A SOLUBLE FORM OF SD(A)-BETA-1,4-N-ACETYLGALACTOSAMINYL-TRANSFERASE IS RELEASED BY DIFFERENTIATED HUMAN COLON-CARCINOMA CACO-2 CELLS

We have previously shown that human colon carcinoma CaCo-2 cells express the Sd(a)-beta 1,4-N-acetylgalactosaminyltransferase (Sd(a)-beta GalNAc-transferase) and that the enzyme activity correlates with the degree of enterocytic differentiation. Here we report that a large amount of this glycosyltransferase is released in soluble form, particularly when CaCo-2 cells are maintained in culture for more than 3 weeks in order to ensure an higher degree of enterocyte differentiation. The soluble enzyme was concentrated and partially purified by Blue-Sepharose and fetuin-Sepharose chromatography. The substrate specificity of the partially purified enzyme was similar to that of Sda-enzyme from epithelial cells of colon mucosa, and for its activity strictly required the presence in accepters of NeuAc in alpha 2,3-linkage to subterminal galactose. Among the low molecular glycans tested, NeuAc alpha 2,3Gal beta 1,4GlcNAc appeared to be the best acceptor, whereas sialyl-Lewis(x) and sialyl-Lewis(a) did not serve as accepters, indicating that the fucosylation of sub-terminal GlcNAc hindered the transferase activity. Contrary to this, the activity towards a disialylated acceptor such as di-sialyl-lacto-N-tetraose was reduced but not abolished. When CaCo-2 cells were cultured on porous membranes and the transferase activity assayed in medium collected from chambers corresponding to either the apical or basolateral face of highly differentiated CaCo-2 cells, a preferential release from the basolateral surface was found. Considering that Sd(a)-beta GalNAc-transferase is mainly located in the large intestine, current results support the notion that colonic cells largely contribute to the presence of the enzyme in human plasma.