RESUMO
This report concerns the stepwise biosynthesis in vitro of Sialyl Lewis X, (SA-Le(x)), a carcinoembryonic antigen, in human colon carcinoma KM12 cells exhibiting different metastatic behaviors. The significance of SA-Le(x) has become even more apparent since the detection of its terminal epitope NeuAc(alpha 2-3)Gal beta 1-4(Fuc alpha 1-3)GlcNAc-, as the binding ligand of the selectin family member ELAM-1. The activity level of galactosyltransferase GalT-4 which catalyzes the formation of core nLcOse4Cer (Gal beta 1-4GlcNAc beta 1-3Gal beta 1-4Glc-Cer) is very high in all the metastatic lines tested with highly metastatic lines (KM12-SM) exhibiting the highest activity. The same activity pattern for galactosyltransferase is also observed when tested with iLcOse5Cer (GlcNAc beta 1-3nLcOse4Cer), the precursor for polylactosamine glycolipid. Sialyltransferase SAT-3 which catalyzes the formation of LM1 (NeuAc alpha 2-3nLcOse4Cer), the precursor for SA-Le(x), is also present in all the metastatic cell lines although the activity levels are much lower compared to galactosyltransferase. The fucosyltransferase FucT-3, which catalyzes the formation of R'-Gal-Fuc(alpha 1-3)GlcNAc-R linkage, is active with both nonsialylated substrate, nLcOse4Cer, and sialylated substrate, LM1 (NeuAc alpha 2-3nLcOse4Cer) with the formation of either Le(x) (Gal beta 1-4(Fuc alpha 1-3)GlcNAc beta 1-3Gal beta 1-4Glc-Cer) or SA-Le(x) (NeuAc alpha 2-3nLcOse4Cer). However, the sialylated substrate LM1 is preferred to enzymatic activity since it exhibited lower Km (46 microM) than that of nLcOse4Cer (67 microM).(ABSTRACT TRUNCATED AT 250 WORDS)