Human Lewis alpha 1-->3/4fucosyltransferase: specificity of fucose transfer to GlcNAc beta 1-->3Gal beta 1-->4Glc beta 1-->1Cer (LcOse3Cer).

Biosynthesis of the Lex series of carbohydrate antigens proceeds by fucose transfer in alpha 1-->3-linkage to the pen-ultimate GlcNAc residue of a neolacto-series oligosaccharide acceptor, a reaction catalysed by multiple enzymes expressed in human tissues. Particularly broad acceptor specificity, including the ability to catalyse fucose transfer to both lacto- and neolacto-series acceptors as well as the precursor Lc3 structure (where Lc3, lactotriaosylceramide, is GlcNAc beta 1-->3Gal beta 1-->4Glc beta 1-->1Cer), exists for one human fucosyltransferase form, the Lewis alpha 1-->3/4fucosyltransferase (FucT-III). To determine if fucose transfer to Lc3 may represent an alternate early step in Le(x) or Le(a) antigen biosynthesis with this enzyme, the chemical structure of the fucosylated Lc3 reaction product formed by the Lewis alpha 1-->3/4fucosyltransferase from Colo 205 cells has been defined. Transfer of [14C]fucose to Lc3 yielded a labelled product migrating as a tetrasaccharide on thin layer chromatography plates. This product remained an acceptor for both beta 1-->3- and beta 1-->4-galactosyl transfer on the terminal GlcNAc residue. The product was degraded to a fucosylated trisaccharide derivative by bovine kidney beta-N-acetylglucosaminidase. Fast atom bombardment mass spectrometry and methylation analysis confirmed that the product was composed exclusively of the following structure containing a fucose linked to the 3-position of the internal Glc residue: [formula; see text] Such a structure does not represent an intermediate in Le(x) or Le(a) antigen biosynthesis. Thus, the evidence suggests that Le(x) or Le(a) antigen synthesis results exclusively from fucosylation of complete core chains.