-linked sugar chains of carcinoembryonic antigen and its normal counterparts and suggest that in the process of differentiated carcinogenesis of human colonic tissues, the expression of GlcNAc:beta1-->3 galactosyltransferase is negatively regulated.